Sebastian Sanchez | Universidad Nacional Autónoma de México (original) (raw)

Papers by Sebastian Sanchez

Astronomy & Astrophysics, 2016

We measured the distribution in absolute magnitude-circular velocity space for a well-defined sam... more We measured the distribution in absolute magnitude-circular velocity space for a well-defined sample of 199 rotating galaxies of the Calar Alto Legacy Integral Field Area Survey (CALIFA) using their stellar kinematics. Our aim in this analysis is to avoid subjective selection criteria and to take volume and large-scale structure factors into account. Using stellar velocity fields instead of gas emission line kinematics allows including rapidly rotating early-type galaxies. Our initial sample contains 277 galaxies with available stellar velocity fields and growth curve r-band photometry. After rejecting 51 velocity fields that could not be modelled because of the low number of bins, foreground contamination, or significant interaction, we performed Markov chain Monte Carlo modelling of the velocity fields, from which we obtained the rotation curve and kinematic parameters and their realistic uncertainties. We performed an extinction correction and calculated the circular velocity v circ accounting for the pressure support of a given galaxy. The resulting galaxy distribution on the M r −v circ plane was then modelled as a mixture of two distinct populations, allowing robust and reproducible rejection of outliers, a significant fraction of which are slow rotators. The selection effects are understood well enough that we were able to correct for the incompleteness of the sample. The 199 galaxies were weighted by volume and large-scale structure factors, which enabled us to fit a volume-corrected Tully-Fisher relation (TFR). More importantly, we also provide the volume-corrected distribution of galaxies in the M r − v circ plane, which can be compared with cosmological simulations. The joint distribution of the luminosity and circular velocity space densities, representative over the range of −20 > M r > −22 mag, can place more stringent constraints on the galaxy formation and evolution scenarios than linear TFR fit parameters or the luminosity function alone.

The Astrophysical Journal, 2007

We report the discovery of an almost complete (∼ 300 •) Einstein ring of diameter 10 ′′ in Sloan ... more We report the discovery of an almost complete (∼ 300 •) Einstein ring of diameter 10 ′′ in Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). Spectroscopic data from the 6m telescope of the Special Astrophysical Observatory reveals that the deflecting galaxy has a line-of-sight velocity dispersion in excess of 400 kms −1 and a redshift of 0.444, whilst the source is a star-forming galaxy with a redshift of 2.379. From its color, luminosity and velocity dispersion, we argue that this is the most massive galaxy lens hitherto discovered.

Monthly Notices of the Royal Astronomical Society, 2009

We present discovery images, together with follow-up imaging and spectroscopy, of two large-separ... more We present discovery images, together with follow-up imaging and spectroscopy, of two large-separation gravitational lenses found by our survey for wide arcs [the CAmbridge Sloan Survey Of Wide ARcs in the skY (CASSOWARY)]. The survey exploits the multicolour photometry of the Sloan Digital Sky Survey to find multiple blue components around red galaxies. CASSOWARY 2 (or 'the Cheshire Cat') is composed of two massive early-type galaxies at z = 0.426 and 0.432, respectively, lensing two background sources, the first a star-forming galaxy at z = 0.97 and the second a high-redshift galaxy (z > 1.4). There are at least three images of the former source and probably four or more of the latter, arranged in two giant arcs. The mass enclosed within the larger arc of radius ∼11 arcsec is ∼33 × 10 12 M. CASSOWARY 3 comprises an arc of three bright images of a z = 0.725 source, lensed by a foreground elliptical at z = 0.274. The radius of the arc is ∼4 arcsec and the enclosed mass is ∼2.5 × 10 12 M. Together with earlier discoveries like the Cosmic Horseshoe and the 8 o'clock Arc, these new systems, with separations intermediate between the arcsecondseparation lenses of typical strong galaxy lensing and arcminute-separation cluster lenses, probe the very high end of the galaxy mass function.

Annual Review of Astronomy and Astrophysics, 2020

I review the spatially resolved spectroscopic properties of low-redshift star-forming galaxies (a... more I review the spatially resolved spectroscopic properties of low-redshift star-forming galaxies (and their retired counterparts) using results from the most recent optical integral field spectroscopy galaxy surveys. First, I briefly summarize the global spectroscopic properties of these galaxies, discussing the main ionization processes and the global relations described by the star-formation rates, gas-phase oxygen abundances, and average properties of their stellar populations (age and metallicity) in comparison with the stellar mass. Second, I present the local distribution of the ionizing processes down to kiloparsec scales, and I show how the global scaling relations found using integrated parameters (like the star-formation main sequence, mass–metallicity relation, and Schmidt–Kennicutt law) have local/resolved counterparts, with the global ones being, for the most part, just integrated/average versions of the local ones. I discuss the local/resolved star-formation histories (S...

The Astrophysical Journal, 2014

We present an overview of a new integral field spectroscopic survey called MaNGA (Mapping Nearby ... more We present an overview of a new integral field spectroscopic survey called MaNGA (Mapping Nearby Galaxies at Apache Point Observatory), one of three core programs in the fourth-generation Sloan Digital Sky Survey (SDSS-IV) that began on 2014 July 1. MaNGA will investigate the internal kinematic structure and composition of gas and stars in an unprecedented sample of 10,000 nearby galaxies. We summarize essential characteristics of the instrument and survey design in the context of MaNGA's key science goals and present prototype observations to demonstrate MaNGA's scientific potential. MaNGA employs dithered observations with 17 fiber-bundle integral field units that vary in diameter from 12 (19 fibers) to 32 (127 fibers). Two dual-channel spectrographs provide simultaneous wavelength coverage over 3600-10300Å at R∼2000. With a typical integration time of 3 hr, MaNGA reaches a target r-band signal-to-noise ratio of 4-8 (Å −1 per 2 fiber) at 23 AB mag arcsec −2 , which is typical for the outskirts of MaNGA galaxies. Targets are selected with M * 10 9 M using SDSS-I redshifts and i-band luminosity to achieve uniform radial coverage in terms of the effective radius, an approximately flat distribution in stellar mass, and a sample spanning a wide range of environments. Analysis of our prototype observations demonstrates MaNGA's ability to probe gas ionization, shed light on recent star formation and quenching, enable dynamical modeling, decompose constituent components, and map the composition of stellar populations. MaNGA's spatially resolved spectra will enable an unprecedented study of the astrophysics of nearby galaxies in the coming 6 yr.

Multiwavelength AGN Surveys, 2004

We have mapped the AGN luminosity function and its evolution between z = 1 and z = 5 down to appa... more We have mapped the AGN luminosity function and its evolution between z = 1 and z = 5 down to apparent magnitudes of R < 24. Within the GEMS project we have analysed HST-ACS images of many AGN in the Extended Chandra Deep Field South, enabling us to assess the evolution of AGN host galaxy properties with cosmic time.

Multiwavelength Mapping of Galaxy Formation and Evolution

Astrophysical Journal Letters, 2009

We perform a comprehensive estimate of the frequency of galaxy mergers and their impact on star f... more We perform a comprehensive estimate of the frequency of galaxy mergers and their impact on star formation over z ∼ 0.24-0.80 (lookback time T b ∼ 3-7 Gyr) using ∼ 3600 (M ≥ 1 × 10 9 M ⊙) galaxies with GEMS HST , COMBO-17, and Spitzer data. Our results are: (1) Among ∼ 790 high mass (M ≥ 2.5 × 10 10 M ⊙) galaxies, the visually-based merger fraction over z ∼ 0.24-0.80, ranges from 9% ± 5% to 8% ± 2%. Lower limits on the major merger and minor merger fraction over this interval range from 1.1% to 3.5% , and 3.6% to 7.5%, respectively. This is the first, albeit approximate, empirical estimate of the frequency of minor mergers over the last 7 Gyr. Assuming a visibility timescale of ∼ 0.5 Gyr, it follows that over T b ∼ 3-7 Gyr, ∼ 68% of high mass systems have undergone a merger of mass ratio > 1/10, with ∼ 16%, 45%, and 7% of these corresponding respectively to major, minor, and ambiguous 'major or minor' mergers. The average merger rate is ∼ a few ×10 −4 galaxies Gyr −1 Mpc −3. Among ∼ 2840 blue cloud galaxies of mass M ≥ 1.0 × 10 9 M ⊙ , similar results hold. (2) We compare the empirical merger fraction and merger rate for high mass galaxies to three ΛCDM-based models: halo occupation distribution models, semi-analytic models, and hydrodynamic SPH simulations. We find qualitative agreement between observations and models such that the (major+minor) merger fraction or rate from different models bracket the observations, and show a factor of five dispersion. Near-future improvements can now start to rule out certain merger scenarios. (3) Among ∼ 3698 M ≥ 1.0 × 10 9 M ⊙ galaxies, we find that the mean SFR of visibly merging systems is only modestly enhanced compared to non-interacting galaxies over z ∼ 0.24-0.80. Visibly merging systems only account for a small fraction (< 30%) of the cosmic SFR density over T b ∼ 3-7 Gyr. This complements the results of Wolf et al. (2005) over a shorter time interval of T b ∼ 6.2-6.8 Gyr, and suggests that the behavior of the cosmic SFR density over the last 7 Gyr is predominantly shaped by non-interacting galaxies.

The Astrophysical Journal, 2005

We combine imaging from the Hubble Space Telescope Advanced Camera for Surveys, as part of the GE... more We combine imaging from the Hubble Space Telescope Advanced Camera for Surveys, as part of the GEMS (Galaxy Evolution from Morphologies and SEDs) survey, with redshifts and rest-frame quantities from COMBO-17 to study the evolution of morphologically early-type galaxies with red colors since z = 1. From 0.5 • × 0.5 • imaging, we draw a large sample of 728 galaxies with centrally-concentrated radial profiles (i.e. n ≥ 2.5 from Sérsic fits) and rest-frame (U − V) colors on the red sequence. We explore how the correlations of rest-frame V-band luminosity and of stellar mass with intrinsic halflight size change over the last half of cosmic time. By appropriate comparison with the well-defined local relations from the Sloan Digital Sky Survey, we find that the luminosity-size and stellar mass-size relations evolve in a manner that is consistent with the passive aging of ancient stellar populations. By itself, this result is consistent with a completely passive evolution of the red early-type galaxy population. If instead, as demonstrated by a number of recent surveys, the early-type galaxy population builds up in mass by roughly a factor of two since z ∼ 1, our results imply that new additions to the early-type galaxy population follow similar luminosity-size and stellar mass-size correlations, compared to the older subset of early-type galaxies. Adding early-type galaxies to the red sequence through the fading of previously prominent disks appears to be consistent with the data. Through comparison with models, the role of dissipationless merging is limited to < 1 major merger on average since z = 1 for the most massive galaxies. Predictions from models of gas-rich mergers are not yet mature enough to allow a detailed comparison to our observations. We find tentative evidence that the amount of luminosity evolution depends on galaxy stellar mass, such that the least massive galaxies show stronger luminosity evolution compared to more massive early types. This could reflect a different origin of low-mass earlytype galaxies and/or younger stellar populations; the present data is insufficient to discriminate between these possibilities.

Proceedings of the International Astronomical Union, 2004

We present our cosmic shear analysis of the Galaxy Evolution from Morphology and SEDs (GEMS) surv... more We present our cosmic shear analysis of the Galaxy Evolution from Morphology and SEDs (GEMS) survey. Imaged with the Advanced Camera for Surveys (ACS) on HST, GEMS provides high resolution imaging spanning some 800 square arcmins in the Chandra Deep Field South (CDFS). We discuss the benefits of using space-based data for weak lensing studies and show that the ACS is a very powerful instrument in this regard. We find that we are not limited by systematic errors arising from the anisotropic ACS point spread function distortion and use our cosmic shear results to place joint constraints on the matter density parameter Ω m and the amplitude of the matter power spectrum σ8, finding σ8(Ωm /0.3) 0.62 = 0.73 ± 0.12. To investigate the impact of atmospheric seeing on weak lensing analysis we compare the shear measured from CDFS galaxies resolved by the COMBO-17 survey and imaged by GEMS. We find good agreement between the two surveys and a higher dispersion in the intrinsic ellipticity distribution of COMBO-17. This dispersion implies that a space-based cosmic shear analysis would yield higher signal-to-noise results compared to a ground-based cosmic shear analysis of the same galaxy sample.

Eso Astrophysics Symposia

We report on integral field spectrocopy observations, performed with the PPAK module of the PMAS ... more We report on integral field spectrocopy observations, performed with the PPAK module of the PMAS spectrograph, covering a field-of-view of ∼74"×64" centered on the core of the galaxy cluster Abell 2218. A total of 43 objects were detected, 27 of them galaxies at the redshift of the cluster. We deblended and extracted the integrated spectra of each of the objects in the field using an adapted version of galfit for 3D spectroscopy (galfit3d). We use these spectra, in combination with morphological parameters derived from deep HST/ACS images, to study the stellar population and evolution of galaxies in the core of this cluster.

The Astrophysical Journal, 2008

Surveys of distant galaxies with the Hubble Space Telescope and from the ground have shown that t... more Surveys of distant galaxies with the Hubble Space Telescope and from the ground have shown that there is only mild evolution in the relationship between radial size and stellar mass for galactic disks from z ∼ 1 to the present day. Using a sample of nearby disk-dominated galaxies from the Sloan Digital Sky Survey (SDSS), and high redshift data from the GEMS (Galaxy Evolution from Morphology and SEDs) survey, we investigate whether this result is consistent with theoretical expectations within the hierarchical paradigm of structure formation. The relationship between virial radius and mass for dark matter halos in the ΛCDM model evolves by about a factor of two over this interval. However, N-body simulations have shown that halos of a given mass have less centrally concentrated mass profiles at high redshift. When we compute the expected disk size-stellar mass distribution, accounting for this evolution in the internal structure of dark matter halos and the adiabatic contraction of the dark matter by the self-gravity of the collapsing baryons, we find that the predicted evolution in the mean size at fixed stellar mass since z ∼ 1 is about 15-20 percent, in good agreement with the observational constraints from GEMS. At redshift z ∼ 2, the model predicts that disks at fixed stellar mass were on average only 60% as large as they are today. Similarly, we predict that the rotation velocity at a given stellar mass (essentially the zero-point of the Tully-Fisher relation) is only about 10 percent larger at z ∼ 1 (20 percent at z ∼ 2) than at the present day.

The Astrophysical Journal, 2005

We combine HST imaging from the GEMS 1 survey with photometric redshifts from COMBO-17 to explore... more We combine HST imaging from the GEMS 1 survey with photometric redshifts from COMBO-17 to explore the evolution of disk-dominated galaxies since z 1.1. The sample is comprised of all GEMS galaxies with Sérsic indices n < 2.5, derived from fits to the galaxy images. We account fully for selection effects through careful analysis of image simulations; we are limited by the depth of the redshift and HST data to the study of galaxies with M V −20, or equivalently log (M/M ⊙) 10. We find strong evolution in the magnitude-size scaling relation for galaxies with M V −20, corresponding to a brightening of ∼1 mag arcsec −2 in rest-frame V-band by z ∼ 1. Yet, disks at a given absolute magnitude are bluer and have lower stellar mass-to-light ratios at z ∼ 1 than at the present day. As a result, our findings indicate weak or no evolution in the relation between stellar mass and effective disk size for galaxies with log (M/M ⊙) 10 over the same time interval. This is strongly inconsistent with the most naive theoretical expectation, in which disk size scales in proportion to the halo virial radius, which would predict that disks are a factor of two denser at fixed mass at z ∼ 1. The lack of evolution in the stellar mass-size relation is consistent with an "inside-out" growth of galaxy disks on average (galaxies increasing in size as they grow more massive), although we cannot rule out more complex evolutionary scenarios.

The Astrophysical Journal, 2006

We have used the 28 ′ × 28 ′ Hubble Space Telescope image mosaic from the GEMS (Galaxy Evolution ... more We have used the 28 ′ × 28 ′ Hubble Space Telescope image mosaic from the GEMS (Galaxy Evolution from Morphology and SEDs) survey in conjunction with the COMBO-17 photometric redshift survey to constrain the incidence of major mergers between spheroid-dominated galaxies with little cold gas (dry mergers) since z = 0.7. A set of N-body merger simulations was used to explore the morphological signatures of such interactions: they are recognizable either as < 5 kpc separation close pairs or because of broad, low surface brightness tidal features and asymmetries. Data with the depth and resolution of GEMS are sensitive to dry mergers between galaxies with M V −20.5 for z 0.7; dry mergers at higher redshifts are not easily recovered in single-orbit HST imaging. Six dry mergers (12 galaxies) with luminosity ratios between 1:1 and 4:1 were found from a sample of 379 red early-type galaxies with M V < −20.5 and 0.1 < z < 0.7. The simulations suggest that the morphological signatures of dry merging are visible for ∼ 150 Myr and we use this timescale to convert the observed merger incidence into a rate. On this basis we find that present day spheroidal galaxies with M V < −20.5 on average have undergone between 0.5 and 2 major dry mergers since z ∼ 0.7. We have compared this result with the predictions of a Cold Dark Matter based semi-analytic galaxy formation model. The model reproduces the observed declining major merger fraction of bright galaxies and the space density of luminous early-type galaxies reasonably well. The predicted dry merger fraction is consistent with our observational result. Hence, hierarchical models predict and observations now show that major dry mergers are an important driver of the evolution of massive early-type galaxies in recent epochs.

The Astrophysical Journal, 2004

We present a detailed comparison of structural properties in the rest-frame V-band of cluster and... more We present a detailed comparison of structural properties in the rest-frame V-band of cluster and field galaxies, selected and analyzed in the same manner, to test the hypothesis that much of the current cluster galaxy membership resulted from the fairly rapid (1-2 gigayears) transformation of infalling, field spirals into red, cluster early-types. Specifically, we have selected ∼ 140 galaxies from three nearby Abell clusters (A85, A496 and A754) that have colors significantly bluer than the red sequence population, and compared them to ∼ 80 field galaxies with similar colors and luminosities from Jansen et al. (2000, ApJS, 126, 271). The comparison is based on the hypothesis that recent (1-4 gigayears) cluster arrivals were originally blue and star-forming, then stopped forming stars to dim and redden in a few gigayears. For the comparison we quantify galaxy internal structure and morphology from two-dimensional bulge/disk decompositions using GIM2D. We observe structural differences between blue galaxies in local (z < 0.06) clusters, compared to field environments. All cluster galaxies have spectroscopic membership. The majority of blue cluster members, presumably recent additions, are physically smaller and fainter than their equally-colored field counterparts. At a matched size and luminosity, the newer cluster arrivals are quantifiably smoother in appearance, yet their total light is as disk-dominated as in normal field spirals. Moreover, half of the blue cluster members appear to have blue cores or globally blue color profiles, in contrast with field spirals, which typically exhibit red inward color gradients. Blue cores suggest enhanced nuclear star formation, possibly a starburst, while uniformly blue profiles are consistent with an episode of fairly strong global star formation in the past few gigayears. Our previous work (McIntosh et al. 2004, ApJ submitted) shows that the blue membership of local clusters is a recently infalling population that has yet to encounter the dense core. In a Universe without environmental dependent evolution outside of cluster cores, we would expect blue disk galaxies inhabiting field and cluster regions to have similar morphology, size, and color gradient distributions. Our findings show conclusively that not only the abundance of red and blue galaxies depends on environment, but also that fundamental structural and morphological galaxy properties do indeed reflect the environment in which the galaxy is found. Moreover, the data show that the transformation of accreted galaxies is not confined to the dense cluster core. The overall properties of bluer cluster members are best explained by environment-driven transformation of accreted field spirals, and our results suggest that the processes that govern color and morphological evolution occur separately.

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The Astrophysical Journal, 2003

We have used the 30 ′ × 30 ′ Hubble Space Telescope image mosaic from the Galaxy Evolution from M... more We have used the 30 ′ × 30 ′ Hubble Space Telescope image mosaic from the Galaxy Evolution from Morphology and SEDs (GEMS) project in conjunction with the COMBO-17 deep photometric redshift survey to define a sample of nearly 1500 galaxies with 0.65 ≤ z ≤ 0.75. With this sample, we can study the distribution of rest-frame V-band morphologies more than 6 Gyr ago, without differential bandpass shifting and surface brightness dimming across this narrow redshift slice. Focusing on red-sequence galaxies at z ∼ 0.7, we find that 85% of their combined restframe V-band luminosity density comes from visually-classified E/S0/Sa galaxies down to M V − 5 log 10 h −19.5. Similar results are obtained if automated classifiers are used. This fraction is identical to that found at the present day, and is biased by less than 10% by large scale structure and the morphology-density relation. Under the assumption that peculiar and edge-on disk galaxies are red by virtue of their dust content, we find that less than 13% of the total rest-frame V-band luminosity of the z ∼ 0.7 red galaxy population is from dusty galaxies.

The Astrophysical Journal, 2008

We explore the amount of obscured star-formation as a function of environment in the A901/902 sup... more We explore the amount of obscured star-formation as a function of environment in the A901/902 supercluster at z = 0.165 in conjunction with a field sample drawn from the A901 and CDFS fields, imaged with HST as part of the STAGES and GEMS surveys. We combine the COMBO-17 near-UV/optical SED with Spitzer 24µm photometry to estimate both the unobscured and obscured star formation in galaxies with M * > 10 10 M ⊙. We find that the star formation activity in massive galaxies is suppressed in dense environments, in agreement with previous studies. Yet, nearly 40% of the star-forming galaxies have red optical colors at intermediate and high densities. These red systems are not starbursting; they have star formation rates per unit stellar mass similar to or lower than blue starforming galaxies. More than half of the red star-forming galaxies have low IR-to-UV luminosity ratios, relatively high Sersic indices and they are equally abundant at all densities. They might be gradually quenching their starformation, possibly but not necessarily under the influence of gas-removing environmental processes. The other 40% of the red star-forming galaxies have high IR-to-UV luminosity ratios, indicative of high dust obscuration. They have relatively high specific star formation rates and are more abundant at intermediate densities. Our results indicate that while there is an overall suppression in the star-forming galaxy fraction with density, the small amount of star formation surviving the cluster environment is to a large extent obscured, suggesting that environmental interactions trigger a phase of obscured star formation, before complete quenching.

The Astrophysical Journal, 2004

We have performed HST imaging of a sample of 23 high-redshift (1.8 < z < 2.75) Active Galactic Nu... more We have performed HST imaging of a sample of 23 high-redshift (1.8 < z < 2.75) Active Galactic Nuclei, drawn from the combo-17 survey. The sample contains moderately luminous quasars (M B ∼ −23). The data are part of the gems imaging survey that provides high resolution optical images obtained with the Advanced Camera for Surveys in two bands (F606W and F850LP), sampling the rest-frame UV flux of the targets. To deblend the AGN images into nuclear and resolved (host galaxy) components we use a PSF subtraction technique that is strictly conservative with respect to the flux of the host galaxy. We resolve the host galaxies in both filter bands in 9 of the 23 AGN, whereas the remaining 14 objects are considered non-detections, with upper limits of less than 5 % of the nuclear flux. However, when we coadd the unresolved AGN images into a single high signal-to-noise composite image we find again an unambiguously resolved host galaxy. The recovered host galaxies have apparent magnitudes of 23.0 < F606W < 26.0 and 22.5 < F850LP < 24.5 with rest-frame UV colours in the range −0.2 < (F606W − F850LP) obs < 2.3. The rest-frame absolute magnitudes at 200 nm are −20.0 < M 200 nm < −22.2. The photometric properties of the composite host are consistent with the individual resolved host galaxies. We find that the UV colors of all host galaxies are substantially bluer than expected from an old population of stars with formation redshift z ≤ 5, independent of the assumed metallicities. These UV colours and luminosities range up to the values found for Lyman-break galaxies (LBGs) at z = 3. Our results suggest either a recent starburst, of e.g. a few per cent of the total stellar mass and 100 Myrs before observation, with mass-fraction and age strongly degenerate, or the possibility that the detected UV emission may be due to young stars forming continuously. For the latter case we estimate star formation rates of typically ∼6 M ⊙ yr −1 (uncorrected for internal dust attenuation), which again lies in the range of rates implied from the UV flux of LBGs. Our results agree with the recent discovery of enhanced blue stellar light in AGN hosts at lower redshifts.

The Astrophysical Journal, 2009

Both observations and simulations show that major tidal interactions or mergers between gas-rich ... more Both observations and simulations show that major tidal interactions or mergers between gas-rich galaxies can lead to intense bursts of star formation. Yet, the average enhancement in star formation rate (SFR) in major mergers and the contribution of such events to the cosmic SFR are not well estimated. Here we use photometric redshifts, stellar masses and UV SFRs from COMBO-17, 24µm SFRs from Spitzer and morphologies from two deep Hubble Space Telescope (HST) cosmological survey fields (ECDFS/GEMS and A901/STAGES) to study the enhancement in SFR as a function of projected galaxy separation. We apply two-point projected correlation function techniques, which we augment with morphologically-selected very close pairs (separation < 2 ′′) and merger remnants from the HST imaging. Our analysis confirms that the most intensely star-forming systems are indeed interacting or merging. Yet, for massive (M * ≥ 10 10 M ⊙) star-forming galaxies at 0.4 < z < 0.8, we find that the SFRs of galaxies undergoing a major interaction (mass ratios ≤ 1 : 4 and separations ≤ 40 kpc) are only 1.80 ± 0.30 times higher than the SFRs of non-interacting galaxies when averaged over all interactions and all stages of the interaction, in good agreement with other observational works. Our results also agree with hydrodynamical simulations of galaxy interactions, which produce some mergers with large bursts of star formation on ∼ 100 Myr timescales, but only a modest SFR enhancement when averaged over the entire merger timescale. We demonstrate that these results imply that only 10% of star formation at 0.4 ≤ z ≤ 0.8 is triggered directly by major mergers and interactions; these events are not important factors in the build-up of stellar mass since z = 1.

The Astronomical Journal, 2006

We present aperture-matched PSF-corrected BV i ′ z ′ JH photometry and Bayesian photometric redsh... more We present aperture-matched PSF-corrected BV i ′ z ′ JH photometry and Bayesian photometric redshifts (BPZ) for objects detected in the Hubble Ultra Deep Field (UDF), 8,042 of which are detected at the 10-σ level (e.g., i ′ < 29.01 or z ′ < 28.43). Most of our objects are defined identically to those in the public STScI catalogs, enabling straightforward object-by-object comparison. We have combined detections from i ′ , z ′ , J+H, and B+V +i ′ +z ′ images into a single comprehensive segmentation map. Using a new program called SExSeg we are able to force this segmentation map into SExtractor for photometric analysis. The resulting photometry is corrected for the wider NIC3 PSFs using our ColorPro software. We also correct for the ACS z ′-band PSF halo. Offsets are applied to our NIC3 magnitudes, which are found to be too faint relative to the ACS fluxes. Based on BPZ SED fits to objects of known spectroscopic redshift, we derived corrections of −0.30 ± 0.03 mag in J and −0.18 ± 0.04 mag in H. Our offsets appear to be supported by a recent recalibration of the UDF NIC3 images combined with non-linearity measured in NICMOS itself. The UDF reveals a large population of faint blue galaxies (presumably young starbursts), bluer than those observed in the original Hubble Deep Fields (HDF).

Astronomy & Astrophysics, 2016

We measured the distribution in absolute magnitude-circular velocity space for a well-defined sam... more We measured the distribution in absolute magnitude-circular velocity space for a well-defined sample of 199 rotating galaxies of the Calar Alto Legacy Integral Field Area Survey (CALIFA) using their stellar kinematics. Our aim in this analysis is to avoid subjective selection criteria and to take volume and large-scale structure factors into account. Using stellar velocity fields instead of gas emission line kinematics allows including rapidly rotating early-type galaxies. Our initial sample contains 277 galaxies with available stellar velocity fields and growth curve r-band photometry. After rejecting 51 velocity fields that could not be modelled because of the low number of bins, foreground contamination, or significant interaction, we performed Markov chain Monte Carlo modelling of the velocity fields, from which we obtained the rotation curve and kinematic parameters and their realistic uncertainties. We performed an extinction correction and calculated the circular velocity v circ accounting for the pressure support of a given galaxy. The resulting galaxy distribution on the M r −v circ plane was then modelled as a mixture of two distinct populations, allowing robust and reproducible rejection of outliers, a significant fraction of which are slow rotators. The selection effects are understood well enough that we were able to correct for the incompleteness of the sample. The 199 galaxies were weighted by volume and large-scale structure factors, which enabled us to fit a volume-corrected Tully-Fisher relation (TFR). More importantly, we also provide the volume-corrected distribution of galaxies in the M r − v circ plane, which can be compared with cosmological simulations. The joint distribution of the luminosity and circular velocity space densities, representative over the range of −20 > M r > −22 mag, can place more stringent constraints on the galaxy formation and evolution scenarios than linear TFR fit parameters or the luminosity function alone.

The Astrophysical Journal, 2007

We report the discovery of an almost complete (∼ 300 •) Einstein ring of diameter 10 ′′ in Sloan ... more We report the discovery of an almost complete (∼ 300 •) Einstein ring of diameter 10 ′′ in Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). Spectroscopic data from the 6m telescope of the Special Astrophysical Observatory reveals that the deflecting galaxy has a line-of-sight velocity dispersion in excess of 400 kms −1 and a redshift of 0.444, whilst the source is a star-forming galaxy with a redshift of 2.379. From its color, luminosity and velocity dispersion, we argue that this is the most massive galaxy lens hitherto discovered.

Monthly Notices of the Royal Astronomical Society, 2009

We present discovery images, together with follow-up imaging and spectroscopy, of two large-separ... more We present discovery images, together with follow-up imaging and spectroscopy, of two large-separation gravitational lenses found by our survey for wide arcs [the CAmbridge Sloan Survey Of Wide ARcs in the skY (CASSOWARY)]. The survey exploits the multicolour photometry of the Sloan Digital Sky Survey to find multiple blue components around red galaxies. CASSOWARY 2 (or 'the Cheshire Cat') is composed of two massive early-type galaxies at z = 0.426 and 0.432, respectively, lensing two background sources, the first a star-forming galaxy at z = 0.97 and the second a high-redshift galaxy (z > 1.4). There are at least three images of the former source and probably four or more of the latter, arranged in two giant arcs. The mass enclosed within the larger arc of radius ∼11 arcsec is ∼33 × 10 12 M. CASSOWARY 3 comprises an arc of three bright images of a z = 0.725 source, lensed by a foreground elliptical at z = 0.274. The radius of the arc is ∼4 arcsec and the enclosed mass is ∼2.5 × 10 12 M. Together with earlier discoveries like the Cosmic Horseshoe and the 8 o'clock Arc, these new systems, with separations intermediate between the arcsecondseparation lenses of typical strong galaxy lensing and arcminute-separation cluster lenses, probe the very high end of the galaxy mass function.

Annual Review of Astronomy and Astrophysics, 2020

I review the spatially resolved spectroscopic properties of low-redshift star-forming galaxies (a... more I review the spatially resolved spectroscopic properties of low-redshift star-forming galaxies (and their retired counterparts) using results from the most recent optical integral field spectroscopy galaxy surveys. First, I briefly summarize the global spectroscopic properties of these galaxies, discussing the main ionization processes and the global relations described by the star-formation rates, gas-phase oxygen abundances, and average properties of their stellar populations (age and metallicity) in comparison with the stellar mass. Second, I present the local distribution of the ionizing processes down to kiloparsec scales, and I show how the global scaling relations found using integrated parameters (like the star-formation main sequence, mass–metallicity relation, and Schmidt–Kennicutt law) have local/resolved counterparts, with the global ones being, for the most part, just integrated/average versions of the local ones. I discuss the local/resolved star-formation histories (S...

The Astrophysical Journal, 2014

We present an overview of a new integral field spectroscopic survey called MaNGA (Mapping Nearby ... more We present an overview of a new integral field spectroscopic survey called MaNGA (Mapping Nearby Galaxies at Apache Point Observatory), one of three core programs in the fourth-generation Sloan Digital Sky Survey (SDSS-IV) that began on 2014 July 1. MaNGA will investigate the internal kinematic structure and composition of gas and stars in an unprecedented sample of 10,000 nearby galaxies. We summarize essential characteristics of the instrument and survey design in the context of MaNGA's key science goals and present prototype observations to demonstrate MaNGA's scientific potential. MaNGA employs dithered observations with 17 fiber-bundle integral field units that vary in diameter from 12 (19 fibers) to 32 (127 fibers). Two dual-channel spectrographs provide simultaneous wavelength coverage over 3600-10300Å at R∼2000. With a typical integration time of 3 hr, MaNGA reaches a target r-band signal-to-noise ratio of 4-8 (Å −1 per 2 fiber) at 23 AB mag arcsec −2 , which is typical for the outskirts of MaNGA galaxies. Targets are selected with M * 10 9 M using SDSS-I redshifts and i-band luminosity to achieve uniform radial coverage in terms of the effective radius, an approximately flat distribution in stellar mass, and a sample spanning a wide range of environments. Analysis of our prototype observations demonstrates MaNGA's ability to probe gas ionization, shed light on recent star formation and quenching, enable dynamical modeling, decompose constituent components, and map the composition of stellar populations. MaNGA's spatially resolved spectra will enable an unprecedented study of the astrophysics of nearby galaxies in the coming 6 yr.

Multiwavelength AGN Surveys, 2004

We have mapped the AGN luminosity function and its evolution between z = 1 and z = 5 down to appa... more We have mapped the AGN luminosity function and its evolution between z = 1 and z = 5 down to apparent magnitudes of R < 24. Within the GEMS project we have analysed HST-ACS images of many AGN in the Extended Chandra Deep Field South, enabling us to assess the evolution of AGN host galaxy properties with cosmic time.

Multiwavelength Mapping of Galaxy Formation and Evolution

Astrophysical Journal Letters, 2009

We perform a comprehensive estimate of the frequency of galaxy mergers and their impact on star f... more We perform a comprehensive estimate of the frequency of galaxy mergers and their impact on star formation over z ∼ 0.24-0.80 (lookback time T b ∼ 3-7 Gyr) using ∼ 3600 (M ≥ 1 × 10 9 M ⊙) galaxies with GEMS HST , COMBO-17, and Spitzer data. Our results are: (1) Among ∼ 790 high mass (M ≥ 2.5 × 10 10 M ⊙) galaxies, the visually-based merger fraction over z ∼ 0.24-0.80, ranges from 9% ± 5% to 8% ± 2%. Lower limits on the major merger and minor merger fraction over this interval range from 1.1% to 3.5% , and 3.6% to 7.5%, respectively. This is the first, albeit approximate, empirical estimate of the frequency of minor mergers over the last 7 Gyr. Assuming a visibility timescale of ∼ 0.5 Gyr, it follows that over T b ∼ 3-7 Gyr, ∼ 68% of high mass systems have undergone a merger of mass ratio > 1/10, with ∼ 16%, 45%, and 7% of these corresponding respectively to major, minor, and ambiguous 'major or minor' mergers. The average merger rate is ∼ a few ×10 −4 galaxies Gyr −1 Mpc −3. Among ∼ 2840 blue cloud galaxies of mass M ≥ 1.0 × 10 9 M ⊙ , similar results hold. (2) We compare the empirical merger fraction and merger rate for high mass galaxies to three ΛCDM-based models: halo occupation distribution models, semi-analytic models, and hydrodynamic SPH simulations. We find qualitative agreement between observations and models such that the (major+minor) merger fraction or rate from different models bracket the observations, and show a factor of five dispersion. Near-future improvements can now start to rule out certain merger scenarios. (3) Among ∼ 3698 M ≥ 1.0 × 10 9 M ⊙ galaxies, we find that the mean SFR of visibly merging systems is only modestly enhanced compared to non-interacting galaxies over z ∼ 0.24-0.80. Visibly merging systems only account for a small fraction (< 30%) of the cosmic SFR density over T b ∼ 3-7 Gyr. This complements the results of Wolf et al. (2005) over a shorter time interval of T b ∼ 6.2-6.8 Gyr, and suggests that the behavior of the cosmic SFR density over the last 7 Gyr is predominantly shaped by non-interacting galaxies.

The Astrophysical Journal, 2005

We combine imaging from the Hubble Space Telescope Advanced Camera for Surveys, as part of the GE... more We combine imaging from the Hubble Space Telescope Advanced Camera for Surveys, as part of the GEMS (Galaxy Evolution from Morphologies and SEDs) survey, with redshifts and rest-frame quantities from COMBO-17 to study the evolution of morphologically early-type galaxies with red colors since z = 1. From 0.5 • × 0.5 • imaging, we draw a large sample of 728 galaxies with centrally-concentrated radial profiles (i.e. n ≥ 2.5 from Sérsic fits) and rest-frame (U − V) colors on the red sequence. We explore how the correlations of rest-frame V-band luminosity and of stellar mass with intrinsic halflight size change over the last half of cosmic time. By appropriate comparison with the well-defined local relations from the Sloan Digital Sky Survey, we find that the luminosity-size and stellar mass-size relations evolve in a manner that is consistent with the passive aging of ancient stellar populations. By itself, this result is consistent with a completely passive evolution of the red early-type galaxy population. If instead, as demonstrated by a number of recent surveys, the early-type galaxy population builds up in mass by roughly a factor of two since z ∼ 1, our results imply that new additions to the early-type galaxy population follow similar luminosity-size and stellar mass-size correlations, compared to the older subset of early-type galaxies. Adding early-type galaxies to the red sequence through the fading of previously prominent disks appears to be consistent with the data. Through comparison with models, the role of dissipationless merging is limited to < 1 major merger on average since z = 1 for the most massive galaxies. Predictions from models of gas-rich mergers are not yet mature enough to allow a detailed comparison to our observations. We find tentative evidence that the amount of luminosity evolution depends on galaxy stellar mass, such that the least massive galaxies show stronger luminosity evolution compared to more massive early types. This could reflect a different origin of low-mass earlytype galaxies and/or younger stellar populations; the present data is insufficient to discriminate between these possibilities.

Proceedings of the International Astronomical Union, 2004

We present our cosmic shear analysis of the Galaxy Evolution from Morphology and SEDs (GEMS) surv... more We present our cosmic shear analysis of the Galaxy Evolution from Morphology and SEDs (GEMS) survey. Imaged with the Advanced Camera for Surveys (ACS) on HST, GEMS provides high resolution imaging spanning some 800 square arcmins in the Chandra Deep Field South (CDFS). We discuss the benefits of using space-based data for weak lensing studies and show that the ACS is a very powerful instrument in this regard. We find that we are not limited by systematic errors arising from the anisotropic ACS point spread function distortion and use our cosmic shear results to place joint constraints on the matter density parameter Ω m and the amplitude of the matter power spectrum σ8, finding σ8(Ωm /0.3) 0.62 = 0.73 ± 0.12. To investigate the impact of atmospheric seeing on weak lensing analysis we compare the shear measured from CDFS galaxies resolved by the COMBO-17 survey and imaged by GEMS. We find good agreement between the two surveys and a higher dispersion in the intrinsic ellipticity distribution of COMBO-17. This dispersion implies that a space-based cosmic shear analysis would yield higher signal-to-noise results compared to a ground-based cosmic shear analysis of the same galaxy sample.

Eso Astrophysics Symposia

We report on integral field spectrocopy observations, performed with the PPAK module of the PMAS ... more We report on integral field spectrocopy observations, performed with the PPAK module of the PMAS spectrograph, covering a field-of-view of ∼74"×64" centered on the core of the galaxy cluster Abell 2218. A total of 43 objects were detected, 27 of them galaxies at the redshift of the cluster. We deblended and extracted the integrated spectra of each of the objects in the field using an adapted version of galfit for 3D spectroscopy (galfit3d). We use these spectra, in combination with morphological parameters derived from deep HST/ACS images, to study the stellar population and evolution of galaxies in the core of this cluster.

The Astrophysical Journal, 2008

Surveys of distant galaxies with the Hubble Space Telescope and from the ground have shown that t... more Surveys of distant galaxies with the Hubble Space Telescope and from the ground have shown that there is only mild evolution in the relationship between radial size and stellar mass for galactic disks from z ∼ 1 to the present day. Using a sample of nearby disk-dominated galaxies from the Sloan Digital Sky Survey (SDSS), and high redshift data from the GEMS (Galaxy Evolution from Morphology and SEDs) survey, we investigate whether this result is consistent with theoretical expectations within the hierarchical paradigm of structure formation. The relationship between virial radius and mass for dark matter halos in the ΛCDM model evolves by about a factor of two over this interval. However, N-body simulations have shown that halos of a given mass have less centrally concentrated mass profiles at high redshift. When we compute the expected disk size-stellar mass distribution, accounting for this evolution in the internal structure of dark matter halos and the adiabatic contraction of the dark matter by the self-gravity of the collapsing baryons, we find that the predicted evolution in the mean size at fixed stellar mass since z ∼ 1 is about 15-20 percent, in good agreement with the observational constraints from GEMS. At redshift z ∼ 2, the model predicts that disks at fixed stellar mass were on average only 60% as large as they are today. Similarly, we predict that the rotation velocity at a given stellar mass (essentially the zero-point of the Tully-Fisher relation) is only about 10 percent larger at z ∼ 1 (20 percent at z ∼ 2) than at the present day.

The Astrophysical Journal, 2005

We combine HST imaging from the GEMS 1 survey with photometric redshifts from COMBO-17 to explore... more We combine HST imaging from the GEMS 1 survey with photometric redshifts from COMBO-17 to explore the evolution of disk-dominated galaxies since z 1.1. The sample is comprised of all GEMS galaxies with Sérsic indices n < 2.5, derived from fits to the galaxy images. We account fully for selection effects through careful analysis of image simulations; we are limited by the depth of the redshift and HST data to the study of galaxies with M V −20, or equivalently log (M/M ⊙) 10. We find strong evolution in the magnitude-size scaling relation for galaxies with M V −20, corresponding to a brightening of ∼1 mag arcsec −2 in rest-frame V-band by z ∼ 1. Yet, disks at a given absolute magnitude are bluer and have lower stellar mass-to-light ratios at z ∼ 1 than at the present day. As a result, our findings indicate weak or no evolution in the relation between stellar mass and effective disk size for galaxies with log (M/M ⊙) 10 over the same time interval. This is strongly inconsistent with the most naive theoretical expectation, in which disk size scales in proportion to the halo virial radius, which would predict that disks are a factor of two denser at fixed mass at z ∼ 1. The lack of evolution in the stellar mass-size relation is consistent with an "inside-out" growth of galaxy disks on average (galaxies increasing in size as they grow more massive), although we cannot rule out more complex evolutionary scenarios.

The Astrophysical Journal, 2006

We have used the 28 ′ × 28 ′ Hubble Space Telescope image mosaic from the GEMS (Galaxy Evolution ... more We have used the 28 ′ × 28 ′ Hubble Space Telescope image mosaic from the GEMS (Galaxy Evolution from Morphology and SEDs) survey in conjunction with the COMBO-17 photometric redshift survey to constrain the incidence of major mergers between spheroid-dominated galaxies with little cold gas (dry mergers) since z = 0.7. A set of N-body merger simulations was used to explore the morphological signatures of such interactions: they are recognizable either as < 5 kpc separation close pairs or because of broad, low surface brightness tidal features and asymmetries. Data with the depth and resolution of GEMS are sensitive to dry mergers between galaxies with M V −20.5 for z 0.7; dry mergers at higher redshifts are not easily recovered in single-orbit HST imaging. Six dry mergers (12 galaxies) with luminosity ratios between 1:1 and 4:1 were found from a sample of 379 red early-type galaxies with M V < −20.5 and 0.1 < z < 0.7. The simulations suggest that the morphological signatures of dry merging are visible for ∼ 150 Myr and we use this timescale to convert the observed merger incidence into a rate. On this basis we find that present day spheroidal galaxies with M V < −20.5 on average have undergone between 0.5 and 2 major dry mergers since z ∼ 0.7. We have compared this result with the predictions of a Cold Dark Matter based semi-analytic galaxy formation model. The model reproduces the observed declining major merger fraction of bright galaxies and the space density of luminous early-type galaxies reasonably well. The predicted dry merger fraction is consistent with our observational result. Hence, hierarchical models predict and observations now show that major dry mergers are an important driver of the evolution of massive early-type galaxies in recent epochs.

The Astrophysical Journal, 2004

We present a detailed comparison of structural properties in the rest-frame V-band of cluster and... more We present a detailed comparison of structural properties in the rest-frame V-band of cluster and field galaxies, selected and analyzed in the same manner, to test the hypothesis that much of the current cluster galaxy membership resulted from the fairly rapid (1-2 gigayears) transformation of infalling, field spirals into red, cluster early-types. Specifically, we have selected ∼ 140 galaxies from three nearby Abell clusters (A85, A496 and A754) that have colors significantly bluer than the red sequence population, and compared them to ∼ 80 field galaxies with similar colors and luminosities from Jansen et al. (2000, ApJS, 126, 271). The comparison is based on the hypothesis that recent (1-4 gigayears) cluster arrivals were originally blue and star-forming, then stopped forming stars to dim and redden in a few gigayears. For the comparison we quantify galaxy internal structure and morphology from two-dimensional bulge/disk decompositions using GIM2D. We observe structural differences between blue galaxies in local (z < 0.06) clusters, compared to field environments. All cluster galaxies have spectroscopic membership. The majority of blue cluster members, presumably recent additions, are physically smaller and fainter than their equally-colored field counterparts. At a matched size and luminosity, the newer cluster arrivals are quantifiably smoother in appearance, yet their total light is as disk-dominated as in normal field spirals. Moreover, half of the blue cluster members appear to have blue cores or globally blue color profiles, in contrast with field spirals, which typically exhibit red inward color gradients. Blue cores suggest enhanced nuclear star formation, possibly a starburst, while uniformly blue profiles are consistent with an episode of fairly strong global star formation in the past few gigayears. Our previous work (McIntosh et al. 2004, ApJ submitted) shows that the blue membership of local clusters is a recently infalling population that has yet to encounter the dense core. In a Universe without environmental dependent evolution outside of cluster cores, we would expect blue disk galaxies inhabiting field and cluster regions to have similar morphology, size, and color gradient distributions. Our findings show conclusively that not only the abundance of red and blue galaxies depends on environment, but also that fundamental structural and morphological galaxy properties do indeed reflect the environment in which the galaxy is found. Moreover, the data show that the transformation of accreted galaxies is not confined to the dense cluster core. The overall properties of bluer cluster members are best explained by environment-driven transformation of accreted field spirals, and our results suggest that the processes that govern color and morphological evolution occur separately.

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The Astrophysical Journal, 2003

We have used the 30 ′ × 30 ′ Hubble Space Telescope image mosaic from the Galaxy Evolution from M... more We have used the 30 ′ × 30 ′ Hubble Space Telescope image mosaic from the Galaxy Evolution from Morphology and SEDs (GEMS) project in conjunction with the COMBO-17 deep photometric redshift survey to define a sample of nearly 1500 galaxies with 0.65 ≤ z ≤ 0.75. With this sample, we can study the distribution of rest-frame V-band morphologies more than 6 Gyr ago, without differential bandpass shifting and surface brightness dimming across this narrow redshift slice. Focusing on red-sequence galaxies at z ∼ 0.7, we find that 85% of their combined restframe V-band luminosity density comes from visually-classified E/S0/Sa galaxies down to M V − 5 log 10 h −19.5. Similar results are obtained if automated classifiers are used. This fraction is identical to that found at the present day, and is biased by less than 10% by large scale structure and the morphology-density relation. Under the assumption that peculiar and edge-on disk galaxies are red by virtue of their dust content, we find that less than 13% of the total rest-frame V-band luminosity of the z ∼ 0.7 red galaxy population is from dusty galaxies.

The Astrophysical Journal, 2008

We explore the amount of obscured star-formation as a function of environment in the A901/902 sup... more We explore the amount of obscured star-formation as a function of environment in the A901/902 supercluster at z = 0.165 in conjunction with a field sample drawn from the A901 and CDFS fields, imaged with HST as part of the STAGES and GEMS surveys. We combine the COMBO-17 near-UV/optical SED with Spitzer 24µm photometry to estimate both the unobscured and obscured star formation in galaxies with M * > 10 10 M ⊙. We find that the star formation activity in massive galaxies is suppressed in dense environments, in agreement with previous studies. Yet, nearly 40% of the star-forming galaxies have red optical colors at intermediate and high densities. These red systems are not starbursting; they have star formation rates per unit stellar mass similar to or lower than blue starforming galaxies. More than half of the red star-forming galaxies have low IR-to-UV luminosity ratios, relatively high Sersic indices and they are equally abundant at all densities. They might be gradually quenching their starformation, possibly but not necessarily under the influence of gas-removing environmental processes. The other 40% of the red star-forming galaxies have high IR-to-UV luminosity ratios, indicative of high dust obscuration. They have relatively high specific star formation rates and are more abundant at intermediate densities. Our results indicate that while there is an overall suppression in the star-forming galaxy fraction with density, the small amount of star formation surviving the cluster environment is to a large extent obscured, suggesting that environmental interactions trigger a phase of obscured star formation, before complete quenching.

The Astrophysical Journal, 2004

We have performed HST imaging of a sample of 23 high-redshift (1.8 < z < 2.75) Active Galactic Nu... more We have performed HST imaging of a sample of 23 high-redshift (1.8 < z < 2.75) Active Galactic Nuclei, drawn from the combo-17 survey. The sample contains moderately luminous quasars (M B ∼ −23). The data are part of the gems imaging survey that provides high resolution optical images obtained with the Advanced Camera for Surveys in two bands (F606W and F850LP), sampling the rest-frame UV flux of the targets. To deblend the AGN images into nuclear and resolved (host galaxy) components we use a PSF subtraction technique that is strictly conservative with respect to the flux of the host galaxy. We resolve the host galaxies in both filter bands in 9 of the 23 AGN, whereas the remaining 14 objects are considered non-detections, with upper limits of less than 5 % of the nuclear flux. However, when we coadd the unresolved AGN images into a single high signal-to-noise composite image we find again an unambiguously resolved host galaxy. The recovered host galaxies have apparent magnitudes of 23.0 < F606W < 26.0 and 22.5 < F850LP < 24.5 with rest-frame UV colours in the range −0.2 < (F606W − F850LP) obs < 2.3. The rest-frame absolute magnitudes at 200 nm are −20.0 < M 200 nm < −22.2. The photometric properties of the composite host are consistent with the individual resolved host galaxies. We find that the UV colors of all host galaxies are substantially bluer than expected from an old population of stars with formation redshift z ≤ 5, independent of the assumed metallicities. These UV colours and luminosities range up to the values found for Lyman-break galaxies (LBGs) at z = 3. Our results suggest either a recent starburst, of e.g. a few per cent of the total stellar mass and 100 Myrs before observation, with mass-fraction and age strongly degenerate, or the possibility that the detected UV emission may be due to young stars forming continuously. For the latter case we estimate star formation rates of typically ∼6 M ⊙ yr −1 (uncorrected for internal dust attenuation), which again lies in the range of rates implied from the UV flux of LBGs. Our results agree with the recent discovery of enhanced blue stellar light in AGN hosts at lower redshifts.

The Astrophysical Journal, 2009

Both observations and simulations show that major tidal interactions or mergers between gas-rich ... more Both observations and simulations show that major tidal interactions or mergers between gas-rich galaxies can lead to intense bursts of star formation. Yet, the average enhancement in star formation rate (SFR) in major mergers and the contribution of such events to the cosmic SFR are not well estimated. Here we use photometric redshifts, stellar masses and UV SFRs from COMBO-17, 24µm SFRs from Spitzer and morphologies from two deep Hubble Space Telescope (HST) cosmological survey fields (ECDFS/GEMS and A901/STAGES) to study the enhancement in SFR as a function of projected galaxy separation. We apply two-point projected correlation function techniques, which we augment with morphologically-selected very close pairs (separation < 2 ′′) and merger remnants from the HST imaging. Our analysis confirms that the most intensely star-forming systems are indeed interacting or merging. Yet, for massive (M * ≥ 10 10 M ⊙) star-forming galaxies at 0.4 < z < 0.8, we find that the SFRs of galaxies undergoing a major interaction (mass ratios ≤ 1 : 4 and separations ≤ 40 kpc) are only 1.80 ± 0.30 times higher than the SFRs of non-interacting galaxies when averaged over all interactions and all stages of the interaction, in good agreement with other observational works. Our results also agree with hydrodynamical simulations of galaxy interactions, which produce some mergers with large bursts of star formation on ∼ 100 Myr timescales, but only a modest SFR enhancement when averaged over the entire merger timescale. We demonstrate that these results imply that only 10% of star formation at 0.4 ≤ z ≤ 0.8 is triggered directly by major mergers and interactions; these events are not important factors in the build-up of stellar mass since z = 1.

The Astronomical Journal, 2006

We present aperture-matched PSF-corrected BV i ′ z ′ JH photometry and Bayesian photometric redsh... more We present aperture-matched PSF-corrected BV i ′ z ′ JH photometry and Bayesian photometric redshifts (BPZ) for objects detected in the Hubble Ultra Deep Field (UDF), 8,042 of which are detected at the 10-σ level (e.g., i ′ < 29.01 or z ′ < 28.43). Most of our objects are defined identically to those in the public STScI catalogs, enabling straightforward object-by-object comparison. We have combined detections from i ′ , z ′ , J+H, and B+V +i ′ +z ′ images into a single comprehensive segmentation map. Using a new program called SExSeg we are able to force this segmentation map into SExtractor for photometric analysis. The resulting photometry is corrected for the wider NIC3 PSFs using our ColorPro software. We also correct for the ACS z ′-band PSF halo. Offsets are applied to our NIC3 magnitudes, which are found to be too faint relative to the ACS fluxes. Based on BPZ SED fits to objects of known spectroscopic redshift, we derived corrections of −0.30 ± 0.03 mag in J and −0.18 ± 0.04 mag in H. Our offsets appear to be supported by a recent recalibration of the UDF NIC3 images combined with non-linearity measured in NICMOS itself. The UDF reveals a large population of faint blue galaxies (presumably young starbursts), bluer than those observed in the original Hubble Deep Fields (HDF).