Morphologies and stellar populations of galaxies in the core of Abell 2218 (original) (raw)
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Photometric and Morphological Properties of Nearby Cluster Galaxies
The Astrophysical Journal Supplement Series, 2004
Photometric observations are presented for 8747 galaxies in the fields of 28 clusters with redshifts in the range 0:05 < z < 0:11 and spanning a wide range in Abell richness class. The primary motivation for this study is to provide a nearby sample of clusters whose galaxy populations can be used to provide an anchor point for evolutionary studies of cluster galaxy populations. The present analysis focuses on the sequence of luminous red galaxies that form a tight correlation in the colormagnitude plane. An ensemble cluster constructed from the full sample shows the red sequence clearly and the observed dispersion-not corrected for observational errors-is (V À I) ¼ 0:071 AE 0:006 down to M I(AB) ¼ À19:5. Selecting only early-type galaxies on the basis of their fractional bulge luminosity (B=T) reduces the observed dispersion to (V À I) ¼ 0:050 AE 0:006 mag, a significantly smaller dispersion than the full sample at more than the 95% confidence level. The slope of the color-magnitude relation in (V À I)=I is À0:023 AE 0:006. This set of clusters shows a red sequence that has the same mean color in all clusters (V À I) AB ¼ 0:91 AE 0:03 (measured at M I(AB) ¼ À23) and has the same intrinsic dispersion, (V À I) ¼ 0:04 AE 0:01, in all clusters within the observational errors. Thus, these clusters form a remarkably homogeneous family as represented by their luminous red galaxy populations.
The colour-magnitude relations of ClJ1226.9+3332, a massive cluster of galaxies at z= 0.89
Monthly Notices of the Royal Astronomical Society, 2006
The colour-magnitude relations of one of the most massive (≈ 10 15 M ⊙ ), high redshift (z = 0.89) clusters of galaxies known have been studied. Photometry has been measured in the V, R, I, z, F606W, F814W, J and K bands to a depth of K ≈ 20.5 = K * + 2.5 and spectroscopy confirms 27 K band selected galaxies as members of the cluster. The V − K colours are equivalent to a rest-frame colour of ≈ 2700Å − J, and provide a very sensitive measure of star-formation activity. Hubble Space Telescope imaging with the Advanced Camera for Surveys has been used to morphologically classify the galaxies.
The physical parameters of the evolving population of faint galaxies
Monthly Notices of the Royal Astronomical Society, 1998
The excess numbers of blue galaxies at faint magnitudes is a subject of much controversy. Recent Hubble Space Telescope results has revealed a plethora of galaxies with peculiar morphologies tentatively identified as the evolving population. We report the results of optical spectroscopy and near-infrared photometry of a sample of faint HST galaxies from the Medium Deep Survey to ascertain the physical properties of the faint morphological populations. We find four principal results: Firstly that the population of objects classified as 'peculiar' are intrinsically luminous in the optical (M B ∼ −19). Secondly these systems tend to be strong sources of [OII] line luminosity. Thirdly the optical-infrared colours of the faint population (a) confirm the presence of a population of compact blue galaxies and (b) show the stellar populations of Irregular/Peculiar galaxies encompass a wide range in age. Finally a surface-brightness comparison with the local galaxy sample of Frei et al. shows that these objects are not of anomalously low surface brightness, rather we find that all morphological classes have evolved to a higher surface brightness at higher-redshifts (z > 0.3).
Multiwavelength observations of a rich galaxy cluster at z [Formula: see text][Formula: see text] 1
Astronomy and Astrophysics, 2009
Context. XMMU J1229+0151 is a rich galaxy cluster with redshift z=0.975, that was serendipitously detected in X-rays within the scope of the XMM-Newton Distant Cluster Project. HST/ACS observations in the i 775 and z 850 passbands, as well as VLT/FORS2 spectroscopy were further obtained, in addition to follow-up Near-Infrared (NIR) imaging in J-and Ks-bands with NTT/SOFI. Aims. We investigate the photometric, structural and spectral properties of the early-type galaxies in the high-redshift cluster XMMU J1229+0151. Methods. Source detection and aperture photometry are performed in the optical and NIR imaging. Galaxy morphology is inspected visually and by means of Sersic profile fitting to the 21 spectroscopically confirmed cluster members in the ACS field of view. The i 775 -z 850 colour-magnitude relation (CMR) is derived with a method based on galaxy magnitudes obtained by fitting the surface brightness of the galaxies with Sersic models. Stellar masses and formation ages of the cluster galaxies are derived by fitting the observed spectral energy distributions (SED) with models based on Bruzual & Charlot 2003. Star formation histories of the early-type galaxies are constrained through the analysis of the stacked spectrophotometric data. Results. The structural Sersic index n obtained with the model fitting is in agreement with the visual morphological classification of the confirmed members, indicating a clear predominance of elliptical galaxies (15/21). The i 775 -z 850 colour-magnitude relation of the spectroscopic members shows a very tight red-sequence with a zero point of 0.86±0.04 mag and intrinsic scatter equal to 0.039 mag. The CMR obtained with the galaxy models has similar parameters. By fitting both the spectra and SED of the early-type population we obtain a star formation weighted age of 4.3 Gyr for a median mass of 7.4 ×10 10 M ⊙ . Instead of an unambiguous brightest cluster galaxy (BCG), we find three bright galaxies with a similar z 850 magnitude, which are, in addition, the most massive cluster members, with ∼2 ×10 11 M ⊙ . Our results strengthen the current evidence for a lack of significant evolution of the scatter and slope of the red-sequence out to z ∼ 1.
Unravelling the Morphologies of Luminous Compact Galaxies Using the HST/ACS GOODS Survey
Astronomy & Astrophysics, 2007
Context. Luminous Compact Galaxies (LCGs) (M B ≤ −20, R 1/2 ≤ 4.5 kpc and EW 0 (OII) ≥15Å) constitute one of the most rapidly evolving galaxy populations over the last ∼8 Gyr history of the universe. Due to their inherently compact sizes, any detailed quantitative analysis of their morphologies has proved to be difficult in the past. Hence, the morphologies and thereby the local counterparts of these enigmatic sources have been hotly debated. Aims. Our aim is to use the high angular resolution, deep, multiband HST/ACS imaging data, from the HST/ACS GOODS survey, to study the quantitative morphology of a complete sample of LCGs in the redshift range 0.5 ≤ z ≤ 1.2. Methods. We have derived structural parameters for a representative sample of 39 LCGs selected from the GOODS-S HST/ACS field, using full 2-dimensional surface brightness profile fitting of the galaxy images in each of the four filters available. B 435W − z 850LP color maps are constructed for the sample to aid in the morphological classification. We then use the rest frame B band bulge flux fraction (B/T) to determine the morphological class of galaxies which are well fit by a bulge+disk two dimensional structure. Mergers were essentially identified visually by the presence of multiple maxima of comparable intensity in the rest frame B band images, aided by the color maps to distinguish them from HII regions. We also make use of the Spitzer 24µm source catalog of sources in the CDFS to derive the dust enshrouded star formation rates (SFR) for some of the sample LCGs Results. We derive the following morphological mix for our sample of intermediate redshift LCGs: Mergers: ∼36%, Disk dominated: ∼22%, S0: ∼20%, Early types: ∼7%, Irr/tadpole: ∼15%. We establish that our sample LCGs are intermediate mass objects with stellar mass ranging from 9.44 ≤ Log 10 (M/M ⊙ ) ≤ 10.96, with a median mass of Log 10 (M/M ⊙ ) = 10.32. We also derive SFR values ranging from a few to ∼ 65 M ⊙ /year as expected for this class of objects. We find that LCGs account for ∼26% of the M B ≤ −20 galaxy population in the redshift range 0.5 ≤ z ≤ 1.2. We estimate a factor ∼11 fall in the comoving number density of blue LCGs from redshifts 0.5 ≤ z ≤ 1.2 to the current epoch, even though this number is subject to large uncertainities given the small sample size at zero redshift available from the literature. Conclusions. The strong redshift evolution exhibited by LCGs, and the fact that a significant fraction of LCGs are in merging systems, seem to indicate that LCGs might be an important phase in the hierarchical evolution of galaxies. We envisage that some of the LCGs that are classified as merging systems, might go on to rebuild their disks and evolve into disk galaxies in the local universe.
THE EVOLUTION OF EARLY- AND LATE-TYPE GALAXIES IN THE COSMIC EVOLUTION SURVEY UP TO z ≈ 1.2
The Astrophysical Journal, 2009
The Cosmic Evolution Survey (COSMOS) allows for the first time a highly significant census of environments and structures up to redshift one, as well as a full morphological description of the galaxy population. In this paper we present a study aimed to constrain the evolution, in the redshift range 0.2 < z < 1.2, of the mass content of different morphological types and its dependence on the environmental density. We use a deep multicolor catalog, covering an area of ∼0.7 ⊓ ⊔ • inside the COSMOS field, with accurate photometric redshifts (i ∼ < 26.5 and ∆z/(z spec + 1) ≈ 0.035). We estimate galaxy stellar masses by fitting the multi-color photometry to a grid of composite stellar population models. We quantitatively describe the galaxy morphology by fitting PSF convolved Sérsic profiles to the galaxy surface brightness distributions down to F814 = 24 mag for a sample of 41300 objects.
The bright end of the colour-magnitude relation of cluster galaxies
Monthly Notices of The Royal Astronomical Society, 2011
We investigate the physical processes involved in the development of the red sequence (RS) of cluster galaxies by using a combination of cosmological N-body simulations of clusters of galaxies and a semi-analytic model of galaxy formation. Results show good agreement between the general trend of the simulated RS and the observed colour-magnitude relation (CMR) of early-type galaxies in different magnitude planes. However, in many clusters, the most luminous galaxies (?) depart from the linear fit to observed data, as traced by less luminous ones, displaying almost constant colours. With the aim of understanding this particular behaviour of galaxies in the bright end of the RS, we analyse the dependence with redshift of the fraction of stellar mass contributed to each galaxy by different processes, i.e. quiescent star formation, and starbursts triggered by disc instability and merger events. We find that the evolution of galaxies in the bright end since z≈ 2 is mainly driven by minor and major dry mergers, while minor and major wet mergers are relevant in determining the properties of less luminous galaxies. Since the most luminous galaxies have a narrow spread in ages (1.0 × 1010 yr < t < 1.2 × 1010 yr), their metallicities are the main factor that affects their colours. Their mean iron abundances are close to the solar value and have already been reached at z≈ 1. This fact is consistent with several observational evidence that favour a scenario in which both the slope and scatter of the CMR are in place since z≈ 1.2. Galaxies in the bright end reach an upper limit in metallicity as a result of the competition of the mass of stars and metals provided by the star formation occurring in the galaxies themselves and by the accretion of merging satellites. Star formation activity in massive galaxies (stellar mass M★≳ 1010 M⊙) that takes place at low redshifts contributes with stellar components of high metallicity, but the fraction of stellar mass contributed since z≈ 1 is negligible with respect to the total mass of the galaxy at z= 0. On the other hand, mergers contribute with a larger fraction of stellar mass (≈10-20 per cent), but the metallicity of the accreted satellites is lower by ≈0.2 dex than the mean metallicity of galaxies they merge with. The effect of dry mergers is to increase the mass of galaxies in the bright end, without significantly altering their metallicities. Hence, very luminous galaxies present similar colours that are bluer than those expected if recent star formation activity were higher, thus giving rise to a break in the RS. These results are found for simulated clusters with different virial masses (1014- 1015 h-1 M⊙), supporting the idea of the universality of the CMR in agreement with observational results.
Astronomy & Astrophysics, 2009
We present a comprehensive analysis of the stellar population properties (age, metallicity and the alpha-element enhancement [E/Fe]) and morphologies of red-sequence galaxies in 24 clusters and groups from z ∼ 0.75 to z ∼ 0.45. The dataset, consisting of 215 spectra drawn from the ESO Distant Cluster Survey, constitutes the largest spectroscopic sample at these redshifts for which such an analysis has been conducted. Analysis reveals that the evolution of the stellar population properties of red-sequence galaxies depend on their mass: while the properties of most massive are well described by passive evolution and high-redshift formation, the less massive galaxies require a more extended star formation history. We show that these scenarios reproduce the index-σ relations as well as the galaxy colours. The two main results of this work are (1) the evolution of the line-strength indices for the red-sequence galaxies can be reproduced if 40% of the galaxies with σ < 175 kms −1 entered the red-sequence between z = 0.75 to z = 0.45, in agreement with the fraction derived in studies of the luminosity functions, and (2) the percentage the red-sequence galaxies exhibiting early-type morphologies (E and S0) decreases by 20% from z = 0.75 to z = 0.45. This can be understood if the red-sequence gets populated at later times with disc galaxies whose star formation has been quenched. We conclude that the processes quenching star formation do not necessarily produce a simultaneous morphological transformation of the galaxies entering the red-sequence.
Astronomy and Astrophysics, 2006
We constrain the evolution of the galaxy mass and luminosity functions from the analysis of (public) multi-wavelength data in the Chandra Deep Field South (CDFS) area, obtained from the GOODS and other projects, and including very deep high-resolution imaging by HST/ACS. Our reference catalogue of faint high-redshift galaxies, which we have thoroughly tested for completeness and reliability, comes from a deep (S3.6 ≥ 1 µJy) image by IRAC on the Spitzer Observatory. These imaging data in the field are complemented with extensive optical spectroscopy by the ESO VLT/FORS2 and VIMOS spectrographs, while deep K-band VLT/ISAAC imaging is also used to derive further complementary statistical constraints and to assist the source identification and SED analysis. We have selected a highly reliable IRAC 3.6µm sub-sample of 1478 galaxies with S3.6 ≥ 10 µJy, 47% of which have spectroscopic redshift, while for the remaining objects both COMBO-17 and Hyperz are used to estimate the photometric redshift. This very extensive dataset is exploited to assess evolutionary effects in the galaxy luminosity and stellar mass functions, while luminosity/density evolution is further constrained with the number counts and redshift distributions. The deep ACS imaging allows us to differentiate these evolutionary paths by morphological type, which our simulations show to be reliable at least up to z ∼ 1.5 for the two main early-(E/S0) and late-type (Sp/Irr) classes. These data, as well as our direct estimate of the stellar mass function above M * h 2 = 10 10 M⊙ for the spheroidal subclass, consistently evidence a progressive dearth of such objects to occur starting at z ∼ 0.7, paralleled by an increase in luminosity. A similar trend, with a more modest decrease of the mass function, is also shared by spiral galaxies, while the irregulars/mergers show an increased incidence at higher z. Remarkably, this decrease of the comoving density with redshift of the total population appears to depend on galaxy mass, being stronger for moderate-mass, but almost absent until z = 1.4 for high-mass galaxies, thus confirming previous evidence for a "downsizing" effect in galaxy formation. Our favoured interpretation of the evolutionary trends for the two galaxy categories is that of a progressive morphological transformation (due to gas exhaustion and, likely, merging) from the star-forming to the passively evolving phase, starting at z ≥ 2 and keeping on down to z ∼ 0.7. The rate of this process appears to depend on galaxy mass, being already largely settled by z ∼ 1 for the most massive systems.
Star-forming Galaxies at Intermediate Redshifts: Morphology, Ages, and Sizes
The Astronomical Journal, 2006
We present the analysis of the deepest near-UV image obtained with Hubble Space Telescope using the WFPC2(F300W) as part of the parallel observations of the Hubble Ultra Deep Field campaign. The U-band 10σ limiting magnitude measured over 0.2 arcsec 2 is m AB =27.5 which is 0.5 magnitudes deeper than that in the Hubble Deep Field North. We matched the U-band catalog with those in the ACS images (B, V, i, z) taken during the Great Observatories Origins Deep Survey observations of the Chandra Deep Field South and obtained photometric redshifts for 306 matched objects. We find that the UV-selected galaxies span all the major morphological types at 0.2 <z phot < 1.2. However, disks are more common at lower redshifts, 0.2 <z phot < 0.8. Higher redshift objects (0.7 <z phot < 1.2) are on average bluer than lower−z and have spectral type typical of starbursts. Their morphologies are compact, peculiar or low surface brightness galaxies. Despite the UV-selection, 13 objects have spectral types of early-type galaxies; two of them are spheroids with blue cores. The evolutionary synthesis code, Starburst99, was used to age-date the UV-selected galaxies which were found to have rest-frame colors typical of stellar populations with intermediate ages > 100 Myr. The average halflight radius (rest-frame 1200-1800Å) of the UV-selected galaxies at 0.66<z phot <1.5 is 0.26 ± 0.01 arcsec (2.07 ± 0.08 kpc). The UV-selected galaxies are on average fainter (M B =-18.43±0.13) than Lyman Break Galaxies (M B =-23±1). Our sample includes early-type galaxies that are presumably massive and forming stars only in their cores, as well as starburst-type systems that are more similar to the LBGs, although much less luminous. This implies that even the starbursts in our sample are either much less massive than LBGs or are forming stars at a much lower rate or both. The low surface brightness galaxies have no overlap with the LBGs and form an interesting new class of their own.