B. Holwerda - Academia.edu (original) (raw)

Papers by B. Holwerda

The Astrophysical Journal, 2014

We have mapped the distribution of young and old stars in the gaseous H I warp of NGC 4565. We fi... more We have mapped the distribution of young and old stars in the gaseous H I warp of NGC 4565. We find a clear correlation of young stars (< 600 Myr) with the warp, but no coincident old stars (> 1 Gyr), which places an upper limit on the age of the structure. The formation rate of the young stars, which increased ∼ 300 Myr ago relative to the surrounding regions, is (6.3 +2.5 −1.5 ) × 10 −5 M yr −1 kpc −2 . This implies a ∼ 60 ± 20 Gyr depletion time of the H I warp, similar to the timescales calculated for the outer H I disks of nearby spiral galaxies. While some stars associated with the warp fall into the asymptotic giant branch (AGB) region of the color magnitude diagram, where stars could be as old as 1 Gyr, further investigation suggests that they may be interlopers rather than real AGB stars. We discuss the implications of these age constraints for the formation of H I warps, and the gas fueling of disk galaxies.

The Astrophysical Journal, 2016

Monthly Notices of the Royal Astronomical Society, 2015

The Astrophysical Journal, 2015

The remarkable HST datasets from the CANDELS, HUDF09, HUDF12, ERS, and BORG/HIPPIES programs have... more The remarkable HST datasets from the CANDELS, HUDF09, HUDF12, ERS, and BORG/HIPPIES programs have allowed us to map out the evolution of the rest-frame UV luminosity function from z ∼ 10 to z ∼ 4. We develop new color criteria that more optimally utilize the full wavelength coverage from the optical, near-IR, and mid-IR observations over our search fields, while simultaneously minimizing the incompleteness and eliminating redshift gaps. We have identified 5991, 3391, 940, 598, 225, and 6 galaxy candidates at z ∼ 4, z ∼ 5, z ∼ 6, z ∼ 7, z ∼ 8, and z ∼ 10, respectively from the ∼1000 arcmin 2 area covered by these datasets. This sample of >11000 galaxy candidates at z ≥ 4 is by far the largest assembled to date with HST. The large z ∼ 4-8 samples we have identified in the five CANDELS fields allow us to assess the cosmic variance; the largest variations are at z ≥ 7. Our new LF determinations at z ∼ 4 and z ∼ 5 span a 6-mag baseline and reach down to −16 AB mag. These determinations agree well with previous estimates, but the larger samples and the larger volumes probed here result in a more reliable sampling of > L * galaxies and allow us to re-assess the form of the UV LFs. Our new LF results strengthen our earlier findings to 4.5σ significance for a steeper U V LF at z > 4, with α evolving from α = −1.64 ± 0.04 at z ∼ 4 to α = −2.06 ± 0.12 at z ∼ 7 (and α ∼ −1.9 at z ∼ 8). The observed steepening of the U V LF is consistent with that expected from the evolution of the halo mass function. With our improved constraints at the bright end, we find less evolution in the characteristic luminosity M * over the redshift range z ∼ 4 to z ∼ 7 consistent with current models; the observed evolution in the LF is now largely represented by changes in φ * . Even with the much larger sample of bright galaxies, we find no evidence for the LF having a non-Schechter-like form at z ∼ 4-8. A simple conditional luminosity function model based on halo growth and a modest evolution in the M/L (∝ (1 + z) −1.5 ) of halos provides a good representation of the evolution of the UV LF. * More details on the observational data we use for each of these search fields is provided in Appendix A. a The 5σ depths are based on the light within a 0.35 ′′ -diameter aperture. No correction is made for the light outside this aperture. This is in contrast to many other studies where the quoted depths are corrected for the missing light (which can result in a ∼0.3 mag and ∼0.5 mag correction to the quoted depths for the ACS and WFC3/IR data, respectively, but depend upon the profile assumed). b The XDF refers to the 4.7 arcmin 2 region over the HUDF with ultra-deep near-IR observations from the HUDF09 and HUDF12 programs . It includes all ACS and WFC3/IR observations acquired over this region for the 10-year period 2002 to 2012. c The present XDF reduction ) is typically ∼0.2 mag deeper than the original reduction of the HUDF ACS data provided by . d The JH 140 observations are from the 3D-HST and GO-11600 (PI: Weiner) programs. e Total sensitivity assuming a flat fν source and adding in quadrature all the ground-based optical data we consider. f Only the highest quality (longer exposure) BORG/HIPPIES fields are considered in our analysis (see Appendix A.5). For inclusion, we require search fields to have an average exposure time in the J 125 and H 160 bands of at least 1200 seconds and with longer exposure times in the optical V 606 + V 600LP bands than the average exposure time in the near-infrared J 125 + H 160 observations. This data set also includes similar observations obtained by the COS GTO team, follow-up of the putative BORG protocluster , and WFC3 parallels to Abell 1689 (GO 11710: Alamo-Martínez et al. 2013).

Monthly Notices of the Royal Astronomical Society, 2014

Supernovae Type Ia display a complex relation with their host galaxies. An important prior to the... more Supernovae Type Ia display a complex relation with their host galaxies. An important prior to the fit of the supernovae's lightcurve is the distribution of host galaxy extinction values that can be encountered. The SDSS-SN project has published light curve fits using both MLCS2k2 and SALT2. We use the former fits extinction parameter (A V ) to map this distribution of extinction values.

Astronomy & Astrophysics, 2014

Context. In recent years multi-wavelength observations have shown the presence of substructures r... more Context. In recent years multi-wavelength observations have shown the presence of substructures related to merging events in a high fraction of galaxy clusters. Clusters can be roughly grouped into two categories -relaxed and non-relaxed -and a proper characterisation of the dynamical state of these systems is of crucial importance both for astrophysical and cosmological studies. Aims. In this paper we investigate the use of a number of morphological parameters (Gini, M 20 , Concentration, Asymmetry, Smoothness, Ellipticity and Gini of the second order moment, G M ) introduced to automatically classify clusters as relaxed or dynamically disturbed systems. Methods. We apply our method to a sample of clusters at different redshifts extracted from the Chandra archive and we investigate possible correlations between morphological parameters and other X-ray gas properties. We conclude that a combination of the adopted parameters is a very useful tool to properly characterise the X-ray cluster morphology. Results. According to our results three parameters -Gini, M 20 and Concentration -are very promising for identifying cluster mergers. The Gini coefficient is a particularly powerful tool, especially at high redshift, being independent from the choice of the position of the cluster centre. We find that high Gini (> 0.65), high Concentration (> 1.55) and low M 20 (< -2.0) values are associated with relaxed clusters, while low Gini (< 0.4), low Concentration (< 1.0) and high M 20 (> -1.4) characterise dynamically perturbed systems. We also estimate the X-ray cluster morphological parameters in the case of radio loud clusters. In excellent agreement with previous analyses we confirm that diffuse intracluster radio sources are associated with major mergers.

The halo of NGC 891 has been the subject of studies for more than a decade. One of its most strik... more The halo of NGC 891 has been the subject of studies for more than a decade. One of its most striking features is the large asymmetry in H-$\alpha$ emission. We have taken a quantitative look at this asymmetry at different wavelengths for the first time. We propose that NGC 891 is intrinsically almost symmetric, as seen in Spitzer observations, and the large asymmetry in H-$\alpha$ emission is mostly due to dust attenuation. We quantify the additional optical depth needed to cause the observed H-$\alpha$ asymmetry. A comparison of large strips on the North East side of the galaxy with strips covering the same area in the South West we can quantify and analyze the asymmetry in the different wavelengths. From the 24 mu\mumum emission we find that the intrinsic asymmetry in star-formation in NGC 891 is small i.e., approximately 30%. The additional asymmetry in H-alpha is modeled as additional symmetric dust attenuation which extends up to ~ 40'' (1.9 kpc) above the plane of the ...

Recently, we have developed and calibrated the Synthetic Field Method to derive the total extinct... more Recently, we have developed and calibrated the Synthetic Field Method to derive the total extinction through disk galaxies. The method is based on the number counts and colours of distant background field galaxies that can be seen through the foreground object. Here, we investigate how large (10-m) and very large (20 to 30-m), diffraction-limited, optical and infrared telescopes in space would improve the detection of background galaxies behind Local Group objects, including the Galactic bulge. We find that, besides and perhaps more important than telescope size, a well-behaved, well-characterized PSF would facilitate in general the detection of faint objects in crowded fields, and greatly benefit several other important research areas, like the search for extrasolar planets, the study of quasar hosts and, most relevant for this meeting, the surveying of nearby large-scale structure in the Zone of Avoidance, in particular behind the Galactic bulge.

The Astronomical Journal, 2007

The quantity of dust in a spiral disk can be estimated using the dust's typical emission or the e... more The quantity of dust in a spiral disk can be estimated using the dust's typical emission or the extinction of a known source. In this paper, we compare two techniques, one based on emission and one on absorption, applied on sections of fourteen disk galaxies. The two measurements reflect, respectively the average and apparent optical depth of a disk section. Hence, they depend differently on the average number and optical depth of ISM structures in the disk.

Astronomy and Astrophysics, 2005

The opacity of spiral galaxy disks, from counts of distant galaxies, is compared to HI column den... more The opacity of spiral galaxy disks, from counts of distant galaxies, is compared to HI column densities. The opacity measurements are calibrated using the "Synthetic Field Method" from , ApJ, 506, 152), Holwerda et al. (2005a, AJ, 129, 1381. When compared for individual disks, the HI column density and dust opacity do not seem to be correlated as HI and opacity follow different radial profiles. To improve statistics, an average radial opacity profile is compared to an average HI profile. Compared to dust-to-HI estimates from the literature, more extinction is found in this profile. This difference may be accounted for by an underestimate of the dust in earlier measurements due to their dependence on dust temperature. Since the SFM is insensitive to the dust temperature, the ratio between the SFM opacity and HI could very well be indicative of the true ratio. Earlier claims for a radially extended cold dust disk were based on sub-mm observations. A comparison between sub-mm observations and counts of distant galaxies is therefore desirable. We present the best current example of such a comparison, M 51, for which the measurements seem to agree. However, this remains an area where improved counts of distant galaxies, sub-mm observations and our understanding of dust emissivity are needed.

Astronomy & Astrophysics, 2013

Context. Edge-on spiral galaxies with prominent dust lanes provide us with an excellent opportuni... more Context. Edge-on spiral galaxies with prominent dust lanes provide us with an excellent opportunity to study the distribution and properties of the dust within them. The HEROES project was set up to observe a sample of seven large edge-on galaxies across various wavelengths for this investigation. Aims. Within this first paper, we present the Herschel observations and perform a qualitative and quantitative analysis on them, and we derive some global properties of the far infrared and submillimetre emission. Methods. We determine horizontal and vertical profiles from the Herschel observations of the galaxies in the sample and describe the morphology. Modified black-body fits to the global fluxes, measured using aperture photometry, result in dust temperatures and dust masses. The latter values are compared to those that are derived from radiative transfer models taken from the literature. Results. On the whole, our Herschel flux measurements agree well with archival values. We find that the exponential horizontal dust distribution model often used in the literature generally provides a good description of the observed horizontal profiles. Three out of the seven galaxies show signatures of extended vertical emission at 100 and 160 µm at the 5σ level, but in two of these it is probably due to deviations from an exactly edge-on orientation. Only for NGC 4013, a galaxy in which vertically extended dust has already been detected in optical images, we can detect vertically extended dust, and the derived scaleheight agrees with the value estimated through radiative transfer modelling. Our analysis hints at a correlation between the dust scaleheight and its degree of clumpiness, which we infer from the difference between the dust masses as calculated from modelling of optical data and from fitting the spectral energy distribution of Herschel datapoints.

Astronomy and Astrophysics, 2007

Context. The halo of NGC 891 has been the subject of studies for more than a decade. One of its m... more Context. The halo of NGC 891 has been the subject of studies for more than a decade. One of its most striking features is the large asymmetry in Hα emission. In this letter, we will take a quantitative look at this asymmetry at different wavelengths for the first time.

Edge-on spiral galaxies offer a unique perspective on disks. One can accurately determine the hei... more Edge-on spiral galaxies offer a unique perspective on disks. One can accurately determine the height distribution of stars and ISM and the lineof-sight integration allows for the study of faint structures. The Spitzer IRAC camera is an ideal instrument to study both the ISM and stellar structure in nearby galaxies; two of its channels trace the old stellar disk with little extinction and the 8 micron channel is dominated by the smallest dust grains (Polycyclic Aromatic Hydrocarbons, PAHs). probed the link between the appearance of dust lanes and the disk stability. In a sample of bulge-less disks they show how in massive disks the ISM collapses into the characteristic thin dust lane. Less massive disks are gravitationally stable and their dust morphology is fractured. The transition occurs at 120 km/s for bulgeless disks. Here we report on our results of our Spitzer/IRAC survey of nearby edgeon spirals and its first results on the NIR Tully-Fischer relation, and ISM disk stability.

Photometric analysis of backlit galaxies is a powerful way to measure the extinction in disks, bu... more Photometric analysis of backlit galaxies is a powerful way to measure the extinction in disks, but has hitherto been limited by the number of suitable pairs near enough for analysis. New searches of the SDSS imagery (mostly from the public galaxyzoo.org project) have revealed hundreds of candidate pairs, opening the possibility of tracing extinction properties across Hubble type and surface

Photometric analysis of backlit galaxies is a powerful way to measure the extinction in disks, bu... more Photometric analysis of backlit galaxies is a powerful way to measure the extinction in disks, but has hitherto been limited by the number of suitable pairs near enough for analysis. New searches of the SDSS imagery (mostly from the public galaxyzoo.org project) have revealed over 1800 candidate pairs, opening the possibility of tracing extinction properties across Hubble type and surface

ABSTRACT NGC 1097 has an extensive, unique network of jet-like extended structures that stretch o... more ABSTRACT NGC 1097 has an extensive, unique network of jet-like extended structures that stretch out for dozens of kiloparsecs. Their origin has remained a mystery for decades. The evidence for their association with the AGN is weak, and the lack of HI emission in the vicinity makes it unlikely that they are the product of tidal interactions or streams off the main disk of the galaxy. Recent modeling has shown that the structures may be the remnants of a dwarf galaxy, though the type of dwarf remains unclear, and such interactions are complicated by the fact that the main spiral has a large bar. We propose Spitzer observations of these features to determine the streams' true extent, to age-date individual structures by focusing on the old stellar population, estimate their masses, and calculate their metallicities. We will use Spitzer's sensitivity and the degeneracy-breaking IRAC colors to unravel the history of this unusual object. This proposal is part of broader studies by members of this team on galaxy outskirts and interactions, including mergers, streams, shells, asymmetries and lopsidedness, all of which contribute to our understanding of galaxy evolution in the nearby Universe.