arXiv subjects

H. -W. Rix

Publications and source records attributed to H. -W. Rix.

At least 127 records · Page 7Linked to original sources

arXiv2005

IRAC Mid-Infrared Imaging of the Hubble Deep Field South: Star Formation Histories and Stellar Masses of Red Galaxies at z>2

We present deep 3.6 - 8 micron imaging of the Hubble Deep Field South with IRAC on the Spitzer Space Telescope. We study Distant Red Galaxies (DRGs) at z>2 selected by Js - Ks > 2.3 and compare them to a sample of Lyman Break Galaxies (LBGs) at z=2-3. The observed UV-to-8 micron spectral energy distributions are fit with stellar population models to constrain star formation histories and derive stellar masses. We find that 70% of the DRGs are best described by dust-reddened star forming models and 30% are very well fit with old and ``dead'' models. Using only the I - Ks and Ks - 4.5 micron colors we can effectively separate the two groups. The dead systems are among the most massive at z~2.5 (mean stellar mass = 0.8 x 10^11 Msun) and likely formed most of their stellar mass at z>5. To a limit of 0.5 x 10^11 Msun their number density is ~10 x lower than that of local early-type galaxies. Furthermore, we use the IRAC photometry to derive rest-frame near-infrared J, H, and K fluxes. The DRGs and LBGs together show a large variation (a factor of 6) in the rest-frame K-band mass-to-light ratios (M/L_K), implying that even a Spitzer 8 micron-selected sample would be very different from a mass-selected sample. The average M/L_K of the DRGs is about three times higher than that of the LBGs, and DRGs dominate the high-mass end. The M/L_K ratios and ages of the two samples appear to correlate with derived stellar mass, with the most massive galaxies being the oldest and having the highest mass-to-light ratios, similar as found in the low-redshift universe.

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arXiv2005

GEMS: which galaxies dominate the z~0.7 ultraviolet luminosity density?

We combine high-resolution images from GEMS with redshifts and spectral energy distributions from COMBO-17 to explore the morphological types of galaxies that dominate the z~0.7 UV luminosity density. We analysed rest-frame 280 nm and V-band luminosities of 1483 galaxies with 0.65<z< 0.75, combining these with visual morphological classifications from F850LP images (approximately rest-frame V-band) taken with HST/ACS on the Extended Chandra Deep Field South. We derive UV luminosity functions and j_280 luminosity densities for spheroid-dominated galaxies, spiral galaxies, Magellanic irregulars, and clearly-interacting galaxies with morphologies suggestive of ongoing major mergers. We check the reliability of GEMS morphologies against the deeper GOODS images and quantify an incompleteness of the GEMS merger identification at the faint end. We derive the fractions of the global UV luminosity density j_280 originating from the galaxy types, and find that spiral galaxies and Magellanic irregulars dominate with about 40% each. Interacting and merging galaxies account for roughly 20% of j_280, while the contribution of early types is negligible. These results imply that the strong decline in the UV luminosity density of the Universe observed from z~1 until today is dominated by the decreasing UV luminosities of normal spiral galaxies, accompanied by the migration of UV-luminous star formation in irregular galaxies to systems of progressively lower mass and luminosity. (abridged)

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arXiv2005

Mass-to-Light Ratios of Field Early-Type Galaxies at z~1 from Ultra-Deep Spectroscopy: Evidence for Mass-dependent Evolution

We present an analysis of the Fundamental Plane for a sample of 27 field early-type galaxies in the redshift range 0.6 2 x 10^11 Msol$, we find no significant difference between the evolution of field and cluster galaxies: $Delta ln (M/L_B) = -1.20+/-0.18z for field galaxies and $Delta ln (M/L_B) = -1.12+/-0.06z$ for cluster galaxies. The relation between the measured M/L evolution and mass is partially due to selection effects. However, even when taking selection effects into account, we still find a relation between M/L evolution and mass, which is most likely caused by a lower mean age and a larger intrinsic scatter for low mass galaxies. Results from lensing early-type galaxies, which are mass-selected, show a very similar trend with mass. This, combined with our findings, provides evidence for down-sizing. Previous studies of the rate of evolution of field early-type galaxies found a large range of mutually exclusive values. We show that these differences are largely caused by the differences between fitting methods. (Abridged)

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arXiv2005

The Fundamental Plane of Cluster Ellipticals at z = 1.25

Using deep HST ACS imaging and VLT FORS2 spectra, we determined the velocity dispersions, effective radii and surface brightnesses for four early-type galaxies in the $z=1.237$ cluster \cl. All four galaxies are massive, $> 10^{11} M_{\sun}$. These four galaxies, combined with three from \lynx at $z=1.276$, establish the Fundamental Plane of massive early-type cluster galaxies at $\bar{z}=1.25$. The offset of the Fundamental Plane shows that the luminosity evolution in rest-frame $B$ is $Δ\ln M/L_B = (-0.98 \pm 0.06) z$ for galaxies with $M > 10^{11.5} M_{\sun}$. To reproduce the observed mass-to-light ratio ($M/L$) evolution, we determine the characteristic age of the stars in these $M > 10^{11.5} M_{\sun}$ galaxies to be $3.0^{+0.3}_{-0.3}$ Gyrs, {\em i.e.} $z_{*}=3.4^{+0.5}_{-0.4}$. Including selection effects caused by morphological bias (the ``progenitor bias''), we estimate an age of $2.1^{+0.2}_{-0.2}$ Gyrs, or $z_{*} = 2.3^{+0.2}_{-0.2}$ for the elliptical galaxy population. Massive cluster early-type galaxies appear to have a large fraction of stars that formed early in the history of the universe. However, there is a large scatter in the derived $M/L$ values, which is confirmed by the spread in the galaxies' colors. Two lower mass galaxies in our $\bar{z}=1.25$ sample have much lower $M/L$ values, implying significant star-formation close to the epoch of observation. Thus, even in the centers of massive clusters, there appears to have been significant star formation in some massive, $M \simeq 10^{11} M_{\sun}$, galaxies at $z\simeq 1.5$.

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arXiv2004

Weak lensing measurements of dark matter halos of galaxies from COMBO-17

We present a measurement of mass estimates for dark matter halos around galaxies from the COMBO-17 survey using weak gravitational lensing. COMBO-17 is particularly useful for this kind of investigation because it covers observations in 17 optical filters from which accurate photometric redshifts and spectral classification for objects with $R<24$ are derived. This allows us to select lens and source galaxies from their redshifts and to thus avoid any uncertainties from estimates of the source redshift distribution. We study galaxy lenses at redshifts $z_\mathrm{d}=0.2-0.7$ by fitting the normalization of either singular isothermal spheres (SIS) or Navarro-Frenk-White (NFW) profiles to the whole lens sample; we then consider halos around blue and red subsamples separately. We also constrain the scaling of halo mass with light. For the NFW model, we find virial masses $M_\mathrm{vir}^*=3.9^{+3.3}_{-2.4}\times 10^{11}h^{-1}M_{\sun}$ (1-$σ$) for blue and $M_\mathrm{vir}^*=7.1^{+7.1}_{-3.8}\times 10^{11}h^{-1}M_{\sun}$ for red galaxies of $L_\star=10^{10}h^{-2}L_{\sun}$, respectively. The derived mass-to-light scaling relations suggest that the mass-to-light ratio might decrease with increasing luminosity for blue galaxies but increase with increasing luminosity for red galaxies. However, these differences between blue and red galaxies are only marginally significant and both subsamples are consistent with having the same mass-to-light ratio at all luminosities. Finally, we compare our results to those obtained from the Red-Sequence Cluster Survey (RCS) and the Sloan Digital Sky Survey (SDSS). Taking differences in the actual modelling into account, we find very good agreement with these surveys.

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arXiv2004

The influence of redshift information on galaxy-galaxy lensing measurements

We investigate how galaxy-galaxy lensing measurements depend on the knowledge of redshifts for lens and source galaxies. Galaxy-galaxy lensing allows one to study dark matter halos of galaxies statistically using weak gravitational lensing. Redshift information is required to reliably distinguish foreground lens galaxies from background source galaxies and to convert the measured shear into constraints on the lens model. Without spectroscopy or multi-colour information, redshifts can be drawn from independently estimated probability distributions. The COMBO-17 survey provides redshifts for both lens and source galaxies. It thus offers the unique possibility to do this investigation with observational data. We find that it is of great importance to know the redshifts of individual lens galaxies in order to constrain the properties of their dark matter halos. Whether the redshifts are derived from $UBVRI$ or the larger number of filters available in COMBO-17 is not very important. In contrast, knowledge of individual source redshifts improves the measurements only very little over the use of statistical source redshift distributions as long as the source redshift distribution is known accurately.

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arXiv2004

Bar Evolution Over the Last Eight Billion Years: A Constant Fraction of Strong Bars in GEMS

One third of present-day spirals host optically visible strong bars that drive their dynamical evolution. However, the fundamental question of how bars evolve over cosmological times has yet to be addressed, and even the frequency of bars at intermediate redshifts remains controversial. We investigate the frequency of bars out to z~1.0 drawing on a sample of 1590 galaxies from the GEMS survey, which provides morphologies from HST ACS two-color images, and highly accurate redshifts from the COMBO-17 survey. We identify spiral galaxies using the Sersic index, concentration parameter, and rest-frame color. We characterize bars and disks by fitting ellipses to F606W and F850LP images, taking advantage of the two bands to minimize bandpass shifting. We exclude highly inclined (i>60 deg) galaxies to ensure reliable morphological classifications, and apply completeness cuts of M_v <= -19.3 and -20.6. More than 40% of the bars that we detect have semi major axes a<0.5" and would be easily missed in earlier surveys without the small PSF of ACS. The bars that we can reliably detect are fairly strong (with ellipticities e>=0.4) and have a in the range ~1.2-13 kpc. We find that the optical fraction of such strong bars remains at ~(30% +- 6%) from the present-day out to look-back times of 2-6 Gyr (z~0.2-0.7) and 6-8 Gyr (z~0.7-1.0); it certainly shows no sign of a drastic decline at z>0.7. Our findings of a large and similar bar fraction at these three epochs favor scenarios in which cold gravitationally unstable disks are already in place by z~1, and where on average bars have a long lifetime (well above 2 Gyr). The distributions of structural bar properties in the two slices are, however, not statistically identical and therefore allow for the possibility that the bar strengths and sizes may evolve over time.

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arXiv2004

Masses of star clusters in the nuclei of bulge-less spiral galaxies

In the last decade star clusters have been found in the centers of spiral galaxies across all Hubble types. We here present a spectroscopic study of the exceptionally bright (10^6 - 10^8 Lsun) but compact (Re ~ 5 pc) nuclear star clusters in very late type spirals with UVES at the VLT. We find the velocity dispersions of the nine clusters in our sample to range from 13 to 34 km/s. Using photometric data from the HST/WFPC2 and spherically symmetric dynamical models we determine masses between 8*10^5 and 6*10^7 Msun. The mass to light ratios range from 0.2 to 1.5 in the I band. This indicates a young mean age for most clusters, in agreement with previous studies. Given their high masses and small sizes we find that nuclear clusters are among the objects with the highest mean surface density known (up to 10^5 Msun pc^-2). From their dynamical properties we infer that, rather than small bulges, the closest structural kin of nuclear clusters appear to be massive compact star clusters. This includes such different objects as globular clusters, "super star clusters", ultra compact dwarf galaxies and the nuclei of dwarf elliptical galaxies. It is a challenge to explain why, despite the wildly different current environments, all different types of massive star clusters share very similar and structural properties. A possible explanation links UCDs and massive globular clusters to nuclear star clusters through stripping of nucleated dwarf galaxies in a merger event. The extreme properties of this type of clusters would then be a consequence of their location in the centers of their respective host galaxies.

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arXiv2004

Evolution and Impact of Bars over the Last Eight Billion Years: Early Results from GEMS

Bars drive the dynamical evolution of disk galaxies by redistributing mass and angular momentum, and they are ubiquitous in present-day spirals. Early studies of the Hubble Deep Field reported a dramatic decline in the rest-frame optical bar fraction f_opt to below 5% at redshifts z>0.7, implying that disks at these epochs are fundamentally different from present-day spirals. The GEMS bar project, based on ~8300 galaxies with HST-based morphologies and accurate redshifts over the range 0.2-1.1, aims at constraining the evolution and impact of bars over the last 8 Gyr. We present early results indicating that f_opt remains nearly constant at ~30% over the range z=0.2-1.1,corresponding to lookback times of ~2.5-8 Gyr. The bars detected at z>0.6 are primarily strong with ellipticities of 0.4-0.8. Remarkably, the bar fraction and range of bar sizes observed at z>0.6 appear to be comparable to the values measured in the local Universe for bars of corresponding strengths. Implications for bar evolution models are discussed.

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arXiv2004

A Substantial Population of Red Galaxies at z > 2: Modeling of the Spectral Energy Distributions of an Extended Sample

We investigate the nature of the substantial population of high-z galaxies with Js-Ks>2.3 discovered as part of our FIRES survey. This colour cut efficiently isolates z>2 galaxies with red rest-frame optical colors ("Distant Red Galaxies" or DRGs). We select objects in the 2.5'x2.5' HDF-South (HDF-S) and 5'x5' field around the MS1054-03 cluster; the surface densities at Ks<21 are 1.6+-0.6 and 1.0+-0.2 arcmin^-2. We discuss the 34 DRGs at 2<z<3.5: 11 at Ks<22.5 in HDF-S and 23 at Ks<21.7 in the MS1054-03 field. We analyze the SEDs constructed from our deep near-infrared (NIR) and optical imaging from the ESO VLT and HST. We develop diagnostics involving I-Js, Js-H, and H-Ks to argue that the red NIR colors of DRGs cannot be attributed solely to extinction and require for many an evolved stellar population with prominent Balmer/4000A break. In the rest-frame, the optical colours of DRGs fall within the envelope of normal nearby galaxies and the UV colours suggest a wide range in star formation activity and/or extinction. This contrasts with the much bluer and more uniform SEDs of Lyman break galaxies (LBGs). From evolutionary synthesis models with constant star formation, solar metallicity, Salpeter IMF, and Calzetti et al. extinction law, we derive for the HDF-S (MS1054-03 field) DRGs median ages of 1.7(2.0) Gyr, A_V = 2.7(2.4) mag, stellar masses 0.8(1.6)x10^11 Msun, M/L_V = 1.2(2.3) Msun/LVsun, and SFR = 120(170) Msun/yr. Models assuming declining SFRs with e-folding timescales of 10Myr-1Gyr generally imply younger ages, lower A_V's and SFRs, but similar stellar masses within a factor of two. Compared to LBGs at similar redshifts and rest-frame L_V's, DRGs are older, more massive, and more obscured for any given star formation history. [ABRIDGED]

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arXiv2004

UV light from young stars in GEMS quasar host galaxies at 1.8<z<2.75

We have performed HST imaging of a sample of 23 high-redshift (1.8<z<2.75) AGN, 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 ACS 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_200nm<-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 agree with the recent discovery of enhanced blue stellar light in AGN hosts at lower redshifts. [abridged]

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arXiv2004

Colors of AGN host galaxies at 0.5<z<1.1 from GEMS

We present the results from a study of the host galaxies of 15 optically selected AGNs with 0.5<z<1.1 from GEMS. GEMS is a Hubble Space Telescope imaging survey of a ~28"x28" contiguous field centered on the Chandra Deep Field South in the F606W and F850LP filter bands. It incorporates the SEDs and redshifts of ~10000 objects, obtained by the COMBO-17 project. We have detected the host galaxies of all 15 AGNs in the F850LP-band (and 13/15 in the F606W-band), recovering their fluxes, morphologies and structural parameters. We find that 80% of the host galaxies have early-type (bulge-dominated) morphologies, while the rest have structures characteristic of late-type (disk-dominated) galaxies. We find that 25% of the early types, and 30% of the late types, exhibit disturbances consistent with galaxy interactions. The hosts show a wide range of colors, from those of red-sequence galaxies to blue colors consistent with ongoing star formation. Roughly 70% of the morphologically early-type hosts have rest-frame blue colors, a much larger fraction than those typical of non-active morphologically early-type galaxies in this redshift and luminosity range. Yet, we find that the early-type hosts are structurally similar to red-sequence ellipticals, inasmuch as they follow an absolute magnitude versus half-light size correlation that are consistent with the mean relation for early-type galaxies at similar redshifts.

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arXiv2004

Evolution of optically faint AGN from COMBO-17 and GEMS

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.

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arXiv2004

A catalogue of the Chandra Deep Field South with multi-colour classification and photometric redshifts from COMBO-17

We present the COMBO-17 object catalogue of the Chandra Deep Field South for public use, covering a field which is 31.5' x 30' in size. This catalogue lists astrometry, photometry in 17 passbands from 350 to 930 nm, and ground-based morphological data for 63,501 objects. The catalogue also contains multi-colour classification into the categories 'Star', 'Galaxy' and 'Quasar' as well as photometric redshifts. We include restframe luminosities in Johnson, SDSS and Bessell passbands and estimated errors. The redshifts are most reliable at R<24, where the sample contains approximately 100 quasars, 1000 stars and 10000 galaxies. We use nearly 1000 spectroscopically identified objects in conjunction with detailed simulations to characterize the performance of COMBO-17. We show that the selection of quasars, more generally type-1 AGN, is nearly complete and minimally contaminated at z=[0.5,5] for luminosities above M_B=-21.7. Their photometric redshifts are accurate to roughly 5000 km/sec. Galaxy redshifts are accurate to 1% in dz/(1+z) at R<21. They degrade in quality for progressively fainter galaxies, reaching accuracies of 2% for galaxies with R~222 and of 10% for galaxies with R>24. The selection of stars is complete to R~23, and deeper for M stars. We also present an updated discussion of our classification technique with maps of survey completeness, and discuss possible failures of the statistical classification in the faint regime at R>24.

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arXiv2004

The Nuclei of Late-type Spiral Galaxies

We summarize some recent results from our observational campaign to study the central regions of spiral galaxies of late Hubble type (Scd - Sm). These disk-dominated, bulgeless galaxies have an apparently uneventful merger history. The evolution of their nuclei thus holds important constraints on the mechanisms that are responsible for bulge formation and nuclear activity in spiral galaxies. We discuss the structural properties, stellar populations, and dynamical masses of the compact, luminous star cluster that is found in the nuclei of most late-type spiral galaxies. Although preliminary, our results strongly indicate that many galaxies of our sample experience repeated periods of nuclear star formation. While the exact mechanism that leads to the required high gas densities in the galaxy nucleus remains unclear, results from our recent CO survey of late-type spirals demonstrate that in most cases, the central kpc contains enough molecular gas to support repetitive nuclear starbursts.

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arXiv2003

Observations and Theoretical Implications of the Large Separation Lensed Quasar SDSS J1004+4112

We study the recently discovered gravitational lens SDSS J1004+4112, the first quasar lensed by a cluster of galaxies. It consists of four images with a maximum separation of 14.62''. The system has been confirmed as a lensed quasar at z=1.734 on the basis of deep imaging and spectroscopic follow-up observations. We present color-magnitude relations for galaxies near the lens plus spectroscopy of three central cluster members, which unambiguously confirm that a cluster at z=0.68 is responsible for the large image separation. We find a wide range of lens models consistent with the data, but they suggest four general conclusions: (1) the brightest cluster galaxy and the center of the cluster potential well appear to be offset by several kpc; (2) the cluster mass distribution must be elongated in the North--South direction, which is consistent with the observed distribution of cluster galaxies; (3) the inference of a large tidal shear (~0.2) suggests significant substructure in the cluster; and (4) enormous uncertainty in the predicted time delays between the images means that measuring the delays would greatly improve constraints on the models. We also compute the probability of such large separation lensing in the SDSS quasar sample, on the basis of the CDM model. The lack of large separation lenses in previous surveys and the discovery of one in SDSS together imply a mass fluctuation normalization σ_8=1.0^{+0.4}_{-0.2} (95% CL), if cluster dark matter halos have an inner slope -1.5. Shallower profiles would require higher values of σ_8. Although the statistical conclusion might be somewhat dependent on the degree of the complexity of the lens potential, the discovery is consistent with the predictions of the abundance of cluster-scale halos in the CDM scenario. (Abridged)

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arXiv2003

A Gravitationally Lensed Quasar with Quadruple Images Separated by 14.62 Arcseconds

Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of large-scale structures predicts the existence of quasars gravitationally lensed by concentrations of dark matter so massive that the quasar images would be split by over 7 arcsec. Numerous searches for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known, including the first lensed quasar discovered, have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004+4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations based on the cold-dark-matter model.

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arXiv2003

The Fundamental Plane of Field Early Type Galaxies at z=1

We present deep VLT spectra of early type galaxies at z~1 in the Chandra Deep Field South, from which we derive velocity dispersions. Together with structural parameters from Hubble Space Telescope imaging, we can study the Fundamental Plane for field early type galaxies at that epoch. We determine accurate mass-to-light ratios and colors for four field early type galaxies in the redshift range 0.96 2). The lower mass galaxies at z~1 have a lower average mass-to-light ratio, and one is a genuine 'E+A' galaxy. The mass-to-light ratios indicate that their luminosity weighted ages are a factor of three younger at the epoch of observation, due to either a late formation redshift, or due to late bursts of star formation contributing 20-30% of the mass.

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