arXiv subjects

Hans-Walter Rix

Publications and source records attributed to Hans-Walter Rix.

At least 433 records · Page 24Linked to original sources

arXiv2005

The Lensed Arc Production Efficiency of Galaxy Clusters: A Comparison of Matched Observed and Simulated Samples

We compare the statistical properties of giant gravitationally lensed arcs produced in matched simulated and observed cluster samples. The observed sample consists of 10 X-ray selected clusters at redshifts z ~ 0.2 imaged with HST by Smith et al. The simulated dataset is produced by lensing the Hubble Deep Field, which serves as a background source image, with 150 realizations (different projections and shifts) of five simulated z = 0.2 clusters from a LambdaCDM N-body simulation. The real and simulated clusters have similar masses, the real photometric redshift is used for each background source, and all the observational effects influencing arc detection in the real dataset, including light from cluster galaxies, are simulated in the artificial dataset. We develop, and apply to both datasets, an objective automatic arc-finding algorithm. We find consistent arc statistics in the real and in the simulated sample, with an average of ~ 1 detected giant (length to width ratio >= 10) arc per cluster and ~ 0.2 giant luminous (R<22.3 mag) arc per cluster. Thus, taking into account a realistic source population and observational effects, the clusters predicted by LambdaCDM have the same arc-production efficiency as the observed clusters. If, as suggested by other studies, there is a discrepancy between the predicted and the observed total number of arcs on the sky, it must be the result of differences between the redshift dependent cluster mass functions, and not due to differences in the lensing efficiency of the most massive clusters.

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arXiv2005

Cosmological weak lensing with the HST GEMS survey

We present our cosmic shear analysis of GEMS, one of the largest wide-field surveys ever undertaken by the Hubble Space Telescope. Imaged with the Advanced Camera for Surveys (ACS), GEMS spans 795 square arcmin in the Chandra Deep Field South. We detect weak lensing by large-scale structure in high resolution F606W GEMS data from ~60 resolved galaxies per square arcminute. We measure the two-point shear correlation function, the top-hat shear variance and the shear power spectrum, performing an E/B mode decomposition for each statistic. We show that we are not limited by systematic errors and use our results to place joint constraints on the matter density parameter Omega_m and the amplitude of the matter power spectrum sigma_8. We find sigma_8(Omega_m/0.3)^{0.65}=0.68 +/- 0.13 where the 1sigma error includes both our uncertainty on the median redshift of the survey and sampling variance. Removing image and point spread function (PSF) distortions are crucial to all weak lensing analyses. We therefore include a thorough discussion on the degree of ACS PSF distortion and anisotropy which we characterise directly from GEMS data. Consecutively imaged over 20 days, GEMS data also allows us to investigate PSF instability over time. We find that, even in the relatively short GEMS observing period, the ACS PSF ellipticity varies at the level of a few percent which we account for with a semi-time dependent PSF model. Our correction for the temporal and spatial variability of the PSF is shown to be successful through a series of diagnostic tests.

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arXiv2005

The Closest View of a Dwarf Galaxy: New Evidence on the Nature of the Canis Major over-density

We present a first deep colour-magnitude diagram of the putative central region (0.5 deg x 0.5 deg) of the Canis Major stellar over-density (l,b)=(240,-8) found by Martin et al. (2004), which has been proposed as the remnant of a dwarf satellite accreted onto the Milky Way on a near-equatorial orbit. We find a narrow (in apparent magnitude) main-sequence, extending 6 magnitudes below the turn-off to our limiting magnitude of B ~ 24.5 mag. This main sequence has very high constrast (>3) with respect to the thin/thick disk/halo background; its narrowness at brighter magnitudes clearly implies the presence of a distinct and possibly still bound stellar system. We derived the line-of-sight size (r_{1/2}) of this system based on the B-band width of the lower main sequence, obtaining 0.94 +/- 0.18 (random) +/- 0.18 (systematic) kpc. That size matches a model prediction for the main body of the parent galaxy of the Monoceros tidal stream. The high density contrast and limited spatial extent in the radial direction are very hard to reconcile with the alternative explanation put forward to explain the Canis Major stellar-overdensity: a flared or warped Galactic disk viewed in projection (Momamy et al. 2004). We also derived a central surface brightness of mu_{V,0}= 23.3 +/- 0.1 mag arcsec^{-2} and an absolute magnitude of M_{V}=-14.5 +/- 0.1 mag. These values place the Canis Major object in the category of dwarf galaxy in the the L_{V}--size and M_V - mu_{V} planes for such objects. However, like the Sagittarius dwarf, it is an outlier in the[Fe/H] -M_V plane in the sense that it is too metal rich for its estimated absolute magnitude. This suggests that the main mechanism driving its recent and current star formation history (possibly tidal stripping) is different to that of isolated dwarfs.

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arXiv2005

Discovery of a Fifth Image of the Large Separation Gravitationally Lensed Quasar SDSS J1004+4112

We report the discovery of a fifth image in the large separation lensed quasar system SDSS J1004+4112. A faint point source located 0.2'' from the center of the brightest galaxy in the lensing cluster is detected in images taken with the Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope. The flux ratio between the point source and the brightest lensed component in the ACS image is similar to that in the NICMOS image. The location and brightness of the point source are consistent with lens model predictions for a lensed image. We therefore conclude that the point source is likely to be a fifth image of the source quasar. In addition, the NICMOS image reveals the lensed host galaxy of the source quasar, which can strongly constrain the structure of the lensing critical curves and thereby the mass distribution of the lensing cluster.

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arXiv2005

GEMS: The Size Evolution of Disk Galaxies

We combine HST imaging from the GEMS 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 Sersic 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 absolute magnitudes M(V)<-20, or equivalently stellar masses log(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 per sqarcsec 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)>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.

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arXiv2005

Towards an understanding of the rapid decline of the cosmic star formation rate

We present a first analysis of deep 24 micron observations with the Spitzer Space Telescope of a sample of nearly 1500 galaxies in a thin redshift slice, 0.65 2x10^10 solar masses) are undergoing a period of intense star formation above their past-averaged SFR. In contrast, less than 1% of equally-massive galaxies in the local universe have similarly intense star formation activity. Morphologically-undisturbed galaxies dominate the total infrared luminosity density and SFR density: at z~0.7, more than half of the intensely star-forming galaxies have spiral morphologies, whereas less than \~30% are strongly interacting. Thus, a decline in major-merger rate is not the underlying cause of the rapid decline in cosmic SFR since z~0.7. Physical properties that do not strongly affect galaxy morphology - for example, gas consumption and weak interactions with small satellite galaxies - appear to be responsible.

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arXiv2004

Discovery of Two Gravitationally Lensed Quasars with Image Separations of 3 Arcseconds from the Sloan Digital Sky Survey

We report the discovery of two doubly-imaged quasars, SDSS J100128.61+502756.9 and SDSS J120629.65+433217.6, at redshifts of 1.838 and 1.789 and with image separations of 2.86'' and 2.90'', respectively. The objects were selected as lens candidates from the Sloan Digital Sky Survey (SDSS). Based on the identical nature of the spectra of the two quasars in each pair and the identification of the lens galaxies, we conclude that the objects are gravitational lenses. The lenses are complicated; in both systems there are several galaxies in the fields very close to the quasars, in addition to the lens galaxies themselves. The lens modeling implies that these nearby galaxies contribute significantly to the lens potentials. On larger scales, we have detected an enhancement in the galaxy density near SDSS J100128.61+502756.9. The number of lenses with image separation of ~3'' in the SDSS already exceeds the prediction of simple theoretical models based on the standard Lambda-dominated cosmology and observed velocity function of galaxies.

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arXiv2004

The Evolution of Early-type Red Galaxies with the GEMS Survey: Luminosity-size and Stellar Mass-size Relations Since z=1

We combine HST/ACS imaging from the GEMS 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. We use a new large sample of 728 galaxies with centrally-concentrated radial profiles (Sersic n>2.5) and rest-frame U-V colors on the red sequence. By appropriate comparison with the local relations from SDSS, we find that the luminosity-size (L-R) and stellar mass-size (M-R) relations evolve in a manner that is consistent with the passive aging of ancient stars. 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 a factor of 2 since z=1, our results imply that new additions to the early-type galaxy population follow similar L-R and M-R correlations, compared to the older subset of early-type galaxies. Adding early-type galaxies to the red sequence through disk fading 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 early-type galaxies and/or younger stellar populations; the present data is insufficient to discriminate between these possibilities. (abridged)

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arXiv2004

The Stellar Populations in the Central Parsecs of Galactic Bulges Central Stellar Populations

We present Hubble Space Telescope blue spectra at intermediate spectral resolution for the nuclei of 23 nearby disk galaxies. These objects were selected to have nebular emission in their nuclei, and span a range of emission-line classifications as well as Hubble types. In this paper we focus on the stellar population as revealed by the continuum spectral energy distribution measured within the central 0."13 (~8pc) of these galaxies. The data were modeled with linear combinations of single-age stellar population synthesis models. The large majority (~80%) of the surveyed nuclei have spectra whose features are consistent with a predominantly old (>5x10^9 yr) stellar population. Approximately 25% of these nuclei show evidence of a component with age younger than 1 Gyr, with the incidence of these stars related to the nebular classification. Successful model fits imply an average reddening corresponding to A_V~0.4 mag and stellar metallicity of (1--2.5) Z_sun. We discuss the implications of these results for the understanding of star formation history in the environment of quiescent and active supermassive black holes. Our findings reinforce the picture wherein Seyfert nuclei and the majority of low-ionization nuclear emission-line regions (LINERs) are predominantly accretion-powered, and suggest that much of the central star formation in Hii nuclei is actually circumnuclear.

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arXiv2004

Testing the Universality of the (U-V) Color-Magnitude Relations for Nearby Clusters of Galaxies

We present a detailed U-V CMR analysis for 3 local (z<0.06) Abell (A85, A496, A754) galaxy clusters, with 637 total spectroscopic members down to 0.1L^star. We find that these clusters have similar CMRs with a mean color of U-V=1.4 at M_V=-20+5logh, and narrow limits of intrinsic color scatter [0.047,0.079] and slope [-0.094,-0.075]. The core cluster population of red galaxies follow CMRs that are in close agreement with that of Coma. We test how sensitive the CMR uniformity is to variations in color aperture size and sample selection, and find slightly wider limits of scatter [0.047,0.112] and slope [-0.104,-0.054]. This upper limit for U-V scatter is consistent with z_form>1.2 for the bulk of the stars in red cluster galaxies, and a maximum age spread of 5.2 Gyr. We note that none of the slopes exhibit a break as claimed by Metcalf et al., and using colors from equal-light apertures does not remove the CMR slope. Our findings expand the single Coma data point and provide a much-needed z=0 base-line for comparisons to high-z cluster U-V CMRs. The CMR scatter range in low-z clusters is consistent with that found in high-z clusters, suggesting little evolution in the stars of red cluster galaxies out to z~1. To identify the most recently accreted galaxies, we divide each cluster's membership into 3 galaxy populations based on CMR-relative U-V color. Blue galaxies make up 18-23% of each cluster's galaxy counts above 0.1L^star. We test the hypothesis that CMR-relative color represents rough cluster infall times and find that blue galaxies are spatially, kinematically, and morphologically distinct from red cluster members. These distinctions provide further evidence that bluer members are indeed more recently accreted field spirals as expected in an hierarchical universe. (abridged)

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arXiv2004

Galaxy-galaxy lensing studies from COMBO-17

We study the dark matter halos of galaxies with galaxy-galaxy lensing using the COMBO-17 survey. This survey offers an unprecedented data set for studying lens galaxies at z=0.2-0.7 including redshift information and spectral classification from 17 optical filters for objects brighter than R=24. So far, redshifts and classification for the lens galaxies have mainly been available for local surveys like the Sloan Digital Sky Survey (SDSS). Further, redshifts for the source galaxies have typically not been available at all but had to be estimated from redshift probability distribution which -- for faint surveys -- even had to be extrapolated. To study the dark matter halos we parametrize the lens galaxies as singular isothermal spheres (SIS) or by Navarro-Frenk-White (NFW) profiles. In both cases we find a dependence of the velocity dispersion or virial radius, respectively, on lens luminosity and colour. For the SIS model, we are able to reproduce the Tully-Fisher/Faber-Jackson relation on a scale of 150h^-1kpc. For the NFW profile we also calculate virial masses, mass-to-light ratios and rotation velocities. Finally, we investigate differences between the three survey fields used here.

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arXiv2004

Andromeda IX: A New Dwarf Spheroidal Satellite of M31

We report the discovery of a new dwarf spheroidal satellite of M31, Andromeda IX, based on resolved stellar photometry from the Sloan Digital Sky Survey (SDSS). Using both SDSS and public archival data we have estimated its distance and other physical properties, and compared these to the properties of a previously known dwarf spheroidal companion, Andromeda V, also observed by SDSS. Andromeda IX is the lowest surface brightness galaxy found to date (mu_{V} ~ 26.8 mag arcsec^-2), and at the distance we estimate from the position of the tip of Andromeda IX's red giant branch, (m - M)_{0} ~ 24.5 (805 kpc), Andromeda IX would also be the faintest galaxy known (M_{V} ~ -8.3).

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arXiv2004

A New Giant Stellar Structure in the Outer Halo of M31

The Sloan Digital Sky Survey has revealed an overdensity of luminous red giant stars ~ 3 degrees (40 projected kpc) to the northeast of M31, which we have called Andromeda NE. The line-of-sight distance to Andromeda NE is within approximately 50 kpc of M31; Andromeda NE is not a physically unrelated projection. Andromeda NE has a g-band absolute magnitude of ~ -11.6 and central surface brightness of ~ 29 mag/sq.arcsec, making it nearly two orders of magnitude more diffuse than any known Local Group dwarf galaxy at that luminosity. Based on its distance and morphology, Andromeda NE is likely undergoing tidal disruption. Andromeda NE's red giant branch color is unlike that of M31's present-day outer disk or the stellar stream reported by Ibata et al. (2001), arguing against a direct link between Andromeda NE and these structures. However, Andromeda NE has a red giant branch color similar to that of the G1 clump; it is possible that these structures are both material torn off of M31's disk in the distant past, or that these are both part of one ancient stellar stream.

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arXiv2004

Stellar Populations and Kinematics of Red Galaxies at z>2: Implications for the Formation of Massive Galaxies

We recently identified a substantial population of galaxies at z>2 with red rest-frame optical colors. These distant red galaxies (DRGs) are efficiently selected by the simple observed color criterion J-K>2.3. In this paper we present NIR spectroscopy with Keck/NIRSPEC of six DRGs at 2.4<z<3.2. We detect continuum emission and emission lines of all observed galaxies. Equivalent widths of H alpha are 20-30 Ang, smaller than measured for LBGs and nearby LIRGs, and comparable to normal nearby galaxies. The modest equivalent widths imply that the galaxies either have a decreasing star formation rate, or that they are very dusty. Fitting both the photometry and the H alpha lines, we find continuum extinction A_V=1-2 mag, ages 1-2.5 Gyr, star formation rates 200-400 solar masses/yr, and stellar masses 1-5x10^11 solar masses for models with constant star formation rates. From [NII]/H alpha ratios we infer that the metallicities are high, 1-1.5 x Solar. For four galaxies we can determine line widths from the optical emission lines. The widths are high, ranging from 130-240 km/s, and by combining data for LBGs and DRGs we find significant correlations between linewidth and restframe U-V color, and between linewidth and stellar mass. The latter correlation has a similar slope and offset as the ``baryonic Tully-Fisher relation'' for nearby galaxies. The median dynamical mass is ~2x10^11 solar masses, supporting the high stellar masses inferred from the photometry. We find that the median M/L_V ~ 0.8, a factor of ~5 higher than measured for LBGs. We infer from our small sample that DRGs are dustier, more metal rich, more massive, and have higher ages than z=3 LBGs of the same rest-frame V-band luminosity. Their high M/L ratios imply that they contribute significantly to the stellar mass density at z~2.5. [ABRIDGED]

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arXiv2004

Structural Evidence for Environment-Driven Transformation of the Blue Galaxies in Local Abell Clusters - A85, A496, and A754

We compare V-band structural properties of cluster and field galaxies, to test whether much of the current cluster membership resulted from the transformation of infalling spirals into red early types. We use 140 galaxies from 3 z<0.6 Abell clusters (A85,A496,A754 - spectroscopic membership) with blue colors, and compare them to 80 field galaxies with similar colors and luminosities. Our previous work (McIntosh et al. 2004, ApJ submitted) shows that the blue galaxies in these local clusters are a recently infalling population that has yet to encounter the dense core. We quantify galaxy internal structure from 2D B/D decompositions using GIM2D. We observe structural differences between blue galaxies in clusters, compared to the field. Most blue cluster members are physically smaller and fainter than their field counterparts. At a matched size and luminosity, recent arrivals are smoother in appearance, yet their total light is as disk-dominated as in normal field spirals. Moreover, 50% of blue cluster galaxies have blue cores or globally blue color profiles, in contrast with field spirals that have typically red color gradients. Without environment dependent evolution outside of cluster cores, we expect blue disk galaxies in clusters and the field to have similar morphologies, sizes, and color gradients. We show conclusively that galaxy properties do reflect the environment in which the galaxy is found. The data show that the transformation of accreted galaxies is not confined to the dense cluster core. The overall properties of bluer cluster galaxies 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. (abridged)

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arXiv2004

Nearly 5000 Distant Early-Type Galaxies in COMBO-17: a Red Sequence and its Evolution since z~1

We present the rest-frame colors and luminosities of ~25000 m_R<24 galaxies in the redshift range 0.2<z<1.1, drawn from 0.78 square degrees of the COMBO-17 survey. We find that the rest-frame color distribution of these galaxies is bimodal at all redshifts out to z~1. This bimodality permits a model-independent definition of red, early-type galaxies and blue, late-type galaxies at any given redshift. The colors of the blue peak become redder towards the present day, and the number density of blue luminous galaxies has dropped strongly since z~1. Focusing on the red galaxies, we find that they populate a color-magnitude relation. Such red sequences have been identified in galaxy cluster environments, but our data show that such a sequence exists over this redshift range even when averaging over all environments. The mean color of the red galaxy sequence evolves with redshift in a way that is consistent with the aging of an ancient stellar population. The rest-frame B-band luminosity density in red galaxies evolves only mildly with redshift in a Lambda-dominated cold dark matter universe. Accounting for the change in stellar mass-to-light ratio implied by the redshift evolution in red galaxy colors, the COMBO-17 data indicate an increase in stellar mass on the red sequence by a factor of two since z~1. The largest source of uncertainty is large-scale structure, implying that considerably larger surveys are necessary to further refine this result. We explore mechanisms that may drive this evolution in the red galaxy population, finding that both galaxy merging and truncation of star formation in some fraction of the blue, star-forming population are required to fully explain the properties of these galaxies.

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arXiv2004

On the Black Hole Mass - Bulge Mass Relation

We have re-examined the relation between the mass of the central black holes in nearby galaxies, M_bh, and the stellar mass of the surrounding spheroid or bulge, M_bulge. For a total of 30 galaxies bulge masses were derived through Jeans equation modeling or adopted from dynamical models in the literature. In stellar mass-to-light ratios the spheroids and bulges span a range of a factor of eight. The bulge masses were related to well-determined black hole masses taken from the literature. With these improved values for M_bh, compared to Magorrian et al. (1998), and our redetermination of M_bulge, we find the M_bh-M_bulge relation becomes very tight. We find M_bh ~ M_bulge^(1.12+/-0.06) with an observed scatter of < 0.30 dex, a fraction of which can be attributed to measurement errors. The scatter in this relation is therefore comparable to the scatter in the relations of M_bh with sigma and the stellar concentration. These results confirm and refine the work of Marconi and Hunt (2003). For M_bulge~5x10^10 M_sun the median black hole mass is 0.14% +/- 0.04% of the bulge mass.

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arXiv2004

GEMS: Galaxy Evolution from Morphologies and SEDs

GEMS, Galaxy Evolution from Morphologies and SEDs, is a large-area (800 arcmin2) two-color (F606W and F850LP) imaging survey with the Advanced Camera for Surveys on HST. Centered on the Chandra Deep Field South, it covers an area of ~28'x28', or about 120 Hubble Deep Field areas, to a depth of m_AB(F606W)=28.3 (5sigma and m_AB(F850LP)=27.1 (5sigma) for compact sources. In its central ~1/4, GEMS incorporates ACS imaging from the GOODS project. Focusing on the redshift range 0.2<=z<=1.1, GEMS provides morphologies and structural parameters for nearly 10,000 galaxies where redshift estimates, luminosities and SEDs exist from COMBO-17. At the same time, GEMS contains detectable host galaxy images for several hundred faint AGN. This paper provides an overview of the science goals, the experiment design, the data reduction and the science analysis plan for GEMS.

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