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Hans-Walter Rix

Publications and source records attributed to Hans-Walter Rix.

At least 451 records · Page 25Linked to original sources
arXiv2003

WFI J2026-4536 and WFI J2033-4723: Two New Quadruple Gravitational Lenses

We report the discovery of two new gravitationally lensed quasars, WFI J2026-4536 and WFI J2033-4723, at respective source redshifts of z=2.23 and z=1.66. Both systems are quadruply imaged and have similar PG1115-like image configurations. WFI J2026-4536 has a maximum image separation of 1.4", a total brightness of g = 16.5, and a relatively simple lensing environment, while WFI J2033-4723 has a maximum image separation of 2.5", an estimated total brightness of g = 17.9, and a more complicated environment of at least six galaxies within 20". The primary lensing galaxies are detected for both systems after PSF subtraction. Several of the broadband flux ratios for the two lenses show a strong (0.1-0.4 mags) trend with wavelength, suggesting either microlensing or differential extinction through the lensing galaxy. For WFI J2026-4536, the total quasar flux has dimmed by 0.1 mag in the blue but only half as much in the red over three months, suggestive of microlensing-induced variations. For WFI J2033-4723, resolved spectra of some of the quasar components reveal emission line flux ratios that agree better with the macromodel predictions than either the broadband or continuum ratios, also indicative of microlensing. The predicted differential time delays for WFI J2026-4536 are short, ranging from 1-2 weeks for the long delay, but are longer for WFI J2033-4723, ranging from 1-2 months. Both systems hold promise for future monitoring campaigns aimed at microlensing or time delay studies.

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arXiv2003

The Luminosity-Size and Mass-Size Relations of Galaxies out to z~3

The luminosity-size and stellar mass-size distributions of galaxies out to z~3 is presented. We use very deep near-infrared images of the Hubble Deep Field South in the J_s, H, and K_s bands, taken as part FIRES at the VLT, to follow the evolution of the optical rest-frame sizes of galaxies. For a total of 168 galaxies with K_{s,AB}<23.5, we find that the rest-frame V-band sizes r_{e,V} of luminous galaxies ( ~2x10^{10}h^{-2}L_sun) at 2 ~2x10^{10}h^{-2}M_sun, has changed by 20(+-20)% since z~2.5. Both results can be reconciled by the fact that the stellar M/L ratio is lower in the luminous high z galaxies than in nearby ones because they have young stellar populations. The lower incidence of large galaxies at z~3 seems to reflect the rarity of galaxies with high stellar mass.

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arXiv2003

GEMS Imaging of Red Sequence Galaxies at z~0.7: Dusty or Old?

We have used the 30'x30' 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 rest-frame V-band luminosity density comes from visually-classified E/S0/Sa galaxies (at V-band magnitudes brighter than -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.

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arXiv2003

Exploring spiral galaxy potentials with hydrodynamical simulations

We study how well the complex gas velocity fields induced by massive spiral arms are modelled by the hydrodynamical simulations we used to constrain the dark matter fraction in nearby spiral galaxies (Kranz et al. 2001, 2003). More specifically, we explore the dependence of the positions and amplitudes of features in the gas flow on the temperature of the interstellar medium (assumed to behave as a one-component isothermal fluid), the non-axisymmetric disk contribution to the galactic potential, the pattern speed, Omega_p, and finally the numerical resolution of the simulation. We argue that, after constraining the pattern speed reasonably well by matching the simulations to the observed spiral arm morphology, the amplitude of the non-axisymmetric perturbation (the disk fraction) is left as the primary parameter determining the gas dynamics. However, due to the sensitivity of the positions of the shocks to modeling parameters, one has to be cautious when quantitatively comparing the simulations to observations. In particular, we show that a global least squares analysis is not the optimal method for distinguishing different models as it tends to slightly favor low disk fraction models. Nevertheless, we conclude that, given observational data of reasonably high spatial resolution and an accurate shock-resolving hydro-code this method tightly constrains the dark matter content within spiral galaxies. We further argue that even if the perturbations induced by spiral arms are weaker than those of strong bars, they are better suited for this kind of analysis because the spiral arms extend to larger radii where effects like inflows due to numerical viscosity and morphological dependence on gas sound speed are less of a concern than they are in the centers of disks.

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arXiv2003

The Rest-Frame Optical Luminosity Density, Color, and Stellar Mass Density of the Universe from z=0 to z=3

We present the evolution of the rest-frame optical luminosity density, of the integrated rest-frame optical color, and of the stellar mass density for a sample of Ks-band selected galaxies in the HDF-S. We derived the luminosity density in the rest-frame U, B, and V-bands and found that the luminosity density increases by a factor of 1.9+-0.4, 2.9+-0.6, and 4.9+-1.0 in the V, B, and U rest-frame bands respectively between a redshift of 0.1 and 3.2. We derived the luminosity weighted mean cosmic (U-B)_rest and (B-V)_rest colors as a function of redshift. The colors bluen almost monotonically with increasing redshift; at z=0.1, the (U-B)_rest and (B-V)_rest colors are 0.16 and 0.75 respectively, while at z=2.8 they are -0.39 and 0.29 respectively. We derived the luminosity weighted mean M/LV using the correlation between (U-V)_rest and log_{10} M/LV which exists for a range in smooth SFHs and moderate extinctions. We have shown that the mean of individual M/LV estimates can overpredict the true value by ~70% while our method overpredicts the true values by only ~35%. We find that the universe at z~3 had ~10 times lower stellar mass density than it does today in galaxies with LV>1.4 \times 10^{10} h_{70}^-2 Lsol. 50% of the stellar mass of the universe was formed by $z~1-1.5. The rate of increase in the stellar mass density with decreasing redshift is similar to but above that for independent estimates from the HDF-N, but is slightly less than that predicted by the integral of the SFR(z) curve.

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arXiv2003

Sagittarius Tidal Debris 90 kpc from the Galactic Center

A new overdensity of A-colored stars in distant parts of the Milky Way's stellar halo, at a dereddened SDSS magnitude of g_0 = 20.3, is presented. Identification of associated variable RR Lyrae candidates supports the claim that these are blue horizontal branch stars. The inferred distance of these stars from the Galactic center is 90 kpc, assuming the absolute magnitude of these stars is M_g_0 = 0.7 and that the Sun is 8.5 kpc from the Galactic center. The new tidal debris is within 10 kpc of same plane as other confirmed tidal debris from the disruption of the Sagittarius dwarf galaxy, and could be associated with the trailing tidal arm. Distances to the Sagittarius stream estimated from M stars are about 13% smaller than our inferred distances. The tidal debris has a width of at least 10 degrees, and is traced for more than 20 degrees across the sky. The globular cluster NGC 2419 is located within the detected tidal debris, and may also have once been associated with the Sagittarius dwarf galaxy.

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arXiv2003

The dependence on environment of the color-magnitude relation of galaxies

The distribution in color and absolute magnitude is presented for 55,158 galaxies taken from the Sloan Digital Sky Survey in the redshift range $0.08<z<0.12$, as a function of galaxy number overdensity in a line-of-sight cylinder of transverse radius $1 h^{-1} \mathrm{Mpc}$. In all environments, bulge-dominated galaxies (defined to be those with radial profiles best fit with large Sérsic indices) form a narrow, well defined color--magnitude relation. Although the most luminous galaxies reside preferentially in the highest density regions, there is only a barely detectable variation in the color (zero-point) or slope of the color--magnitude relation ($<0.02 \mathrm{mag}$ in $^{0.1}[g-r]$ or $[B-V]$). These results constrain variations with environmental density in the ages or metallicities of typical bulge-dominated galaxies to be under 20 percent.

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arXiv2003

The Tully-Fisher Relation of Barred Galaxies

We present new data exploring the scaling relations, such as the Tully-Fisher relation (TFR), of bright barred and unbarred galaxies. A primary motivation for this study is to establish whether barredness correlates with, and is a consequence of, virial properties of galaxies. Various lines of evidence suggest that dark matter is dominant in disks of bright unbarred galaxies at 2.2 disk scale lengths, the point of peak rotation for a pure exponential disk. We test the hypothesis that the TF plane of barred high surface brightness galaxies is offset from the mean TFR of unbarred galaxies, as might be expected if barred galaxies are ``maximal'' in their inner parts. We use existing and new TF data to search for basic structural differences between barred and unbarred galaxies. Our new data consist of 2-dimensional Halpha velocity fields derived from SparsePak integral field spectroscopy (IFS) and V,I-band CCD images collected at the WIYN Observatory for 14 strongly barred galaxies. We use WIYN/SparsePak (2-D) velocity fields to show that long-slit (1-D) spectra yield reliable circular speed measurements at or beyond 2.2 disk scale lengths, far from any influence of the bar. This enables us to consider line width measurements from extensive TF surveys which include barred and nonbarred disks and derive detailed scaling relation comparisons. We find that for a given luminosity, barred and unbarred galaxies have comparable structural and dynamical parameters, such as peak velocities, scale lengths, or colors. In particular, the location of a galaxy in the TF plane is independent of barredness. In a global dynamical sense, barred and unbarred galaxies behave similarly and are likely to have, on average, comparable fractions of luminous and dark matter at a given radius. (abridged)

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arXiv2003

The redshift distribution of gravitational lenses revisited: Constraints on galaxy mass evolution

The redshifts of lens galaxies in known gravitational lens systems probe the volume distribution of lensing mass. Following earlier work by Kochanek, we re-derive the lens redshift probability distribution, allowing for mass and number density evolution of the lensing galaxies, and apply this test to a much enlarged sample of lens systems. From a literature survey of all known lenses, we have selected an unbiased sample of 15 lenses with complete redshift information. For a flat Universe and no lens evolution, we can only put an upper limit on the cosmological constant of Omega_lambda<0.89 at the 95% CL. Omega_lambda~0.7 and no evolution is consistent with the data. Allowing for evolution in an Omega_m=0.3, Omega_lambda=0.7 cosmology, we find that the best-fit evolution in sigma* (i.e., the characteristic velocity dispersion in a Schechter-like function) of early-type galaxies, in the redshift range z~0 to 1, is d[log sigma*(z)]/dz=-0.10+/-0.06. This is consistent with no evolution and implies that, at 95% CL, sigma* of early-type galaxies at z~1 was at least 63% of its current value. Alternatively, if there is no mass evolution, a present-day value of sigma*>175 km/s for elliptical galaxies is required (95% CL).

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arXiv2003

Galaxy-galaxy lensing results from COMBO-17

We use galaxy-galaxy lensing to study the statistical properties of the dark matter halos of galaxies. Our data is taken from the COMBO-17 survey which has imaged 1 square degree in 17 optical filters giving accurate photometric redshifts and spectral classification down to R=24. This allows us to study lens galaxies at $z=0.2-0.7$. Shapes are measured from deep $R$-band images taken at the best seeing conditions ($0\farcs 8$ PSF). We model the lens galaxies as singular isothermal spheres (SIS). Investigating the dependence of the velocity dispersion on the luminosities of the lens galaxies we are able to reproduce the Tully-Fisher/Faber-Jackson relation. Further we find a larger lensing signal for early-type galaxies than for the late-types. However, when testing the dependence of the velocity dispersion on radius we see clear deviations from the SIS model. The velocity dispersion is first rising and then declining again. This is exactly what is expected for lens galaxies that can be modeled by Navarro-Frenk-White (NFW) profiles.

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arXiv2003

A Significant Population of Red, Near-IR Selected High Redshift Galaxies

We use very deep near-infrared photometry of the Hubble Deep Field South taken with ISAAC/VLT to identify a population of high redshift galaxies with rest- frame optical colors similar to those of nearby galaxies. The galaxies are chosen by their infrared colors Js-Ks > 2.3, aimed at selecting galaxies with redshifts above 2. When applied to our dataset, we find 14 galaxies with Ks < 22.5, corresponding to a surface density of 3+-0.8 /arcmin**2. The photometric redshifts all lie above 1.9, with a median of 2.6 and a rms of 0.7. The spectral energy distributions of these galaxies show a wide range: one is very blue in the rest-frame UV, and satisfies the normal Lyman-break criteria for high redshift, star-forming galaxies. Others are quite red throughout the observed spectral range, and are extremely faint in the optical, with a median V = 26.6. Hence these galaxies would not be included in photometric samples based on optical ground-based data, and spectroscopic follow-up is difficult. The spectral energy distributions often show a prominent break, identified as the Balmer break or 4000 Ang. break. The median age is 1 Gyr when fit with a constant star formation model with dust, or 0.7 Gyr when fit with a single burst model. Although significantly younger ages cannot be excluded when a larger range of models is allowed, the results indicate that these galaxies are among the oldest at these redshifts. The volume density to Ks=22.5 is half that of Lyman-break galaxies at z = 3. Since the mass-to-light ratios of the red galaxies are likely to be higher, the stellar mass density is inferred to be comparable to that of Lyman-break galaxies. These red galaxies may be the descendants of galaxies which started to form stars at very high redshifts, and they may evolve into the most massive galaxies at low redshift.

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arXiv2003

Spectroscopic Confirmation of a Substantial Population of Luminous Red Galaxies at Redshifts z>2

We confirm spectroscopically the existence of a population of galaxies at z>~2 with rest-frame optical colors similar to normal nearby galaxies. The galaxies were identified by their red near-infrared colors in deep images obtained with ISAAC on the Very Large Telescope. Redshifts of six galaxies with Js-Ks>2.3 were measured from optical spectra obtained with the Keck Telescope. Five out of six are in the range 2.43<=z<=3.52, demonstrating that the Js-Ks color selection is quite efficient. The rest-frame ultraviolet spectra of confirmed z>2 galaxies display a range of properties, with two galaxies showing emission lines characteristic of AGN, two having Ly-alpha in emission, and one showing interstellar absorption lines only. Their full spectral energy distributions are well described by constant star formation models with ages 1.4-2.6 Gyr, except for one galaxy whose colors indicate a dusty starburst. The confirmed z>2 galaxies are very luminous, with Ks=19.2-19.9. Assuming that our bright spectroscopic sample is representative for the general population of Js-Ks selected objects, we find that the surface density of red z>~2 galaxies is ~0.9/arcmin^2 to Ks=21. The surface density is comparable to that of Lyman-break selected galaxies with Ks<21, when corrections are made for the different redshift distributions of the two samples. Although there will be some overlap between the two populations, most 'optical-break' galaxies are too faint in the rest-frame ultraviolet to be selected as Lyman-break galaxies. The most straightforward interpretation is that star formation in typical optical-break galaxies started earlier than in typical Lyman-break galaxies. Optical-break galaxies may be the oldest and most massive galaxies yet identified at z>2, and could evolve into early-type galaxies and bulges.

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arXiv2002

Dark matter within high surface brightness spiral galaxies

We present results from a detailed dynamical analysis of five high surface brightness, late type spirals, studied with the aim to quantify the luminous-to-dark matter ratio inside their optical radii. The galaxies' stellar light distribution and gas kinematics have been observed and compared to hydrodynamic gas simulations, which predict the 2D gas dynamics arising in response to empirical gravitational potentials, which are combinations of differing stellar disk and dark halo contributions. The gravitational potential of the stellar disk was derived from near-infrared photometry, color-corrected to constant (M/L); the dark halo was modelled by an isothermal sphere with a core. Hydrodynamic gas simulations were performed for each galaxy for a sequence of five different mass fractions of the stellar disk and for a wide range of spiral pattern speeds. These two parameters mainly determine the modelled gas distribution and kinematics. The agreement between the non-axisymmetric part of the simulated and observed gas kinematics permitted us to conclude that the galaxies with the highest rotation velocities tend to possess near-maximal stellar disks. In less massive galaxies, with v_max<200 km/s, the mass of the dark halo at least equals the stellar mass within 2-3 R_disk. The simulated gas morphology provides a powerful tool to determine the dominant spiral pattern speed. The corotation radius for all galaxies was found to be constant at R_corotation ~ 3 R_disk and encloses the strong part of the stellar spiral in all cases.

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arXiv2002

A Survey for Large Separation Lensed FIRST Quasars, II. Magnification Bias and Redshift Distribution

The statistics of large-separation gravitational lensing are a powerful tool to probe mass distributions on the scale of galaxy clusters. In this paper we refine the analysis of our survey for large-separation (>5'') lensed FIRST quasars (Ofek et al. 2001) by estimating the magnification bias and the source redshift distribution. Finding no large separation lens among 8000 likely quasars in that sample, implies an upper bound on the lensed fraction of 3.7 x 10^-4 at 95% CL. From a published deep 1.4 GHz radio survey of the Hubble Deep Field, and corresponding optical searches for faint quasars, we calculate a lower limit to the `double flux magnification bias' affecting our radio-optically selected sample, of B>1.1. From the four-colour information in the SDSS Early Data Release, we calculate the photometric redshift distribution of a sample of FIRST quasar candidates and compare it with the redshift distribution from the FIRST Bright Quasar Survey. We find that the median redshift of the quasars in our sample is about 1.4. With these new results, we find that for all plausible cosmologies, the absence of lensed quasars in our survey is consistent with a model based on an empirical, non-evolving, cluster mass function, where clusters are represented by singular isothermal spheres. On the other hand, comparison of our results to the lensing predictions of published N-body-ray-tracing simulations (Wambsganss et al. 1995, 1998) rejects the COBE normalised Ω_{0}=1 CDM model at 99.9% confidence.

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arXiv2002

Kinematic study of the disrupting globular cluster Palomar 5 using VLT spectra

Wide-field photometric data from the Sloan Digital Sky Survey have recently revealed that the Galactic globular cluster Palomar 5 is in the process of being tidally disrupted (Odenkirchen et al. 2001). Here we investigate the kinematics of this sparse remote star cluster using high resolution spectra from the Very Large Telescope (VLT). Twenty candidate cluster giants located within 6 arcmin of the cluster center have been observed with the UV-Visual Echelle Spectrograph (UVES) on VLT-UT2. The spectra provide radial velocities with a typical accuracy of 0.15 km/s. We find that the sample contains 17 certain cluster members with very coherent kinematics, two unrelated field dwarfs, and one giant with a deviant velocity, which is most likely a cluster binary showing fast orbital motion. From the confirmed members we determine the heliocentric velocity of the cluster as -58.7 +- 0.2 km/s. The total line-of-sight velocity dispersion of the cluster stars is 1.1 +- 0.2 km/s (all members) or 0.9 +- 0.2 km/s (stars on the red giant branch only). This is the lowest velocity dispersion that has so far been measured for a stellar system classified as a globular cluster. The shape of the velocity distribution suggests that there is a significant contribution from orbital motions of binaries and that the dynamical part of the velocity dispersion is therefore still substantially smaller than the total dispersion. ... (abridged)

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arXiv2002

Dynamical Confirmation of SDSS Weak Lensing Scaling Laws

Galaxy masses can be estimated by a variety of methods; each applicable in different circumstances, and each suffering from different systematic uncertainties. Confirmation of results obtained by one technique with analysis by another is particularly important. Recent SDSS weak lensing measurements of the projected-mass correlation function reveal a linear relation between galaxy luminosities and the depth of their dark matter halos (measured on 260 \hinv kpc scales). In this work we use an entirely independent dynamical method to confirm these results. We begin by assembling a sample of 618 relatively isolated host galaxies, surrounded by a total of 1225 substantially fainter satellites. We observe the mean dynamical effect of these hosts on the motions of their satellites by assembling velocity difference histograms. Dividing the sample by host properties, we find significant variations in satellite velocity dispersion with host luminosity. We quantify these variations using a simple dynamical model, measuring \mtsd a dynamical mass within 260 \hinv kpc. The appropriateness of this mass reconstruction is checked by conducting a similar analysis within an N-body simulation. Comparison between the dynamical and lensing mass-to-light scalings shows reasonable agreement, providing some quantitative confirmation for the lensing results.

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arXiv2002

PKS 1830-211: A Face-On Spiral Galaxy Lens

We present new Hubble Space Telescope images of the gravitational lens PKS 1830-211, which allow us to characterize the lens galaxy and update the determination of the Hubble constant from this system. The I-band image shows that the lens galaxy is a face-on spiral galaxy with clearly delineated spiral arms. The southwestern image of the background quasar passes through one of the spiral arms, explaining the previous detections of large quantities of molecular gas and dust in front of this image. The lens galaxy photometry is consistent with the Tully-Fisher relation, suggesting the lens galaxy is a typical spiral galaxy for its redshift. The lens galaxy position, which was the main source of uncertainty in previous attempts to determine H_0, is now known precisely. Given the current time delay measurement and assuming the lens galaxy has an isothermal mass distribution, we compute H_0 = 44 +/- 9 km/s/Mpc for an Omega_m = 0.3 flat cosmological model. We describe some possible systematic errors and how to reduce them. We also discuss the possibility raised by Courbin et al. (2002), that what we have identified as a single lens galaxy is actually a foreground star and two separate galaxies.

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