arXiv subjects

Eric F. Bell

Publications and source records attributed to Eric F. Bell.

At least 271 records · Page 15Linked to original sources

arXiv2008

The dark matter environment of the Abell 901/902 supercluster: a weak lensing analysis of the HST STAGES survey

We present a high resolution dark matter reconstruction of the z=0.165 Abell 901/902 supercluster from a weak lensing analysis of the HST STAGES survey. We detect the four main structures of the supercluster at high significance, resolving substructure within and between the clusters. We find that the distribution of dark matter is well traced by the cluster galaxies, with the brightest cluster galaxies marking out the strongest peaks in the dark matter distribution. We also find a significant extension of the dark matter distribution of Abell 901a in the direction of an infalling X-ray group Abell 901alpha. We present mass, mass-to-light and mass-to-stellar mass ratio measurements of the structures and substructures that we detect. We find no evidence for variation of the mass-to-light and mass-to-stellar mass ratio between the different clusters. We compare our space-based lensing analysis with an earlier ground-based lensing analysis of the supercluster to demonstrate the importance of space-based imaging for future weak lensing dark matter 'observations'.

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arXiv2008

The evolving disk galaxy population

In this contribution, I present a simplified overview of the evolution of the disk galaxy population since z=1, and a brief discussion of a few open questions. Galaxy evolution surveys have found that the disk galaxy population forms stars intensely at intermediate redshift. In particular, they dominate the cosmic star formation rate at z<1 -- the factor of ten drop in cosmic average comoving star formation rate in the last 8 Gyr is driven primarily by disk physics, not by a decreasing major merger rate. Despite this intense star formation, there has been little change in the stellar mass density in disk galaxies since z=1; large numbers of disk galaxies are being transformed into non-star-forming spheroid-dominated galaxies by galaxy interactions, AGN feedback, environmental effects, and other physical processes. Finally, despite this intense activity, the scaling relations of disk galaxies appear to evolve little. In particular, as individual galaxies grow in mass through the formation of stars, they appear to grow in radius (on average, the population grows inside-out), and they appear to evolve towards somewhat higher rotation velocity (i.e., mass is added at both small and large radii during this inside-out growth).

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arXiv2007

A deep Large Binocular Telescope view of the Canes Venatici I dwarf galaxy

We present the first deep color-magnitude diagram of the Canes Venatici I (CVnI) dwarf galaxy from observations with the wide field Large Binocular Camera on the Large Binocular Telescope. Reaching down to the main-sequence turnoff of the oldest stars, it reveals a dichotomy in the stellar populations of CVnI: it harbors an old (> 10 Gyr), metal-poor ([Fe/H] ~ -2.0) and spatially extended population along with a much younger (~ 1.4-2.0 Gyr), 0.5 dex more metal-rich, and spatially more concentrated population. These young stars are also offset by 64_{-20}^{+40} pc to the East of the galaxy center. The data suggest that this young population, which represent ~ 3-5 % of the stellar mass of the galaxy within its half-light radius, should be identified with the kinematically cold stellar component found by Ibata et al. (2006). CVnI therefore follows the behavior of the other remote MW dwarf spheroidals which all contain intermediate age and/or young populations: a complex star formation history is possible in extremely low-mass galaxies.

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arXiv2007

An Explanation for the Observed Weak Size Evolution of Disk Galaxies

Surveys of distant galaxies with the Hubble Space Telescope and from the ground have shown that there is only mild evolution in the relationship between radial size and stellar mass for galactic disks from z~1 to the present day. Using a sample of nearby disk-dominated galaxies from the Sloan Digital Sky Survey (SDSS), and high redshift data from the GEMS (Galaxy Evolution from Morphology and SEDs) survey, we investigate whether this result is consistent with theoretical expectations within the hierarchical paradigm of structure formation. The relationship between virial radius and mass for dark matter halos in the LCDM model evolves by about a factor of two over this interval. However, N-body simulations have shown that halos of a given mass have less centrally concentrated mass profiles at high redshift. When we compute the expected disk size-stellar mass distribution, accounting for this evolution in the internal structure of dark matter halos and the adiabatic contraction of the dark matter by the self-gravity of the collapsing baryons, we find that the predicted evolution in the mean size at fixed stellar mass since z~1 is about 15-20 percent, in good agreement with the observational constraints from GEMS. At redshift z~2, the model predicts that disks at fixed stellar mass were on average only 60% as large as they are today. Similarly, we predict that the rotation velocity at a given stellar mass (essentially the zero-point of the Tully-Fisher relation) is only about 10 percent larger at z~1 (20 percent at z~2) than at the present day.

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arXiv2007

The Elongated Structure of the Hercules dSph from Deep LBT Imaging

We present a deep, wide-field photometric survey of the newly-discovered Hercules dwarf spheroidal galaxy, based on data from the Large Binocular Telescope. Images in B, V and r were obtained with the Large Binocular Camera covering a 23' times 23' field of view to a magnitude of ~25.5 (5 sigma). This permitted the construction of colour-magnitude diagrams that reach approximately 1.5 magnitudes below the Hercules main sequence turnoff. Three-filter photometry allowed us to preferentially select probable Hercules member stars, and examine the structure of this system at a previously unattained level. We find that the Hercules dwarf is highly elongated (3:1), considerably more so than any other dSph satellite of the Milky Way except the disrupting Sagittarius dwarf. While we cannot rule out that the unusual structure is intrinsic to Hercules as an equilibrium system, our results suggest tidal disruption as a likely cause of this highly elliptical structure. Given the relatively large Galactocentric distance of this system (132 +/- 12 kpc), signs of tidal disruption would require the Hercules dwarf to be on a highly eccentric orbit around the Milky Way.

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arXiv2007

Infrared Spectral Energy Distributions of z~0.7 Star-Forming Galaxies

We analyze the infrared (IR) spectral energy distributions (SEDs) for 10micron < lambda(rest) < 100micron for ~600 galaxies at z~0.7 in the extended Chandra Deep Field South by stacking their Spitzer 24, 70 and 160micron images. We place interesting constraints on the average IR SED shape in two bins: the brightest 25% of z~0.7 galaxies detected at 24micron, and the remaining 75% of individually-detected galaxies. Galaxies without individual detections at 24micron were not well-detected at 70micron and 160micron even through stacking. We find that the average IR SEDs of z~0.7 star-forming galaxies fall within the diversity of z~0 templates. While dust obscuration Lir/Luv seems to be only a function of star formation rate (SFR; ~ Lir+Luv), not of redshift, the dust temperature of star-forming galaxies (with SFR ~ 10 solar mass per year) at a given IR luminosity was lower at z~0.7 than today. We suggest an interpretation of this phenomenology in terms of dust geometry: intensely star-forming galaxies at z~0 are typically interacting, and host dense centrally-concentrated bursts of star formation and warm dust temperatures. At z~0.7, the bulk of intensely star-forming galaxies are relatively undisturbed spirals and irregulars, and we postulate that they have large amounts of widespread lower-density star formation, yielding lower dust temperatures for a given IR luminosity. We recommend what IR SEDs are most suitable for modeling intermediate redshift galaxies with different SFRs.

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arXiv2007

GEMS: Galaxy fitting catalogues and testing parametric galaxy fitting codes

In the context of measuring structure and morphology of intermediate redshift galaxies with recent HST/ACS surveys, we tune, test, and compare two widely used fitting codes (GALFIT and GIM2D) for fitting single-component Sersic models to the light profiles of both simulated and real galaxy data. We find that fitting accuracy depends sensitively on galaxy profile shape. Exponential disks are well fit with Sersic models and have small measurement errors, whereas fits to de Vaucouleurs profiles show larger uncertainties owing to the large amount of light at large radii. We find that both codes provide reliable fits and little systematic error, when the effective surface brightness is above that of the sky. Moreover, both codes return errors that significantly underestimate the true fitting uncertainties, which are best estimated with simulations. We find that GIM2D suffers significant systematic errors for spheroids with close companions owing to the difficulty of effectively masking out neighboring galaxy light; there appears to be no work around to this important systematic in GIM2D's current implementation. While this crowding error affects only a small fraction of galaxies in GEMS, it must be accounted for in the analysis of deeper cosmological images or of more crowded fields with GIM2D. In contrast, GALFIT results are robust to the presence of neighbors because it can simultaneously fit the profiles of multiple companions thereby deblending their effect on the fit to the galaxy of interest. We find GALFIT's robustness to nearby companions and factor of >~20 faster runtime speed are important advantages over GIM2D for analyzing large HST/ACS datasets. Finally we include our final catalog of fit results for all 41,495 objects detected in GEMS.

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arXiv2007

Star Formation and the Growth of Stellar Mass

Recent observations have demonstrated a significant growth in the integrated stellar mass of the red sequence since z=1, dominated by a steadily increasing number of galaxies with stellar masses M* < 10^11 M_sun. In this paper, we use the COMBO-17 photometric redshift survey in conjunction with deep Spitzer 24 micron data to explore the relationship between star formation and the growth of stellar mass. We calculate `star formation rate functions' in four different redshift slices, splitting also into contributions from the red sequence and blue cloud for the first time. We find that the growth of stellar mass since z=1 is consistent with the integrated star formation rate. Yet, most of the stars formed are in blue cloud galaxies. If the stellar mass already in, and formed in, z<1 blue cloud galaxies were to stay in the blue cloud the total stellar mass in blue galaxies would be dramatically overproduced. We explore the expected evolution of stellar mass functions, finding that in this picture the number of massive M* > 3x10^10 M_sun blue galaxies would also be overproduced; i.e., most of the new stars formed in blue cloud galaxies are in the massive galaxies. We explore a simple truncation scenario in which these `extra' blue galaxies have their star formation suppressed by an unspecified mechanism or mechanisms; simple cessation of star formation in these extra blue galaxies is approximately sufficient to build up the red sequence at M*<10^11 M_sun.

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arXiv2007

The dependence of Star Formation on Galaxy Stellar Mass

We combine Spitzer 24micron observations with data from the COMBO-17 survey for ~15,000 0.2 we consider both the ultraviolet (UV) and the infrared (IR) luminosities, and account for the contributions of galaxies that are individually undetected at 24micron through image stacking. For all redshifts we find that higher-mass galaxies have substantially lower specific SFR, / , than lower-mass ones. However, we find the striking result that the rate of decline in cosmic SFR with redshift is nearly the same for massive and low-mass galaxies, i.e. NOT a strong function of stellar mass. This analysis confirms one version of what has been referred to as `downsizing', namely that the epoch of major mass build-up in massive galaxies is substantially earlier than the epoch of mass build-up in low-mass galaxies. Yet it shows that star formation activity is NOT becoming increasingly limited to low-mass galaxies towards the present epoch. We argue that this suggests that heating by AGN-powered radio jets is not the dominant mechanism responsible for the decline in cosmic SFR since z~1, which is borne out by comparison with semi-analytic models that include this effect.

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arXiv2006

The merger rate of massive galaxies

We calculate the projected two point correlation function for samples of luminous and massive galaxies in the COMBO-17 photometric redshift survey, focusing particularly on the amplitude of the correlation function at small projected radii and exploring the constraints such measurements can place on the galaxy merger rate. For nearly volume-limited samples with 0.4 2.5e10 M_sun is 5+/-1%. Incorporating close pair fractions from the literature, the 2dFGRS and the SDSS, we find a fairly rapid evolution of the merger fraction of massive galaxies between z=0.8 and the present day. Assuming that the major merger timescale is of order the dynamical timescale for close massive galaxy pairs, we tentatively infer that ~50% (70%) of all galaxies with present-day masses M_*>5e10 M_sun (remnants of mergers between galaxies with M_*>2.5e10 M_sun) have undergone a major merger since z=0.8(1): major mergers between massive galaxies are a significant driver of galaxy evolution over the last eight billion years.

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arXiv2006

A weak lensing estimate from GEMS of the virial to stellar mass ratio in massive galaxies to z~0.8

We present constraints on the evolution of the virial to stellar mass ratio of galaxies with high stellar masses in the redshift range 0.2 10.5, where the majority show an early-type morphology, we find that the virial mass to stellar mass ratio is given by M_vir/M_star = 53^{+13}_{-16}. Assuming a baryon fraction from the concordance cosmology, this corresponds to a stellar fraction of baryons in massive galaxies of Omega_b^*/Ω_b = 0.10 +/- 0.03. Analysing the galaxy sample in different redshift slices, we find little or no evolution in the virial to stellar mass ratio, and place an upper limit of ~2.5 on the growth of massive galaxies through the conversion of gas into stars from z=0.8 to the present day.

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arXiv2006

Eight billion years of disk galaxy evolution: no galaxy is an island

We present a brief discussion of the evolution of disk galaxy stellar masses, sizes, rotation velocities, and star formation rates over the last eight billion years. Recent observations have failed to detect significant evolution in the stellar mass Tully-Fisher relation, stellar mass-size relation, and the stellar mass function of disk galaxies. Yet, most z<1 star formation is in disks, and this star formation would be expected to drive a rapid growth of the total stellar mass (and therefore mass function) of disks in the last eight billion years. Such a build-up is not seen; instead, a rapid build-up in the total stellar mass in non-star-forming spheroid-dominated galaxies is observed. Large numbers of disk-dominated galaxies are systematically shutting off their star formation and building up a spheroid (or losing a disk) in the epoch 0<z<1.

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arXiv2006

The stellar masses of 25000 galaxies at 0.2<z<1.0 estimated by the COMBO-17 survey

We present an analysis of stellar mass estimates for a sample of 25000 galaxies from the COMBO-17 survey over the interval 0.2 60%) of massive galaxies with M_* > 10^{11} solar masses at all z<1 are non-star-forming; blue star-forming galaxies dominate at lower masses. We have used these mass estimates to explore the evolution of the stellar mass function since z=1. We find that the total stellar mass density of the universe has roughly doubled since z~1. Our measurements are consistent with other measurements of the growth of stellar mass with cosmic time and with estimates of the time evolution of the cosmic star formation rate. Intriguingly, the integrated stellar mass of blue galaxies with young stars has not significantly changed since z~1, even though these galaxies host the majority of the star formation: instead, the growth of the total stellar mass density is dominated by the growth of the total mass in the largely passive galaxies on the red sequence.

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arXiv2006

Andromeda X, A New Dwarf Spheroidal Satellite of M31: Photometry

We report the discovery of Andromeda X, a new dwarf spheroidal satellite of M31, based on stellar photometry from the Sloan Digital Sky Survey (SDSS). Using follow-up imaging data we have estimated its distance and other physical properties. We find that Andromeda X has a dereddened central surface brightness of mu_V,0 ~ 26.7 mag arcsec^-2 and a total apparent magnitude of V_tot ~ 16.1, which at the derived distance modulus, (m - M)_0 ~ 24.12 - 24.34, yields an absolute magnitude of M_V ~ -8.1 +/- 0.5; these values are quite comparable to those of Andromeda IX, a previously-discovered low luminosity M31 satellite. The discoveries of Andromeda IX and Andromeda X suggest that such extremely faint satellites may be plentiful in the Local Group.

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arXiv2006

Color, Structure, and Star Formation History of Dwarf Galaxies over the last ~3 Gyr with GEMS and SDSS

We present a study of the colors, structural properties, and star formation histories for a sample of ~1600 dwarfs over look-back times of ~3 Gyr (z=0.002-0.25). The sample consists of 401 distant dwarfs drawn from the Galaxy Evolution from Morphologies and SEDs (GEMS) survey, which provides high resolution Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) images and accurate redshifts, and of 1291 dwarfs at 10-90 Mpc compiled from the Sloan Digitized Sky Survey (SDSS). The sample is complete down to an effective surface brightness of 22 mag arcsec^-2 in z and includes dwarfs with M_g=-18.5 to -14 mag. Rest-frame luminosities in Johnson UBV and SDSS ugr filters are provided by the COMBO-17 survey and structural parameters have been determined by Sérsic fits. We find that the GEMS dwarfs are bluer than the SDSS dwarfs by ~0.13 mag in g-r, which is consistent with the color evolution over ~2 Gyr of star formation histories involving moderate starbursts and long periods of continuous star formation. The full color range of the samples cannot be reproduced by single starbursts of different masses or long periods of continuous star formation alone. Furthermore, an estimate of the mechanical luminosities needed for the gas in the GEMS dwarfs to be completely removed from the galaxies shows that a significant number of low luminosity dwarfs are susceptible to such a complete gas loss, if they would experience a starburst. On the other hand, a large fraction of more luminous dwarfs is likely to retain their gas. We also estimate the star formation rates per unit area for the GEMS dwarfs and find good agreement with the values for local dwarfs.

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arXiv2005

Studying distant dwarf galaxies with GEMS and SDSS

We study the colors, structural properties, and star formation histories of a sample of ~1600 dwarfs over look-back times of ~3 Gyr (z=0.002-0.25). The sample consists of 401 distant dwarfs drawn from the Galaxy Evolution from Morphologies and SEDs (GEMS) survey, which provides high resolution Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) images and accurate redshifts, and of 1291 dwarfs at 10-90 Mpc compiled from the Sloan Digitized Sky Survey (SDSS). We find that the GEMS dwarfs are bluer than the SDSS dwarfs, which is consistent with star formation histories involving starbursts and periods of continuous star formation. The full range of colors cannot be reproduced by single starbursts or constant star formation alone. We derive the star formation rates of the GEMS dwarfs and estimate the mechanical luminosities needed for a complete removal of their gas. We find that a large fraction of luminous dwarfs are likely to retain their gas, whereas fainter dwarfs are susceptible to a significant gas loss, if they would experience a starburst.

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arXiv2005

Very Small-Scale Clustering and Merger Rate of Luminous Red Galaxies

We present the small-scale (0.01<r<8 h^{-1} Mpc) projected correlation function w_p(r_p) and real space correlation function xi(r) of 24520 luminous early-type galaxies from the Sloan Digital Sky Survey Luminous Red Galaxy (LRG) sample (0.16<z<0.36). ``Fiber collision'' incompleteness of the SDSS spectroscopic sample at scales smaller than 55 arcseconds prevents measurements of the correlation function for LRGs on scales smaller than ~0.3 Mpc by the usual methods. In this work, we cross-correlate the spectroscopic sample with the imaging sample, with a weighting scheme to account for the collisions, extensively tested against mock catalogs. We correct for photometric biases in the SDSS imaging of close galaxy pairs. We find that the correlation function xi(r) is surprisingly close to a r^{-2} power law over more than 4 orders of magnitude in separation r. This result is too steep at small scales to be explained in current versions of the halo model for galaxy clustering. We infer an LRG-LRG merger rate of less than or similar to 0.6 Gyr^{-1} Gpc^{-3} for this sample. This result suggests that the LRG-LRG mergers are not the main mode of mass growth for LRGs at z<0.36.

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arXiv2005

Nature and completeness of galaxies detected in the Two Micron All Sky Survey

We cross correlate the well-defined and very complete spectroscopic Main Galaxy Sample (MGS) of 156,000 bright (r<17.5 mag) galaxies from the SDSS with 2MASS sources to explore the nature and completeness of the 2MASS K-band selection of nearby galaxies. 2MASS detects 90% of the MGS brighter than r=17 mag. For r<16, 93.1% of the MGS is found in the 2MASS Extended Source Catalog (XSC). These detections span the representative range of optical and near-IR galaxy properties, but with a surface brightness-dependent bias to preferentially miss the most blue and low-concentration sources, consistent with the most morphologically late-type galaxy population. An XSC completeness of 97.5% is achievable at bright magnitudes, with blue LSBs being the only major source of incompleteness, if one follows our careful matching criteria and weeds out spurious SDSS sources. We conclude that the rapid drop in XSC completeness at r>16 reflects the sharp surface-brightness limit of the extended source detection algorithm in 2MASS. As a result, the r>16 galaxies found in the XSC are over-representative in red early types and under-representative in blue latetypes. At r>16 the XSC suffers an additional selection effect from the 2-3" spatial resolution limit of 2MASS. Therefore, 2MASS continues to detect 90% of of the MGS at 16<r<17, but with a growing fraction found in the Point Source Catalog (PSC) only. Overall, one third of the MGS is detected in the 2MASS PSC but not the XSC. A combined K<13.57 and r<16 selection provides the most representative inventory of galaxies in the local cosmos with near-IR and optical measurements, and 90.8% completeness. Using data from SDSS-DR2, this sample contains 19,156 galaxies with a median redshift of 0.052. (abridged)

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