Galaxy Evolution From Morphology And SEDs Research Articles

A Research on the Evolution of Star Formation Activities of Early-type Galaxies

The observational investigation of the evolution of the star formation activities of early-type galaxies (ETGs) with redshifts helps us to understand the formation and evolution of this kind of galaxies. Combined with the highresolution images from HST/ACS (Hubble Space Telescope/Advanded Camera for Surveys) of the GEMS (Galaxy Evolution fromMorphology and SEDs) survey and the multi-band data from Spitzer, GALEX (Galaxy Evolution Explorer) and so on in the CDFS (Chandra Deep Field South) field, a complete sample including 456 ETGs with their redshifts in the range of 0 .2 ≤ z ≤ 1 .0 is selected on the basis of morphology, color and stellar mass. By using the stacking technique, the ultraviolet and infrared average luminosities of sample galaxies are measured, and the star formation rates of ETGs are estimated. The results indicate that the star formation rates of ETGs are relatively low (< 3 M yr −1) and decrease with decreasing redshifts. The mass contributed by the star formation since z = 1 is less than 15%. The analyses of stellar populations also confirm that the bulk of the population of massive ETGs was formed in the early universe ( z > 2).

SIZES OF LYα-EMITTING GALAXIES AND THEIR REST-FRAME ULTRAVIOLET COMPONENTS ATz= 3.1

We present a rest-frame ultraviolet analysis of ~120 z~3.1 Lyman Alpha Emitters (LAEs) in the Extended Chandra Deep Field South (ECDF-S). Using Hubble Space Telescope (HST) images taken as part of the Galaxy Evolution From Morphology and SEDS (GEMS) survey, Great Observatories Origins Deep Survey (GOODS), and Hubble Ultradeep Field (HUDF) surveys, we analyze the sizes of LAEs, as well as the spatial distribution of their components, which are defined as distinct clumps of UV-continuum emission. We set an upper limit of ~1 kpc (~0.1") on the rms offset between the centroids of the continuum and Lyman-alpha emission. The star formation rates of LAE components inferred from the rest-frame ultraviolet continuum range from ~0.1 M_{sun}/yr to ~5$ M_{sun}/yr. A subsample of LAEs with coverage in multiple surveys (at different imaging depths) suggests that one needs a signal-to-noise ratio, S/N>~30, in order to make a robust estimate of the half-light radius of an LAE system. The majority of LAEs have observed half-light radii <~2 kpc, and LAE components typically have observed half-light radii <~1.5 kpc (<~ 0.2"). Although only ~50% of the detected LAE components are resolved at GOODS depth, the brightest (V<~26.3) are all resolved in both GOODS and GEMS. Since we find little evidence for a correlation between the rest-UV sizes and magnitudes of LAEs, the majority should be resolved in a deeper survey at the ~0.05" angular resolution of HST. Most of the multi-component LAEs identified in shallow frames become connected in deeper images, suggesting that the majority of the rest-UV "clumps" are individual star-forming regions within a single system.

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 ΛCDM model evolves by about a factor of 2 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%, 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% larger at z ∼ 1 (20% at z ∼ 2) than at the present day.

The X‐Ray Evolution of Early‐Type Galaxies in the Extended Chandra Deep Field–South

We investigate the evolution over the last 6.3 Gyr of cosmic time (i.e., since z ≈ 0.7) of the average X-ray properties of early-type galaxies within the Extended Chandra Deep Field-South (E-CDF-S). Our early-type galaxy sample includes 539 objects with red sequence colors and Sérsic indices larger than n = 2.5, which were selected jointly from the COMBO-17 (Classifying Objects by Medium-Band Observations in 17 Filters) and GEMS (Galaxy Evolution from Morphologies and SEDs) surveys. We utilize the deep Chandra observations over the E-CDF-S and X-ray stacking analyses to constrain primarily the average X-ray emission from ``normal'' early-type galaxies (i.e., those that are not dominated by luminous AGNs). We study separately optically luminous (LB ≈ 1010-1011 LB,☉) and faint (LB ≈ 109.3-1010 LB,☉) galaxy samples, which we expect to have soft (0.5-2.0 keV) X-ray emission dominated by hot (~1 keV) interstellar gas and low-mass X-ray binary (LMXB) populations, respectively. The AGN fraction of our optically luminous sample evolves with redshift in a manner consistent with the (1 + z)3 evolution observed in other investigations of X-ray-selected AGNs. We find that the X-ray-to-B-band mean luminosity ratio (LX/LB) for normal optically luminous galaxies does not evolve significantly over z ≈ 0.0-0.7. This lack of X-ray evolution implies a general balance between the heating and cooling of the hot gas. If transient AGN activity is largely responsible for maintaining this balance, then we infer that mechanical power must be dominating the feedback out to z ≈ 0.7. Furthermore, in this scenario the average mechanical AGN power must remain roughly constant over the last half of cosmic time. For our optically faint early-type galaxies, we find suggestive evidence that LX/LB increases with redshift over z ≈ 0.0-0.5.

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 S\'ersic 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.

Dry Mergers in GEMS: The Dynamical Evolution of Massive Early‐Type Galaxies

We have used the 28' × 28' Hubble Space Telescope image mosaic from the GEMS (Galaxy Evolution from Morphology and SEDs) survey in conjunction with the COMBO-17 photometric redshift survey to constrain the incidence of major mergers between spheroid-dominated galaxies with little cold gas (dry mergers) since z = 0.7. A set of N-body merger simulations was used to explore the morphological signatures of such interactions: they are recognizable either as <5 kpc separation close pairs or because of broad, low surface brightness tidal features and asymmetries. Data with the depth and resolution of GEMS are sensitive to dry mergers between galaxies with MV -20.5 for z 0.7; dry mergers at higher redshifts are not easily recovered in single-orbit HST imaging. Six dry mergers (12 galaxies) with luminosity ratios between 1 : 1 and 4 : 1 were found from a sample of 379 red early-type galaxies with MV < -20.5 and 0.1 < z < 0.7. The simulations suggest that the morphological signatures of dry merging are visible for ~150 Myr, and we use this timescale to convert the observed merger incidence into a rate. On this basis we find that present-day spheroidal galaxies with MV < -20.5 on average have undergone between 0.5 and 2 major dry mergers since z ~ 0.7. We have compared this result with the predictions of a cold dark matter-based semianalytic galaxy formation model. The model reproduces the observed declining major merger fraction of bright galaxies and the space density of luminous early-type galaxies reasonably well. The predicted dry merger fraction is consistent with our observational result. Hence, hierarchical models predict and observations now show that major dry mergers are an important driver of the evolution of massive early-type galaxies in recent epochs.

The Evolution of Early‐Type Red Galaxies with the GEMS Survey: Luminosity‐Size and Stellar Mass–Size Relations Sincez = 1

We combine imaging from the Hubble Space Telescope Advanced Camera for Surveys, as part of the GEMS (Galaxy Evolution from Morphologies and SEDs) 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. From 05 ? 05 imaging, we draw a large sample of 728 galaxies with centrally concentrated radial profiles (i.e., n ? 2.5 from S?rsic fits) and rest-frame (U - V) colors on the red sequence. We explore how the correlations of rest-frame V-band luminosity and of stellar mass with intrinsic half-light size change over the last half of cosmic time. By appropriate comparison with the well-defined local relations from the Sloan Digital Sky Survey, we find that the luminosity-size and stellar mass-size relations evolve in a manner that is consistent with the passive aging of ancient stellar populations. 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 roughly a factor of 2 since z ~ 1, our results imply that new additions to the early-type galaxy population follow similar luminosity-size and stellar mass-size correlations, compared to the older subset of early-type galaxies. Adding early-type galaxies to the red sequence through the fading of previously prominent disks 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 are insufficient to discriminate between these possibilities.

Toward an Understanding of the Rapid Decline of the Cosmic Star Formation Rate

We present a first analysis of deep 24 μm observations with the Spitzer Space Telescope of a sample of nearly 1500 galaxies in a thin redshift slice, 0.65 ≤ z < 0.75. We combine the infrared data with redshifts, rest-frame luminosities, and colors from COMBO-17 and with morphologies from Hubble Space Telescope images collected by the Galaxy Evolution from Morphology and SEDs (GEMS) and Great Observatories Origins Deep Survey (GOODS) projects. To characterize the decline in star formation rate (SFR) since z ~ 0.7, we estimate the total thermal IR luminosities, SFRs, and stellar masses for the galaxies in this sample. At z ~ 0.7, nearly 40% of intermediate- and high-mass galaxies (with stellar masses ≥2 × 1010 M☉) 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.

GEMS: The Surface Brightness and Surface Mass Density Evolution of Disk Galaxies

We combine HST imaging from the GEMS (Galaxy Evolution from Morphologies and SEDs) survey with photometric redshifts from COMBO-17 to explore the evolution of disk-dominated galaxies since z ≲ 1.1. The sample is composed of all GEMS galaxies with Sérsic 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 MV ≲ -20, or equivalently, log ≳ 10. We find strong evolution in the magnitude-size scaling relation for galaxies with MV ≲ -20, corresponding to a brightening of ~1 mag arcsec-2 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 ≳ 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 2 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.

Weak Lensing Results from GEMS

We present our cosmic shear analysis of the Galaxy Evolution from Morphology and SEDs (GEMS) survey. Imaged with the Advanced Camera for Surveys (ACS) on HST, GEMS provides high resolution imaging spanning some 800 square arcmins in the Chandra Deep Field South (CDFS). We discuss the benefits of using space-based data for weak lensing studies and show that the ACS is a very powerful instrument in this regard. We find that we are not limited by systematic errors arising from the anisotropic ACS point spread function distortion and use our cosmic shear results to place joint constraints on the matter density parameter ??m and the amplitude of the matter power spectrum ??8, finding ??8(??m/0.3)0.62=0.73 ?? 0.12.