Stijn Wuyts

Structure and Star Formation in Galaxies out to z = 3: Evidence for Surface Density Dependent Evolution and Upsizing* **

We present an analysis of galaxies in the CDF-South. We find a tight relation to -->z = 3 between color and size at a given mass, with red galaxies being small, and blue galaxies being large. We show that the relation is driven by stellar surface density or inferred velocity dispersion: galaxies with high surface density are red and have low specific star formation rates, and galaxies with low surface density are blue and have high specific star formation rates. Surface density and inferred velocity dispersion are better correlated with specific star formation rate and color than stellar mass. Hence stellar mass by itself is not a good predictor of the star formation history of galaxies. In general, galaxies at a given surface density have higher specific star formation rates at higher redshift. Specifically, galaxies with a surface density of -->(1?3) ? 109 M? kpc?2 are red and dead at low redshift, approximately 50% are forming stars at -->z = 1, and almost all are forming stars by -->z = 2. This provides direct additional evidence for the late evolution of galaxies onto the red sequence. The sizes of galaxies at a given mass evolve like -->1/(1 + z)0.59 ? 0.10. Hence galaxies undergo significant upsizing in their history. The size evolution is fastest for the highest mass galaxies and quiescent galaxies. The persistence of the structural relations from -->z = 0 to -->z = 2.5, and the upsizing of galaxies imply that a relation analogous to the Hubble sequence exists out to -->z = 2.5, and possibly beyond. The star-forming galaxies at -->z ? 1.5 are quite different from star-forming galaxies at -->z = 0, as they have likely very high gas fractions, and star formation timescales comparable to the orbital time.

FIREWORKS U38-to-24 μm Photometry of the GOODS Chandra Deep Field-South: Multiwavelength Catalog and Total Infrared Properties of Distant Ks-selected Galaxies

We present a -->Ks-selected catalog, dubbed FIREWORKS, for the Chandra Deep Field-South (CDF-S) containing photometry in the -->U38, -->B435, B, V, -->V606, R, -->i775, I, -->z850, J, H, -->Ks, [3.6 ?m], [4.5 ?m], [5.8 ?m], [8.0 ?m], and MIPS [24 ?m] bands. The imaging has a typical -->Ktots,AB limit of 24.3 mag (5 ?) and coverage over 113 arcmin2 in all bands and 138 arcmin2 in all bands but H. We cross-correlate our catalog with the 1 Ms X-ray catalog by Giacconi et al. (2002) and with all available spectroscopic redshifts to date. We find and explain systematic differences in a comparison with the -->z850 + Ks-selected GOODS-MUSIC catalog that covers ~90% of the field. We exploit the -->U38-to-24 ?m photometry to determine which -->Ks-selected galaxies at -->1.5 < z < 2.5 have the brightest total IR luminosities and which galaxies contribute most to the integrated total IR emission. The answer to both questions is that red galaxies are dominating in the IR. This is true no matter whether color is defined in the rest-frame UV, optical, or optical-near-IR. We do find, however, that among the reddest galaxies in the rest-frame optical, there is a population of sources with only little mid-IR emission, suggesting a quiescent nature.

DISSIPATION AND EXTRA LIGHT IN GALACTIC NUCLEI. IV. EVOLUTION IN THE SCALING RELATIONS OF SPHEROIDS

We develop a model for the physical origin and redshift evolution of spheroid scaling relations. We consider spheroid sizes, velocity dispersions, dynamical masses, profile shapes (Sersic indices), stellar and supermassive black hole (BH) masses, and their related scalings. Our approach combines advantages of prior observational constraints in halo occupation models and libraries of high-resolution hydrodynamic simulations of galaxy mergers. This allows us to separate the relative roles of dissipation, dry mergers, formation time, and evolution in progenitor properties, and identify their impact on observed scalings at each redshift. We show that, at all redshifts, dissipation is the most important factor determining spheroid sizes and fundamental plane scalings, and can account (at z = 0) for the observed fundamental plane tilt and differences between observed disk and spheroid scaling relations. Because disks (spheroid progenitors) at high redshift have characteristically larger gas fractions, this predicts more dissipation in mergers, yielding systematically more compact, smaller spheroids. In detail, this gives rise to a mass-dependent evolution in the sizes of spheroids of a given mass, which agrees well with observations. This relates to a subtle weakening of the tilt of the early-type fundamental plane with redshift, important for a number of studies that assume a nonevolving stellar mass fundamental plane. This also predicts evolution in the BH-host mass relations, toward more massive BHs at higher redshifts. Dry mergers are also significant, but only for large systems which form early—they originate as compact systems, but undergo a number of dry mergers (consistent with observations) such that they have sizes at any later observed redshift similar to systems of the same mass formed more recently. Most of the observed, compact high-redshift ellipticals will become the cores of present brightest cluster galaxies, and we show how their sizes, velocity dispersions, and BH masses evolve to become consistent with observations. We also predict what fraction might survive intact from early formation and identify their characteristic z = 0 properties. We make predictions for residual correlations as well, e.g., the correlation of size and fundamental plane residuals with formation time of a given elliptical, that can be used as additional tests of these models.

Discriminating between the physical processes that drive spheroid size evolution

Observations have shown that massive galaxies at high redshift have much smaller effective radii than galaxies of similar mass today; however, recent work has shown that they have similar central densities. The primary growth of size, therefore, relates to the apparent relative abundance of low-density material at low redshifts. But various models have been proposed to accomplish this, and the exact contribution of these mechanisms, relative to others that would, for example, lower the density of the system uniformly, or relate to possible observational misestimates of the stellar mass distribution, remain uncertain, as does the degree to which this evolution is driven by processes of initial spheroid formation versus subsequent ‘dry’ assembly of spheroids. These different possibilities also yield dramatically different constraints on any possible evolution in the MBH–σ relation. Here, we compile observations of spheroid properties as a function of redshift and use them to test the different proposed models, each of which we have calibrated and studied in a suite of high-resolution hydrodynamic simulations. We show that the evolution in progenitor disc gas fractions with redshift gives rise to the initial formation of smaller spheroids at high redshift. We then consider how these early-forming systems must evolve to be consistent with the larger sizes of old spheroids today. We consider (1) equal-density ‘dry’ mergers, (2) later major or minor ‘dry’ mergers with less dense galaxies, (3) adiabatic expansion, after significant gas mass loss, (4) gradients in stellar mass-to-light ratios from young nuclear stellar populations (yielding smaller Re at early times, which vanish as the system fades), (5) biases in the stellar mass estimation of high-redshift (young) systems (from e.g. uncertain asymptotic giant branch starlight contributions) and (6) observational effects (possible biases in fitting or missed light from surface brightness dimming, or the effects of different definitions of effective radii). In principle, any of these models could be tuned to explain any observed effective radius evolution. However, the predicted evolution in velocity dispersions, central stellar mass surface densities and profile shape are very distinct. Comparing with observations, only model (2), later or minor ‘dry’ mergers with less dense systems, is consistent with the constraints as an explanation of the entire effect. Moreover, it is the only model which allows for any evolution in MBH–σ towards more massive black holes (BHs) at high redshift. Still, the amount of merging needed for this to explain the observed factor of ∼6 size evolution is larger than that predicted by hierarchical growth and clustering constraints. We, therefore, consider a cosmologically motivated model with high-resolution simulations, in which the initial galaxy forms in a gas rich merger and is observed at an appropriate age under representative conditions, then evolves undergoing a ‘typical’ level of dry merging and mass loss. We show that this case is consistent with all the observational constraints without tension with cosmological expectations. Effect (2), which builds up an extended, low-density envelope, dominates the evolution (giving factors ∼2–3 size evolution), but effects (1), (3), (4) and (possibly) (6) each contribute an additional ∼20 per cent size evolution (net factor of ∼2), together bringing the natural cosmological predictions into good agreement with the combination of observational constraints. We discuss implications for the evolution in correlations between BH and host bulge properties and show that this naturally predicts some evolution similar to that observed; better observations of BH masses could also constrain host galaxy merger histories.

ON SIZES, KINEMATICS, M/L GRADIENTS, AND LIGHT PROFILES OF MASSIVE COMPACT GALAXIES AT z ∼ 2

We present a detailed analysis of the structure and resolved stellar populations of simulated merger remnants, and compare them to observations of compact quiescent galaxies at z � 2. We find that major merging is a viable mechanism to produce systems of � 10 11 Mand � 1 kpc size, provided the gas fraction at the time of final coalescence is high (� 40%), and provided that the progenitors are compact star-forming galaxies, as expected at high redshift. Their integrated spectral energy distributions and velocity dispersions are in good agreement with the observations, and their position in the (vmaj=�;�) diagram traces the upper envelope of the distribution of lower redshift early-type galaxies. The simulated merger remnants show time- and sightline-dependent M=L ratio gradients that result from a superposition of radially dependent stellar age, stellar metallicity, and extinction. The median ratio of effective radius in rest-frame V -band light to that in mass surface density is � 2 during the quiescent remnant phase. This is typically expressed by a negative color gradient (i.e., red core), which we expect to correlate with the integrated color of the system. Finally, the simulations differ from the observations in their surface brightness profile shape. The simulated remnants are typically best fit by high (n � 4) Sersic indices, whereas observed quiescent galaxies at z � 2 tend to be less cuspy (hni � 2:3). Limiting early star formation in the progenitors may be required to prevent the simulated merger remnants from having extended wings. Subject headings: galaxies: evolution, galaxies: formation - galaxies: structure - galaxies: stellar content

A Near-Infrared Spectroscopic Survey of K-Selected Galaxies at z ~ 2.3: Redshifts and Implications for Broadband Photometric Studies

Using the Gemini Near-Infrared Spectrograph (GNIRS), we have completed a near-infrared spectroscopic survey for -->K-bright galaxies at -->z ~ 2.3 selected from the MUSYC survey. We derived spectroscopic redshifts from emission lines or from continuum features and shapes for all 36 observed galaxies. The continuum redshifts are driven by the Balmer/4000 A break and have an uncertainty in -->Δ z/(1 + z) of Δ z/(1 + z) . The systematic error can be reduced by using optimal templates and deep photometry; the random error, however, will be hard to reduce below 5%. The spectra lead to significantly improved constraints for stellar population parameters. For most quantities this improvement is about equally driven by the higher spectral resolution and by the much reduced redshift uncertainty. Properties such as the age, -->AV, current star formation rate, and the star formation history are generally very poorly constrained with broadband data alone. Interestingly, stellar masses and mass-to-light ratios are among the most stable parameters from broadband data. Nevertheless, photometric studies may overestimate the number of massive galaxies at -->2 z = 2-3.

The Rest-Frame Optical Luminosity Functions of Galaxies at 2?z?3.5

We present the rest-frame optical (B, V, and R band) luminosity functions (LFs) of galaxies at 2 ≤ z ≤ 3.5, measured from a K-selected sample constructed from the deep NIR MUSYC, the ultradeep FIRES, and the GOODS-CDFS. This sample is unique for its combination of area and range of luminosities. The faint-end slopes of the LFs at z > 2 are consistent with those at z ~ 0. The characteristic magnitudes are significantly brighter than the local values (e.g., ~1.2 mag in the R band), while the measured values for Φ are typically ~5 times smaller. The B-band luminosity density at z ~ 2.3 is similar to the local value, and in the R band it is ~2 times smaller than the local value. We present the LF of distant red galaxies (DRGs), which we compare to that of non-DRGs. While DRGs and non-DRGs are characterized by similar LFs at the bright end, the faint-end slope of the non-DRG LF is much steeper than that of DRGs. The contribution of DRGs to the global densities down to the faintest probed luminosities is 14%-25% in number and 22%-33% in luminosity. From the derived rest-frame U - V colors and stellar population synthesis models, we estimate the mass-to-light ratios (M/L) of the different subsamples. The M/L ratios of DRGs are ~5 times higher (in the R and V bands) than those of non-DRGs. The global stellar mass density at 2 ≤ z ≤ 3.5 appears to be dominated by DRGs, whose contribution is of order ~60%-80% of the global value. Qualitatively similar results are obtained when the population is split by rest-frame U - V color instead of observed J - K color.

Recovering stellar population properties and redshifts from broadband photometry of simulated galaxies: Lessons for SED modeling

We present a detailed analysis of our ability to determine stellar masses, ages, reddening, and extinction values, and star formation rates (SFRs) of high-redshift galaxies by modeling broadband spectral energy distributions (SEDs) with stellar population synthesis. In order to do so, we computed synthetic optical-to-NIR SEDs for model galaxies taken from hydrodynamical merger simulations placed at redshifts 1.5 ≤ z ≤ 2.9. Viewed under different angles and during different evolutionary phases, the simulations represent a wide variety of galaxy types (disks, mergers, spheroids). We show that simulated galaxies span a wide range in SEDs and color, comparable to those of observed galaxies. In all star-forming phases, dust attenuation has a large effect on colors, SEDs, and fluxes. The broadband SEDs were then fed to a standard SED modeling procedure, and resulting stellar population parameters were compared to their true values. Disk galaxies generally show a decent median correspondence between the true and estimated mass and age, but suffer from large uncertainties (Δlog M = –0.06+0.06 –0.13, Δlog age w = +0.03+0.19 –0.42). During the merger itself, we find larger offsets: Δlog M = –0.13+0.10 –0.14 and Δlog age w = –0.12+0.40 –0.26. E(B – V) values are generally recovered well, but the estimated total visual absorption AV is consistently too low, increasingly so for larger optical depths (ΔAV = –0.54+0.40 –0.46 in the merger regime). Since the largest optical depths occur during the phases of most intense star formation, it is for the highest SFRs that we find the largest underestimates (Δlog SFR = –0.44+0.32 –0.31 in the merger regime). The masses, ages, E(B – V), AV , and SFRs of merger remnants (spheroids) are very well reproduced. We discuss possible biases in SED modeling results caused by mismatch between the true and template star formation history (SFH), dust distribution, metallicity variations, and active galactic nucleus contribution. Mismatch between the real and template SFH, as is the case during the merging event, drives the age, and consequently mass estimate, down with respect to the true age and mass. However, the larger optical depth toward young stars during this phase reduces the effect considerably. Finally, we tested the photometric redshift code EAZY on the simulated galaxies placed at high redshift. We find a small scatter in Δz/(1 + z) of 0.031.

THE EVOLUTION OF THE FIELD AND CLUSTER MORPHOLOGY-DENSITY RELATION FOR MASS-SELECTED SAMPLES OF GALAXIES

The Sloan Digital Sky Survey (SDSS) and photometric/spectroscopic surveys in the GOODS-South field (the Chandra Deep Field-South, CDF-S) are used to construct volume-limited, stellar-mass-selected samples of galaxies at redshifts 0 2.5. The fraction of E + S0 galaxies is 43% ? 3% at z ~ 0.03 and 48% ? 7% at z ~ 0.8; i.e., it has not changed significantly since z ~ 0.8. When combined with recent results for cluster galaxies in the same redshift range, we find that the morphology-density relation for galaxies more massive than 0.5M* has remained constant since at least z ~ 0.8. This implies that galaxies evolve in mass, morphology, and density such that the morphology-density relation does not change. In particular, the decline of star formation activity and the accompanying increase in the stellar mass density of red galaxies since z ~ 1 must happen without large changes in the early-type galaxy fraction in a given environment.

Dense cores in galaxies out to z = 2.5 in SDSS, UltraVISTA, and the five 3D-HST/CANDELS fields

The dense interiors of massive galaxies are among the most intriguing environments in the universe. In this paper,we ask when these dense cores were formed and determine how galaxies gradually assembled around them. We select galaxies that have a stellar mass >3 × 1010 M ☉ inside r = 1 kpc out to z = 2.5, using the 3D-HST survey and data at low redshift. Remarkably, the number density of galaxies with dense cores appears to have decreased from z = 2.5 to the present. This decrease is probably mostly due to stellar mass loss and the resulting adiabatic expansion, with some contribution from merging. We infer that dense cores were mostly formed at z > 2.5, consistent with their largely quiescent stellar populations. While the cores appear to form early, the galaxies in which they reside show strong evolution: their total masses increase by a factor of 2-3 from z = 2.5 to z = 0 and their effective radii increase by a factor of 5-6. As a result, the contribution of dense cores to the total mass of the galaxies in which they reside decreases from ~50% at z = 2.5 to ~15% at z = 0. Because of their early formation, the contribution of dense cores to the total stellar mass budget of the universe is a strong function of redshift. The stars in cores with M 1 kpc > 3 × 1010 M ☉ make up ~0.1% of the stellar mass density of the universe today but 10%-20% at z ~ 2, depending on their initial mass function. The formation of these cores required the conversion of ~1011 M ☉ of gas into stars within ~1 kpc, while preventing significant star formation at larger radii.