arXiv subjects

Casey Papovich

Publications and source records attributed to Casey Papovich.

At least 307 records · Page 17Linked to original sources

arXiv2005

Spitzer Observations of Red Galaxies: Implication for High-Redshift Star Formation

My colleagues and I identified distant red galaxies (DRGs) with J-K>2.3 mag in the GOODS-S field. These galaxies reside at z~1-3.5, ( =2.2) and based on their ACS (0.4-1 micron), ISAAC (1-2.2 micron), and IRAC (3-8 micron) photometry, they typically have inferred stellar masses > 10^11 solar masses. Interestingly, more than 50% of these objects have 24 micron flux densities >50 micro-Jy. Attributing the IR emission to star-formation implies SFRs of \~100-1000 solar masses per year. As a result, galaxies with stellar masses >10^11 solar masses have specific SFRs equal to or exceeding the global value at z~1.5-3. In contrast, galaxies with >10^11 solar masses z~0.3-0.75 have specific SFRs less than the global average, and more than an order of magnitude lower than that for massive DRGs at z~1.5-3. Thus, the bulk of star formation in massive galaxies is largely complete by z~1.5. The red colors and large inferred stellar masses in the DRGs suggest that much of the star formation in these galaxies occurred at redshifts z>5-6. Using model star-formation histories that match the DRG colors and stellar masses at z~2-3, and measurements of the UV luminosity density at z>5-6, we consider what constraints exist on the stellar initial mass function in the progenitors of the massive DRGs at z~2-3.

astro-ph↗

arXiv2005

Spitzer Observations of Massive Red Galaxies at High Redshift

We investigate the properties of massive galaxies at z=1-3.5 using HST observations, ground-based near-IR imaging, and Spitzer Space Telescope observations at 3-24 micron. We identify 153 distant red galaxies (DRGs) with J-K > 2.3 mag (Vega) in the southern GOODS field. This sample is approximately complete in stellar mass for passively evolving galaxies above 10^11 solar masses and z < 3. The galaxies identified by this selection are roughly split between objects whose optical and near-IR rest-frame light is dominated by evolved stars combined with ongoing star formation, and galaxies whose light is dominated by heavily reddened starbursts. Very few of the galaxies (< 10%) have no indication of current star formation. Using SFR estimates that include the reradiated IR emission, the DRGs at z=1.5-3 with stellar masses > 10^11 solar masses have specific SFRs (SFRs per unit stellar mass) ranging from 0.2 to 10 Gyr^-1, with a mean value of ~2.4 Gyr^-1. The DRGs with stellar masses > 10^11 solar masses and 1.5 < z < 3 have integrated specific SFRs greater the global value over all galaxies. In contrast, we find that galaxies at z = 0.3-0.75 with these stellar masses have integrated specific SFRs less than the global value, and more than an order of magnitude lower than that for massive DRGs at z = 1.5-3. At z < 1, lower-mass galaxies dominate the overall cosmic mass assembly. This suggests that the bulk of star formation in massive galaxies occurs at early cosmic epochs and is largely complete by z~1.5. [Abridged]

astro-ph↗

arXiv2005

Building Galaxies with Simulations

We present an overview of some of the issues surrounding current models of galaxy formation, highlighting recent insights obtained from cosmological hydrodynamic simulations. Detailed examination of gas accretion processes show a hot mode of gas cooling from near the halo's virial temperature, and a previously underappreciated cold mode where gas flows in along filaments on dynamical timescales, emitting its energy in line radiation. Cold mode dominates in systems with halo masses slightly smaller than the Milky Way and below, and hence dominates the global accretion during the heydey of galaxy formation. This rapid accretion path enables prompt assembly of massive galaxies in the early universe, and results in $z\sim 4$ galaxy properties in broad agreement with observations, with the most massive galaxies being the most rapid star formers. Massive galaxies today are forming stars at a much reduced rate, a trend called downsizing. The trend of downsizing is naturally reproduced in simulations, owing to a transition from cold mode accretion in the early growth phase to slower hot mode accretion once their halos grow large. However, massive galaxies at the present epoch are still observed to have considerably redder colors than simulations suggest, suggesting that star formation is not sufficiently truncated in models by the transition to hot mode, and that another process not included in current simulations is required to suppress star formation.

astro-ph↗

arXiv2005

Rest-frame Ultraviolet-to-Optical Properties of Galaxies at z ~ 6 and 5 in the Hubble Ultra Deep Field: from Hubble to Spitzer

We use data from the first epoch of observations with the IRAC/Spitzer for the GOODS to detect and study a collection of LBGs at z ~ 6 to 5 in the HUDF, six of which have spectroscopic confirmation. At these redshifts, IRAC samples rest-frame optical light in the range 0.5 to 0.8 um, where the effects of dust extinction are smaller and the sensitivity to light from evolved stars is greater than at shorter, rest-frame UV observable from the ground or with the HST. As such, it provides useful constraints on the ages and masses of these galaxies' stellar populations. We find that the SEDs for many of these galaxies are best fitted by models of stellar populations with masses of a few x 1e10 M_sun, and with ages of a few hundred Myrs, values quite similar to those derived for typical LBGs at z ~ 3. When the universe was only 1 Gyr old, some galaxies had already formed a mass of stars approaching that of the present-day Milky Way, and that they started forming those stars at z > 7, and in some cases much earlier. We find that the lower limits to the space density for galaxies in this mass range are consistent with predictions from recent hydrodynamic simulations of structure formation in a LCDM universe. All objects in our samples are consistent with having solar metallicity, suggesting that they might have already been significantly polluted by metals. The values for dust reddening derived from the model fitting are low or zero, and we find that some of the galaxies have rest-frame UV colors that are even bluer than those predicted by the stellar population models to which we compare them. These colors might be attributed to the presence of very massive stars (> 100 M_sun), or by weaker intergalactic HI absorption than what is commonly assumed.

astro-ph↗

arXiv2005

Spitzer view on the evolution of star-forming galaxies from z=0 to z~3

We use a 24 micron selected sample containing more than 8,000 sources to study the evolution of star-forming galaxies in the redshift range from z=0 to z~3. We obtain photometric redshifts for most of the sources in our survey using a method based on empirically-built templates spanning from ultraviolet to mid-infrared wavelengths. The accuracy of these redshifts is better than 10% for 80% of the sample. The derived redshift distribution of the sources detected by our survey peaks at around z=0.6-1.0 (the location of the peak being affected by cosmic variance), and decays monotonically from z~1 to z~3. We have fitted infrared luminosity functions in several redshift bins in the range 0 1.e11 L_sun) to the total SFR density increases steadily from z~0 up to z~2.5, forming at least half of the newly-born stars by z~1.5. Ultraluminous infrared galaxies (L_TIR>1.e12 L_sun) play a rapidly increasing role for z>~1.3.

astro-ph↗

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.

astro-ph↗

arXiv2005

The Assembly of Diversity in the Morphologies and Stellar Populations of High-Redshift Galaxies

We have studied the evolution in the morphologies, sizes, stellar-masses, colors, and internal color dispersion (ICD) of galaxies at z=1 and 2.3, using a near-IR, flux-limited catalog for the HDF-N. At z=1 most luminous galaxies have morphologies of early-to-mid Hubble-types, and many show transformations between their rest-frame UV-optical morphologies. Galaxies at z=2.3 have compact and irregular morphologies with no clearly evident Hubble-sequence candidates. The mean galaxy size grows from z=2.3 to 1 by 40%, and the density of galaxies larger than 3 kpc increases by 7 times. At z=1, the size-luminosity distribution is broadly consistent with that of local galaxies, with passive evolution. However, galaxies at z=2.3 are smaller than the large present-day galaxies, and must continue to grow in size and stellar mass. We have measured the galaxies' UV-optical ICD, which quantifies differences in morphology and the relative amount of on-going star-formation. The mean and scatter in galaxies' total colors and ICD increase from z=2.3 to 1. At z=1 many galaxies with large ICD are spirals, with a few irregular systems. Few z=2.3 galaxies have high ICD, and those that do are actively merging. We interpret this as evidence for the presence of older and more diverse stellar populations at z=1 that are not generally present at z>2. We conclude that the star-formation histories of galaxies at z>2 are dominated by discrete, recurrent bursts, which quickly homogenize the galaxies' stellar content, and are possibly associated with mergers. The increase in the stellar-population diversification by z<1.4 implies that merger-induced starbursts occur less frequently than at higher redshifts, and more quiescent star-forming modes dominate. This transition coincides with the emergence of Hubble-sequence galaxies. [Abridged]

astro-ph↗

arXiv2004

The Luminosity, Stellar Mass, and Number Density Evolution of Field Galaxies of Known Morphology from z = 0.5 - 3

The evolution of rest-frame B-band luminosities, stellar masses, and number densities of field galaxies in the Hubble Deep Field North and South are studied as a function of rest-frame B-band morphological type out to redshifts z~3 using a sample of 1231 I < 27 galaxies with spectroscopic and photometric redshifts. We find that the co-moving number, and relative number, densities of ellipticals and spirals declines rapidly at z > 1, although examples exist at z > 2. The number and number fraction of peculiar galaxies consistent with undergoing major mergers rises consistently at redshifts z > 2. Through simulations we argue that this decline is robust at the 4 sigma level against morphological k-corrections and redshift effects. We trace the evolution of rest-frame B-band luminosity density as a function of morphology out to z~3 finding that the luminosity density is steadily dominated by peculiars at z > 1.5 with a peak fraction of 60-90% at z~3. By z~0.5 B-band luminosity fractions are similar to their local values. The stellar mass density follows a similar trend as the luminosity density, with some important exceptions. At high redshifts, z > 2, over 60-80% of stellar mass is attached to peculiars, while at z< 1, 80-95% of stellar mass is attached to ellipticals and spirals. In contrast to the luminosity density, the stellar mass density of ellipticals is greater than spirals at z < 1.5, and the stellar masses in both types grow together at z < 1, while number densities remain constant. (abridged)

astro-ph↗

arXiv2004

Studying Distant Infrared-Luminous Galaxies with Spitzer and Hubble

New surveys with the Spitzer space telescope identify distant star-forming and active galaxies by their strong emission at far-infrared wavelengths, which provides strong constraints on these galaxies' bolometric energy. Using early results from Spitzer surveys at 24 micron, we argue that the faint sources correspond to the existence of a population of infrared-luminous galaxies at z > 1 that are not expected from predictions based on previous observations from ISO and IRAS. Combining Spitzer images with deep ground-based optical and Hubble Space Telescope imaging, we discuss the properties of galaxies selected at 24 micron in the region of the Chandra Deep Field South, including redshift and morphological distributions. Galaxies with z < 1 constitute roughly half of the faint 24 micron sources. Infrared-luminous galaxies at these redshifts span a wide variety of normal to strongly interacting/merging morphologies, which suggests that a range of mechanisms produce infrared activity. Large-area, joint surveys between Spitzer and HST are needed to understand the complex relation between galaxy morphology, structure, environment and activity level, and how this evolves with cosmic time. We briefly discuss strategies for constructing surveys to maximize the legacy of these missions.

astro-ph↗

arXiv2004

The 24 Micron Source Counts in Deep Spitzer Surveys

Galaxy source counts in the infrared provide strong constraints on the evolution of the bolometric energy output from distant galaxy populations. We present the results from deep 24 micron imaging from Spitzer surveys, which include approximately 50,000 sources to an 80% completeness of 60 uJy. The 24 micron counts rapidly rise at near-Euclidean rates down to 5 mJy, increase with a super-Euclidean rate between 0.4 - 4 mJy, and converge below 0.3 mJy. The 24 micron counts exceed expectations from non-evolving models by a factor >10 at 0.1 mJy. The peak in the differential number counts corresponds to a population of faint sources that is not expected from predictions based on 15 micron counts from ISO. We argue that this implies the existence of a previously undetected population of infrared-luminous galaxies at z ~ 1-3. Integrating the counts to 60 uJy, we derive a lower limit on the 24 micron background intensity of 1.9 +/- 0.6 nW m-2 sr-1 of which the majority (~ 60%) stems from sources fainter than 0.4 mJy. Extrapolating to fainter flux densities, sources below 60 uJy contribute 0.8 {+0.9/-0.4} nW m-2 sr-1 to the background, which provides an estimate of the total 24 micron background of 2.7 {+1.1/-0.7} nW m-2 sr-1.

astro-ph↗

arXiv2003

Evolution in the Colors of Lyman-Break Galaxies from Z~4 to Z~3

The integrated colors of distant galaxies provide a means for interpreting the properties of their stellar content. Here, we use rest-frame UV-to-optical colors to constrain the spectral-energy distributions and stellar populations of color-selected, B-dropout galaxies at z ~ 4 in the Great Observatories Origins Deep Survey. We combine the ACS data with ground-based near-infrared images, which extend the coverage of galaxies at z ~ 4 to the rest-frame B-band. We observe a color-magnitude trend in the rest-frame m(UV) - B versus B diagram for the z ~ 4 galaxies that has a fairly well-defined "blue-envelope", and is strikingly similar to that of color-selected, U-dropout galaxies at z ~ 3. We also find that although the co-moving luminosity density at rest-frame UV wavelengths (1600 Angstroms) is roughly comparable at z ~ 3 and z ~ 4, the luminosity density at rest-frame optical wavelengths increases by about one-third from z ~ 4 to z ~ 3. Although the star-formation histories of individual galaxies may involve complex and stochastic events, the evolution in the global luminosity density of the UV-bright galaxy population corresponds to an average star-formation history with a star-formation rate that is constant or increasing over these redshifts. This suggests that the evolution in the luminosity density corresponds to an increase in the stellar-mass density of more than 33%.

astro-ph↗

arXiv2003

Searching for Star Formation Beyond Reionization

The goal of searching back in cosmic time to find star formation during the epoch of reionization will soon be within reach. We assess the detectability of high-redshift galaxies by combining cosmological hydrodynamic simulations of galaxy formation, stellar evolution models appropriate for the first generations of stars, and estimates of the efficiency for Lyman alpha to escape from forming galaxies into the intergalactic medium. Our simulated observations show that Lyman alpha emission at z ~ 8 may be observable in the near-infrared with 8-meter class telescopes and present-day technology. Not only is the detection of early star-forming objects vital to understanding the underlying cause of the reionization of the universe, but the timely discovery of a z > 7 star-forming population -- or even an interesting upper limit on the emergent flux from these objects -- will have implications for the design of the next generation of ground- and space-based facilities.

astro-ph↗

arXiv2003

Morphologies and Spectral Energy Distributions of Extremely Red Galaxies in the GOODS-South Field

Using U'- through Ks-band imaging data in the GOODS-South field, we construct a large, complete sample of 275 ``extremely red objects'' (EROs; K_s<22.0, R-K_s>3.35; AB), all with deep HST/ACS imaging in B_435, V_606, i_775, and z_850, and well-calibrated photometric redshifts. Quantitative concentration and asymmetry measurements fail to separate EROs into distinct morphological classes. We therefore visually classify the morphologies of all EROs into four broad types: ``Early'' (elliptical-like), ``Late'' (disk galaxies), ``Irregular'' and ``Other'' (chain galaxies and low surface brightness galaxies), and calculate their relative fractions and comoving space densities. For a broad range of limiting magnitudes and color thresholds, the relative number of early-type EROs is approximately constant at 33-44%, and the comoving space densities of Early- and Late-type EROs are comparable. Mean rest-frame spectral energy distributions (SEDs) at wavelengths between 0.1 and 1.2 um are constructed for all EROs. The SEDs are extremely similar in their range of shapes, independent of morphological type. The implication is that any differences between the broad-band SEDs of Early-type EROs and the other types are relatively subtle, and there is no robust way of photometrically distinguishing between different morphological types with usual optical/near-infrared photometry.

astro-ph↗

arXiv2003

The Redshift Distribution of Near-IR Selected Galaxies in the Great Observatories Origins Deep Survey as a Test of Galaxy Formation Scenarios

The redshift distribution of near-IR selected galaxies is often used to attempt to discriminate between the classical view of galaxy formation, in which present-day luminous galaxies were assembled at early times and evolve due to the passive aging of their stellar populations, and that of hierarchical structure formation, in which galaxies were assembled more recently via the merging of smaller objects. We carry out such a test here, by computing the distribution of photometric redshifts of K_AB<22 galaxies in the Great Observatories Origins Deep Field Survey GOODS Southern Field, and comparing the results with predictions from a semi-analytic model based on hierarchical structure formation, and a classical `passive evolution' model. We find that the redshift distributions at z <~ 1.5 of both the hierarchical and passive models are very similar to the observed one. At z>~1.5, the hierarchical model shows a deficit of galaxies, while the passive model predicts an excess. We investigate the nature of the observed galaxies in the redshift range where the models diverge, and find that the majority have highly disturbed morphologies, suggesting that they may be merger-induced starbursts. While the hierarchical model used here does not produce these objects in great enough numbers, the appearance of this population is clearly in better qualitative agreement with the hierarchical picture than with the classical passive evolution scenario. We conclude that the observations support the general framework of hierarchical formation, but suggest the need for new or modified physics in the models.

astro-ph↗

arXiv2003

The z ~ 4 Lyman Break Galaxies: Colors and Theoretical Predictions

We investigate several fundamental properties of z ~ 4 Lyman-break galaxies by comparing observations with the predictions of a semi-analytic model based on the Cold Dark Matter theory of hierarchical structure formation. We use a sample of B_{435}-dropouts from the Great Observatories Origins Deep Survey, and complement the ACS optical B_{435}, V_{606}, i_{775}, and z_{850} data with the VLT ISAAC J, H, and K_{s} observations. We extract B_{435}-dropouts from our semi-analytic mock catalog using the same color criteria and magnitude limits that were applied to the observed sample. We find that the i_{775} - K_{s} colors of the model-derived and observed B_{435}-dropouts are in good agreement. However, we find that the i_{775}-z_{850} colors differ significantly, indicating perhaps that either too little dust or an incorrect extinction curve have been used. Motivated by the reasonably good agreement between the model and observed data we present predictions for the stellar masses, star formation rates, and ages for the z ~ 4 Lyman-break sample. We find that according to our model the color selection criteria used to select our z ~ 4 sample surveys 67% of all galaxies at this epoch down to z_{850} < 26.5. We find that our model predicts a roughly 40% mass build-up between the z \~ 4 and z ~ 3 epochs for the UV rest-frame L* galaxies. Furthermore, according to our model, at least 50% of the total stellar mass resides in relatively massive UV-faint objects that fall below our observational detection limit.

astro-ph↗

arXiv2003

The Internal Ultraviolet-to-Optical Color Dispersion: Quantifying the Morphological K-Correction

We present a quantitative measure of the internal color dispersion within galaxies, which quantifies differences in morphology as a function of wavelength. We apply this statistic to a local galaxy sample with archival images at 1500 and 2500 Angstroms from the Ultraviolet Imaging Telescope, and ground-based B-band observations to investigate how the color dispersion relates to global galaxy properties. The intenal color dispersion generally correlates with transformations in galaxy morphology as a function of wavelength, i.e., it quantifies the morphological K-correction. Mid-type spiral galaxies exhibit the highest dispersion in their internal colors, which stems from differences in the bulge, disk, and spiral-arm components. Irregulars and late-type spirals show moderate internal color dispersion, which implies that young stars generally dominate the colors. Ellipticals, lenticulars, and early-type spirals generally have low or negligible internal color dispersion, which indicates that the stars contributing to the UV-to-optical emission have a very homogeneous distribution. We discuss the application of the internal color dispersion to high-redshift galaxies in deep, Hubble Space Telescope images. By simulating local galaxies at cosmological distances, many of the galaxies have luminosities that are sufficiently bright at rest--frame optical wavelengths to be detected within the limits of the currently deepest near-infrared surveys even with no evolution. Under assumptions that the luminosity and color evolution of the local galaxies conform with the measured values of high-redshift objects, we show that galaxies' intrinsic internal color dispersion remains measurable out to z ~ 3.

astro-ph↗

arXiv2003

A Direct Measurement of Major Galaxy Mergers at z < 3

This paper presents direct evidence for hierarchical galaxy assembly out to redshifts z ~ 3. We identify major mergers using the model-independent CAS (concentration, asymmetry, clumpiness) physical morphological system on galaxies detected, and photometrically selected, in the WFPC2 and NICMOS Hubble Deep Field North. We specifically use the asymmetric distributions of rest-frame optical light measured through the asymmetry parameter (A) to determine the fraction of galaxies undergoing major mergers as a function of redshift (z), stellar mass (M_{*}), and absolute magnitude (M_{B}). We find that the fraction of galaxies consistent with undergoing a major merger increases with redshift for all galaxies, but most significantly, at 5 - 10 sigma confidence, for the most luminous and massive systems. The highest merger fractions we find are 40% - 50% for galaxies with M_{B} < -21, or M_{*} > 10^10 M_0 at z > 2.5, i.e., objects identified as Lyman-break galaxies. We model the merger fraction evolution in the form: f x (1+z)^m, finding m values ~4-6 for the most luminous and massive galaxies. We use these merger fractions combined with merger time scales calculated from N-body simulations to derive galaxy merger rates to z ~ 3. We also use stellar masses of HDF-N galaxies to determine the mass accretion rate of field galaxies involved in major mergers, and how this changes with redshift. We further find that the fraction of stellar mass density in galaxies involved in major mergers increases with redshift. By comparing merger fractions predicted in Cold Dark Matter semi-analytic models with our results there is a reasonably good agreement for the largest and brightest systems, although there are more low-mass galaxy mergers at lower redshifts than what these models predict. (abridged)

astro-ph↗

arXiv2002

The Evolution of the Global Stellar Mass Density at 0 < z < 3

The build-up of stellar mass in galaxies is the consequence of their past star formation and merging histories. Here we report measurements of rest-frame optical light and calculations of stellar mass at high redshift based on an infrared-selected sample of galaxies from the Hubble Deep Field North. The bright envelope of rest-frame B-band galaxy luminosities is similar from 0 >2.

astro-ph↗