arXiv subjects

Christopher J. Conselice

Publications and source records attributed to Christopher J. Conselice.

At least 307 records · Page 17Linked to original sources

arXiv2007

Strong size evolution of the most massive galaxies since z~2

Using the combined capabilities of the large near-infrared Palomar/DEEP-2 survey, and the superb resolution of the ACS HST camera, we explore the size evolution of 831 very massive galaxies (M*>10^{11}h_{70}^{-2}M_sun) since z~2. We split our sample according to their light concentration using the Sersic index n. At a given stellar mass, both low (n<2.5) and high (n>2.5) concentrated objects were much smaller in the past than their local massive counterparts. This evolution is particularly strong for the highly concentrated (spheroid-like) objects. At z~1.5, massive spheroid-like objects were a factor of 4(+-0.4) smaller (i.e. almost two orders of magnitudes denser) than those we see today. These small sized, high mass galaxies do not exist in the nearby Universe, suggesting that this population merged with other galaxies over several billion years to form the largest galaxies we see today.

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arXiv2007

New Constraints on the Lyman Continuum Escape Fraction at z~1.3

We examine deep far-ultraviolet (1600 Angstrom) imaging of the Hubble Deep Field-North (HDFN) and the Hubble Ultra Deep Field (HUDF) to search for leaking Lyman continuum radiation from starburst galaxies at z~1.3. There are 21 (primarily sub-L*) galaxies with spectroscopic redshifts between 1.1<z<1.5 and none are detected in the far-UV. We fit stellar population templates to the galaxies' optical/near-infrared SEDs to determine the starburst age and level of dust attenuation, giving an accurate estimate of the intrinsic Lyman continuum ratio, f_1500/f_700, and allowing a conversion from f_700 limits to relative escape fractions. We show that previous high-redshift studies may have underestimated the amplitude of the Lyman Break, and thus the relative escape fraction, by a factor of ~2. Once the starburst age and intergalactic HI absorption are accounted for, 18 galaxies in our sample have limits to the relative escape fraction, f_esc,rel < 1.0 with some limits as low as f_esc,rel < 0.10 and a stacked limit of f_esc,rel < 0.08. This demonstrates, for the first time, that most sub-L* galaxies at high redshift do not have large escape fractions. When combined with a similar study of more luminous galaxies at the same redshift we show that, if all star-forming galaxies at z~1 have similar relative escape fractions, the value must be less than 0.14 (3 sigma). We also show that less than 20% (3 sigma) of star-forming galaxies at z~1 have relative escape fractions near unity. These limits contrast with the large escape fractions found at z~3 and suggest that the average escape fraction has decreased between z~3 and z~1. (Abridged)

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arXiv2007

The DEEP2 Galaxy Redshift Survey: The Role of Galaxy Environment in the Cosmic Star-Formation History

Using galaxy samples drawn from the Sloan Digital Sky Survey and the DEEP2 Galaxy Redshift Survey, we study the relationship between star formation and environment at z ~ 0.1 and z ~ 1. We estimate the total star-formation rate (SFR) and specific star-formation rate (sSFR) for each galaxy according to the measured [O II] nebular line luminosity, corrected using empirical calibrations to match more robust SFR indicators. Echoing previous results, we find that in the local Universe star formation depends on environment such that galaxies in regions of higher overdensity, on average, have lower star-formation rates and longer star-formation timescales than their counterparts in lower-density regions. At z ~ 1, we show that the relationship between specific SFR and environment mirrors that found locally. However, we discover that the relationship between total SFR and overdensity at z ~ 1 is inverted relative to the local relation. This observed evolution in the SFR-density relation is driven, in part, by a population of bright, blue galaxies in dense environments at z ~ 1. This population, which lacks a counterpart at z ~ 0, is thought to evolve into members of the red sequence from z ~ 1 to z ~ 0. Finally, we conclude that environment does not play a dominant role in the cosmic star-formation history at z < 1: the dependence of the mean galaxy SFR on local galaxy density at constant redshift is small compared to the decline in the global SFR space density over the last 7 Gyr.

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arXiv2007

The Assembly History of Massive Galaxies: What Do We Know?

Understanding the formation history of massive galaxies is one of most popular and longstanding problems in astronomy, with observations and theory addressing how and when these systems assembled. Since the most massive galaxies in today's universe, with M_*> 10^11 M_0, are nearly all elliptical with uniform old stellar populations, we must probe higher redshifts to discover their full origins. A recent consensus has developed that nearly all M_* > 10^11 M_0 galaxies we see today were established by z~1, with at most a factor of two growth in stellar mass and number densities at lower redshifts. We review the evidence for this, and discuss how recent observations of star formation rates, colors, and morphologies of massive galaxies at z < 1 with M_* > 10^11 M_0 show that these systems are still experiencing some evolution. Massive galaxies undergo on average a single major merger at z < 1.5, and roughly half are experiencing star formation at the same redshifts. The highest mass galaxies, with M_* > 10^11.5 M_0, appear in similar abundance at z < 2, suggesting that extremely massive galaxies are mostly formed very early in the universe. Observations at z > 1.5 demonstrate that major galaxy mergers are the primary method for assembling these massive galaxies, with nearly all of this merging occurring at z > 2, with on average 4 to 5 major mergers taking place at z = 1.5 - 3.

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arXiv2007

The DEEP2 Galaxy Redshift Survey: AEGIS Observations of a Dual AGN at z=0.7

We present evidence for a dual Active Galactic Nucleus (AGN) within an early-type galaxy at z=0.709 in the Extended Groth Strip. The galaxy lies on the red sequence, with absolute magnitude M_B=-21.0 (AB, with h=0.7) and rest-frame color U-B=1.38. Its optical spectrum shows strong, double-peaked [OIII] emission lines and weak Hbeta emission, with Seyfert-like line ratios. The two narrow peaks are separated by 630 km/s in velocity and arise from two distinct regions, spatially resolved in the DEIMOS spectrum, with a projected physical separation of 1.2 kpc. HST/ACS imaging shows an early-type (E/S0) galaxy with hints of disturbed structure, consistent with the remnant of a dissipationless merger. Multiwavelength photometric information from the AEGIS consortium confirms the identification of a dust-obscured AGN in an early-type galaxy, with detections in X-ray, optical, infrared and radio wavebands. These data are most readily explained as a single galaxy harboring two AGN--the first such system to be observed in an otherwise typical early-type galaxy.

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arXiv2007

AEGIS: Enhancement of Dust-enshrouded Star Formation in Close Galaxy Pairs and Merging Galaxies up to z ~ 1

Using data from the DEEP2 Galaxy Redshift Survey and HST/ACS imaging in the Extended Groth Strip, we select nearly 100 interacting galaxy systems including kinematic close pairs and morphologically identified merging galaxies. Spitzer MIPS 24 micron fluxes of these systems reflect the current dusty star formation activity, and at a fixed stellar mass (M_{*}) the median infrared luminosity (L_{IR}) among merging galaxies and close pairs of blue galaxies is twice (1.9 +/- 0.4) that of control pairs drawn from isolated blue galaxies. Enhancement declines with galaxy separation, being strongest in close pairs and mergers and weaker in wide pairs compared to the control sample. At z ~ 0.9, 7.1% +/- 4.3% of massive interacting galaxies (M_{*} > 2*10^{10} M_{solar}) are found to be ULIRGs, compared to 2.6% +/- 0.7% in the control sample. The large spread of IR luminosity to stellar mass ratio among interacting galaxies suggests that this enhancement may depend on the merger stage as well as other as yet unidentified factors (e.g., galaxy structure, mass ratio, orbital characteristics, presence of AGN or bar). The contribution of interacting systems to the total IR luminosity density is moderate (<= 36 %).

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arXiv2006

Galaxy Mergers and Interactions at High Redshift

In this review we discuss the evidence for galaxy interactions and mergers in the distant universe and the role of mergers in forming galaxies. Observations show that the fraction of massive (M> M_*) galaxies involved in major mergers is roughly 5-10% at z~1. The merger fraction however increases steeply for the most massive galaxies up to z~3, where the merger fraction is 50+/-20%. Using N-body models of the galaxy merger process at a variety of merger conditions, merger mass ratios, and viewing angles this merger fraction can be converted into a merger rate, and mass accretion rate due to mergers. A simple integration of the merger rate shows that a typical massive galaxy at z~3 will undergo 4-5 major mergers between z~3 and z~0, with most of this activity, and resulting mass assembly, occurring at z > 1.5.

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arXiv2006

The Fundamental Properties of Galaxies and a New Galaxy Classification System

We present in this paper a new three-dimensional galaxy classification system designed to account for the diversity of galaxy properties in the nearby universe. To construct this system we statistically analyse a sample of >22,000 galaxies at v<15,000 km/s (z<0.05) with Spearman rank and principal component analyses. Fourteen major galaxy properties are considered, including: Hubble type, size, colour, surface brightness, magnitude, stellar mass, internal velocities, HI gas content, and an index that measures dynamical disturbances. We find, to a high degree, that most galaxy properties are correlated, with in particular Hubble type, colour, and stellar mass all strongly related. We argue that this tight 3-way correlation is a result of evolutionary processes that depend on galaxy mass, as we show that the relation between colour and mass is independent of Hubble type. Various principal component analyses reveal that most of the variation in nearby galaxy properties can be accounted for by eigenvectors dominated by: (i) the scale of a galaxy, such as its stellar mass, (ii) the spectral type, and (iii) the degree of dynamical disturbances. We suggest that these three properties: mass, star formation, and interactions/mergers are the major features that determine a galaxy's physical state, and should be used to classify galaxies. As shown in Conselice (2003), these properties are measurable within the CAS (concentration, asymmetry, clumpiness) structural system, thus providing an efficient mechanism for classifying galaxies in optical light within a physical meaningful framework. We furthermore discuss the fraction and number density of galaxies in the nearby universe as a function of Hubble type, for comparison with higher redshift populations.

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arXiv2006

The History of Galaxy Formation in Groups: An Observational Perspective

We present a pedagogical review on the formation and evolution of galaxies in groups, utilizing observational information from the Local Group to galaxies at z~6. The majority of galaxies in the nearby universe are found in groups, and galaxies at all redshifts up to z~6 tend to cluster on the scale of nearby groups (~1 Mpc). This suggests that the group environment may play a role in the formation of most galaxies. The Local Group, and other nearby groups, display a diversity in star formation and morphological properties that puts limits on how, and when, galaxies in groups formed. Effects that depend on an intragroup medium, such as ram-pressure and strangulation, are likely not major mechanisms driving group galaxy evolution. Simple dynamical friction arguments however show that galaxy mergers should be common, and a dominant process for driving evolution. While mergers between L_* galaxies are observed to be rare at z < 1, they are much more common at earlier times. This is due to the increased density of the universe, and to the fact that high mass galaxies are highly clustered on the scale of groups. We furthermore discus why the local number density environment of galaxies strongly correlates with galaxy properties, and why the group environment may be the preferred method for establishing the relationship between properties of galaxies and their local density.

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arXiv2006

Evolution in the Halo Masses of Isolated Galaxies between z~1 and z~0: From DEEP2 to SDSS

We measure the evolution in the virial mass-to-light ratio (M_{200}/L_B) and virial-to-stellar mass ratio (M_{200}/M_\ast) for isolated ~ L* galaxies between z~1 and z~0 by combining data from the DEEP2 Galaxy Redshift Survey and the Sloan Digital Sky Survey. Utilizing the motions of satellite galaxies around isolated galaxies, we measure line-of-sight velocity dispersions and derive dark matter halo virial masses for these host galaxies. At both epochs the velocity dispersion of satellites correlates with host galaxy stellar mass, σ\propto M_\ast^{0.4+/-0.1}, while the relation between satellite velocity dispersion and host galaxy B-band luminosity may grow somewhat shallower from σ\propto L_B^{0.6+/-0.1} at z~1 to σ\propto L_B^{0.4+/-0.1} at z~0. The evolution in M_200/M_\ast from z~1 to z~0 displays a bimodality insofar as host galaxies with stellar mass below M_\ast ~10^{11} M_Sun/h maintain a constant ratio (the intrinsic increase is constrained to a factor of 1.1+/-0.7) while host galaxies above M_\ast ~10^{11} M_Sun/h experience a factor of 4+/-3 increase in their virial-to-stellar mass ratio. This result can be easily understood if galaxies below this stellar mass scale continue to form stars while star formation in galaxies above this scale is quenched and the dark matter halos of galaxies both above and below this scale grow in accordance with LCDM cosmological simulations. Host galaxies that are red in U-B color have larger satellite dispersions and hence reside on average in more massive halos than blue galaxies at both z~1 and z~0. The redshift and host galaxy stellar mass dependence of M_200/M_\ast agrees qualitatively with the Millennium Run semi-analytic model of galaxy formation. (ABRIDGED)

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arXiv2006

The Mass Assembly History of Field Galaxies: Detection of an Evolving Mass Limit for Star Forming Galaxies

We characterize the mass-dependent evolution in a large sample of more than 8,000 galaxies using spectroscopic redshifts drawn from the DEEP2 Galaxy Redshift Survey in the range 0.4 < z < 1.4 and stellar masses calculated from K-band photometry obtained at Palomar Observatory. Using restframe (U-B) color and [OII] equivalent widths, we distinguish star-forming from passive populations in order to explore the nature of "downsizing''--a pattern in which the sites of active star formation shift from high mass galaxies at early times to lower mass systems at later epochs. Over the redshift range probed, we identify a mass limit, M_Q, above which star formation appears to be quenched. The physical mechanisms responsible for downsizing can thus be empirically quantified by charting the evolution in this threshold mass. We find that M_Q decreases with time by a factor of ~3 across the redshift range sampled according with a redshift dependence of (1+z)^3.5. To further constrain possible quenching mechanisms, we investigate how this downsizing signal depends on local galaxy environment. For the majority of galaxies in regions near the median density, there is no significant correlation between downsizing and environment. However, a trend is observed in the comparison between more extreme environments that are more than 3 times overdense or underdense relative to the median. Here, we find that downsizing is accelerated in overdense regions which host higher numbers of massive, early-type galaxies and fewer late-types as compared to the underdense regions. Our results significantly constrain recent suggestions for the origin of downsizing and indicate that the process for quenching star formation must, primarily, be internally driven. (Abridged)

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arXiv2005

Constraining Galaxy Formation Models with Dwarf Ellipticals in Clusters

Recent observations demonstrate that dwarf elliptical (dE) galaxies in clusters, despite their faintness, are likely a critical galaxy type for understanding the processes behind galaxy formation. Dwarf ellipticals are the most common galaxy type, and are particularly abundant in rich galaxy clusters. The dwarf to giant ratio is in fact highest in rich clusters of galaxies, suggesting that cluster dEs do not form in groups that later merge to form clusters. Dwarf ellipticals are potentially the only galaxy type whose formation is sensitive to global, rather than local, environment. The dominant idea for explaining the formation of these systems, through Cold Dark Matter models, is that dEs form early and within their present environments. Recent results suggest that some dwarfs appear in clusters after the bulk of massive galaxies form, a scenario not predicted in standard hierarchical structure formation models. Many dEs have younger and more metal rich stellar populations than dwarfs in lower density environments, suggesting processes induced by rich clusters play an important role in dE formation. Several general galaxy cluster observations, including steep luminosity functions, and the origin of intracluster light, are natural outcomes of this delayed formation.

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arXiv2005

Keck Spectroscopy of Globular Clusters in the Virgo Cluster Dwarf Elliptical VCC 1386

We present the results of a Keck spectroscopic study of globular clusters associated with the Virgo Cluster dwarf elliptical VCC 1386. We analyze blue spectroscopic absorption lines from 3500-5500 A for 13 globular cluster candidates and confirm that five are associated with VCC 1386. By comparing metal and Balmer line indices of these globular clusters with alpha-enhanced single stellar population models we find that these systems are metal poor with [Fe/H] < -1.35 and old, with ages > 5 Gyr at 3 sigma confidence, placing their formation at z >1. This is one of the first spectroscopic studies of globular clusters surrounding dwarfs in a cluster, revealing that some low mass galaxies in rich environments form at least part of their stellar mass early in the history of the universe. We further find that the luminosity weighted stellar population of VCC 1386 itself is younger, and more metal rich than its globular clusters, consistent with (V-I)_0 colors from Hubble Space Telescope imaging. This implies that VCC 1386, like Local Group dEs, has had multiple episodes of star formation. Globular clusters associated with low luminosity systems however appear to be roughly as old as those associated with giant galaxies, contrary to the `downsizing' formation of their bulk stellar populations.

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arXiv2005

Early and Rapid Merging as a Formation Mechanism of Massive Galaxies: Empirical Constraints

We present the results of a series of empirical computations regarding the role of major mergers in forming the stellar masses of modern galaxies based on measurements of galaxy merger and star formation histories from z~0.5-3. We re-construct the merger history of normal field galaxies from z~3 to z~0 as a function of initial mass using published pair fractions and merger fractions from structural analyses. We calibrate the observed merger time-scale and mass ratios for galaxy mergers using self-consistent N-body models of mergers, composed of dark matter and stars, with mass ratios from 1:1 to 1:5 with various orbital properties and viewing angles. We use these simulations to determine the time-scales and mass ratios that produce structures that would be identified as major mergers. Based on these calculations we argue that a typical massive galaxy at z~3 with M_{*} > 10^{10} M_0 undergoes 4.4^{+1.6}_{-0.9} major mergers at z > 1. We find that by z~1.5 the stellar mass of an average massive galaxy is relatively established, a scenario qualitatively favored in a lambda dominated universe. We argue that the final masses of these systems increases by as much as a factor of 100 allowing Lyman-break galaxies, which tend to have low stellar masses, to become the most massive galaxies in today's universe with M > M^{*}. Induced star formation however only accounts for 10-30% of the stellar mass formed in these galaxies at z < 3. A comparison to semi-analytic models of galaxy formation shows that Cold Dark Matter (CDM) models consistently under-predict the merger fraction, and rate of merging, of massive galaxies at high redshift. This suggests that massive galaxy formation occurs through more merging than predicted in CDM models, rather than a rapid early collapse.

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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.

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arXiv2005

Total Galaxy Magnitudes and Effective Radii from Petrosian Magnitudes and Radii

Petrosian magnitudes were designed to help with the difficult task of determining a galaxy's total light. Although these magnitudes (taken here as the flux within 2R_P, with the inverted Petrosian index 1/eta(R_P)=0.2) can represent most of an object's flux, they do of course miss the light outside of the Petrosian aperture (2R_P). The size of this flux deficit varies monotonically with the shape of a galaxy's light-profile, i.e., its concentration. In the case of a de Vaucouleurs R^{1/4} profile, the deficit is 0.20 mag; for an R^{1/8} profile this figure rises to 0.50 mag. Here we provide a simple method for recovering total (Sersic) magnitudes from Petrosian magnitudes using only the galaxy concentration (R_90/R_50 or R_80/R_20) within the Petrosian aperture. The corrections hold to the extent that Sersic's model provides a good description of a galaxy's luminosity profile. We show how the concentration can also be used to convert Petrosian radii into effective half-light radii, enabling a robust measure of the mean effective surface brightness. Our technique is applied to the SDSS DR2 Petrosian parameters, yielding good agreement with the total magnitudes, effective radii, and mean effective surface brightnesses obtained from the NYU--VAGC Sersic R^{1/n} fits by Blanton et al. (2005). Although the corrective procedure described here is specifically applicable to the SDSS DR2 and DR3, it is generally applicable to all imaging data where any Petrosian index and concentration can be constructed.

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arXiv2005

Evolution of the Near-Infrared Tully-Fisher Relation: Constraints on the Relationship Between the Stellar and Total Masses of Disk Galaxies since z=1

Using a combination of Keck spectroscopy and near-infrared imaging, we investigate the K-band and stellar mass Tully-Fisher relation for 101 disk galaxies at 0.2 < z < 1.2, with the goal of placing the first observational constraints on the assembly history of halo and stellar mass. Our main result is a lack of evolution in either the K-band or stellar mass Tully-Fisher relation from z = 0 - 1.2. Furthermore, although our sample is not statistically complete, we consider it suitable for an initial investigation of how the fraction of total mass that has condensed into stars is distributed with both redshift and total halo mass. We calculate stellar masses from optical and near-infrared photometry and total masses from maximum rotational velocities and disk scale lengths, utilizing a range of model relationships derived analytically and from simulations. We find that the stellar/total mass distribution and stellar-mass Tully-Fisher relation for z > 0.7 disks is similar to that at lower redshift, suggesting that baryonic mass is accreted by disks along with dark matter at z < 1, and that disk galaxy formation at z < 1 is hierarchical in nature. We briefly discuss the evolutionary trends expected in conventional structure formation models and the implications of extending such a study to much larger samples.

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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]

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