arXiv subjects

Casey Papovich

Publications and source records attributed to Casey Papovich.

At least 253 records · Page 14Linked to original sources

arXiv2014

Galaxy Stellar Mass Functions from ZFOURGE/CANDELS: An Excess of Low-Mass Galaxies Since z=2 and the Rapid Buildup of Quiescent Galaxies

Using observations from the FourStar Galaxy Evolution Survey (ZFOURGE), we obtain the deepest measurements to date of the galaxy stellar mass function at 0.5 < z < 2.5. ZFOURGE provides well-constrained photometric redshifts made possible through deep medium-bandwidth imaging at 1-2um . We combine this with HST imaging from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS), allowing for the efficient selection of both blue and red galaxies down to stellar masses ~10^9.5 Msol at z ~ 2.5. The total surveyed area is 316 arcmin^2 distributed over three independent fields. We supplement these data with the wider and shallower NEWFIRM Medium-Band Survey (NMBS) to provide stronger constraints at high masses. Several studies at z<=1 have revealed a steepening of the slope at the low-mass end of the stellar mass function (SMF), leading to an upturn at masses <10^10 Msol that is not well-described by a standard single-Schechter function. We find evidence that this feature extends to at least z ~ 2, and that it can be found in both the star-forming and quiescent populations individually. The characteristic mass (M*) and slope at the lowest masses (alpha) of a double-Schechter function fit to the SMF stay roughly constant at Log(M/Msol) ~ 10.65 and ~-1.5 respectively. The SMF of star-forming galaxies has evolved primarily in normalization, while the change in shape is relatively minor. This is not the case for quiescent galaxies: the depth of our imaging allows us to show for the first time significantly more evolution at Log(M/Msol) < 10.5 than at higher masses. We find that the total mass density (down to 10^9 Msol) in star-forming galaxies has increased by a factor of ~2.2 since z ~ 2.5, whereas in quiescent galaxies it has increased by a factor of ~12 .

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arXiv2013

A Critical Assessment of Photometric Redshift Methods: A CANDELS Investigation

We present results from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) photometric redshift methods investigation. In this investigation, the results from eleven participants, each using a different combination of photometric redshift code, template spectral energy distributions (SEDs) and priors, are used to examine the properties of photometric redshifts applied to deep fields with broad-band multi-wavelength coverage. The photometry used includes U-band through mid-infrared filters and was derived using the TFIT method. Comparing the results, we find that there is no particular code or set of template SEDs that results in significantly better photometric redshifts compared to others. However, we find codes producing the lowest scatter and outlier fraction utilize a training sample to optimize photometric redshifts by adding zero-point offsets, template adjusting or adding extra smoothing errors. These results therefore stress the importance of the training procedure. We find a strong dependence of the photometric redshift accuracy on the signal-to-noise ratio of the photometry. On the other hand, we find a weak dependence of the photometric redshift scatter with redshift and galaxy color. We find that most photometric redshift codes quote redshift errors (e.g., 68% confidence intervals) that are too small compared to that expected from the spectroscopic control sample. We find that all codes show a statistically significant bias in the photometric redshifts. However, the bias is in all cases smaller than the scatter, the latter therefore dominates the errors. Finally, we find that combining results from multiple codes significantly decreases the photometric redshift scatter and outlier fraction. We discuss different ways of combining data to produce accurate photometric redshifts and error estimates.

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arXiv2013

CANDELS Observations of the Environmental Dependence of the Color-Mass-Morphology Relation at z = 1.6

We study the environmental dependence of color, stellar mass, and morphology by comparing galaxies in a forming cluster to those in the field at z = 1:6 with Hubble Space Telescope near-infrared imaging in the CANDELS/UDS field. We quantify the morphology of the galaxies using the effective radius, reff, and Sérsic index, n. In both the cluster and field, approximately half of the bulge-dominated galaxies (n > 2) reside on the red sequence of the color-magnitude diagram, and most disk-dominated galaxies (n < 2) have colors expected for star-forming galaxies. There is weak evidence that cluster galaxies have redder rest-frame U - B colors and higher stellar masses compared to the field. Star-forming galaxies in both the cluster and field show no significant differences in their morphologies. In contrast, there is evidence that quiescent galaxies in the cluster have larger median effective radii and smaller Sérsic indices compared to the field with a significance of 2?. These differences are most pronounced for galaxies at clustercentric distances 1 Mpc < Rproj < 1.5 Mpc, which have low Sérsic indices and possibly larger effective radii, more consistent with star-forming galaxies at this epoch and in contrast to other quiescent galaxies. We argue that star-forming galaxies are processed under the influence of the cluster environment at distances greater than the cluster-halo virial radius. Our results are consistent with models where gas accretion onto these galaxies is suppressed from processes associated with the cluster environment.

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arXiv2013

Spitzer Spectroscopy of Infrared-Luminous Galaxies: Diagnostics of AGN and Star Formation and Contribution to Total Infrared Luminosity

We use Spitzer IRS spectroscopy to study the nature 65 IR-luminous galaxies at 0.02 < z < 0.6 with F(24micron) > 1.2mJy. The IRS spectra cover wavelengths spanning the PAH features and important atomic diagnostic lines. Our sample corresponds to L(IR) = L(8-1000micron) = 10^10-10^12 Lsun. We divide our galaxies into those with Spitzer IRAC colors indicative of warm dust heated by an AGN (IRAGN) and those whose colors indicate star-formation processes (non-IRAGN). Compared to the non-IRAGN, the IRAGN show smaller PAH emission EWs, which we attribute to an increase in mid-IR continuum from the AGN. We find that in both the IRAGN and non-IRAGN samples, the PAH luminosities correlate strongly with the [Ne II] emission, from which we conclude that the PAH luminosity directly traces the instantaneous SFR in both the IRAGN and non-IRAGN galaxies. We compare the ratio of PAH luminosity to the total IR luminosity and show that for most IRAGN star-formation accounts for 10-50% of the L(IR). We also find no measurable difference between the PAH luminosity ratios of L(11.3)/L(7.7) and L(6.2)/L(7.7) for the IRAGN and non-IRAGN, suggesting that AGN do not significantly excite or destroy PAH molecules on galaxy-wide scales. A small subset of galaxies show excess of [O IV] emission compared to their PAH emission, which indicates the presence of heavily-obscured AGN, including 3 galaxies that are not otherwise selected as IRAGN. The low PAH emission and low [Ne II] emission of the IRAGN and [O IV]-excess objects imply they have low SFRs and their IR luminosity is dominated by processes associated with the AGN.

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arXiv2012

Luminous and High Stellar Mass Candidate Galaxies at z ~ 8 Discovered in CANDELS

One key goal of the Hubble Space Telescope Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey is to track galaxy evolution back to z ~ 8. Its two-tiered "wide and deep" strategy bridges significant gaps in existing near-infrared surveys. Here we report on z ~ 8 galaxy candidates selected as F105W-band dropouts in one of its deep fields, which covers 50.1 square arcmin to 4 ks depth in each of three near-infrared bands in the Great Observatories Origins Deep Survey southern field. Two of our candidates have J<26.2 mag, and are > 1 mag brighter than any previously known F105W-dropouts. We derive constraints on the bright-end of the rest-frame ultraviolet luminosity function of galaxies at z ~ 8, and show that the number density of such very bright objects is higher than expected from the previous Schechter luminosity function estimates at this redshift. Another two candidates are securely detected in Spitzer Infrared Array Camera images, which are the first such individual detections at z ~ 8. Their derived stellar masses are on the order of a few x 10^9 M_sun, from which we obtain the first measurement of the high-mass end of the galaxy stellar mass function at z ~ 8. The high number density of very luminous and very massive galaxies at z ~ 8, if real, could imply a large stellar-to-halo mass ratio and an efficient conversion of baryons to stars at such an early time.

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arXiv2012

The Massive Distant Clusters of WISE Survey: The First Distant Galaxy Cluster Discovered by WISE

We present spectroscopic confirmation of a z=0.99 galaxy cluster discovered using data from the Wide-field Infrared Survey Explorer (WISE). This is the first z~1 cluster candidate from the Massive Distant Clusters of WISE Survey (MaDCoWS) to be confirmed. It was selected as an overdensity of probable z>~1 sources using a combination of WISE and SDSS-DR8 photometric catalogs. Deeper follow-up imaging data from Subaru and WIYN reveal the cluster to be a rich system of galaxies, and multi-object spectroscopic observations from Keck confirm five cluster members at z=0.99. The detection and confirmation of this cluster represents a first step towards constructing a uniformly-selected sample of distant, high-mass galaxy clusters over the full extragalactic sky using WISE data.

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arXiv2012

CANDELS: The Evolution of Galaxy Rest-Frame Ultraviolet Colors from z = 8 to 4

We study the evolution of galaxy rest-frame ultraviolet (UV) colors in the epoch 4 < z < 8. We use new wide-field near-infrared data in GOODS-S from the CANDELS, HUDF09 and ERS programs to select galaxies via photometric redshift measurements. Our sample consists of 2812 candidate galaxies at z > 3.5, including 113 at z = 7 to 8. We fit the observed spectral energy distribution to a suite of synthetic stellar population models, and measure the value of the UV spectral slope (beta) from the best-fit model spectrum. The median value of beta evolves significantly from -1.82 (+0.00,-0.04) at z = 4, to -2.37 (+0.26,-0.06) at z = 7. Additionally, we find that faint galaxies at z = 7 have beta = -2.68 (+0.39,-0.24) (~ -2.4 after correcting for observational bias); this is redder than previous claims in the literature, and does not require "exotic" stellar populations to explain their colors. This evolution can be explained by an increase in dust extinction, with the timescale consistent with low-mass AGB stars forming the bulk of the dust. We find no significant (< 2-sigma) correlation between beta and M_UV when measuring M_UV at a consistent rest-frame wavelength of 1500 A. This is particularly true at bright magnitudes, though our results do show evidence for a weak correlation at faint magnitudes when galaxies in the HUDF are considered separately, hinting that dynamic range in sample luminosities may play a role. We do find a strong correlation between beta and the stellar mass at all redshifts, in that more massive galaxies exhibit redder colors. The most massive galaxies in our sample have red colors at each redshift, implying that dust can build up quickly in massive galaxies, and that feedback is likely removing dust from low-mass galaxies at z > 7. Thus the stellar-mass - metallicity relation, previously observed up to z ~ 3, may extend out to z = 7 - 8.

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arXiv2012

A Tale of Dwarfs and Giants: Using a z=1.62 Cluster to Understand How the Red Sequence Grew Over The Last 9.5 Billion Years

We study the red sequence in a cluster of galaxies at z=1.62 and follow its evolution over the intervening 9.5 Gyr to the present day. Using deep YJKs imaging with the HAWK-I instrument on the VLT we identify a tight red sequence and construct its rest-frame i-band luminosity function (LF). There is a marked deficit of faint red galaxies in the cluster that causes a turnover in the LF. We compare the red sequence LF to that for clusters at z<0.8 correcting the luminosities for passive evolution. The shape of the cluster red sequence LF does not evolve between z=1.62 and z=0.6 but at z<0.6 the faint population builds up significantly. Meanwhile, between z=1.62 to 0.6 the inferred total light on the red sequence grows by a factor of about 2 and the bright end of the LF becomes more populated. We construct a simple model for red sequence evolution that grows the red sequence in total luminosity and matches the constant LF shape at z>0.6. In this model the cluster accretes blue galaxies from the field that are then quenched and subsequently allowed to merge. We find that 3--4 mergers among cluster galaxies during the 4 Gyr between z=1.62 and z=0.6 matches the observed luminosity function evolution between the two redshifts. The inferred merger rate is consistent with other studies of this cluster. Our result supports the picture that galaxy merging during the major growth phase of massive clusters is an important process in shaping the red sequence population at all luminosities.

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arXiv2012

CANDELS: The Contribution of the Observed Galaxy Population to Cosmic Reionization

We present measurements of the specific ultraviolet luminosity density from a sample of 483 galaxies at 6 50%. Combining our observations with constraints on the emission rate of ionizing photons from Ly-alpha forest observations at z=6, we can constrain f_esc<34% (2-sigma) if the observed galaxies are the only contributors to reionization, or <13% (2-sigma) if the luminosity function extends to M_UV = -13. These escape fractions are sufficient to complete reionization by z=6. These constraints imply that the volume ionized fraction of the IGM becomes less than unity at z>7, consistent with a number of complementary reionization probes. If faint galaxies dominate reionization, future JWST observations will probe deep enough to see them, providing an indirect constraint on the ionizing photon escape fraction [abridged].

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arXiv2012

What turns galaxies off? The different morphologies of star-forming and quiescent galaxies since z~2 from CANDELS

We use HST/WFC3 imaging from the CANDELS Multicycle Treasury Survey, in conjunction with the Sloan Digital Sky Survey, to explore the evolution of galactic structure for galaxies with stellar masses >3e10M_sun from z=2.2 to the present epoch, a time span of 10Gyr. We explore the relationship between rest-frame optical color, stellar mass, star formation activity and galaxy structure. We confirm the dramatic increase from z=2.2 to the present day in the number density of non-star-forming galaxies above 3e10M_sun reported by others. We further find that the vast majority of these quiescent systems have concentrated light profiles, as parametrized by the Sersic index, and the population of concentrated galaxies grows similarly rapidly. We examine the joint distribution of star formation activity, Sersic index, stellar mass, inferred velocity dispersion, and stellar surface density. Quiescence correlates poorly with stellar mass at all z<2.2. Quiescence correlates well with Sersic index at all redshifts. Quiescence correlates well with `velocity dispersion' and stellar surface density at z>1.3, and somewhat less well at lower redshifts. Yet, there is significant scatter between quiescence and galaxy structure: while the vast majority of quiescent galaxies have prominent bulges, many of them have significant disks, and a number of bulge-dominated galaxies have significant star formation. Noting the rarity of quiescent galaxies without prominent bulges, we argue that a prominent bulge (and perhaps, by association, a supermassive black hole) is an important condition for quenching star formation on galactic scales over the last 10Gyr, in qualitative agreement with the AGN feedback paradigm.

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arXiv2012

How Do Star-Forming Galaxies at Z>3 Assemble Their Masses?

We investigate how star-forming galaxies typically assemble their masses at high redshift. Using the deep multi-wavelength coverage of the GOODS dataset, we measure stellar mass of a large sample of star-forming galaxies at z~4 and 5, and make a robust determination of stellar mass function (SMF). We report a broad correlation between stellar mass and UV luminosity, such that more UV-luminous galaxies are, on average, more massive. However, the correlation has a substantial intrinsic scatter evidenced by a non-negligible number of UV-faint but massive galaxies. Furthermore, the low-mass end of the SMF does not rise as steeply as the UV luminosity function (alpha_UVLF} -(1.7-1.8), alpha_SMF -(1.3-1.4)) of the same galaxies. In a smooth formation scenario where star formation (SF) is sustained at the observed rates for a long time, these galaxies would have accumulated more mass (by a factor of ~3) than observed and therefore the SMF would mirror more closely that of the UVLF. The relatively shallow slope of the SMF is due to the fact that many of the UV-selected galaxies are not massive enough, and therefore are too faint in their rest-frame optical bands, to be detected in the current observations. Our results favor a episodic formation history in which SF in low-mass galaxies vary significantly over time, a scenario favored by galaxy clustering. Our findings for the UV-faint galaxies are in contrast with those found for more UV-luminous galaxies, which exhibit tighter SFR-M_star correlations. The discrepancy may suggest that galaxies at different luminosities may have different evolutionary paths.

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arXiv2012

A large scale structure traced by [OII] emitters hosting a distant cluster at z=1.62

We present a panoramic narrow-band imaging survey of [OII] emitters in and around the ClG J0218.3-0510 cluster at z=1.62 with Suprime-Cam on Subaru telescope. 352 [OII] emitters were identified on the basis of narrow-band excesses and photometric redshifts. We discovered a huge filamentary structure with some clumps traced by [OII] emitters and found that the ClG J0218.3-0510 cluster is embedded in an even larger super-structure than the one reported previously. 31 [OII] emitters were spectroscopically confirmed with the detection of H-alpha and/or [OIII] emission lines by FMOS observations. In the high density regions such as cluster core and clumps, star-forming [OII] emitters show a high overdensity by a factor of more than 10 compared to the field region. Although the star formation activity is very high even in the cluster core, some massive quiescent galaxies also exits at the same time. Furthermore, the properties of the individual [OII] emitters, such as star formation rates, stellar masses and specific star formation rates, do not show a significant dependence on the local density, either. Such lack of environmental dependence is consistent with our earlier result by Hayashi et al. (2011) on a z=1.5 cluster and its surrounding region. The fact that the star-forming activity of galaxies in the cluster core is as high as that in the field at z~1.6 may suggest that the star-forming galaxies are probably just in a transition phase from a starburst mode to a quiescent mode, and are thus showing comparable level of star formation rates to those in lower density environments. We may be witnessing the start of the reversal of the local SFR--density relation due to the "biased" galaxy formation and evolution in high density regions at high this redshift, beyond which massive galaxies would be forming vigorously in a more biased way in proto-cluster cores.

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arXiv2012

First Results from Z-FOURGE: Discovery of a Candidate Cluster at z = 2.2 in COSMOS

We report the first results from the Z-FOURGE survey: the discovery of a candidate galaxy cluster at z = 2.2 consisting of two compact overdensities with red galaxies detected at >20σ above the mean surface density. The discovery was made possible by a new deep (Ks < 24.8 AB 5σ) Magellan/FourStar near-IR imaging survey with 5 custom medium-bandwidth filters. The filters pinpoint the location of the Balmer/4000Å break in evolved stellar populations at 1.5 < z < 3.5, yielding significantly more accurate photometric redshifts than possible with broadband imaging alone. The overdensities are within 1' of each other in the COSMOS field and appear to be embedded in a larger structure that contains at least one additional overdensity (\sim10σ). Considering the global properties of the overdensities, the z = 2.2 system appears to be the most distant example of a galaxy cluster with a population of red galaxies. A comparison to a large ΛCDM simulation suggests that the system may consist of merging subclusters, with properties in between those of z > 2 protoclusters with more diffuse distributions of blue galaxies and the lower-redshift galaxy clusters with prominent red sequences. The structure is completely absent in public optical catalogs in COSMOS and only weakly visible in a shallower near-IR survey. The discovery showcases the potential of deep near-IR surveys with medium-band filters to advance the understanding of environment and galaxy evolution at z > 1.5.

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arXiv2012

CANDELS Observations of the Structural Properties and Evolution of Galaxies in a Cluster at z=1.62

We discuss the structural and morphological properties of galaxies in a z=1.62 proto-cluster using near-IR imaging data from Hubble Space Telescope Wide Field Camera 3 data of the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS). The cluster galaxies exhibit a clear color-morphology relation: galaxies with colors of quiescent stellar populations generally have morphologies consistent with spheroids, and galaxies with colors consistent with ongoing star formation have disk-like and irregular morphologies. The size distribution of the quiescent cluster galaxies shows a deficit of compact (< 1kpc), massive galaxies compared to CANDELS field galaxies at z=1.6. As a result the cluster quiescent galaxies have larger average effective sizes compared to field galaxies at fixed mass at greater than 90% significance. Combined with data from the literature, the size evolution of quiescent cluster galaxies is relatively slow from z~1.6 to the present, growing as (1+z)^(-0.6+/-0.1). If this result is generalizable, then it implies that physical processes associated with the denser cluster region seems to have caused accelerated size growth in quiescent galaxies prior to z=1.6 and slower subsequent growth at z<1.6 compared to galaxies in the lower density field. The quiescent cluster galaxies at z=1.6 have higher ellipticities compared to lower redshift samples at fixed mass, and their surface-brightness profiles suggest that they contain extended stellar disks. We argue the cluster galaxies require dissipationless (i.e., gas-poor or "dry") mergers to reorganize the disk material and to match the relations for ellipticity, stellar mass, size, and color of early-type galaxies in z<1 clusters.

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arXiv2011

AKARI Observation of the North Ecliptic Pole (NEP) Supercluster at z = 0.087: mid-infrared view of transition galaxies

We present the mid-infrared (MIR) properties of galaxies within a supercluster in the North Ecliptic Pole region at z?0.087 observed with the AKARI satellite. We use data from the AKARI NEP-Wide (5.4 deg2) IR survey and the CLusters of galaxies EVoLution studies (CLEVL) mission program. We show that near-IR (3 μm)-mid- IR (11 μm) color can be used as an indicator of the specific star formation rate and the presence of intermediate age stellar populations. From the MIR observations, we find that red-sequence galaxies consist not only of passively evolving red early-type galaxies, but also of 1) "weak-SFG" (disk-dominated star-forming galaxies which have star formation rates lower by \sim 4 \times than blue-cloud galaxies), and 2) "intermediate- MXG" (bulge-dominated galaxies showing stronger MIR dust emission than normal red early-type galaxies). Those two populations can be a set of transition galaxies from blue, star-forming, late-type galaxies evolving into red, quiescent, early-type ones. We find that the weak-SFG are predominant at intermediate masses (1010M\odot < M\star < 1010.5M\odot) and are typically found in local densities similar to the outskirts of galaxy clusters. As much as 40% of the supercluster member galaxies in this mass range can be classified as weak-SFGs, but their proportion decreases to < 10% at larger masses (M\star > 1010.5 M\odot) at any galaxy density. The fraction of the intermediate-MXG among red- sequence galaxies at 1010M\odot < M\star < 1011M\odot also decreases as the density and mass increase. In particular, \sim42% of the red-sequence galaxies with early-type morphologies are classified as intermediate-MXG at intermediate densities. These results suggest that the star formation activity is strongly dependent on the stellar mass, but that the morphological transformation is mainly controlled by the environment.

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arXiv2011

Probing the Star Formation History and Initial Mass Function of the z~2.5 Lensed Galaxy SMM J163554.2+661225 with Herschel

We present the analysis of Herschel SPIRE far-infrared (FIR) observations of the z = 2.515 lensed galaxy SMM J163554.2+661225. Combining new 250, 350, and 500 micron observations with existing data, we make an improved fit to the FIR spectral energy distribution (SED) of this galaxy. We find a total infrared (IR) luminosity of L(8--1000 micron) = 6.9 +/- 0.6x10^11 Lsol; a factor of 3 more precise over previous L_IR estimates for this galaxy, and one of the most accurate measurements for any galaxy at these redshifts. This FIR luminosity implies an unlensed star formation rate (SFR) for this galaxy of 119 +/- 10 Msol per yr, which is a factor of 1.9 +/- 0.35 lower than the SFR derived from the nebular Pa-alpha emission line (a 2.5-sigma discrepancy). Both SFR indicators assume identical Salpeter initial mass functions (IMF) with slope Gamma=2.35 over a mass range of 0.1 - 100 Msol, thus this discrepancy suggests that more ionizing photons may be necessary to account for the higher Pa-alpha-derived SFR. We examine a number of scenarios and find that the observations can be explained with a varying star formation history (SFH) due to an increasing star formation rate (SFR), paired with a slight flattening of the IMF. If the SFR is constant in time, then larger changes need to be made to the IMF by either increasing the upper-mass cutoff to ~ 200 Msol, or a flattening of the IMF slope to 1.9 +/- 0.15, or a combination of the two. These scenarios result in up to double the number of stars with masses above 20 Msol, which produce the requisite increase in ionizing photons over a Salpeter IMF with a constant SFH.

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arXiv2011

Color and Stellar Population Gradients in Passively Evolving Galaxies at z~2 from HST/WFC3 Deep Imaging in the Hubble Ultra Deep Field

We report the detection of color gradients in six massive (stellar mass > 10^{10} M_{sun}) and passively evolving (specific SFR < 10^{-11}/yr) galaxies at redshift 1.3<z<2.5 identified in the HUDF using HST ACS and WFC3/IR images. After matching different PSFs, we obtain color maps and multi-band optical/near-IR photometry (BVizYJH) in concentric annuli, from the smallest resolved radial (~1.7 kpc) up to several times the H-band effective radius. We find that the inner regions of these galaxies have redder rest-frame UV--optical colors than the outer parts. The slopes of the color gradients mildly depend on the overall dust obscuration and rest-frame (U-V) color, with more obscured or redder galaxies having steeper color gradients. The z~2 color gradients are also steeper than those of local early-types. The gradient of a single parameter (age, extinction or metallicity) cannot fully explain the observed color gradients. Fitting spatially resolved HST seven-band photometry to stellar population synthesis models, we find that, regardless of assumptions for metallicity gradient, the redder inner regions of the galaxies have slightly higher dust obscuration than the bluer outer regions, although the magnitude depends on the assumed extinction law. The derived age gradient depends on the assumptions for metallicity gradient. We discuss the implications of a number of assumptions for metallicity gradient on the formation and evolution of these galaxies. We find that the evolution of the mass--size relationship from z~2 to z~0 cannot be driven by in--situ extended star formation, implying that accretion or merger is mostly responsible for the evolution. The lack of a correlation between color gradient and stellar mass argues against the metallicity gradient predicted by the monolithic collapse, which would require significant major mergers to evolve into the one observed at z~0. (Abridged)

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arXiv2011

The mid-IR Luminosity Function at z<0.3 from 5MUSES: Understanding the Star-formation/AGN Balance from a Spectroscopic View

We present rest-frame 15 and 24 um luminosity functions and the corresponding star-forming luminosity functions at z<0.3 derived from the 5MUSES sample. Spectroscopic redshifts have been obtained for ~98% of the objects and the median redshift is ~0.12. The 5-35 um IRS spectra allow us to estimate accurately the luminosities and build the luminosity functions. Using a combination of starburst and quasar templates, we quantify the star-formation and AGN contributions in the mid-IR SED. We then compute the star-formation luminosity functions at 15 um and 24 um, and compare with the total 15 um and 24 um luminosity functions. When we remove the contribution of AGN, the bright end of the luminosity function exhibits a strong decline, consistent with the exponential cutoff of a Schechter function. Integrating the differential luminosity function, we find that the fractional contribution by star formation to the energy density is 58% at 15 um and 78% at 24 um, while it goes up to ~86% when we extrapolate our mid-IR results to the total IR luminosity density. We confirm that the active galactic nuclei play more important roles energetically at high luminosities. Finally, we compare our results with work at z~0.7 and confirm that evolution on both luminosity and density is required to explain the difference in the LFs at different redshifts.

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