arXiv subjects

Ryan F. Quadri

Publications and source records attributed to Ryan F. Quadri.

34 records · Page 2Linked to original sources

arXiv2014

Observations of environmental quenching in groups in the 11 Gyr since z=2.5: different quenching for central and satellite galaxies

We present direct observational evidence for star formation quenching in galaxy groups in the redshift range 0<z<2.5. We utilize a large sample of nearly 6000 groups, selected by fixed cumulative number density from three photometric catalogs, to follow the evolving quiescent fractions of central and satellite galaxies over roughly 11 Gyr. At z~0, central galaxies in our sample range in stellar mass from Milky Way/M31 analogs (M=6.5x10^10 M\solar) to nearby massive ellipticals (M=1.5x10^11 M\solar). Satellite galaxies in the same groups reach masses as low as twice that of the Large Magellanic Cloud (M=6.5x10^9 M\solar). Using statistical background subtraction, we measure the average rest-frame colors of galaxies in our groups and calculate the evolving quiescent fractions of centrals and satellites over seven redshift bins. Our analysis shows clear evidence for star formation quenching in group halos, with a different quenching onset for centrals and their satellite galaxies. Using halo mass estimates for our central galaxies, we find that star formation shuts off in centrals when typical halo masses reach between 10^12 and 10^13 M\solar, consistent with predictions from the halo quenching model. In contrast, satellite galaxies in the same groups most likely undergo quenching by environmental processes, whose onset is delayed with respect to their central galaxy. Although star formation is suppressed in all galaxies over time, the processes that govern quenching are different for centrals and satellites. While mass plays an important role in determining the star formation activity of central galaxies, quenching in satellite galaxies is dominated by the environment in which they reside.

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arXiv2013

The Structural Evolution of Milky Way-like Star Forming Galaxies since z~1.3

We follow the structural evolution of star forming galaxies (SFGs) like the Milky Way by selecting progenitors to z~1.3 based on the stellar mass growth inferred from the evolution of the star forming sequence. We select our sample from the 3D-HST survey, which utilizes spectroscopy from the HST WFC3 G141 near-IR grism and enables precise redshift measurements for our sample of SFGs. Structural properties are obtained from Sersic profile fits to CANDELS WFC3 imaging. The progenitors of z=0 SFGs with stellar mass M=10^{10.5} Msun are typically half as massive at z~1. This late-time stellar mass assembly is consistent with recent studies that employ abundance matching techniques. The descendant SFGs at z~0 have grown in half-light radius by a factor of ~1.4 since z~1. The half-light radius grows with stellar mass as r_e M^{0.29}. While most of the stellar mass is clearly assembling at large radii, the mass surface density profiles reveal ongoing mass growth also in the central regions where bulges and pseudobulges are common features in present day late-type galaxies. Some portion of this growth in the central regions is due to star formation as recent observations of H-alpha maps for SFGs at z~1 are found to be extended but centrally peaked. Connecting our lookback study with galactic archeology, we find the stellar mass surface density at R=8 kpc to have increased by a factor of ~2 since z~1, in good agreement with measurements derived for the solar neighborhood of the Milky Way.

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arXiv2013

Stellar Kinematics of z~2 Galaxies and the Inside-Out Growth of Quiescent Galaxies

Using stellar kinematics measurements, we investigate the growth of massive, quiescent galaxies from z~2 to today. We present X-Shooter spectra from the UV to NIR and dynamical mass measurements of 5 quiescent massive (>10^11 Msun) galaxies at z~2. This triples the sample of z>1.5 galaxies with well constrained (dsigma <100 km/s) velocity dispersion measurements. From spectral population synthesis modeling we find that these galaxies have stellar ages that range from 0.5-2 Gyr, with no signs of ongoing star formation. We measure velocity dispersions (290-450 km/s) from absorption lines and find that they are 1.6-2.1 times higher than those of galaxies in the SDSS at the same mass. Sizes are measured using GALFIT from HST-WFC3 H160 and UDS K-band images. The dynamical masses correspond well to the SED based stellar masses, with dynamical masses that are ~15% higher. We find that M_*/M_dyn may decrease slightly with time, which could reflect the increase of the dark matter fraction within an increasing effective radius. We combine different stellar kinematic studies from the literature, and examine the structural evolution from z~2 to z~0: we confirm that at fixed dynamical mass, the effective radius increases by a factor of ~2.8, and the velocity dispersion decreases by a factor of ~1.7. The mass density within one effective radius decreases by a factor of ~20, while within a fixed physical radius (1 kpc) it decreases only mildly (~2). When we allow for an evolving mass limit by selecting a population of galaxies at fixed number density, a stronger size growth with time is found (~4), velocity dispersion decreases by a factor of ~1.4, and interestingly, the mass density within 1 kpc is consistent with no evolution. This finding suggests that massive quiescent galaxies at ~2 grow inside-out, consistent with the expectations from minor mergers.

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arXiv2013

HST/WFC3 Confirmation of the Inside-Out Growth of Massive Galaxies at 0<z<2 and Identification of their Star Forming Progenitors at z~3

We study the structural evolution of massive galaxies by linking progenitors and descendants at a constant cumulative number density of n_c=1.4x10^{-4} Mpc^{-3} to z~3. Structural parameters were measured by fitting Sersic profiles to high resolution CANDELS HST WFC3 J_{125} and H_{160} imaging in the UKIDSS-UDS at 1<z<3 and ACS I_{814} imaging in COSMOS at 0.25<z<1. At a given redshift, we selected the HST band that most closely samples a common rest-frame wavelength so as to minimize systematics from color gradients in galaxies. At fixed n_c, galaxies grow in stellar mass by a factor of ~3 from z~3 to z~0. The size evolution is complex: galaxies appear roughly constant in size from z~3 to z~2 and then grow rapidly to lower redshifts. The evolution in the surface mass density profiles indicates that most of the mass at r<2 kpc was in place by z~2, and that most of the new mass growth occurred at larger radii. This inside-out mass growth is therefore responsible for the larger sizes and higher Sersic indices of the descendants toward low redshift. At z<2, the effective radius evolves with the stellar mass as r_e M^{2.0}, consistent with scenarios that find dissipationless minor mergers to be a key driver of size evolution. The progenitors at z~3 were likely star-forming disks with r_e~2 kpc, based on their low Sersic index of n~1, low median axis ratio of b/a~0.52, and typical location in the star-forming region of the U-V versus V-J diagram. By z~1.5, many of these star-forming disks disappeared, giving rise to compact quiescent galaxies. Toward lower redshifts, these galaxies continued to assemble mass at larger radii and became the local ellipticals that dominate the high mass end of the mass function at the present epoch.

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arXiv2012

The Carnegie-Spitzer-IMACS Redshift Survey of Galaxy Evolution since z=1.5: I. Description and Methodology

We describe the Carnegie-Spitzer-IMACS (CSI) Survey, a wide-field, near-IR selected spectrophotometric redshift survey with the Inamori Magellan Areal Camera and Spectrograph (IMACS) on Magellan-Baade. By defining a flux-limited sample of galaxies in Spitzer 3.6micron imaging of SWIRE fields, the CSI Survey efficiently traces the stellar mass of average galaxies to z~1.5. This first paper provides an overview of the survey selection, observations, processing of the photometry and spectrophotometry. We also describe the processing of the data: new methods of fitting synthetic templates of spectral energy distributions are used to derive redshifts, stellar masses, emission line luminosities, and coarse information on recent star-formation. Our unique methodology for analyzing low-dispersion spectra taken with multilayer prisms in IMACS, combined with panchromatic photometry from the ultraviolet to the IR, has yielded 37,000 high quality redshifts in our first 5.3 sq.degs of the SWIRE XMM-LSS field. We use three different approaches to estimate our redshift errors and find robust agreement. Over the full range of 3.6micron fluxes of our selection, we find typical uncertainties of sigma_z/(1+z) < 0.015. In comparisons with previously published VVDS redshifts, for example, we find a scatter of sigma_z/(1+z) = 0.012 for galaxies at 0.8< z< 1.2. For galaxies brighter and fainter than i=23 mag, we find sigma_z/(1+z) = 0.009 and sigma_z/(1+z) = 0.025, respectively. Notably, our low-dispersion spectroscopy and analysis yields comparable redshift uncertainties and success rates for both red and blue galaxies, largely eliminating color-based systematics that can seriously bias observed dependencies of galaxy evolution on environment.

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arXiv2011

The diminishing importance of major galaxy mergers at higher redshifts

Using mass-selected galaxy samples from deep multiwavelength data we investigate the incidence of close galaxy pairs between z=0.4-2. Many such close pairs will eventually merge, and the pair fraction is therefore related to the merger rate. Over this redshift range the mean pair fraction is essentially constant (evolving as f_pair (1+z)^{-0.4 +/- 0.6}) with about 6+/-1% of massive galaxies having a 1:4 or greater companion within 30h^-1 kpc. Assuming the timescale over which pairs merge is not a strong function of redshift, this implies a similarly constant merger rate (per unit time) out to z=2. Since about three times as much cosmic time passes at z<1 as between z=1-2, this implies that correspondingly more mergers occur in the low-redshift universe. When minor companions (1:10 mass ratio or greater) are included, the pair fraction increases to ~20% and still does not evolve strongly with redshift. We also use a rest-frame color criterion to select pairs containing only quiescent galaxies (major "dry merger" progenitors), and find them to be similarly rare and constant with 4-7% of massive quiescent galaxies exhibiting a nearby companion. Thus, even though other studies find major mergers to be relatively uncommon since z=1, our results suggest that few additional mergers occur in the 1<z<2 range and other mechanisms may be required to explain the mass and size growth of galaxies over this epoch.

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arXiv2011

Redshift Evolution of the Galaxy Velocity Dispersion Function

We present a study of the evolution of the galaxy Velocity Dispersion Function (VDF) from z=0 to z=1.5 using photometric data from the UKIDSS Ultra Deep Survey (UDS) and Newfirm Medium Band Survey (NMBS) COSMOS surveys. The VDF has been measured locally using direct kinematic measurements from the Sloan Digital Sky Survey, but direct studies of the VDF at high redshift are difficult as they require velocity dispersion measurements of many thousands of galaxies. Taylor et al. (2010) demonstrated that dynamical and stellar mass are linearly related when the structure of the galaxy is accounted for. We show that the stellar mass, size and Sérsic index can reliably predict the velocity dispersions of SDSS galaxies. We apply this relation to galaxies at high redshift and determine the evolution of the inferred VDF. We find that the VDF at z~0.5 is very similar to the VDF at z=0. At higher redshifts, we find that the number density of galaxies with dispersions <~200 km/s is lower, but the number of high dispersion galaxies is constant or even higher. At fixed cumulative number density, the velocity dispersions of galaxies with logN[Mpc^{-3}]<-3.5 increase with time by a factor of ~1.4 from z~1.5-0, whereas the dispersions of galaxies with lower number density are approximately constant or decrease with time. The VDF appears to show less evolution than the stellar mass function, particularly at the lowest number densities. We note that these results are still somewhat uncertain and we suggest several avenues for further calibrating the inferred velocity dispersions.

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arXiv2011

The NEWFIRM Medium-band Survey: Photometric Catalogs, Redshifts and the Bimodal Color Distribution of Galaxies out to z~3

We present deep near-infrared (NIR) medium-bandwidth photometry over the wavelength range 1-1.8 microns in the All-wavelength Extended Groth strip International Survey (AEGIS) and Cosmic Evolution Survey (COSMOS) fields. The observations were carried out as part of the NEWFIRM Medium-Band Survey (NMBS), an NOAO survey program on the Mayall 4m telescope on Kitt Peak using the NOAO Extremely Wide-Field Infrared Imager (NEWFIRM). In this paper, we describe the full details of the observations, data reduction and photometry for the survey. We also present a public K-selected photometric catalog, along with accurate photometric redshifts. The redshifts are computed with 37 (20) filters in the COSMOS (AEGIS) fields, combining the NIR medium-bandwidth data with existing ultraviolet (UV; Galaxy Evolution Explorer), visible and NIR (Canada-France-Hawaii Telescope and Subaru) and mid-IR (Spitzer/IRAC) imaging. We find excellent agreement with publicly available spectroscopic redshifts, with sigma_z/(1+z)~1-2% for ~4000 galaxies at z=0-3. The NMBS catalogs contain ~13,000 galaxies at z>1.5 with accurate photometric redshifts and rest-frame colors. Due to the increased spectral resolution obtained with the five NIR medium-band filters, the median 68% confidence intervals of the photometric redshifts of both quiescent and star-forming galaxies are a factor of ~2 times smaller when comparing catalogs with medium-band NIR photometry to NIR broadband photometry. We show evidence for a clear bimodal color distribution between quiescent and star-forming galaxies that persists to z~3, a higher redshift than has been probed so far.

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arXiv2011

The number density and mass density of star-forming and quiescent galaxies at 0.4 < z < 2.2

We study the build-up of the bimodal galaxy population using the NEWFIRM Medium-Band Survey, which provides excellent redshifts and well-sampled spectral energy distributions of ~27,000 galaxies with K<22.8 at 0.4 < z < 2.2. We first show that star-forming galaxies and quiescent galaxies can be robustly separated with a two-color criterion over this entire redshift range. We then study the evolution of the number density and mass density of quiescent and star-forming galaxies, extending the results of the COMBO-17, DEEP2, and other surveys to z=2.2. The mass density of quiescent galaxies with M > 3 10^11 solar masses increases by a factor of ~10 from z=2 to the present day, whereas the mass density in star-forming galaxies is flat or decreases over the same time period. Modest mass growth by a factor of 2 of individual quiescent galaxies can explain roughly half of the strong density evolution at M>10^11 solar masses, due to the steepness of the exponential tail of the mass function. The rest of the density evolution of massive, quiescent galaxies is likely due to transformation (e.g. quenching) of the massive star-forming population, a conclusion which is consistent with the density evolution we observe for the star-forming galaxies themselves, which is flat or decreasing with cosmic time. Modest mass growth does not explain the evolution of less massive quiescent galaxies (~10^10.5 solar masses), which show a similarly steep increase in their number densities. The less massive quiescent galaxies are therefore continuously formed by transforming galaxies from the star-forming population

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arXiv2011

Tracing the Star Formation-Density Relation to z~2

Recent work has shown that the star formation-density relation -- in which galaxies with low star formation rates are preferentially found in dense environments -- is still in place at z~1, but the situation becomes less clear at higher redshifts. We use mass-selected samples drawn from the UKIDSS Ultra-Deep Survey to show that galaxies with quenched star formation tend to reside in dense environments out to at least z~1.8. Over most of this redshift range we are able to demonstrate that this star formation-density relation holds even at fixed stellar mass. The environmental quenching of star formation appears to operate with similar efficiency on all galaxies regardless of stellar mass. Nevertheless, the environment plays a greater role in the build-up of the red sequence at lower masses, whereas other quenching processes dominate at higher masses. In addition to a statistical analysis of environmental densities, we investigate a cluster at z=1.6, and show that the central region has an elevated fraction of quiescent objects relative to the field. Although the uncertainties are large, the environmental quenching efficiency in this cluster is consistent with that of galaxy groups and clusters at z~0. In this work we rely on photometric redshifts, and describe some of the pitfalls that large redshift errors can present.

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arXiv2010

The Most Massive Galaxies at 3.0<z<4.0 in the NEWFIRM Medium-Band Survey: Properties and Improved Constraints on the Stellar Mass Function

[Abridged] We use the NEWFIRM Medium-Band Survey (NMBS) to characterize the properties of a mass-complete sample of 14 galaxies at 3.0 2.5x10^11 Msun, and to derive more accurate measurements of the high-mass end of the stellar mass function (SMF) of galaxies at z=3.5, with significantly reduced contributions from photometric redshift errors and cosmic variance to the total error budget of the SMF. The typical very massive galaxy at z=3.5 is red and faint in the observer's optical, with median r=26.1, and rest-frame U-V=1.6. About 60% of the sample have optical colors satisfying either the U- or the B-dropout color criteria, although ~50% of these galaxies have r>25.5. About 30% of the sample has SFRs from SED modeling consistent with zero. However, >80% of the sample is detected at 24 micron, with total infrared luminosities in the range (0.5-4.0)x10^13 Lsun. This implies the presence of either dust-enshrouded starburst activity (with SFRs of 600-4300 Msun/yr) and/or highly-obscured active galactic nuclei (AGN). The contribution of galaxies with M_star>2.5x10^11 Msun to the total stellar mass budget at z=3.5 is ~8%. We find an evolution by a factor of 2-7 and 3-22 from z~5 and z~6, respectively, to z=3.5. The previously found disagreement at the high-mass end between observed and model-predicted SMFs is now significant at the 3sigma level. However, systematic uncertainties dominate the total error budget, with errors up to a factor of ~8 in the densities, bringing the observed SMF in marginal agreement with the predicted SMF. Additional systematic uncertainties on the high-mass end could be introduced by either 1) the intense star-formation and/or the very common AGN activities as inferred from the MIPS 24 micron detections, and/or 2) contamination by a significant population of massive, old, and dusty galaxies at z~2.6.

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arXiv2010

The evolving relations between size, mass, surface density, and star formation in 3x10^4 galaxies since z=2

The presence of massive, compact, quiescent galaxies at z>2 presents a major challenge for theoretical models of galaxy formation and evolution. Using one of the deepest large public near-IR surveys to date, we investigate in detail the correlations between star formation and galaxy structural parameters (size, stellar mass, and surface density) from z=2 to the present. At all redshifts, massive quiescent galaxies (i.e. those with little or no star formation) occupy the extreme high end of the surface density distribution and follow a tight mass-size correlation, while star-forming galaxies show a broad range of both densities and sizes. Conversely, galaxies with the highest surface densities comprise a nearly-homogeneous population with little or no ongoing star formation, while less dense galaxies exhibit high star-formation rates and varying levels of dust obscuration. Both the sizes and surface densities of quiescent galaxies evolve strongly from z=2-0; we parameterize this evolution for both populations with simple power law functions and present best-fit parameters for comparison to future theoretical models. Higher-mass quiescent galaxies undergo faster structural evolution, consistent with previous results. Interestingly, star-forming galaxies' sizes and densities evolve at rates similar to those of quiescent galaxies. It is therefore possible that the same physical processes drive the structural evolution of both populations, suggesting that "dry mergers" may not be the sole culprit in this size evolution.

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arXiv2009

Quantifying Photometric Redshift Errors in the Absence of Spectroscopic Redshifts

Much of the science that is made possible by multiwavelength redshift surveys requires the use of photometric redshifts. But as these surveys become more ambitious, and as we seek to perform increasingly accurate measurements, it becomes crucial to take proper account of the photometric redshift uncertainties. Ideally the uncertainties can be directly measured using a comparison to spectroscopic redshifts, but this may yield misleading results since spectroscopic samples are frequently small and not representative of the parent photometric samples. We present a simple and powerful empirical method to constrain photometric redshift uncertainties in the absence of spectroscopic redshifts. Close pairs of galaxies on the sky have a significant probability of being physically associated, and therefore of lying at nearly the same redshift. The difference in photometric redshifts in close pairs is therefore a measure of the redshift uncertainty. Some observed close pairs will arise from chance projections along the line of sight, but it is straightforward to perform a statistical correction for this effect. We demonstrate the technique using both simulated data and actual observations, and discuss how its usefulness can be limited by the presence of systematic photometric redshift errors. Finally, we use this technique to show how photometric redshift accuracy can depend on galaxy type.

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arXiv2009

An ultra-deep near-infrared spectrum of a compact quiescent galaxy at z=2.2

Several recent studies have shown that about half of the massive galaxies at z~2 are in a quiescent phase. Moreover, these galaxies are commonly found to be ultra-compact with half-light radii of ~1 kpc. We have obtained a ~29 hr spectrum of a typical quiescent, ultra-dense galaxy at z=2.1865 with the Gemini Near-Infrared Spectrograph. The spectrum exhibits a strong optical break and several absorption features, which have not previously been detected in z>2 quiescent galaxies. Comparison of the spectral energy distribution with stellar population synthesis models implies a low star formation rate (SFR) of 1-3 Msol/yr, an age of 1.3-2.2 Gyr, and a stellar mass of ~2x10^11 Msol. We detect several faint emission lines, with emission-line ratios of [NII]/Halpha, [SII]/Halpha and [OII]/[OIII] typical of low-ionization nuclear emission-line regions. Thus, neither the stellar continuum nor the nebular emission implies active star formation. The current SFR is <1% of the past average SFR. If this galaxy is representative of compact quiescent galaxies beyond z=2, it implies that quenching of star formation is extremely efficient and also indicates that low luminosity active galactic nuclei (AGNs) could be common in these objects. Nuclear emission is a potential concern for the size measurement. However, we show that the AGN contributes <8% to the rest-frame optical emission. A possible post-starburst population may affect size measurements more strongly; although a 0.5 Gyr old stellar population can make up <10% of the total stellar mass, it could account for up to ~40% of the optical light. Nevertheless, this spectrum shows that this compact galaxy is dominated by an evolved stellar population.

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arXiv2008

Detection of quiescent galaxies in a bicolor sequence from z=0-2

We investigate the properties of quiescent and star-forming galaxy populations to z~2 with purely photometric data, employing a novel rest-frame color selection technique. From the UKIDSS Ultra-Deep Survey Data Release 1, with matched optical and mid-IR photometry taken from the Subaru XMM Deep Survey and Spitzer Wide-Area Infrared Extragalactic Survey respectively, we construct a K-selected galaxy catalog and calculate photometric redshifts. Excluding stars, objects with uncertain z_phot solutions, those that fall in bad or incomplete survey regions, and those for which reliable rest-frame colors could not be derived, 30108 galaxies with K<22.4 (AB) and z<2.5 remain. The galaxies in this sample are found to occupy two distinct populations in the rest-frame U-V vs. V-J color space: a clump of red, quiescent galaxies (analogous to the red sequence) and a track of star-forming galaxies extending from blue to red U-V colors. This bimodal behavior is seen up to z~2. Due to a combination of measurement errors and passive evolution, the color-color diagram is not suitable to distinguish the galaxy bimodality at z>2 for this sample, but we show that MIPS 24um data suggest that a significant population of quiescent galaxies exists even at these higher redshifts. At z=1-2, the most luminous objects in the sample are divided roughly equally between star-forming and quiescent galaxies, while at lower redshifts most of the brightest galaxies are quiescent. Moreover, quiescent galaxies at these redshifts are clustered more strongly than those actively forming stars, indicating that galaxies with early-quenched star formation may occupy more massive host dark matter halos. This suggests that the end of star formation is associated with, and perhaps brought about by, a mechanism related to halo mass.

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arXiv2008

A Confirmation of the Strong Clustering of Distant Red Galaxies at 2 < z <3

Recent studies have shown that distant red galaxies (DRGs), which dominate the high-mass end of the galaxy population at z~2.5, are more strongly clustered than the population of blue star-forming galaxies at similar redshifts. However these studies have been severely hampered by the small sizes of fields having deep near-infrared imaging. Here we use the large UKIDSS Ultra Deep Survey to study the clustering of DRGs. The size and depth of this survey allows for an unprecedented measurement of the angular clustering of DRGs at 2<z_phot<3 and K<21. The correlation function shows the expected power law behavior, but with an apparent upturn at theta<~10". We deproject the angular clustering to infer the spatial correlation length, finding 10.6+-1.6 h^-1 Mpc. We use the halo occupation distribution framework to demonstrate that the observed strong clustering of DRGs is not consistent with standard models of galaxy clustering, confirming previous suggestions that were based on smaller samples. Inaccurate photometric redshifts could artificially enhance the observed clustering, however significant systematic redshift errors would be required to bring the measurements into agreement with the models. Another possibility is that the underlying assumption that galaxies interact with their large-scale environment only through halo mass is not valid, and that other factors drive the evolution of the oldest, most massive galaxies at z~2.

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