arXiv subjects

Rachel Bezanson

Publications and source records attributed to Rachel Bezanson.

At least 181 records · Page 10Linked to original sources

arXiv2020

ALMA measures rapidly depleted molecular gas reservoirs in massive quiescent galaxies at z~1.5

We present ALMA CO(2-1) spectroscopy of 6 massive (log$_{10}$M$_{\rm{*}}/\rm{M}_\odot>$11.3) quiescent galaxies at $z\sim1.5$. These data represent the largest sample using CO emission to trace molecular gas in quiescent galaxies above $z>1$, achieving an average 3$σ$ sensitivity of M$_{\rm{H_{2}}}\sim10^{10}\rm{M}_\odot$. We detect one galaxy at 4$σ$ significance and place upper limits on the molecular gas reservoirs of the other 5, finding molecular gas mass fractions M$_{\rm{H_{2}}}$/M$_{\rm{*}}$=f$_{\rm{H_{2}}}<2-6$% (3$σ$ upper limits). This is 1-2 orders of magnitude lower than coeval star-forming galaxies at similar stellar mass, and comparable to galaxies at $z=0$ with similarly low sSFR. This indicates that their molecular gas reservoirs were rapidly and efficiently used up or destroyed, and that gas fractions are uniformly low ($<$6%) despite the structural diversity of our sample. The implied rapid depletion time of molecular gas (t$_{\rm{dep}}<0.6$ Gyr) disagrees with extrapolations of empirical scaling relations to low sSFR. We find that our low gas fractions are instead in agreement with predictions from both the recent SIMBA cosmological simulation, and from analytical "bathtub" models for gas accretion onto galaxies in massive dark matter halos (log$_{10}M_{\rm{halo}}/\rm{M}_\odot\sim14$ at $z=0$). Such high mass halos reach a critical mass of log$_{10}M_{\rm{halo}}/\rm{M}_\odot>12$ by $z\sim4$ that halt the accretion of baryons early in the Universe. Our data is consistent with a simple picture where galaxies truncate accretion and then consume the existing gas at or faster than typical main sequence rates. Alternatively, we cannot rule out that these galaxies reside in lower mass halos, and low gas fractions may instead reflect either stronger feedback, or more efficient gas consumption.

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arXiv2020

Tightly coupled morpho-kinematic evolution for massive star-forming and quiescent galaxies across 7 Gyr of cosmic time

We use the Fundamental Plane (FP) to measure the redshift evolution of the dynamical mass-to-light ratio ($M_{\mathrm{dyn}}/L$) and the dynamical-to-stellar mass ratio ($M_{\mathrm{dyn}}/M_*$). Although conventionally used to study the properties of early-type galaxies, we here obtain stellar kinematic measurements from the Large Early Galaxy Astrophysics Census (LEGA-C) Survey for a sample of $\sim1400$ massive ($\log( M_*/M_\odot) >10.5$) galaxies at $0.6<z<1.0$ that span a wide range in star formation activity. In line with previous studies, we find a strong evolution in $M_{\mathrm{dyn}}/L_g$ with redshift. In contrast, we find only a weak dependence of the mean value of $M_{\mathrm{dyn}}/M_*$ on the specific star formation rate, and a redshift evolution that likely is explained by systematics. Therefore, we demonstrate that star-forming and quiescent galaxies lie on the same, stable mass FP across $0<z<1$, and that the decrease in $M_{\mathrm{dyn}}/L_g$ toward high redshift can be attributed entirely to evolution of the stellar populations. Moreover, we show that the growth of galaxies in size and mass is constrained to occur within the mass FP. Our results imply either minimal structural evolution in massive galaxies since $z\sim1$, or a tight coupling in the evolution of their morphological and dynamical properties, and establish the mass FP as a tool for studying galaxy evolution with low impact from progenitor bias.

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arXiv2020

SQuIGG$\vec{L}$E Survey: Massive z$\sim$0.6 Post-Starburst Galaxies Exhibit Flat Age Gradients

We present Gemini GMOS IFU observations of six massive ($M_\star\geq10^{11} \ M_\odot$) A-star dominated post-starburst galaxies at $z\sim0.6$. These galaxies are a subsample of the SQuIGG$\vec{L}$E Survey, which selects intermediate-redshift post-starbursts from the Sloan Digital Sky Survey spectroscopic sample (DR14) with spectral shapes that indicate they have recently shut off their primary epoch of star formation. Using $Hδ_A$ absorption as a proxy for stellar age, we constrain five of the galaxies to have young ($\sim 600$ Myr) light-weighted ages at all radii and find that the sample on average has flat age gradients. We examine the spatial distribution of mass-weighted properties by fitting our profiles with a toy model including a young, centrally concentrated burst superimposed on an older, extended population. We find that galaxies with flat $Hδ_A$ profiles are inconsistent with formation via a central secondary starburst. This implies that the mechanism responsible for shutting off this dominant episode of star formation must have done so uniformly throughout the galaxy.

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arXiv2020

Dust Attenuation Curves at z $\sim$ 0.8 from LEGA-C: Precise Constraints on the Slope and 2175$Å$ Bump Strength

We present a novel approach to measure the attenuation curves of 485 individual star-forming galaxies with M$_*$ $>$ 10$^{10}$ M$_{\odot}$ based on deep optical spectra from the VLT/VIMOS LEGA-C survey and multi-band photometry in the COSMOS field. Most importantly, we find that the attenuation curves in the rest-frame $3000-4500$A range are typically almost twice as steep as the Milky Way, LMC, SMC, and Calzetti attenuation curves, which is in agreement with recent studies of the integrated light of present-day galaxies. The attenuation at $4500$A and the slope strongly correlate with the galaxy inclination: face-on galaxies show less attenuation and steeper curves compared to edge-on galaxies, suggesting that geometric effects dominate observed variations in attenuation. Our new method produces $2175$A UV bump detections for 260 individual galaxies. Even though obvious correlations between UV bump strength and global galaxy properties are absent, strong UV bumps are most often seen in face-on, lower-mass galaxies (10 $<$ log$_{10}$(M$_*$/M$_{\odot}$) $<$ 10.5) with low overall attenuation. Finally, we produce a typical attenuation curve for star-forming galaxies at $z\sim0.8$; this prescription represents the effect of dust on the integrated spectral energy distributions of high-redshift galaxies more accurately than commonly used attenuation laws.

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arXiv2020

REQUIEM-2D: Spatially Resolved Stellar Populations from HST 2D Grism Spectroscopy

We present a novel Bayesian methodology to jointly model photometry and deep Hubble Space Telescope (HST) 2d grism spectroscopy of high-redshift galaxies. Our requiem2d code measures both unresolved and resolved stellar populations, ages, and star-formation histories (SFHs) for the ongoing REQIUEM (REsolving QUIEscent Magnified) Galaxies Survey, which targets strong gravitationally lensed quiescent galaxies at z~2. We test the accuracy of \texttt{requiem2d} using a simulated sample of massive galaxies at z~2 from the Illustris cosmological simulation and find we recover the general trends in SFH and median stellar ages. We further present a pilot study for the REQUIEM Galaxies Survey: MRG-S0851, a quintuply-imaged, massive ($\log M_* / M_\odot = 11.02 \pm 0.04$) red galaxy at $z=1.883\pm 0.001$. With an estimated gravitational magnification of $μ= 5.7^{+0.4}_{-0.2}$, we sample the stellar populations on 0.6 kpc physical size bins. The global mass-weighted median age is constrained to be $1.8_{-0.2}^{+0.3}$ Gyr, and our spatially resolved analysis reveals that MRG-S0851 has a flat age gradient in the inner 3 kpc core after taking into account the subtle effects of dust and metallicity on age measurements, favoring an early formation scenario. The analysis for the full REQUIEM-2D sample will be presented in a forthcoming paper with a beta-release of the requiem2d code.

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arXiv2020

The Role of Active Galactic Nuclei in the Quenching of Massive Galaxies in the SQuiGGLE Survey

We study the incidence of nuclear activity in a large sample of massive post-starburst galaxies at z~0.7 selected from the Sloan Digital Sky Survey, and identify active galactic nuclei based on radio continuum and optical emission lines. Over our mass range of 10^10.6-10^11.5 Msun, the incidence of radio activity is weakly dependent on stellar mass and independent of stellar age, while radio luminosity depends strongly on stellar mass. Optical nuclear activity incidence depends most strongly on the Dn4000 line index, a proxy for stellar age, with an active fraction that is ~ten times higher in the youngest versus oldest post-starburst galaxies. Since a similar trend is seen between age and molecular gas fractions, we argue that, like in local galaxies, the age trend reflects a peak in available fueling rather than feedback from the central black hole on the surrounding galaxy.

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arXiv2020

Inverse stellar population age gradients of post-starburst galaxies at z=0.8 with LEGA-C

We use deep, spatially resolved spectroscopy from the LEGA-C Survey to study radial variations in the stellar population of 17 spectroscopically-selected post-starburst (PSB) galaxies. We use spectral fitting to measure two Lick indices, $Hδ_A$ and $Fe4383$, and find that, on average, PSB galaxies have radially decreasing $Hδ_A$ and increasing $Fe4383$ profiles. In contrast, a control sample of quiescent, non-PSB galaxies in the same mass range shows outwardly increasing $Hδ_A$ and decreasing $Fe4383$. The observed gradients are weak ($\approx-0.2$ Å/$R_e$), mainly due to seeing convolution. A two-SSP model suggests intrinsic gradients are as strong as observed in local PSB galaxies ($\approx -0.8$ Å$/R_e$). We interpret these results in terms of inside-out growth (for the bulk of the quiescent population) vs star formation occurring last in the centre (for PSB galaxies). At $z\approx0.8$, central starbursts are often the result of gas-rich mergers, as evidenced by the high fraction of PSB galaxies with disturbed morphologies and tidal features (40%). Our results provide additional evidence for multiple paths to quiescence: a standard path, associated with inside-out disc formation and with gradually decreasing star-formation activity, without fundamental structural transformation, and a fast path, associated with centrally-concentrated starbursts, leaving an inverse age gradient and smaller half-light radius.

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arXiv2020

Stellar Kinematics and Environment at z~0.8 in the LEGA-C Survey: Massive, Slow-Rotators are Built First in Overdense Environments

In this letter, we investigate the impact of environment on integrated and spatially-resolved stellar kinematics of a sample of massive, quiescent galaxies at intermediate redshift ($0.6<z<1.0$). For this analysis, we combine photometric and spectroscopic parameters from the UltraVISTA and Large Early Galaxy Astrophysics Census (LEGA-C) surveys in the COSMOS field and environmental measurements. We analyze the trends with overdensity (1+$δ$) on the rotational support of quiescent galaxies and find no universal trends at either fixed mass or fixed stellar velocity dispersion. This is consistent with previous studies of the local Universe; rotational support of massive galaxies depends primarily on stellar mass. We highlight two populations of massive galaxies ($\log M_\star/M_\odot\geq11$) that deviate from the average mass relation. First, the most massive galaxies in the most under-dense regions ($(1+δ)\leq1$) exhibit elevated rotational support. Similarly, at the highest masses ($\log M_\star/M_\odot\geq11.25$) the range in rotational support is significant in all but the densest regions. This corresponds to an increasing slow-rotator fraction such that only galaxies in the densest environments ($(1+δ)\geq3.5$) are primarily (90$\pm$10\%) slow-rotators.This effect is not seen at fixed velocity dispersion, suggesting minor merging as the driving mechanism: only in the densest regions have the most massive galaxies experienced significant minor merging, building stellar mass and diminishing rotation without significantly affecting the central stellar velocity dispersion. In the local Universe, most massive galaxies are slow-rotators, regardless of environment, suggesting minor merging occurs at later cosmic times $(z\lesssim0.6)$ in all but the most dense environments.

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arXiv2019

The colors and sizes of recently quenched galaxies: a result of compact starburst before quenching

We analyze the colors and sizes of 32 quiescent (UVJ-selected) galaxies with strong Balmer absorption ($\mbox{EW}(Hδ) \geq 4$Å) at $z\sim0.8$ drawn from DR2 of the LEGA-C survey to test the hypothesis that these galaxies experienced compact, central starbursts before quenching. These recently quenched galaxies, usually referred to as post-starburst galaxies, span a wide range of colors and we find a clear correlation between color and half-light radius, such that bluer galaxies are smaller. We build simple toy models to explain this correlation: a normal star-forming disk plus a central, compact starburst component. Bursts with exponential decay timescale of $\sim$~100 Myr that produce $\sim10\%$ to more than 100\% of the pre-existing masses can reproduce the observed correlation. More significant bursts also produce bluer and smaller descendants. Our findings imply that when galaxies shut down star formation rapidly, they generally had experienced compact, starburst events and that the large, observed spread in sizes and colors mostly reflects a variety of burst strengths. Recently quenched galaxies should have younger stellar ages in the centers; multi-wavelength data with high spatial resolution are required to reveal the age gradient. Highly dissipative processes should be responsible for this type of formation history. While determining the mechanisms for individual galaxies is challenging, some recently quenched galaxies show signs of gravitational interactions, suggesting that mergers are likely an important mechanism in triggering the rapid shut-down of star-formation activities at $z\sim0.8$.

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arXiv2019

Discovery of a dark, massive, ALMA-only galaxy at z~5-6 in a tiny 3-millimeter survey

We report the serendipitous detection of two 3 mm continuum sources found in deep ALMA Band 3 observations to study intermediate redshift galaxies in the COSMOS field. One is near a foreground galaxy at 1.3", but is a previously unknown dust-obscured star-forming galaxy (DSFG) at probable $z_{CO}=3.329$, illustrating the risk of misidentifying shorter wavelength counterparts. The optical-to-mm spectral energy distribution (SED) favors a grey $λ^{-0.4}$ attenuation curve and results in significantly larger stellar mass and SFR compared to a Calzetti starburst law, suggesting caution when relating progenitors and descendants based on these quantities. The other source is missing from all previous optical/near-infrared/sub-mm/radio catalogs ("ALMA-only"), and remains undetected even in stacked ultradeep optical ($>29.6$ AB) and near-infrared ($>27.9$ AB) images. Using the ALMA position as a prior reveals faint $SNR\sim3$ measurements in stacked IRAC 3.6+4.5, ultradeep SCUBA2 850$μ$m, and VLA 3GHz, indicating the source is real. The SED is robustly reproduced by a massive $M^*=10^{10.8}$M$_\odot$ and $M_{gas}=10^{11}$M$_\odot$, highly obscured $A_V\sim4$, star forming $SFR\sim300$ M$_{\odot}$yr$^{-1}$ galaxy at redshift $z=5.5\pm$1.1. The ultrasmall 8 arcmin$^{2}$ survey area implies a large yet uncertain contribution to the cosmic star formation rate density CSFRD(z=5) $\sim0.9\times10^{-2}$ M$_{\odot}$ yr$^{-1}$ Mpc$^{-3}$, comparable to all ultraviolet-selected galaxies combined. These results indicate the existence of a prominent population of DSFGs at $z>4$, below the typical detection limit of bright galaxies found in single-dish sub-mm surveys, but with larger space densities $\sim3 \times 10^{-5}$ Mpc$^{-3}$, higher duty cycles $50-100\%$, contributing more to the CSFRD, and potentially dominating the high-mass galaxy stellar mass function.

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arXiv2019

Evidence for Inside-Out Galaxy Growth and Quenching of a z~2 Compact Galaxy from High-Resolution Molecular Gas Imaging

We present high spatial resolution imaging of the CO(1-0) line from the Karl G. Jansky Very Large Array (VLA) of COSMOS27289, a massive, compact star forming galaxy at z=2.234. This galaxy was selected to be structurally similar to z~2 passive galaxies. Our previous observations showed that it is very gas-poor with respect to typical star-forming galaxies at these redshifts, consistent with a rapid transition to quiescence as the molecular gas is depleted. The new data show that both the molecular gas fraction f_H2 = M_H2/M_star and the molecular gas depletion time t_dep = M_H2/SFR are lower in the central 1-2kpc of the galaxy and rise at larger radii ~2-4kpc. These observations are consistent with a scenario in which COSMOS27289 will imminently cease star formation in the inner regions before the outskirts, i.e. inside-out quenching, the first time this phenomenon has been seen via observations of molecular gas in the high-redshift universe. We find good qualitative and quantitative agreement with a hydrodynamical simulation of galaxy quenching, in which the central suppression of molecular gas arises due to rapid gas consumption and outflows that evacuate the central regions of gas. Our results provide independent evidence for inside-out quenching of star formation as a plausible formation mechanism for z~2 quiescent galaxies.

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arXiv2019

Stellar Metallicities and Elemental Abundance Ratios of z~1.4 Massive Quiescent Galaxies

The chemical composition of galaxies has been measured out to z~4. However, nearly all studies beyond z~0.7 are based on strong-line emission from HII regions within star-forming galaxies. Measuring the chemical composition of distant quiescent galaxies is extremely challenging, as the required stellar absorption features are faint and shifted to near-infrared wavelengths. Here, we present ultra-deep rest-frame optical spectra of five massive quiescent galaxies at z~1.4, all of which show numerous stellar absorption lines. We derive the abundance ratios [Mg/Fe] and [Fe/H] for three out of five galaxies; the remaining two galaxies have too young luminosity-weighted ages to yield robust measurements. Similar to lower-redshift findings, [Mg/Fe] appears positively correlated with stellar mass, while [Fe/H] is approximately constant with mass. These results may imply that the stellar mass-metallicity relation was already in place at z~1.4. While the [Mg/Fe]-mass relation at z~1.4 is consistent with the z<0.7 relation, [Fe/H] at z~1.4 is ~0.2 dex lower than at z<0.7. With a [Mg/Fe] of 0.44(+0.08,-0.07) the most massive galaxy may be more alpha-enhanced than similar-mass galaxies at lower redshift, but the offset is less significant than the [Mg/Fe] of 0.6 previously found for a massive galaxy at z=2.1. Nonetheless, these results combined may suggest that [Mg/Fe] in the most massive galaxies decreases over time, possibly by accreting low-mass, less alpha-enhanced galaxies. A larger galaxy sample is needed to confirm this scenario. Finally, the abundance ratios indicate short star-formation timescales of 0.2-1.0 Gyr.

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arXiv2019

Rejuvenation in $z\sim0.8$ quiescent galaxies in LEGA-C

We use reconstructed star-formation histories (SFHs) of quiescent galaxies at $z=0.6-1$ in the LEGA-C survey to identify secondary star-formation episodes that, after an initial period of quiescence, moved the galaxies back to the star-forming main sequence (blue cloud). $16\pm3$\% of the $z\sim0.8$ quiescent population has experienced such rejuvenation events in the redshift range $0.7<z<1.5$ after reaching quiescence at some earlier time. On average, these galaxies first became quiescent at $z=1.2$, and those that rejuvenated, remained quiescent for $\sim1$Gyr before their secondary SF episode which lasted $\sim0.7$Gyr. The stellar mass attributed to rejuvenation is on average 10\% of the galaxy stellar mass, with rare instances of an increase of more than a factor 2. Overall, rejuvenation events only contribute $\sim2$\% of the total stellar mass in $z\sim0.8$ quiescent galaxies and we conclude that rejuvenation is not an important evolutionary channel when considering the growth of the red sequence. However, our results complicate the interpretation of galaxy demographics in color space: the galaxies with rejuvenation events tend to lie in the so-called `green valley', yet their progenitors were quiescent at $z\sim2$.

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arXiv2019

Extremely Low Molecular Gas Content in a Compact, Quiescent Galaxy at z=1.522

One of the greatest challenges to theoretical models of massive galaxy formation is the regulation of star formation at early times. The relative roles of molecular gas expulsion, depletion, and stabilization are uncertain as direct observational constraints of the gas reservoirs in quenched or quenching galaxies at high redshift are scant. We present ALMA observations of CO(2-1) in a massive ($\log M_{\star}/M_{\odot}=11.2$), recently quenched galaxy at $z=1.522$. The optical spectrum of this object shows strong Balmer absorption lines, which implies that star formation ceased $\sim$0.8 Gyr ago. We do not detect CO(2-1) line emission, placing an upper limit on the molecular $\mathrm{H_2}$ gas mass of 1.1$\times10^{10}\,M_{\odot}$. The implied gas fraction is $f_{\rm{H_2}}{\equiv M_{H_2}/M_{\star}}<7\%$, $\sim10\times$ lower than typical star forming galaxies at similar stellar masses at this redshift, among the lowest gas fractions at this specific star formation rate at any epoch, and the most stringent constraint on the gas contents of a $z>1$ passive galaxy to date. Our observations show that the depletion of $\mathrm{H_2}$ from the interstellar medium of quenched objects can be both efficient and fairly complete, in contrast to recent claims of significant cold gas in recently quenched galaxies. We explore the variation in observed gas fractions in high-$z$ galaxies and show that galaxies with high stellar surface density have low $f_{\rm{H_2}}$, similar to recent correlations between specific star formation rate and stellar surface density.

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arXiv2019

An Absence of Radio-Loud Active Galactic Nuclei in Geometrically Flat Quiescent Galaxies: Implications for Maintenance-Mode Feedback Models

Maintenance-mode feedback from low-accretion rate AGN, manifesting itself observationally through radio-loudness, is invoked in all cosmological galaxy formation models as a mechanism that prevents excessive star-formation in massive galaxies (M$_*$ $\gtrsim$ 3$\times$10$^{10}$ M$_{\odot}$). We demonstrate that at a fixed mass the incidence of radio-loud AGN (L $>$ 10$^{23}$ WHz$^{- 1}$) identified in the FIRST and NVSS radio surveys among a large sample of quiescent (non-star forming) galaxies selected from the SDSS is much higher in geometrically round galaxies than in geometrically flat, disk-like galaxies. As found previously, the RL AGN fraction increases steeply with stellar velocity dispersion $σ_*$ and stellar mass, but even at a fixed velocity dispersion of 200-250 kms$^{-1}$ this fraction increases from 0.3% for flat galaxies (projected axis ratio of q $<$ 0.4) to 5% for round galaxies (q $>$ 0.8). We rule out that this strong trend is due to projection effects in the measured velocity dispersion. The large fraction of radio-loud AGN in massive, round galaxies is consistent with the hypothesis that such AGN deposit energy into their hot gaseous halos, preventing cooling and star-formation. However, the absence of such AGN in disk-like quiescent galaxies -- most of which are not satellites in massive clusters, raises important questions: is maintenance-mode feedback a generally valid explanation for quiescence; and, if so, how does that feedback avoid manifesting at least occasionally as a radio-loud galaxy?

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arXiv2018

1D Kinematics from stars and ionized gas at $z\sim0.8$ from the LEGA-C spectroscopic survey of massive galaxies

We present a comparison of the observed, spatially integrated stellar and ionized gas velocity dispersions of $\sim1000$ massive ($\log M_{\star}/M_{\odot}\gtrsim\,10.3$) galaxies in the Large Early Galaxy Astrophysics Census (LEGA-C) survey at $0.6\lesssim\,z\lesssim1.0$. The high $S/N\sim20{\rmÅ^{-1}}$ afforded by 20 hour VLT/VIMOS spectra allows for joint modeling of the stellar continuum and emission lines in all galaxies, spanning the full range of galaxy colors and morphologies. These observed integrated velocity dispersions (denoted as $σ'_{g, int}$ and $σ'_{\star, int}$) are related to the intrinsic velocity dispersions of ionized gas or stars, but also include rotational motions through beam smearing and spectral extraction. We find good average agreement between observed velocity dispersions, with $\langle\log(σ'_{g, int}/σ'_{\star, int})\rangle=-0.003$. This result does not depend strongly on stellar population, structural properties, or alignment with respect to the slit. However, in all regimes we find significant scatter between $σ'_{g, int}$ and $σ'_{\star, int}$, with an overall scatter of 0.13 dex of which 0.05 dex is due to observational uncertainties. For an individual galaxy, the scatter between $σ'_{g, int}$ and $σ'_{\star, int}$ translates to an additional uncertainty of $\sim0.24\rm{dex}$ on dynamical mass derived from $σ'_{g, int}$, on top of measurement errors and uncertainties from Virial constant or size estimates. We measure the $z\sim0.8$ stellar mass Faber-Jackson relation and demonstrate that emission line widths can be used to measure scaling relations. However, these relations will exhibit increased scatter and slopes that are artificially steepened by selecting on subsets of galaxies with progressively brighter emission lines.

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arXiv2018

Complete IRAC mapping of the CFHTLS-DEEP, MUSYC AND NMBS-II FIELDS

The IRAC mapping of the NMBS-II fields program is an imaging survey at 3.6 and 4.5$μ$m with the Spitzer Infrared Array Camera (IRAC). The observations cover three Canada-France-Hawaii Telescope Legacy Survey Deep (CFHTLS-D) fields, including one also imaged by AEGIS, and two MUSYC fields. These are then combined with archival data from all previous programs into deep mosaics. The resulting imaging covers a combined area of about 3 $deg^2$, with at least $\sim$2 hr integration time for each field. In this work, we present our data reduction techniques and document the resulting coverage maps at 3.6 and 4.5$μ$m. All of the images are W-registered to the reference image, which is either the z-band stack image of the 25\% best seeing images from the CFHTLS-D for CFHTLS-D1, CFHTLS-D3, and CFHTLS-D4, or the K-band images obtained at the Blanco 4-m telescope at CTIO for MUSYC1030 and MUSYC1255. We make all images and coverage maps described herein publicly available via the Spitzer Science Center.

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arXiv2018

Fast and slow paths to quiescence: ages and sizes of 400 quiescent galaxies from the LEGA-C survey

We analyze stellar age indicators (D$_n$4000 and EW(H$δ$)) and sizes of 467 quiescent galaxies with $M_\ast \geq 10^{10} M_\odot$ at $z\sim0.7$ drawn from DR2 of the LEGA-C survey. Interpreting index variations in terms of equivalent single stellar population age, we find that the median stellar population is younger for larger galaxies at fixed stellar mass. The effect is significant, yet small; the ages of the larger and the smaller subsets differ by only $<500$ Myr, much less than the age variation among individual galaxies ($\sim1.5$ Gyr). At the same time, quiescent galaxies with the strongest H$δ$ absorption --- those experienced recent and rapid quenching events --- tend to be smaller than the average. These co-existing trends unify seemingly contradictory results in the literature; the complex correlations between size and age indicators revealed by our large sample of galaxies with high-quality spectra suggest that there are multiple evolutionary pathways to quiescence. Regardless of the specific physical mechanisms responsible for the cessation of star formation in massive galaxies, the large scatter in D$_n$4000 and EW(H$δ$) immediately implies that galaxies follow a large variety in evolutionary pathways. On the one hand, we see evidence for a process that slowly shuts off star-formation and transforms star-forming galaxies to quiescent galaxies without necessarily changing their structures. On the other hand, there is likely a mechanism that rapidly quenches galaxies, an event that coincides with dramatic structural changes, producing small post-starburst galaxies.

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