Fading AGN in Post-Starburst Galaxies

The role of AGN in quenching galaxies and driving the evolution from star-forming to quiescent remains a key question in galaxy evolution. We present evidence from the Mapping Nearby Galaxies at APO (MaNGA) survey for fading AGN activity in 6/93 post-starburst galaxies. These six galaxies show extended emission line regions (EELRs) consistent with ionization from past AGN activity, analogous to "Hanny's voorwerp" and other systems where the OIII5007 emission is bright enough to be visible in broadband imaging. Using the infrared luminosities from IRAS to estimate the current AGN luminosity, we find that 5/6 of the post-starburst galaxies have current AGN which have faded from the peak luminosity required to have ionized the EELRs. Given the rate at which we observe EELRs, the typical EELR visibility timescale, and an estimate of how often EELRs would be visible, we estimate the duty cycle of AGN activity during the post-starburst phase. The timescale for the galaxy to cycle between peaks in AGN luminosity is $t_{\rm EELR}\sim1.1-2.3\times10^5$ yr. Given the rate at which we observe current AGN activity during this phase, we estimate that the AGN spends only 5.3% of this time (or $t_{\rm ON} = 0.6-1.3\times10^4$ yr) in its luminous phase, with the rest of the time spent "off" or in a low-luminosity phase. The length of this duty cycle may explain why so few luminous AGN have been observed during the post-starburst phase, despite evidence for AGN feedback at work.Comment: Re-submitted to ApJ after referee report. 20 pages, 13 figure

Star Formation Suppression by Tidal Removal of Cold Molecular Gas from an Intermediate-redshift Massive Post-starburst Galaxy

Observations and simulations have demonstrated that star formation in galaxies must be actively suppressed to prevent the formation of overly massive galaxies. Galactic outflows driven by stellar feedback or supermassive black hole accretion are often invoked to regulate the amount of cold molecular gas available for future star formation but may not be the only relevant quenching processes in all galaxies. We present the discovery of vast molecular tidal features extending up to 64 kpc outside of a massive z = 0.646 post-starburst galaxy that recently concluded its primary star-forming episode. The tidal tails contain (1.2 ± 0.1) × 1010 M⊙ of molecular gas, 47% ± 5% of the total cold gas reservoir of the system. Both the scale and magnitude of the molecular tidal features are unprecedented compared to all known nearby or high-redshift merging systems. We infer that the cold gas was stripped from the host galaxies during the merger, which is most likely responsible for triggering the initial burst phase and the subsequent suppression of star formation. While only a single example, this result shows that galaxy mergers can regulate the cold gas contents in distant galaxies by directly removing a large fraction of the molecular gas fuel, and plausibly suppress star formation directly, a qualitatively different physical mechanism than feedback-driven outflows

The Compact Structures of Massive z ∼ 0.7 Post-starburst Galaxies in the SQuIGGL⃗E Sample

We present structural measurements of 145 spectroscopically selected intermediate-redshift (z ∼ 0.7), massive (M⋆ ∼ 1011 M⊙) post-starburst galaxies from the $\mathrm{SQuIGG}\vec{L}{\rm{E}}$ sample measured using wide-depth Hyper Suprime-Cam i-band imaging. This deep imaging allows us to probe the sizes and structures of these galaxies, which we compare to a control sample of star-forming and quiescent galaxies drawn from the LEGA-C Survey. We find that post-starburst galaxies systematically lie ∼0.1 dex below the quiescent mass–size (half-light radius) relation, with a scatter of ∼0.2 dex. This finding is bolstered by nonparametric measures, such as the Gini coefficient and the concentration, which also reveal these galaxies to have more compact light profiles than both quiescent and star-forming populations at similar mass and redshift. The sizes of post-starburst galaxies show either negative or no correlation with the time since quenching, such that more recently quenched galaxies are larger or similarly sized. This empirical finding disfavors the formation of post-starburst galaxies via a purely central burst of star formation that simultaneously shrinks the galaxy and shuts off star formation. We show that the central densities of post-starburst and quiescent galaxies at this epoch are very similar, in contrast with their effective radii. The structural properties of z ∼ 0.7 post-starburst galaxies match those of quiescent galaxies that formed in the early universe, suggesting that rapid quenching in the present epoch is driven by a similar mechanism to the one at high redshift

SQuIGGLE: Studying Quenching in Intermediate-z Galaxies -- Gas, AnguLar Momentum, and Evolution

We describe the SQuIGGLE survey of intermediate-redshift post-starburst galaxies. We leverage the large sky coverage of the SDSS to select ~1300 recently-quenched galaxies at 0.5<z<~0.9 based on their unique spectral shapes. These bright, intermediate-redshift galaxies are ideal laboratories to study the physics responsible for the rapid quenching of star formation: they are distant enough to be useful analogs for high-redshift quenching galaxies, but low enough redshift that multi-wavelength follow-up observations are feasible with modest telescope investments. We use the Prospector code to infer the stellar population properties and non-parametric star formation histories of all galaxies in the sample. We find that SQuIGGLE galaxies are both very massive (M* ~ 10^11.25 Msun) and quenched, with inferred star formation rates <~1Msun/yr, more than an order of magnitude below the star-forming main sequence. The best-fit star formation histories confirm that these galaxies recently quenched a major burst of star formation: >75% of SQuIGGLE galaxies formed at least a quarter of their total stellar mass in the recent burst, which ended just ~200Myr before observation. We find that SQuIGGLE galaxies are on average younger and more burst-dominated than most other z<~1 post-starburst samples. This large sample of bright post-starburst galaxies at intermediate redshift opens a wide range of studies into the quenching process. In particular, the full SQuIGGLE survey will investigate the molecular gas reservoirs, morphologies, kinematics, resolved stellar populations, AGN incidence, and infrared properties of this unique sample of galaxies in order to place definitive constraints on the quenching process.Comment: 23 pages, 16 figures, accepted to Ap

Squiggle: Studying Quenching in Intermediate-z Galaxies—Gas, Angular Momentum, and Evolution

We describe the Studying Quenching in Intermediate-z Galaxies: Gas, ${\rm{angu}}\overrightarrow{L}{\rm{ar}}$ momentum, and Evolution ($\mathrm{SQuIGG}\vec{L}{\rm{E}}$) survey of intermediate-redshift post-starburst galaxies. We leverage the large sky coverage of the Sloan Digital Sky Survey to select ∼ 1300 recently quenched galaxies at 0.5 75% of $\mathrm{SQuIGG}\vec{L}{\rm{E}}$ galaxies formed at least a quarter of their total stellar mass in the recent burst, which ended just ∼200 Myr before observation. We find that $\mathrm{SQuIGG}\vec{L}{\rm{E}}$ galaxies are on average younger and more burst-dominated than most other z ≲ 1 post-starburst galaxy samples. This large sample of bright post-starburst galaxies at intermediate redshift opens a wide range of studies into the quenching process. In particular, the full $\mathrm{SQuIGG}\vec{L}{\rm{E}}$ survey will investigate the molecular gas reservoirs, morphologies, kinematics, resolved stellar populations, active galactic nucleus incidence, and infrared properties of this unique sample of galaxies in order to place definitive constraints on the quenching process

Merger Signatures are Common, but not Universal, in Massive, Recently Quenched Galaxies at z ∼ 0.7

We present visual classifications of merger-induced tidal disturbances in 143 M _* ∼ 10 ^11 M _⊙ post-starburst galaxies at z ∼ 0.7 identified in the $\mathrm{SQuIGG}\vec{L}{\rm{E}}$ Sample. This sample spectroscopically selects galaxies from the Sloan Digital Sky Survey that have stopped their primary epoch of star formation within the past ∼500 Myr. Visual classifications are performed on Hyper Suprime-Cam imaging. We compare to a control sample of mass- and redshift-matched star-forming and quiescent galaxies from the Large Early Galaxy Census and find that post-starburst galaxies are more likely to be classified as disturbed than either category. This corresponds to a factor of ${3.6}_{-1.3}^{+2.9}$ times the disturbance rate of older quiescent galaxies and ${2.1}_{-.73}^{+1.9}$ times the disturbance rate of star-forming galaxies. Assuming tidal features persist for ≲500 Myr, this suggests merging is coincident with quenching in a significant fraction of these post-starbursts. Galaxies with tidal disturbances are younger on average than undisturbed post-starburst galaxies in our sample, suggesting tidal features from a major merger may have faded over time. This may be exacerbated by the fact that, on average, the undisturbed subset is fainter, rendering low-surface-brightness tidal features harder to identify. However, the presence of 10 young (≲150 Myr since quenching) undisturbed galaxies suggests that major mergers are not the only fast physical mechanism that shut down the primary epoch of star formation in massive galaxies at intermediate redshift