• Clocking the Evolution of Post-starburst Galaxies: Methods and First Results

      French, K. Decker; Yang, Yujin; Zabludoff, Ann I.; Tremonti, Christy A.; Univ Arizona, Steward Observ (IOP PUBLISHING LTD, 2018-07-20)
      Detailed modeling of the recent star formation histories (SFHs) of post-starburst (or "E+A") galaxies is impeded by the degeneracy between the time elapsed since the starburst ended (post-burst age), the fraction of stellar mass produced in the burst (burst strength), and the burst duration. To resolve this issue, we combine GALEX ultraviolet photometry, SDSS photometry and spectra, and new stellar population synthesis models to fit the SFHs of 532 post-starburst galaxies. In addition to an old stellar population and a recent starburst, 48% of the galaxies are best fit with a second recent burst. Lower stellar mass galaxies (log M-*/M-circle dot<10.5) are more likely to experience two recent bursts, and the fraction of their young stellar mass is more strongly anticorrelated with their total stellar mass. Applying our methodology to other, younger post-starburst samples, we identify likely progenitors to our sample and examine the evolutionary trends of molecular gas and dust content with post-burst age. We discover a significant (4 sigma) decline, with a 117-230 Myr characteristic depletion time, in the molecular gas to stellar mass fraction with the post-burst age. The implied rapid gas depletion rate of 2-150 M(circle dot)yr(-1) cannot be due to current star formation, given the upper limits on the current star formation rates in these post-starbursts. Nor are stellar winds or supernova feedback likely to explain this decline. Instead, the decline points to the expulsion or destruction of molecular gas in outflows, a possible smoking gun for active galactic nucleus feedback.
    • Continuum-fitting the X-Ray Spectra of Tidal Disruption Events

      Wen, Sixiang; Jonker, Peter G.; Stone, Nicholas C.; Zabludoff, Ann I.; Psaltis, Dimitrios; Univ Arizona (IOP PUBLISHING LTD, 2020-07-06)
      We develop a new model for X-ray emission from tidal disruption events (TDEs), applying stationary general relativistic "slim disk" accretion solutions to supermassive black holes (SMBHs) and then ray-tracing the photon trajectories from the image plane to the disk surface, including gravitational redshift, Doppler, and lensing effects self-consistently. We simultaneously and successfully fit the multi-epoch XMM-Newton X-ray spectra for two TDEs: ASASSN-14li and ASASSN-15oi. We test explanations for the observed, unexpectedly slow X-ray brightening of ASASSN-15oi, including delayed disk formation and variable obscuration by a reprocessing layer. We propose a new mechanism that better fits the data: a "slimming disk" scenario in which accretion onto an edgeon disk slows, reducing the disk height and exposing more X-rays from the inner disk to the sightline over time. For ASASSN-15oi, we constrain the SMBH mass to 4.0(-3.1)(+2.5) x 10(6)M(circle dot). For ASASSN-14li, the SMBH mass is 10(-7)(+1) x 10(6) M-circle dot, and the spin is >0.3. For both TDEs, our fitted masses are consistent with independent estimates; for ASASSN-14li, application of the external mass constraint narrows our spin constraint to >0.85. The mass accretion rate of ASASSN-14li decays slowly, as proportional to t(-1.1), perhaps due to inefficient debris circularization. Over approximate to 1100 days, its SMBH has accreted Delta M approximate to 0.17M(circle dot), implying a progenitor star mass of >0.34M(circle dot), i.e., no "missing energy problem." For both TDEs, the hydrogen column density declines to the host galaxy plus Milky Way value after a few hundred days, suggesting a characteristic timescale for the depletion or removal of obscuring gas.
    • Detecting Exoplanets Using Eclipsing Binaries as Natural Starshades

      Bellotti, Stefano; Zabludoff, Ann I.; Belikov, Ruslan; Guyon, Olivier; Rathi, Chirag; Univ Arizona, Steward Observ (IOP PUBLISHING LTD, 2020-09)
      We investigate directly imaging exoplanets around eclipsing binaries using the eclipse as a natural tool for dimming the binary and thus increasing the planet to star brightness contrast. At eclipse, the binary becomes pointlike, making coronagraphy possible. We select binaries where the planet-star contrast would be boosted by >10x during eclipse, making it possible to detect a planet that is greater than or similar to 10x fainter or in a star system that is similar to 2-3x more massive than otherwise. Our approach will yield insights into planet occurrence rates around binaries versus individual stars. We consider both self-luminous (SL) and reflected light (RL) planets. In the SL case, we select binaries whose age is young enough so that an orbiting SL planet would remain luminous; in U Cep and AC Sct, respectively, our method is sensitive to SL planets of similar to 4.5 and similar to 9M(J)with current ground- or near-future space-based instruments and similar to 1.5 and similar to 6M(J)with future ground-based observatories. In the RL case, there are three nearby (less than or similar to 50 pc) systems-V1412 Aql, RR Cae, and RT Pic-around which a Jupiter-like planet at a planet-star separation of greater than or similar to 20 mas might be imaged with future ground- and space-based coronagraphs. A Venus-like planet at the same distance might be detectable around RR Cae and RT Pic. A habitable Earth-like planet represents a challenge; while the planet-star contrast at eclipse and planet flux are accessible with a 6-8 m space telescope, the planet-star separation is 1/3-1/4 of the angular separation limit of modern coronagraphy.
    • Evidence for Late-time Feedback from the Discovery of Multiphase Gas in a Massive Elliptical at z = 0.4

      Zahedy, Fakhri S.; Chen, Hsiao-Wen; Boettcher, Erin; Rauch, Michael; Decker French, K.; Zabludoff, Ann I.; Univ Arizona, Dept Astron; Univ Arizona, Steward Observ (IOP PUBLISHING LTD, 2020-11-19)
      We report the first detection of multiphase gas within a quiescent galaxy beyond z approximate to 0. The observations use the brighter image of doubly lensed QSO HE 0047-1756 to probe the interstellar medium (ISM) of the massive (M-star approximate to 10(11) M-circle dot) elliptical lens galaxy at z(gal) = 0.408. Using Hubble Space Telescope's Cosmic Origins Spectrograph (COS), we obtain a medium-resolution FUV spectrum of the lensed QSO and identify numerous absorption features from H-2 in the lens ISM at projected distance d = 4.6 kpc. The H-2 column density is log N(H-2)/cm(-2) 17.8(-0.3)(+0.1) with a molecular gas fraction of f(H2) = 2%-5% , roughly consistent with some local quiescent galaxies. The new COS spectrum also reveals kinematically complex absorption features from highly ionized species O VI and N V with column densities log N(O VI) cm(-2) = 15.2 +/- 0.1 and log N(N V) cm(-2) = 14.6 +/- 0.1, among the highest known in external galaxies. Assuming the high-ionization absorption features originate in a transient warm (T similar to 105 K) phase undergoing radiative cooling from a hot halo surrounding the galaxy, we infer a mass accretion rate of similar to 0.5-1.5 M-circle dot yr(-1). The lack of star formation in the lens suggests that the bulk of this flow is returned to the hot halo, implying a heating rate of similar to 1048 erg yr(-1). Continuous heating from evolved stellar populations (primarily SNe Ia but also winds from AGB stars) may suffice to prevent a large accumulation of cold gas in the ISM, even in the absence of strong feedback from an active nucleus.
    • The Evolution of the Interstellar Medium in Post-starburst Galaxies

      Li, Zhihui; French, K. Decker; Zabludoff, Ann I.; Ho, Luis C.; Univ Arizona, Steward Observ (IOP PUBLISHING LTD, 2019-07-16)
      We derive dust masses (M-dust) from the spectral energy distributions of 58 post-starburst galaxies (PSBs). There is an anticorrelation between specific dust mass (M-dust/M-star) and the time elapsed since the starburst ended, indicating that dust was either destroyed, expelled, or rendered undetectable over the similar to 1 Gyr after the burst. The M-dust/M-star depletion timescale, 205(-37)(+58) Myr, is consistent with that of the CO-traced M-H2/M-star, suggesting that dust and gas are altered via the same process. Extrapolating these trends leads to the M-dust/M-star and M-H2/M-star values of early-type galaxies (ETGs) within 1-2 Gyr, a timescale consistent with the evolution of other PSB properties into ETGs. Comparing M-dust and M-H2 for PSBs yields a calibration, log M-H2 = 0.45 log M-dust + 6.02, that allows us to place 33 PSBs on the Kennicutt-Schmidt (KS) plane, Sigma SFR-Sigma M-H2. Over the first similar to 200-300 Myr, the PSBs evolve down and off of the KS relation, as their star formation rate (SFR) decreases more rapidly than M-H2. Afterwards, M-H2 continues to decline whereas the SFR levels off. These trends suggest that the star formation efficiency bottoms out at 10-11 yr(-1) and will rise to ETG levels within 0.5-1.1 Gyr afterwards. The SFR decline after the burst is likely due to the absence of gas denser than the CO-traced H-2. The mechanism of the M-dust/M-star and M-H2/M-star decline, whose timescale suggests active galactic nucleus/low-ionization nuclear emission-line region feedback, may also be preventing the large CO-traced molecular gas reservoirs from collapsing and forming denser star-forming clouds.
    • The Next Generation Virgo Cluster Survey. XVII. A Search for Planetary Nebulae in Virgo Cluster Globular Clusters

      Sun, Weijia; Peng, Eric W.; Ko, Youkyung; Côté, Patrick; Ferrarese, Laura; Lee, Myung Gyoon; Liu, Chengze; Longobardi, Alessia; Chilingarian, Igor V.; Spengler, Chelsea; et al. (IOP PUBLISHING LTD, 2019-11-11)
      The occurrence of planetary nebulae (PNe) in globular clusters (GCs) provides an excellent chance to study lowmass stellar evolution in a special (low-metallicity, high stellar density) environment. We report a systematic spectroscopic survey for the [O III] 5007 angstrom emission line of PNe in 1469 Virgo GCs and 121 Virgo ultra-compact dwarfs (UCDs), mainly hosted in the giant elliptical galaxies M87, M49, M86, and M84. We detected zero PNe in our UCD sample and discovered one PN (M-5007 = -4.1 mag) associated with an M87 GC. We used the [O III] detection limit for each GC to estimate the luminosity-specific frequency of PNe, a, and measured a in the Virgo cluster GCs to be alpha similar to 3.9(-0.7)(+5.2) x 10(-8) PN/(circle dot). The value of a in the Virgo GCs is among the lowest reported in any environment, due in part to the large sample size, and it is 5-6 times lower than that for the Galactic GCs. We suggest that a decreases toward brighter and more massive clusters, sharing a similar trend as the binary fraction, and the discrepancy between the Virgo and Galactic GCs can be explained by the observational bias in extragalactic surveys toward brighter GCs. This low but nonzero efficiency in forming PNe may highlight the important role played by binary interactions in forming PNe in GCs. We argue that a future survey of less massive Virgo GCs will be able to determine whether PN production in the Virgo GCs is governed by an internal process (mass, density, binary fraction) or if it is largely regulated by the external environment.
    • The Structure of Tidal Disruption Event Host Galaxies on Scales of Tens to Thousands of Parsecs

      French, K. Decker; Arcavi, Iair; Zabludoff, Ann I.; Stone, Nicholas; Hiramatsu, Daichi; van Velzen, Sjoert; McCully, Curtis; Jiang, Ning; Univ Arizona, Steward Observ (IOP PUBLISHING LTD, 2020-03-09)
      We explore the galaxy structure of four tidal disruption event (TDE) host galaxies on 30 pc to kiloparsec scales using Hubble Space Telescope WFC3 multiband imaging. The star formation histories of these hosts are diverse, including one post-starburst galaxy (ASASSN-14li), two hosts with recent weak starbursts (ASASSN-14ae and iPTF15af), and one early-type galaxy (PTF09ge). Compared to early-type galaxies of similar stellar masses, the TDE hosts have higher central surface brightnesses and stellar mass surface densities on 30-100 pc scales. The TDE hosts do not show the large, kiloparsec-scale tidal disruptions seen in some post-starburst galaxies; the hosts have low morphological asymmetries similar to those of early-type galaxies. The lack of strong asymmetries is inconsistent with a recent major (similar to 1:1 mass) merger, although minor (less than or similar to 1:3) mergers are possible. Given the time elapsed since the end of the starbursts in the three post-burst TDE hosts and the constraints on the merger mass ratios, it is unlikely that a bound supermassive black hole binary (SMBHB) has had time to coalesce. The TDE hosts have low central (<140 pc) ellipticities compared to early-type galaxies. The low central ellipticities disfavor a strong radial anisotropy as the cause for the enhanced TDE rate, although we cannot rule out eccentric disks at the scale of the black hole gravitational radius of influence (similar to 1 pc). These observations suggest that the high central stellar densities are a more important driver than SMBHBs or radial anisotropies in increasing the TDE rate in galaxies with recent starbursts.
    • Why Post-starburst Galaxies Are Now Quiescent

      French, K. Decker; Zabludoff, Ann I.; Yoon, Ilsang; Shirley, Yancy; Yang, Yujin; Smercina, Adam; Smith, J. D.; Narayanan, Desika; Univ Arizona, Steward Observ (IOP PUBLISHING LTD, 2018-07-10)
      Post-starburst or "E + A" galaxies are rapidly transitioning from star-forming to quiescence. While the current star formation rate (SFR) of post-starbursts is already at the level of early-type galaxies, we recently discovered that many have large CO-traced molecular gas reservoirs consistent with normal star-forming galaxies. These observations raise the question of why these galaxies have such low SFRs. Here we present an ALMA search for the denser gas traced by HCN (1-0) and HCO+ (1-0) in two CO-luminous, quiescent post-starburst galaxies. Intriguingly, we fail to detect either molecule. The upper limits are consistent with the low SFRs and with earlytype galaxies. The HCN/CO luminosity ratio upper limits are low compared to star-forming and even many earlytype galaxies. This implied low dense gas mass fraction explains the low SFRs relative to the CO-traced molecular gas and suggests that the state of the gas in post-starburst galaxies is unusual, with some mechanism inhibiting its collapse to denser states. We conclude that post-starbursts galaxies are now quiescent because little dense gas is available, in contrast to the significant CO-traced lower density gas reservoirs that still remain.