Sun City Center, United States
Sun City Center, United States

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Cales S.L.,University of Concepción | Cales S.L.,University of Wyoming | Brotherton M.S.,University of Wyoming | Shang Z.,University of Wyoming | And 10 more authors.
Astrophysical Journal | Year: 2013

We present optical spectroscopy of a sample of 38 post-starburst quasars (PSQs) at z ∼ 0.3, 29 of which have morphological classifications based on Hubble Space Telescope imaging. These broad-lined active galactic nuclei (AGNs) possess the spectral signatures of massive intermediate-aged stellar populations, making them potentially useful for studying connections between nuclear activity and host galaxy evolution. We model the spectra in order to determine the ages and masses of the host stellar populations, and the black hole masses and Eddington fractions of the AGNs. Our model components include an instantaneous starburst, a power law, and emission lines. We find that the PSQs have M BH ∼ 108 M⊙ accreting at a few percent of Eddington luminosity and host ∼1010.5 M ⊙ stellar populations which are several hundred Myr to a few Gyr old. We investigate relationships among these derived properties, spectral properties, and morphologies. We find that PSQs hosted in spiral galaxies have significantly weaker AGN luminosities, older starburst ages, and narrow emission-line ratios diagnostic of ongoing star formation when compared to their early-type counterparts. We conclude that the early-type PSQs are likely the result of major mergers and were likely luminous infrared galaxies in the past, while spiral PSQs with more complex star formation histories are triggered by less dramatic events (e.g., harassment, bars). We provide diagnostics to distinguish the early-type and spiral hosts when high spatial resolution imaging is not available. © 2013. The American Astronomical Society. All rights reserved..


Fang T.,Xiamen University | Fang T.,University of California at Irvine | Bullock J.,University of California at Irvine | Boylan-Kolchin M.,University of California at Irvine | Boylan-Kolchin M.,Center for Galaxy Evolution
Astrophysical Journal | Year: 2013

The Milky Way appears to be missing baryons, as the observed mass in stars and gas is well below the cosmic mean. One possibility is that a substantial fraction of the Galaxy's baryons are embedded within an extended, million-degree hot halo, an idea supported indirectly by observations of warm gas clouds in the halo and gas-free dwarf spheroidal satellites. X-ray observations have established that hot gas does exist in our Galaxy beyond the local hot bubble; however, it may be distributed in a hot disk configuration. Moreover, recent investigations into the X-ray constraints have suggested that any Galactic corona must be insignificant. Here we re-examine the observational data, particularly in the X-ray and radio bands, in order to determine whether it is possible for a substantial fraction of the Galaxy's baryons to exist in ∼106 K gas. In agreement with past studies, we find that a baryonically closed halo is clearly ruled out if one assumes that the hot corona is distributed with a cuspy Navarro-Frenk-White profile. However, if the hot corona of the galaxy is in an extended, low-density distribution with a large central core, as expected for an adiabatic gas in hydrostatic equilibrium, then it may contain up to 1011 M⊙ of material, possibly accounting for all of the missing Galactic baryons. We briefly discuss some potential avenues for discriminating between a massive, extended hot halo and a local hot disk. © 2013. The American Astronomical Society. All rights reserved.


Diamond-Stanic A.M.,University of California at San Diego | Diamond-Stanic A.M.,Center for Galaxy Evolution | Moustakas J.,University of California at San Diego | Tremonti C.A.,University of Wisconsin - Madison | And 5 more authors.
Astrophysical Journal Letters | Year: 2012

We present the discovery of compact, obscured star formation in galaxies at z ∼ 0.6 that exhibit ≳ 1000kms-1 outflows. Using optical morphologies from the Hubble Space Telescope and infrared photometry from the Wide-field Infrared Survey Explorer, we estimate star formation rate (SFR) surface densities that approach ΣSFR ≈ 3000 M ⊙yr-1kpc-2, comparable to the Eddington limit from radiation pressure on dust grains. We argue that feedback associated with a compact starburst in the form of radiation pressure from massive stars and ram pressure from supernovae and stellar winds is sufficient to produce the high-velocity outflows we observe, without the need to invoke feedback from an active galactic nucleus. © © 2012. The American Astronomical Society. All rights reserved.


Kirby E.N.,University of California at Irvine | Kirby E.N.,Center for Galaxy Evolution | Boylan-Kolchin M.,University of California at Irvine | Boylan-Kolchin M.,Center for Galaxy Evolution | And 4 more authors.
Astrophysical Journal | Year: 2013

Segue 2, discovered by Belokurov et al., is a galaxy with a luminosity of only 900 L ⊙. We present Keck/DEIMOS spectroscopy of 25 members of Segue 2 - a threefold increase in spectroscopic sample size. The velocity dispersion is too small to be measured with our data. The upper limit with 90% (95%) confidence is σv < 2.2 (2.6) km s-1, the most stringent limit for any galaxy. The corresponding limit on the mass within the three-dimensional half-light radius (46 pc) is M 1/2 < 1.5 (2.1) × 105 M ⊙. Segue 2 is the least massive galaxy known. We identify Segue 2 as a galaxy rather than a star cluster based on the wide dispersion in [Fe/H] (from -2.85 to -1.33) among the member stars. The stars' [α/Fe] ratios decline with increasing [Fe/H], indicating that Segue 2 retained Type Ia supernova ejecta despite its presently small mass and that star formation lasted for at least 100 Myr. The mean metallicity, 〈[Fe/H]〉 = -2.22 ± 0.13 (about the same as the Ursa Minor galaxy, 330 times more luminous than Segue 2), is higher than expected from the luminosity-metallicity relation defined by more luminous dwarf galaxy satellites of the Milky Way. Segue 2 may be the barest remnant of a tidally stripped, Ursa Minor-sized galaxy. If so, it is the best example of an ultra-faint dwarf galaxy that came to be ultra-faint through tidal stripping. Alternatively, Segue 2 could have been born in a very low mass dark matter subhalo (v max < 10 km s-1), below the atomic hydrogen cooling limit. © 2013. The American Astronomical Society. All rights reserved.


Ly C.,NASA | Rigby J.R.,NASA | Cooper M.,Center for Galaxy Evolution | Yan R.,University of Kentucky
Astrophysical Journal | Year: 2015

We report on the discovery of 28 z ≈ 0.8 metal-poor galaxies in DEEP2. These galaxies were selected for their detection of the weak [O III] λ4363 emission line, which provides a "direct" measure of the gas-phase metallicity. A primary goal for identifying these rare galaxies is to examine whether the fundamental metallicity relation (FMR) between stellar mass, gas metallicity, and star formation rate (SFR) holds for low stellar mass and high SFR galaxies. The FMR suggests that higher SFR galaxies have lower metallicity (at fixed stellar mass). To test this trend, we combine spectroscopic measurements of metallicity and dust-corrected SFR with stellar mass estimates from modeling the optical photometry. We find that these galaxies are 1.05 ± 0.61 dex above the z ∼ 1 stellar mass-SFR relation and 0.23 ± 0.23 dex below the local mass-metallicity relation. Relative to the FMR, the latter offset is reduced to 0.01 dex, but significant dispersion remains (0.29 dex with 0.16 dex due to measurement uncertainties). This dispersion suggests that gas accretion, star formation, and chemical enrichment have not reached equilibrium in these galaxies. This is evident by their short stellar mass doubling timescale of ≈100+310-75 Myr, which suggests stochastic star formation. Combining our sample with other z ∼ 1 metal-poor galaxies, we find a weak positive SFR-metallicity dependence (at fixed stellar mass) that is significant at 94.4% confidence. We interpret this positive correlation as recent star formation that has enriched the gas but has not had time to drive the metal-enriched gas out with feedback mechanisms. © 2015. The American Astronomical Society. All rights reserved.

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