Browsing by Author "Evans, A. S."

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    A hard X-ray view of luminous and ultra-luminous infrared galaxies in GOALS - I. AGN obscuration along the merger sequence
    (2021) Ricci, C.; Privon, G. C.; Pfeifle, R. W.; Armus, L.; Iwasawa, K.; Torres-Albà, N.; Satyapal, S.; Bauer, F. E.; Treister, E.; Ho, L. C.; Aalto, S.; Arévalo, P.; Barcos-Muñoz, L.; Charmandaris, V.; Diaz-Santos, T.; Evans, A. S.; Gao, T.; Inami, H.; Koss, M. J.; Lansbury, G.; Linden, S. T.; Medling, A.; Sanders, D. B.; Song, Y.; Stern, D.; U, V.; Ueda, Y.; Yamada, S.
    The merger of two or more galaxies can enhance the inflow of material from galactic scales into the close environments of active galactic nuclei (AGNs), obscuring and feeding the supermassive black hole (SMBH). Both recent simulations and observations of AGN in mergers have confirmed that mergers are related to strong nuclear obscuration. However, it is still unclear how AGN obscuration evolves in the last phases of the merger process. We study a sample of 60 luminous and ultra-luminous IR galaxies (U/LIRGs) from the GOALS sample observed by NuSTAR. We find that the fraction of AGNs that are Compton thick (CT;N-H >= 10(24)cm(-2) ) peaks at at a late merger stage, prior to coalescence, when the nuclei have projected separations (d(sep)) of 0.4-6 kpc. A similar peak is also observed in the median N-H [[(1.6 +/- 0.5) x 10(24) cm(-2)].]. The vast majority (85(-9)(+7) per cent)) of the AGNs in the final merger stages (d(sep) less than or similar to 10 kpc) are heavily obscured (N-H = 10(23) cm(-2)), and the median N-H of the accreting SMBHs in our sample is systematically higher than that of local hard X-ray-selected AGN, regardless of the merger stage. This implies that these objects have very obscured nuclear environments, with the gas almost completely covering the AGN in late mergers. CT AGNs tend to have systematically higher absorption-corrected X-ray luminosities than less obscured sources. This could either be due to an evolutionary effect, with more obscured sources accreting more rapidly because they have more gas available in their surroundings, or to a selection bias. The latter scenario would imply that we are still missing a large fraction of heavily obscured, lower luminosity (L2-10 less than or similar to 10(43) erg s(-1)) AGNs in U/LIRGs.
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    A Herschel/PACS Far-infrared Line Emission Survey of Local Luminous Infrared Galaxies
    (IOP PUBLISHING LTD, 2017) Diaz Santos, T.; Armus, L.; Charmandaris, V.; Lu, N.; Stierwalt, S.; Stacey, G.; Malhotra, S.; van der Werf, P. P.; Howell, J. H.; Privon, G. C.; Mazzarella, J. M.; Goldsmith, P. F.; Murphy, E. J.; Barcos Munoz, L.; Linden, S. T.; Inami, H.; Larson, L.; Evans, A. S.; Appleton, P.; Iwasawa, K.; Lord, S.; Sanders, D. B.; Surace, J. A.
    We present an analysis of [O I](63), [O III](88), [N II](122), and [C II](158) far-infrared (FIR) fine-structure line observations obtained with Herschel/PACS, for similar to 240 local luminous infrared galaxies (LIRGs) in the Great Observatories All-sky LIRG Survey. We find pronounced declines ("deficits") of line-to-FIR continuum emission for [N II](122), [O I](63), and [C II](158) as a function of FIR color and infrared luminosity surface density, Sigma(IR). The median electron density of the ionized gas in LIRGs, based on the [N II](122)/[N II](205) ratio, is n(e) = 41 cm(-3). We find that the dispersion in the [C II](158) deficit of LIRGs is attributed to a varying fractional contribution of photodissociation regions (PDRs) to the observed [C II](158) emission, f ([C II](158)(PDR)) = [C II](158)(PDR)/C II](158), which increases from similar to 60% to similar to 95% in the warmest LIRGs. The [O I](63)/[C II](158)(PDR) ratio is tightly correlated with the PDR gas kinetic temperaturein sources where [O I] 63 is not optically thick or self-absorbed. For each galaxy, we derive the average PDR hydrogen density, n(H), and intensity of the interstellar radiation field, G, in units of G(0) and find G/n(H) ratios of similar to 0.1-50 G(0) cm(3), with ULIRGs populating the upper end of the distribution. There is a relation between G/n(H) and Sigma(IR), showing a critical break at Sigma(IR)* similar or equal to 5 x 10(10) L-circle dot kpc(-2). Below Sigma(IR)*, G/n(H) remains constant, similar or equal to 0.32G(0) cm(3), and variations in Sigma(IR) are driven by the number density of star-forming regions within a galaxy, with no change in their PDR properties. Above Sigma(IR)*, G/n(H) increases rapidly with Sigma(IR), signaling a departure from the typical PDR conditions found in normal star-forming galaxies toward more intense/harder radiation fields and compact geometries typical of starbursting sources.
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    Massive Star Cluster Formation and Destruction in Luminous Infrared Galaxies in GOALS
    (IOP PUBLISHING LTD, 2017) Linden, S. T.; Evans, A. S.; Rich, J.; Larson, K. L.; Armus, L.; Diaz Santos, T.; Privon, G. C.; Howell, J.; Inami, H.; Kim, D. C.; Chien, L. H.; Vavilkin, T.; Mazzarella, J. M.; Modica, F.; Surace, J. A.; Manning, S.; Abdullah, A.; Blake, A.; Yarber, A.; Lambert, T.
    We present the results of a Hubble Space Telescope ACS/HRC FUV, ACS/WFC optical study into the cluster populations of a sample of 22 Luminous Infrared Galaxies in the Great Observatories All-Sky LIRG Survey. Through integrated broadband photometry, we have derived ages and masses for a total of 484 star clusters contained within these systems. This allows us to examine the properties of star clusters found in the extreme environments of LIRGs relative to lower luminosity star-forming galaxies in the local universe. We find that by adopting a Bruzual & Charlot simple stellar population model and Salpeter initial mass function, the age distribution of the clusters declines as dN/d tau = tau(-0.9+/-0.3), consistent with the age distribution derived for the Antennae Galaxies, and interpreted as evidence for rapid cluster disruption occurring in the strong tidal fields of merging galaxies. The large number of 10(6) M-circle dot young clusters identified in the sample also suggests that LIRGs are capable of producing more high-mass clusters than what is observed to date in any lower luminosity star-forming galaxy in the local universe. The observed cluster mass distribution of dN/dM = M-1.95+/-0.11 is consistent with the canonical -2 power law used to describe the underlying initial cluster mass function (ICMF) for a wide range of galactic environments. We interpret this as evidence against mass-dependent cluster disruption, which would flatten the observed CMF relative to the underlying ICMF distribution.
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    MORPHOLOGY AND MOLECULAR GAS FRACTIONS OF LOCAL LUMINOUS INFRARED GALAXIES AS A FUNCTION OF INFRARED LUMINOSITY AND MERGER STAGE
    (IOP PUBLISHING LTD, 2016) Larson, K. L.; Sanders, D. B.; Barnes, J. E.; Ishida, C. M.; Evans, A. S.; Mazzarella, J. M.; Kim, D. C.; Privon, G. C.; Mirabel, I. F.; Flewelling, H. A.
    We present a new, detailed analysis of the morphologies and molecular gas fractions (MGFs) for a complete sample of 65 local luminous infrared galaxies from Great Observatories All-Sky Luminous Infrared Galaxies (LIRG) Survey using high resolution I-band images from The Hubble Space Telescope, the University of Hawaii 2.2 m Telescope and the Pan-STARRS1 Survey. Our classification scheme includes single undisturbed galaxies, minor mergers, and major mergers, with the latter divided into five distinct stages from pre-first pericenter passage to final nuclear coalescence. We find that major mergers of molecular gas-rich spirals clearly play a major role for all sources with L-IR > 10(11.5)L(circle dot); however, below this luminosity threshold, minor mergers and secular processes dominate. Additionally, galaxies do not reach L-IR > 10(12.0)L(circle dot) until late in the merger process when both disks are near final coalescence. The mean MGF (MGF = M-H2(M*+ M-H2)) for non-interacting and early-stage major merger LIRGs is 18 +/- 2%, which increases to 33 +/- 3%, for intermediate stage major merger LIRGs, consistent with the hypothesis that, during the early-mid stages of major mergers, most of the initial large reservoir of atomic gas (HI) at large galactocentric radii is swept inward where it is converted into molecular gas (H-2).