Morphology and Molecular Gas Fractions of Local Luminous Infrared Galaxies As a Function of Infrared Luminosity and Merger Stage K
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The Astrophysical Journal, 825:128 (16pp), 2016 July 10 doi:10.3847/0004-637X/825/2/128 © 2016. The American Astronomical Society. All rights reserved. MORPHOLOGY AND MOLECULAR GAS FRACTIONS OF LOCAL LUMINOUS INFRARED GALAXIES AS A FUNCTION OF INFRARED LUMINOSITY AND MERGER STAGE K. L. Larson1,2, D. B. Sanders1,3, J. E. Barnes1,4, C. M. Ishida1,5,A.S.Evans6,7,8,V.U9, J. M. Mazzarella2, D.-C. Kim7, G. C. Privon10,11, I. F. Mirabel12,13, and H. A. Flewelling1 1 University of Hawaii, Institute for Astronomy, 2680 Woodlawn Dr., Honolulu, HI 96822, USA; [email protected] 2 Infrared Processing and Analysis Center, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA 3 Visiting Astronomer, Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia 4 Visiting Professor, Yukawa Institute for Theoretical Physics, Kyoto University, Kitashirakawa Oiwakecho, Sakyo-ku, Kyoto 606-8502, Japan 5 Department of Physics and Astronomy, University of Hawaii at Hilo, Hilo, HI 96720-4091, USA 6 Department of Astronomy, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904, USA 7 National Radio Astronomy Observatory, Edgemont Road, Charlottesville, VA 22904, USA 8 Visiting Astronomer, Infrared Processing and Analysis Center, California Institute of Technology, 1200 E. California Boulevard, Pasadena, CA 91125, USA 9 Department of Physics and Astronomy, University of California, Riverside, 900 University Avenue, Riverside, CA 92521, USA 10 Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile 11 Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, 306, Santiago 22, Chile 12 Instituto de Astronomía y Física del Espacio. CONICET. Universidad de Buenos Aires. Ciudad Universitaria.1428 Buenos Aires, Argentina 13 Laboratoire AIM, CEA/DSM/Irfu/Service d’Astrophysique, Centre de Saclay, Bât. 709, F-91191 Gif-sur-Yvette Cedex, France Received 2015 June 24; accepted 2016 April 4; published 2016 July 12 ABSTRACT 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 11.5 all sources with LIR > 10L ; however, below this luminosity threshold, minor mergers and secular processes 12.0 dominate. Additionally, galaxies do not reach LIR > 10 L until late in the merger process when both disks are near final coalescence. The mean MGF (MGF =+MM() M) for non-interacting and early-stage major HH22* 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 (H2). Key words: galaxies: evolution – galaxies: interactions – infrared: galaxies 1. INTRODUCTION the subject of intense research and debate (e.g., Joseph 1999; Sanders 1999). While a degree of consensus has been reached Luminous infrared galaxies (LIRGs, L (–8 1000m m )> IR in understanding the origin and evolution of (U)LIRGs in the 1011L ) are galaxies where intense infrared emission is fueled local universe, there continue to be conflicting views of the by star formation and active galactic nuclei (AGN)(e.g., Rieke origin and evolution of (U)LIRGs at higher redshift (e.g., Elbaz &Low1972; Joseph & Wright 1985; Soifer et al. 1987;Sanders et al. 2007; Tacconi et al. 2008; Hung et al. 2013), where the et al. 1988a). Although relatively rare in the local universe ( ) surface brightness dimming and decreased spatial resolution are z < 0.3 , their number density still exceeds that of optically the major complicating factors in interpreting the multi- selected starburst and Seyfert galaxies at comparable redshift and ( ) wavelength properties of individual sources. Deeper, and bolometric luminosity e.g., Soifer et al. 1987 . For the more higher resolution data will clearly enhance our ability to ( extreme ultra-luminous objects ULIRGs: LIR (–8 1000m m )> understand the nature of (U)LIRGs at high redshift, but it is 12 ) 10 L , whose infrared luminosity is equal to the bolometric also important to continue studies of nearby sources in order to ( ) luminosity of optically selected quasi-stellar objects QSOs ,the have a well understood sample for comparison with their high- number density remains a factor of ~´1.5– 3 larger than that for redshift counterparts. QSOs at comparable bolometric luminosity (e.g., Sanders According to the hierarchical formation model of galaxies, et al. 2003). galaxies build up mass over time through interactions and Deep infrared surveys have shown that the number density mergers (White & Rees 1978; Barnes & Hernquist 1992). Disk of (U)LIRGs increases rapidly with increasing redshift, e.g., galaxy mergers create gravitational torques that cause gaseous Fµ+()zz (1 )3-5, where the exponent increases with increas- dissipation and inflows in the galaxies (Barnes & Hernquist ing LIR (Kim & Sanders 1998).Atz > 1, the bolometric 1996; Mihos & Hernquist 1996).Gaseousinflows then induce infrared luminosity of the population of LIRGs exceeds that of star formation and can feed powerful AGN. To truly understand the total UV–optical luminosity output of all galaxies at a given the processes involved in galaxy evolution we need to trace how redshift, while the relative contribution from ULIRGs increases galaxy properties change with galaxy morphology over time. to where they equal or exceed that from LIRGs (e.g., Le Floc’h A useful sample for studying the properties of local (U)LIRGs et al. 2005). What powers (U)LIRGs, as well as understanding is the Great Observatories All-Sky LIRG Survey (GOALS: their relationship to galaxy evolution in general, continues to be Armus et al. 2009),aflux-limited sample (S60m m > 5.24 Jy) of 1 The Astrophysical Journal, 825:128 (16pp), 2016 July 10 Larson et al. 11 203 galaxies with LIR > 10 L selected from the IRAS Revised sky RBGS, 203 are LIRGs, i.e., log (LLIR ) > 11.0, and 90 of Bright Galaxy Survey (RBGS: Sanders et al. 2003). Extensive these LIRGs are in our northern region of the sky. multi-wavelength (radio to X-ray) imaging and spectroscopic data Ishida (2004) used the UH 2.2 m telescope to observe a total have been obtained for different subsamples of the GOALS14 of 65 objects from the complete flux-limited sample of 90 sources, with the most extensive coverage being that for the objects in the northern LIRG Sample. The northern observa- sample of 65 GOALS objects discussed in this paper. Observa- tions are complete above log (LLIR ) > 11.54 with all 38 tions have shown that all ULIRGs and many LIRGS in the local objects being observed. For the 42 lower luminosity LIRGs universe involve strong tidal interactions and mergers between with log (LLIR )–= 11.01 11.54, approximately half molecular gas-rich disk galaxies (e.g., Haan et al. 2011).These (2752= 52%) were observed by Ishida (2004). The galaxies may represent an evolutionary stage in the formation of availability of deep I-band wide-field imaging (Ishida 2004) quasars and massive ellipticals from gas-rich mergers (Sanders and total galaxy stellar masses U et al. (2012) for the northern et al. 1988a;Genzeletal.2001). LIRG Sample, was critical for determining the morphology Detailed morphology study of LIRGs is important to determine classifications and computing the MGFs, respectively, that are the role of interactions and mergers in the evolution of infrared presented in the current paper. Table 1 gives the object names, luminosities and molecular gas fractions (MGFs). Our previous data references, and lists the general galaxy properties for all 65 studies of LIRG morphology either focused on only the most objects in the northern LIRG sample. luminous sources (e.g., Sanders et al. 1988a; Veilleux et al. 2002, This paper is the third in a series of papers describing the 2006;Kimetal.2013) or relied on lower resolution (~2) data multi-wavelength properties of the northern LIRG sample. The (e.g., Stierwalt et al. 2013). Here we use higher resolution Hubble original optical imaging data were presented by Ishida (2004).A Space Telescope (HST), The University of Hawaii (UH) 2.2 m second paper (Uetal.2012) presented complete spectral energy Telescope and Pan-STARRS1 (PS1) optical data to determine the distributions using the large amount of multi-wavelength data galaxy merger stage across the full range on LIRGs now available in GOALS, and used these data to compute more 11 12.5 (10 LL <<IR 10 L). accurate galaxy stellar masses (M*) and infrared luminosities The total mass of molecular gas represents the fuel immediately (LIR). The current paper presents a more detailed analysis of available for forming new stars which are presumably responsible galaxy morphology along with an analysis of the variation of for powering the large observed infrared luminosity. The galaxy galaxy properties. A companion paper (fourth in this series) MGF, or the ratio of total mass of molecular gas to the total mass compares the visual classifications with classifications derived (gasmassplusmassofpreviouslyformedstars), MGF = using automated methods (e.g., Gini, M20, Compactness, MM()+ M, is also an important parameter that can be )( ) HH22* Assymetry, Smoothness K. Larson et al. 2016, in preparation . used to estimate e-folding times for the growth of galaxy stellar The fifth and final paper in this series (K.