ENHANCED DENSE GAS FRACTION IN ULTRALUMINOUS INFRARED GALAXIES

Journal Article · Sun Dec 20 04:00:00 EST 2009 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/707/2/1217· OSTI ID:21392572

Juneau, S; Shirley, Y L; Bussmann, R S ^[1]; Narayanan, D T ^[2]; Moustakas, J ^[3]; Kennicutt, R C ^[4]

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1. Steward Observatory, Department of Astronomy, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721 (United States)
2. Harvard-Smithsonian Center for Astrophysics, 60 Garden St. MS 51, Cambridge, MA, 02138 (United States)
3. Department of Physics, New York University, 4 Washington Place, New York, NY 10003 (United States)
4. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA (United Kingdom)

We present a detailed analysis of the relation between infrared luminosity and molecular line luminosity, for a variety of molecular transitions, using a sample of 34 nearby galaxies spanning a broad range of infrared luminosities (10{sup 10} L{sub sun} < L{sub IR} < 10{sup 12.5} L{sub sun}). We show that the power-law index of the relation is sensitive to the critical density of the molecular gas tracer used, and that the dominant driver in observed molecular line ratios in galaxies is the gas density. As most nearby ultraluminous infrared galaxies (ULIRGs) exhibit strong signatures of active galactic nuclei (AGNs) in their center, we revisit previous claims questioning the reliability of HCN as a probe of the dense gas responsible for star formation in the presence of AGNs. We find that the enhanced HCN(1-0)/CO(1-0) luminosity ratio observed in ULIRGs can be successfully reproduced using numerical models with fixed chemical abundances and without AGN-induced chemistry effects. We extend this analysis to a total of 10 molecular line ratios by combining the following transitions: CO(1-0), HCO{sup +}(1-0), HCO{sup +}(3-2), HCN(1-0), and HCN(3-2). Our results suggest that AGNs reside in systems with higher dense gas fraction, and that chemistry or other effects associated with their hard radiation field may not dominate (NGC 1068 is one exception). Galaxy merger could be the underlying cause of increased dense gas fraction, and the evolutionary stage of such mergers may be another determinant of the HCN/CO luminosity ratio.

• OSTI ID: 21392572
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 707; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABUNDANCE
EVOLUTION
GALACTIC EVOLUTION
GALAXIES
GALAXY NUCLEI
LUMINOSITY
OPTICAL PROPERTIES
PHYSICAL PROPERTIES
STARS