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Title:
A Dense Gas Trigger for OH Megamasers
Authors:
Darling, Jeremy
Affiliation:
AA(Center for Astrophysics and Space Astronomy, Department of Astrophysical and Planetary Sciences, University of Colorado, 389 UCB, Boulder, CO 80309-0389 .)
Publication:
The Astrophysical Journal, Volume 669, Issue 1, pp. L9-L12. (ApJL Homepage)
Publication Date:
11/2007
Origin:
UCP
ApJ Keywords:
Galaxies: Interactions, Galaxies: Nuclei, Galaxies: Starburst, Masers, Radio Lines: Galaxies
DOI:
10.1086/523756
Bibliographic Code:
2007ApJ...669L...9D

Abstract

HCN and CO line diagnostics provide new insight into the OH megamaser (OHM) phenomenon, suggesting a dense gas trigger for OHMs. We identify three physical properties that differentiate OHM hosts from other starburst galaxies: (1) OHMs have the highest mean molecular gas densities among starburst galaxies; nearly all OHM hosts have n¯(H2)=103-104 cm-3 (OH line-emitting clouds likely have n(H2)>104 cm-3). (2) OHM hosts are a distinct population in the nonlinear part of the IR-CO relation. (3) OHM hosts have exceptionally high dense molecular gas fractions, LHCN/LCO>0.07, and comprise roughly half of this unusual population. OH absorbers and kilomasers generally follow the linear IR-CO relation and are uniformly distributed in dense gas fraction and LHCN, demonstrating that OHMs are independent of OH abundance. The fraction of non-OHMs with high mean densities and high dense gas fractions constrains beaming to be a minor effect: OHM emission solid angle must exceed 2π steradians. Contrary to conventional wisdom, IR luminosity does not dictate OHM formation; both star formation and OHM activity are consequences of tidal density enhancements accompanying galaxy interactions. The OHM fraction in starbursts is likely due to the fraction of mergers experiencing a temporal spike in tidally driven density enhancement. OHMs are thus signposts marking the most intense, compact, and unusual modes of star formation in the local universe. Future high-redshift OHM surveys can now be interpreted in a star formation and galaxy evolution context, indicating both the merging rate of galaxies and the burst contribution to star formation.
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