Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aas...185.7702g&link_type=abstract
American Astronomical Society, 185th AAS Meeting, #77.02; Bulletin of the American Astronomical Society, Vol. 26, p.1437
Astronomy and Astrophysics
Astronomy
Scientific paper
We have carried out systematic HCN (1-0) observations (complimentary with CO observation) in a sample of ~ 40 IR/CO bright/luminous galaxies using the NRAO 12-m, IRAM 30-m and FCRAO 14-m telescopes. HCN traces much higher density molecular gas ( ~ 10(5) cm(-3) ) than that of CO. LHCN is a measure of the mass of dense molecular gas and and LHCN/LCO is an indicator of the fraction of dense molecular gas. For 10 nearby galaxies we present HCN maps along the major axis. A high detection rate of nearly ~ 75 % is obtained. This is one of the largest HCN surveys undertaken so far. We show that all galaxies observed so far follow the correlations between LIR and LHCN and between LIR/LCO and LHCN/LCO proposed by Solomon et al (1992). This suggests that the star formation efficiency depends upon the fraction of dense molecular gas and the ratio of LIR/LHCN is similar in all galaxies, whether normal, starburst or ultraluminous. Dense molecular gas is not confined to the inner ~ 1 kpc nuclear regions, and can be detected to a radius as large as ~ 5 kpc, perhaps to a diameter of ~ D25/4, although the highest concentration of dense molecular gas is in the central region. A significant fraction of dense molecular gas is distributed in the inner disks of galaxies outside the nuclear or inner ring starburst region. The starburst cores have the highest fraction of dense molecular gas with surface brightness ratio IHCN/ICO comparable to that of ultraluminous IR galaxies, whereas this ratio falls off quickly in the galaxy disks at larger radii (M82 is probably an exception). HCN emission is thus a much better indicator of star forming molecular gas than is CO emission.
Gao Yanfang
Solomon Philip M.
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