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Title:
Variation of Molecular Line Ratios and Cloud Properties in the ARP 299 Galaxy Merger
Authors:
Aalto, S.; Radford, Simon J. E.; Scoville, N. Z.; Sargent, A. I.
Publication:
Astrophysical Journal Letters v.475, p.L107 (ApJL Homepage)
Publication Date:
02/1997
Origin:
APJ
ApJ Keywords:
GALAXIES: EVOLUTION, GALAXIES: INDIVIDUAL NAME: ARP 299, GALAXIES: ISM, GALAXIES: STARBURST, RADIO LINES: GALAXIES, RADIO LINES: ISM
DOI:
10.1086/310475
Bibliographic Code:
1997ApJ...475L.107A

Abstract

High-resolution observations of 12CO (2."3), 13CO (3."9), and HCN (5."4) J = 1--0 in the galaxy merger Arp 299 (IC 694 and NGC 3690) show that the line ratios vary dramatically across the system. The 12CO/13CO ratio is unusually large, 60 +/- 15, at the IC 694 nucleus, where 12CO emission is very strong, and much smaller, 10 +/- 3, in the southern extended disk of that galaxy. Elsewhere, the 12CO/13CO line ratio is 5--20, typical of spiral galaxies. The line ratio variation in the overlap between the two galaxies is smaller, ranging from 10 +/- 3 in the east to 20 +/- 4 in the west. The 12CO/HCN line ratio also varies across Arp 299, although to a lesser degree. HCN emission is bright toward each galaxy nucleus and in the extranuclear region of active star formation; it was not detected in the IC 694 disk or the eastern part of the overlap region, leading to lower limits of 25 and 20, respectively. By contrast, at the nuclei of IC 694 and NGC 3690 the ratios are 9 +/- 1 and 14 +/- 3, respectively. In the western part of the overlap region it is 11 +/- 3. The large 12CO/13CO 1--0 intensity ratio at the nucleus of IC 694 can primarily be attributed to a low-to-moderate optical depth ( tau <~ 1) in the 12CO 1--0 line. These data support the hypothesis that unusually high 12CO/13CO line ratios (>20) are associated with extremely compact molecular distributions in the nuclei of merging galaxies. Relative to 12CO, 13CO 1--0 is brightest in quiescent regions of low 12CO surface brightness and weakest in starburst regions and the galactic nuclei. A medium consisting of dense (n = 104--105 cm-3) and warm (Tk > 50 K) gas will reproduce the extreme line ratios observed in the nucleus of IC 694, where the area filling factor must be at least 20%.
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