High-resolution observations of the molecular gas in luminous infrared galaxies
Abstract (Summary)I have performed a high-resolution imaging survey of the molecular gas in seven luminous infrared galaxies using the Owens Valley millimeter array. This work has produced 2"-resolution maps of the CO 1â0 emission. Further high-resolution data has been obtained for subsets of these galaxies in CO 2â1, HCN 1â0, HCO[superscript +] 1â0 and CS 1â0. The CO maps reveal a systematic variation in the morphology of the molecular gas with the interaction state of the system. The three mergers with a single IR/radio nucleus show very bright and compact CO cores that peak at the stellar nucleus. The two mergers with double IR/radio nuclei also possess bright CO cores, but these peak roughly midway between the nuclei and show an extent nearly equal to the nuclear separation. In these objects, the gas cores of the individual merging galaxies appear to be coalescing while the stellar nuclei still remain distinct. Based on dynamical arguments, the Galactic conversion factor from CO luminosity to molecular gas mass is overestimating the gas mass in Mrk 231 by at least a factor of 3.6 and in NGC 6240 by at least a factor of 1.5. Nonetheless, the molecular gas likely dominates the nuclear gravitational potential. The probable cause of the severe overestimate in Mrk 231 is the high brightness temperature of the CO 1â0 emission (T[subscript b] > 34 K). Starbursts remain a viable explanation for the powerful output of luminous infrared galaxies. Upper limits to the free-free mm-wave continuum are used to constrain a starburst model from the literature, with the result that starbursts can reasonably explain all the systems studied here except perhaps Mrk 231. In several cases, though, they are required to be older than several 10[superscript 7] yr. Furthermore, the extreme mid-IR optical depths implied by the CO surface brightnesses confirm a prediction of the compact starburst model of Condon et al. (1991). The trend of increasing L[subscript FIR]/L[subscript CO] ratio with increasing CO surface brightness is confirmed. The high concentrations of molecular gas thus appear intimately related to the high luminosities of these systems. The CO/HCN ratio varies by an order of magnitude even in this small sample of luminous mergers, suggesting that the gas properties are dramatically variable.
Advisor:Nick Z. Scoville
School Location:USA - California
Source Type:Master's Thesis
Date of Publication:09/16/1996