The Molecular Interstellar Medium in Ultraluminous Infrared Galaxies

Abstract

We present CO observations of a large sample of ultraluminous IR galaxies out to z = 0.3. Most of the galaxies are interacting, but not completed mergers. All but one have high CO(1-0) luminosities, log(Lco [K-km/s-pc^2]) = 9.92 +/- 0.12. The dispersion in Lco is only 30%, less than that in the FIR luminosity. The integrated CO intensity correlates Strongly with the 100 micron flux density, as expected for a black body model in which the mid and far IR radiation are optically thick. We use this model to derive sizes of the FIR and CO emitting regions and the enclosed dynamical masses. Both the IR and CO emission originate in regions a few hundred parsecs in radius. The median value of Lfir/Lco = 160 Lsun/(K-km/s-pc^2), within a factor of two of the black body limit for the observed FIR temperatures. The entire ISM is a scaled up version of a normal galactic disk with densities a factor of 100 higher, making even the intercloud medium a molecular region. Using three different techniques of H2 mass estimation, we conclude that the ratio of gas mass to Lco is about a factor of four lower than for Galactic molecular clouds, but that the gas mass is a large fraction of the dynamical mass. Our analysis of CO emission reduces the H2 mass from previous estimates of 2-5e10 Msun to 0.4-1.5e10 Msun, which is in the range found for molecular gas rich spiral galaxies. A collision involving a molecular gas rich spiral could lead to an ultraluminous galaxy powered by central starbursts triggered by the compression of infalling preexisting GMC's.Comment: 34 pages LaTeX with aasms.sty, 14 Postscript figures, submitted to ApJ Higher quality versions of Figs 2a-f and 7a-c available by anonymous FTP from ftp://sbast1.ess.sunysb.edu/solomon/

Evidence of ongoing AGN-driven feedback in a quiescent post-starburst E+A galaxy

“…Using table 4 in Genzel et al (2015), and assuming that the total gas mass is 1.4 times the molecular gas mass, we obtain Mgas ≈ 10 10.43 M . This value is consistent with gas mass observations in ULIRGs with similar properties (e.g., Solomon et al 1997). …”

ALMA observations of cold molecular gas filaments trailing rising radio bubbles in PKS 0745−191

“…ULIRG and starburst galaxy environments are known to have kinematically disturbed molecular gas with higher temperatures and velocity dispersions causing reductions in X CO by a factor of a few to ten (eg. Solomon et al 1997;Downes & Solomon 1998;Bolatto, Wolfire & Leroy 2013). However, as discussed in section 3.5, the low gas velocities in PKS 0745-191 and narrow velocity dispersions observed for molecular clouds in BCGs are inconsistent with the conditions in starburst galaxies.…”

“…Although the BCG in PKS 0745-191 falls short of the massive starbursts found in ULIRGs, the molecular gas mass could be overestimated by a factor of a few (Solomon et al 1997;Downes & Solomon 1998). For kinematically disturbed and turbulent gas associated with outflows where the gas may be optically thin even lower X CO factors are expected (for a review see Bolatto, Wolfire & Leroy 2013).…”

LIGHTING THE DARK MOLECULAR GAS: H2 AS A DIRECT TRACER

“…a CO,Gal therefore gives an overestimate of H 2 gas masses in these galaxies (Downes et al 1993;Bryant & Scoville 1999). Moreover, a CO,Gal can yield molecular gas masses greater than the observed dynamical mass (Solomon et al 1997). To avoid this, a lower value of conversion factor is suggested for ULIRGs and other merger systems-a CO =0.8  M (K km s −1 ) -pc 2 1 , 5.5×lower than the standard Galactic value (Downes & Solomon 1998).…”