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Molecular Gas in Interacting Galaxies*

Ming Zhu

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Doctor of Philosophy 2001
Graduate Department of Astronomy and Astrophysics, University of Toronto

A systematic study of the molecular gas properties in strongly interacting galaxies (SIGs) has been undertaken, which includes two parts: (1) a statistical study of a large, optically-selected, complete sample of SIGs; (2) a case study of the nearest colliding pair NGC 4038/9 (the "Antennae") with multi-transition data of both 12CO and 13CO.

Consisting of 126 galaxies in 92 systems, our complete sample of SIGs includes all the SIGs in the northern sky with optical magnitude BT < 14.5. CO data have been collected for 95 SIGs (59 of which were observed by us) as well as for comparison samples of 59 weakly interacting and 69 isolated spiral galaxies. The statistical analysis of the samples shows that the SIGs, especially the colliding and merging systems, have a higher CO luminosity than isolated spiral galaxies. However, there is no significant difference in the atomic gas contents between the samples. This indicates that the excess CO emission is not due to the conversion of atomic gas to molecular gas, but may more plausibly be accounted for by a lower CO-to-H2 conversion factor X.

For the Antennae galaxies, we have obtained high quality, fully sampled, single dish maps at 12CO J=1-0 and 3-2 transitions with an angular resolution of 15" (1.5 kpc) This is so far the highest resolution single dish data at both the J=1-0 and 3-2 transitions. 12CO J=2-1 data at the positions of the two nuclei as well as in part of the overlap region with 20" angular resolution have also been obtained. We find twice as much 12CO J=1--0 flux in the Nobeyama 45m single dish map as was reported by Wilson et. al. (2000) using the OVRO interferometer. The 12CO J=1--0, 2--1, 3--2 emission all peak in an off-nucleus region adjacent to where the two disks overlap. Using the conventional X factor yields approximately 4 x 109 M_sun molecular gas mass in the overlap region. Such large amounts of molecular gas are almost impossible for this particular region to accumulate given the relatively short lifetime of molecular clouds and the limited period of time for this region to overlap.

The 13CO J=2--1 and 3--2 line emission is detected at selected points in the two nuclei and the overlap region. Both the 12CO/13CO J=2--1 and 3--2 integrated intensity ratios are remarkably high in the overlap region. This is the first published case in which such high 12CO/13CO J=2--1 and 3--2 ratios are found outside a galactic nucleus. Detailed LVG modeling indicates that the 12CO and 13CO emission come from different spatial components. The 12CO emission originates from a non-virialized low density gas component with a large velocity gradient. Such a large velocity gradient can produce "over luminous" CO emission and the model derived X factor is an order of magnitude lower than the conventional value. We suggest that the apparently strong CO emission in the overlap region of the Antennae galaxies is due to increased radiative efficiency, possibly because of the large velocity dispersion.

A comparison of the CO J=3--2 emission with the SCUBA 850 um continuum in the Antennae galaxies shows that CO line emission on average contributes 46% of the 850 um continuum flux and the ratio of 12CO J=3--2 to SCUBA 850 um flux varies by a factor of two across the system. After correcting for the 12CO J=3--2 contamination, the dust emission at 850 um detected by SCUBA is consistent with the thermal emission from a single warm dust component with a mass of 1.7 x 107 M_sun. This value is more than six times lower than that estimated by Haas et. al. (2000) using the uncorrected 850 um data.

* This is a longer abstract than appears in the print version of the thesis.
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Reproduced with permission by Marlene Cummins library@astro.utoronto.ca
March 14 2001