Dense Molecular Gas in the Bulges of External Galaxies

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Session 68 -- Star Formation
Oral presentation, Thursday, 10:30-12:00, Zellerbach Playhouse Room

[68.07] Dense Molecular Gas in the Bulges of External Galaxies

Tamara T. Helfer and Leo Blitz (UMD)

The pressures in the bulges of galaxies may be two to three orders of magnitude higher than the pressures in galactic disks (Spergel and Blitz 1992). Such an extraordinary difference in gas pressures should be revealed as a larger fraction of dense gas or broader linewidths in galactic bulges when compared with their disks. We present here the results of a preliminary study of the dense gas emission from a sample of 19 mostly normal spiral galaxies: we observed the 3 mm emission from the molecules HCN and CS, which trace gas densities of $\sim$ 10$^5$ cm$^{-3}$. Our high detection rate (68\% in HCN, 58\% in CS) suggests that most spiral galaxies, not just starburst galaxies, contain appreciable quantities of dense gas in their bulges. The HCN/CO ratio measured in the bulge of a given source can be quite sensitive to the resolution of the instrument used for the observations; the ratio tends to be significantly higher for observations made using a telescope with a smaller beam. The CS/CO ratios measured in extragalactic bulges are consistent with the ratio observed in the the bulge of the Milky Way, but they appear to be at least a factor of two larger than seen in local disk GMCs and at least an order of magnitude higher than the ratio in high latitude clouds in the Milky Way. Since the dense gas ratio measured in local GMCs is an upper limit to the local disk ratio in the Milky Way, we conclude that nuclear molecular clouds are fundamentally different than disk molecular clouds. Despite the similar critical excitation densities of the two molecules we studied, we find that the HCN emission is consistently stronger than is the CS by an average of $\sim$ 2.4 in the integrated intensities.

This research was partially supported by NSF grant AST8918912.

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