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Session 65 - ISM in Nearby Galaxies.
Oral session, Tuesday, January 16
Salon del Rey South, Hilton
\def\alwaysmath#1\ifmmode #1 \else #1\mkern-5mu \fi \def\hiiH \small II \def\lsim\raisebox-.7ex\stackrel\textstyle <\sim \def\jth\alwaysmathJ\!=\!3\!\rightarrow\!2 \def\hcop\alwaysmathHCO^+ I observed the molecular emission from the dense gas in starburst galaxies to determine the physical properties of their nuclear clouds, and to compare them with clouds in normal galaxies and the Milky Way. The project is comprised of several sections: single-dish observations of galaxies, wide-field mapping of the Galactic Center, and millimeter interferometry of starburst galaxies.
The single-dish project involved mapping the millimeter and submillimeter emission from high density tracing molecules such as HCN, \hcop\ and CS from starburst galaxies to find the extent of the dense gas in their nuclei, and to determine the physical properties of their molecular clouds. I also searched for HCN \jth\ emission from nearly 20 galaxies that span a range of star formation activity. The survey allows me to place the starburst data into the proper context and to understand how different starbursts are from normal galaxies. I find that the average molecular gas density in galactic nuclei is correlated with its star formation efficiency.
The spatial resolution of single-dish observations of galaxies is poor, and to compare properly the gas in different sources, we must examine them on the same spatial scales. Therefore, we made wide-field maps of the HCN, CS, and CO emission from the Milky Way, and intereferometric maps of the dense gas in starburst galaxies to compare the properties of the Galactic Center with those of starburst and other normal galaxies. I find a loose association between dense clouds in starbursts and star forming regions (radio continuum). Some of these radio sources may be thermal, in which case they could have, within a small volume (\lsim 10 pc), 20--100 times the number of massive stars than the most luminous \hii\ region complexes in the Galaxy. I also find large-scale (> 100 pc) gradients in chemical abundances, gas densities and temperatures. In general the average molecular gas density within the nuclear region of a starburst is higher than that of normal galaxies. Also, despite surprisingly similar morphologies, the emission from dense gas is much more luminous and extended in starburst galaxies than in the Milky Way.
