From G.Dahmen@qmw.ac.uk Thu Jul  3 12:52:00 1997
From: Gereon Dahmen <G.Dahmen@qmw.ac.uk>
Date: Thu, 3 Jul 1997 17:50:10 +0100 (BST)
To: gcnews@astro.umd.edu
Subject: C18O Paper II - endlich
Cc: G.Dahmen@qmw.ac.uk
X-Status: A


\documentstyle{laa}
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\topmargin3cm
\hyphenation{Ra-dio-astro-no-mie}
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\begin{document}
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\thesaurus{Interstellar Matter (03.20.9, 04.19.1, 09.13.2., 09.19.1, 
10.03.1, 13.19.3)}
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\title{Molecular Gas in the Galactic Center Region}
\subtitle{II. The Large Scale Structure and Physics based on a 
C$^{18}$O($J = 1 \to 0$) Survey}
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\author{G.~Dahmen\inst{1,3}, S.~H\"uttemeister\inst{2,4,1}, 
T.L.~Wilson\inst{1}}
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\offprints{S.\ H\"uttemeister, Radioastronomisches Institut,
           Universit\"at Bonn}
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\institute{
 Max-Planck-Institut f\"ur Radioastronomie, 
 Auf dem H\"ugel 69, 53121 Bonn, Germany 
\and
 Harvard-Smithsonian Center for Astrophysics, 
 60 Garden Street, Cambridge, MA 02138, U.S.A. 
\and
 Physics Department, Queen Mary \& Westfield College, University of London, 
 Mile End Road, London E1 4NS, England 
\and
 Radioastronomisches Institut, Universit\"at Bonn, 
 Auf dem H\"ugel 71, 53121 Bonn, Germany 
}
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\date{Submitted on 3$^{\rm rd}$ July 1997 to A\&A}
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\maketitle
\markboth{G.\ Dahmen et al.: Molecular Gas in the Galactic Center Region, 
          II. The Large Scale Structure and Physics~\dots}{ }
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\begin{abstract}
The large scale structure and physics of molecular gas in the Galactic
center region is discussed based on the detailed analysis of a
9\arcmin{} resolution survey of the Galactic center region in the $J =
1 \to 0$ line of C$^{18}$O. Emphasis is placed on the comparison with
\mbox{$^{12}$CO(1--0)} data.  The line shapes of \mbox{C$^{18}$O(1--0)}
and \mbox{$^{12}$CO(1--0)} differ significantly.  The ratio of the
intensities of the two isotopomers in the Galactic center region is
generally higher than the value of $\sim$\,15 expected from the
``Standard Conversion Factor'' (SCF) of $^{12}$CO integrated line
intensity to H$_2$ column density.  In the 9\arcmin{}-beam, this ratio
is in the range from 30 to 200, mostly $\sim$\,60 to 80. From LVG
calculations, we estimate that the large scale \mbox{$^{12}$CO(1--0)}
emission in the Galactic center region is of moderate ($\tau \grsim 1$)
or low optical depth ($\tau < 1$). Higher optical depths ($\tau \ge
10$) are restricted to very limited regions such as Sgr\,B2.  In
addition, we estimate H$_2$ densities and kinetic temperatures for
different ranges of intensity ratios.  A considerable amount of
molecular mass is in a widespread molecular gas component with low
densities and high kinetic temperatures. From our C$^{18}$O
measurements and from results based on dust measurements, the total
molecular mass is found to be $(3^{+2}_{-1}) \cdot 10^7 \, {\rm
M_{\odot}}$. We show that the SCF is {\it not\/} valid toward the
Galactic bulge. It overestimates the H$_2$ column density by an order
of magnitude because the assumptions required for this factor of
optically thick $^{12}$CO emission and virialization of the molecular
clouds are not fulfilled for a significant fraction of the molecular
gas. Therefore, one can also not apply a modified conversion factor to
the Galactic center region since the $^{12}$CO luminosity is not
directly related to the H$_2$ mass.  A comparison with results from
bulge regions of external galaxies indicates that the $^{12}$CO
emission is generally not a suitable tracer of H$_2$ masses in the
central regions of galaxies.
\end{abstract} 
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\keywords{Radiative transfer --- ISM: molecules, structure --- Galaxy:
center --- Galaxies: nuclei --- Radio lines: ISM}
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\end{document} 
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