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 McGary2002.gcnews.tex ApJ (2002), in press
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\begin{document}
 
\title{NH\3 in the Central 10 pc of the Galaxy. II. Determination of Opacity for Gas with Large Linewidths}
%\title{Determination of Opacity of NH\3 Rotation Inversion Transitions for Gas with Large Linewidths}
 
\author{
Robeson S. McGary\altaffilmark{1} and
Paul T.P. Ho\altaffilmark{1}}

\altaffiltext{1}{Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, \\rmcgary@cfa.harvard.edu, pho@cfa.harvard.edu}

%\slugcomment{Draft of 05 Mar 2002}

\begin{abstract}
The 23~GHz emission lines from the NH\3 rotation inversion transitions
are widely used to investigate the kinematics and physical conditions
in dense molecular clouds.  The line profile is composed of hyperfine
components which can be used to calculate the opacity of the gas (Ho
\& Townes 1983).  For intrinsic linewidths of a few \kms, the 18
magnetic hyperfine components blend together to form a line profile
composed of five quadrupole hyperfine lines.  If the intrinsic
linewidth exceeds one half of the separation of these quadrupole
hyperfine components ($\sim5-10$~\kms) these five lines blend together
and the observed linewidths greatly overestimate the intrinsic
linewidths.  If uncorrected, these artificially broad linewidths will
lead to artificially high opacities.  We have observed this effect in
our NH\3 data from the central 10 pc of the Galaxy where uncorrected
NH\3 (1,1) linewidths of $\sim30$ \kms ~exaggerate the intrinsic
linewidths by more than a factor of two (Genzel \& Townes 1987).
Models of the effect of blending on the line profile enable us to
solve for the intrinsic linewidth and opacity of NH\3 using the
observed linewidth and intensity of two NH\3 rotation inversion
transitions.  By using the observed linewidth instead of the entire
line profile, our method may also be used to correct linewidths in
historical data where detailed information on the shape of the line
profile is no longer available.  We present the result of the
application of this method to our Galactic Center data.  We
successfully recover the intrinsic linewidth
($\langle\dvi\rangle\approx15$~\kms) and opacity of the gas. Clouds
close to the nucleus in projected distance as well as those that are
being impacted by Sgr A East show the highest intrinsic linewidths.
The cores of the ``southern streamer'' (Ho et al., 1991; Coil \& Ho
1999,2000) and the ``50 \kms'' giant molecular cloud (GMC) have the
highest opacities.

\end{abstract}

%\keywords{Galaxy: center --- ISM:clouds ---ISM:molecules --- radio lines:ISM}

\section{Introduction}

\end{document}