ABSTRACT
We study the properties of the dense molecular gas
in the Galactic center using maps of CS J=3->2 emission. Since CS emission
traces gas with densities > 100 times larger than that traced by CO J=1->0,
the CS luminosity measures the mass of dense gas. We create a mosaic of
32 15'×15' maps made with the ``on-the-fly'' (OTF) technique at the
National Radio Astronomy Observatory (NRAO) 12 m at Kitt Peak, Arizona
in 1995 March and October. The map covers the central 730×75 pc (l×b
=4°.9×0°.5) of the Galaxy including the anomalous velocity
cloud Clump 2. For this poster we present preliminary results of these
observations.
INTRODUCTION
Since stars form only in the dense cores of molecular clouds, dense gas plays a fundamental role in galaxy development. Despite extensive study over the last two decades (see review by Mauersberger & Henkel 1993), only a few galaxies have been observed in enough detail to make specific conclusions about their gas densities and temperatures (e.g., Paglione et al. 1998; Jackson et al. 1995; Brouillet & Schilke 1993; Wild et al. 1992; Wall et al. 1991; Downes et al. 1992). Unfortunately, most of these galaxies contain starburst nuclei. Studying the dense gas in normal galaxies like the Milky Way is important for quantifying the differences in the properties of the star forming clouds in starburst and normal galaxies.
To this end, we mapped the CS J=3->2 emission, an effective
probe of the dense gas in star forming regions (e.g., Plume et al. 1997),
in the central ~ 700 pc of the Galactic center. A wide-field map is important
for comparing the Milky Way to other galaxies on the same spatial scales.
Our goals are the following:
We observed the CS J=3->2 emission (rest frequency of 146.97 GHz) from the Galactic center with the NRAO 12 m at Kitt Peak, AZ in 1995 March and October, and 1996 February. We used the facility 2 mm receiver, which had typical system temperatures of ~ 500 K. We used the facility filterbank spectrometers which consist of 256 channels of 2 MHz width (4 km s¯¹). The main beam efficiency and FWHM were 0.5 and 45''. Calibration was done using the chopper wheel method, and with observations of W49 and Sgr B2 in position-switching mode.
For fast mapping, we used the ``on-the-fly'' (OTF) technique to make maps 7', 10', 15', and 20' square. The maps were created by scanning in right ascension at a rate of 30'' s¯¹. The observations are sampled every 0.1 s as the dish continues to scan. The position of the dish is monitored every .01 s to ensure accurate placement of observed data points in the final map. Each map row is separated by 14'' to achieve an oversampling in declination of roughly 3.
The OTFUV and SDGRD routines of the NRAO AIPS reduction
package are used to grid the data into cubes with 256 velocity channels.
The map pixels are separated by 20''. A parabolic baseline is removed from
each gridded spectrum. Each map has been carefully examined for referencing
problems which cause ``striping.'' Some stripes simply require removal
of a higher order baseline. Others, offset by the referencing error, require
correcting them to the noise level of surrounding rows (equivalent to removing
a spatial baseline). To create the final 730×75 pc (l×b
=4°.9×0°.5) mosaic(1,
2, 3),
each map is first rotated and regridded to Galactic coordinates. The maps
are then averaged together weighted by their r.m.s. noise levels.
RESULTS
The integrated intensity map of the CS emission, and spectra at selected positions, are shown above. Well-known features such as the Sagittarius clouds, Clump 2 , Sgr C , and the l=1°.5 complex are prominent. Many of the spectra show self-absorption indicating temperature gradients or cooler foreground material. The broad line width of the circumnuclear ring in Sgr A is evident. The longitude-velocity plot is also shown.
At a few positions, especially near
Sgr B, the different
K
transitions of the CH3CN J=8->7
line are seen. A map of
Sgr
B in CH3CN, integrated over all K transitions, is
shown. The peak occurs at the location of the compact regions.
We are presently beginning analysis of these data.
The many CH3CN lines are used to estimate temperatures and column
densities with rotation diagrams. Maps of HCN and CO J=1->0, (Jackson et
al. 1996), CS J=2->1 and CH3CN J=6->5 (Bally et al. 1987) over
roughly the same regions are available. Ratios of these lines, specifically
the CS J=3->2/J=2->1 ratio, are used to estimate density. Principal Component
Analysis (Heyer & Schloerb 1997), is used to gauge the importance of
various spectral features and determine the kinematics of the gas.
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