Milky Way in Molecular Clouds:
A New Complete CO Survey
Dame, Hartmann, & Thaddeus (2001)
ApJ, 547, 792 (ADS link)
Background
After preliminary Galactic surveys in the mid-1970's revealed the vast extent of CO emission on the sky, it became clear that even with the relatively large beams of the 1.2 meter telescopes a sensitive, well-sampled survey of the entire Galaxy would require many years. For that reason, during the period 1979-1986, the telescopes carried out a series of "superbeam" CO surveys in which angular resolution was sacrificed for the sake of coverage and speed. Most of these studies were conducted at an effective angular resolution of 1/2°, achieved by stepping through a 4 x 4 grid of positions on the sky separated by 1/8° (slightly less than one beamwidth) during the accumulation of a single spectrum. These low-resolution surveys, in total comprising over 31,000 spectra and sampling nearly a fifth of the entire sky (~7700 sq-deg), were combined into the first complete CO survey of the Milky Way by Dame et al.(1987)
Our "superbeam" CO survey was widely used for studies of star formation and Galactic structure. One measure of its impact is the number of citations to the Dame et al. (1987) paper, which according to the ADS Abstract Service now stands at 752 (the median number of citations per paper cataloged by the ADS is 4). According to a 2000 study of astronomy citations by D. Burstein (astro-ph/0005277), this paper is the 2nd most citied astronomy research paper for 1987 and in the top 100 for all years.
The New Survey
Even before the "superbeam" surveys were undertaken, our two telescopes began to survey the Galaxy and its nearest neighbors at several times higher angular resolution--typically every beamwidth, but sometimes half or twice that--and at 5-10 times higher sensitivity per solid angle. Such observations now cover the entire Galactic plane over a 4-10 degree band in latitude, all large local clouds at higher latitude, as well as the Large Magellanic Cloud and M31. In Dame, Hartmann, & Thaddeus (2001) we combine all of the full-resolution observations, a total of 488,000 spectra, into a new composite CO survey of the Galaxy.
* Table 1 of Trimble & Zaich (2006) on the “Productivity and Impact of Radio Telescopes”, updated with the ADS through 2009.
The Whole-Galaxy CO Maps
The centerpieces of the paper are spatial and longitude-velocity maps of the entire Galaxy in CO. These maps appear on opposite sides of a 5-page color foldout in the Astrophysical Journal; a similar foldout with contour maps appeared in the Dame et al. (1987) paper.
FITS image
A FITS-format version of the spatial map can be downloaded below, but first please read the following:
As discussed in Section 2.2 of the paper, both the angular resolution and the sensitivity varies from region to region in this map: see Figure 1 and Table 1. Moment masking and clipping were used as necessary to keep the noise in the map below ~1.5 K km/s. For small-scale studies needing the most accurate possible column densities and uncertainties, you should use one of the individual CO surveys listed in Table 1 of the paper.
ascii header: Wco_DHT2001.hdr
Uncompressed (3.3 Mb): Wco_DHT2001.fits
gzipped (593K): Wco_DHT2001.fits.gz
Old vs. New
The alternating maps below compare the second Galactic quadrant as mapped in CO by the old "superbeam" survey of Dame et al. (1987) and by the new survey of Dame, Hartmann, & Thaddeus (2001). The 1/2° square "beam" of the old survey is clearly visible, while the 1/8° beam of the new survey is less than the size of a pixel in the gif image.
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