The properties of Galactic molecular clouds tabulated by Solomon et al. (1987) (SRBY) are re-examined using the Boston University-FCRAO Galactic Ring Survey of 13 CO J=1-0 emission. These new data provide a lower opacity tracer of molecular clouds and improved angular and spectral resolution compared to previous surveys of molecular line emission along the Galactic Plane. We calculate GMC masses within the SRBY cloud boundaries assuming LTE conditions throughout the cloud and a constant H 2 to 13 CO abundance, while accounting for the variation of the 12 C/ 13 C with Galacto-centric radius. The LTE derived masses are typically five times smaller than the SRBY virial masses. The corresponding median mass surface density of molecular hydrogen for this sample is 42 M ⊙ pc −2 , which is significantly lower than the value derived by SRBY (median 206 M ⊙ pc −2 ) that has been widely adopted by most models of cloud evolution and star formation. This discrepancy arises from both the extrapolation by SRBY of velocity dispersion, size, and CO luminosity to the 1 K antenna temperature isophote that likely overestimates the GMC masses and our assumption of constant 13 CO abundance over the projected area of each cloud. Owing to the uncertainty of molecular abundances in the envelopes of clouds, the mass surface density of giant molecular clouds could be larger than the valued derived from our 13 CO measurements. From velocity dispersions derived from the 13 CO data, we find that the coefficient of the cloud structure functions, v • = σ v /R 1/2 , is not constant, as required to satisfy Larson's scaling relationships, but rather systematically varies with the surface density of the cloud as ∼ Σ 0.5 as expected for clouds in self-gravitational equilibrium.

We investigate the relation between star formation rate (SFR) and gas surface densities in Galactic star forming regions using a sample of young stellar objects (YSOs) and massive dense clumps. Our YSO sample consists of objects located in 20 large molecular clouds from the Spitzer cores to disks (c2d) and Gould's Belt (GB) surveys. These data allow us to probe the regime of low-mass star formation essentially invisible to tracers of high-mass star formation used to establish extragalactic SFR-gas relations. We estimate the gas surface density (Σ gas ) from extinction (A V ) maps and YSO SFR surface densities (Σ SFR ) from the number of YSOs, assuming a mean mass and lifetime. We also divide the clouds into evenly spaced contour levels of A V , counting only Class I and Flat SED YSOs, which have not yet migrated from their birthplace. For a sample of massive star forming clumps, we derive SFRs from the total infrared luminosity and use HCN gas maps to estimate gas surface densities. We find that c2d and GB clouds lie above the extragalactic SFR-gas relations (e.g., Kennicutt-Schmidt Law) by factors up to 17. Cloud regions with high Σ gas lie above extragalactic relations up to a factor of 54 and overlap with high-mass star forming regions. We use 12 CO and 13 CO gas maps of the Perseus and Ophiuchus clouds from the COMPLETE survey to estimate gas surface densities and compare to measurements from A V maps. We find that 13 CO, with the standard conversions to total gas, underestimates the A Vbased mass by factors of ∼4-5. 12 CO may underestimate the total gas mass at Σ gas 200 M ⊙ pc −2 by 30%; however, this small difference in mass estimates does not explain the large discrepancy between Galactic and extragalactic relations. We find evidence for a threshold of star formation (Σ th ) at 129±14 M ⊙ pc −2 . At Σ gas > Σ th , the Galactic SFR-gas relation is linear. A possible reason for the difference between Galactic and extragalactic relations is that much of Σ gas is below Σ th in extragalactic studies, which detect all the CO-emitting gas. If the Kennicutt-Schmidt relation (Σ SFR ∝ Σ 1.4 gas ) and a linear relation between dense gas and star formation is assumed, the fraction of dense star forming gas (f dense ) increases as ∼ Σ 0.4 gas . When Σ gas reaches ∼300Σ th , the fraction of dense gas is ∼1, creating a maximal starburst.

We report the results of a 100 deg 2 survey of the Taurus molecular cloud region in 12 CO and 13 CO J ¼ 1 ! 0. The image of the cloud in each velocity channel includes '3 ; 10 6 Nyquist-sampled pixels on a 20 00 grid. The high sensitivity and large spatial dynamic range of the maps reveal a very complex, highly structured cloud morphology, including filaments, cavities, and rings. The axes of the striations seen in the 12 CO emission from relatively diffuse gas are aligned with the direction of the magnetic field. We have developed a statistical method for analyzing the pixels in which 12 CO but not 13 CO is detected, which allows us to determine the CO column in the diffuse portion of the cloud, as well as in the denser regions in which we detect both isotopologues. Using a column-density-dependent model for the CO fractional abundance, we derive the mass of the region mapped to be 2:4 ; 10 4 M , more than twice as large as would be obtained using a canonical fixed fractional abundance of 13 CO, and a factor of 3 greater than would be obtained considering only the high column density regions. We determine that half the mass of the cloud is in regions having column density below 2:1 ; 10 21 cm À2. The distribution of young stars in the region covered is highly nonuniform, with the probability of finding a star in a pixel with a specified column density rising sharply for N (H 2 ) ¼ 6 ; 10 21 cm À2 . We determine a relatively low star formation efficiency (mass of young stars/mass of molecular gas), between 0.3% and 1.2%, and an average star formation rate during the past 3 Myr of 8 ; 10 À5 stars yr À1.

The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey is a new survey of Galactic 13 CO J ¼ 1 ! 0 emission. The survey used the SEQUOIA multipixel array on the Five College Radio Astronomy Observatory 14 m telescope to cover a longitude range of l ¼ 18 55N7 and a latitude range of jbj < 1 , a total of 75.4 deg 2 . Using both position-switching and On-The-Fly mapping modes, we achieved an angular sampling of 22 00 , better than half of the telescope's 46 00 angular resolution. The survey's velocity coverage is À5 to 135 km s À1 for Galactic longitudes l 40 and À5 to 85 km s À1 for Galactic longitudes l > 40 . At the velocity resolution of 0.21 km s À1 , the typical rms sensitivity is (T Ã A ) $ 0:13 K. The survey comprises a total of 1,993,522 spectra. We show integrated intensity images (zeroth moment maps), channel maps, position-velocity diagrams, and an average spectrum of the completed survey data set. We also discuss the telescope and instrumental parameters, the observing modes, the data reduction processes, and the emission and noise characteristics of the data set. The Galactic Ring Survey data are available to the community online or in DVD form by request.

We derive the physical properties of 580 molecular clouds based on their 12 CO and 13 CO line emission detected in the University of Massachusetts-Stony Brook (UMSB) and Galactic Ring surveys. We provide a range of values of the physical properties of molecular clouds, and find a power-law correlation between their radii and masses, suggesting that the fractal dimension of the ISM is around 2.36. This relation, M = (228±18) R 2.36±0.04 , allows us to derive masses for an additional 170 GRS molecular clouds not covered by the UMSB survey. We derive the Galactic surface mass density of molecular gas and examine its spatial variations throughout the Galaxy. We find that the azimuthally averaged Galactic surface density of molecular gas peaks between Galactocentric radii of 4 and 5 kpc. Although the Perseus arm is not detected in molecular gas, the Galactic surface density of molecular gas is enhanced along the positions of the Scutum-Crux and Sagittarius arms. This may indicate that molecular clouds form in spiral arms and are disrupted in the inter-arm space. Last, we find that the CO excitation temperature of molecular clouds decreases away from the Galactic center, suggesting a possible decline in the star formation rate with Galactocentric radius. There is a marginally significant enhancement in the CO excitation temperature of molecular clouds at a Galactocentric radius of about 6 kpc, which in the longitude range of the GRS corresponds to the Sagittarius arm. This temperature increase could be associated with massive star formation in the Sagittarius spiral arm.