Molecular clouds and stellar clusters in the far outer galaxy.
SNELL R.L., CARPENTER J.M. and HEYER M.H.
Abstract (from CDS):
We present a study of the molecular cloud content and star formation activity in the far outer Galaxy at Galactocentric radii greater than 13.5 kpc. The properties of star-forming regions associated with far outer Galaxy clouds are examined within a 60 deg2 area toward the second Galactic quadrant based on the FCRAO CO Survey of the Outer Galaxy, the IRAS Point Source Catalog, and follow-up 13CO(J=1-0) and K'-band near-infrared imaging. This region contains 63 far outer Galaxy molecular clouds; the most massive clouds have molecular masses of ∼104 M☉. The K'-band imaging of 10 IRAS point sources associated with the far outer Galaxy clouds resulted in the detection of 11 stellar clusters with Galactocentric distances between 13.5 and 17.3 kpc. Several of these clusters are comparable to the well-studied clusters found within 1 kpc of the Sun in terms of the number of stars. We have also examined a much larger sample of far outer Galaxy molecular clouds using the entire FCRAO CO survey. The mass spectrum of 246 far outer Galaxy clouds found within a 300 deg2 area has a power-law slope of -1.88, similar to, although slightly steeper than that found for molecular clouds inside the solar circle. Global measures of the star formation activity, as traced by the ratio of far-infrared luminosity to molecular cloud mass, indicate that these far outer Galaxy clouds are equally active sites of massive star formation as molecular clouds associated with the W3/W4/W5 region and clouds found in the inner Galaxy. Therefore, despite the different environment expected in the far outer Galaxy, the cloud mass spectrum and star formation activity per unit mass are similar to that found throughout the Galaxy. Finally, based on Two Micron All Sky Survey data, we identify 31 additional candidate far outer Galaxy star-forming regions within the larger survey area.