The Astrophysical Journal, 653:1325Y1335, 2006 December 20 # 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.
THE CHARACTERIZATION AND GALACTIC DISTRIBUTION OF INFRARED DARK CLOUDS R. Simon,1 J. M. Rathborne, R. Y. Shah, J. M. Jackson, and E. T. Chambers Institute for Astrophysical Research, Boston University, 725 Commonwealth Avenue, Boston, MA 02215;
[email protected],
[email protected],
[email protected],
[email protected],
[email protected] Received 2005 September 14; accepted 2006 September 5
ABSTRACT Using 13CO J ¼ 1 ! 0 molecular line emission from the Boston UniversityYFive College Radio Astronomy Observatory Galactic Ring Survey (BU-FCRAO GRS), we have established kinematic distances to 313 infrared dark clouds (IRDCs) by matching the morphology of the molecular line emission in distinct velocity channels to their midinfrared extinction. The Galactic distribution of IRDCs shows an enhancement toward the Galaxy’s most massive and active star-forming structure, the so-called 5 kpc ring. IRDCs have typical sizes of 5 pc, peak column densities of 1022 cm2, LTE masses of 5 ; 103 M , and volume-averaged H2 densities of 2 ; 103 cm3. The similarity of these properties to those of molecular clumps associated with active star formation suggests that IRDCs represent the densest clumps within giant molecular clouds where clusters may eventually form. Subject headingg s: dust, extinction — Galaxy: structure — infrared: ISM — radio lines: ISM — stars: formation
1. INTRODUCTION
( Teyssier et al. 2002) show extended molecular line emission associated with IRDCs and thus suggest that they are condensations within larger molecular clouds. Moreover, the prominent IRDC near l ¼ 28N35 and b ¼ 0N05 appears to be in the middle of a large CO cloud and even larger H i complex (Minter et al. 2001). One problem that has hampered a thorough understanding of IRDCs is the paucity of distance measurements. Without distances, it is impossible to ascertain their sizes, masses, and Galactic distribution. Unfortunately, distances cannot be measured with IR continuum data alone. At present, distances to IRDCs can only be measured kinematically using molecular line emission. This kinematic distance technique morphologically associates the IRDC with molecular line emission. The molecular line’s LSR velocity then establishes a kinematic distance through the use of a suitable Galactic rotation curve. To date, approximately 20 IRDCs have known kinematic distances (Carey et al. 1998; Teyssier et al. 2002). These studies suggest that IRDCs have sizes of a few parsecs and masses >1000 M . In order to establish the general characteristics of IRDCs, however, it is important to measure the distances for a larger sample using a large, homogeneous molecular line database. Galactic CO surveys at millimeter wavelengths offer excellent data sets to establish the physical properties of IRDCs. Although several molecular line surveys exist (e.g., Columbia/CfA, Dame et al. 2001; Bell Labs, Lee et al. 2001; University of MassachusettsY Stony Brook, Sanders et al. 1986), the BU-FCRAO GRS Simon et al. 2001; Jackson et al. 2006) is perhaps the best database for a large-scale study of IRDCs. The GRS is a new survey of the 13CO J ¼ 1 ! 0 line emission in the first Galactic quadrant covering 18 < l < 55 and jbj < 1 ( limited data for 14 < l < 18 also exist; Jackson et al. 2006). Compared with previous molecular line surveys of the inner Galaxy, the GRS offers excellent sensitivity (105 cm3), cold (1A53 (twice the GRS beam). These 379 clouds constitute our sample.
3. DETERMINING KINEMATIC DISTANCES TO IRDCs In the inner Galaxy, there are typically several molecular clouds along any line of sight. Because each IRDC is associated with a significant column of molecular gas, it should be readily observable in 13CO. Therefore, to ensure that we isolate the molecular line emission actually associated with the IRDC, we require a morphological match between the 13CO line emission and the mid-IR extinction. Because of this requirement, high-resolution imaging surveys of both the IR continuum and the molecular line emission are necessary. In the MSX 8 m images (see example in Simon et al. 2006), the extinction boundaries are very sharp and usually unresolved. Therefore, in practice it is easy to establish a morphological match, and hence, to assign radial velocities to each IRDC. To convert the radial velocities into kinematic distances, we assume circular motions and the Clemens ( 1985) rotation curve with (R0 , 0 ) ¼ (8:5 kpc, 220 km s1). Since IRDCs are seen as extinction features, they probably lie in front of the bulk of the Galactic diffuse mid-IR background. Consequently, we resolve the near /far kinematic distance ambiguity by assuming that all IRDCs lie at the near kinematic distance. Although noncircular or streaming motions introduce errors, such motions tend to be small (
