Determining Distances to Infrared Point Sources

    We search the first six square degrees of the Milky Way Galactic Ring Survey (GRS) for correlations between molecular and infrared emission.  Extended infrared emission correlates well with extended ¹³CO emission (Fig. 1) Many compact infrared sources identified by MSX and IRAS are also often coincident with ¹³CO clumps.  These sources are probably embedded young stars and protostars.  By associating infrared point sources with ¹³CO emission, we estimate their kinematic distances.  We resolve the near/far distance ambiguity using H I self-absorption data from the BUAO survey. Using the estimated distances, we calculate total far infrared luminosities.  Once the distances to enough faint young stars and protostars have been estimated,  we will determine their luminosity functions and spatial distributions throughout the Inner Galaxy.
 

Fig. 1: Overlay of contours from 6 square degrees of the GRS ¹³CO channel map for emission between 50 and 70 km/s on a false color infrared image obtained with the MSX satellite (Band A, 6.8-10.8 micron). The image has been background subtracted which removes both Zodiacal and very large scale Galactic emission.  The contours plotted are for 5, 10, 17 and 25 K km/s.  The angular resolution of the infrared image is 18" compared with the 46" resolution of the GRS.  Darker contours in ¹³CO correspond to intensity peaks.  IRAS point sources are marked with yellow stars, and IRAS point sources associated with ¹³CO clumps are marked with green stars.

    How well is the ¹³CO emission correlated with infrared emission?  How many IRAS point sources are obviously associated with ¹³CO clumps? Can we use molecular kinematics to find distances to infrared sources?  Is the association sample large enough to measure its luminosity function and spatial distribution?

We superpose contours of ¹³CO emission on MSX images (Egan et al., 1998) to look for correlation between molecular and infrared emission (Fig. 1).  We find roughly 500 IRAS point sources coincident with ¹³CO features, and 118 of these are clearly associated with bright, isolated ¹³CO clumps.  For these unambiguous cases, we assign LSR velocities to the infrared point sources using the ¹³CO velocities.  After resolving the near/far distance ambiguity using the HI data, we calculate distances to the IRAS point sources assuming the rotation curve of Clemens et al. (1988).  These distances allow us to estimate the total far infrared luminosity from the IRAS 60 and 100 micron fluxes (Fig. 2).  To explore the spatial distribution of these 118 ¹³CO-IRAS point source associations, we mark their positions on a face-on map of the Galaxy, together with the distribution of ¹³CO clumps (Fig. 3). Galactic distribution of ¹³CO LTE mass (Fig. 3).
 

Fig. 2: Luminosity histogram for IRAS point sources associated with bright isolated ¹³CO clumps.  The luminosities, ln(L/Lsun), range from 1.6 to 6.68.  with an average of 3.7. The average distance is 6.558 kpc.

Fig. 3: Face-on Galactic distribution of molecular gas and IR sources. The shading is proportional to ¹³CO clump LTE mass.  The red stars represent IRAS point sources, and their sizes are proportional to their luminosities. (Note that both the molecular gas and the infrared point sources show clear concentrations and gaps. The ¹³CO shows clear concentrations and gaps, and the distribution of infrared sources is different.)  The face-on Galactic distribution of molecular gas and IR sources for the region covered by the GRS to date. The axes represent distances in kpc; the sun and the Galactic Center are marked.  The greyscale shading of each pixel is proportional to the total mass of clumps within that pixel.  The red stars mark the positions of IRAS point sources and their sizes are proportional to log(L).  The horizontal line is an artifact produced by the assignment of all forbidden velocity clouds to the tangent point.
 
 

Figure 1 reveals an excellent correlation between ¹³CO emission and many extended infrared features.  The correlation is particularly striking in the region of G45.5+0.1, a star-forming molecular cloud at a distance of roughly 6 kpc.  The infrared features that appear not to have molecular counterparts are associated with ¹³CO emission at different velocities, hence at different distances, than the contour map shown in figure 1.  For example, the extremely bright infrared feature at l = 43.2 that appears not to have a ¹³CO counterpart is W49.    W49 is at a distance of 11.6 kpc, and has an LSR velocity of 10 km/s.  We find total far infrared luminosities of IRAS point sources as faint as a few hundred solar luminosities and as bright as a few million solar luminosities.  These luminosities correspond to spectral types A1 to O4 (Panagia, 1973).
 
 

We find distances and luminosities for the most obvious 118 correlations between IRAS point sources and ¹³CO clumps.  Most of the remaining IRAS point sources are coincident with morphologically complex ¹³CO emission or multiple ¹³CO clumps along the line of sight at different velocities.  We plan to compare GRS data with high resolution infrared data (MSX, 2MASS, ISO) to associate these point sources with some of the more complex ¹³CO features.  When the GRS is finished, we expect roughly 700 simple and a few thousand complex IRAS point source correlations.
 
 

Overall, there is a remarkable correlation between extended emission in ¹³CO and the infrared. We find distances and luminosities for nearly 25% of IRAS point sources coincident with ¹³CO emission in the GRS region. When the GRS is finished, we expect roughly 700 simple IRAS point source correlations with which we can begin to determine the luminosity function for infrared sources in the Inner Galaxy.  The large scale spatial distribution of these infrared point sources is significantly different from Galactic structures seen in ¹³CO.

REFERENCES:
Clemens, D.P., et al., 1988, ApJ, 327, 139
Egan, M.P., et al., 1998, ApJ, 494, L199
Panagia, Nino, 1973, AJ, 78, 929

A full size version of the poster can be obtained here (gzipped postscript file)

The GRS is supported by the NSF via grant AST-9800334 and AST-0098562