A Census of Star Formation in the Outer Galaxy II: The GLIMPSE360 Field

Abstract

We have conducted a study of star formation in the outer Galaxy from 65\degr$<l<$265\degr~in the region observed by the GLIMPSE360 program. This {\it Spitzer} warm mission program mapped the plane of the outer Milky Way with IRAC at 3.6 and 4.5~$\mu$m. We combine the IRAC, {\it WISE}, and 2MASS catalogs and our previous results from another outer Galaxy survey and identify a total of 47,338 Young Stellar Objects (YSOs) across the field spanning $>$180\degr\ in Galactic longitude. Using the $DBSCAN$ method on the combined catalog, we identify 618 clusters or aggregations of YSOs having 5 or more members. We identify 10,476 Class I, 29,604 Class II, and 7,325 anemic Class II/Class III YSOs. The ratio of YSOs identified as members of clusters was 25,528/47,338, or 54\%. We found 100 of the clusters identified have previously measured distances in the {\it WISE} \ion{H}{2} survey. We used these distances in our spectral energy distribution (SED) fitting of the YSOs in these clusters, of which 96 had YSOs with $<3\sigma$ fits. We used the derived masses from the SED model fits to estimate the initial mass function (IMF) in the inner and outer Galaxy clusters: dividing the clusters by Galactocentric distances, the slopes were $\Gamma =1.87\pm 0.31$ above 3~M${}_{\odot }$ for $R_{Gal}<11.5$~kpc and $\Gamma =1.15\pm 0.24$ above 3~M${}_{\odot }$ for $R_{Gal}>11.5$~kpc. The slope of the combined IMF was found to be $\Gamma =1.92\pm 0.42$ above 3~M${}_{\odot }$. These values are consistent with each other within the uncertainties, and with literature values in the inner Galaxy high-mass star formation regions. The slopes are likely also consistent with a universal Salpeter IMF.

25 pages, 14 figures

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