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

Abstract

In this paper we undertake a study of the 21 square degree SMOG field, a Spitzer cryogenic mission Legacy program to map a region of the outer Milky Way towards the Perseus and Outer spiral arms with the IRAC and MIPS instruments. We identify 4648 YSOs across the field. Using the DBSCAN method we identify 68 clusters or aggregations of YSOs in the region, having 8 or more members. We identify 1197 class Is, 2632 class IIs, 819 class IIIs, of which 45 are candidate transition disk objects, utilizing the MIPS 24 photometry. The ratio of YSOs identified as members of clusters was 2872/4648, or 62%. The ratios of class I to class II YSOs in the clusters are broadly consistent with those found in the inner galactic and nearby Gould's Belt young star formation regions. The clustering properties indicate that the protostars may be more tightly bound to their natal sites than the class IIs, and the class IIIs are generally widely distributed. We further perform an analysis of the WISE data of the SMOG field to determine how the lower resolution and sensitivity of WISE affects the identification of YSOs as compared to Spitzer: we identify 931 YSOs using combined WISE and 2MASS photometry, 931/4648 or 20% of the total number identified with Spitzer. Performing the same clustering analysis finds 31 clusters which reliably trace the larger associations identified with the Spitzer data. Twelve of the clusters identified have previously measured distances from the WISE HII survey. SEDFitter modeling of these YSOs is reported, leading to an estimation of the IMF in the aggregate of these clusters which approximates that found in the inner galaxy, implying that the processes behind stellar mass distribution during star formation are not widely affected by the lower density and metallicity of the outer galaxy.

SPICY: The Spitzer/IRAC Candidate YSO Catalog for the Inner Galactic Midplane

“…The difference in magnitude distributions-ours peaking at [4.5] = 13 mag and theirs peaking at [4.5] = 14.5 mag-may be a limitation related to our training set which was dominated by objects from the shallower GLIMPSE survey areas (Section 2). In spatial distribution (Figure 26, left), our candidate YSOs tend to be more spatially clustered than those from Winston et al (2019).…”

“…The YSO selection is largely based on IRAC photometry from GLIMPSE (Benjamin et al 2003;Churchwell et al 2009) and related surveys that used similar observing strategies and data reduction methodologies. 8 These include GLIMPSE I (31,184,509 sources), GLIMPSE II (19,067,533 sources),and GLIMPSE 3D (20,403,915 sources), the Vela-Carina (2,001,032 sources) (Majewski et al 2007;Zasowski et al 2009), Cygnus X (3,913,559 sources) (Beerer et al 2010), and SMOG (2,512,099 sources) (Winston et al 2019) surveys. Spitzer's observations of the Galactic Center (Stolovy et al 2006) were included in the GLIMPSE II Catalog.…”

“…As discussed in the introduction, a variety of strategies have been used to identify YSOs from IRAC colors. The SMOG field, which was published by Winston et al (2019) using a modified version of the Gutermuth et al (2009) color selection rules, provides an excellent testbed for such a comparison. Comparison between our SMOG candidates (1524 objects) and theirs (4648 objects) shows that our selection methodology is more restrictive; 97% of our YSO candidates were also classified as YSOs by Winston et al (2019), while only 32% of their YSO candidates were classified as YSOs by us.…”

“…The SMOG field, which was published by Winston et al (2019) using a modified version of the Gutermuth et al (2009) color selection rules, provides an excellent testbed for such a comparison. Comparison between our SMOG candidates (1524 objects) and theirs (4648 objects) shows that our selection methodology is more restrictive; 97% of our YSO candidates were also classified as YSOs by Winston et al (2019), while only 32% of their YSO candidates were classified as YSOs by us. Figure 26 (right) shows that objects from their list that are not included by us tend to be either objects with bluer colors or objects fainter than most of our sample.…”

“…In our study, we relax these criteria to identify significantly more YSOs candidates, but use patterns in the IRAC photometry to better distinguish between YSOs and contaminants. Our survey area also overlaps YSO catalogs for Cygnus X (Beerer et al 2010;Winston et al 2020) and the Spitzer Mapping of the Outer Galaxy (SMOG; Winston et al 2019) survey; we use the latter as a benchmark to compare to our results.…”