Star Formation Activity in the Long, Filamentary Infrared Dark Cloud
  G53.2
  We present star formation activity in the infrared dark cloud (IRDC) G53.2, a
remarkable IRDC located at Galactic coordinates
$(l,b)\sim(53^{\circ}.2,\,0^{\circ}.0)$ based on the census of young stellar
object (YSO) candidates. IRDC G53.2 was previously identified as several IRDCs
in mid-IR images, but it is in fact a long ($\gtrsim$45 pc) cloud, well
consistent with a CO cloud at $v\sim23$ \kms\ (or at $d\sim$1.7 kpc). We
present a point-source catalog of IRDC G53.2 that contains $\sim$370 sources
from our photometry of the {\it Spitzer} MIPS 24 \um\ data and Galactic Legacy
Infrared Mid-Plane Survey Extraordinaire Catalog. The classification of the
identified sources based on their spectral index and control field analysis to
remove field star contamination reveals that IRDC G53.2 is an active
star-forming region with $\sim$300 YSO candidates. We compare the YSO
classification based on spectral index, mid-IR colors, and the wavelength range
used, which results in consistent classification, except for flat-spectrum
objects, with some ambiguity between Class I and II. Comparison of the YSO
population in IRDC G53.2 with those of other nearby star-forming clusters
indicates that they are similar in age; on the other hand, stronger association
with mid-IR stellar sources in IRDC G53.2 compared with other IRDCs indicates
that IRDC G53.2 is at a later evolutionary stage among IRDCs. Spatial
distribution of the YSO candidates in IRDC G53.2 shows a good correlation with
$^{13}$CO column density and far-IR emission, and earlier-class objects tend to
be more clustered in the regions with higher density.
