The Extended Environment of M17: A Star Formation History
  M17 is one of the youngest and most massive nearby star-formation regions in
the Galaxy. It features a bright H II region erupting as a blister from the
side of a giant molecular cloud (GMC). Combining photometry from the Spitzer
GLIMPSE survey with complementary infrared (IR) surveys, we identify candidate
young stellar objects (YSOs) throughout a 1.5 deg x 1 deg field that includes
the M17 complex. The long sightline through the Galaxy behind M17 creates
significant contamination in our YSO sample from unassociated sources with
similar IR colors. Removing contaminants, we produce a highly-reliable catalog
of 96 candidate YSOs with a high probability of association with the M17
complex. We fit model spectral energy distributions to these sources and
constrain their physical properties. Extrapolating the mass function of 62
intermediate-mass YSOs (M >3 Msun), we estimate that >1000 stars are in the
process of forming in the extended outer regions of M17.
  From IR survey images from IRAS and GLIMPSE, we find that M17 lies on the rim
of a large shell structure ~0.5 deg in diameter (~20 pc at 2.1 kpc). We present
new maps of CO and 13CO (J=2-1) emission, which show that the shell is a
coherent, kinematic structure associated with M17 at v = 19 km/s. The shell is
an extended bubble outlining the photodissociation region of a faint, diffuse H
II region several Myr old. We provide evidence that massive star formation has
been triggered by the expansion of the bubble. The formation of the massive
cluster ionizing the M17 H II region itself may have been similarly triggered.
We conclude that the star formation history in the extended environment of M17
has been punctuated by successive waves of massive star formation propagating
through a GMC complex.
