2002ApJ...577..245J

We conducted a deep JHK_s-band imaging survey of the M17 region, using a near-infrared camera, the Simultaneous 3-color InfraRed Imager for Unbiased Survey (SIRIUS), mounted on the InfraRed Survey Facility (IRSF) 1.4 m telescope at the South African Astronomical Observatory. This survey covers an area of ∼200 arcmin² with 10 σ limiting magnitudes of J∼18.7, H∼18.2, and K_s∼17.5. The near-infrared (NIR) images reveal an unprecedented view of the region. The NIR nebulae are highly structured, with two nebular bars corresponding to, but a little larger than, the H II region defined by Felli, Massi, & Churchwell, constructing a conical shape. Fine structures are found all over the nebular area. The central region contains a congregation of intermediate- to high-mass stars. From the slope of the K_s-band luminosity function and the frequency of young stellar objects (YSOs) we infer that the central cluster has an age less than 3 Myr. The central OB cluster provides tremendous energy that heats and ionizes its surrounding materials, triggering the star formation of second-generation in the nebular bars. The second generation stars are so numerous that could they affect the star formation efficiency in the whole region. To the southwest of the central cluster and the nebular bars, where a giant molecular cloud core is located, a large number of red stars are detected. We argue that these red stars are most probably associated YSOs with intrinsic color excesses, not normal field stars reddened by the molecular cloud in front of them. Being located beyond the photodissociation region, the star-forming process in the molecular region could be independent of the impact of the central cluster.