Near‐Infrared and CO (J = 1–0) Observations of Photodissociation Regions in M17

Abstract

We have carried out near-infrared mapping observations of photodissociation regions in M17 with the Wide Field Cryogenic Telescope and CO (J ¼ 1 0) observations in three isotope lines with the '' NANTEN '' telescope. The observations covered an area of 20 0 Â 20 0 with a spatial resolution of 5>6 for near-infrared wavelengths and with a half-power beamwidth of 2<7 for millimeter wavelengths. We detected 38 sources brighter than 7 mag at 3.67 lm (L n band), five of which show signs of young stellar objects. We have detected two emission bars (the N bar and the S bar) in all four near-infrared bands (J, K, L n , and 3.3 lm). Their spatial distributions differ considerably from band to band, and we have compared them with the radio continuum, the mid-infrared data, and the CO molecular line emission. The different brightness and spectral energy distributions at near-infrared wavelengths can be well explained by emission from hot dust and ionized gas together with obscuration by local cold dust with a steep gradient from north to south. In the N bar, the freefree emission from ionized gas dominates at shorter wavelengths (J and K) and there is little extinction, whereas in the S bar, the free-free emission is attenuated at shorter wavelengths by the heavy local extinction. In both the N and S bars, the thermal emission from hot dust at around 1000 K dominates in the L n band. The 3.3 lm unidentified infrared (UIR) emission delineates photodissociation regions between the H ii regions and the surrounding molecular clouds. The UIR intensity decreases exponentially from the UIR peak toward the molecular clouds, with scale lengths of 88 00 and 100 00 , or 0.9 and 1.0 pc, at the N and the S bars, respectively. Far-ultraviolet photons, which excite UIR emission, penetrate into the molecular clouds for $1 pc, in the nearly edge-on geometry. The 12 CO contours are elongated in the direction northwest-southeast, while the C 18 O contours are round. Far-ultraviolet photons erode the tenuous portions (as seen in 12 CO) of the surface of the cloud and penetrate deeply toward the denser inside, forming complex structures in the photodissociation regions bordering the molecular cloud.

Constraining the Initial Mass Function in Extreme Environments: Detecting Young Low-Mass Stars in Unresolved Starbursts

“…Since free-free emission from such a young stellar population becomes increasingly dominant at longer wavelengths, one could measure the free-free emission directly at centimeter wavelengths where it is expected to dominate dust, and compare the inferred amount in the near-IR to the predictions of Starburst99. Spatially resolved observations of the H ii regions in M17 by Ando et al (2002) indicate qualitative agreement between observed IR emission and predicted free-free emission extrapolated from centimeter observations. An additional check on this extrapolation would be the predicted reddening-corrected Brg line flux given the observed free-free emission.…”

Properties of active galactic star-forming regions probed by imaging spectroscopy with the Fourier transform spectrometer (FTS) onboard AKARI

“…11. Ando et al (2002) pointed out that a double ridge of the northern bar is distinct in the MSX band-A and the 3.3 μm UIR band emission corresponds to the outer ridge, again consistent with band-A tracing PAHs, hence PDR interfaces. Figure 15a shows that the [C II] 158 μm emission is also strong at the outer ridge, which indicates the presence of the PDR region.…”

“…Stutzki et al (1988) investigated the clumpiness of the edge-on H II region/PDR interface using the spatial distribution of [C II] 158 μm and CO rotational line emission. The PDR structures and morphology were also investigated by Ando et al (2002) and Povich et al (2007).…”

“…14e,f). This indicates that the northern bar has a much lower column density than the southern bar (Elmegreen & Lada 1976;Stutzki et al 1988;Ando et al 2002;Wilson et al 2003) …”

Low upper limits on the O2abundance from the Odin satellite

“…Further Odin observations are underway to settle this point. Onishi et al (1996), (2) Dutrey et al (1996), (3) Aoyama et al (2001), (4) Pagani et al (2001), (5) Tachihara et al (2000), (6) Kraemer & Jackson (1999), (7) Schwartz et al (1989), (8) Dickel et al (1977), (9) Dahmen et al (1997), (10) Dahmen et al (1998), (11) Ando et al (2002), (12) McMullin et al (2000), (13) Churchwell et al (1992), (14) Matthews et al (1987), (15) Little et al (1994), (16) Kim et al (2001).…”