Parit Mehta
University of Cologne
Optical Depth Effects in the Extended [CII] and CO (3→2) Maps of the M17 nebula
PARIT MEHTA (1), JÜRGEN STUTZKI (1), CRISTIAN GUEVARA (1), NICOLA SCHNEIDER (1), ROLF GÜSTEN (2)
Affiliations: (1)Institute for Astrophysics, University of Cologne, 50937 Cologne, Germany (2) Max-Planck-Institut für Radioastronomie, 53121 Bonn, Germany
Abstract: M17 is one of the most massive star forming regions in our galaxy: a young stellar nursery with over 100 OB stars embedded in dense gas, illuminating two prominent Photon Dominated Regions (PDRs). M17 and its central cluster, NGC 6618, have been studied extensively with a large amount of data available in the literature for CO and other molecular lines, dust, as well as data from other wavelength regimes. The [CII] fine structure line is the dominant gas coolant in PDRs, and thus an important tracer to study the star forming gas. High optical depths and self-absorption of [CII] were confirmed by Guevara et al. (2020), pointing to the possible existence of a cold absorbing foreground in front of the warm PDR background layer. We present a new large-scale map of velocity-resolved [CII] 158μm emission across an area of approx. 11x12 sq. pc towards the M17 nebula as part of the SOFIA FEEDBACK legacy program, significantly extending the former GREAT and velocity-integrated FIFI-LS maps of the M17 SW interface; the FIFI-LS maps allow to cross-calibrate the fine structure line intensities. These data are complemented by [OI] 63μm data from upGREAT HFA observed simultaneously with [CII] and new extended maps of 12CO and 13CO (J=3→2) emission in M17 from APEX. The high spatial and spectral resolution, along with efficient large-scale mapping capabilities offered by upGREAT onboard SOFIA in [CII] reveal for the first time the complex structure of the extended [CII] emission in unprecedented detail. Owing to the good S/N achieved in the appropriately spatially and spectrally smoothed SOFIA/upGREAT data, we are able to detect the fainter spectral signature of the outer [13CII] hyperfine satellites. Comparison with the main [12CII] transition allows investigation of the optical depth in the extended M17 region and possible evidence for signatures of self-absorption in M17. 12CO (3→2) also exhibits complex line profiles. Many areas in the map show absorption dips in both 12CO (3→2) and 13CO (3→2) along with large optical depths. We discuss physical conditions deduced from the CO (3→2) and [OI] 63μm data in comparison to the [CII] line emission. The aforementioned work will form the core of the doctoral thesis of P. Mehta.
