Star formation activity throughout the Galactic disk depends on the thermal and dynamical state of the interstellar gas, which in turn depends on heating and cooling rates, modulated by the gravitational potential and shock and turbulent pressures. Molecular cloud formation, and thus the star formation, may be regulated by pressures in the interstellar medium (ISM). To understand these processes we need information about the properties of the diffuse atomic and diffuse molecular gas clouds, and Photon Dominated Regions (PDR). An important tracer of these regions is the CII line at 158 microns (1900.5 GHz). We propose a pencil-beam survey of CII with HIFI band 7b, based on deep integrations and systematic sparse sampling of the Galactic disk plus selected targets, totaling over 900 lines of sight. We will detect both emission and, against the bright inner Galaxy and selected continuum sources, absorption lines. These spectra will provide the astronomical community with a large rich statistical database of the diffuse cloud properties throughout the Galaxy for understanding the Milky Way ISM and, by extension, other galaxies. It will be extremely valuable for determining the properties of the atomic gas, the role of barometric pressure and turbulence in cloud evolution, and the properties of the interface between the atomic and molecular clouds. The CII line is one of the major ISM cooling lines and is present throughout the Galactic plane. It is the strongest far-IR emission line in the Galaxy, with a total luminosity about a 1000 times that of the CO J=1-0 line. Combined with other data, it can be used to determine density, pressure, and radiation environment in gas clouds, and PDRs, and their dynamics via velocity fields. HSO is the best opportunity over the next several years to probe the ISM in this tracer and will provide a template for large-scale surveys with dedicated small telescopes and future surveys of other important ISM tracers.
Publication
A Herschel [C ii] Galactic plane survey. II. CO-dark H2 in clouds . Langer W. D. et al. . Astronomy & Astrophysics, Volume 561, id.A122, 20 pp. . 561 . 10.1051\/0004-6361\/201322406 . 2014A&A...561A.122L , A sample of [C II] clouds tracing dense clouds in weak FUV fields observed by Herschel . Pineda J. L. et al. . Astronomy and Astrophysics, Volume 521, id.L19, 4 pp. . 521 . 10.1051\/0004-6361\/201015089 . 2010A&A...521L..19P , [CII] observations of H2 molecular layers in transition clouds . Velusamy T. et al. . Astronomy and Astrophysics, Volume 521, id.L18, 4 pp. . 521 . 10.1051\/0004-6361\/201015091 . 2010A&A...521L..18V , A Herschel [C II] Galactic plane survey. III. [C II] as a tracer of star formation . Pineda J. L. et al. . Astronomy & Astrophysics, Volume 570, id.A121, 13 pp. . 570 . 10.1051\/0004-6361\/201424054 . 2014A&A...570A.121P , A Herschel [C ii] Galactic plane survey. I. The global distribution of ISM gas components . Pineda J. L. et al. . Astronomy & Astrophysics, Volume 554, id.A103, 28 pp. . 554 . 10.1051\/0004-6361\/201321188 . 2013A&A...554A.103P , Origin and z-distribution of Galactic diffuse [C II] emission . Velusamy T. et al. . Astronomy & Astrophysics, Volume 572, id.A45, 20 pp. . 572 . 10.1051\/0004-6361\/201424350 . 2014A&A...572A..45V , Collisional Excitation of the [C II] Fine Structure Transition in Interstellar Clouds . Goldsmith Paul F. et al. . The Astrophysical Journal Supplement, Volume 203, Issue 1, article id. 13, 11 pp. (2012).... . 203 . 10.1088\/0067-0049\/203\/1\/13 . 2012ApJS..203...13G , The first results from the Herschel-HIFI mission . van der Tak Floris et al. . Advances in Space Research, Volume 49, Issue 10, p. 1395-1407. . 49 . 10.1016\/j.asr.2012.02.027 . 2012AdSpR..49.1395V , C+ detection of warm dark gas in diffuse clouds . Langer W. D. et al. . Astronomy and Astrophysics, Volume 521, id.L17, 4 pp. . 521 . 10.1051\/0004-6361\/201015088 . 2010A&A...521L..17L , [C ii] 158 μm line detection of the warm ionized medium in the Scutum-Crux spiral arm tangency . Velusamy T. et al. . Astronomy & Astrophysics, Volume 541, id.L10, 4 pp. . 541 . 10.1051\/0004-6361\/201219303 . 2012A&A...541L..10V , Physical properties of CO-dark molecular gas traced by C+ . Tang Ningyu et al. . Astronomy & Astrophysics, Volume 593, id.A42, 13 pp. . 593 . 10.1051\/0004-6361\/201528055 . 2016A&A...593A..42T ,
Instrument
HIFI_HifiPoint_load, HIFI_HifiMapping_load-raster
Temporal Coverage
2010-04-17T00:27:41Z/2011-05-11T09:37:17Z
Version
SPG v14.2.0
Mission Description
Herschel was launched on 14 May 2009! It is the fourth 'cornerstone' mission in the ESA science programme. With a 3.5 m Cassegrain telescope it is the largest space telescope ever launched. It is performing photometry and spectroscopy in approximately the 55-671 µm range, bridging the gap between earlier infrared space missions and groundbased facilities.
European Space Agency, 2012, State Of The Diffuse Ism: Galactic Observations Of The Terahertz Cii Line Openpargot Cplusclosepar, SPG v14.2.0, European Space Agency, https://doi.org/10.5270/esa-l1y70l9
