We propose to detect and map rotational line emission from water and CO in the edge-on spiral galaxy, NGC 891.Both water and CO rotational lines are important coolants for low velocity C-shocks resulting from micro-turbulence in molecular clouds. The water lines are of particular interest, as they are definitive shock tracers on galactic scales. Micro-turbulent shocks are an important process through which molecular clouds dispel their turbulent kinetic energy enabling them to collapse and to form stars. NGC 891 is the ideal source for detecting shock tracers from the more quiescent ISM within a normal spiral galaxy: it is presented to us edge-on so that column densities along the line of sight are quite large, and it is nearby so that the edge-on geometry still nearly fills the Herschel spectrometer beams. The combination of high columns, and good beam match enables the detection of particularly weak lines. Our search is motivated byour recent study of NGC 891 in its H2 rotational line and OI, CII, and NII fine-structure line emission that provides strong evidence for micro-turbulent shock excitation of the S(2) and S(1) rotational lines. These lines are bright, and within C-shock models, several water lines are predicted to be nearly equally bright, and readily detectable with Herschel/PACS and HIFI,and the rotational ladder of CO is detectable with SPIRE. The proposed Herschel observations of water CO will provide the first definitive indicators of the micro-turbulent shocks that along with H2 line emission are the dominant coolants for molecular cloudinteriors. Herschel is the only facility that can observe these water lines that are totally obscurred by the Earth.s atmosphere even at aircraft altitudes.
Publication
On the far-infrared metallicity diagnostics: applications to high-redshift galaxies | Rigopoulou D. et al. | Monthly Notices of the Royal Astronomical Society Volume 473 Issue 1 p.20-29 | 473 | 10.1093\\/mnras\\/stx2311 | 2018MNRAS.473...20R | http://adsabs.harvard.edu/abs/2018MNRAS.473...20R
The Herschel SPIRE Fourier Transform Spectrometer Spectral Feature Finder - II. Estimating radial velocity of SPIRE spectral observation sources | Scott Jeremy P. et al. | Monthly Notices of the Royal Astronomical Society | null | null | 2020MNRAS.496.4894S |
The distribution of dust in edge-on galaxies: I. The global structure | Mosenkov Aleksandr V. et al. | Monthly Notices of the Royal Astronomical Society | null | null | 2022MNRAS.515.5698M |
The Herschel SPIRE Fourier Transform Spectrometer Spectral Feature Finder - III. Line identification and off-axis spectra | Benson Chris S. et al. | Monthly Notices of the Royal Astronomical Society | null | null | 2020MNRAS.496.4906B |
The Herschel SPIRE Fourier Transform Spectrometer Spectral Feature Finder I. The Spectral Feature Finder and Catalogue | Hopwood R. et al. | Monthly Notices of the Royal Astronomical Society | null | null | 2020MNRAS.496.4874H |
High-J CO versus Far-infrared Relations in Normal and Starburst Galaxies | Liu Daizhong et al. | The Astrophysical Journal Letters Volume 810 Issue 2 article id. L14 6 pp. (2015). | 810 | 10.1088\\/2041-8205\\/810\\/2\\/L14 | 2015ApJ...810L..14L | http://adsabs.harvard.edu/abs/2015ApJ...810L..14L
Insights into gas heating and cooling in the disc of NGC 891 from Herschel far-infrared spectroscopy | Hughes T. M. et al. | Astronomy & Astrophysics Volume 575 id.A17 25 pp. | 575 | 10.1051\\/0004-6361\\/201424732 | 2015A&A...575A..17H | http://adsabs.harvard.edu/abs/2015A%26A...575A..17H
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, stacey et al., 2012, 'Water and CO: Signatures of Microturbulent Shocks in NGC 891', SPG v14.2.0, European Space Agency, https://doi.org/10.5270/esa-jcl6pdk
