| Description |
We propose to carry out a HIFI survey of the fundamental transition of hydrogen fluoride (HF) at 1.232 THz toward 16 nearby IRAS-bright galaxies to probe their ISM physical conditions. Our proposal is motivated by recent Herschel observations revealing ubiquitous absorption by HF in the Milky Way galaxy. The HF J=1-0 transition has been observed in the spectrum of almost every bright continuum source in the Galactic plane and reveled to be an extremely sensitive probe of the diffuse molecular gas. We intend to make use of this unique probe by conducting a high spectral resolution (10-20 km/s) HIFI absorption survey toward continuum-bright external galactic nuclei exhibiting a wide range of physical properties (AGN, starbursts, mergers). HF is chemically very strongly bound and therefore resistant to photodissociation. Our study is facilitated by this resistance of HF molecules to destruction, which will occur in the extreme environments in the galactic nuclei. HF will thus be a very useful probe in regions of the ISM where more traditional gas probes, such as CO, are more prone to error (e.g., the use of the X-factor). With the simplifying assumption that all fluorine is likely to be locked up in HF, and that the HF molecules will reside in the ground rotational state, we can measure the hydrogen column density and mass of the nearby IRAS-bright galaxies. Observations of the local galaxies proposed here is the first step toward using HF as a tracer of the gas in high-redshift galaxies. By looking at extragalactic continuum-bright nuclei, we will also be able, through the use of the HIFI Wide Band Spectrometer, to simultaneously search for absorption through the Milky Way halo cloud population. Here, the rapid formation rate of HF and its strong molecular bond will allow us to detect HF absorption toward the tenuous and quiescent mostly HI clouds making up the galactic halo. These diffuse, cold regions may not otherwise be detectable in CO emission or other commonly uses tracers. |
