Other
Scientific paper
Jan 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21541204p&link_type=abstract
American Astronomical Society, AAS Meeting #215, #412.04; Bulletin of the American Astronomical Society, Vol. 42, p.246
Other
Scientific paper
Hydrogen recombination lines were observed in starburst galaxies in a broad spectral range, from optical to cm radio. Since these lines are due to ionization, they have the potential to be a probe of star formation rate (SFR). However, the observed intensity of most of these lines is affected by either strong interstellar dust extinction (in visible and near-IR), or the competing fluctuations of strong emission background created by dust (far IR), or else by maser effects (radio domain). Some authors argue that mm radio recombination lines may be practically free of maser effects and thus may be good probes of the SFR. However there were several reports of possible observed masing in some of these lines. Here we present, based on our observations with the 12m ARO radio telescope (Kitt Peak, AZ), strong evidence of masing in the H35α line at 2 mm in the prototype starburst galaxy M82. We demonstrate that the flux ratio of H35α and H40α lines strongly varies with the radial velocity of the emitting clouds and that at some radial velocity it is considerably (up to a factor of 3) higher than the ratio for spontaneous optically thin emission (1.9). This cannot be explained otherwise than by masing in the H35α (or both of these lines). Masing in H35α requires a high density of ionized gas ( 106 -107 cm-3), which may take place in the numerous dense supernova shells or supercompact HII regions of this galaxy. Possible masing makes the 2mm recombination lines unreliable probes of SFR. We argue that the best recombination line probes of SFR should be in the 1 cm domain (≈H60 α) and the mid-IR domain (≈H6α). This project was supported by the NSF/REU grant AST-0851892 and the Nantucket Maria Mitchell Association.
Pequette Naomi
Strelnitski Vladimir
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