using lws guaranteed time in the core program hsmith_irbs, we obtained lws01 spectra from 43 to 196 um of the nuclei of the nearby starburst galaxies, m82 and ngc 253. the m82 spectrum contains the expected 7 bright far-ir fine structure lines, but also contains obvious absorption lines at 119.3 and 149.2 um that we identify as the fundamental rotational transitions of oh and ch respectively. there also are probable detections of the hd rotational lines in emission at 112.1 um (j=1-0), and 56.2 um (j=2-1). these molecular rotational lines are apparent in the ngc 253 spectrum as well. we propose to use the lws fabry-perot to spectrally resolve the 119, 149 and 112 um lines, and the lws to get ten times better s/n on the 56 um line for both galaxies, thereby confirming the line identifications. each of these lines can be used for abundance analyses. the ch and oh absorption lines lead directly column density, hence abundance, thereby testing the chemical models of molecular clouds exposed to the very large (virgul 10^3 isrf) far-uv radiation fields, and high cosmic ray fluxes associated with the starburst. we will compared the observed far-ir oh line profiles to radio observations of the 18 cm line to determine the excitation temperature of the radio transitions. most of the radio profile is in absorption, so that the excitation temperature is likely largely determined by collisional processes. the line comparison will therefore yield molecular cloud densities as a function of radial velocity. for m82 at a lsr velocity virgul 75 km s^-1, however, the 18 cm lines are in emission, and likely masing, so that determining the excitation temperature connects to models of the pumping scheme for masers. confirming observations of the hd lines is especially exciting. the pair of lines yields the physical parameters of the emitting gas, hence column density of hd along the line of sight. since d is only destroyed by normal stellar process, the observed d abundance is a lower limit to the primordial d abundance, hence upper limit to the baryon density of the universe. our initial line analysis for m82 indicates the hd lines come from gas at t virgul 120 k, and that the d/h ratio virgul 1.6 e-5.
Instrument
LWS01 , LWS02 , LWS03
Temporal Coverage
1997-06-07T09:24:18Z/1997-09-04T01:07:46Z
Version
1.0
Mission Description
The Infrared Space Observatory (ISO) was the worlds first true orbiting infrared observatory. Equipped with four highly-sophisticated and versatile scientific instruments, it was launched by Ariane in November 1995 and provided astronomers world-wide with a facility of unprecedented sensitivity and capabilities for a detailed exploration of the Universe at infrared wavelengths.
European Space Agency, SMITH et al., 1999, 'LWS MOLECULAR LINE OBSERVATIONS OF M82 AND NGC 253', 1.0, European Space Agency, https://doi.org/10.5270/esa-ioq98vd
