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Session 89 - Starburst Galaxies.
Oral session, Wednesday, January 15
We have obtained fully sampled far-infrared (far-IR) 44 to 196 um spectra of the nuclei of the nearby starburst galaxies NGC 253 and M82. These spectra were obtained with the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). The LWS has a resolving power 200.
Both spectra show the seven bright far-IR fine-structure lines ([OI] 63 and 145 um, [OIII] 52 and 88 um, [NII] 122 um, [NIII] 57 um, and [CII] 158 um) expected from prior Kuiper Airborne Observatory (KAO) spectroscopy. These lines characterize the physical conditions of the ionized gas, photodissociation regions (PDRs) at the surfaces of far-UV exposed molecular clouds, and the atomic ISM. They also characterize the stellar radiation field, and N/O abundance ratio. Relative to the far-IR continuum, these lines are much brighter for M82. Some of the weaker lines (e.g. [OI] 145 um) are only marginally detected in NGC 253. We will discuss the implications of these observations in terms of the age and size of the nuclear starbursts.
There are also several very interesting low-J rotational transitions from abundant molecules apparent in these spectra. The most obvious are absortion lines of OH at 119 and 53 um. These lines trace the column density of OH along the line of sight, hence the abundance of OH. We therefore can test chemical models of molecular clouds exposed to the high UV fields and cosmic ray fluxes associated with the starburst. The spectra also show OH emission at 163 um, and possible CH and H2O absorption lines. The implications of the line strengths for molecular cloud studies will be discussed.
Program listing for Wednesday