Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: ISO Far-IR Spectroscopy of IR-Bright Galaxies and ULIRGs   Based on far-infrared spectroscopy of a small sample of nearby
infrared-bright and ultraluminous infrared galaxies (ULIRGs) with the ISO Long
Wavelength Spectrometer, we find a dramatic progression in ionic/atomic
fine-structure emission line and molecular/atomic absorption line
characteristics in these galaxies extending from strong [O III]52,88 and [N
III]57 micron line emission to detection of only faint [C II]158 micron line
emission from gas in photodissociation regions in the ULIRGs. The molecular
absorption spectra show varying excitation as well, extending from galaxies in
which the molecular population mainly occupies the ground state to galaxies in
which there is significant population in higher levels. In the case of the
prototypical ULIRG, the merger galaxy Arp 220, the spectrum is dominated by
absorption lines of OH, H2O, CH, and [O I]. Low [O III]88 micron line flux
relative to the integrated far-infrared flux correlates with low excitation and
does not appear to be due to far-infrared extinction or to density effects. A
progression toward soft radiation fields or very dusty HII regions may explain
these effects.

1
The categories of this paper are: cs
37
The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
20
The categories of this paper are: acc-phys
22
The categories of this paper are: chao-dyn
7
The categories of this paper are: hep-th
14
The categories of this paper are: math-ph
30
The categories of this paper are: q-alg
36
The categories of this paper are: cmp-lg
11
The categories of this paper are: hep-lat
33
The categories of this paper are: plasm-ph
9
The categories of this paper are: nlin
12
The categories of this paper are: nucl-th
28
The categories of this paper are: alg-geom
3
The categories of this paper are: physics
25
The categories of this paper are: patt-sol