ISO measurements of [CII] line variations in galaxies
msra(1997)
摘要
We report measurements of the [CII] fine structure line at 157.714 micron in
30 normal star-forming galaxies with the Long Wavelength Spectrometer (LWS) on
the Infrared Space Observatory (ISO). The ratio of the line to total
far-infrared luminosity, [CII]/FIR, measures the ratio of the cooling of gas to
that of dust; and thus the efficiency of the grain photoelectric heating
process. This ratio varies by a factor of 40 in the current sample. About
two-thirds of the galaxies have [CII]/FIR = 2-7 x 10^{-3}. The other one-third
show trends of decreasing [CII]/FIR with increasing dust temperature, indicated
by the ratio of infrared emission at 60 and 100 microns; and with increasing
star-formation activity, measured by the ratio of far-infrared and blue band
luminosity. We find three FIR bright galaxies with undetected [CII] line at
3-sigma upper limits of [CII]/FIR < 0.5-2 x 10^{-4}. The trend in the [CII]/FIR
ratio with the temperature of dust and with star-formation activity may be due
to decreased efficiency of photoelectric heating of gas at high UV radiation
intensity as dust grains become positively charged, decreasing the yield and
the energy of the photoelectrons. The three galaxies with no observed PDR lines
have among the highest ratios of FIR to Blue luminosity and the ratio of 60 and
100 micron emission. Their lack of [CII] lines may be due to a continuing trend
of decreasing [CII]/FIR with increasing star-formation activity and dust
temperature seen in one-third of the sample with warm IRAS colors. In that case
the upper limits on [CII]/FIR imply a ratio of UV flux to gas density G_0/n >
10 cm$^3. The low [CII]/FIR could also be due to either weak [CII] because of
self-absorption or strong FIR continuum from regions weak in [CII], such as
dense HII regions or plasma ionized by hard radiation of AGNs.
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关键词
far infrared,star formation,infrared,fine structure
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