Issue |
A&A
Volume 375, Number 2, August IV 2001
|
|
---|---|---|
Page(s) | 566 - 578 | |
Section | The Sun | |
DOI | https://doi.org/10.1051/0004-6361:20010817 | |
Published online | 15 August 2001 |
Global physical conditions of the interstellar medium in nearby galaxies*
1
Department of Astronomy, School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2
MS245-6, NASA Ames Research Center, Moffett Field, CA94035-1000, USA
Corresponding author: T. Onaka, onaka@astron.s.u-tokyo.ac.jp
Received:
20
April
2001
Accepted:
1
June
2001
Far-infrared spectra (43-197 μm) of 34 nearby galaxies obtained by
the Long Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory
(ISO) were analyzed to investigate the general properties of interstellar
matter
in galaxies. The present sample includes not only normal galaxies but also
starbursts and active galactic nuclei (AGNs).
Far-infrared forbidden lines, such as [Cii]158 μm,
[Oi]63 μm, [Nii]122 μm, and
[Oiii]88 μm,
were detected in most of the sample galaxies.
[Oi]145 μm line was detected in 13 galaxies.
The line fluxes of [Cii]158 μm and [Nii]122 μm
relative
to the total far-infrared flux (FIR) decrease as the far-infrared color
becomes
bluer, while the ratio of the [Oi]63 μm
flux to FIR does not show a systematic trend with
the color. The [Oiii]88 μm to FIR ratio shows a large
scatter with a weak trend of increase with the color.
AGNs do not show any distinguishable trend from normal and starburst
galaxies in
the far-infrared spectra, suggesting that the far-infrared emission is
mainly driven by star-formation activities even in AGNs.
We estimate the physical conditions of photodissociation regions (PDRs) in
the sample galaxies, such as the far-ultraviolet radiation field intensity
and the gas density n
by assuming that all the observed [Oi]63 μm and far-infrared
continuum emissions come from PDRs.
Comparison with PDR models indicates that
ranges from 102-104 and
-
. The
present results also suggest
that n varies proportionally with
.
The ratio of [Cii] 158 μm to CO (
-0) line emission supports
the linear increase in n with
.
We estimate that about a half of [Cii]158 μm
emission originates from PDRs and
attribute the rest to the emission as coming from low-density diffuse
ionized gas.
The estimated intensity of [Cii]158 μm from the ionized gas
is compatible with the observed intensity of
[Nii]122 μm if both lines come from the same diffuse ionized
gas. The present analysis suggests that the decrease in
[Cii]158 μm
with the
far-infrared color may not be accounted for by the decrease in the
photoelectric heating efficiency owing to the increase in positive charges
of dust grains
because a measure of the efficiency,
, is found to stay
constant with the far-infrared color. Instead the decrease
can be interpreted in terms of either the increase in the collisional
de-excitation of
the [Cii] transition due to the increase in the gas density or the
decrease in the
ionized component relative to the far-infrared intensity suggested by the
decrease in [Nii]122 μm
.
Based on the present analysis, we derive average relations of the
far-infrared
color with
and n in galaxies, which can be applied to the
investigation
of
interstellar matter in distant galaxies.
Key words: galaxies: ISM / infrared: ISM / ISM: lines and bands / radiation mechanisms: thermal
© ESO, 2001
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