Normal irregular galaxies seem to be unusual in having vigorous star formation yet lacking the many dark nebulae typical of spirals. The Infrared Astronomy Satellite (IRAS) observations of a large sample of irregulars are used to explore the dust contents of these galaxies. Compared to normal spirals, the irregulars generally have higher L sub IR/L sub B ratios, warmer f(100)/f(60) dust color temperatures, and lower globally-averaged dust/gas ratios. The relationship between the infrared data and various global optical properties of the galaxies is discussed

Gallagher, J.

Gillett, F.

Hunter, D.

Rice, W.

English

NASA Technical Reports Server

IRAS Observations of Irregular Galaxies
N87- 24299
D. Hunter
Dept. of Terrestrial Magnetism, CIW
Washington, D.C.
W. Rice
Infrared Processing and Analysis Center, CIT
Pasadena, CA
J. Gallagher and F. Gillett
Kitt Peak National Observatory, NOAO
Tucson, AZ
Abstract
Normal irregular galaxies seem to be unusual in having vigorous star
formation yet lacking the many dark nebulae typical of spirals. IRAS
observations of a large sample of irregulars are used to explore the dust
contents of these galaxies. Compared to normal spirals, the irregulars
generally have higher LTR/L R ratios, warmer f(100)/f(60) dust color
temperatures, and lower-_lobally-averaged dust/gas ratios. The relationship
between the infrared data and various global optical properties of the galaxies
is discussed.
In spiral galaxies dust seems to be intimately connected with star-forming
regions. In normal irregular galaxies, however, there appears to be a dearth
of dark nebulae and reddenings are low relative to spirals. This is in spite
of the vigorous, apparently normal, star formation often found in irregular
galaxies. We present preliminary results from an investigation of the dust
content of a large sample of irregulars based on IRAS (co-added survey)
infrared observations at 12, 25, 60, and 100 um (see also Hunter et al. 1986).
The irregular galaxies are divided into four groups: I ) "Dwarf"-- low
surface brightness Magellanic-type irregulars, 2) "Giant"-- high surface
brightness Magellanic-types, 3) "Amorphous"-- noted for their smoothness and
lack of resolution into stars and clusters, and 4) "Distant"-- distant, clumpy
irregulars. Systems in the first three grouups are primarily nearby,
non-interacting galaxies. The distant irregulars, however, are at such large
distances that morphological distinctions are more difficult and many of the
systems have been found to be interacting. The galaxies in these four groups
have many similar optical properties which are summarized and compared by
Hunter and Gallagher (1986). For comparison with spiral galaxies we have
chosen systems from Kennicutt's (1983) list of Ha observations and taken
infrared data from the Cataloged Galaxies and Quasars Observed in the IRAS
Survey.
In comparing the different groups of irregular galaxies, we find that the
distant irregulars tend to have higher ratios of L_/L.. and L_/L_ and
somewhat warmer f(100)/f(60) dust color temperatures. _owever, differences in
the infrared properties could be due to the fact that many of these systems are
* See erratum, page 257.
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 253 PRECEDING PAGE F::_._'_ NOT FIIAMED
https://ntrs.nasa.gov/search.jsp?R=19870014866 2020-03-20T10:57:14+00:00Z
D. A. HUNTER ET AL.
interacting. The dwarf irregulars tend to have somewhat lower LIR/L B ratios,
but otherwise the groups have fairly similar infrared properties.
• dwarfIrr 1.2
x giantIn-
o amorphous Irr .8
• distantIrr
- _ 0
o ,.,.,.,. x, F1, F1
0 40 80 120 180 200 240 280 220
c(m)/_(_)
Fig. l-Ratio of infrared to Ha
luminosity of the irregulars.
' I ' I ' I ' Io'
- 4 4 0
-- x@ OA 0 xa
- ,"-F , I , I , I ,
.8 .6 .4 .2 0
sOoo)/s(6o)
Fig. 2-The outlined region shows the area
occupied by normal spiral galaxies (de Jong
et al. 1984).
In a comparison of irregulars with spiral galaxies, on the other hand,
there are some general differences. When irregulars are put on the plot of log
LTR/L B vs. log f(1OO)/f(60) of de Jong et al. (1984), one finds that on average
t_e irregulars have warmer f(100)/f(60) dust color temperatures and somewhat
higher LTR/L R ratios than the spirals. However, in terms of the f(25)/f(12)
dust co16_ t_mperature, the spirals are on average somewhat warmer. Also,
spirals generally have higher LIR/LH_ ratios.
Because dust at a range of temperatures contributes to the global IRAS
fluxes, the ratio f(100)/f(60) does not refer to a single temperature of the
dust. However, the ratio does give an idea of the average temperature of the
dust contributing to those passbands. One can then ask why this
temperature-sensitive ratio varies and why irregulars are warmer in general
than many spirals. One factor affecting the dust color temperature is the
composition of the dust grains. One might expect that, if composition of the
grains is the dominant factor affecting the overall temperature variation in
the dust between galaxies, that the temperature ratio f(1OO)/f(60) would be
correlated with the metallicity of the galaxies. Irregulars are in general
more metal poor than spirals. However, a plot of f(100)/f(60) against the
oxygen abundance shows that the average temperature of the dust does not depend
on the metal abundance of the irregular in any simple fashion.
Dust masses have also been estimated using f(60) and a silicate model. It
is important to note that the dust and gas masses are global measurements
rather than measurements of individual star-forming regions; and the gas beyond
the optical galaxy is not expected to contain dust that contributes to the IRAS
infrared flux. Keeping this in mind, one finds that irregulars generally have
lower global dust/gas ratios than spirals. Studies (Werner et al. 1978;
Viallefond, Goss, and Allen 1982; Viallefond, Donas, and Goss 1983; references
254
IRAS OBSERVATIONS OF IRREGULAR GALAXIES
0
:I Irre_._lm.s
-@
-@
Zo o
1 2 3 4 5• 8 7 8 9 10_,1i
lo M_
Fig. 3-The dust/hydrogen gas ratio. Dust
masses have been estimated using f(60)
and a silicate model. These are
globally-averaged values.
I0
8
56
4
2
0
2.4
2
_" 1.8
_--_ 1.2
.8
"1 I I I I" I I i I"
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&
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xeor_•
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x • @
• x @
_ @x AA
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Xo A •
0
-I I I I I I [ I I-
_ 2 3 _ 50/He 7 8''o4
Fig. 4-Dust color temperatures and
global dust/hydrogen gas ratios
vs. oxygen abundance of the
irregular.
in Koornneef 1984) of a few individual star-forming regions in the LMC suggest
that local dust/gas ratios are also lower than regions in spirals although
large optical depths due to dust can occur (Gatley et al. 1981). There is no
correlation of the dust/gas ratio with the metallicity of the irregular.
Traditionally people have thought that radiation from stars in
star-forming regions is responsible for heating the dust (cf. Wynn-Williams and
Becklin 1974). But, with IRAS people have found that "cirrus" (Low et
al. 1984) can also contribute to a galaxy's total far-IR flux. It is possible
then that part of the far-IR flux of a galaxy comes from stars not connected
with the current star-forming activity (see also Helou 1986; Persson and Helou,
1986). The problem in dealing with integrated fluxes of galaxies
is in determining the relative contributions of these two sources.
The SFR (star formation rate)/area is a measure of the star-formation
related radiation field in the galaxy, and the optical surface brightness is
more a measure of the general stellar radiation field. There is no correlation
between the temperature-indicators, f(25)/f(12) and f(100)/f(60), and the
SFR/area although there may be a trend between f(1OO)/f(60) and surface
brightness. There are also hints of trends of both SFR/area and surface
brightness vs. LIR/LB, but they are far from clean. When spirals are added to
these plots, they increase the scatter. Part of the difficulty in using these
255
D.A.HUNTERETAL.
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Figs. 5,6,7-The star formation rate per unit area and the blue optical surface
brightness vs. infrared properties of the galaxies.
parameters is that we are looking a_ local processes through global averages.
In addition the irregular galaxies have formed stars at approximately constant
rates, so the stellar radiation field is not independent of the current SFR and
it is difficult to separate the two. (See Gallagher and Hunter, these
proceedings, for a discussion of star-formation rates and histories).
We are grateful to all of the people involved in the IRAS project for an
excellent infrared data base and to the people at IPAC for their assistance in
extracting the data. This work is supported in part by a grant from NASA as
part of the IRAS General Investigator program.
References
de Jong, et al. 1984, Ap. J. (Lett), 278, L67.
Gatley, I., Becklin, E. E., Hyland, A. R., and Jones, T. J. 1981,
M. N. R. A. S., 197, 17p.
Helou, G. 1986, Ap. J. (Lett), submitted.
Hunter, D. A. and Gallagher, J. S. 1986, Pub. A. S. P., 98, 5.
Hunter, D. A., Gillett, F. C., Gallagher, J. S., Rice, W. L., and Low, F. J.
1986, A._., 303, 171.
Kennicutt, R. C. 1983, Ap. J., 272, 54.
Koornneef, J. 1984, in Structure and Evolution of the Magellanic Clouds, eds.
S. van den Bergh and K. de Boer (Dordrecht: Reidel), p. 333.
Low, F. J., et al. 1984, Ap. J. (Lett), 278, LI9.
Persson, C. J. and Helou, G. 1986, Ap. J., in press.
Viallefond, F., Donas, J., and Goss, W. M. 1983, Astr. Ap., 119, 185.
Viallefond, F., Goss, W. M., and Allen, R. J. 1982, Astr. Ap., 115, 373.
Werner, M. W., Becklin, E. E., Gatley, I., Ellis, M. J., Hyland, A. R.,
Robinson, G., and Thomas, J. A. 1978, M. N. R. A. S., 184, 365.
Wynn-Williams, C. G., and Becklin, E. E. 1974, Pub. A. S. P., 86, 5.
256


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