Two regions of recent star formation in blue irregular galaxies were observed with the IUE in the short wavelength, low dispersion mode. The spectra indicate that the massive star content is similar in these regions and is best fit by massive stars formed in a burst and now approximately 2.5 to 3.0 million years old

Gallagher, J. S., III

Hunter, D. A.

Lamb, S. A.

English

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N87- 24 301
IUE OBSERVATIONS OF LUMINOUS BLUE STAR ASSOCIATIONS IN IRREGULAR GALAXIES
S.A. Lamb
NASA-Ames Research Center and
Departments of Physics and Astronomy
University of Illinois at Urbana-Champaign
Urbana, IL 61801
D.A. Hunter
Department of Terrestrial Magnetism
Carnegie Institution of Washington
5241 Broad Branch Road
Washington, D.C. 20015
J.S. Gallagher, Ill
National Optica] Astronomy Observatories
Kitt Peak National Observatory
P.O. Box 26732
Tucson, AZ 85726-6732
ABSTR ACT
Two regions of recent star" formation in blue irregular galaxies have been
observed with the IUE in the short wavelength, low dispersion mode. The
spectra indicate that the massive star content is similar in these regions
and is best6fit by massive stars formed in a burst and now approximately
2.5-3.0 x10 years old.
1. INTRODUCTION
Ir'reguJar galaxies offer a special opportunity for the study of massive star
formation. They are the bluest of galaxies, as a class, and this, together with
the optical emission line spectra observed for some, indicates that massive stars
are currently being formed. We are particularly interested in exploring the high
mass end of the initial mass function (IMF) and the spatial patterns of star
formation for the massive star's in these systems.
The irregular galaxies span a considerable range in morphology. The
majority are blue, ciumpy lm (Magellanic type) galaxies which have very obvious
lIIl regions and associated OB associations. The remaining irregulars are
classified I0 or Irr II, some of which are red, iike M82. An interesting subset
of the latter class are the Amorphous galaxies, which were first classified by
Sandage and Brucato (1979). These Amorphous systems can be as blue or bluer than
Im galaxies, but OB associations are not resolved, despite the relative proximity
of the galaxies to us. Extensive HII regions are observed in these systems,
however.
Ultraviolet spectra provide a means of exploring the hot stellar content of
these galaxies. In a previous paper (Lamb et al. 1985) we investigated the
massive star content of two amorphous irregulars - NGC 1705 and NGC 1800, through
IUE (International Ultraviolet Explorer' Satellite) observations, together with
optical spectra. In this paper we continue the exploration of irregular galaxies
through IUE ultraviolet spectra of two systems which have very different star-
forming patterns: I) NGC 1140 is a typical Amorphous system. It contains a
large blob of ionized gas at its center upon which the IUE aperture was placed.
2) NGC 4449 is a classical, well-resolved Magellanic Irr with many star-forming
Carol J. Lonsdale Persson (Editor)
Star Formation in Galaxies 259
https://ntrs.nasa.gov/search.jsp?R=19870014868 2020-03-20T10:57:11+00:00Z
S.A.LAMBETAL.
2.4 × 10 13
2,0 X 10 -13
¢?
E 16 X 10 -13
1.2 X 10 13
0.0 × 10 -14
t _ n i i i i i i
NGC 4449
1; 1" 1,_o 1_o ,_o 1,_o 10oo' 1o"
WAVELENGTH _.
6×10 14
'-. 5 × 10 -14
'E
4 × 10 -14
3 x 10 -14
2 x 10 -14 I
I I I I
NGC 1140
WAVELENGTH A
Fig. 1. IUE short wavelength spectra of NGC 4449 and NGC 1140. The two spectra
of NGC 1140 were summed with a weighting proportional to their signal-to-noise.
The _pectrum for each galaxy was smoothed with a 5-point smoothing function.
Lines of primarily stellar origin are _dentified above the spectra.
HII regions. One large H]I region at the southwest end of the bar is in the form
of an arc with a cluster of stars nestled in this arc {see Hunter and Gallagher
1985) and the IUE aperture was centered on this cluster.
Short-wavelength ultraviolet spectra were obtained at low-dispersion of
these two galaxies in 1984 June and December. Setting with the I0"x20" aperture
was by blind offset from bright stars. SWP 23262 of NGC 4449 is a 210 minute
exposure, and SWP 24623 and SWP 24624 of NGC 1140 are respectively 180 and 185
minute exposures.
Data reductions and analysis were done using the Goddard Space Flight Center
Regional Data Analaysis Facility. The results are shown in Figure I, and we see
that the primary stellar lines present are Si IV X 1394, 1403_, C IV _ 1548,
1551_ and N IV X 1720_. The other prominent absorption features arise from
interstellar' sources within the external galaxies, and there can be a substantial
but u_known contribution from the interstellar medium of our own galaxy.
2. THE MASSIVE STAR CONTENT
2.1 Fitting the Ultraviolet Spectra
We have chosen to fit the overall shapes of the spectral lines, including
P Cygni profiles when they are present, and to give considerable weight to the
ratio of the equivalent widths of the SiIV and CIV features. This latter has
been shown by Walborn and Panek (1984 a,b) to be a sensitive indicator of whether
an individual massive star is on the main sequence or evolved into a
supergiant. They have also demonstrated that the presence of SiIV P Cygni
profiles is another indication of the luminosity class, as these develop only as
the massive stars move off the main sequence.
We have constructed spectra of stellar clusters using the IUE stellar atlas
(Wu et al. 1983) and relative contributions of stellar groups based on a standard
260
LUMINOUSBLUESTARASSOCIATIONSIN IRREGULARS
Salpeter (1955) initial mass function. The weighting factor is a product of the
star's relative luminosity, number (the initial mass function), and lifetime (for
evolved models). The relative luminosity at k 1400:1500_ for a star was
determined from the ratio of LFx (1400-1500_) to Te4 , where Fk was determined
from Kurucz (1979) model stellar atmospheres (Z=Zo, log g=4). The relationship
between Te and stellar spectral class, mass and luminosity were obtained from
Humphreys and McElroy (1984, see their Fig. I), and the lifetimes of the stars
were taken from models by Maeder (1981, 1983). Six stellar masses
(9,15,30,60,85,120 Mo) were chosen as representative of six stellar groups, and
average main sequence and evolved properties were determined for them. The atlas
stellar spectra were extracted as point sources, corrected for reddening, and
normalized to -1500_.
In constructing the synthesised models we attempted to span reasonable
possibilities in the upper mass cut-off and the time dependence of the star
formation rate. We did not vary the IMF, but our results are such that we can
exclude a very flat IMF for the most massive stars. We constructed nine
models. These included single burst models which varied from all main sequence
stars to a burst now 3xI0 v years old, and extended production models in which
the massive stars are either constantly formed or in w_ich the production rate
has fallen off by a factor of five from a maximum 3xI0 v years ago.
The best fit to our spectra of NGC 4449 and NGC 1140 were obtained from
synthesised spectra of single bursts of star formation. The burst of age -2.5 x
I0_ vear s with an upper mass cut off6of -120MQ gave an equally good fit to that
of the single burst of age ~3.0 x 10 years wlth an upper mass cut off of-70-80
Mo. The main features needed to provide a good fit are plenty of massive
supergiants and a lack of the most massive (M>5OMn) main sequence O stars. The
emission hump in the spectra of NGC 1140 at k 140_-1500_ was not fit by our'
synthesised spectra. This feature is due to Wolf Rayet stars (see Nussbaumer et
al. 1982). Their spectra are not available in the IUE spectral atlas and are not
included in our models.
2.2 Comparison with Spectra of Giant HI1 Regions.
Rosa et al. (1984) have published IUE spectra for a variety of extragalactic
}III regions and we compare these to our spectra. The majority of the systems
have quite similar short-wavelength spectra. That is, most giant extragalactic
HII regions have spectra which are similar to those of late O, early B stars. A
few systems have weaker stellar absorption features without having the strong
emission lines of very hot stars, and a few systems are of higher excitation,
having NIV] _ 1483,1486 _ emission (These latter systems include NGC 1140.)
Thus we conclude, from the similarity between our results and those for
extragalactic HII regions, that the OB star population is roughly comparable in
these systems. Small variations observed in these spectra indicate varying mixes
of evolved massive star's which can most easily be attributed to an age spread
from -2.5 x I0_ years to I0" years amoung the various regions.
In addition to the stellar features the energy distributions, i.e. the
slopes of the continua, are all roughly similar for the giant H]I regions and for
our Amorphous galaxies. The similarities in the ultraviolet continua are actually
quite remarkable. One might expect that dust contents and distributions would
vary greatly from region to region and with time within a given region. We
261
S.A.LAMBET AL.
conclude that dust does not play a large role in those parts of the galaxies
which we are observing in the ultraviolet.
3. THE SPATIAL PATTERNS OF STAR FORMATION.
Among the five irr galaxies we have investigated (the two discussed here,
plus NGC 1705, NGC 18OO and DDO 50, see Lamb et al. 1985 and Lamb et al. 1986),
none are resolved into multiple clumps in the ultraviolet. Two of these
galaxies, NGC 4449 and DDO 50, are Im type and do have a clumpy appearance in the
optical. Although unresolved into clumps, the ultraviolet sources are broader
than a point source. Our other three galaxies (NGC 1140, NGC 1705 and NGC 1800)
are of the bJue Amorphous type. These galaxies are more distant than our Im
galaxies (ranging from 8.7 Mpe to 30 Mpc), and the large aperture of the IUE
covers a considerable fraction of the galaxy. In each case the aperture was
centered on the optical center of the object and in each case the ultraviolet
source was point like.
It appears that the ultraviolet spectra are dominated by the youngest and
brightest regions within the star forming complexes in all these galaxies. The
Amorphous systems appear to experience massive star production in very condensed
regions but the population of massive stars is not obviously different from that
in normal Magellanic type irregulars.
We wish to acknowledge support from NASA grant NAG5-404 and to thank the
staff at the IUE observatory and at the Data Reduction Facility at Goddard Space
Flight Center for their help. S.A.L. was a NRC senior fellow during the
completion of this work.
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Humphreys, R. M. and McElroy, D.B. (1984), Ap.J., 284, 565.
Hunter, D.A. and Gallagher, J.S. (1985)Astron, J., 90, 80.
Kurucz, R. (1979)Ap.J. Suppl., 40,1.
Lamb, S.A., Gallagher, 3.S., Hjellming, M.S., and Hunter, D.A. (1985)Ap.J.,
291, 63.
Lamb, S.A., Hunter, D.A. and Gallagher, J.S. (1986) in preparation.
Meader, A. (1981) Astron. Astrophys., 102, 401.
Maeder, A. (1983)Astron. Astrophys., 120, 113.
Nussbaumer, H., Schmutz, W., Smith, L.J., and Willis, A.J. (1982), Astron.
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Rosa, M., doubert, M., and Benvenuti, P. (1984)Astron. Astrophys. Suppl.
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IUE observations of luminous blue star associations in irregular galaxies

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