Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.
Reginald J. Dufour
Department of Space Physics & Astronomy, Rice University,
Houston, TX 77005-1892 USA
Donald R. Garnett & Evan D. Skillman
Department of Astronomy, University of Minnesota, 116 Church St. SE,
Minneapolis, MN 55455 USA
Gregory A. Shields
Department of Astronomy, University of Texas, Austin, TX 78712 USA
40Myr. A nearby Im galaxy to the NW is also resolved into stars and
is likely a companion system. The FOS spectroscopy permit us to derive the
first accurate C/O value for the NW HII of IZw18: C/O=0.20
0.08.
A scenario for the recent star formation history of IZw18 is discussed.
The dwarf irregular galaxy IZw18 has the lowest O/H abundance known among
star-forming irregulars (e.g., Skillman & Kennicutt 1993, SK93). As
such, it is an exceptionally important object for understanding galactic
chemical evolution, star formation in very low metallicity (and presumably
dust-free) gas, and determining the primordial helium abundance (Y
).
It is also relatively nearby, its radial velocity, V
740
,
suggests a distance of only 
10 Mpc (assuming 
=75).
Consequently, numerous observational and theoretical investigations of IZw18
have been made during the past two decades; some of the most recent include
the deep ground-based imagery by Dufour & Hester (1990, DH90), a detailed
theoretical discussion of possible scenarios for its star formation history by
Kunth, Matteucci & Marconi (1995, KMM95), and characteristics of its
brightest stars from HST planetary camera images by Hunter & Thronson
(1995).
Figure: Deep combined images of IZw18: 
is a V+R+H
image
from our WF camera imagery of 1994 November showing the various diffuse
systems to the NW of the main body, and 
is a closeup of the stars
in the main body from the combination of our B+V+R PC images of 1995 March.
Three sets of observations of IZw18 were obtained with HST as part of
the Cycle 4 General Observer Program GO-5434. In 1994 October, we obtained
HST FOS spectroscopy of the NW HII region with the G190H and G570H
gratings. In 1994 November, we obtained CR-SPLIT imagery of IZw18 and
surrounding systems with WFPC2 through the F450W, F555W, F702W, F502N, &
F658N filters. Finally, in 1995 March, we obtained additional WFPC2 imagery
through the F439W, F555W, and F675W filters. The WFPC2 images were calibrated
using the standard procedures and calibration data available at STScI at the
time they were taken. The multiple images obtained in each filter were
combined and cosmic rays removed by both automated cross-correlation
techniques and inspection. The 1994 November imagery concentrated on deep
exposures of the main body and diffuse objects to the NW in the wide-field
camera frames (WF, 0 resolution), while the 1995 March imagery had the
main body centered in the planetary camera (PC, 0 resolution) field.
At an assumed distance of 10 Mpc for IZw18, our WFC imagery reached
V
28, corresponding to M
-2 stars in the galaxy, with a
spatial resolution of 0 corresponding to 
5 pc spatial resolution.
The FOS spectroscopy were taken through one of the 0 square apertures
centered on a position just ESE of the NW lobe star cluster of the main body
of IZw18. Two gratings were used: G190H to cover the UV 1650--2300Å range
in order to measure CIII] 1909Å and SiIII] 1883,92Å; and
G570H to cover 4600--6800Å, where the prominent lines of H
,
[OIII], and H
could be measured for exactly the same position.
These FOS spectra were later reprocessed to use specific calibration data for
the gratings and 0 square apertures, when it became available in late
1994.
Figure 1 gives two composite WFPC2 images of IZw18 from our data. The left
frame is a deep (combined F555W+F702W+F658N) image covering an 80
wide
(=3.9 kpc at a 10 Mpc distance) field with the bright main body at the lower
left and the several nearby diffuse objects designated in DH90 identified.
Only ``A'' (the main body) and ``C'' (an Im galaxy) showed resolution into
stars. The right frame gives a closeup of the main body from the combined
(F439W+F555W+F675W) PC images. Color PostScript and GIF versions of these
(and other) images can be obtained by anonymous ftp from spacsun.rice.edu
(128.42.10.5; ``cd pub/rjd/IZw18'' after login) and are very revealing in the
structure they show, some of which we discuss below.
In the ground-based imagery papers of Davidson et al. (1989) and of
DH90, attention was drawn to several diffuse objects in the vicinity of the
main body of IZw18, particularly a line of systems along
PA
210
. From the images in Figure 1, only diffuse object,
``C,'' is resolved into stars. All of the other diffuse objects
in the field appear to be distant galaxies based on their appearance and
colors (with the possible exception of ``G''). Information on these are
tabulated in Table 1, where we have corrected the observed magnitudes and
colors for foreground reddening, estimated to be A
=0.25, E(B-V)=0.07, &
E(V-R)=0.04, based on previous spectroscopy (SK93) of IZw18. Aside from its
large size and resolution into stars, further evidence that galaxy C may be
near the main body comes from the reanalysis of VLA HI observations by
Skillman et al. (1995). They show that the HI cloud associated
with IZw18 extends from the main body northwest to C. For the present
discussion, we will assume that C is at the distance of IZw18, 10 Mpc.
Table 1: Positional & Photometric Properties of IZw18 and Nearby Diffuse
Objects (corrected for foreground reddening).
The stars in the main body of IZw18 can be characterized by three
distributions: 
a dense star cluster in the NW, containing a rich
population of both blue- and red-supergiants, 
a more scattered
star-forming region in the SE, including several resolved HII emission
spheres, and 
blue- and red-supergiant stars around and in between the NW
and SE star-forming areas. In Figure 2 
we identify the locations of
the brighter blue- and red-supergiants in the main body, along with several of
the more interesting morphological features. With respect to the history of
star formation in IZw18, it is significant that there also exist numerous
bright isolated blue- and red-supergiants not associated with the
current starbursts in the NW or SE lobes of the main body. It is plausible
that these could have formed earlier in the history of the galaxy than the
current bursts in the NW and SE lobes, and be the source of ionization for the
more distant HII ``arcs'' seen on the imagery (Fig. 1
).
Figure: (left) F555W HST WFC emission-line subtracted image of the
main body of IZw18 with interesting stellar and nebular features labeled, and
a V vs. B-V CMD of stars in the main body, corrected for foreground
reddening. Contours of the H
emission are overlaid onto the F555W
image for reference.
Figure 2 
presents a V vs. B-V color-magnitude-diagram (CMD) of all
stars in the main body based on our emission-line subtracted F450W and F555W
wide-field images. The photometry was done using DAOPHOT PSF-fitting
photometry and the WFPC2 calibrations from Holtzman et al. (1995).
Our nominal rms photometric errors were 
0.1
down to
V
26.5 and B
26.0. Completeness seemed to be near 100% down
to those limits, but rapidly deteriorated for fainter stars in the NW cluster
due to severe overcrowding there. Therefore, our main body B-V CMD is
truncated near V
26. We overplot several cluster evolutionary tracks
from Bertelli et al. (1994) for Z=0.0004 metallicity [numbers give
(age)]. Note that we have applied a small reddening correction:
E(B-V)=0.07 and A
=0.25 (assumed to be Galactic foreground). The brightest
stars in IZw18 are at V
22, or 
for a distance of 10 Mpc
(reasonable for bright B- and A-supergiants in metal-poor galaxies). The
whole-galaxy CMD shows many evolved blue-and red-supergiants (about a 3:1
ratio) above V=25 (
), as well as a detectable concentration along
an upper main sequence around V
24 and below. This broad range of
magnitudes for the various yellow/red-supergiants seen in the CMD suggests a
significant dispersion in the ages of the main body stars. While not given
here, our PC CMD for the NW cluster alone shows a similar result. We
interpret the CMD as indicating that the star formation in the main body began
at least 30Myr ago, possibly as long as 50Myr ago, and continues to the
present (as apparent in the SE component).
Figure: Illustration of several prominent stellar and nebular components of
the companion galaxy to IZw18 
and a CMD of all detected stars in B
&V for the galaxy 
. Contours of the H
emission are overlaid
onto the F555W (15
square field); the location of the most prominent blue
(``B'') and red (``R'') stars are labeled. At the top-left is an insert of an
H
image of two emission regions detected.
The ionized gas in the main body of IZw18 also shows an interesting
three-component morphology. Most prominent is a 100 pc diameter ``bubble''
surrounding the NW cluster, presumably driven by stellar winds from the
cluster. The fact that the stars in this part of IZw18 have strong winds is
evident from the small cavity surrounding a star to the ESE of the cluster
(labeled as ``W'' in Figure 2 
; for which V=22.86 & B-V=0.34,
indicating that it has evolved off the main sequence). If we assume that the
expansion velocity of this bubble is 
50
, then the age of the
bubble is only about 1Myr. However, the numerous red-supergiants seen
in the NW cluster indicate an age of at least 7--10Myr. We also note that the
center of the NW emission bubble is offset from the cluster to the SW,
indicating that the gas density is likely higher on the eastern side of the
cluster. Indeed, the highest surface brightness emission region is a small
0 (24 pc) diameter ``knot'' just on the NE side of the cluster.
A second prominent HII component is the clumpy emission in the SE,
consisting of numerous knots and a larger (1 diameter) doughnut-shaped
feature containing 5 blue stars, the brightest of which near the center is
labeled ``O'' in Figure 2
. The smaller emission knots, are basically
small HII spheres surrounding newly forming stars, including several for
which no stellar component is detected once the emission-line contribution is
subtracted from the wide-band images. A third component is the outlying
fainter HII arcs surrounding the entire main body, which may have been
ionized and blown outward by an earlier generation of stars surrounding the
two current star-forming regions of the main body. The best example of this
is the faint ``bubble'' surrounding two B-supergiants to the NNE of the
main-body NW cluster, which has a diameter of 
275 pc, corresponding to a
``blow-out'' age of 
3Myr. Even farther out is an ``arm'' of emission
south and west of the main body (Fig. 1
), extending as far as 650 pc
from the NW cluster. To the north of the cluster are two bubbles; the
brighter one surrounding two blue stars mentioned earlier and a second fainter
bubble extending to the north as far as 700 pc from the NW cluster. If these
features were the result of 50
wind-driven gas from the starbursting in
the NW cluster, their ages would be approximately 13--15Myr.
Galaxy C is an elongated irregular Im galaxy of similar angular extent
(
15
= 730 pc) to the main body of IZw18 with its major axis roughly
in line with the main body. The SE part contains a B-star association and
there is a prominent compact ``cluster'' near its center (V=22.0,
B-V=-0.1). Figure 3 
gives the F555W image of the system with
several features identified, as well as a CMD of some 150 stars detected. Two
fainter clusters (one blue and one red) are located at the NW tip of the
system. The brightest blue stars in C are seen at V
24.5 and show a
well defined upper main sequence in the CMD. The brightest red stars are seen
at V
26.5 and are located in the NW half of the system. Two
emission-line nebulae are seen: the brightest is a small knot associated with
the central blue cluster, appearing essentially unresolved in both H
and [OIII]; the other is a larger 2
(= 100 pc) diameter ring just
north of the central cluster, which is barely detectable in [OIII].
Near the center of the ring is a prominent blue star with V=26.5 and
B-V=-0.3, suggesting that the ring is most likely a small photoionized
HII region of low ionization.
Our CMD for C shows a well defined upper main sequence indicating that the age
of the youngest blue stars is approximately 40Myr (very dependent on our
adopted 10 Mpc distance). However, the numerous red stars seen at V
26.5
and fainter correspond to 100--300Myr isochrones. Therefore, the CMD supports
the idea that the Im system has an older stellar population, at least several
hundred Myr old, but also has experienced a modest starburst recently in the
SE half.
The FOS spectra provide direct determination of the abundance ratios of
C
/O
and Si
/O
from the measured line ratios of
1909/5007 and 
(1883+1892)/5007.
The line strengths were measured and the H
/H
ratio was used
to derive the line-of-sight extinction, estimated to be C(H
)=0.10
0.09. The line flux ratios were then corrected for extinction and
abundance ratios were calculated using T
=19,600 
900 K, as derived
from [OIII] for the NW HII region by SK93. We obtained
N(C
)/N(O
)=0.18
0.07 and
N(Si
)/N(O
)=0.030
0.018.
Using a model for the NW HII region previously developed in
the study of C/O in extragalactic H II regions by Garnett et al. (1995a;
G95a), we derived ionization correction factors to derive the final C/O and
Si/O ratios. The results were:
Taking the N/O ratio for the NW H II region derived by SK93, we further find:
Figure: Plots of C/O & C/N for the NW HII region of IZw18, derived
from our FOS spectra, compared with other metal-poor galaxies, the Orion
Nebula, B stars, and the sun (c.f. Garnett et al. 1995a,b).
Figure 4 shows plots of [C/O] and [C/N] versus [O/H] for IZw18 compared with those of several other metal-poor extragalactic HII regions from the study of G95a, and includes Galactic objects such as the Sun, B stars, and the Orion Nebula. C/O, and particularly C/N, is higher in IZw18 than was found for the three lowest metallicity systems in that study. By contrast, N/O (SK93) and Si/O (Garnett et al. 1995b) are similar to the ratios found in several other very metal-poor irregular galaxies studied with FOS UV spectra.
The many components noted above combine to complicate the simple picture that IZw18 is simply a short period starburst from a primordial HI cloud. While the main body is dominated by a starbursting cluster in the NW, it is clearly not a system formed in a single short starburst that began only a few Myr ago. Numerous red supergiants, both in the NW cluster and surrounding areas of the main body suggest a more extended period of star formation, possibly beginning as long as 30--50Myr ago. In addition, the numerous B- and A-supergiants separated From the two main body star forming regions suggest a modest rate of star formation occurred before the current bursts in the NW and SE lobes. These outlying blue stars may also be responsible for ionizing the extended HII arcs seen a distances as great as 1 kpc from the main body.
Such an extended period of star
formation would solve some of problems in accounting for the rather high N/O
and C/O ratios observed in IZw18. KMM95 noted that the ``nitrogen'' problem
requires two short bursts of star formation separated by 
1Gyr, or one
extended starburst of 
50Myr duration. Our data do not show evidence
supporting (such as a red halo) or refuting an underlying old (
1Gyr)
stellar population for the main body of IZw18. However, the data are
consistent with an extended current starburst scenario for IZw18, in which
some chemical enrichment would have to come from winds and SNe ejecta from
stars forming in the main body beginning several tens of Myr ago.
We thank the STScI ``A-teams'' assigned to GO-5434: Jeff Hayes and Jen Christensen (FOS) and Andy Fruchter, Inge Heyer, and Rick White for assistance during the early processing of the data. This research has been supported in part by AURA/STScI grants related to G0-5434, a NASA LTSA (EDS), and a Hubble Fellowship (DRG).
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R. J. Dufour, D. R. Garnett, E. D. Skillman, and G. A. ShieldsDufour, Garnett, Skillman, and ShieldsHST Imagery and Spectroscopy of IZw18