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
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, V740 , 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 V28, 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 (184.108.40.206; ``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 PA210. 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 V26.5 and B26.0. 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 V26. 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 V22, 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 V24 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 V24.5 and show a well defined upper main sequence in the CMD. The brightest red stars are seen at V26.5 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 V26.5 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)=0.180.07 and N(Si)/N(O)=0.0300.018. 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