View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CERN Document Server To app ear in the Astrophysical Journal HIGH CARBON IN I ZWICKY 18: NEW RESULTS FROM HUBBLE SPACE 1 TELESCOPE SPECTROSCOPY 2; 2 3 4 D. R. Garnett E. D. Skillman , R. J. Dufour , and G. A. Shields ABSTRACT We present new measurments of the gas-phase C/O abundance ratio in b oth the NW and SE comp onents of the extremely metal-p o or dwarf irregular galaxy I Zw 18, based on ultraviolet sp ectroscopy of the two H II regions using the Faint Ob ject Sp ectrograph on the Hubble Space Telescope. We determine values of log C/O = 0.630.10 for the NW comp onent and log C/O = 0.560.09 for the SE comp onent. In comparison, log C/O = 0.37 in the sun, while log C/O = 0.850.07 in the three most metal-p o or irregular galaxies measured by Garnett et al. (1995a). Our measurements show that C/O in I Zw 18 is signi cantly higher than in other comparably metal-p o or irregular galaxies, and ab ove predictions for the exp ected C/O from massive star nucleosynthesis. These results suggest that carb on in I Zw 18 has b een enhanced by an earlier p opulation of lower-mass carb on pro ducing stars; this idea is supp orted by stellar photometry of I Zw 18 and its companion, which demonstrate that the current bursts of massive stars were not the rst. Despite its very low metallicity,itislikely that I Zw 18 is not a \primeval" galaxy. Subject headings: Galaxies: abundances { galaxies: evolution { galaxies: irregular { galaxies: individual: I Zw 18 { galaxies: ISM { H I I regions 1. Intro duction The dwarf emission-line galaxy I Zw 18 plays an imp ortant role in studies of the prop erties and evolution of dwarf irregular galaxies b ecause of its extreme prop erties. Searle & Sargent (1972) 1 Based on observations with the NASA/ESA Hubble Space Telescop e obtained at Space Telescop e Science Institute, which is op erated by the Asso ciation of Universities for Research in Astronomy, under NASA contract NAS5-26555. 2 Astronomy Department, University of Minnesota, 116 Church St., S. E., Minneap olis MN 55455; e-mail: [email protected], [email protected] 3 Department of Space Physics and Astronomy, Rice University, Houston, TX 77251-1892; e-mail: [email protected] 4 Astronomy Department, UniversityofTexas, Austin, TX 78712; e-mail: [email protected] {2 { determined early on that I Zw 18 (and its cousin II Zw 40) had a very low oxygen abundance; 5 since then the O/H value has b een re ned to the current b est estimate of (1.470.15)10 in the NW H I I region (Skillman & Kennicutt 1993; hereafter SK93), the lowest value measured in any emission-line galaxy. The colors of I Zw 18 are very blue, dominated byyoung massive stars, and no evidence has b een found to date for an older red stellar p opulation (see Hunter & Thronson 1995). The oxygen de ciency and blue colors led to the conclusion that I Zw 18 and similar galaxies are either very young, exp eriencing their rst episo des of star formation, or have exp erienced only sp oradic episo des of star formation over their history (Searle, Sargent, & Bagnuolo 1973). Meanwhile, surveys for other very metal-p o or emission-line dwarf galaxies (Kunth & Joub ert 1985, Terlevichet al. 1991) have failed to nd any more metal-p o or than I Zw 18. This failure led Kunth & Sargent (1986) to p ostulate that the H I I regions in I Zw 18 were \self-p olluted" by oxygen-rich ejecta from sup ernovae that had explo ded during the lifetime of the H I I region, and they prop osed that the surrounding neutral gas should b e essentially pristine material (but see Tenorio-Tagle 1996 for ob jections to this scheme). If the ionized gas in I Zw 18 is pristine material which has b een contaminated by Typ e II sup ernova ejecta, the heavy element abundance ratios should show values characteristic of massive star nucleosynthesis. In particular, the gas should be relatively de cient in elements pro duced mainly in low- and intermediate-mass stars, such as carb on and nitrogen, compared to oxygen, which is pro duced in massive stars alone. Yet Dufour, Garnett, & Shields (1988) derived surprisingly high (essentially solar) values for C/O and N/O in I Zw 18 from IUE UV and ground-based optical sp ectroscopy (since revised downward by Dufour & Hester 1990 and SK93), suggesting that IZw18 did exp erience contamination from an older generation of stars. On the other hand, Kunth et al. (1994) inferred from high-resolution GHRS sp ectra an oxygen abundance in the neutral gas that was much smaller than in the H I I region in I Zw 18 , and concluded that the ionized gas has b een enriched by the present starburst (but see Pettini & Lippman 1995 for a criticism of their analysis). Thus, there remains con icting evidence over the nature of I Zw 18. Because of the intimate connection b etween the pro duction of He, C, and N in stars, accurate measurements of C, N, and O abundances in metal-p o or galaxies are vital for determining the primordial He abundance; ultimately, one might exp ect that a mo del for the metallicityevolution of a galaxy combined with observed CNO abundances will provide the b est estimates for the stellar He contamination and thus an accurate value for Y (Balb es, Boyd, & Mathews 1993). In p this pap er, we rep ort on new measurements of the C/O abundance ratio in I Zw 18 obtained with the Hubble Space Telescope (HST), and show that C/O in I Zw 18 is signi cantly higher than observed in other very metal-p o or dwarf galaxies, suggesting that I Zw 18 has b een enriched in carb on from an earlier generation of intermediate-mass stars. {3 { 2. Observations and Analysis We observed b oth the NW and SE comp onents of I Zw 18 with the Faint Ob ject Sp ectrograph 00 on HST, using the red Digicon. The NW H I I region was observed with the 0.86 square (C-1) ap erture and gratings G190H (exp osure time 3600s) and G570H (300s) in Cycle 4, while the SE 00 region was observed in Cycle 5 with the 0.86 circular (B-3) ap erture and gratings G190H (5170s), G570H (540s), and G400H (600s). Figure 1 (Plate XXX) shows the lo cations of the ap ertures sup erp osed on H and F555W images of the galaxy taken with WFPC2. The sp ectra for the SE region were pro cessed using the standard FOS pip eline reduction routines. The NW region sp ectra were repro cessed using the at- elds and calibration observations taken through the C-1 ap ertures in July 1994; comparison of the pip eline and repro cessed sp ectra showed di erences in relative photometry of no more than a few p ercent. The FOS sp ectra of the SE knot are displayed in Figure 2. We measured uxes for the emission lines by direct integration of the emission line pro les. Interstellar reddening was estimated from H /H ratios measured from the G570H sp ectra; we measured values for C(H ) = log F (intrinsic) log F (observed) of 0.100.02 for the NW H H p osition and 0.030.04 for the SE p osition. (These compare with the values 0.100.05 and 0.200.05 measured by SK93 for the NW and SE regions, resp ectively, suggesting that there is variable reddening within I Zw 18.) The observed line uxes were corrected for reddening using these values for C(H ) with the average Galactic interstellar reddening curve of Seaton (1979), assuming all of the reddening is foreground. If one assumes the reddening is internal, and that an SMC-like reddening law applies, the C I I I]/H ratio in the NW region is reduced by approximately 10%; the uxes for the SE region would b e essentially unchanged. We list our reddening-corrected line uxes for b oth regions in Table 1. The 1 uncertainties in the line uxes in Table 1 were determined by adding in quadrature the error contributions from the following sources: the statistical noise in the lines plus lo cal continuum, determined from the raw counts; the uncertainty in the photometric calibration of the FOS, approximately 3% (Bohlin 1995); and the error due to the uncertainty in the reddening correction. Statistical noise dominates the uncertainty in the C I I I] line ux. To determine C and O abundances from the sp ectra, we adopted electron temp eratures and densities for the two regions determined by SK93, listed in Table 1. Garnett et al. (1995; G95a) were able to measure lines from b oth C I I I] and O III] 1666 in other H I I regions, and thus to +2 +2 deriveC /O abundance ratios with only small uncertainties from reddening and temp erature errors. Here we were not able to measure O I I I] 1666, however, so we must compare the C III] +2 +2 5 emission with the optical [O III] lines to determine the C /O ratio.