The HII Regions of Sextans A

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1 Publications of the Astronomical Society of the Pacific 6: , 1994 July The HII Regions of Sextans A Paul Hodge 1 Astronomy Department, University of Washington, Seattle, Washington Electronic mail: hodge@astro.washington.edu Robert C. Kennicutt 1 Steward Observatory, University of Arizona, Tucson, Arizona Nicolas Strobel Astronomy Department, University of Washington, Seattle, Washington Electronic mail: strobel@astro.washington.edu Received 1993 November 15; accepted 1994 April 26 ABSTRACT. Calibrated Ha and continuum CCD frames are used to measure the emission-line luminosities, sizes, and morphological characteristics of 25 Hu regions in the dwarf irregular galaxy Sextans A, a probable member of the Local Group. The luminosity function fits a power law with slope 1.70 and the size distribution is roughly exponential with a scale length of 62 pc. We identify 56 candidate exciting stars, from which we find that the estimated ultraviolet flux in the HII regions is approximately proportional to the Ha flux. Comparison with the HI distribution shows good positional agreement, except with an approximate 300 pc offset of Ha peak from HI peak. 1. INTRODUCTION Sextans A (=DD075) is one of the most thoroughly studied dwarf galaxies in or near the Local Group. Its distance of 1.3 Mpc (Sandage and Carlson 1982, 1985; Visvanathan 1989; Madore and Freedman 1991), based on Cepheid variables, makes it a probable member of the Group, but if so, it lies in the outer regions of the Group. In addition to the work on Cepheids (of which it has five confirmed examples) cited above, there have been three photometric studies of its resolved stars (Hoessel, Schommer, and Danielson 1983; Aparicio et al. 1987; Walker 1987), a discovery of a planetary nebula (Jacoby and Lesser 1981), a search for CO emission (Rowan-Robinson, Phillips, and White 1980), a study of the dust emission from IRAS imaging (Hunter et al. 1989), and a high-resolution VLA study of its HI distribution and velocities (Skillman et al. 1988). The HII regions of Sex A have also been studied previously. An early map of the brightest three HII regions was described by Hodge (19) and subsequent studies were published by Hunter and Gallagher (1990, 1992) and Hunter, Hawley, and Gallagher (1993), who detected faint emission in the form of what they termed interstellar froth and supergiant filaments. 2 Spectra of the H II regions have been used to determine abundances by Skillman, Kennicutt, and Hodge (1989). The present paper is based on observational material obtained for the purposes of identifying HII regions to be used in the Skillman et al. (1989) spectroscopic study. 3 In this paper we will adopt the distance derived by Madore and Freedman (1991), who give a distance of 1.31 Mpc. Their adopted foreground extinction is A B = Visiting Astronomer, Kitt Peak National Observatory, operated by AURA, Inc., under contract to the National Science Foundation. 2 Their filament numbers 2 and 3 are our diffuse H n regions numbers 12 and 8, respectively. Their number 1 was not detected on our frames. 3 In that paper, spectra were described for 4 H n regions in Sextans A. Numbers 1-4 there correspond to H n regions, 20, 19, and, respectively, in this paper. 2. OBSERVATIONS A set of CCD frames was obtained at the Kitt Peak National Observatory with the 0.9-m telescope on 1987 March 20. The night was photometric and calibration was achieved by comparison with the spectrophotometric standard Feige 34. The Ha filter had a HPBW of 38 Â and the comparison off-ha frame was exposed through a 400 Â wide filter centered at 6000 Â. Further details of the photometric procedure can be found in Strobel et al. (1991). The continuum-subtracted image was examined with various contrast and intensity levels to identify the HII regions. Individual HII regions were defined in terms most likely to be physically meaningful, such that significant peaks, with at least a twofold contrast between peaks and any valleys of emission separating peaks, were interpreted as having separate exciting stars. (Less well-separated peaks could be merely the result of gas density fluctuations or variable extinction.) Most of the HII regions are well-enough separated that this did not turn out to be a crucial matter. A total of 25 separate HII regions were identified (Fig. 1). Tests on the detection efficiency, as well as consideration of the possibility of missed overlapped Hu regions in the two more crowded areas of the galaxy, lead us to estimate that our detection is complete to a flux level of about 14 ergs cm -2 s _1. The faintest HII region detected in Sextans A is three times fainter than this completeness limit. 3. LUMINOSITIES AND THE LUMINOSITY FUNCTION The luminosities of the HII regions (Table 1) were measured on a calibrated frame from which the continuum emission had been removed photometrically. The boundaries were set at a surface brightness of ~2X~ erg cm -2 s -1 arcsec 2 (Fig. 1). The flux values include [N n] emission, for which we could not correct in detail without Astronomical Society of the Pacific

2 766 HODGE, KENNICUTT, AND STROBEL PIXELS Fig. 1 Composite of two deep Hor images (continuum-subtracted) of Sextans A. Each square area is 5X5 arcmin. North is up and east is to the left. H n regions are identified. individual spectra. The [N il] contribution is probably small, on the order of 5%-%, as the heavy element abundances in Sextans A are small (Skillman et al. 1989). Internal photometric errors are on the order of 3%, but the choice of borders is such that flux measurements can be as much as % different for different choices. Therefore, our estimate is that the total uncertainties for the H n regions fluxes is 15%. Using the adopted distance and reddening, we have calculated total Ha luminosities for each Hu region, which range from 7X 35 ergs s _1 for No. 11 to 5X 37 ergs s -1 for No.. The luminosity function is plotted in Fig. 2, where it is compared with that for NGC 6822, a similar irregular galaxy (Hodge, Lee, and Kennicutt 1989). For HII regions brighter than ~ 365 ergss -1 (below which our sample is surely incomplete), the data can be fit by a power law of slope 1.70, in good agreement with slopes found for other irregular galaxies (Kennicutt, Edgar, and Hodge 1989). 4. SIZES AND THE SIZE DISTRIBUTION It has been shown that the size distribution of H II regions (defined in terms of a particular surface brightness) generally fits an exponential relationship (van den Bergh 1981). Figure 3 shows the size distribution for Sextans A HII regions. Incompleteness affects the distribution for sizes less than about

3 H il REGIONS OF SEXTANS A 767 Table 1 H il Region Characteristics of Sextans A Position (1950) Diameter S (P c Flux (xlo -15 ) (erg cm -2 sec. -1 ) Morphology 1 h 08 m 22 B '41" * * Identified by Jacoby and Lesser (1981) as a planetary nebula. 50 pc. A least-squares fit to the data (Strobel 1990) results in a scale length of 62±3 pc. This value is similar to those for other galaxies of the same type, distance, and luminosity (Strobel et al. 1991). 5. H II REGION MORPHOLOGIES Although the morphological characteristics of HII regions can be complex, we have found that a simple classification based on inspection of the frames provides a convenient way to compare HII region morphological populations in different kinds of galaxies (e.g., see Hodge and Lee 1990). An astrophysical discussion of NGC 6822 s Hu region morphologies measured quantitatively, based on photometric maps of each Hll region, is given elsewhere (Collier and Hodge 1992, 1994), where these morphological classes are compared to quantitative physical properties of the objects. Table 1 gives morphological classifications for the Hll regions, as defined in Hodge and Lee (1990), where the scheme is as follows: Composite: c Bright compact: b Faint compact: / Loop or ring-shaped: / Diffuse: d 6. EXCITING STARS Because of the availability of several multicolor photometric studies of Sextans A it is possible to identify possible exciting stars for the HII regions. Unlike giant galaxies, Sextans A has a low dust content and star-formation regions are expected to be relatively transparent (Hunter et al. 1989). Thus the hot stars exciting the gas should be visible optically. We have searched the lists of Sandage and Carlson (1982), Hoessel et al. (1983), Walker (1987), and Aparicio et al. (1987) to identify candidate exciting stars, basing our selection on central position within an H n region and color. At the adopted distance of Sextans A, an unreddened star of spectral type BO would have a magnitude of U=19.2 to V=.9, depending on its luminosity class, and a color of U V = 1.4. We have chosen stars according to the criteria that U-VC-1.0 or that -V<-0.2 in cases where no U magnitude is available. Table 2 lists the resulting candidates, identified by Aparicio et al. (1987) numbers and using their photometry, which is the most complete of those published. The absolute magnitudes (neglecting any internal reddening) range from M v = 1.8 to M v = 6.2 and values of U V range from 1.01 to All but one of the Hll regions (No. 13) have candidates and 16 of the 25 HII regions have multiple candidates. It is likely that these, which include most of the largest clouds, are composite regions, the UV flux coming from two or more hot stars within them. Unfortunately, spectral types are not available for stars in Sextans A and the available photometry is not accurate enough to derive reliable temperatures (for example, star 1370, the candidate exciting star for HII region No. 1, has a measured B V of 0.77, corresponding to a star even bluer than one with an infinite surface temperature). As a first step for looking at the physical conditions in the HII regions, we Diam. (pc) Fig. 2 The luminosity function of H n regions in Sextans A compared to that for NGC 6822 (Hodge et al. 1989). Fig. 3 The size distribution for H n regions in Sextans A.

4 768 HODGE, KENNICUTT, AND STROBEL H II Region Table 2 Candidate Exciting Stars Star No. U-V M a Fig. A The relationship between the total U magnitudes of candidate exciting stars and the Hi* fluxes of the H n regions. The line (not a fit) indicates a slope appropriate to direct proportionality. 7. COMPARISON WITH HI Detailed HI mapping exists for Sex A (Skillman et al. 1988). A comparison of their HI contours with our HII map (Fig. 5) shows that there is a close overall correlation between the positions of the neutral and the excited gas, with two strong maxima at opposite ends of the optical image, which is also the location of the two recent star-forming areas (Aparicio et al. 1987; Hoessel et al. 1983). However, the positional correspondence is not perfect. The brightest HII regions near the HI maxima are displaced from these maxima by a distance of approximately 300 pc in each case. This kind of displacement is known to occur in other dwarf irregular galaxies, such as IC (Hodge and Lee 1990) for which the displacement is also approximately 300 pc. As we have suggested in that paper, this fact is consistent with a pattern of sequential star formation (Elmegreen and Lada 1977). a Assuming no internal reddening. have used the U magnitudes of the stars as a rough guide to stellar UV flux and have looked for any correlation with Ha flux, as is expected and as was found for GR8 (Hodge, Lee, and Kennicutt 1989), another nearly dust-free galaxy. For multiple candidates, we have added the U fluxes to produce a composite U magnitude. Figure 4 shows the relationship between the U magnitudes and the Ha flux. There is considerable scatter, but an approximate proportionality exists. The scatter is probably the result of the relative insensitivity of even the U magnitude to the true UV flux, and possibly also reflects any internal reddening, which we have neglected. Of course, true UV spectra of the stars would be much more informative for this comparison RA Fig. 5 A comparison of the H i contours (Skillman et al. 1988) with the positions of the H n regions (shaded areas).

5 H il REGIONS OF SEXTANS A CONCLUSIONS The assemblage of H il regions in the Local Group dwarf galaxy Sextans A has a normal luminosity function and size distribution, indicating that the physical process of star formation in this galaxy is normal, as measured by these parameters. Candidate exciting stars have been identified from published UBV photometry. The total UV flux involved in each H II region is estimated from the stellar photometry and it is roughly proportional to the Ha flux. The H n regions cluster towards regions of high H I density, but are displaced from the peaks in the H I contours. We are indebted to the NSF for partial support through grants AST-958 (to P.H.) and AST (to R.C.K., Jr.). REFERENCES Aparicio, A., Garcia-Pelayo, J. M., Moles, M., and Melnick, J. 1987, AApS, 71, 297 Collier, J., and Hodge, P. 1992, BAAS, 24, 1201 Collier, J., and Hodge, P. 1994, ApJ, in press Elmegreen, B. G., and Lada, C. J. 1977, ApJ, 4, 725 Hodge, P. W. 19, ApJS, 27, 113 Hodge, P W., Lee, M. G., and Kennicutt, R. C. 1989, PASP, 1, 32 Hodge, P. W., and Lee, M. G. 1990, PASP, 2, 26 Hoessel, J. G., Schommer, R. A., and Danielson, G. E. 1983, ApJ, 2, 577 Hunter, D. A., and Gallagher, J. S. 1990, ApJ, 362, 480 Hunter, D. A., and Gallagher, J. S. 1992, ApJ, 391, L9 Hunter, D. A., Gallagher, J. S., Rice, W. L., and Gillett, F. C. 1989, ApJ, 336, 152 Hunter, D. A., Hawley, W. N., and Gallagher, J. S. 1993, AJ, 6, 97 Jacoby, G. H., and Lesser, M. P 1981, AJ, 86, 5 Kennicutt, R. C., Edgar, B. K., and Hodge, P. W. 1989, ApJ, 337, 761 Madore, B. E, and Freedman, W. L. 1991, PASP, 3, 933 Rowan-Robinson, M., Phillips, T. G., and White, G. 1980, A&A, 82, 381 Sandage, A. R., and Carlson, G. 1982, ApJ, 258, 439 Sandage, A. R., and Carlson, G. 1985, AJ, 90, 19 Skillman, E. D., Kennicutt, R. C., and Hodge, P W. 1989, ApJ, 347, 875 Skillman, E. D., Terlevich, R., Teuben, P. T, and van Woerden, H. 1988, A&A, 198, 33 Strobel, N. V. 1990, BAAS, 23, 909 Strobel, N. V., Hodge, P. W., and Kennicutt, R. C. 1991, ApJ, 383, 148 van den Bergh, S. 1981, AJ, 86, 1464 Visvanathan, N. 1989, ApJ, 346, 629 Walker, A. R. 1987, MNRAS, 224, 935