THE ASTRONOMICAL JOURNAL, 116:2341È2362, 1998 November ( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.

THE RECENT STAR FORMATION IN SEXTANS A SCHUYLER D. VAN DYK1 Infrared Processing and Analysis Center, California Institute of Technology, Mail Code 100-22, Pasadena, CA 91125; vandyk=ipac.caltech.edu DANIEL PUCHE1 Tellabs Transport Group, Inc., 3403 Griffith Street, Ville St. Laurent, Montreal, Quebec H4T 1W5, Canada; Daniel.Puche=tellabs.com AND TONY WONG1 Department of Astronomy, University of California at Berkeley, 601 Cambell Hall, Berkeley, CA 94720-3411; twong=astro.berkeley.edu Received 5 February 1998; revised 1998 July 8 ABSTRACT We investigate the relationship between the spatial distributions of stellar populations and of neutral and ionized gas in the Local Group dwarf irregular galaxy Sextans A. This galaxy is currently experi- encing a burst of localized star formation, the trigger of which is unknown. We have resolved various populations of stars via deepUBV (RI) imaging over an area with [email protected]. We have compared our photometry with theoretical isochronesC appropriate for Sextans A, in order to determine the ages of these populations. We have mapped out the history of star formation, most accurately for times [100 Myr. We Ðnd that star formation in Sextans A is correlated both in time and space, especially for the most recent([12 Myr) times. The youngest stars in the galaxy are forming primarily along the inner edge of the large H I shell. Somewhat older populations,[50 Myr, are found inward of the youngest stars. Progressively older star formation, from D50È100 Myr, appears to have some spatially coherent structure and is more centrally concentrated. The oldest stars we can accurately sample appear to have approximately a uniform spatial distribution, which extends beyond a surface brightness of kB ^ 25.9 mag arcsec~2 (or, a radiusr ^ [email protected]). Although other processes are also possible, our data provide support for a mechanism of supernova-driven expansion of the neutral gas, resulting in cold-gas pileup and com- pression along the H I shell and sequential star formation in recent times. Key words: galaxies: dwarf È galaxies: evolution È galaxies: individual (Sextans A, DDO 75) È galaxies: photometry È galaxies: stellar content È Local Group

1. INTRODUCTION gas and the history of star formation. The gas can be traced and analyzed by mapping the neutral, molecular, and Dwarf galaxies, both irregular and spheroidal, are the ionized components, and the star formation history can best most numerous galaxies in the universe. Dwarf irregular be traced by the distributions of resolved stars of various galaxies play a key role in our understanding of galactic ages. evolution, especially with the discoveries of faint, blue gal- A program to study the interstellar medium (ISM) in axies in high-redshift imaging surveys such as the Hubble dwarf irregular galaxies was undertaken by D. Puche, Deep Field. Since dwarf irregular galaxies are simpler, less D. Westpfahl, and their collaborators, with the goal of evolved systems, it should be possible to observe short-lived determining whether the neutral component of the ISM is evolutionary properties in nearby systems, which would be distributed di†erently in low-mass systems than in grand- more easily destroyed in more massive, complex systems, design spiral galaxies.Puche et al. (1992), Westpfahl & such as grand-design spiral galaxies. Knowledge of the star Puche(1994), and Puche & Westpfahl (1994) found that, formation history of dwarf irregular galaxies is of particular based on their high-resolution H I data for a number of importance;Hunter (1997) recently presented a com- dwarf galaxies, the neutral gas component shows an prehensive discussion of the topic. unprecedented amount of structure in the form of shells or Dwarf irregular galaxies show a larger range in star for- bubbles (Heiles1979, 1984; Tenorio-Tagle & Bodenheimer mation rates than do spiral galaxies. Studies of the stellar 1988). Low-rotation velocities for dwarf galaxies means less populations in dwarf irregular galaxies (e.g.,Tosi et al. shear, and thus, longer lifetimes for these large-scale shells. 1991; Greggioet al. 1993; Marconi et al. 1995) show that Furthermore, the gas scale-height in dwarf galaxies must be most of these galaxies evolve with approximately constant much larger than in high-mass spiral galaxies, resulting in a star formation rates, either high or low, interrupted by lower gas density and a higher efficiency for making large variations of factors 2È3 in these rates over the past few holes(Heiles 1979). Also important, of course, is the absence gigayears(Hunter 1997). The question, then, is what of spiral density waves (seeHunter, Elmegreen, & Baker governs and regulates star formation in dwarf irregular gal- 1998). axies. A vital clue to answering this question can be found in Pucheet al. (1992), Puche & Westpfahl (1994), and the relationship between the structure and kinematics of the Westpfahl& Puche (1994) found that these structures are expanding and argued that they surround hot, low-density ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ bubbles formed as the result of the combined e†ects of 1 Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by the Association of photoionization, stellar winds, and multiple supernovae Universities for Research in Astronomy (AURA), Inc., under contract with from sequential formation of massive stars(Cash et al. the National Science Foundation. 1980). Supernovae would likely dominate all other sources 2341 2342 VAN DYK, PUCHE, & WONG Vol. 116 of energy input, although winds from massive stars also rotator than these other small dwarfs(v ^ 30 km s~1 substantially contribute (e.g.,Leitherer, Robert, & Drissen for Sextans A, compared with D15 km s~1rotmaxfor Ho I and D5 1992).The models of Chevalier (1974) and Koo & McKee km s~1 for M81dwA;Puche & Westpfahl 1994), consistent (1992a, 1992b) predict that about 100 supernovae exploding with the central large cavity appearing to have undergone in preexisting wind-blown bubbles would provide about some shearing and twisting. 1053 ergs to the ISM and can produce shells with radii of This galaxy has also been the object of previous intense hundreds of parsecs, expansion velocities of D10È20 km optical study, both from the ground and, recently, from s~1, and lifetimes of D108 yr, consistent with those space. Its resolved stellar populations have been observed in observed. a number of di†erent bands. Sandage & Carlson (1982, If supernovae are producing expanding bubbles, Puche & hereafterSC82; 1985, hereafter SC85; SC85 revises the pho- Westpfahl have argued, the current star formation in these tometry presented inSC82) published an early photometric galaxies should be concentrated near the high-density study consisting of photographic BV ;Hoessel, Schommer, regions of the expanding H I shell at the inner edge of the &Danielson (1983, hereafter HSD) obtained CCD Gunn- bubble. This is consistent with theories of sequential star Thuan gri photometry;Walker (1987, hereafter, W87) formation (e.g.,Gerola & Seiden 1978; Gerola, Seiden, & published CCD BV photometry of bright stars;Aparicio et Schulman1980), in which the compression associated with al. (1987,hereafter A87) presented deep CCD UBV photo- the shell triggers the star formation process. If the metry; andSMF recently analyzed deep CCD BV I photo- supernova-driven scenario holds, then we would expect star metry. Based on their deep UBV imaging data, A87 formation episodes to be not only correlated in age, but also presented a history of recent star formation in Sextans A, in spatial distribution, along the H I shell structures. but they considered the star formation over only quite Limited, low-resolution optical data tend to suggest that broad regions of the galaxy.SMF, although their BV I this scenario is possibly taking place: Ha appears concen- imaging was quite deep, did not address in depth the star trated on the edges of the nearly circular H I shells in Holm- formation in Sextans A, dealing primarily with their dis- berg II(Puche et al. 1992; Hodge, Strobel, & Kennicutt tance estimate. 1994b),Holmberg I (Westpfahl & Puche 1994), and IC 2574 Recently, Dohm-Palmer et al.(1997a, 1997b) presented (Martimbeau,Carignan, & Roy 1994). Another, more local, reports on photometric results based on deep Hubble Space example where this also may be occurring is constellation Telescope (HST ) imaging.Dohm-Palmer et al. (1997b) III and the supershell LMC 4 in the LMC (Dopita, provide a detailed description of the star formation history Mathewson, & Ford1985; Grebel & Brandner 1998; in Sextans A over the last 1 Gyr and illustrated the corre- although seeReid, Mould, & Thompson 1987; Braun et al. lation of the history both in time and in space. Their results 1997; Olsenet al. 1997). In addition, Puche et al. (1992) are unhampered by the problems of crowding and blending claim that broadband imaging shows the possible presence that beset ground-based studies, and the number of stars of stars or clusters at the centers of some of the largest shells that they can statistically analyze is truly formidable. in Holmberg II, which could be the populations associated Based on their results,Dohm-Palmer et al. (1997b) con- with the supernovae progenitors, which remain long after clude that from 100È600 Myr ago, the star formation rate the explosions. (D2000M_ Myr~1 kpc~2) was about 6 times larger than One of the galaxiesPuche & Westpfahl (1994) sampled the rate (D310M_ Myr~1 kpc~2) averaged over the life- was Sextans A (DDO 75, UGC 205, A1008[04), a low time of the galaxy and that since about 80È100 Myr ago, the surface brightness dwarf irregular galaxy with angular rate has become remarkably higher (by a factor D20, at diameter of onlyD D [email protected] (Ables 1971; Tully 1988) and D5000M_ Myr~1 kpc~2) and more densely concentrated with an oddly square,25 or, more appropriately, diamond than in the past; this suggests that some unusual event has shape. A large number of resolved stars can be seen on a occurred in the past 100 Myr to trigger the recent star number of published ground-based (and space-based) formation. (Their movie of star formation in Dohm-Palmer images of the galaxy. The galaxy is seen nearly face-on et al.1998 seems to indicate that the trigger might have (inclination i D 36¡,Skillman et al. 1988; to 45¡, Graham & begun as early as D350 Myr ago.) Westpfahl1998). A recent estimate of the distance to Dohm-Palmer et al. speculate on possible scenarios for Sextans A bySakai, Madore, & Freedman (1996, hereafter this mysterious event, including a tidal interaction with SMF), based on the tip of the red giant branch (TRGB) neighboring Sextans B; however, they point out that these brightness, is 1.42 Mpc (m [ M \ 25.74), placing it near the two galaxies may be separated by nearly 300 kpc, a distance edge of the Local Group. At this distance, the angular diam- over which the dynamical e†ects would be minimal. The eter corresponds to a linear diameter of only D1.8 kpc for trigger could also be the recent onset of a bar potential, the galaxy. The galaxy hasBT \ 11.93 and (B[V )T \ 0.30 whichSkillman et al. (1988) suggest possibly exists in (somewhat bluer than the mean for Magellanic irregular Sextans A, based on their data; although Puche disagrees, galaxies;Ables 1971); at the TRGB distance modulus, D. J. Westpfahl (1996, private communication) states that I I MBT \[13.81. The morphology of Sextans A in H had the gas kinematics based on their recent H data could also been initially studied bySkillman et al. (1988). Skillman et indicate the presence of a bar.Graham & Westpfahl (1998) al. estimate a gas mass of M D 5.8 ] 107 M_ and a total suggest a possible merger or accretion event. Finally, the mass of M Z 1.6 ] 108 M_. supernova-induced star formation mechanism could be the From higher resolution and greater sensitivity maps, trigger for recent star formation. Combinations of one or Puche& Westpfahl (1994) found that Sextans A looks more of these scenarios is, of course, also possible. similar in H I morphology to other small dwarf irregular In this paper, we present deepUBV (RI) imaging of galaxies, such as Holmberg I and M81dwA(Westpfahl & Sextans A, in which a large number of starsC are resolved. Puche1994), in that one H I hole appears to dominate the Through our ground-based Ðve-band imaging we have overall structure, although Sextans A is a slightly faster sampled nearly the entire main body of the optical galaxy, No. 5, 1998 STAR FORMATION IN SEXTANS A 2343 and therefore we can more completely describe the tempo- are the only emission lines dominating the R bandpass (see ral and spatial history of star formation over a large area, Waller 1990). The scale factor for the subtraction was particularly for the last 100 Myr, and (with far less derived from the instrumental photometry of the Ðeld stars certainty) back to about 1 Gyr. Furthermore, we examine in common between the two sets of images. The subtracted the correlation of the recent star formation history with image was not Ñux calibrated. both the neutral H I and the ionized gas components. 2.2. Photometry Methods 2. DATA The DAOPHOT and ALLSTAR (Stetson 1987; Stetson, 2.1. Observations and Reductions Davis, & Crabtree1990) routines in IRAF were used to The observations were conducted at the Kitt Peak derive the magnitudes of the resolved stars on all the National Observatory (KPNO) 2.1 m telescope in UBV (RI) images made of Sextans A (including those made UBV (RI) on 1994 Feb 11 UT, as part of a program to with the KPNOC 2.1 m, the Lick 1 m, and the KPNO 0.9 m image severalC dwarf irregular galaxies showing signiÐcant telescopes). These routines perform multiple Ðttings of a structure in H I. The KPNO ““ T1KA ÏÏ 10242 CCD was used semiempirical point-spread function (PSF) to the individual in direct imaging mode at f/7.5; the pixel scale was 0A.31, stellar proÐles. The PSF was established by assuming a resulting in a Ðeld size [email protected]. An area of Sextans A with Mo†at analytical function, plus a table of residuals, using a [email protected] could therefore be imaged on the chip. All number (D30) of well shaped stars on each image. Figure 1 images exhibited large numbers of resolved stars in the shows the residuals of the Ðtting, p, given by ALLSTAR for galaxy. Total exposures in each band were divided into all stars in each band on the images made at the KPNO 2.1 three separate exposures, dithering the telescope slightly m telescope (after elimination of probable foreground stars, between exposures to reduce pixel-to-pixel and position- obviously blended stars, and erroneous detections; see ° 3 dependent e†ects on stellar proÐles and magnitudes. Total below). These p values are representative of the internal exposure times were 1800 s in each of the Ðve Ðlters. Unfor- errors in the photometry presented in this paper. tunately, although the seeing was good(D1A.1), the night Our Lick 1 m telescope observations and ShupingÏs was not photometric (nor was the entire run). KPNO 0.9 m telescope observations not only consisted of Observations were subsequently made on 1995 April 24 images of Sextans A, but also images of a number of UT using the Lick Observatory Nickel 1.0 m Telescope in standard-star Ðelds fromLandolt (1992) spanning a wide BV (RI) under photometric conditions for the purpose of range of colors. The standard-star instrumental magnitudes calibration.C The Nickel Telescope imaging system, consist- were determined through large (radius D7A) synthetic aper- ing of an Orbit 20482 CCD (binned 2 ] 2), with pixel scale tures, which signiÐcantly included the wings of the stellar 0A.37and Ðeld size [email protected], however, has essentially no response proÐles, and sky annuli with radii larger than the apertures. in U. Fortunately, a service observing campaign (1996 April The photometric transformations to the absolute system 1È5 UT) was undertaken during bright time at the KPNO were computed using these instrumental magnitudes. 0.9 m telescope by R. Young Shuping to provide calibration The ALLSTAR PSF-Ðtting magnitudes for stars in observations for previous KPNO imaging programs con- Sextans A imaged during the two sets of calibration obser- ducted under nonphotometric conditions. Shuping vations were corrected to synthetic aperture magnitudes, obtainedUBV (RI) images for us. Shorter (600È900 s) using the aperture growth-curve method (Stetson 1990). exposures of SextansC A were made during both sets of cali- However, this method was only reliable for the D15 bright- bration observations. est stars (or fewer) on these shorter exposure images of the In addition, we obtained Ha ] [N II] and R-band images galaxy in each band. The ALLSTAR magnitudes from the of Sextans A at the old KPNO ““ No. 1 ÏÏ 0.9 m telescope in KPNO 2.1 m images were, of course, not aperture- 1989 May 31 UT. These observations employed the old corrected, since these observations were not photometric, ““ RCA3 ÏÏ 508 ] 312 chip, with pixel scale0A.86 and Ðeld size but had to be ““ bootstrapped ÏÏ to the absolute system using [email protected] ] [email protected]. Three individual images through a narrowband the observations of Sextans A during the calibration runs. Ðlter with central wavelength 6563ÓÓ (bandpass 75) were made. The R-band image taken during that observing run 2.3. Transformation to the Standard Johnson-Cousins had an exposure time of 300 s. System The usual bias and Ñat-Ðeld images were obtained during For the Lick calibration run, an adequate number (30) of all observing runs. All image processing was performed standard stars were observed over a large range in air mass. usingIRAF.2 As mentioned above, the separate 2.1 m expo- The aperture magnitudes of the standard stars were then sures of Sextans A were combined into a single total image used to determine the atmospheric extinction in each band using cosmic-ray rejection for each Ðlter. In addition, the and the transformation to Johnson-Cousins magnitudes. three Ha ] [N II] images were combined into a single The transformation equations, derived using the 1500 s image. PHOTCAL tasks in IRAF for the Lick Nickel 1 m run are A continuum-subtracted Ha ] [N II] image was produc- ed using the combined narrowband ““ on-line ÏÏ observation b \ B ] (5.234 ^ 0.051) ] (0.315 ^ 0.032)X and the broadband R observation, in which R was [ (0.057 ^ 0.070)(B[V ) [ (0.008 ^ 0.040)(B[V )X , employed as the ““ o†-line ÏÏ Ðlter, assuming that Ha ] [N II] (1) ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ v \ V ] (4.926 ^ 0.019) ] (0.196 ^ 0.012)X 2 IRAF is distributed by the National Optical Astronomy Observa- tories, which are operated by the Association of Universities for Research ] (0.044 ^ 0.025)(B[V ) [ (0.005 ^ 0.016)(B[V )X , in Astronomy, Inc., under cooperative agreement with the National Science Foundation. (2) 2344 VAN DYK, PUCHE, & WONG Vol. 116

FIG. 1.ÈDistribution as a function of magnitude of the standard errors derived by ALLSTAR in DAOPHOT for theUBV (RI) bands C r \ R ] (4.858 ^ 0.029) ] (0.134 ^ 0.019)X ables. We therefore chose to Ðx the air mass corrections to C the nominal values appropriate for KPNO, as given in the ] (0.035 ^ 0.070)(V [R ) ] (0.001 ^ 0.042)(V [R )X , C C Direct Imaging Manual for Kitt Peak(Massey et al. 1997). (3) The transformation equations are then i \ I ] (4.908 ^ 0.053) ] (0.112 ^ 0.029)X u \ U ] (6.972 ^ 0.030) ] 0.500X [ (0.076 ^ 0.248) C ] (0.069 ^ 0.127)(R[I) [ (0.049 ^ 0.070)(R[I) X , ] (U[B) [ (0.055 ^ 0.167)(U[B)X , (5) C C (4) b \ B ] (4.902 ^ 0.007) ] 0.250X [ (0.019 ^ 0.039) where the lowercase letters represent the aperture ] (B[V ) [ (0.064 ^ 0.028)(B[V )X , (6) (instrumental) magnitudes, the uppercase letters represent the true Johnson-Cousins magnitudes, and X is the air mass v \ V ] (4.665 ^ 0.004) ] 0.150X ] (0.134 ^ 0.020) of the observation. The formal errors in the coefficients are (B V ) (0.095 0.013)(B V )X (7) also given. ] [ [ ^ [ Although standard stars over a large range of color were r \ R ] (4.579 ^ 0.007) ] 0.100X ] (0.115 ^ 0.064) observed during ShupingÏs photometric night on the C ] (V [R ) [ (0.124 ^ 0.047)(V [R )X , (8) KPNO 0.9 m, a smaller number (17) of stars were observed C C without adequately sampling over air mass. As a result, we i \ I ] (5.353 ^ 0.007) ] 0.070X ] (0.005 ^ 0.058) had difficulty achieving a reasonable solution for the photo- C metric coefficients, while allowing them all to be free vari- ] (R[I) [ (0.082 ^ 0.044)(R[I) X , (9) C C No. 5, 1998 STAR FORMATION IN SEXTANS A 2345

TABLE 1 where the symbols are as above. The formal are also given COMPARISON WITH PHOTOMETRY FROM PREVIOUS STUDIES for those coefficients treated as free parameters. The aperture-corrected ALLSTAR magnitudes for the Authors Source Di†erence stars resolved on the images of Sextans A made during the V : Lick 1 m and KPNO 0.9 m runs were then transformed to HSD...... CCD gri 0.350 ^ 1.024 the Johnson-Cousins system using the PHOTCAL routines SC85 ...... Photographic BV 0.533 ^ 0.711 within IRAF. We compared the Ðnal photometry results A87...... CCD UBV [0.025 ^ 0.494 from the Lick 1 m and KPNO 0.9 m observations for stars W87 ...... CCD BV 0.013 ^ 0.098 in Sextans A in common between the two sets (we were SMF...... CCD BV I 0.035 ^ 0.031 B[V : limited to a comparison for only the brightest stars, i.e., HSD...... CCD gri [0.140 ^ 0.516 those withV [ 17) and found that the BV RI magnitudes SC85 ...... Photographic BV 0.185 ^ 0.251 and colors agreed between the two sets to the following A87...... CCD UBV [0.125 ^ 0.443 degree: [0.003 ^ 0.024 (V ), 0.013 ^ 0.033 (B[V ), W87 ...... CCD BV 0.042 ^ 0.082 0.019 0.014 (V R), and 0.000 0.025 (R I) (the U[B: [ ^ [ ^ [ A87...... CCD UBV 0.076 ^ 0.438 sense of the sign is ““ KPNO 0.9 m [ Lick 1 m ÏÏ). One can V [I: see that the agreement is quite good. We therefore used the SMF...... CCD BV I 0.046 ^ 0.054 photometry from the Lick run to transform the ALLSTAR R[I: magnitudes obtained from the deep KPNO 2.1 m imaging HSD...... CCD gri 0.093 0.198 ^ in BV (RI) . C

0.4

0.2

0

-0.2

-0.4 17 18 19 20 21 0.6 0.5 0.4 0.2 0 0 -0.2 -0.5 -0.4 -0.6 -0.5 0 0.5 1 -0.5 0 0.5 1 1.5 2

1 0.2

0 0

-1 -0.2

-1012 -0.5 0 0.5 1 1.5 2

FIG. 2.ÈComparison of our photometry with the photometry from other investigators in the various bands. See text for author abbreviations. V is represented by squares(SMF), triangles (A87), pentagons (W87), stars (SC85), and crosses (HSD).B[V is represented by squares (A87), triangles (W87), pentagons(SC85), and stars (HSD).U[B is represented by squares (A87).R[I is represented by squares (HSD).V [I is represented by squares (SMF). 2346 VAN DYK, PUCHE, & WONG Vol. 116

We had no choice for U; the agreement between the TABLE 2 BV (RI) calibration obtained from the Lick1mand COMPARISON OF ACTUAL AND PREDICTED FOREGROUND STAR COUNTS KPNOC 0.9 m imaging provided us with conÐdence that the U calibration done on the KPNO 0.9 m was reasonable. VB[V\0.8 0.8 \ B[V \ 1.3 1.3 \ B[V We therefore transformed the U photometry from the Actual counts: KPNO 2.1 m images using the photometry from the KPNO 18...... 8 1 4 0.9 m image. 20...... 196 20 40 22...... 725 472 83 24...... 55 209 54 Predicted counts:a 2.4. Comparison with Previous Photometry 18...... 1.1 1.4 1.9 20...... 2.4 1.1 5.5 In any study of this kind, it is always appropriate to 22...... 2.4 2.4 10.8 compare the new photometry with older published photo- 24...... 0.9 2.8 17.7 metry when available. Fortunately, as mentioned above, a Based on interpolating the model counts fromRatnatunga & Bahcall Sextans A has been the subject of a good number of recent (1985) for the Ðeld appropriate for Sextans A. optical studies. Here, we can compare subsets of our photo- metry with the results fromSC82, SC85, HSD (their gri magnitudes were transformed into Johnson BV RI magni- IRAF and the Digitized SkySurvey3 image of the Sextans A tudes via the transformations inHoessel & Melnick 1980 Ðeld obtained from the Canadian Astronomy Data Center andWade et al. 1979), W87, A87, and SMF. (Dohm-Palmer (CADC), from which absolute coordinates for stars on the et al.1997a and 1997b present BV I photometry using HST, Palomar Observatory Sky Survey can be measured. The but they do not provide their data in tabular form, so we do accuracy obtained in these absolute positions by this tech- not make a comparison with their photometry.) nique (D1AÈ2A) is adequate for our comparison with the H I The stars that have been most used as the basis of com- radio map, given the relatively lower resolution of the radio parison in all these previous studies are the bright stars data (see ° 4). listed inSC82. In Table 1, we list the mean di†erences of our measurements for the various colors for theSC82 stars with the measurements from these other studies. InFigure 2, we 3. ANALYSIS OF THE PHOTOMETRY show a comparison of our magnitudes and colors for the We present here a brief description of our photometric SC82 stars with those obtained by the other studies. results. We have removed obvious foreground stars from One can see that our V magnitudes are in very good our star list, e.g., the notoriously bright red foreground star agreement with those ofSMF, A87, and W87 but di†er in the northeast quadrant of Sextans A, and those stars that substantially fromSC85 and HSD, the former likely A87and SC82 previously identiÐed as foreground stars (e.g., because of the zero-point problems associated with the pho- those thatSC82 measured as proper-motion stars). These tographic photometry, and the latter likely because of the are essentially all stars with V [ 17.5. uncertainty in the transformation from the Gunn-Thuan We can estimate how many foreground stars fainter than system to the Johnson system, as noted bySMF. Our B V [ this V should be in our Ðeld, from theRatnatunga & colors agree very well with those inW87, but less so with Bahcall(1985) model for the galactic coordinates of Sextans those from the other studies. Our U B colors are in rea- [ A (we interpolate the model appropriate for Sextans A from sonably good agreement with those fromA87, but in the the nearby Ðelds including the galaxies Pal 3 and Leo I). In case for both U B and B V , we point out the large [ [ Table 2 we list, for our 27 arcmin Ðeld, the predicted amount of scatter for even the brightest stars studied in 2 number of stars in three color bins and four magnitude bins, A87. along with the estimated number of actual stars found One can see very good agreement of our V I colors with [ within these bins. Of course, without identiÐcation spectra those fromSMF, with an o†set of D0.05 between our for individual stars, we have no means of eliminating pos- colors and theirs (although the rms in the comparison is sible foreground interlopers from our star list, but the com- roughly equivalent to this possible o†set; in addition, we parison inTable 2 should be kept in mind throughout the notice a similar o†set between the I magnitudes from SMF paper. and those fromDohm-Palmer et al. 1997a in the latterÏs Finally, we have also used our color-color diagrams Fig. 7 forI [ 20.3). Finally, we compare our R I (Cousins [ (° 3.2) to isolate and remove stars from our list that clearly system) colors with the R I (Johnson system) colors from [ have grossly discordant colors, indicating possible blends, HSD. The o†set of D0.1 agrees with the zero point of 0.12 unresolved nebulosity, or other photometric errors. We in the transformation from the Johnson to the Kron- have eliminated erroneous detections along the two bad Cousins system derived using asymptotic giant branch stars rows on the combined images and within the saturated byCosta & Frogel (1996). In all, we believe the agreement of our photometry with previous photometry to be quite satisfactory. ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 3 The Digitized Sky Surveys were produced at the Space Telescope Science Institute (STScI) under US government grant NAGW-2166. The images of these surveys are based on photographic data obtained using the 2.5. Transformation to Absolute Coordinates Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Absolute star positions were derived by converting the Telescope. The places were processed into the present compressed digital form with the permission of these institutions. The National Geographic pixel centroids of the stars on our KPNO 2.1 m images Society-Palomar Observatory Sky Atlas (POSS-I) was made by the Cali- from ALLSTAR into absolute coordinates using the Guide fornia Institute of Technology with grants from the National Geographic Star Astrometric Program (GASP) software in STSDAS/ Society. No. 5, 1998 STAR FORMATION IN SEXTANS A 2347 image of the aforementioned bright red foreground star; we This is consistent with the slope for other dwarf irregular have also removed stars that are too close to the edge of the galaxies, such as WLM, Sextans B, and NGC 6822 (see, e.g., CCD in any band to provide reliable photometry. Tosiet al. 1991; Greggio et al. 1993; Marconi et al. 1995) We arrive at 2525 stars detected in V and at least one and implies that Sextans A has a star formation rate and other band. Because of space limitations, we do not list the initial mass function that are not unlike other dwarf irregu- magnitudes and colors for all the stars here. An electronic lar galaxies studied so far. As pointed out byHunter & tabulation of magnitudes, colors, and absolute positions of Plummer(1996), Sextans A currently has a star formation the stars can be obtained courtesy of the CDS. rate (D0.006M_ yr~1), when normalized to its linear scale, which is typical of other dwarf irregular galaxies (see also 3.1. L uminosity Functions Hunter& Gallagher 1986), being neither very low nor We can derive a luminosity function (LF) for the stars in excessively high. the galaxy and compare it with those for other dwarf irregu- Interestingly enough, similar slopes are found for the LFs lar galaxies. InFigure 3, we show the global di†erential LF for all stars in each of the bands for which the photometry is for all detected stars in each of the bands. Crowding and complete (0.44 for all stars in V ; 0.44 in B; 0.53 in U; 0.48 in blending of stars in our images limit the depth and accuracy R, and 0.53 in I). This implies that the mass function aver- of the photometry we have obtained. This can be represent- aged over more than 1 Gyr has been relatively constant in ed by the completeness of our photometry. One can see that Sextans A. We have also isolated just those stars along the incompleteness signiÐcantly a†ects the photometry in each putative bar in the galaxy; we Ðnd that the slope of the LF of the bands at a given magnitude. Our photometry is most for these stars in V is 0.51, similar to the slopes in V and complete in the V band, then most complete in R, then I, B, other bands for all stars in Sextans A. and Ðnally, U. The photometry is seriously a†ected by incompleteness at V D 23, R D 22.5, I D 22, B D 21.7, and 3.2. Color-Color Diagrams U D 20.7. InFigure 4, we present the color-color diagrams for We also derive the LF for only the main-sequence stars. Sextans A. The solid line in each diagram shows the loca- Following the recommendation byFreedman (1985), we tion of the (unreddened) main-sequence, giant, and super- separate these stars from the evolved ““ blue-loop ÏÏ super- giant branches, based on theoretical models (see° 3.4). We giants (which are indistinguishable from the main-sequence have indicated in these color-color diagrams the direction stars in the blue plume on the (V , B V ) CMD in Fig. 5 [ of the reddening vector. We also show representative uncer- below) by selecting those stars withU V [ 1.05 (we use [ [ tainties in the observed colors. On the (U B, B V ) a similar criterion in U B in° 3.3 to distinguish the hot [ [ [ diagram we can see two clumps of stars, the bluer main- main-sequence stars). InFigure 3, we show the di†erential sequence stars and the somewhat redder blue supergiants. LF for only those 241 stars ( Ðlled circles). A least-squares Ðt On the (B V , V I) diagram, we again see a blue clump of to the data for the main-sequence stars brighter than [ [ stars, which is a mixture of main-sequence stars and super- V 22 (fainter than which incompleteness clearly a†ects \ giants and a smaller clumping of red supergiant stars. The our statistics) results in a slope, d log N/dV 0.48. \ general agreement between the model tracks and the observed color distributions indicates that the amount of reddening to and within the galaxy must be in general small (see° 3.3). But, some reddening is clearly necessary, based on these diagrams, on the amount of Galactic foreground reddening, and on the results of other investigators from colors of individual stars (e.g.,SMF). The reddening also appears somewhat variable.

3.3. Color-Magnitude Diagrams The most direct information on the stellar populations and relative star formation histories in galaxies can be obtained from the analysis of color-magnitude diagrams (CMDs) based on deep photometry of the resolved popu- lations. In this paper we have observed at Ðve optical bands: our bluest colors can very adequately trace out the youngest, hottest stars in the galaxy, while, asAparicio & Gallart(1995) have demonstrated in their study of the Pegasus dwarf galaxy, photometry in red colors is also necessary to derive information about stars of all ages. InFigure 5 we present the CMDs for Sextans A in the various colors based on our photometry. What is imme- FIG. 3.ÈDi†erential LFs for Sextans A in U (dotted line), B (dashed diately evident from examining all of the CMDs, as has line), V (solid line), R (short dashed-dotted line) and I (long dashed-dotted been found in previous studies of Sextans A, is the rich and line) for all detected stars. Also shown is the function for only main- various array of populations of di†erent ages, from main- sequence stars ( Ðlled circles), i.e., those withU[V [ [1.05. The line rep- sequence stars to red supergiants to asymptotic giant resents a least-squares Ðt to the main-sequence points for stars with V [ branch (AGB) stars. Such populations have been identiÐed 22, resulting in a slope d log N/dV \ 0.48, which is consistent with that found for other dwarf irregular galaxies and also with the functions for all in this and other dwarf galaxies from the ground, but never stars in all the bands. before with this color baseline. 2348 VAN DYK, PUCHE, & WONG

FIG.4a FIG.4b

FIG. 4.ÈColor-color diagrams for Sextans A. In (a) we show the (U[B, B[V ) diagram, and in (b) we show the (B[V , V [I) diagram. For each diagram we show the unreddened 4 Myr isochrone fromBertelli et al. (1994) with metallicity Z \ 0.001; for (b) we also show the 100 Myr isochrone (note the poorer agreement of the AGB stars with the model). For both Ðgures we also show representative uncertainties in the colors and the direction of the reddening vector. The practical detection limit for our photometry is V [ Drissen,Roy, & Robert (1997) in NGC 2366. This star 23. In our deep ground-based images, a di†use background clearly deserves more attention. of faint stars in the galaxy is also discernible below this One can also see on the (V , B[V ) CMD, and more so limit. with the addition of the R band in the (V , V [R) CMD, that On the (V , B[V ) and (V , V [R) CMDs, one can clearly the number of blue-loop stars is relatively larger than the see the blue sequence, or ““ plume,ÏÏ of stars, with B[V [ number of red supergiants, in a ““ red plume ÏÏ of stars with 0.2(V [R [ 0.3). The greatest limitation to separating out B[V Z 0.5 (V [R Z 0.6) andV [ 22. More blue super- the various populations in Sextans A, based on our imaging giants than red supergiants may exist in this galaxy. This is is, of course, crowding and blending. Despite this limitation consistent with the fact that, at low metallicity, stars spend in resolution, however, our (V , U[B) CMD clearly shows more time as blue supergiants than red supergiants during that this blue plume separates into the main sequence (with their He core-burning phase(Bertelli et al. 1994). U[B [ [0.7), down to about spectral type B1 (V D 22), On the (I, R[I) and (I, V [I) CMDs the conspicuous and the evolved blue-loop supergiant stars, with red plume of stars are again visible at 0.8 [ V [I [ 1.6 [0.7 [ U[B [ 0.3, which are burning helium (He) at their (0.3 [ R[I [ 0.8) andI [ 22.3, with two or three con- cores, although some small contamination exists from spicuous clumps of red supergiants, as noted bySMF, at main-sequence turno† stars. (This blue loop is not as I D 21.2, 20.6, and 19.8. Also visible is the broad clump of evident in our [V , B[V ] and [V , V [R] CMDs, in part red stars at 0.8 [ V [I [ 1.8 (0.3 [ R[I [ 0.8), I D 22, because of the above-mentioned crowding and blending of which is the top of whatAparicio & Gallart (1995) refer to main-sequence and evolved stars on the CCD images, but as the ““ red tangle ÏÏ of RGB stars, old and intermediate-age also because of the fact that U[B a†ords a much better AGB stars, and intermediate-age blue-loop stars. The temperature resolution for luminous blue stars and there- TRGB, although likely contaminated by intermediate-age fore assists in overcoming the degeneracy in the blue AGB stars, is at I D 21.8 and V [I D 1.3 (SMF). plume.) The stars fainter than I D 19 with V [I [ 2(R[I[0.9) We therefore have been able to detect the brighter (V [ are whatAparicio & Gallart (1995) refer to as the ““ red 22) blue-loop population based on our ground-based data, tails ÏÏ of intermediate-age and old AGB stars. Aparicio & even in a galaxy as distant as Sextans A, through deep UBV Gallart point out that the ““ length ÏÏ of the ““ tail ÏÏ (i.e., how imaging. red this feature extends on the CMD) is a function of metal- One notes on the blue CMDs the very bright star with licity: the longer the tail, the greater the metallicity. The V \ 17.51, U[B \[0.03, and B[V \ 0.14;A87 point to observed tails for Sextans A do not extend as red in color as it as likely being a supergiant member of Sextans A pri- do, for example, those on the CMDs for the Pegasus dwarf marily because of its location in the bright blue clustering of galaxy(Aparicio & Gallart 1995), consistent with the metal- stars in the southeast. The PSF for this object does appear licity di†erence between Sextans A and Pegasus (Skillman, stellar, so it is not likely to be a blend. For this star MV ^ Bomans, & Kobulnicky1997; Aparicio, Gallart, & Bertelli [8.3 for a distance modulus of 25.8 (see° 3.4). It appears 1997). therefore to be D2 mag brighter than the blue straggler Of particular curiosity are the very bright red stars seen binary stars found in the young SMC cluster NGC 330 on the red CMDs. Although they appear to be especially, (Grebel,Roberts, & Brandner 1996). It is more likely to be a and possibly anomalously, bright, to be members of Sextans luminous blue variable star, similar to the one found by A, we have no particular evidence to the contrary; for FIG. 5.ÈColor-magnitude diagrams for the stars in Sextans A. In (a) we have also plotted the theoretical isochrones fromBertelli et al. (1994) with metallicity Z \ 0.001 for the ages 4, 8, 12, 25, 40, and 80 Myr. In (b) we also include the 125 Myr isochrone. In (c) we have plotted the isochrones for 8, 12, 25, 40, and 125 Myr. In (d) and (e) we have plotted the isochrones in (c), but have also included the isochrones with ages 60, and 0.3, 1, and 3 Gyr. On each we show the known Cepheids(Piotto et al. 1994) with open circles surrounding the data points. A distance modulus of 25.8 mag and an average extinction of AV \ 0.16 have been assumed. 2350 VAN DYK, PUCHE, & WONG Vol. 116 example, they are not among the proper-motion stars in The redder CMDs show three noticeable clumps of red SC82. All but one of the stars is in or near a region of recent supergiants, with ages D40, D60, and D80 Myr. The star formation. Short of having spectra for these stars, we number of red supergiants both older and younger than consider them to be members of the galaxy. these ages appears to be signiÐcantly smaller. These stars We also show on all CMDs, highlighted with open circles, must have formed in three di†erent bursts of recent star the magnitudes and colors of the known Cepheids from formation. The clump of RGB stars (red tangle), for which Piotto,Capaccioli, & Pellegrini (1994). we can resolve only the top, appears to correspond to an age ofZ100 Myr. The oldest populations we can resolve 3.4. Stellar Population Ages (the di†use quantity of red stars withV [I Z 1.5, compris- On the CMDs inFigure 5, we have compared our photo- ing the various red AGB tails) appear to have ages from metry with a series of theoretical isochrones calculated by D0.2È0.3 Gyr to several Gyr, although we can only probe the Padova group (seeBertelli et al. 1994 and references these populations with accuracy to ages 1È2 Gyr. Our pho- therein), which we use to estimate the ages of the stellar tometry is not deep or complete enough to resolve stars populations in Sextans A. We have chosen those isochrones older than this. computed with a metallicity Z \ 0.001, which is consistent From our CMDs we Ðnd that the Cepheids in Sextans A with that inferred from the oxygen abundance for this have agesZ40 Myr and appear to be younger than D100 galaxy(Skillman, Kennicutt, & Hodge 1989). We do not Myr. extensively sample the oldest populations, where consider- SPATIAL DISTRIBUTION OF STELLAR POPULATIONS ation of signiÐcantly lower metallicity would be important. 4. We have found that isochrones of higher metallicity do not First, we examine the spatial distribution of the youngest match the positions of the stars in color and magnitude population of stars in Sextans A. We consider stars with nearly as well as those of the chosen metallicity. ages[50 Myr. Dohm-Palmer et al. (1997b; see their Fig. Before further comparison with the isochrones is made, it 13) have found that the star formation rate beginning D50 is Ðrst necessary to choose a distance to the galaxy and to Myr ago has been a factor of D20 larger than the time- consider the amount of reddening that the observed stars averaged rate for the galaxy, and therefore we illustrate the are experiencing in order to estimate ages for the various extent of the most vigorous star formation over the last 1 populations. We therefore adjust the isochrones on the Gyr. InFigure 6, we represent on the V -band image of the CMDs accordingly. galaxy four young populations of stars: (1) those stars with First, we have adopted a distance modulus to Sextans A, [1.2 [ U[B [ [1.0 andV [ 22, which are the youn- which is a compromise between the modulus derived from gest, bluest main-sequence stars (plus signs); (2) those young the TRGB and Cepheid brightness methods given by SMF, stars with magnitudes and colors, particularly in U[B, that is, m [ M ^ 25.8 mag (which corresponds to a distance which make them likely main-sequence turno† stars and of 1.44 Mpc). Second, we have assumed a reddening for the supergiants with ages[12 Myr, including the very bright galaxy of E(B[V ) \ 0.05. The amount of Galactic blue and red supergiant stars mentioned in° 3 (crosses); (3) reddening is E(B[V ) \ 0.02(Burstein & Heiles 1984), but, those blue He-burning stars (with [0.7 [ U[B [ 0.3) asSMF point out, E(B[V ) ^ 0.05 is likely appropriate for withV [20.7, i.e., those with ages[50 Myr (circles); the main body of the galaxy, where most of the young stars and (4) the corresponding red He-burning stars, seen on and gas are found(SMF Ðnd a higher E[B[V ] for the the (I, V [I) CMD, with1.2 [ V [I [ 1.6 and I [ 19.7 older red giant population in the galaxy). So, we have (squares). assumed this value, which, assuming a normal reddening AsDohm-Palmer et al. (1997b) emphasize, the blue He- law, i.e.,AV \ 3.1E(B[V ), corresponds toAV ^ 0.16. Note burning stars follow a somewhat redder, nearly vertical that this is somewhat larger than the reddening assumed by track parallel to the main sequence and are about 2 mag Dohm-Palmer et al.(1997a, 1997b), which is essentially only brighter than the corresponding main-sequence turno† due to the Galactic component. Based on our reddening stars of the same age. The position in magnitude of a blue assumption, we followCardelli, Clayton, & Mathis (1989) in He-burning star on the CMD is determined by the starÏs deriving the extinction for the other bands and assume that mass and, therefore, its age. The blue He-burning stars are E(U[B) ^ 0.7E(B[V ). clearly very useful age probes for stellar populations, back From examining all the CMDs it is evident, regardless of to about 600 Myr in Sextans A. This is only true at these their exact ages, that a number of stellar populations of low metallicities for blue He-burning stars with ages older various ages coexist in Sextans A, as has been noted in than D20 Myr, orV Z 19.3 for Sextans A, since the He- previous studies (e.g.,A87; Dohm-Palmer et al. 1997b). The burning phase for stars with M [ 15M_ is not well under- star formation does not seem to have been continuous but stood(Chiosi, Bertelli, & Bressan 1992; Dohm-Palmer et al. instead appears to have occurred more in bursts or epi- 1997b). sodes. We do not try to execute a thorough and exact One immediately can see that the recent star formation analysis of the population ages through more sophisticated over the last 50 Myr has occurred in speciÐc regions in means such as synthetic CMDs (e.g.,Aparicio et al. 1996). Sextans A and is not uniformly distributed. In particular, Based on our more cursory analysis with the theoretical the very young, hot, blue main-sequence stars are primarily isochrones, it can be seen from the CMDs constructed from found near surface brightnesskB^25.9 mag arcsec~2,ora the U, B, and V bands that the blue plume of stars consists radiusr ^ [email protected] (Ables 1971). As pointed out previously by of several young populations, from very young of roughly HSD, A87, and others, these hot stars are primarily found in only a few megayears) to signiÐcantly older populations the bright clusters in the southeast and to the west. But they with ages[100 Myr. Clear gaps in stellar age within this are not exclusively in just these two locations. Another sig- time range exist; this is best seen in the redder populations niÐcant clustering of these stars is to the northwest, and a (i.e., with B[V Z 0.4). small cluster is to the northeast. A smaller number of these No. 5, 1998 STAR FORMATION IN SEXTANS A 2351

FIG. 6.ÈOn the V -band image of Sextans A we show the following young populations of stars: (1) stars with[1.2 [ U[B [ [1.0 andV [ 22, which are the youngest, bluest main-sequence stars (pluses); (2) stars with magnitudes and colors, particularly in U[B, which make them likely main-sequence turno† stars and supergiants with ages[12 Myr (crosses); (3) blue He-burning stars (with[0.7 [ U[B [ 0.3) withV [ 20.7, i.e., with ages[50 Myr (circles); and (4) the corresponding red He-burning stars, with1.2 [ V [I [ 1.6 andI [ 19.7 (squares). stars are found within the main optical body of the galaxy. magnitude and color [V \ 18.92, B[V \ 0.52, Large areas of the galaxy clearly exist where no recent star U[B \ 0.31] are consistent with other bright supergiants formation has taken place. seen within this radius.) But these stars, as plotted in Figure What is most intriguing is that some of these stars are 6, have ages up to D12 Myr, and some are therefore older also found beyond kB ^ 25.9mag arcsec~2 (r ^ [email protected]) in the than the youngest population represented (the hot main northeast and the southwest. Based onTable 2, we consider sequence). Some of these supergiants are seen in regions of the likelihood of foreground stars in such numbers super- the galaxy not directly associated with the youngest stars posed on this small Ðeld to be very small. We conclude that and therefore likely trace out regions of somewhat older recent star formation appears to even be occurring beyond star formation. the main optical surface brightness of Sextans A. Massive The evolved He-burning stars, both blue and red with stars may form seemingly ““ isolated; ÏÏ though this does not ages up to D50 Myr are also found nearr ^ [email protected], but, partic- occur often, it is not unusual (e.g.,Massey, Johnson, & ularly in the west, their spatial distribution is appreciably DeGioia-Eastwood1995; Massey et al. 1995). inward, located more toward the galaxyÏs optical center, The bright young supergiants, not surprisingly, have a extending from the northwest down to the southwest. Many very similar spatial distribution to the very young main- of these stars are also found in the bright cluster of stars in sequence stars. (One supergiant star can be seen in the the southeast, indicating that star formation in that region extreme southeast well beyondr ^ [email protected]; it is possible that has been occurring for at least D50 Myr. The brighter stars this is a foreground star, based onTable 2, although its with ages D50 Myr in this cluster appear to form a ““ ring,ÏÏ 2352 VAN DYK, PUCHE, & WONG Vol. 116 with radius of D300 pc. Several of these blue-loop stars are northwest. The youngest large concentration of star forma- also found in the smaller cluster of stars in the northwest. tion has been in the clustering in the west. Noticeable large But, most noticeably, only one bright blue He-burning star holes can be seen in the distribution of young stars. No is found in the galaxyÏs second-largest star-forming region, appreciable recent star formation has occurred at the in the west, and this star, from its position on the CMDs optical center of the galaxy. (with V \ 19.23, B[V \ 0.12, and U[B \[0.03), is Next, we examine the spatial distribution of older popu- possibly as young as D15 Myr. This region of star forma- lations in Sextans A. InFigure 7, we again show the V tion, which Dohm-Palmer et al.(1997a, 1997b) could not image of the galaxy. Shown on this Ðgure are two popu- study, is clearly one of the youngest in the galaxy, along lations of stars: (1) those blue He-burning stars with ages with the small clustering of hot stars in the northeast. Star D50È100 Myr, withV Z 20.7 to V D 22 (the faintest blue formation in these regions could be as recent as[20 Myr. He-burning stars that we can conÐdently resolve; squares) In summary, the recent massive star formation over the and (2) the corresponding red He-burning stars with 19.7 Z last 50 Myr, which has been so pronounced in Sextans A, I Z 21.3, with ages to D100 Myr (circles). has been conÐned toward the edge of the main body of the The more central concentration of the distribution of the galaxy, nearkB ^ 25.9 mag arcsec~2, and may have prog- stars with ages 50È100 Myr is more noticeable than for the ressed even beyond this point. In particular, in the west, it younger populations. Some structure and clustering also has been progressing outward from more central regions. appears in the distribution of these older stars, which does Much of the recent star formation has been concentrated in not appears to be random. We have used a large age bin the large clustering of stars in the southeast, and to the (D50 Myr wide) for these stars, so small timescale details in

FIG. 7.ÈOn the V -band image of Sextans A we show the following populations of stars: (1) those blue He-burning stars with ages D50È100 Myr, with V Z 20.7(squares); and (2) the corresponding red He-burning stars with 19.7 Z I Z 21.3, with ages to D100 Myr (circles). No. 5, 1998 STAR FORMATION IN SEXTANS A 2353

FIG. 8.ÈOn the V -band image of Sextans A we show the following older populations: (1) red giants, at roughly I D 22 and V [I D 1.1, and some AGB stars, with ages between D100 and D600 Myr (crosses); and (2) those red giants and AGB stars with agesZ600 Myr (the older red tangle and red-tail stars with ages possibly up to D3 Gyr; plus signs). their distribution as a function of age are lost. Nonetheless, seen in the isophotes with position angle141¡.5; Skillman et the distribution of this population illustrates the advanced al.1988 describe a possible H I bar with position angle stage of dissolution of parent OB associations. Regions in 105¡.) The integrated color of the bar feature, which appears which these stars are missing are noticeable in the Ðgure. particularly in the isophotal photometry, is B[V \ 0.22, SpeciÐcally, relatively fewer of the He-burning stars with excluding the star complex in the southeast (Ables 1971) ages greater than 50 Myr are found at the sites of the most and, thus, should be a mix of both blue and red stars. These recent star formation (the bright blue clusters) to the east evolved He-burning stars should therefore comprise the and west. In fact, no such stars are seen in the cluster to the brighter populations along the bar, although we do not west, again, indicating that this region is very young. In consider the optical bar structure to be entirely obvious addition, the older He-burning stars near the large cluster of from the distribution of these populations inFigure 7. A few young stars in the southeast tend to be to the southwest of the red He-burning stars are also visible beyond the main edge of the cluster, indicating a possible age gradient in this optical galaxy; although their colors and magnitudes are large star complex. quite consistent with those of other similar stars within the The He-burning stars appear to mostly extend from the galaxy, these could well be foreground stars. In summary, far north down to the far south in the galaxy, with apparent the distribution of these older stars is clearly di†erent from clustering toward the galaxyÏs optical center. Some of these that of the younger stars. stars appear along a putative bar (see, e.g., Ables 1971; A87), Finally, inFigure 8 we once again show the V image with although they are clearly not exclusive to that region, if it two older populations: 1) RGB stars at roughly I D 22 and exists.(Ables 1971 describes this barlike feature as being V [I D 1.1 and some AGB stars in the red tangle with ages 2354 VAN DYK, PUCHE, & WONG Vol. 116 between D100 and D600 Myr (crosses); and 2) those RGB consider only the ionized and neutral components of the gas stars and AGB stars with agesZ600 Myr (the older red in the galaxy, since molecular gas has not been detected tangle and red-tail stars with ages possibly up to D3 Gyr; (Ohtaet al. 1993 found only a low upper limit for the lumi- pluses). We chose the age ranges of 100È600 Myr and nosity of CO). greater than 600 Myr becauseDohm-Palmer et al. (1997b) have found that the star formation rate between about 100È 5.1. Ionized Hydrogen 600 Myr ago was a factor of D6 higher than the time- First, we examine the distribution of blue stars relative to averaged rate, likely declining for ages older than this the ionized gas, as seen from our Ha ] [N II] image. In range; this is illustrated in their Figure 14. Figure 9, we show our Ha map, which is qualitatively No particular pattern or structure is obvious in the similar to that inAparicio & Rodriguez-Ulloa (1992) and spatial distribution for the RGB stars. However, few of Hodge,Kennicutt, & Strobel (1994a). All the Ha emission these stars are seen in or near the youngest star formation appears to be nonstellar. The H II regions are distributed in regions. Again, we are using a large age bin to increase our two arcs separated by nearly 4@ (with the brightest patches statistics, and therefore we cannot describe small timescale of emission in the southeast). Fainter emission can also be variations in the distribution. These stars have likely dis- seen. Our Ha image, although continuum-subtracted, is not persed from their birthplaces over an area of diameter D5@. Ñux-calibrated, but comparing our map with that in Hodge Note the approximate similarity of the distribution of RGB et al.(1994a), we estimate that our detection limit for H II stars that we Ðnd and that found byDohm-Palmer et al. regions is D2.5 ] 10~14 ergs cm~2 s~1, which, at a distance (1997b; their Fig. 16) for their limited Ðeld of view, espe- of 1.4 Mpc, corresponds to a luminosity of D6 ] 1036 ergs cially the possible clustering of RGB stars just east of the s~1. Some of the fainter structures seen on the deep Ha map center. inHunter & Gallagher (1997), such as the long Ðlaments One interesting aspect of the distributions for these stars (e.g., their Ðlaments 1 and 2) and fainter shells (e.g., their is that they appear to also be found outsidekB ^ 25.9 mag shell 4) are just barely visible on our map and are somewhat arcsec~2. Again, based onTable 2, one might suspect that below our detection limit. D15%È30% of these stars are merely in the foreground. InFigure 9, we show the blue main-sequence stars (as in But, again, the colors and magnitudes for the stars found Fig. 6, with[1.2 [ U[B [ [1.0 andV [ 22) relative to outside the main body of the galaxy are consistent with the Ha emission. These stars are roughly spectral type B0 those red giants well within the galaxy, and a large fraction and hotter, and[8È10 Myr old (at the assumed metallicity therefore could be members of Sextans A. These stars may of Sextans A, this corresponds to a main-sequence turno† comprise a more extended halolike structure, which is also mass of D20M_). Stars cooler than this do not contribute found for a number of dwarf irregular galaxies (e.g., Minniti signiÐcantly to the UV continuum Ñux that can produce & Zijlstra 1996). H II regions in the galaxy. One can see that the hot, massive The older RGB and AGB stars with agesZ600 Myr are stars, as expected, are well correlated with the ionized gas, also uniformly distributed across and beyond the main both bright and faint (in fact, a cluster of six stars cannot be body of the optical galaxy, with no apparent clustering or distinguished from the dark contrast inFig. 9 for the H II structure. Again, the stars seen beyondkB ^ 25.9 mag region ““ No. 17,ÏÏ following the Hodge et al. numbering arcsec~2 have colors and magnitudes consistent with those scheme, or ““ No. 7,ÏÏ following that ofAparicio & within the galaxy, but probably up to about one-third of Rodriguez-Ulloa1992). Thus, we have accounted for the these stars could be in the foreground. The quite homoge- ionizing sources for the majority of the H II gas. neous distribution is what one would expect for an older A smaller number of presumed OB stars are not associ- halo Ðeld population.Hunter & Plummer (1996) point out ated with any H II regions above the detection limit of our that star formation in Sextans A must once have occurred study. This is true also for the regions detected by Aparicio further out in the galaxy than is seen today, given the size of & Rodriguez-Ulloa(1992) and Hodge et al. (1994a), the Holmberg radius. They also point out that in the although some appear to be associated with some of the southwest they can trace faint stars as far out as 1.5 kpc fainter emission seen in theHunter & Gallagher (1997) map. from the center. In addition, some of the fainter emission features, based on What we Ðnd for stars older than 100 Myr is consistent closer inspection of our images, may be ionized by unre- with the picture developed by Dohm-Palmer et al. (1997b, solved small clusters of hot stars, as is also likely the case for 1998), in which star formation, at a signiÐcantly lower rate the bright knotty emission region, which isHodge et al. than the current rate, percolates in coherent regions and (1994a)H II region No. 20 and Aparicio & Rodriguez-Ulloa propagates to other neighboring regions within their Ðeld of (1992) No. 2. However, the inner area of the galaxy is rela- study during the time range of D100È600 Myr. The rate is tively devoid of massive stars and H II regions (the inner likely lower for ages older than 600 Myr, but the pattern portion of the galaxy may have several faint Ðlaments of and propagation of star formation appear from our data to emission, and, asHunter & Gallagher 1997 point out, these have been similar in nature. In particular,Dohm-Palmer et Ðlaments might comprise a larger shell structure associated al.(1998) show that from 300È400 Myr, star formation with the bright H II complex in the southeast). The bulk of migrated from the northeast to the center, peaking there the massive star formation within the last D20 Myr has around 350 Myr ago. Our results seem to imply that this been spatially and temporally correlated along the edge of centrally concentrated star formation continued to peak the main surface brightness for the galaxy. from[100 Myr. 5.2. Neutral Hydrogen RELATIONSHIP OF THE STARS TO THE GAS 5. Next, we investigate the relationship of the stars to the We now examine the spatial distribution of the various neutral gas. We have utilized a radio 21 cm H I map from stellar populations to that of the gas in Sextans A. We can Graham& Westpfahl (1998). Here we dispense with the o etn .Wa etfh osdra consider Westpfahl & H an within hole,ÏÏ ““ or depression, central What A. Sextans for data. al. et Skillman the more were to than sensitive emission are extended maps the conÐguration, compact most (1988). o ,19 TRFRAINI ETN 2355 A SEXTANS IN FORMATION STAR 1998 5, No. vial nteWrdWd e thttp://cfa-www.harvard.edu/ at Web Wide World the projects/nga/sexa.html. on available by agreement cooperative under Inc. Universities, operated Associated Foundation Science National ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ H their of details hntoefrteH the for those than a hw ee)Teetost fdt eete com- then the were resolution in with data map included of weighted 34 not naturally sets frequency a are redshifted in two resulting the but D- These bined, at June here.) the 1994 shown and in observations map May made (B-conÐguration the also 1992 July. for were in 1992 appropriate hydrogen in C-conÐguration neutral conÐguration the of in line National cm galaxy the 21 of the (VLA) of Array Large Astronomy Very Radio the using made beam 5 4 nw hwtettlH total the show we In A ] niiilaayi ftedt yD uh,icuigmmn as is maps, moment including Puche, the D. by of data the of facility analysis initial a An is Observatory Astronomy Radio National The iue10, Figure

~1 DECLINATION (B1950) -04 2500 23 F nadto,wt h hr pcnssmldi the in sampled spacings short the with addition, In ohterslto n h estvt r higher are sensitivity the and resolution the Both . IG A 29 00 28 00 27 00 26 00 .Dsrbto fsasi etn ihadrltv oteinzdhdoe (H hydrogen ionized the to relative and with A Sextans in stars of 9.ÈDistribution . n m os ntecanlmp f13 mJy 1.36 of maps channel the in noise rms and 10 0840 30 30 30 30 I aaadol oeta hsH this that note only and data Observatory uh (1994) Puche I a rsne ne al. et in presented map 4 I oundniymap density column 35 I Skillman hl sclearly is shell [ 1.2 I [ a was map U 5 [ B RIGHT ASCENSION(B1950) [ [ 1.0 ta.adta h H the that and presentation complete a for (Puche spirals for that than thicker al. likely et is disk gas the Westpfahl that & H here dwarf these of defer of of ÏÏ we sample hook Again, ““ their Zee, west. for the the found is to to disk map the the from extending of gas feature fascinating what A galaxies. and galaxies Giovanelli & normal Haynes, other with consistent H the tr si ( in as stars el nadto,oecnseta H that see can one addition, In well. nti a niae usrcuigo mle,fainter H larger smaller, the of within holes substructuring and clouds, indicates or map clumps, this in a ye l n ihihstenearly the highlights and al. et of center is the hole in the density in column galaxy The the shell. or ring complete by map et(ekcolumn, (peak west iil;e l ecieteH the describe the toward column, structures (peak arclike optical or southeast spurlike main density large column two entire highest the The forms galaxy. the central the al. of a nearly brightness et surface around encompasses structure diameter, hole, This ÏÏ boundary. in ringlike outer sharp or a with horseshoe minimum ““ a as visible; 30 V nw loso h oiin ftesm blue same the of positions the show also we In rhm(1998) Graham [ iue10 Figure 1992) 22 I klmn(1988) Skillman igo hl,a ela h anotclgalaxy, optical main the as well as shell, or ring klmn(1988) Skillman I iue9 Figure data. D pluses (1995) 1.2 D D 25 ] .Wa hudb eti idis mind in kept be should What ). I 1.2 1.6 hl sruhyshrcl What spherical. roughly is shell 10 ] ] 22 10 10 cm 21 22 a ~2 ] I cm cm xed elbeyond well extends ,a lose nthe in seen also as ), [N ~2 ~2 II emission) ] h n detail Ðne The . n h north- the and ) I morphology 20 D van I oeas hole . kpc 1.5 otes n h otws.Frfwro hs tr are H stars these of locus. this fewer of outside Far or within H northwest. seen of the complexes and large southeast two the with, coincident otws,aentcicdn ihtebihetH brightest the with coincident not are northwest, 116 Vol. WONG & PUCHE, DYK, VAN 2356 w ag H large two found are stars H youngest large the bluest, along the predominantly and that preferentially is striking most is n&WspalTeclrbrgvsteapoiaeH approximate the gives bar color The Westpfahl & in ihtefc httems eetsa omto is formation star recent most H the the H of edge of that inner distribution the the along fact primarily that occurring noted the H the with as far as extend Plummer the straddle fact, in but, tr oehr si n a e htteoverall the that see can the One to similar to axis up H symmetry the and ages of shape distribution with current a have stars to of appears He-burning distribution and spatial in supergiants, as together, stars, stars main-sequence blue, a rhm(1998). Graham F noecnas e,ntuepcel,ta the that unexpectedly, not see, also can one In nw hwtedsrbtoso h young, the of distributions the show we In IG msini ln h ne de fteH the of edges inner the along is emission 0Èitiuino h tr nrltv onurlhdoe (H hydrogen neutral to relative in stars the of 10.ÈDistribution . iue11 Figure iue12, Figure I oe atclryaogteinreg f n not and of, edge inner the along particularly hole, (1996) II

eincmlxs ntesuhatadthe and southeast the in complexes, region DECLINATION (B1950) -04 15 35 30 25 20 iue6. Figure I 0 inner a itiuin hsi consistent is This distribution. gas 10 0900 I deo ohpas & peaks. both of edge a n oteyugs stars youngest the to and gas i.9 Fig. inner 08 45 I I deo the of edge a D I a othe to gas Hunter oe The hole. hole. osnot does 0Myr 50 I peaks, 0.5 RIGHT ASCENSION(B1950) I oundniyi 10 in density column I ssonfo oundniympdrvdfo 1c L observations VLA cm 21 from derived map density column a from shown as ) 30 usrcuigta oteyugrsasi We H the within regions in that stars out younger point the do H than the substructuring with relationship little relatively bear since center, galaxyÏs albeit the stars, formed to recently closer most stars the expect as distribution might spatial to we up ages then with expanded, homologously gas in dispersion velocity peak the If H radius. the smaller a with but itnl omn ln h ne deo h H the of edge con- inner been the have along stars gas forming If the sistently rate. of expansion ( expansion constant hole a possible the assuming a H of represents large size the current form to outward Westpfahl unn tr,a nwt ages with are H the stars of older center these the that toward is more evident concentrated immediately is in What as stars, burning Hwvr Bascain ol aedsesdsomewhat a been dispersed over have would have associations OB (However, would hole nw hwtedsrbtoso h le He- older the of distributions the show we In iue13 Figure D I ( 22 D 0Mrtimescale.) Myr 50 cm 1998) 9 È ~2 0k s km 10 15 . D 0Mrt hr iial ring-shaped similarly a share to Myr 50 1 iue7, Figure ~1 ta.& al. et ; D klmn18;Graham 1988; Skillman D . p)wudtake would kpc) 1.5 00 . p mle nradius. in smaller kpc 0.5 I oe hnt rdc the produce to then hole, D I 0MraoteH the ago Myr 50 oeeitweeno where exist hole I D oe hl,and shell, hole, iue12. Figure 50 I È oea the as hole D I 0 Myr. 100 oeand hole 0Myr, 80 I rmterbrhlcs h ouain fodrstars older of from observed populations ages the to with H relative The distributed stars uniformly these nearly birthplaces. appear of dispersal their neutral substantial the from the of with structure consistent current the gas, and stars these between in as stars, D AGB and RGB older wr reua aais ta.con- al. 1998 et in movie the an (although less, state its or into high Dohm-Palmer Myr galaxy the star-forming 100 launched last that present transpired the events or within event sometime other in that occurring clude formation galaxies. star irregular of particular dwarf bursts this has the both puzzle and this of galaxy to understanding recent answer our The the for of A? ramiÐcations concentration Sextans in spatial formation and star rate the in increase o ,19 TRFRAINI ETN 2357 A SEXTANS IN FORMATION STAR 1998 5, No. eryteetr H entire the nearly concen- now. centrally is more it been than past have the must in gas trated It the formed. have that to clear appear is generations older these from stars a a a elhv xse admysileit there. exist) still may (and molecu- existed star-forming have the well may that gas beyond implies lar shell well the ago outside long disk formed H had current stars the that are H however, the beyond gas 0 y o1Gro o eew e orelationship no see we Here so. or Gyr 1 to Myr 100 htcudhv enrsosbefrtesubstantial the for responsible been have could What ial,i eso h itiuin fthe of distributions the show we in Finally, I lhuhorotclyiae edencompasses Ðeld imaged optically our Although . DECLINATION (B1950) -04 2500 F IG 29 00 28 00 27 00 26 00 1Èitiuino oie yrgn(H hydrogen ionized of 11.ÈDistribution . iue14 Figure I 30 30 30 30 hl tutr.TeH The structure. shell ta.mgtipyta hsevent this that imply might al. et I I oe tde o vra uhwt the with much overlap not does it hole, 10 0840 hl.Teidctosi iue14, Figure in indications The shell. 6 . DISCUSSION omPle (1997b) Dohm-Palmer iue8, Figure I a nteextended the in gas a ] 35 [N II msin nSxasA(hw nrltv oteH the to relative in (shown A Sextans in emission) ] RIGHT ASCENSION(B1950) cin&HrqitI addi- In Hernquist & 1996) H one-half have about to in disk tend a in galaxies inter- (Hunter form tidal can late-type or bar merger A a tion, of result a as action period rotation one to inter- Miller from than & H arise mass neighboring possibly small may greater with and action of common (generally are A) galaxies systems. Sextans infer- massive irregular more make the in can for known Bars we is what galaxies, on larger based ences other for than galaxies formation star recent prominent most occurring. is the where ends, its at rsmbycnet h w rnucdmxm nthe in maxima pronounced two H the connects presumably aaadfo h a ieaiset Magellanic for true H is The kinematics A. gas Sextans as than the massive feature from center, more bar and mor- galaxies The o† irregular bar. optical a H somewhat the of the of indication with appears misalignment an A possibly, typically is and, phology, axis kinematical the isophotal optical the al. in both pos- evident data a feature, of existence barlike the sible stressed have authors Several galaxy. two inter- the between tidal a separations galaxies. as A large above the dismissed mechanisms. of been because possible has possibility B the Sextans nearby of with action some on here as early as occurred lhuhls skonaotbr ndafirregular dwarf in bars about known is less Although n osblt stepeec fabrptnili the in potential bar a of presence the is possibility One I 1988). itiuinadmyb soitdwt h a pileup gas the with associated be may and distribution Als17)(Skillman 1971) (Ables 30 Ngci18;Bre 1991). Barnes 1987; (Noguchi otsa ihsa omto occurring formation star with Gottesman & 1996). i.9 i.10) Fig. 9) Fig. D 5 y g) ebiÑ speculate brieÑy We ago). Myr 350 25 I lusLehman, clouds I I opooia xswith axis morphological sonin (shown (Wilcots, I a-ihbars gas-rich 20 I bar 38VNDK UH,&WN o.116 Vol. WONG & PUCHE, DYK, VAN 2358 neato racnrlacmlto fmass of Yet, accumulation distribution. gas current the central explain could ring tidal a a a into after ring 1996). or (Athanassoula gaseous 1996), the outer with long-lived period interaction rotation a account one could of about a in formation However, destroyed inter- be periods. of can absence rotation bar the several in requires bar, likely H a a the around action, of cen- However, shell evolution ago. more a The Myr once in hole. concentrated central was currently A is Sextans formation star in gas (Hunter Induced the concentrated evolution. trally that fact bar (e.g., the bar for for trend a the along galaxy timescale gas be the of to in necessary inÑow appears formation which star the and gas and of distribution the are example. (Odewahn 4618 NGC galaxies 1991) irregular Magellanic the for bar the along un†sasadsprinsaerpeetdwt rage,rte hncoss h oo a stesm si i.10. Fig. in as same the is bar color The crosses. than rather triangles, with represented are supergiants and stars turno† F h andfiute ihabrepaainfrSxasA Sextans for explanation bar a with difficulties main The IG 2Èitiuino h tr nrltv onurlhdoe (H hydrogen neutral to relative in stars the of 12.ÈDistribution . n G 49&Trno for Thronson & 4449 NGC and

Pilp 1996), (Phillips DECLINATION (B1950) -04 22 Z 31 30 29 28 27 26 25 24 23 eitiuino h a rmtebar the from gas the of Redistribution 50 10 0845 0 cwr 1984) Schwarz i.6Fg 6, Fig. 6 Fig. 40 I 35 a and gas 0.5 RIGHT ASCENSION(B1950) I 30 .Tesmosaetesm si xetta h ieymain-sequence likely the that except in as same the are symbols The ). oe a vltoaytmsae,sneteglx a a has galaxy the these since than period longer timescales, rotation is evolutionary A bar Sextans model for timescale dynamical the upre n&WspalEiec for model a Evidence such propose Westpfahl Nomoto & & Kato, gal- dwarf of number a is for that exist A axies. to appears Sextans events for merger mechanism in the past supported is This the formation). in event LMC. the in is environs analo- and bar III associations constellation or optical Shapley association to rem- an gous OB the central extended seeing of an be the merely of presence may nants throughout we The convincing; generally galaxy. not therefore stars the stars these the of of regions of ages the those between then, and di†erence, no Ðnd We huh ob h uaieotclbr(f and Myr. typically are stars bar (cf. the bar of ages optical the putative that Ðnd the be to thought eodpsiiiyi h †c famre raccretion or merger a of e†ect the is possibility second A utemr,w aeioae h eino h galaxy the of region the isolated have we Furthermore, ao (1992) Saio, 25 rhm(1998). Graham D 0 sin 400 20 1 D 0 y wt rwtotbar without or (with Myr 100 i y tal. et Myr 15 Sila 1988). (Skillman 10 be 1971) Ables Z 40 È 50 ou.Ti ol cu ftegswti h H the within gas this the at if compression occur and would pileup This gas locus. cold with consistent is o ,19 TRFRAINI ETN 2359 A SEXTANS IN FORMATION STAR 1998 5, No. xadn uwr noteetne H extended the into outward expanding H suggested the recently of have edge the formation on star mostly occurring current is is the A that Sextans stars in in fact formation massive The cloud for galaxies. of mechanism al. irregular important concentrations et an be by to likely input mecha- by energy driven formation nical star more triggered sequentially for that least at epochs. formation, recent star sequential and expansion A. Sextans for in explanation formation dis- star an of easily o†er distribution not be current also does the cannot alone model are it merger molecular Mergers However, a the missed. merger. Thus, of galaxies. concentration a in central of a gas in result can result galaxy to the the known as for Ðeld explained velocity is the be A that Sextans claim of Westpfahl case & the in H curiosity particular the Of 10. IC for nFg 10. Fig. in F hr osblt stemcaimo supernova-driven of mechanism the is possibility third A IG 3Èitiuino hs tr nrltv onurlhdoe (H hydrogen neutral to relative in stars those of 13.ÈDistribution . I a xedn etadfo h H the from westward extending gas

utr(1998) Hunter DECLINATION (B1950) -04 22 31 30 29 28 27 26 25 24 23 10 0845 0 i.7Fg 7. Fig. 7 Fig. 40 I neoese in seen envelope I ik Graham disk. I 35 oeis hole I 0.5 RIGHT ASCENSION(B1950) hole 30 aiso h a?A icse ni h ekvelocity peak the if H in the discussed in As dispersion gas? the it, of matics compressing gas, the as colder formation star evolves. the galaxy further triggering on and of clouds, 1995) interior pushing forming the 1992) from galaxy, input outward expands have the gas Brandner they & hot (e.g., resulting A, winds The stellar Sextans through of Mathewson, ISM interior Leitherer the into the energy in signiÐcant past the in Tongue LMC the con- Ford in be & 4 also LMC may and example III local stellation A 1994). al. et (Martimbeau H 1992; largest particu- the occurring along formation is larly star formation the star current to the similar of remarkably behavior The data. radio the rhm1998) Graham I .Tesmosaetesm si h oo a stesm as same the is bar color The in as same the are symbols The ). sti ehns luil o etn ,gvntekine- the given A, Sextans for plausible mechanism this Is evolved and formed have populations stellar massive As e lo&WspaladI 2574 IC and Westpfahl & also see 95 rbl1998). Grebel 1985; etfh ersnsapsil expansion possible a represents Westpfahl & 25 ta.adeddterlvsa supernovae. as lives their ended and al. et I 20 1 ( D 9 I È 0k s km 10 oe eni oI tal. et II Ho in seen holes 15 ° ~1 5.2, tal. et ; (Dopita, klmn1988; Skillman 10 (Puche 30VNDK UH,&WN o.116 Vol. WONG & PUCHE, DYK, VAN 2360 ftegst omtelreH large the form to gas the of xadn rn fsa omto uwr rmtecenter. the from outward formation star of front expanding oee,i a edfiutt e uhafauei these data in same feature very a region. the such from density see column to difficult lowest near be the Ðnd may they in it which However, (1994) center peak, galaxyÏs dispersion Westpfahl velocity the & particu- the proÐles, at shell, expanding velocity larly an double-peaked for characteristic evidence convincing i.e., Ðnd not do set, gas, Puche G n G tr r ersne ihtinlsrte hncoss h oo a sa nFg 10. Fig. in as is bar color The crosses. than rather triangles with represented are stars AGB and RGB ol be would rbtoso ouain ihae esta 0Mris the Myr H to 50 the inward stars within than oldest outward less from youngest progression ages dis- a with the of suggestive Furthermore, populations progeni- generations. the of supernova stellar be tributions the those presumably of from would siblings near tors which stars massive A, of less ages Sextans the remaining of with in agrees center increased also and the substantially Myr rate 100 formation last the star the that fact F ao ifclywt hsseai sta,although that, is scenario this with difficulty major A IG rhm(1998), Graham 4Èitiuino hs tr nrltv onurlhdoe (H hydrogen neutral to relative in stars those of 14.ÈDistribution . etfh li vdnefrexpanding for evidence claim Westpfahl & D 0Mr hstmsaei ossetwt the with consistent is timescale This Myr. 80 DECLINATION (B1950) -04 22 31 30 29 28 27 26 25 24 23 10 0845 0 I oe hnteaeo h hole the of age the then hole, I i.8Fg 8, Fig. 8 Fig. oe ossetwt an with consistent hole, 40 35 0.5 RIGHT ASCENSION(B1950) 30 I ose 98 ao1988.) age feasible see a of LMC; populations several the shows imaging in our we that 1987A as Saio SN supernovae, 1988; with massive Woosley witnessed all directly of neces- progenitors have not the may stars be metal- these low sarily galaxies the irregular with dwarf systems of in licities since cluster, light remnant same the a dominate the from might of supergiants red stars (Interestingly, main-sequence age. as than the identiÐed redder, that are signiÐcantly readily CMDs which most our stars, He-burning be from al. evolved the clear would et is at populations Rhode it remnant populations However, whereas main-sequence dwarfs. centers, recently other imaging, hole remnant CCD H and optical for of deep II centers from searched the Ho at in populations remnant holes Ðnd to not claim Westpfahl oas eBe n ta.fudan found al. et and Domgoergen, respectively. 2, LMC and Furthermore, 4 LMC for expansion of regions. absence Boer de & brightness Bomans, low .Tesmosaetesm si xetta the that except in as same the are symbols The ). nte oeta ifclyi htSle,& Salzer, that is difficulty potential Another 25 (1998), 20 19)Pit (1998) Points (1995) ta.FrSxasA ebelieve we A, Sextans For al. et 1 15 D a rgtr and brighter, mag 2 Rhode, 10 D 100 È D 0 Myr 600 I No. 5, 1998 STAR FORMATION IN SEXTANS A 2361 near the center of the large hole, which are suitable candi- locus. The distribution of stars with ages[50 Myr indi- dates for remnant sibling stars to the supernovae progeni- cates an outward progression of star formation with time. tors. Regions exist where the star formation appears to be exclu- Of course, some combination of these various mecha- sively recent and where no star formation has occurred nisms may have been at work during the galaxyÏs history. It within the last 50 Myr. Older star formation, from D50È is possible that the galaxy has experienced the e†ects of a 100 Myr, appears to have some spatially coherent structure merger or accretion event, or of a bar potential sometime in and is more centrally concentrated than the most recent star the past, resulting in a signiÐcant increase in the star forma- formation. The oldest star formation that we can accurately tion rate. The increased massive star formation resulted in sample appears to be uniformly distributed across Sextans increased feedback to the ISM, which may have resulted in A, even beyond a surface brightness ofkB ^ 25.9 mag an expanding bubble of hot gas, which has promoted the arcsec~2 or a radius r ^ [email protected]. current star formation we are witnessing. We simply do not The recent bursts in star formation are not only coherent have enough information. in time but also in space across Sextans A. The trigger of The trigger for the current burst of star formation in these bursts remains unknown; however, we believe we Sextans A remains a mystery, although we have shed addi- have presented the strongest evidence so far that tional new light through our optical imaging of the galaxy supernova-driven expansion of the gas may be responsible and our comparison with the gas components. Dohm- for at least some of the star formation over the galaxyÏs Palmer et al.(1997a, 1997b) have also provided some vital history. The expansion of the gas due to episodic sequential clues, but because of the limited Ðeld size of the HST star formation within the galaxy is a direct trigger for WFPC2 camera, they sampled only about one-half of the further star formation as the galaxy evolves. We note that galaxy, and therefore, their results are only applicable to the expected expansion age of the H I shell is D80 Myr this smaller Ðeld. Since star formation has occurred in (assuming an expansion velocity of D10 km s~1 and a shell bursts in various regions of Sextans A, sampling a larger diameter of D1.5 kpc), which is consistent with the ages of area of the galaxy provides a broader view of the recent star the older, centrally concentrated evolved stars, which may formation history. be the remnants of the populations that included the Unfortunately, one important additional clue that is massive stars responsible for the expansion. missing is the distribution of the molecular gas, about which we have no information other than the low upper limit to We are grateful to Ralph Young Shuping for his photo- the luminosity of the CO tracer(Ohta et al. 1993). We do metric calibration observations of Sextans A at the KPNO not know the mass or the extent of the actual gaseous raw 0.9 m telescope, to NOAO director Sydney Wol† for sched- material for star formation in the galaxy. That information uling these observations, and Dave Summers for his assist- would help resolve questions about the nature and history ance at the KPNO 2.1 m telescope. We thank Maggie of star formation in Sextans A. Graham and Dave Westpfahl for making their total H I CONCLUSIONS map available to us. S. V. D. is grateful for assistance pro- 7. vided by Alex Filippenko and also to the UCLA Division of From our ground-based observations of the Local Group Astronomy and Astrophysics. We thank Jean Turner, dwarf irregular galaxy Sextans A inUBV (RI) and Ha,we Tammy Smecker-Hane, Dave Westpfahl, and Dave Meier have found that current star formation is occurringC mostly for helpful discussions. We also thank the referee for very along the inner edge of the large H I shell surrounding a helpful comments and suggestions. This research was sup- hole, or depression, in the H I column density, consistent ported by a grant from NASA administered by the Amer- with cold gas pileup and compression of the gas at this ican Astronomical Society.

REFERENCES Ables,H. 1971, Publ. US Naval Obs., 2d Ser., 20, Part 4 Drissen,L., Roy, J.-R., & Robert, C. 1997, ApJ, 474, L35 Aparicio,A., & Gallart, C. 1995, AJ, 110, 2105 Freedman,W. L. 1985, ApJ, 299, 74 Aparicio,A., Gallart, C., & Bertelli, G. 1997, AJ, 114, 669 Gerola,H., & Seiden, P. E. 1978, ApJ, 223, 129 Aparicio,A., Gallart, C., Chiosi, C., & Bertelli, G. 1996, ApJ, 469, L97 Gerola,H., Seiden, P. E., & Schulman, L. S. 1980, ApJ, 242, 517 Aparicio, A., Garcia-Pelayo, J. M., Moles, M., & Melnick, J. 1987, A&AS, Graham,M., & Westpfahl, D. 1998, in preparation 71, 297 (A87) Grebel, E. K., & Brandner, W. 1998, in The Magellanic Clouds and Other Aparicio,A., & Rodriguez-Ulloa, J. A. 1992, A&A, 260, 77 Dwarf Galaxies, ed. T. Richtler & J. M. Braun, in press Athanassoula, E. 1996, in ASP Conf. Ser. 91, Barred Galaxies, ed. R. Buta, Grebel,E. K., Roberts, W. J., & Brandner, W. 1996, A&A, 311, 470 D. A. Crocker, & B. G. Elmegreen (San Francisco: ASP), 309 Greggio,L., Marconi, G., Tosi, M., & Focardi, P. 1993, AJ, 105, 894 Barnes,J. E., & Hernquist, L. E. 1991, ApJ, 370, L65 Heiles,C. 1979, ApJ, 229, 533 Bertelli, G., Bressan, A., Chiosi, C., Fagotto, F., & Nasi, E. 1994, A&AS, ÈÈÈ.1984, ApJS, 55, 585 106, 275 Hodge,P., Kennicutt, R. C., & Strobel, N. 1994a, PASP, 106, 765 Braun, J. M., Bomans, D. J., Will, J.-M., & de Boer, K. S. 1997, A&A, 328, Hodge,P., Strobel, N. V., & Kennicutt, R. C. 1994b, PASP, 106, 309 167 Hoessel,J. G., & Melnick, J. 1980, A&A, 84, 317 Burstein,D., & Heiles, C. 1984, ApJS, 54, 33 Hoessel, J. G., Schommer, R. A., & Danielson, G. E. 1983, ApJ, 274, 577 Cardelli,J. A., Clayton, G. C., & Mathis, J. S. 1989, ApJ, 345, 245 (HSD) Cash, W., Charles, P., Bowyer, S., Walter, F., Garmire, G., & Riegler, G. Hunter,D. A. 1997, PASP, 109, 937 1980, ApJ, 238, L71 Hunter,D. A., Elmegreen, B. G., & Baker, A. L. 1998, ApJ, 493, 595 Chevalier,R. A. 1974, ApJ, 188, 501 Hunter,D. A., & Gallagher, J. S. 1986, PASP, 98, 5 Chiosi,C., Bertelli, G., & Bressan, A. 1992, ARA&A, 30, 235 ÈÈÈ.1997, ApJ, 475, 65 Costa,E., & Frogel, J. A. 1996, AJ, 112, 2607 Hunter, J. H., & Gottesman, S. T. 1996, in ASP Conf. Ser. 91, Barred Dohm-Palmer, R. C., Skillman, E. D., Saha, A., Tolstoy, E., Mateo, M., Galaxies, ed. R. Buta, D. A. Crocker, & B. G. Elmegreen (San Francisco, Gallagher, J., Hoessel, J., Chiosi, C., & DuFour, R. J. 1997a, AJ, 114, ASP), 398 2514 Hunter,D. A., & Plummer, J. D. 1996, ApJ, 462, 732 ÈÈÈ.1997b, AJ, 114, 2527 Hunter,D. A., & Thronson, H. A. 1996, ApJ, 461, 202 ÈÈÈ.1998, AJ, 115, 152 Landolt,A. U. 1992, AJ, 104, 340 Domgoergen,H., Bomens, D. J., & de Boer, K. S. 1995, A&A, 296, 523 Koo,B. C., & McKee, C. F. 1992a, ApJ, 388, 93 Dopita,M. A., Mathewson, D. S., & Ford, V. L. 1985, ApJ, 297, 599 ÈÈÈ.1992b, ApJ, 388, 103 2362 VAN DYK, PUCHE, & WONG

Leitherer,C., Robert, C., & Drissen, L. 1992, ApJ, 401, 596 Saio,H., Kato, M., & Nomoto, K. 1988, ApJ, 331, 388 Marconi,G., Tosi, M., Greggio, L., & Focardi, P. 1995, AJ, 109, 173 Saito,M., Sasaki, M., Ohta, K., & Yamada, T. 1992, PASJ, 44, 593 Martimbeau,N., Carignan, C., & Roy, J.-R. 1994, AJ, 107, 543 Sakai,S., Madore, B. F., & Freedman, W. L. 1996, ApJ, 461, 713 (SMF) Massey, P., Armandro†, T., De Veny, J., Claver, C., Harmer, C., Jacoby, G., Sandage,A., & Carlson, G. 1982, ApJ, 258, 439 (SC82) Schoening, B., & Silva, D. 1997, Direct Imaging Manual for Kitt Peak ÈÈÈ.1985, AJ, 90, 1019 (SC85) (Tuscon: KPNO) Schwarz,M. P. 1984, MNRAS, 209, 93 Massey,P., Johnson, K. E., & De Gioia-Eastwood, K. 1995, ApJ, 454, 151 Skillman,E. D., Bomans, D. J., & Kobulnicky, H. A. 1997, ApJ, 474, 205 Massey, P., Lang, C. C., De Gioia-Eastwood, K., & Garmany, C. D. 1995, Skillman,E. D., Kennicutt, R. C., & Hodge, P. W. 1989, ApJ, 347, 875 ApJ, 438, 188 Skillman, E. D., Terlevich, R., Teuben, P. J., & van Woerden, H. 1988, Minniti,D., & Zijlstra, A. A. 1996, ApJ, 467, L13 A&A, 198, 33 Noguchi,M. 1987, MNRAS, 228, 635 Stetson,P. B. 1987, PASP, 99, 191 Odewahn,S. C. 1991, AJ, 101, 8290 ÈÈÈ.1990, PASP, 102, 932 Ohta, K., Tomita, A., Saito, M., Sasaki, M., & Nakai, N. 1993, PASJ, 45, Stetson, P. B., Davis, L. E., & Crabtree, D. R. 1990, in ASP Conf. Ser. 8, L21 CCDs in Astronomy, ed. G. H. Jacoby (San Francisco: ASP), 289 Olsen, K. A. G., Hodge, P. W., Wilcots, E. M., & Pastwick, L. 1997, ApJ, Tenorio-Tagle,G., & Bodenheimer, P. 1988, AR&A, 26, 145 475, 545 Tongue,T. D., & Westpfahl, D. J. 1995, AJ, 109, 2462 Phillips, A. C. 1996, in ASP Conf. Ser. 91, Barred Galaxies, ed. R. Buta, Tosi,M., Greggio, L., Marconi, G., & Focardi, P. 1991, AJ, 102, 951 D. A. Crocker, & B. G. Elmegreen (San Francisco: ASP), 44 Tully, R. B. 1988, Nearby Galaxies Catalog (Cambridge: Cambridge Univ. Piotto,G., Capaccioli, M., & Pellegrini, C. 1994, A&A, 287, 371 Press) Points, S. D., Chu, Y.-H., Kim, S., Smith, R. C., Snowden, S. L., Brandner, vanZee, L., Haynes, M. P., & Giovanelli, R. 1995, AJ, 109, 990 W., & Gruendl, R. A. 1998, ApJ, in press Wade,R. A., Hoessel, J. G., Elias, J. H., & Huchra, J. P. 1979, PASP, 91, 35 Puche, D., & Westpfahl, D. 1994, in Dwarf Galaxies, ed. G. Meylan & Walker,A. R. 1987, MNRAS, 224, 935 (W87) P. Prugniel (Garching: ESO), 273 Waller,W. 1990, PASP, 102, 1217 Puche,D., Westpfahl, D., Brinks, E., & Roy, J.-R. 1992, AJ, 103, 1841 Westpfahl,D. J., & Puche, D. 1994, preprint Ratnatunga,K. U., & Bahcall, J. N. 1985, ApJS, 59, 63 Wilcots,E. M., Lehman, C., & Miller, B. 1996, AJ, 111, 1575 Reid,N., Mould, J., & Thompson, I. 1987, ApJ, 323, 433 Woosley,S. E. 1988, ApJ, 330, 218 Rhode,K. L., Salzer, J. J., & Westpfahl, D. J. 1997, BAAS, 29, 1342