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1989Apj. . .338. .789C the Astrophysical Journal, 338:789-803,1989 March 15 © 1989. the American Astronomical Society. All Righ

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1989Apj. . .338. .789C the Astrophysical Journal, 338:789-803,1989 March 15 © 1989. the American Astronomical Society. All Righ .789C The Astrophysical Journal, 338:789-803,1989 March 15 © 1989. The American Astronomical Society. All rights reserved. Printed in U.S.A. .338. 1989ApJ. STAR FORMATION IN NGC 5253 Nelson Caldwell F. L. Whipple Observatory, Smithsonian Institution AND M. M. Phillips Cerro Tololo InterAmerican Observatory, National Optical Astronomy Observatories1 Received 1988 July 18 ; accepted 1988 August 24 ABSTRACT We present new observations of the nearby galaxy NGC 5253, which is actively forming stars in its nucleus. Our data include multicolor CCD measurements, narrow-band images in Ha, and long-slit Ca n triplet spectra. The halo of the galaxy is incipiently resolved, probably into red giants. The halo light is composite in age; the younger component is probably older than 108 yr and younger than 109 yr. Of the ~100 cataloged star clusters outside of the nucleus, few if any are old globular clusters; most appear to be in the same age range as the halo light. While star formation is now seen primarily in the nucleus, it occurred relatively recent- ly throughout the galaxy. The surface brightness profile of the galaxy is well described by an exponential falloflf, aside from the nuclear star formation site. The maximum stellar rotation rate is barely detectable, at 7 km s-1, whereas the stellar velocity dispersion is high, at 50 km s_1. Before the galaxy began actively forming stars, it was probably a dwarf elliptical. The distribution of ionized gas is shown to be very complex, although there is still no evidence for excitation of the gas by means other than hot stars. An appendix reviews the two Type I supernova that occurred in NGC 5253. Subject headings: galaxies: individual (NGC 5253) — galaxies: nuclei — galaxies: stellar content — stars: formation — stars: supernovae I. INTRODUCTION 5253, based upon the cited findings, as well as a few other NGC 5253 has at various times been classified by astron- reasons that are no longer acceptable (e.g., the radio emission omers as a spiral galaxy (Shapley 1934), an elliptical galaxy from NGC 5253 can indeed be accounted for by the H n (Shapley and Ames 1932), an SO galaxy (Sersic, Carranza, and regions), van den Bergh (1972, 1980) showed that the nuclear Pastoriza 1972), a Magellanic irregular (de Vaucouleurs 1963), absorption-line spectrum was due to early-type stars and that an Irr II (Hodge 1966), an 10 (de Vaucouleurs, de Vaucouleurs, the galaxy was surrounded by a system of star clusters extend- and Corwin 1976), and, most recently, as an Amorphous ing to a radius of 120" (2.2 kpc). Although he had no colors for galaxy (Sandage and Tammann 1981). From the point of view the clusters, he presumed them not to be globulars, but of morphology alone, it would appear that this nearby, moder- younger star clusters instead, which he regarded as “ fossil evi- ately faint galaxy (mB =11, MB = —16.8) has many remark- dence for active star formation in the past,” remnants of the able properties. The early work on this object by Gaposchkin star formation burst now confined to the nucleus. He suggested (1936), Hubble and Lundmark (1923), and Evans (1952) con- that the burst in NGC 5253 was triggered by an encounter not cerned the relation of the galaxy to the supernova Z Cen (SN more than 109 yr ago with the spiral galaxy NGC 5236 (M83), 1895B) that occurred some 25" away from the nucleus. That which lies at a projected distance of at least 100 kpc. work will be briefly discussed in the Appendix. A summary of Work in the infrared on the nucleus by Moorwood and the modern studies follows. Welch (1970) showed that the Glass (1982) and Aitken et al (1982) revealed an intense star nuclear emission-line spectrum of the galaxy was that of an formation region that is obscured in the visible spectrum by 25 H ii region, requiring 104 BO stars to match the emission-line mag of extinction. This region must contain of order 500 04 luminosities (he excluded O stars from consideration, thinking stars to account for the observed strength of the [S iv] emis- that such stars would have been resolved on his plates). sion line at 10.5 gm. Campbell, Terlevich, and Melnick (1986) Prodded by Welch’s work and Hodge’s (1966) paper describing found broad À46S6 emission in the brightest visual nuclear extended irregular emission features in NGC 5253, Sersic, condensation, which they attributed to Wolf-Rayet stars. The Carranza, and Pastoriza (1972) wrote a paper similar in scope K — L nuclear color of 2 mag is the largest known for any to the present one in which they found (1) a nuclear complex of galaxy as is the K — 10 gm color of 7.7 mag. The large mid- dust and very blue luminous condensations (now known to be infrared emissions is best explained as thermal radiation from OB associations), (2) “jet-like” Ha emission features (later dust at temperatures between 100 and 700 K. This particular shown to be more filament-like in Graham 1981), and (3) observation is perplexing in that, while numerous dust lanes chaotic motions among the condensations of the same order as are visible in the optical image of the galaxy (Fig. 1 [Pis. the overall rotation of the galaxy. The authors drew a compari- 9-10]), there is no evidence for any reddening in the Balmer son between the actively star-forming galaxy M82 and NGC decrement in the visual spectrum. 1 Cerro Tololo InterAmerican Observatory, National Optical Astronomy Since the observed X-radiation and the radio emission in Observatories, operated by the Association of Universities for Research in NGC 5253 can be provided by the known ionizing stars Astronomy, Inc., under contract from the National Science Foundation. without recourse to supernova remnants, and the 2 gm emis- 789 © American Astronomical Society • Provided by the NASA Astrophysics Data System .789C PLATE 9 .338. 1989ApJ. Fig. \a Fig. 1.—(a) Blue CCD image of the field 1 (SW) of NGC 5253. Grid along the picture can be used to locate the star clusters of Table 1. Scale is 0"3 per grid unit. The image was taken with the CTIO 4 m telescope, with a total exposure time of 2000 s. (b) Same as (a) for field 2 (NE). Caldwell and Phillips (see 338, 789) © American Astronomical Society • Provided by the NASA Astrophysics Data System .789C PLATE 10 .338. 1989ApJ. Fig. lb Caldwell and Phillips {see 338, 789) © American Astronomical Society • Provided by the NASA Astrophysics Data System .789C 790 CALDWELL AND PHILLIPS Vol. 338 .338. sion is weak compared to that in M82, implying little contribu- B—V colors are redder by 0.05 mag than the CCD values. tion from red giants and supergiants, Moorwood and Glass Since Wegner quotes an external B—V error estimate of 0.05 conclude that massive stars could not have begun to form in mag, we do not regard this discrepancy as meaningful. An 9ApJ. 7 the nucleus more than 10 yr ago. NGC 5253 is therefore error in the CCD B—V calibration of 0.05 mag would not 198 considered to be in the first stage of the evolutionary sequence significantly change the results of this paper anyway. of starburst galaxies outlined in Rieke, Lebofsky, and Walker (1988). The question of whether the recent episode of star formation b) General Results began throughout the galaxy at a low rate and proceeded inward at a much higher rate remains to be answered, and Figures la and lb show the two fields imaged in the B filter. forms the major part of this new study. We examine evidence A grid along the borders indicates the scale, one CCD pixel provided from the star clusters and the diffuse halo light in an being equal to two grid units. Two things are immediately attempt to answer this question. The data include UBV CCD apparent from these images. First, the star cluster system is imaging, in which over 100 clusters in the halo are studied. The quite large, in fact more than twice as large as van den Bergh’s cluster system may be considered to be at an early stage of initial estimate of 50. It is interesting that this number is com- what we now see in the LMC, in that there are clusters of ages parable to the number found in the Small Magellanic Cloud, from 107 to 109 yr, but no clear evidence for a significant which is of comparable absolute magnitude to NGC 5253. number of old (1010 yr) clusters. The halo light comes from a Second, the galaxy is incipiently resolved, either into lumin- composite population, most likely an old underlying popu- ous stars or small star clusters. To better illustrate this pheno- lation, and a younger one. If the younger one is the result of a menon, an image was created by dividing the B images burst, then its age is between 108 and 109 yr. Thus we conclude displayed in Figure 1 by the same image smoothed with a that indeed the recent episode of star formation began 7x7 box median filter. Thus all low-frequency components in throughout the galaxy, but became violent in the nucleus only the galaxy images have been removed. Figures 2a and 2b (Pis. recently (1 x 107yr). 11-12) show the resultant filtered images. The star clusters now We also present surface photometry and stellar kinematic stand out prominently, and the incipient resolution is apparent data that point to a dwarf elliptical as the likely progenitor as a mottling along the major axis, out to a radius of ~90".
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