The Astrophysical Journal, 532:L109–L112, 2000 April 1 ᭧ 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A. THE RADIO SUPERNEBULA IN NGC 5253 Jean L. Turner Department of Physics and Astronomy, UCLA, Los Angeles, CA 90095-1562; [email protected] Sara C. Beck Department of Physics and Astronomy, Tel Aviv University, Ramat Aviv, Israel; [email protected] and Paul T. P. Ho Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; [email protected] Received 1999 December 30; accepted 2000 February 14; published 2000 March 9 ABSTRACT We present 1.3 cm and 2 cm subarcsecond resolution VLA images of the dwarf galaxy NGC 5253. Within ∼ the central starburst, we detect high-brightness [Tb (2 cm) 100–12,000 K] radio continuum sources. These appear to be very dense, “compact” H ii regions. The dominant radio source is a nebula ∼1–2 pc in size, requiring several thousand O stars within the volume to maintain its ionization. This nebula has no obvious optical counterpart. The number of ionizing photons we find for this cluster is nearly 2 orders of magnitude larger than indicated by Ha fluxes, and the deduced stellar content accounts for a significant fraction of the total infrared luminosity of the galaxy. This cluster is a strong candidate for a globular cluster in the process of formation, perhaps the youngest globular cluster known. Subject headings: galaxies: dwarf — galaxies: individual (NGC 5253) — galaxies: peculiar — galaxies: starburst — galaxies: star clusters — radio continuum: galaxies 1. INTRODUCTION 2. OBSERVATIONS NGC 5253 is a dwarf I0/S0 galaxy at a distance of 4 Mpc The 2 and 1.3 cm observations were made with the A con- ∼ # undergoing a starburst of infrared luminosity L IR 1.8 figuration of the VLA on 1998 October 13. Absolute flux cal- 9 10 L, (Beck et al. 1996). The starburst in NGC 5253 is very ibration was based on observations of 3C 286 and is good to young, displaying the optical (Gorjian 1996; Calzetti et al. better than 5%. Phase center was at a =13h39m55.84,s d = 1997) and infrared (Rieke, Lebofsky, & Walker 1988) signa- Ϫ31Њ38Ј29Љ.1 (J2000). The naturally weighted synthesized tures of a starburst no more than a few million years old. Its beams are0Љ.22 # 0Љ.08 (FWHM) at P.A. = Ϫ8Њ for the 1.3 cm radio continuum emission is unique among galaxies for being map and0Љ.33 # 0Љ.12 at P.A.=1Љ.9 for the 2 cm map. The almost entirely thermal (Beck et al. 1996), which suggests that north-south elongation of the beam is caused by the foreshort- the star formation has not persisted for the time it takes to ening of the array, due to the southerly declination of the source. produce a significant number of supernovae. The two bands have different sensitivities to lower spatial fre- Radio observations at 6, 3.6, and 2 cm made with the NRAO1 quencies, so we cannot directly compare total fluxes or compute Very Large Array with 1Љ–2Љ resolution indicated that the radio spectral indices from the maps. Because of the lack of short free-free emission in NGC 5253 is partly optically thick at spacings, the 2 and 1.3 cm maps are sensitive only to structures centimeter wavelengths (Turner, Ho, & Beck 1998). Classical Շ2Љ–4Љ in the east-west direction and Շ4Љ–8Љ north-south. Galactic H ii regions such as Orion are optically thick at wave- These observations were designed to determine brightness lengths around 1 m; the high densities needed for a source to temperatures and the structure and nature of compact radio be optically thick at 2 cm are seen in the Galaxy only in the sources rather than total fluxes, which are available from lower youngest ultracompact H ii regions, of diameter ∼0.01 pc (Hab- resolution observations (Turner et al. 1998). There was an in- ing & Israel 1979; Wood & Churchwell 1989). Thermal sources sufficient signal-to-noise ratio for self-calibration without sig- of this size cannot be detected in other galaxies (Turner & Ho nificant loss of spatial resolution, and so the measured source 1994). The thermal radio continuum emission in NGC 5253 is sizes may be broadened by seeing at the fractional arcsecond 1–2 orders of magnitude stronger than predicted by the ob- level, with a more pronounced effect expected at 1.3 cm. We served Ha emission (Calzetti et al. 1997), which implies that calculate from comparison with lower resolution maps (Turner the extinctions are substantial, another indicator of youth. This et al. 1998) that our 2 cm maps resolve out ∼50% of the total is very likely the location of the youngest part of the starburst, emission within the central20 # 40 , although they appear which is optically invisible. to recover all of the 2 cm emission in the inner6 # 6 (Beck To minimize the possibility of confusion by sources with et al. 1996). The images were deconvolved from the dirty different optical depths or synchrotron emission in the same beams using the CLEAN algorithm. The rms noise levels in beam, we reobserved the radio core of NGC 5253 at 2 cm with the continuum maps are 0.09 and 0.11 mJy beamϪ1 for 2 and the A configuration of the VLA, which gives a synthesized 1.3 cm, respectively. beam of 0Љ.1–0Љ.3 in size. We also extended the spectrum to 1.3 cm. It is on these observations that we report in this Letter. 3. THE SUPERNEBULA AND ITS ENVIRONS 1 The National Radio Astronomy Observatory is a facility of the National In Figure 1 we display the 1.3 and 2 cm VLA maps of the Science Foundation operated under cooperative agreement by Associated Uni- central 3Љ (50 pc) of the starburst in NGC 5253. The maps versities, Inc. show a dominant radio continuum source at both 2 and L109 L110 SUPERNEBULA IN NGC 5253 Vol. 532 # 2n/2,n=00 , 1, 2), times 0.3 mJy beamϪ1 (3 j). The beam size is Љ.33ע Fig. 1.—Left: 2 cm VLA map of NGC 5253. The map is contoured at levels of 0Љ.12, P.A. = 2 . The peak flux density is 11.8 mJy beamϪ1. The size of the main source, which is located at a =13h39m55.964,s d = Ϫ31Њ38Ј24Љ.38, is 0Љ.1 # 0Љ.04, or 2 pc # 1 pc. Right: 1.3 cm map. The contour levels are as for the left panel (2.5 j for this map). The beam size is0Љ.22 # 0Љ.08 , P.A. = Ϫ8 . The peak flux density is 9.1 mJy beamϪ1. The location of the central source is the same as for the 2 cm source; the eastern secondary peak is at a =13h39m55.98,s d = Ϫ31Њ38Ј24Љ.4. 1.3 cm. Weaker sources appear to the north and east of the 13h39m55.964,s d = Ϫ31Њ38Ј24Љ.38, in agreement with lower res- main radio nebula. The sources appear elongated in the north- olution images (Turner et al. 1998). The absolute positions are south direction, which is mostly due to the north-south elon- known to the ∼0Љ.01 accuracy of the phase calibrator and gation of the synthesized beam. In Figure 2, the 1.3 cm map relative positions to 0Љ.002–0Љ.005, based on the signal-to- is superposed on the Ha image of Calzetti et al. (1997). The noise ratio. The 1.3 cm map has a secondary peak at a = dominant radio source does not have an obvious counterpart 13h39m55.982,s d = Ϫ31Њ38Ј24Љ.37. This peak is not evident at Շ in the Ha image. Given the high (Av 35 mag) extinction 2 cm. There is also either a separate source or an extension of (Calzetti et al. 1997) estimated for the region, it is unlikely the main source northward by ∼1Љ, which is apparent in both that the Ha traces these high-density nebulae at all. maps but stronger at 2 cm. The position of the dominant radio source is a = The main source is slightly resolved. The size of the source, deconvolved from the beam assuming a Gaussian source, is or about , 7 ע0Љ.02(FWHM) at P.A.=27 ע 0Љ.10 # 0Љ.05 2pc# 1 pc for an adopted distance of 4 Mpc (Saha et al. ,mJy beamϪ1 1 ע The peak flux fitted to the core is11 .(1995 with the uncertainty largely due to the difficulty of fitting the compact source properties within the lower level extended emission. The observed brightness temperature is therefore K, close to the value of electron temperature 4000 ע 12,000∽ determined for the visible H ii regions in NGC 5253, which is ∼11,000–12,000 K (Campbell, Terlevich, & Melnick 1986; Walsh & Roy 1989; Kobulnicky et al. 1997). This indicates that the free-free emission has a significant optical depth at 2 cm (t տ 1 ). The total flux estimate for the central 3Љ mapped region is ∼22 mJy at 2 cm, in agreement with lower resolution observations (Beck et al. 1996; Turner et al. 1998). At 1.3 cm, the peak flux density for the main source is mJy beamϪ1. The observed brightness temperature of 1 ע 9 this source is ∼4000 K, given the source size measured at 2 cm. Since the 1.3 cm map may be affected by seeing, the true peak flux density may actually be higher. It may also be that the spectrum is turning over and that the source is becoming optically thin at 1.3 cm.