NGC 3125−1: the Most Extreme Wolf-Rayet Star Cluster Known in the Local Universe1

NGC 3125−1: The Most Extreme Wolf-Rayet Star Cluster Known in the Local Universe1 Rupali Chandar and Claus Leitherer Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218 [email protected] & [email protected] and Christy A. Tremonti Steward Observatory, 933 N. Cherry Ave., Tucson, AZ, 85721 [email protected] ABSTRACT We use Space Telescope Imaging Spectrograph long-slit ultraviolet spectroscopy of local starburst galaxies to study the massive star content of a representative sample of \super star" clusters, with a primary focus on their Wolf-Rayet (WR) content as measured from the He II λ1640 emission feature. The goals of this work are three-fold. First, we quantify the WR and O star content for selected massive young star clusters. These results are compared with similar estimates made from optical spectroscopy and available in the literature. We conclude that the He II λ4686 equivalent width is a poor diagnostic measure of the true WR content. Second, we present the strongest known He II λ1640 emission feature in a local starburst galaxy. This feature is clearly of stellar origin in the massive cluster NGC 3125-1, as it is broadened (∼ 1000 km s−1). Strong N IV] λ1488 and N V λ1720 emission lines commonly found in the spectra of individual Wolf-Rayet stars of WN subtype are also observed in the spectrum of NGC 3125-1. Finally, we create empirical spectral templates to gain a basic understanding of the recently observed strong He II λ1640 feature seen in Lyman Break Galaxies (LBG) at redshifts z ∼ 3. The UV field observed in local starbursts provides a good overall match to the continuum and weak photospheric 1Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS5-26555. { 2 { features in LBGs in the spectral range λλ1300 − 1700, but cannot reproduce the He II λ1640 emission seen in the composite LGB sample of Shapley et al. An additional (ad hoc) 10-15% contribution from \extreme" Wolf-Rayet clusters similar to NGC 3125-A on top of the field provides a good match to the strength of this feature. Subject headings: galaxies: individual (NGC 3125) | galaxies: starburst | galaxies: star clusters { galaxies: stellar content 1. INTRODUCTION Understanding the formation and evolution of starbursts is a problem of fundamental importance to astrophysics. Massive bursts of star formation quickly enrich the host with metals, as gas processed in the interiors of massive stars is returned to the ISM by super- novae. Hubble Space Telescope ultraviolet/optical imaging has resolved the sites of active star formation in local (z < 0:1) UV-bright starbursts into numerous compact \super star" clusters (SSCs) contributing ∼ 20% of the total UV light, and extended diffuse UV “field” light, which makes up the remaining 80% (e.g., Meurer et al. 1995). A subset of starburst galaxies show signatures of Wolf-Rayet (WR) stars, which are the evolved descendants of O stars more massive than M > 20 − 30M , and are characterized by high mass loss from fast, dense stellar winds. These may be ideal laboratories for studies of starburst properties, since they are known to harbor the most massive known stars (O stars and their evolved WR descendants), allowing us to probe the youngest stellar populations. The use of the UV spectral range (to study O and WR stars) has a number of advantages over similar studies conducted using optical spectroscopy: i) the massive stars which we are probing are the direct contributors to the UV continuum, which is essentially unaffected by an underlying stellar population, unlike the situation in the optical; ii) the UV spectral region gives more precise ages; iii) WR emission line equivalent widths (EW) in the UV are undiluted by an underlying stellar population; iv) in the UV, it is possible to measure the extinction intrinsic to the starburst directly from the same stars which contribute to the line flux, rather than inferring it from the nebular emission features; and v) a better quantification of the rest-frame UV is essential to understand the properties of high redshift galaxies. Rest-frame UV observations of high redshift galaxies are now beginning to directly reveal the dominant stellar populations forming in the early universe. One specific example is the discovery of strong, broad He II λ1640 emission in the composite spectrum of hundreds of { 3 { z ∼ 3 Lyman Break Galaxies (LBG) (e.g., Shapley et al. 2003). Broad stellar He II emission is primarily produced in fast, dense winds from WR stars. However, when compared with observations of local starbursts (e.g., Heckman et al. 1998), the He II λ1640 emission in the high redshift galaxy composite is significantly stronger. We are conducting an extensive study of the stellar content of local starburst galaxies, using longslit HST spectroscopy in the UV. This allows us to probe the properties of individual clusters, thus isolating single age stellar populations, and relieving concerns arising when dealing with mixed age populations. In this work, we provide a census of the massive (O and WR) star content of a representative sample of SSCs in local starburst galaxies. In order to study the WR content in individual massive young star clusters formed in starburst galaxies, we focus on the He II λ1640 emission feature, which is observed in a number of objects. This paper is organized as follows: section 2 describes sample selection, data reduction, and basic measurements of the stellar content of our SSCs; section 3 compiles properties of individual Galactic and Large Magellanic Cloud (LMC) WR stars for comparison with stellar clusters; section 4 estimates the number of O and WR stars, and compares with existing optical data to form a coherent picture of the massive star content in individual SSCs; in section 5 we use empirical templates created from different local stellar populations (e.g., individual clusters at different ages, stellar field) to understand the composite rest-frame UV spectra of high redshift LBGs from the Shapley et al. (2001b) sample. Finally, in section 6 we present a summary and conclusions. 2. SAMPLE, DATA REDUCTION, AND MEASUREMENTS 2.1. Local starburst galaxy sample As part of a larger project to quantify the stellar and interstellar properties of local galaxies undergoing active star formation (Chandar et al. 2004, in prep.), we have obtained HST STIS long-slit far- and near- ultraviolet spectra for 15 local starburst galaxies; further information on the program can be found under proposal 9036 at http://www.stsci.edu.edu/public/propinfo.html. Target galaxies were selected to cover a broad range of morphologies, chemical composition, and luminosity. Most importantly, target selection was not based on WR content. We utilized the 5200 × 0:200 slit (projected onto the 25002 MAMA detector) and the G140L and G230L gratings to obtain the best compromise between throughput and spectral resolution. This combination of gratings provides continuous wavelength coverage from 1175 to 3100 A,˚ with a velocity resolution of 100 − 200 km s−1. The STIS MAMA detectors have a plate −1 scale of 0:02400 pix−1, and average pixel scales of 0:584 A˚ pix for the FUV/G140L and −1 1:548 A˚ pix for the NUV/G230L gratings. All target galaxies have been chosen to be { 4 { dominated by star formation; the nuclear starbursts in NGC 6764 and NGC 5102 show the clear signature of massive stars, with no evidence for Seyfert/Liner activity in the UV. To this sample we add three starburst galaxies previously observed with STIS: NGC 3049 (observed with the 0:500 slit), NGC 5253 (0:100 slit), and Tololo89 (hereafter Tol89; 0:500 slit) and available in the HST data archive. I Zw18 which was studied by Brown et al. (2002) is not included due to the low S/N of the observations. Basic information on each galaxy used for this study, such as distance, abundance and foreground reddening, as well as the physical area covered by the long-slit observations, are presented in Table 1. When available, we use direct distances from HST observations (e.g., tip of the red giant branch). The remaining distances were also assembled from the literature, but were determined from a variety of techniques. In this work we focus on data taken in the FUV, since the best diagnostics of stellar properties are found in this spectral region. Each of the starburst galaxies contains a number of individual SSCs, which are clearly visible on available HST WFPC2 and FOC images. We focused on bright SSCs in each galaxy by constraining the pointing and orientation of the STIS slit. Here we briefly describe target selection (we have concentrated on luminous clusters in this work; a future paper will present the entire cluster catalog). In NGC 5102 and NGC 6764 the nuclear starbursts were observed; in NGC 3310 the slit included a giant extra-galactic HII region, and in NGC 3049, NGC 4214, and NGC 5253 we study the UV dominant clusters. Starburst regions A in NGC 3125 and He 2-10 are included in this study. Targeting for the rest of the galaxies in our sample was made primarily from HST FOC UV imaging, where UV luminous, compact SSCs were included, with the goal of aligning the slit to cover as many such objects as possible. Cluster coordinates, ages and extinctions intrinsic to the starburst (age and extinction derivation are described below) are given in Table 2. For the purposes of this study, we wish to isolate coeval events of star formation.

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