Astron. Astrophys. 363, 493-506 (2000)

1. Introduction

SBS 0335-052 (Izotov et al. 1990) is a Blue Compact Dwarf (BCD) galaxy that, at an abundance of (Melnick et al. 1992; Izotov et al. 1997), is the second lowest metallicity galaxy known after I Zw 18. SBS 0335-052 hosts an exceptionally powerful episode of star formation, and Thuan et al. (1997) proposed, on the basis of HST observations, that the present burst is possibly the first star-forming episode in the history of the galaxy. The same authors deduce that the star formation occurs in six Super-Star Clusters (SSCs) not older than 25 Myr and located within a region smaller than 2", or 520 pc at a distance of 54.3 Mpc that we will assume in this paper ( km/s, see Izotov et al. 1997). Though the presence of dust mixed with the star forming clusters is evident from the HST images, the ISO observation by Thuan et al. (1999) of a very strong continuum at m was a surprise. This was successfully modeled with a modified black body and a screen extinction of , and interpreted as due to thermal emission by silicate dust (M ) warmed by embedded clusters of star formation completely invisible in the optical.

Because of the lack of chemical enrichment, low-metallicity BCDs such as SBS 0335-052 are also hypothetical candidates for young or primeval galaxies (PGs) undergoing their first burst of star formation. While the majority of BCDs show an underlying extended low-surface-brightness component with colors indicative of an intermediate-age or old stellar population (Loose & Thuan 1986; Kunth et al. 1988; Papaderos et al. 1996; Telles & Terlevich 1997), there is still debate whether such a component has been detected in the two most metal-deficient BCDs presently known, I Zw 18 (/50, Searle & Sargent 1972) and SBS 0335-052. High-resolution HST optical images of I Zw 18 (Hunter & Thronson 1995) show colors of the underlying diffuse component consistent with a sea of unresolved B or early A stars, with no evidence for stars older than a few tens of Myr, although Aloisi et al. (1999) and Östlin (2000) have claimed to detect a 1-4 Gyr stellar population in that BCD. Similar observations of SBS 0335-052 (Thuan et al. 1997) show colors much bluer than those expected for such an intermediate-age stellar population. The optical evidence is only suggestive, however, since a large fraction of the extended emission in SBS 0335-052 is of gaseous origin (Thuan et al. 1997).

Near-infrared (NIR) colors can help resolve the age question, since they are extremely effective indicators of stellar population age, given the metallicity . While NIR colors of evolved stellar populations vary little (Griersmith et al. 1982; Frogel 1985; Giovanardi & Hunt 1988, 1996), recent evolutionary synthesis models (Leitherer et al. 1999 - SB99; Krüger et al. 1995) show that the NIR colors of young stellar populations differ significantly from those of older ones. Both models find that longward of 1 µm, the nebular continuum dominates the emission during the early phases (a few Myr) of the starburst. Therefore NIR colors provide a unique and effective diagnostic, and when combined with optical colors, are good indicators of stellar population age.

NIR spectroscopy has a three-fold importance for our analysis. First, it enables the estimate of the nebular contribution to the emission in order to decontaminate the broadband colors. Second, it is an almost extinction-free probe of the starburst parameters in what are typically dusty environments. Finally, it offers unique constraints on the star-formation history of the galaxy. In particular the equivalent width of is possibly the best age indicator for young starbursts since it measures the ratio between young blue massive stars and the evolved red stellar population. The NIR CO absorption features are the strongest signature of cold evolved stars, as are the He ricombination lines for hot massive stars, and [FeII] for supernova remnants. Finally the infrared is the only spectral region where the molecular gas can be detected in its warm phase through emission lines.

It is therefore of crucial importance to further investigate SBS 0335-052 in the near-infrared to study the stellar populations in this extremely low-metallicity system, as well as investigate the putative hidden star formation suggested by the large extinction derived from the mid-infrared spectral energy distribution. With this aim we have obtained NIR spectra in the H and K bands, and broadband images in J, H, K, and Ks. We present here the results of our new observations. The SBS 0335-052 system consists of two widely separated (22 kpc in the East-West direction) star-forming components within a large 64 21 kpc HI cloud (Pustilnik et al. 2000). We denote the eastern component by SBS 0335-052, and the western one (Lipovetsky et al. 1999) by SBS 0335-052W.

The present paper is organized as follows. In Sect. 2 we present our data along with a description of the observations and data reduction. Sect. 3 is a description of the NIR spectrum and of the properties of the galaxy as can be derived from the analysis of the spectrum. The same is done in Sect. 4 for the near-infrared images. The characteristics of the galaxy derived from the observations are combined with data from the literature, and then used to constrain a starburst model in Sect. 5. In Sect. 6 we summarize the results of our work.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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