1 Introduction

Blue compact dwarf (BCD) galaxies are characterized by ongoing intense star formation (SF) activity as shown by strong nebular emission lines superimposed on a blue continuum (e.g. Sargent & Searle 1970; Lequeux et al. 1979; Kunth & Sargent 1983; Izotov et al. 1994). They are compact and metal-deficient objects with oxygen abundances ranging between 1/50 and 1/3 solar (e.g. Izotov & Thuan 1999) and possess large amounts of neutral gas (Thuan & Martin 1981; van Zee et al. 1998; Thuan et al. 1999b).

Most of the BCDs studied so far have revealed extended low surface brightness red stellar sheets underlying the blue star-forming regions (Thuan 1983; Loose & Thuan 1986). Some of these stellar sheets have been resolved with the Hubble Space Telescope (HST) into red giant stars (Schulte-Ladbeck; Crone & Hopp 1998; Crone et al. 2000). This indicates the presence of an evolved low-mass stellar population in the underlying host galaxies.

The tiny fraction of galaxies with extremely low oxygen abundance in the ionized gas (less than 1/20 (O/H)$_{\odot}$) has been suggested by Izotov & Thuan (1999) to be young galaxies, based on chemical element abundance arguments. Nearly ten such galaxies with good abundance determinations are known to date. The outer parts of these galaxies are blue. The spectral energy distributions and colours of their outskirts are consistent with those of stellar populations with ages less than a few 100 Myr (see the examples of SBS 0335-052 (Izotov et al. 1997b; Thuan et al. 1997; Papaderos et al. 1998) and SBS 1415+437 (Thuan et al. 1999a)).

Detailed spectroscopic and photometric studies of extremely metal-poor BCDs based on observations with large telescopes are useful to shed light on the origin of these galaxies, deduce their star formation history and constrain their ages.

A good candidate for a young galaxy is SBS 0940+544. This galaxy was discovered in the course of the Second Byurakan Survey (SBS, Markarian et al. 1983) and described as a compact extragalactic H II region without an underlying host galaxy, with strong emission lines, a weak continuum and an ultraviolet excess.

A first abundance determination in the brightest H II region of SBS 0940+544 was made by Izotov et al. (1991) who derived an oxygen abundance 12 + log(O/H) = 7.52 $\pm$ 0.12 and a helium mass fraction Y = 0.246 $\pm$ 0.030. Izotov et al. (1997c), using higher S/N ratio observations, subsequently derived a lower value of 12 + log(O/H) = 7.43 $\pm$ 0.01. Because of its very low metal content, SBS 0940+544 was used for the determination of the primordial helium abundance (Izotov et al. 1994; Izotov & Thuan 1998b) and to study heavy element abundances (Thuan et al. 1995; Izotov & Thuan 1999). The galaxy also has been studied spectroscopically by Augarde et al. (1994) and Comte et al. (1994), without an element abundance determination.

B and R surface photometry and colour profiles of SBS 0940+544 have been derived by Doublier et al. (1997) (see also Doublier et al. 1999).

Correcting the 21 cm heliocentric velocity V = 1638 km s^-1 for Virgocentric infall with a velocity of 220 km s^-1 and adopting a Hubble constant of 75 km s^-1 Mpc^-1, Thuan et al. (1999b) derived a distance of D = 26.7 Mpc for SBS 0940+544. At this distance, assumed here throughout, 1 $^{\prime \prime }$ corresponds to a projected linear scale of 129 pc. The total neutral hydrogen H I mass in SBS 0940+544 is log(M(H I)/$M_\odot$) = 8.71 (or M(H I) = 5.1 $\times$ 10⁸ $M_\odot$) (Thuan et al. 1999b).

Figure 1: a) V image of the cometary iI BCD SBS 0940+544. The overlayed contours correspond to surface brightness levels between 23.5 mag arcsec-2 and 27 mag arcsec-2 in steps of 0.5 mag. The inset at the upper right shows a deconvolved 13 $^{\prime \prime }$ $\times$ 11 $^{\prime \prime }$ subframe at the northern tip of the galaxy. Adjacent to the brightest H II region a is a chain of other compact sources (labeled b traceable to $\sim$10 $^{\prime \prime }$ northeast of a. From the continuum-subtracted H$\alpha$ map of SBS 0940+544 (right panel) and from the spectroscopic study (Sect. 4), star-formation activity is found to be also present close to the optical maximum of the LSB body of the BCD (labeled c). The average V-R and V-I colours of the main body increase from $\sim$0.16 mag and $\sim$0.43 mag in region c to $\sim$0.34 mag and $\sim$0.65 mag in region d. b) Continuum-subtracted H$\alpha$ map of SBS 0940+544 overlayed with V band contours between 21.5 mag arcsec-2 and 26 mag arcsec-2 in steps of 0.5 mag. Roughly 80% of the H$\alpha$ emission originates from the compact H II region a. The long-slit positions during the two observations with the Multiple Mirror Telescope (MMT #1 and MMT #2) and with the KeckII telescope (Keck #1) are indicated.

In this paper we present new V,R,I and H$\alpha$ photometry and very high signal-to-noise spectroscopic observations of SBS 0940+544. We use those data to study the stellar populations in the BCD, put constraints on its age and derive elemental abundances in the ionized gas, including the helium abundance. In Sect. 2 we describe the observations and data reduction. The photometric properties of SBS 0940+544 are discussed in Sect. 3. We derive elemental abundances in the brightest H II region in Sect. 4. In Sect. 5 the stellar populations are discussed and constraints on the age of SBS 0940+544 are considered. Section 6 summarizes the results.