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Optical spectroscopy

The MMT optical spectra of SBS 0335-052W were obtained on 1996 January 19 with the Red Channel of the MMT Spectrograph. We used a 1$\times$180 slit; the 300 g/mm grating provides dispersion of 3 Å pixel^-1 and spectral resolution of about 10 Å in first order. To avoid second-order contamination, an L-38 blocking filter was used. The total spectral range was $\lambda$$\lambda$3700-7300 Å. The spectra were rebinned by factor of 2 along the spatial axis, hence, the spatial sampling was 06 pixel^-1. The linear scale is 1 = 263 pc at the distance to SBS 0335-052W of 54.1 Mpc. The total exposure time was 60 min, broken to three exposures of 20 min. The slit was oriented north-south. The seeing during the observations was $\sim$08 and the galaxy was observed at the airmass of 1.25. Standard stars PG 0205+134 and Feige 15 were observed for absolute flux calibration. Spectra of He-Ne-Ar comparison lamps were obtained before and after each observation to provide wavelength calibration.

The Keck II telescope optical spectra of SBS 0335-052W were obtained on 1997 November 10 with Low Resolution Imaging Spectrometer, using the 600g/mm grating which provides a dispersion 1.28 Å pixel^-1 and a spectral resolution of about 4 Å in first order. The total spectral range covered was $\lambda$$\lambda$4100-6800 Å. The slit was 1$\times$180 and oriented in direction of the SBS 0335-052W major axis with position angle P.A. = -80. No binning along the spatial axis has been done, yielding a spatial sampling of 02 pixel^-1. The total exposure time was 90 min, broken to three exposures of 30 min. The seeing during the observations was around 06 and the galaxy was observed at the airmass of 1.25. The standard star GD 50 was observed for absolute flux calibration. The spectrum of a He-Ne-Ar comparison lamp was obtained after observation to facilitate wavelength calibration. The data reduction was carried out at the NOAO headquarters in Tucson using the IRAF⁶ software package. Procedures included bias subtraction, cosmic-ray removal and flat-field correction using exposures of a quartz incandescent lamp. After wavelength mapping and night sky background subtraction each frame was corrected for atmospheric extinction and flux calibrated. The one-dimensional spectra were extracted from each flux-calibrated frame using the APALL routine. The extracted spectra were then co-added, not combined. The cosmic rays hits have been removed manually. To derive the sensitivity curves, we have fitted the observed spectral energy distributions of the standard stars with a high-order polynomial. The MMT spectrum of brightest 1$\times$2 region of SBS 0335-052W is shown in Figure 1. Two-dimensional spectrum obtained with Keck II telescope along the major axis of SBS 0335-052W shows the presence of three emitting regions with the brightest region at the NW side of the galaxy and two fainter SE regions. One-dimensional spectra of each region in apertures 1$\times$2 each are shown in Figure 2. In Figure 3 we show the spatial distribution of the observed flux and equivalent width of H$\alpha$ emission line. The location of regions with spectra presented in Figure 2 is shown by vertical lines. While in the MMT spectrum the [O III] $\lambda$4363 emission line is barely detected and is blended with strong H$\gamma$ emission line, it is evidently present in Keck II telescope spectra of two regions allowing the reliable determination of electron temperature and element abundances.

In Table 1, the observed line intensities and intensities corrected for interstellar extinction for SBS 0335-052W are shown along with extinction coefficient C(H$\beta$), observed flux of the H$\beta$ emission line, the equivalent width EW(H$\beta$) and equivalent width of absorption Balmer hydrogen lines EW(abs). For the lines which have not been detected we show 1$\sigma$ intensity upper limits. To correct for extinction we used the reddening law for our Galaxy by Whitford (1958). The errors of the line intensities listed in Table 1 take into account the noise statistics in the continuum which implicitly includes the uncertainties of data reduction ( flat-field correction, sky subtraction), the errors in placing the continuum and fitting the line profiles with gaussians. These errors have been propagated to calculate element abundances. Skillman et al. (1994) have discussed the uncertainties in the measurements of the line intensities, including different steps of data reduction. However, as it is difficult to take into account all possible uncertainties which may enter during the data reduction process ( Skillman et al. (1994) have taken into account the uncertainties in the flat-field correction, night sky subtraction and flux calibration, but they do not take into account the uncertainties introduced by COMBINE-CRREJECT routine and uncertainties introduced by the extraction of one-dimensional spectra from two-dimensional data due to tilts, distortions and not perfect focusing of telescope and spectrograph ) the best way to estimate real uncertainties is by cross-comparing data taken with different telescopes. The line intensities in the brightest knot derived from the MMT and the Keck telescope observations are in general agreement implying the correctness of the measurements despite that the orientation of slits was different.

To derive the element abundances in SBS 0335-052W we adopted a two-zone photoionized H II region model (Stasinska 1990). The electron temperature, T_e(O III), is derived from the [O III]$\lambda$4363/($\lambda$4959 + 5007) line intensity ratio using five-level atom model, and the electron temperature T_e(O II) from the relation between T_e(O II) and T_e(O III) fitted by Izotov, Thuan & Lipovetsky (1994, hereafter ITL94) for photoionized H II models by Stasinska (1990). The electron temperature for S III ion is obtained using prescriptions by Garnett (1992). The electron number density N_e is derived from the [S II]$\lambda$6717, 6731 lines. The values of T_e(O III), T_e(O II), T_e(S III) and N_e(S II) are shown in Table 2. The ionic and total abundances are derived from Keck II telescope spectra as described by ITL94 and by Izotov, Thuan & Lipovetsky (1997, hereafter ITL97) and are shown in Table 2. However, due to the fact that these spectra cover a small spectral range, we use the intensities of the [O II] $\lambda$3727, [Ne III] $\lambda$3868 and [S II]$\lambda$6717, 6731 emission lines in the MMT spectrum, scaled to Keck II telescope spectra, to derive oxygen, neon and sulfur abundances. The oxygen abundance in both of the brightest regions of SBS 0335-052W is 0.1-0.2 dex smaller than in SBS 0335-052 (Izotov et al. 1997) and is the same as that in I Zw 18 (SK93). The helium abundance derived from He I $\lambda$4471 and $\lambda$5876 emission line intensities is in agreement with He abundances in brighter blue compact galaxies (ITL94, ITL97). The abundances of other elements are close to mean values for extremely metal-deficient galaxies (Thuan, Izotov & Lipovetsky 1995; Izotov et al. 1997; Izotov & Thuan 1998a).