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On the metallicities of UM 133, UM 283 and UM 382

The study of group properties of the extremely metal-deficient gas-rich local dwarfs is very promising for the understanding the galaxy formation process at high redshifts. About 20 such objects have been picked up from the literature in the recent review by Kunth&Ostlin(2000). However part of these galaxies got low metallicity as a result of earlier observations, and can have rather large uncertainties in their cited element abundances. Before to perform the detailed studies of such galaxies as of some extreme group, it is useful to revise their metallicities. We present the results of the SAO 6m telescope spectrophotometry of two Blue Compact Galaxies (BCG) reported from earlier studies as very metal-poor objects. Well measured [O III] line 4363 A allows to deduce the temperature in HII regions and get reliable abundances of chemical elements. For UM 133 we derive 12+log(O/H) = 7.63+/-0.02, coincident with the published value. UM 382, according to our data, is significantly more metal-rich: its 12+log(O/H) = 7.82+/-0.03 in comparison to the published value 7.45. The third galaxy, UM 283 seems have got its very low 12+log(O/H) = 7.59 due to a misprint. We used its published emission line intensities and derived instead the value of 7.95. Thus the latter two galaxies should NOT be considered as the extremely metal-poor BCGs.


Introduction
Since the discovery of low metallicity gas in H ii regions of Blue Compact Galaxies (BCGs) by Searle & Sargent (1972), a debate lasting for decades on possible existence of local truly young galaxies is still continuing. While the great majority of BCGs have metallicities Z in the range of (1/10 -1/3) Z ⊙ , or respectively 12+log(O/H) in the range 7.92 -8.42 1 , very few galaxies have O-abundance as low as 12+log(O/H) = 7.1-7.6, consistent with the expected oxygen enrichment produced in one star formation (SF) episode. Izotov & Thuan (1999) from the analysis of variations of C and N abundances have shown, that namely the BCGs with 12+log(O/H) ≤ 7.6 are the best candidates to the galaxies with the first SF episode. To clear up the true evolution status of these extremely metal-poor BCGs is a serious challenge for observational astrophysics. However, independently on whether their current SF burst is the first or second one, properties of these local well resolved galaxies best approximate those of primeval lowmass galaxies at high redshifts. Therefore systematic studies of the group properties of this small sample are very important from cosmological point of view.
Kunth &Östlin (2000) (KÖ), in their review of the problems related to the most metal-poor galaxies, summarized published up-to-now data and presented the compilative list of these galaxies. In course of the studies of new large samples of BCGs in a broader context, we attempt to find and study as well new very metal-poor galaxies (e.g., Kniazev et al. 1998, 2000a, 2000b. In parallel we check old measurements for several BCGs, claimed to have very low metallicities. In particular, we reobserved two BCGs presented in the KÖ list -UM 133 and UM 382. Reticon spectrophotometry for these two BCGs is originally from Terlevich et al. (1991) and Masegosa et al. (1994).
In this paper we present a new spectrophotometry with the SAO RAS 6 m telescope for UM 133 and UM 382. For UM 283 with its published emission line intensities we revised its claimed very low O abundance. Hubble constant H 0 = 75 km s −1 Mpc −1 was used through the paper.

Observations and data reduction
Some parameters of the studied galaxies either known from the literature or derived in this paper are presented in Table 1. The spectroscopic data were obtained with the 6 m telescope of the Special Astrophysical Observatory of    Table 2 for details). The Long-Slit spectrograph (LSS) (Afanasiev et al. 1995) at the telescope prime focus was equipped with the Photometrics CCD-detector 1024×1024 pixels with 24 × 24µm pixel size. The long-slit (180 ′′ ) spectra were obtained with the gratings of 325 and 650 grooves/mm with corresponding dispersions of 4.6 and 2.4Å/pixel and a spectral resolution 12-15Å (FWHM) and 7-9Å respectivelty. The wavelength ranges of the obtained spectra for different hardware configurations are given in Table 2. The slit widths of 1.3 ′′ and 2 ′′ were used. The scale along the slit was 0.39 ′′ /pixel in all cases. For UM 133 the slit, centered on the brightest SW knot (see in Fig. 1), was oriented along the main body of the galaxy.
Reference spectra of an Ar-Ne-He lamp were recorded before or after each observation to provide wavelength calibration. Spectrophotometric standard stars from Bohlin (1996) were observed for flux calibration. Observations have been conducted mainly under the software package NICE in MIDAS, described by Kniazev & Shergin (1995).
To be confident that we observed the same parts of the galaxies with the different set-ups in different nights we employed the differential method of pointing the telescope to the specific position in the target galaxy. It consisted of the following steps: 1). the positions of both the specific bright knot in the target galaxy and that for off-set bright enough star at the distance <(1-2) ′ were carefully measured from DSS-2; 2). the position angle of the slit for observations of the target was set; 3). the telescope was pointed on the coordinates of the off-set star; 4). the center of the slit was shifted to the center of this off-set star and the respective correction was saved by the telescope pointing program; 5). finally the telescope was pointed on the coordinates of the specific knot in the target galaxy using this correction. The latter was defined for each observation independently, since it depends on concrete azimuth and zenith distance of the target.
Procedures of primary data reduction included cosmicray removal in MIDAS 2 and bias subtraction and flat-field 2 MIDAS is an acronym for the European Southern Observatory package -Munich Image Data Analysis System. correction in IRAF 3 software packages. For the following reduction of the long-slit spectra we used IRAF. After the wavelength mapping and night sky subtraction, each 2D frame was corrected for atmospheric extinction and was flux calibrated. To derive the sensitivity curves, we used the spectral energy distributions of the standard stars. Average sensitivity curves were produced for each observing night. 2D blue and red parts of spectra were extracted using APALL IRAF task and 2D combined spectrum for each object was created by SCOMBINE task.
Finally, the 1D spectrum was extracted from a region along the slit, where I(λ4363Å) > 2σ (σ is the dispersion of a noise statistics around this line). The size of integrated region was 3.5 ′′ for UM 133 and 3.1 ′′ for UM 382. The 1D extracted spectra of UM 133 and UM 382 are shown in Fig. 1 and Fig. 2.
All spectra of UM 133 and UM 382 obtained during the different observational runs (see Table 2) were used for calculation of chemical element abundances. These values of abundances are consistent within the observational uncertainties for different runs. As final results we used the best quality spectra obtained during October 2000 run.
The redshifts and line fluxes were measured with MIDAS software as described in Kniazev et al. (2000b) and Hopp et al. (2000). The errors of the line intensities have been propagated in the calculations of the element abundances. For the simultaneous derivation of C(Hβ) and EW (abs) and correction for extinction we used the procedure described in detail by Izotov et al. (1997). The high S/N ratio 6 m spectra permit to derive the element abundances with a higher precision than in the previous  Gallego et al. (1996).
studies. The abundances of the ionized species and the total abundances of O, Ne and Ar also have been obtained following the procedure detailed in Izotov et al. (1994), Thuan et al. (1995), Izotov et al. (1997. The abundance of O 3+ ion is derived following to Izotov & Thuan (1999) with use of the intensity of He ii λ4686 line. The observed emission line intensities F (λ), and those corrected for the interstellar extinction and underlying stellar absorption I(λ) are presented in Table 3. The observed Hβ equivalent width EW (Hβ), absorption Balmer hydrogen lines equivalent widths EW (abs), Hβ flux and the extinction coefficient C(Hβ) are also shown in Table 3.

Results
The results of the chemical abundance determination for the studied galaxies are presented in Table 4. The comparison of the respective data for Ne abundance for UM 133 and UM 382 with the abundance ratios from Izotov & Thuan (1999) shows that they well agree with the derived average values for the sample of low metallicity BCGs.

UM 133
The galaxy is elongated and resembles on morphology a comet-like object. In fact, according to NED, UM 133 is a bright H ii region at the SW edge of Sc galaxy CGCG 412-024. It looks like an edge-on disk, bent on NE edge. The full range of the velocity curve from our 2D spectrum is about 100 km s −1 . This is slightly lower than W 0.2 =122±11 km s −1 -the full width for the 21 cm line of the integrated Hi emission at the level 0.2 of peak (Thuan  Gallego et al. (1997). b ICF is the ionization correction factor for unseen stages of ionization. The expressions for ICFs are adopted from Izotov et al. (1994Izotov et al. ( ). et al. 1999. We derived from our data the systemic radial velocity of host galaxy of 1620±15 km s −1 , which is well consistent with that derived from the integral Hi profile ) and with optical data from Huchra et al. (1999). The radial velocity of UM 133 itself is on our data 1590 km s −1 . The continuous Hα-emission and a smooth velocity distribution along the galaxy body supports its interpretation as a single galaxy (CGCG 412-024) with a bright Hii region (UM 133) at the SW edge.
As it is well seen on 1D spectrum of UM 133, there is blue bump near λ4700Å, characteristic of WR stars. Its detailed quantitative analysis along with other observational data is the subject of a forthcoming paper by Kniazev et al. (2001).