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4.3.1 Chemical abundances of LSBGs

Despite their faintness, many LSBG contain H II regions and the metallicity of LSBGs have been investigated through the derivation of nebular oxygen abundances. McGaugh (1994) used an empirical determination of the oxygen abundance to find values of 12 + log(O/H) ranging from 7.3 to 8.8, for individual H II regions with a strong peak around 8.4. There was often considerable scatter between H II regions in the same galaxies. Taking weighted averages, eight galaxies have O abundances less than 1/10 of the solar value, ranging down to ~ 1/15, and the absolute B magnitudes of the metal-poor (leq 10% of solar) subsample range from -15.8 to -20.3 (for H0 = 75 km/s/Mpc).

Similarly, Rönnback & Bergvall (1995) found that a sample of LSBGs with MB = -14 to -18.5, and selected to have blue colours, all had low metallicities. Typical oxygen abundances are around 1/10 of solar, extending down below 1/20. They also derive N/O values. Five out of 13 galaxies had an oxygen abundance below 10% of solar, e.g. the remarkable edge-on galaxy ESO 146-G14 (MB = -16.6) with 12 + log(O/H) = 7.6 (see also Bergvall and Rönnback 1995). Several of the galaxies in these two samples fall below the metallicity-luminosity relation for dIs, see Fig. 10.

Van Zee et al. (1997b, c), for a sample of quiescent LSBGs/dIs derive oxygen abundances ranging from 12 + log(O/H) = 7.6 to 8.3, matching perfectly the MB - Z relation for dIs (Fig. 10). A massive neutral hydrogen cloud was found in the Virgo cluster, by Giovanelli and Haynes (1989). It has an optical low surface brightness counterpart, with some central faint H II regions from which Salzer et al. (1991) derived 12 + log(O/H) = 7.66. De Blok and van der Hulst (1998) found no very metal-poor LSBGs in their investigation, but three galaxies examined for abundance gradients were found to have none. This together with small or non existent colour gradients (Rönnback & Bergvall 1994, de Blok et al. 1995, Patterson and Thuan 1996) suggests that the stellar populations are spatially homogeneous in age and metallicity. This is supported also by recent near infrared surface photometry (Bergvall et al. 1999).

There is only a weak correlation between metallicity and absolute blue luminosity (McGaugh 1994, and Bergvall & Rönnback 1994), see however Fig. 10. Metallicity and surface brightness are related in the sense that LSBGs have lower than average abundances for their absolute magnitudes as compared to high surface brightness galaxies. However, neither investigation found any strong correlation between surface brightness and oxygen abundance.

Some LSBGs have a too low surface brightness to be observable spectroscopically at the time of these studies. The new generation of 8-10m class telescopes may be used to investigate if late type galaxies with extremely low surface brightness are even more metal-poor. Also, new surveys for extremely low surface brightness galaxies may yield many new interesting metal-poor candidates, though abundances may be very difficult to obtain if the galaxies do not have H II regions. The most metal-poor LSBGs are included in Table 3.

Table 3. The most metal-poor galaxies. This Table contains all galaxies with 12 + log(O/H) < 7.65 that we in found the literature. The first column gives the most common name that is also recognized by NED. Alternative names are given below in some cases. A broad galaxy classification is given in column two. The coordinates are from NED (except for HS 0822+3542 and CS 0953-174). The heliocentric radial velocities in column 5 come mainly from NED, but we also quote the reference given by NED. Preference was given to H I-velocities if available. The sixth column gives 12 + log(O/H), where uncertain values based on empirical methods (not utilising [O III]lambda 4363) are marked with colon. Values marked by asterisks represent a weighted average of values from different H II regions and/or authors. The seventh column gives the absolute B-magnitude, from direct distance measurements or from radial velocity and H0 = 75 km/s/Mpc. Values for MB based on other values of H0 were rescaled. A colon indicates that the magnitude is uncertain, either beacuse it is based on photographic data or because from other passband than B. The references are given in the footnotes.

Table 3
Alternative names: a UCM 0049+001; b SBS 0335-052; c Markarian 116; d KUG 0940+544; e HS 1013+3809; f CG 1116+51; g Tol 1223-359, ESO 380-G27; h RMB 132, VCC 1313; i UGC 4459, VII Zw 238; j UGC 912; k ESO 594-G4;
References: 1: Kniazev et al. (1999, in prep.), Masegosa private comm. 2: Merlino et al. (1999). 3: Kniazev et al. (1998). 4: Izotov and Thuan (1999). 5: Bergvall et al. (1999). 6: Rönnback and Bergvall (1995). 7: Gallego et al. (1997). 8: Vitores et al. (1996), assuming B - R = 0.6. 9: Masegosa et al. (1994). 10: Telles and Terlevich (1997). 11: Salzer et al. (1989a). 12: Lauberts and Valentijn (1989). 13: magnitude taken from NED. 14: Lipovetsky et al. (1999). 15: Papaderos et al. (1998). 16: Mazarella and Boroson (1993). 17: Kinman and Davidson (1981). 18: Young and Currie (1998). 19: French (1980). 20: Tikhonov and Karachentsev (1993). 21: RC3 (de Vaucoleurs et al. 1991). 22: Skillman et al. (1989). 23: Skillman et al. (1994). 24: Mateo (1998). 25: Arp and O'Connell (1975). 26: Estimated from continum flux in Terlevich et al. (1991), mB = 18.5 ± 1, very uncertain. 27: Galaxy from the Cambridge survey. No data in litterature on magnitude, nor accurate coordinates. 28: van Zee et al. (1997c). 29: Velocity from the NED-team. 30: Masegosa private communication. 31: Terlevich et al. (1991). 32: Gordon and Gottesman (1981). 33: Pustilnik et al. (1999b). 34: Augarde et al. (1994). 35: Pustilnik et al. (1995). 36: Bergvall and Rönnback (1995) shows that if effects of shocks are taken into account O/H may be higher (12 + log(O/H) = 7.68). 37: Kinman and Hintzen (1981). 38: Mathewson and Ford (1996). 39: Gallagher et al. (1995). 40: Matthews and Gallagher (1996). 41: van Zee et al. (1997a). 42: van Zee et al. (1996). 43: van Zee et al. (1997b).

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