SDSS J121811.0+465501.2: a new Low Surface Brightness Galaxy with low metallicity

We serendipitously find a new nearby Low Surface Brightness (LSB) galaxy from SDSS database. We estimate oxygen abundance of its H II region SDSS J121811.0+465501.2 from electron temperature, as well as for another H II region, SDSS J135440.5+535309.6, located in irregular LSB galaxy UGC 8837. These two extragalactic H II regions were classified as stars in the SDSS-DR4 database, and were found occasionally by us in the automatic recognition and classification on stellar spectra.Their optical spectra show obvious emission lines, i.e., strong [O III]4959, 5007, Balmer emission lines, but very weak [N II]6548,6583 and [S II]6317,6731, which could indicate that they are metal-poor star-forming regions. The derived oxygen abundances of the two objects are 12+log(O/H) ~ 7.88+-0.30 and 7.70+-0.30, respectively. The host of the H II region SDSS J121811.0+465501.2 is identified as a new nearly edge-on LSB disc galaxy (almost without bulge) with the B-band central surface brightness mu_0(B) as 23.68 mag arcsec^{-2} and inclination angle as ~75 degree by using the GIM2D software to analyze its g- and r-band images independently. It is a nearby dwarf galaxy with redshift z~0.00157, disk scale-length ~0.40 kpc and B-band absolute magnitude M_B ~ -13.51 mag. The very low oxygen abundances of these two objects confirm the low metallicities of LSB galaxies.