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 UGC8837. 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]λλ4959, 5007, Balmer emission lines, but very weak [N ii]λλ6548,6583 and [S ii]λλ6317,6731, which could indicate that they are metalpoor 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 μ0(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 MB ∼ −13.51 mag. The very low oxygen abundances of these two objects confirm the low metallicities of LSB galaxies. Subject headings: galaxies: abundances — galaxies: dwarf — galaxies: evolution — galaxies: ISM — galaxies: irregular galaxies: spiral —