SDSS J121811.0+465501.2: A New Low Surface Brightness Galaxy with Low Metallicity

Abstract

We have serendipitously found a new nearby low surface brightness (LSB) galaxy from the SDSS database. We estimate the oxygen abundance of its H II region SDSS J121811.0+465501.2 from its electron temperature, as well as that of another H II region, SDSS J135440.5+535309.6, located in the 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 of stellar spectra. Their optical spectra show obvious emission lines, i.e., strong [O III] λλ4959, 5007 and Balmer emission lines, but very weak [N II] λλ6548, 6583 and [S II] λλ6317, 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 disk galaxy (almost without a bulge), with a B-band central surface brightness μ 0 (B) of 23.68 mag arcsec -2 and an inclination angle of ∼75° 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. © 2007. The American Astronomical Society. All rights reserved.