LOW SURFACE BRIGHTNESS GALAXIES SELECTED FROM THE 40% SKY AREA OF THE ALFALFA H I SURVEY. I. SAMPLE AND STATISTICAL PROPERTIES

The population of low surface brightness (LSB) galaxies, which are objects with central surface brightnesses at least one magnitude fainter than the night sky, is crucial for understanding the extremes of galactic formation and evolution of the universe. As LSB galaxies are mostly rich in gas (H I), the a. 40 Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) sample is one of the best survey combinations to select a sample of them in the local universe. Since the sky backgrounds are systematically overestimated for galactic. images by the SDSS photometric pipeline, particularly for luminous galaxies or galaxies with extended LSB outskirts, in this paper, we above all estimated the sky backgrounds of SDSS images accurately in both the g and r bands for each galaxy in the alpha.40 SDSS DR7 sample, using a precise method of sky subtraction. Once subtracting the sky background, we did surface photometry with the Kron elliptical aperture using. the SExtractor software and fitted geometric parameters with an exponential profile model using. the Galfit software for each galactic image in the alpha.40 SDSS DR7 sample. Based. on the photometric and geometric results, we further calculated the B-band central surface brightness, mu(0)(B), for each galaxy and ultimately defined a sample of LSB galaxies consisting of 1129 galaxies with mu(0)(B) > 22.5 mag arcsec(-2) and the axis ratio b/a > 0.3 from the 12,423 alpha.40 SDSS DR7 galaxies. This H I-selected sample of LSB galaxies is a relatively unbiased sample of gas-rich and disk-dominated LSB galaxies, which is complete both in H I observation and the optical magnitude within the limit of the SDSS DR7 photometric survey. This LSB galactic sample spans from 22.5 to 28.3 in mu(0)(B) with a fraction of 4% fainter than 25.0 mag arcsec(-2) in B-band central surface brightness. and distributes from -27.0 to -12.3 mag in absolute magnitude in the B band (M(B)), including the 43 faintest galaxies (M(B) > -17.3 mag). This sample is a. blue LSB galactic sample, of which 98% are bluer than 0.75 mag in B-V color. As for 21 cm H I properties, our LSB galactic sample has a median M-H I/L-B = 0.87, and a large portion (95%) has high mass of HI (M(H I) > 10(7.7)M circle dot), which supports galaxies in this LSB galactic sample being. mostly gas-rich. Additionally, we statistically investigated the environment of our LSB galaxies. and found that up to 92% of the total LSB galaxies have less than eight. neighboring galaxies, which provides strong. evidence. that LSB galaxies prefer to reside in the low-density environment.