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The population of Low Surface Brightness (LSB) galaxies is crucial forunderstanding the extremes of galaxy formation and evolution of the universe.As LSB galaxies are mostly rich in gas (HI), the alpha.40-SDSS DR7 sample isabsolutely one of the best survey combinations to select a sample of them inthe local Universe. Since the sky backgrounds are systematically overestimatedfor galaxy images by the SDSS photometric pipeline, particularly for luminousgalaxies or galaxies with extended low surface brightness outskirts, in thispaper, we above all estimated the sky backgrounds of SDSS images in thealpha.40-SDSS DR7 sample, using a precise method of sky subtraction. Oncesubtracting the sky background, we did surface photometry with the Kronelliptical aperture and fitted geometric parameters with an exponential profilemodel for each galaxy image. Basing on the photometric and geometric results,we further calculated the B-band central surface brightness, mu_{0}(B), foreach galaxy and ultimately defined a sample of LSB galaxies consisting of 1129galaxies with mu_{0}(B) > 22.5 mag arcsec^{-2} and the minor-to-major axisratio b/a > 0.3. This HI-selected LSB galaxy sample from the alpha.40-SDSS DR7is a relatively unbiased sample of gas-rich and disk-dominated LSB galaxies,which is complete both in HI observation and the optical magnitude within thelimit of SDSS DR7 photometric survey. We made analysis on optical and radio21cm HI properties of this LSB galaxy sample. Additionally, we statisticallyinvestigated the environment of our LSB galaxies, and found that up to 92% ofthe total LSB galaxies have less than 8 neighbouring galaxies, which stronglyevidenced that LSB galaxies prefer to reside in the low-density environment.

Low Surface Brightness Galaxies selected from the 40% sky area of the ALFALFA HI survey.I.Sample and statistical properties