We carry out a systematic search for extremely metal poor ( XMP ) galaxies in the spectroscopic sample of Sloan Digital Sky Survey ( SDSS ) data release 7 ( DR7 ) .
The XMP candidates are found by classifying all the galaxies according to the form of their spectra in a region 80 Å wide around H \alpha .
Due to the data size , the method requires an automatic classification algorithm .
We use k-means .
Our systematic search renders 32 galaxies having negligible [ NII ] lines , as expected in XMP galaxy spectra .
Twenty one of them have been previously identified as XMP galaxies in the literature – the remaining eleven are new .
This was established after a thorough bibliographic search that yielded only some 130 galaxies known to have an oxygen metallicity ten times smaller than the Sun ( explicitly , with 12 + \log ( { O / H ) } \leq 7.65 ) .
XMP galaxies are rare ; they represent 0.01 % of the galaxies with emission lines in SDSS/DR7 .
Although the final metallicity estimate of all candidates remains pending , strong-line empirical calibrations indicate a metallicity about one-tenth solar , with the oxygen metallicity of the twenty one known targets being 12 + \log ( { O / H ) } \simeq 7.61 \pm 0.19 .
Since the SDSS catalog is limited in apparent magnitude , we have been able to estimate the volume number density of XMP galaxies in the local universe , which turns out to be ( 1.32 \pm 0.23 ) \cdot 10 ^ { -4 } ~ { } { Mpc } ^ { -3 } .
The XMP galaxies constitute 0.1 % of the galaxies in the local volume , or \sim 0.2 % considering only emission line galaxies .
All but four of our candidates are blue compact dwarf galaxies ( BCDs ) , and 24 of them have either cometary shape or are formed by chained knots .