J/ApJ/743/77ivo://CDS.VizieR/J/ApJ/743/77doi:10.26093/cds/vizier.17430077CDSMorales-Luis A.B.Sanchez Almeida J.Aguerri J.A.L.Munoz-Tunon C.2017-11-29T11:47:58Z2013-06-13T13:44:03ZCDS support team

CDS, Observatoire de Strasbourg, 11 rue de l'Universite, F-67000 Strasbourg, France

cds-question@unistra.frchemical-abundancesgalaxiesspectroscopyvisible-astronomysloan-photometryredshiftedWe 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{AA} 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 11 are new. This was established after a thorough bibliographic search that yielded only some 130 galaxies known to have an oxygen metallicity 10 times smaller than the Sun (explicitly, with 12+log(O/H)<=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 21 known targets being 12+log(O/H)~7.61+/-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+/-0.23)x10^-4^Mpc^-3^. The XMP galaxies constitute 0.1% of the galaxies in the local volume, or ~0.2% considering only emission-line galaxies. All but four of our candidates are blue compact dwarf galaxies, and 24 of them have either cometary shape or are formed by chained knots.2011ApJ...743...77Mhttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/ApJ/743/77CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toII/294 : The SDSS Photometric Catalog, Release 7 (Adelman-McCarthy+, 2009)J/A+A/546/A122 : SDSS XMP emission-line galaxies (Izotov+, 2012)J/ApJ/695/259 : Massive metal-poor galaxies from SDSS (Peeples+, 2009)J/ApJ/662/15 : Abundances in extragalactic HII regions (Izotov+, 2007)J/A+A/448/955 : Abundances of emission galaxies in SDSS-DR3 (Izotov+, 2006)J/ApJ/636/214 : HII regions in dwarf irregular galaxies (van Zee+, 2006)J/ApJS/161/240 : High-ionization emission in metal-deficient BCD (Thuan+, 2005)J/ApJ/602/200 : HII regions abundances in BCD galaxies (Izotov+, 2004)J/A+A/421/539 : VLT spectroscopy of Tol 1214-277 + Tol 65 (Izotov+, 2004)http://www.sdss.org/ : SDSS home pagehttps://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/ApJ/743/77https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/ApJ/743/77http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/ApJ/743/77https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/ApJ/743/77https://vizier.iucaa.in/viz-bin/votable?-source=J/ApJ/743/77http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/ApJ/743/77GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/ApJ/743/77/table1 (XMP candidates found by classifying all SDSS/DR7 galaxies according to their spectra around H{alpha})https://vizier.cds.unistra.fr/viz-bin/conesearch/J/ApJ/743/77/table1?https://vizier.iucaa.in/viz-bin/conesearch/J/ApJ/743/77/table1?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/ApJ/743/77/table1?GETtext/xml+votable180.050000true9.12666670.87627780.005555555555555556Cone search capability for table J/ApJ/743/77/table2 (XMP targets found in the literature)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/ApJ/743/77/table2?https://vizier.iucaa.in/viz-bin/conesearch/J/ApJ/743/77/table2?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/ApJ/743/77/table2?GETtext/xml+votable180.050000true0.093958339.49541670.0055555555555555566/393 684 2130 2622 4149 4251 4667 4707 4948 4975 4983 5091 5116 5283 5287 5359 5520 5617 5646 5754 5924 5964 5983 6010 6072 6115 6419 6619 7208 7251 7281 7769 7799 7946 8028 8897 8914 9060 9112 9182 9290 9362 9630 10399 10494 10587 10666 10689 10722 10762 10811 10847 10881 10999 11041 11137 11177 11178 11403 16560 16687 17491 17494 17558 17751 17816 17832 17857 18004 18010 18011 18019 18041 18066 18072 18077 18081 18091 18093 18198 18235 18392 18772 18844 18853 18858 19051 19086 19112 19115 19172 19417 19570 20008 20476 20605 22890 23078 23188 23190 23204 23206 24696 25594 25851 25916 26165 26858 27090 27199 27351 27409 27418 27620 27654 27765 27775 27926 27943 28500 29699 30039 32402 32440 34430 34544 35452 35990 36861 39602 40303 43754 44710 45518 47080https://cdsarc.cds.unistra.fr/viz-bin/moc/J/ApJ/743/77?format=asciiOpticaldefaultJ/ApJ/743/77/table1XMP candidates found by classifying all SDSS/DR7 galaxies according to their spectra around H{alpha}SimbadNameSimbad column added by the CDSmeta.idcharrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintSeqRunning sequence numbermeta.id;meta.mainintf_Seq[k] k: known XMP galaxies (the rest are new)meta.codecharSDSSSDSS-DR7 name (JHHMMSS.ss+DDMMSS.s) (1)meta.idchargmagSDSS-DR7 g-band magnitudemagphot.mag;em.opt.BfloatzSDSS-DR7 redshiftsrc.redshiftfloat(O/H)Oxygen abundance from SDSS: 12+log(O/H) (2)phys.abundfloat(O/H)l? Oxygen abundance: 12+log(O/H) (3)phys.abundfloatnullableSpObjIDSDSS-DR7 unique spectrum identificationmeta.idCommComment (sketch of galaxy shape)meta.notecharDR7Display details from SDSS-DR7 photometric catalogmeta.ref.urlcharSloanDisplay the SDSS data of the closest objectmeta.ref.urlcharNEDSimbad column added by the CDSmeta.refchar_RAPositions from SDSS names (right ascension part)degpos.eq.ra;meta.main_DEPositions from SDSS names (declination part)degpos.eq.dec;meta.mainJ/ApJ/743/77/table2XMP targets found in the literatureSimbadNameSimbad column added by the CDSmeta.idcharrecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintNameGalaxy namemeta.id;meta.maincharf_Name[k] k: galaxies also identified in this work (Table 1)meta.codecharRAJ2000Hour of right ascension (J2000)pos.eq.raDEJ2000Degree of declination (J2000)pos.eq.decgmag? SDSS-DR7 g-band magnitudemagphot.mag;em.opt.BfloatnullableSpObjID? SDSS-DR7 unique spectrum identificationmeta.idnullable(O/H)Oxygen abundance: 12+log(O/H)phys.abundfloatAuthAuthor's name reference for the oxygen abundance (1)meta.bib.authorcharBibCodeBibcode for the oxygen abundancemeta.bib.bibcodecharf_BibCode[*] *: not explained in the papermeta.codecharDR7Display details from SDSS-DR7 photometric catalogmeta.ref.urlcharSloanDisplay the SDSS data of the closest objectmeta.ref.urlcharNEDSimbad column added by the CDSmeta.refchar_RAPositions from SIMBAD database (if more accurate) or SDSS full names or this reference (right ascension part)degpos.eq.ra;meta.main_DEPositions from SIMBAD database (if more accurate) or SDSS full names or this reference (declination part)degpos.eq.dec;meta.main