J/ApJ/743/77 Extremely metal-poor (XMP) galaxies in SDSS (Morales-Luis+, 2011) ================================================================================ Systematic search for extremely metal-poor galaxies in the Sloan Digital Sky Survey. Morales-Luis A.B., Sanchez Almeida J., Aguerri J.A.L., Munoz-Tunon C. =2011ApJ...743...77M ================================================================================ ADC_Keywords: Abundances ; Galaxies, spectra ; Photometry, SDSS ; Redshifts Keywords: galaxies: abundances - galaxies: formation - galaxies: starburst - galaxies: statistics - methods: data analysis Abstract: 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{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. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table1.dat 87 32 XMP candidates found by classifying all SDSS/DR7 galaxies according to their spectra around H{alpha} table2.dat 111 129 XMP targets found in the literature -------------------------------------------------------------------------------- See also: II/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 page Byte-by-byte Description of file: table1.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 2 I2 --- Seq Running sequence number 4 A1 --- f_Seq [k] k: known XMP galaxies (the rest are new) 6- 9 A4 --- --- [SDSS] 11- 29 A19 --- SDSS SDSS-DR7 name (JHHMMSS.ss+DDMMSS.s) (1) 31- 34 F4.1 mag gmag SDSS-DR7 g-band magnitude 36- 40 F5.3 --- z SDSS-DR7 redshift 42- 45 F4.2 --- (O/H) Oxygen abundance from SDSS: 12+log(O/H) (2) 47- 50 F4.2 --- (O/H)l ? Oxygen abundance: 12+log(O/H) (3) 52- 69 I18 --- SpObjID SDSS-DR7 unique spectrum identification 71- 87 A17 --- Comm Comment (sketch of galaxy shape) -------------------------------------------------------------------------------- Note (1): SDSS J015809.39+000637.2 is a misprint for SDSS J015809.39-000637.2; corrected at CDS. Note (2): Using the calibration by Pettini & Pagel (2004MNRAS.348L..59P), given in our Equation (4). Note (3): From the literature as listed in Table 2. -------------------------------------------------------------------------------- Byte-by-byte Description of file: table2.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 15 A15 --- Name Galaxy name 16 A1 --- f_Name [k] k: galaxies also identified in this work (Table 1) 18- 19 I2 h RAh Hour of right ascension (J2000) 21- 22 I2 min RAm Minute of right ascension (J2000) 24- 25 I2 s RAs Second of right ascension (J2000) 27 A1 --- DE- Sign of declination (J2000) 28- 29 I2 deg DEd Degree of declination (J2000) 31- 32 I2 arcmin DEm Arcminute of declination (J2000) 34- 35 I2 arcsec DEs Arcsecond of declination (J2000) 37- 40 F4.1 mag gmag ? SDSS-DR7 g-band magnitude 42- 59 I18 --- SpObjID ? SDSS-DR7 unique spectrum identification 61- 64 F4.2 --- (O/H) Oxygen abundance: 12+log(O/H) 66- 90 A25 --- Auth Author's name reference for the oxygen abundance (1) 92-110 A19 --- BibCode Bibcode for the oxygen abundance 111 A1 --- f_BibCode [*] *: not explained in the paper -------------------------------------------------------------------------------- Note (1): KO = the galaxy belongs to the compilation by Kunth & Ostlin 2000A&ARv..10....1K -------------------------------------------------------------------------------- History: From electronic version of the journal ================================================================================ (End) Emmanuelle Perret [CDS] 03-May-2013