J/ApJ/770/16ivo://CDS.VizieR/J/ApJ/770/16doi:10.26093/cds/vizier.17700016CDSKirby E.N.Boylan-Kolchin M.Cohen J.G.Geha M.Bullock J.S.,Kaplinghat M.2017-12-15T10:55:25Z2015-01-28T08:31:57ZCDS support team

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

cds-question@unistra.frmetallicityradial-velocityvisible-astronomysloan-photometryspectroscopygalaxiesSegue 2, discovered by Belokurov et al. (2009, Cat. J/MNRAS/397/1748), is a galaxy with a luminosity of only 900L_{sun}_. We present Keck/DEIMOS spectroscopy of 25 members of Segue 2 - a threefold increase in spectroscopic sample size. The velocity dispersion is too small to be measured with our data. The upper limit with 90% (95%) confidence is {sigma}_v_<2.2(2.6)km/s, the most stringent limit for any galaxy. The corresponding limit on the mass within the three-dimensional half-light radius (46pc) is M_1/2_<1.5(2.1)x10^5^M_{sun}_. Segue 2 is the least massive galaxy known. We identify Segue 2 as a galaxy rather than a star cluster based on the wide dispersion in [Fe/H] (from -2.85 to -1.33) among the member stars. The stars' [{alpha}/Fe] ratios decline with increasing [Fe/H], indicating that Segue 2 retained Type Ia supernova ejecta despite its presently small mass and that star formation lasted for at least 100 Myr. The mean metallicity, <[Fe/H]>=-2.22+/-0.13 (about the same as the Ursa Minor galaxy, 330 times more luminous than Segue 2), is higher than expected from the luminosity-metallicity relation defined by more luminous dwarf galaxy satellites of the Milky Way. Segue 2 may be the barest remnant of a tidally stripped, Ursa Minor-sized galaxy. If so, it is the best example of an ultra-faint dwarf galaxy that came to be ultra-faint through tidal stripping. Alternatively, Segue 2 could have been born in a very low mass dark matter subhalo (v_max_<10km/s), below the atomic hydrogen cooling limit.2013ApJ...770...16Khttps://cdsarc.cds.unistra.fr/viz-bin/cat/J/ApJ/770/16CatalogResearchIsServedByTAP VizieR generic serviceIsServedByConesearch servicerelated-toII/294 : The SDSS Photometric Catalog, Release 7 (Adelman-McCarthy+, 2009)J/AJ/147/13 : Disturbers of nearby giant galaxies (Karachentsev+, 2014)J/AJ/145/101 : Updated nearby galaxy catalog (Karachentsev+, 2013)J/ApJ/767/134 : Abundances of red giant stars in UFD gal. (Vargas+, 2013)J/AJ/144/4 : Dwarf galaxies data in the Local Group (McConnachie+, 2012)J/ApJ/751/46 : Spectrosc. observations in VV124 (UGC 4879) (Kirby+, 2012)J/ApJ/733/46 : Velocity measurements in Segue 1 (Simon+, 2011)J/ApJS/191/352 : Abundances in stars of MW dwarf satellites (Kirby+, 2010)J/ApJ/703/2177 : Metal-poor MS turnoff stars summary (Schlaufman+, 2009)J/ApJ/692/1464 : Spectroscopy of Segue 1 (Geha+, 2009)J/AJ/137/4377 : List of SEGUE plate pairs (Yanny+, 2009)J/MNRAS/397/1748 : Spectroscopic study of Segue 2 (Belokurov+, 2009)J/ApJ/682/1217 : Abundances in red giants in globular clusters (Kirby+, 2008)https://cds.unistra.fr/vizier-org/licences_vizier.htmlhttps://vizier.cds.unistra.fr/viz-bin/VizieR-2?-source=J/ApJ/770/16https://vizier.iucaa.in/viz-bin/VizieR-2?-source=J/ApJ/770/16http://vizieridia.saao.ac.za/viz-bin/VizieR-2?-source=J/ApJ/770/16https://vizier.cds.unistra.fr/viz-bin/votable?-source=J/ApJ/770/16https://vizier.iucaa.in/viz-bin/votable?-source=J/ApJ/770/16http://vizieridia.saao.ac.za/viz-bin/votable?-source=J/ApJ/770/16GETtext/xml+votablehttps://tapvizier.cds.unistra.fr/TAPVizieR/tapCone search capability for table J/ApJ/770/16/table2 (List of target stars, with membership)https://vizier.cds.unistra.fr/viz-bin/conesearch/J/ApJ/770/16/table2?https://vizier.iucaa.in/viz-bin/conesearch/J/ApJ/770/16/table2?http://vizieridia.saao.ac.za/viz-bin/conesearch/J/ApJ/770/16/table2?GETtext/xml+votable180.050000true34.589791720.05888890.0055555555555555566/619 705https://cdsarc.cds.unistra.fr/viz-bin/moc/J/ApJ/770/16?format=asciiOpticaldefaultJ/ApJ/770/16/table2List of target stars, with membershiprecnoRecord number assigned by the VizieR team. Should Not be used for identification.meta.recordintSDSSSDSS-DR7 identifier (JHHMMSS.ss+DDMMSS.s)meta.id;meta.mainchargmag[14.6/26.2]? Extinction-corrected SDSS g band magnitudemagphot.mag;em.opt.Bfloatnullablee_gmag? Uncertainty in gmagmagstat.error;phot.mag;em.opt.Bfloatnullableimag[14.1/26.1]? Extinction-corrected SDSS i band magnitudemagphot.mag;em.opt.Ifloatnullablee_imag? Uncertainty in imagmagstat.error;phot.mag;em.opt.IfloatnullableNmNumber of masks on which object was observedmeta.numberintS/N[0/507.2] Signal-to-noise ratio per Angstromstat.snrfloatHRV[-370/232]? Heliocentric radial velocitykm/sspect.dopplerVeloc;pos.heliocentricfloatnullablee_HRV[2/23]? Uncertainty in HRVkm/sstat.errorfloatnullableEWNa? Equivalent width in NaI 8190 doublet0.1nmspect.line.eqWidthfloatnullablee_EWNa? Uncertainty in EWNa0.1nmstat.errorfloatnullable[Fe/H][-2.9/-0.6]? Logarithmic Fe abundance relative to the Sunlog(Sun)phys.abund.Fefloatnullablee_[Fe/H]? Uncertainty in [Fe/H]log(Sun)stat.error;phys.abund.FefloatnullableMm[YN?B] Membership to Segue 2: Y=yes, N=no, "B"=yes, horizontal branch star, ?=unknownmeta.code.member;pos.cartesian.ycharreason? Reason(s) for non-membership (1)meta.noteSloanDisplay the online SDSS data for the nearest objectmeta.ref.urlcharSimbadSimbad column added by the CDSmeta.ref.urlchar_RAPositions from SDSS-DR7 names (right ascension part)degpos.eq.ra;meta.main_DEPositions from SDSS-DR7 names (declination part)degpos.eq.dec;meta.main