Search for Extremely Metal-poor Galaxies in the Sloan Digital Sky Survey. (II). High Electron Temperature Objects

ENRIQUE PÉREZ-MONTERO; RUBÉN GARCÍA-BENITO; ANA BELÉN MORALES-LUIS; JORGE SÁNCHEZ ALMEIDA; SEBASTIÁN E. NUZA; CASIANA MUÑOZ-TUÑÓN; FRANCISCO-SHU KITAURA

ASTROPHYSICAL JOURNAL

IOP PUBLISHING LTD

Lugar: Bristol; Año: 2016 vol. 819

0004-637X

Extremely metal-poor (XMP) galaxies are defined to have a gas-phasemetallicity smaller than a tenth of the solar value(12+{log}[{{O/H}}]< 7.69). They are uncommon, chemically and possiblydynamically primitive, with physical conditions characteristic ofearlier phases of the universe. We search for new XMPs in the SloanDigital Sky Survey (SDSS) in a work that complements Paper I. This time,high electron temperature objects are selected; metals are a maincoolant of the gas, so metal-poor objects contain high-temperature gas.Using the algorithm k-means, we classify 788,677 spectra to select 1281galaxies that have particularly intense [O III]λ4363 with respectto [O III]λ5007, which is a proxy for high electron temperature.The metallicity of these candidates was computed using a hybridtechnique consistent with the direct method, rendering 196 XMPs. A lessrestrictive noise constraint provides a larger set with 332 candidates.Both lists are provided in electronic format. The selected XMP samplehas a mean stellar mass around {10}8 {M}⊙ ,with the dust mass ∼ {10}3{M}⊙ fortypical star-forming regions. In agreement with previous findings, XMPsshow a tendency to be tadpole-like or cometary. Their underlying stellarcontinuum corresponds to a fairly young stellar population (< 1{{Gyr}}), although young and aged stellar populations coexist at thelow-metallicity starbursts. About 10% of the XMPs show large N/O. Basedon their location in constrained cosmological numerical simulations,XMPs have a strong tendency to appear in voids and to avoid galaxyclusters. The puzzling 2%-solar low-metallicity threshold exhibited byXMPs remains.