J/ApJ/770/16 Spectroscopic members of Segue 2 galaxy (Kirby+, 2013)
Segue 2: the least massive galaxy.
Kirby E.N., Boylan-Kolchin M., Cohen J.G., Geha M., Bullock J.S.,
Kaplinghat M.
<Astrophys. J., 770, 16 (2013)>
=2013ApJ...770...16K 2013ApJ...770...16K
ADC_Keywords: Abundances, [Fe/H] ; Radial velocities ; Photometry, SDSS ;
Spectroscopy ; Galaxies, nearby
Keywords: galaxies: abundances; galaxies: dwarf; galaxies: individual: Segue 2;
galaxies: kinematics and dynamics; Local Group
Abstract:
Segue 2, discovered by Belokurov et al. (2009, Cat. J/MNRAS/397/1748),
is a galaxy with a luminosity of only 900L☉. 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 σv<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
M1/2<1.5(2.1)x105M☉. 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' [α/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 (vmax<10km/s), below the atomic hydrogen cooling
limit.
Description:
We observed individual objects in the vicinity of Segue 2 with the
DEIMOS spectrograph on the Keck II telescope in generally excellent
weather during three nights in 2009 and one night in 2013. The
resolution was 1.2Å FWHM, corresponding to a resolving power of
R=7000 at the CaII infrared triplet. From the reduced one-dimensional
spectra, we measured stellar radial velocities and chemical
abundances. Our spectroscopic member sample includes 21 red giants and
subgiants, 4 HB stars, and 1 RR Lyrae star.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2.dat 106 647 List of target stars, with membership
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See also:
II/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)
Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 4 A4 --- --- [SDSS]
6- 24 A19 --- SDSS SDSS-DR7 identifier (JHHMMSS.ss+DDMMSS.s)
26- 31 F6.3 mag gmag [14.6/26.2]?=0 Extinction-corrected SDSS
g band magnitude
33- 37 F5.3 mag e_gmag ?=0 Uncertainty in gmag
39- 44 F6.3 mag imag [14.1/26.1]?=0 Extinction-corrected SDSS
i band magnitude
46- 50 F5.3 mag e_imag ?=0 Uncertainty in imag
52 I1 --- Nm Number of masks on which object was observed
54- 58 F5.1 --- S/N [0/507.2] Signal-to-noise ratio per Angstrom
60- 65 F6.1 km/s HRV [-370/232]? Heliocentric radial velocity
67- 70 F4.1 km/s e_HRV [2/23]? Uncertainty in HRV
72- 76 F5.2 0.1nm EWNa ? Equivalent width in NaI 8190 doublet
78- 81 F4.2 0.1nm e_EWNa ? Uncertainty in EWNa
83- 87 F5.2 [Sun] [Fe/H] [-2.9/-0.6]? Logarithmic Fe abundance
relative to the Sun
89- 92 F4.2 [Sun] e_[Fe/H] ? Uncertainty in [Fe/H]
94 A1 --- Mm [YN?B] Membership to Segue 2: Y=yes, N=no,
"B"=yes, horizontal branch star, ?=unknown
96-106 A11 --- reason ? Reason(s) for non-membership (1)
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Note (1): Reasons for non-membership as follows:
CMD = location in the color-magnitude diagram.
vr = inappropriate radial velocity.
Na = spectrum shows strong NaI λ8190 doublet.
[Fe/H] = the measured metallicity is greater than [Fe/H]=-1.0.
G = spectrum shows emission lines or redshifted Ca H and K lines,
indicating that the object is a galaxy.
Bad = spectral quality was insufficient for radial velocity measurement.
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History:
From electronic version of the journal
(End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 07-Jan-2015