J/MNRAS/458/L59 Draco II stars gi magnitude & radial velocities (Martin+, 2016)
Is Draco II one of the faintest dwarf galaxies?
First study from Keck/DEIMOS spectroscopy.
Martin N.F., Geha M., Ibata R.A., Collins M.L.M., Laevens B.P.M.,
Bell E.F., Rix H.-W., Ferguson A.M.N., Chambers K.C., Wainscoat R.J.,
Waters C.
<Mon. Not. R. Astron. Soc., 458, L59-L63 (2016)>
=2016MNRAS.458L..59M 2016MNRAS.458L..59M (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Photometry, SDSS ; Radial velocities
Keywords: galaxies: individual: Draco II - galaxies: kinematics and dynamics -
Local Group
Abstract:
We present the first spectroscopic analysis of the faint and compact
stellar system Draco II (Dra II, MV=-2.9±0.8,rh=19+8-6pc),
recently discovered in the Panoramic Survey Telescope and Rapid
Response System 1 3{PI} survey. The observations, conducted with
DEIMOS on the Keck II telescope, establish some of its basic
characteristics: the velocity data reveal a narrow peak with nine
member stars at a systemic heliocentric velocity
<vr≥-347.6+1.7-1.8km/s, thereby confirming Dra II is a
satellite of the Milky Way; we infer a velocity dispersion with
σvr=2.9±2.1km/s (<8.4km/s at the 95 per cent confidence
level), which implie log10(M1/2)-5.5+0.4-0.6 and
log10((M/L)1/2)=2.7+0.5-0.8, in Solar units; furthermore, very
weak calcium triplet lines in the spectra of the high signal-to-noise
member stars imply [Fe/H]←2.1, whilst variations in the line
strengths of two stars with similar colours and magnitudes suggest a
metallicity spread in Dra II. These new data cannot clearly
discriminate whether Draco II is a star cluster or amongst the
faintest, most compact, and closest dwarf galaxies. However, the sum
of the three - individually inconclusive - pieces of evidence
presented here seems to favour the dwarf galaxy interpretation.
Description:
One DEIMOS mask was observed on the night of 2015 July 17, placed
close to the centre of Dra II in such a way to optimize the number of
high-priority bright candidate members. The priorities were set as
both a function of spatial location (higher priority towards the
centre of the system) and location in the colour-magnitude diagram
(CMD). All targets are selected using the PS1 photometry and higher
priorities are given to potential main sequence (MS), main-sequence
turn off (MSTO) and red giant branch (RGB) stars selected to follow an
isochrone that best reproduces the CMD features of Dra II. The mask
was drilled with 0.7arcsec slits.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 82 34 Properties of observed stars
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Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 2 I2 --- Seq Sequential number
4- 14 F11.7 deg RAdeg Right ascension (ICRS)
16- 25 F10.7 deg DEdeg Declination (ICRS)
27- 32 F6.3 mag gP1mag PS1 g magnitude
34- 38 F5.3 mag e_gP1mag rms uncertainty on gP1mag
40- 45 F6.3 mag iP1mag PS1 i magnitude
47- 51 F5.3 mag e_iP1mag rms uncertainty on iP1mag
53 A1 --- Memb [Y/N] Member
55- 60 F6.1 km/s RV Radial velocity
62- 64 F3.1 km/s e_RV rms uncertainty on RV
66- 69 F4.1 --- S/N Signal-to-noise ratio
71- 74 F4.1 [-] [Fe/H] ? Tentative [Fe/H]
76- 78 F3.1 [-] E_[Fe/H] ? Error on [Fe/H] (upper value)
80- 82 F3.1 [-] e_[Fe/H] ? Error on [Fe/H] (lower value)
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Acknowledgements:
Nicolas Martin, nicolas.martin(at)astro.unistra.fr
(End) Patricia Vannier [CDS] 25-Nov-2016