J/MNRAS/491/356 Sagittarius II study with the Pristine survey (Longeard+, 2020)
The Pristine Dwarf-Galaxy survey.
II. In-depth observational study of the faint Milky Way satellite
Sagittarius II.
Longeard N., Martin N., Starkenburg E., Ibata R.A., Collins M.L.M.,
Laevens B.P.M., Mackey D., Rich R.M., Aguado D.S., Arentsen A.,
Jablonka P., Gonzalez Hernandez J.I., Navarro J.F., Sanchez-Janssen R.
<Mon. Not. R. Astron. Soc., 491, 356-377 (2020)>
=2020MNRAS.491..356L 2020MNRAS.491..356L (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies, nearby ; Abundances, [Fe/H] ; Radial velocities ;
Proper motions ; Photometry ; Spectroscopy ; Optical
Keywords: galaxies: dwarf - Local Group
Abstract:
We present an extensive study of the Sagittarius II (Sgr II) stellar
system using MegaCam g and i photometry, narrow-band,
metallicity-sensitive calcium H&K doublet photometry and Keck
II/DEIMOS multiobject spectroscopy. We derive and refine the Sgr II
structural and stellar properties inferred at the time of its
discovery. The colour-magnitude diagram implies Sgr II is old
(12.0±0.5Gyr) and metal poor. The CaHK photometry confirms the
metal-poor nature of the satellite ([Fe/H]CaHK=-2.32±0.04dex) and
suggests that Sgr II hosts more than one single stellar population
(σ[Fe/H]CaHK=0.11+0.05-0.03dex). Using the Ca infrared
triplet measured from our highest signal-to-noise spectra, we confirm
the metallicity and dispersion inferred from the Pristine photometric
metallicities ([Fe/H]spectro=-2.23±0.05dex,
σ[Fe/H]spectro=0.10+0.06-0.04dex). The velocity
dispersion of the system is found to be σv=2.7+1.3-1.0km/s
after excluding two potential binary stars. Sgr II's metallicity and
absolute magnitude (MV=-5.7±0.1mag) place the system on the
luminosity-metallicity relation of the Milky Way dwarf galaxies
despite its small size. The low but resolved metallicity and velocity
dispersions paint the picture of a slightly dark-matter-dominated
satellite (M/L=23.0+32.8-23.0M☉/L☉). Furthermore,
using the Gaia Data Release 2, we constrain the orbit of the satellite
and find an apocentre of 118.4+28.4-23.7kpc and a pericentre of
54.8+3.3-6.1kpc. The orbit of Sgr II is consistent with the
trailing arm of the Sgr stream and indicates that it is possibly a
satellite of the Sgr dSph that was tidally stripped from the dwarf's
influence.
Description:
Our photometry consists of deep broad-band gMC and iMC
observations as well as narrow-band observations with the CaHK filter
centred on the metallicity-sensitive Calcium H&K doublet. Sgr II was
observed using the wide-field imager MegaCam on the CFHT (Boulade et
al. 2003SPIE.4841...72B 2003SPIE.4841...72B). The CaHK photometry is part of a larger
survey called Pristine (Starkenburg et al. 2017MNRAS.471.2587S 2017MNRAS.471.2587S).
Observations were conducted in service mode by the CFHT staff during
the night of 2016 July 2, under good seeing conditions (∼0.4arcsec).
The integration times are of 3x700s in gMC, 5x500s in iMC, and
3x705s in CaHK. We refer the reader to Laevens et al.
(2015ApJ...802L..18L 2015ApJ...802L..18L) for the details of the MegaCam data reduction.
Spectroscopic follow-up observations of Sgr II were obtained with Keck
and the Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS)
(Faber et al. 2003SPIE.4841.1657F 2003SPIE.4841.1657F). The standard set-up was used here
with the OG550 filter and the 1200lines/mm grating. The wavelength
range goes from 6500 to 9000Å, for a resolution of ∼8500. 'Mask 1'
was observed on 2015-09-12 and reobserved on 2015-09-18 (97 stars) and
'Mask 2' on 2015-09-08 (110 stars). Mask 2 was designed to be
perpendicular to the other two in order to probe potential Sgr II
members further away in the South/North direction.
To infer the orbit of Sgr II, we first build a sample of possible Sgr
II. The proper motions of those stars are retrieved from the Gaia Data
Release 2 (Gaia Collaboration 2018A&A...616A...1G 2018A&A...616A...1G, Cat. I/345). All
member stars identified as members from our spectroscopy and bright
enough to have a proper motion measurement in Gaia are naturally
present in this sample.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table2.dat 130 117 Properties of our spectroscopic sample
table3.dat 63 201 Velocities and individual metallicities for all
stars observed more than once, per mask
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See also:
I/345 : Gaia DR2 (Gaia Collaboration, 2018)
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 F9.5 deg RAdeg Right ascension (J2000)
11- 19 F9.5 deg DEdeg Declination (J2000)
21- 25 F5.2 mag g0mag Dereddened g-band magnitude
27- 30 F4.2 mag e_g0mag Error on g0mag
32- 36 F5.2 mag i0mag Dereddened i-band magnitude
38- 41 F4.2 mag e_i0mag Error on i0mag
43- 48 F6.3 mag CaHK0mag Dereddened CaHK-band magnitude
50- 53 F4.2 mag e_CaHK0mag Error on CaHK0mag
55- 60 F6.1 km/s RV Radial velocity
62- 65 F4.1 km/s e_RV Error on RV (G1)
67- 73 F7.3 mas/yr pmRA ? Proper motion in right ascension
75- 79 F5.3 mas/yr e_pmRA ? Error on pmRA
81- 87 F7.3 mas/yr pmDE ? Proper motion in declination
89- 93 F5.3 mas/yr e_pmDE ? Error on pmDE
95- 98 F4.1 --- S/N Signal to noise ratio
100-104 F5.2 [-] [Fe/H]ph ? Photometric metallicity obtained by
combining the CaHK0 magnitude with the
broad-band g0 and i0 photometry (1)
106-109 F4.2 [-] e_[Fe/H]ph ? Error on [Fe/H]ph
111 A1 --- f_[Fe/H]ph [*] Flag on [Fe/H]ph (2)
113-117 F5.2 [-] [Fe/H]sp ? Spectroscopic metallicity (G2)
119-122 F4.2 [-] e_[Fe/H]sp ? Error on [Fe/H]sp
124-127 F4.2 --- Pmem Probability membership
129-130 A2 --- Member [Y/N/HB] Membership (3)
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Note (1): The Pristine metallicity of a given star is indicated only if
[Fe/H]ph←1.0
Note (2): Flag as follows:
* = For this star, [Fe/H]ph and its error where equal to -99.0 in the
original table
Note (3): Membership as follows:
Y = Member, stars with Pmem>0.8 are systematically considered as members
(21/117)
N = Non-member (93/117)
HB = Potential horizontal branch stars of Sgr II. Since our CMD fitting
procedure does not account for the horizontal branch (see Section 3.1),
their membership probability is not meaningful (3/117).
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Byte-by-byte Description of file: table3.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 9 F9.5 deg RAdeg Right ascension (J2000)
11- 19 F9.5 deg DEdeg Declination (J2000)
21- 40 A20 --- Mask Mask used for observation (1)
42- 47 F6.1 km/s RV Radial velocity
49- 52 F4.1 km/s e_RV Error on RV (G1)
54- 58 F5.2 [-] [Fe/H]sp ? Spectroscopic redshift (G2)
60- 63 F4.2 [-] e_[Fe/H]sp ? Error on [Fe/H]sp
--------------------------------------------------------------------------------
Note (1): Mask 1 was observed on the 2015-09-12 and reobserved on 2015-09-18,
and mask 2 on 2015-09-08 (respectively 2457283.760868, 2457277.742083,
and 2457273.738102 in Julian dates)
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Global Notes:
Note (G1): The systematic threshold is not included in the velocity
uncertainty
Note (G2): The individual spectroscopic metallicity is reported for stars with
S/N≥12 and g0mag>20.5 only
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History:
From electronic version of the journal
References:
Longeard et al., Paper I 2018MNRAS.480.2609L 2018MNRAS.480.2609L
Longeard et al., Paper III 2021MNRAS.503.2754L 2021MNRAS.503.2754L
Longeard et al., Paper IV 2022MNRAS.516.2348L 2022MNRAS.516.2348L
(End) Ana Fiallos [CDS] 09-Feb-2023