J/MNRAS/479/5343 Nearby dwarf galaxy in Magellanic bridge (Koposov+, 2018)
Snake in the Clouds: A new nearby dwarf galaxy in the Magellanic bridge*.
Koposov S.E., Walker M.G., Belokurov V., Casey A.R., Geringer-Sameth A.,
Mackey D., Da Costa G., Erkal D., Jethwa P., Mateo M., Olszewski E.W.,
Bailey J.I.
<Mon. Not. R. Astron. Soc., 479, 5343-5361 (2018)>
=2018MNRAS.479.5343K 2018MNRAS.479.5343K (SIMBAD/NED BibCode)
ADC_Keywords: Galaxies ; Clusters, globular ; Magellanic Clouds ; Optical
Keywords: stars: general - globular clusters: general - Galaxy: halo -
galaxies: dwarf - Local Group - Magellanic Clouds
Abstract:
We report the discovery of a nearby dwarf galaxy in the constellation
of Hydrus, between the Large (LMC) and the Small Magellanic Clouds
(SMC). Hydrus 1 is a mildly elliptical ultrafaint system with
luminosity MV~-4.7 and size 53±3pc, located 28kpc from the Sun and
24kpc from the LMC. From spectroscopy of ∼30 member stars, we measure
a velocity dispersion of 2.7±0.5 km/s and find tentative evidence
for a radial velocity gradient consistent with 3km/s rotation. Hydrus
1's velocity dispersion indicates that the system is dark matter
dominated, but its dynamical mass-to-light ratio M/L=66+29-20 is
significantly smaller than typical for ultrafaint dwarfs at similar
luminosity. The kinematics and spatial position of Hydrus 1 make it a
very plausible member of the family of satellites brought into the
Milky Way by the Magellanic Clouds. While Hydrus 1's proximity and
well-measured kinematics make it a promising target for dark matter
annihilation searches, we find no evidence for significant gamma-ray
emission from Hydrus 1. The new dwarf is a metal-poor galaxy with a
mean metallicity [Fe/H]=-2.5 and [Fe/H] standard deviation of 0.4dex,
similar to other systems of similar luminosity. α abundances of
Hyi 1 members indicate that star formation was extended, lasting
between 0.1 and 1Gyr, with self-enrichment dominated by supernova Ia.
The dwarf also hosts a highly carbon-enhanced extremely metal-poor
star with [Fe/H]~-3.2 and [C/Fe]~+3.0.
Description:
This work utilizes the photometric data from two DECam programs,
2016A-0618 and 2017B-0906, focusing on the region between the MCs.
On 2018 February 12-13, we used M2FS, a multi-object fiber
spectrograph at the Magellan Clay 6.5m telescope, to acquire
spectra for the selected targets.
Table 2 presents spectroscopic parameter measurements for all the
stars that either have S/N larger than 3 or have RV uncertainty less
than 10km/s. We list measurements of the effective temperature,
surface gravity, metallicity, and heliocentric LOS velocity. We also
provide in the table, the g, r magnitudes and the log odds of being
Hydrus 1 member star, based on the model described in the next
section.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table2.dat 129 139 Spectroscopic measurements for individual stars
with either S/N>3 or RV error less than 10km/s
in the Hydrus 1 field
--------------------------------------------------------------------------------
Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 I3 --- ID Star identification number
5- 12 F8.5 deg RAdeg Right ascension (J2000)
14- 22 F9.5 deg DEdeg Declination (J2000)
24- 27 F4.1 --- S/N Median signal to noise ratio of the spectra
29- 34 F6.1 km/s HRV Heliocentric radial velocity
36- 40 F5.1 km/s e_HRV Radial velocity error
42- 45 I4 K Teff Effective temperature
47- 50 I4 K e_Teff Effective temperature error
52- 55 F4.2 [cm/s2] loggmean Mean Surface gravity
57- 60 F4.2 [cm/s2] e_loggmean Mean Surface gravity error
62- 66 F5.2 --- [Fe/H] Metallicity
68- 71 F4.2 --- e_[Fe/H] Metallicity error
73- 77 F5.2 mag gmag ?=- Not corrected for extinction g-band
photometry from DECam observations
79- 83 F5.2 mag rmag ?=- Not corrected for extinction r-band
photometry from DECam observations
85-106 F22.16 --- logodds ?=- Log-odds ratios of being a Hydrus 1
member star versus being a foreground
contaminant from the chemodynamical model
fit (1)
108-129 F22.18 --- e_logodds ?=- Log-odds error (2)
-------------------------------------------------------------------------------
Note (1): Membership probability for each star. We express these probabilities
in the form of log odds of being a member star log(Pmemb/Pnonmber).
Note (2): The uncertainties in the log odds are obtained after marginalizing
over all model parameters.
--------------------------------------------------------------------------------
Nomenclature Notes: Sources will be identified as [KWB2018] NNN in Simbad.
History:
From electronic version of the journal
(End) Ana Fiallos [CDS] 04-May-2022