A Keck/DEIMOS spectroscopic survey of faint Galactic satellites: searching for the least massive dwarf galaxies

By N. Martin, R. Ibata, S. Chapman, M. Irwin, Geraint F. Lewis. 2007

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We present the results of a spectroscopic survey of the recently discovered faint Milky Way satellites Bootes, Ursa Major I, Ursa Major II and Willman 1 (Will). Using the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, we have obtained samples that contain from ∼ 15 to ∼85 probable members of these satellites for which we derive radial velocities precise to a few km s -1 down to i ∼ 21-22. About half of these stars are observed with a high enough signal-to-noise ratio to estimate their metallicity to within ±0.2 dex. The characteristics of all the observed stars are made available, along with those of the Canes Venatici I dwarf galaxy that have been analysed in a companion paper. From this data set, we show that Ursa Major II is the only object that does not show a clear radial velocity peak. However, the measured systemic radial velocity (υ r = 115 ± 5kms -1 ) is in good agreement with simulations in which this object is the progenitor of the recently discovered Orphan Stream. The three other satellites show velocity dispersions that make them highly dark matter dominated systems (under the usual assumptions of symmetry and virial equilibrium). In particular, we show that despite its small size and faintness, the Will object is not a globular cluster given its metallicity scatter over -2.0 ≤ [Fe/H] ≤ -1.0 and is therefore almost certainly a dwarf galaxy or dwarf galaxy remnant. We measure a radial velocity dispersion of only 4.3 +2.3-1.3 km s -1 around a systemic velocity of -12.3 ± 2.3 km s -1 which implies a mass-to-light ratio of ∼700 and a total mass of ∼5 x 10 5 M O for this satellite, making it the least massive satellite galaxy known to date. Such a low mass could mean that the 10 7 M O limit that had until now never been crossed for Milky Way and Andromeda satellite galaxies may only be an observational limit and that fainter, less massive systems exist within the Local Group. However, more modelling and an extended search for potential extratidal stars are required to rule out the possibility that these systems have not been significantly heated by tidal interaction.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/3e86ae4768145be4686d8cf511c1f4da4067eef3

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Citations in the corpus

The Masses of the Milky Way and Andromeda galaxies (2010, unknown): “…from https://academ ic.oup.com /m nras/article-abstract/406/1/264/1071477 by guest on 06 D ecem ber 2018 Sources: 1, Belokurov et al. (2006); 2, Martin et al. (2007), 3, Martin, de Jong & Rix (2008); 4, Koch et al. (2009); 5, Zucker et al. (2006a); 6, Simon & Geha (2007); 7, Belokurov et al.…”
Hundreds of Milky Way Satellites? Luminosity Bias in the Satellite Luminosity Function (2008, background): “For example,Willman 1 was originally of unclear classification (Willman et al. 2005), but is now generally recognized as being a darkmatterYdominated dSph (Martin et al. 2007; Strigari et al. 2008b).”
The Luminosity Function of the Milky Way Satellites (2007, background): “Of course, the nature of some of the objects we have included in the dwarf galaxy luminosity function is still uncertain – in particular, Willman 1 may be a globular cluster, although Martin et al. (2007) provide evidence for a metallicity spread and dark matter content.”
EIGHT NEW MILKY WAY COMPANIONS DISCOVERED IN FIRST-YEAR DARK ENERGY SURVEY DATA (2015, background): “…photometrically classified or spectroscopically confirmed as gravitationally bound “ultra-faint” galaxies (Kleyna et al. 2005; Muñoz et al. 2006; Martin et al. 2007; Simon & Geha 2007; Adén et al. 2009; Belokurov et al. 2009; Carlin et al. 2009; Geha et al. 2009; Koch et al. 2009; Walker et…”
THE LEAST-LUMINOUS GALAXY: SPECTROSCOPY OF THE MILKY WAY SATELLITE SEGUE 1 (2008, background): “…total luminosities of the ultra-faint satellites are comparable to globular clusters, spectroscopic studies for the majority of the newly discovered objects firmly suggest that these objects are dark matter-dominated dwarf galaxies (Kleyna et al. 2005; Muñoz et al. 2006; Martin et al. 2007, SG07).”
A Second Galaxy Missing Dark Matter in the NGC 1052 Group (2019, methodology): “…method, used extensively for determining the kinematics of dwarf galaxies in the Local Group from the velocities of individual stars (e.g., Martin et al. 2007), is to construct the likelihood function: L = i≤7∏ i=1 1√ 2πσeff exp [ −0.5 ( vi −µ σeff )2] , (1) with vi the velocities of the…”
A NEW FAINT MILKY WAY SATELLITE DISCOVERED IN THE PAN-STARRS1 3π SURVEY (2015, background): “Only the closest DGs would be detected with current photometric surveys (Koposov et al. 2007; Walsh et al. 2009).”
A KINEMATIC STUDY OF THE ANDROMEDA DWARF SPHEROIDAL SYSTEM (2013, methodology): “We plot the MW dSphs as red triangles (Martin et al. 2007; Kirby et al. 2008, 2011; Belokurov et al. 2009; Koch et al. 2009).”
A QUANTITATIVE EXPLANATION OF THE OBSERVED POPULATION OF MILKY WAY SATELLITE GALAXIES (2009, methodology): “…of them at least an order of magnitude less luminous than the faintest of the previously known, “classical” satellites.7 Spectroscopic follow-up (e.g. Martin et al. 2007; Simon & Geha 2007; Geha et al. 2008) for many of them indicates that they are indeed dark matter dominated systems, even though…”
EIGHT ULTRA-FAINT GALAXY CANDIDATES DISCOVERED IN YEAR TWO OF THE DARK ENERGY SURVEY (2015, background): “…et al. 2006a, 2006b; Irwin et al. 2007; Walsh et al. 2007) and dedicated spectroscopic follow-up observations (Kleyna et al. 2005; Muñoz et al. 2006; Martin et al. 2007; Simon & Geha 2007; Adén et al. 2009; Belokurov et al. 2009; Carlin et al. 2009; Geha et al. 2009; Koch et al. 2009; Walker et…”
The Pristine survey - I. Mining the Galaxy for the most metal-poor stars (2017, background): “At present, most of the “mass” measurements assume dynamical equilibrium (e.g., Martin et al. 2007; Simon & Geha 2007; Simon et al. 2011), which is far from being proven for systems within a few tens of kpc.”
CONSTRAINTS ON MACHO DARK MATTER FROM COMPACT STELLAR SYSTEMS IN ULTRA-FAINT DWARF GALAXIES (2016, unknown): “†† References abbreviated as: 1 (Martin et al. 2007); 2 (Martin et al. 2008); 3 (Simon et al. 2011); 4 (Belokurov et al. 2009); 5 (Bechtol et al. 2015); 6 (Koposov et al. 2015a); 7 (Koposov et al. 2015b); 8 (Laevens et al. 2015); 9 (Crnojević et al. 2016) * Geometric means of Koposov et al.…”
A COMPLETE SPECTROSCOPIC SURVEY OF THE MILKY WAY SATELLITE SEGUE 1: THE DARKEST GALAXY (2010, background): “…et al. 2009) remain relatively poorly characterized by observations; for example, the dynamical state of Willman 1 has still has not been established (Martin et al. 2007; Willman et al. 2010), and the velocity dispersion of Boo II is uncertain at the factor of ∼ 5 level (Koch et al. 2009).”
Luminosity function and radial distribution of Milky Way satellites in a ΛCDM Universe (2009, background): “Spectroscopic surveys subsequently measured the velocity dispersions of these systems, and confirmed their galactic nature (Martin et al. 2007, Simon & Geha 2007).”
ASSESSING THE MILKY WAY SATELLITES ASSOCIATED WITH THE SAGITTARIUS DWARF SPHEROIDAL GALAXY (2010, unknown): “Boötes I (ID=62, Λ⊙ = 277 ◦) Boötes I (Boo I; Belokurov et al. 2006b) is located at (l, b) = (358.1◦, 69.6◦) at a distance of 62 kpc with a radial velocity vGSR = 103 ± 3.4 km s −1 (Muñoz et al. 2006; Martin et al. 2007).”
ACCURATE STELLAR KINEMATICS AT FAINT MAGNITUDES: APPLICATION TO THE BOÖTES I DWARF SPHEROIDAL GALAXY (2011, methodology): “Comparing our derived velocity dispersion, and individual velocities, with literature studies (Muñoz et al. 2006; Martin et al. 2007), shows that previous studies have substantially overestimated the velocity dispersion of Boötes I.”
THE NUCLEUS OF THE SAGITTARIUS DSPH GALAXY AND M54: A WINDOW ON THE PROCESS OF GALAXY NUCLEATION (2008, methodology): “The table reports also an alternative estimate of σ obtained with the Gaussian maximum-likelihood method described by Walker et al. (2006), essentially equivalent to that adopted by Martin et al. (2007).”
Dark Matter Subhalos and the Dwarf Satellites of the Milky Way (2008, background): “The data points in the figure include all the previously known dwarfs (Mateo 1998) plus the new circular velocity estimates of the ultra-faint Milky Way satellites from Simon & Geha (2007), plus Boötes (Munoz et al. 2006) and Willman 1 (Martin et al. 2007).”