A New Milky Way Companion: Unusual Globular Cluster or Extreme Dwarf Satellite?

By B. Willman, M. Blanton, Andrew A. West, J. J. Dalcanton, David W. Hogg, D. Schneider, N. Wherry, Brian Yanny, J. Brinkmann. 2004

313 citations recorded by the source corpus.

We report the discovery of SDSS J1049+5103, an overdensity of resolved blue stars at (α2000, δ2000) = (162.°343, 51.°051). This object appears to be an old, metal-poor stellar system at a distance of 45 ± 10 kpc, with a half-light radius of 23 ± 10 pc and an absolute magnitude of MV = -3.0. One star that is likely associated with this Milky Way companion has an SDSS spectrum confirming it as a blue horizontal-branch star at 48 kpc. The color-magnitude diagram of SDSS J1049+5103 contains few, if any, horizontal or red giant branch stars, similar to the anomalously faint globular cluster AM 4. The size and luminosity of SDSS J1049+5103 places it at the intersection of the size-luminosity relationships followed by known globular clusters and by Milky Way dwarf spheroidal galaxies. If SDSS J1049+5103 is a globular cluster, then its properties are consistent with the established trend that the largest radius Galactic globular clusters are all in the outer halo. However, the five known globular clusters with similarly faint absolute magnitudes all have half-mass radii that are smaller than SDSS J1049+5103 by a factor of ≳5. If it is a dwarf spheroidal galaxy, then it is the faintest yet known by 2 orders of magnitude and is the first example of the ultrafaint dwarfs predicted by some theories. The uncertain nature of this new system underscores the sometimes ambiguous distinction between globular clusters and dwarf spheroidal galaxies. A simple friends-of-friends search for similar, blue, small scale length star clusters detected all known globular clusters and dwarfs closer than 50 kpc in the SDSS area but yielded no other candidates as robust as SDSS J1049+5103.

Subfields: Astronomy and Astrophysics

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THE QUENCHING OF THE ULTRA-FAINT DWARF GALAXIES IN THE REIONIZATION ERA (2014, background): “Over the same time period, wide-field surveys revealed the existence of additional dwarf satellites around the Milky Way (e.g., Willman et al. 2005; Zucker et al. 2006; Belokurov et al. 2007) and Andromeda (e.g., Zucker et al. 2004, 2007; McConnachie et al. 2009; Majewski et al. 2007; Irwin et al.…”
The upper bound on the lowest mass halo (2016, background): “In the Milky Way (MW), satellite objects with extremely low luminosities have recently been discovered (e.g. Willman et al. 2005b,a).”
H i IN LOCAL GROUP DWARF GALAXIES AND STRIPPING BY THE GALACTIC HALO (2009, methodology): “The Sloan Digital Sky Survey data (York et al. 2000) have led to the discovery of 13 new satellites of the Milky Way (Belokurov et al. 2006a; Zucker et al. 2006b; Willman et al. 2005a,b; Belokurov et al. 2006b; Irwin et al. 2007; Walsh et al. 2007; Zucker et al. 2006a; Belokurov et al. 2008).”
Globular cluster systems in nearby dwarf galaxies – III. Formation efficiencies of old globular clusters (2010, result): “This is consistent with the fact that the recently found extremely faint Local Group dwarf galaxies do not possess GCs (e.g. Belokurov et al. 2006, 2007; Irwin et al. 2007; Walsh et al. 2007; Willman et al. 2005a,b; Zucker et al. 2006a,b). c© 2009 RAS, MNRAS 000, 1–??”
ASSESSING THE MILKY WAY SATELLITES ASSOCIATED WITH THE SAGITTARIUS DWARF SPHEROIDAL GALAXY (2010, unknown): “While Willman I lies nearer to the T1/T2 wraps, it is more distant than anticipated for the T1/T2 streams (28/26 kpc respectively) and has a radial velocity inconsistent with these wraps by 102/101 km s−1 respectively.”
CONFIRMATION OF FAINT DWARF GALAXIES IN THE M81 GROUP (2013, background): “…these quantities for the Local Group (Irwin & Hatzidimitriou 1995; Mateo 1998; McConnachie & Irwin 2006; Simon & Geha 2007; Belokurov et al. 2007; Willman et al. 2005; Walsh et al. 2007; Zucker et al. 2006; Martin et al. 2006; Ibata et al. 2007; Irwin et al. 2008; McConnachie 2012) along with…”
THE INVISIBLES: A DETECTION ALGORITHM TO TRACE THE FAINTEST MILKY WAY SATELLITES (2008, methodology): “Twelve of these would be the known objects Boötes, Draco, Canes Venatici I and II, Coma Berenices, Leo I, Leo II, Leo IV, Leo V, Hercules, Ursa Major, and Ursa Major II, leaving one (12) possible missing satellite(s).”
SEGUE: A SPECTROSCOPIC SURVEY OF 240,000 STARS WITH g = 14–20 (2009, unknown): “…structure, Galactic halo, and M31 halo science results; see Ivezic et al. (2000); Yanny et al. (2000); Newberg et al. (2002); Rockosi et al. (2002); Willman et al. (2005); Yanny et al. (2003); Newberg et al. (2003); Zucker et al. (2004a,b); Belokurov et al. (2006a,b); Allende Prieto et al. (2006);…”
A COMPLETE SPECTROSCOPIC SURVEY OF THE MILKY WAY SATELLITE SEGUE 1: THE DARKEST GALAXY (2010, methodology): “Because of the extreme lack of bright stars in these systems, most of the faintest dwarfs such as Willman 1 (Willman et al. 2005), Boötes II (Walsh et al. 2007), Segue 1 (Belokurov et al. 2007a), and Segue 2 (Belokurov et al. 2009) remain relatively poorly characterized by observations; for…”
Small Dwarf Galaxies within Larger Dwarfs: Why Some Are Luminous while Most Go Dark (2008, background): “The population of ultra-faint dwarfs around the MW and M31 found in the Sloan Digital Sky Survey (Willman et al. 2005a,b; Zucker et al. 2006) has increased the number of observed satellites by a factor of two.”
A Comprehensive Maximum Likelihood Analysis of the Structural Properties of Faint Milky Way Satellites (2008, methodology): “…of known dwarf galaxies or dwarf galaxy candidates that reside within the Local Group has doubled over the last very few years (Zucker et al. 2004; Willman et al. 2005a,b; Belokurov et al. 2006; Martin et al. 2006; Sakamoto & Hasegawa 2006; Zucker et al. 2006a,b; Belokurov et al. 2007; Ibata et…”
EIGHT ULTRA-FAINT GALAXY CANDIDATES DISCOVERED IN YEAR TWO OF THE DARK ENERGY SURVEY (2015, methodology): “…galaxies with MV −8mag were identified in Sloan Digital Sky Survey (SDSS; York et al. 2000) data through a combination of systematic searches (Willman et al. 2005a, 2005b; Belokurov et al. 2006, 2007, 2008, 2009, 2010; Grillmair 2006, 2009; Sakamoto & Hasegawa 2006; Zucker et al. 2006a,…”
The spatial distribution of the Milky Way and Andromeda satellite galaxies (2006, methodology): “Two other recently identified stellar systems, SDSS J1049+5103 (Willman et al. 2005) and Boötes (Boo, Belokurov et al. 2006), are excluded from the present analysis because of their uncertain physical nature (globular cluster or dSph galaxy).”
MULTI-ELEMENT ABUNDANCE MEASUREMENTS FROM MEDIUM-RESOLUTION SPECTRA. III. METALLICITY DISTRIBUTIONS OF MILKY WAY DWARF SATELLITE GALAXIES (2010, background): “This LZR now includes all MW dwarfs less luminous than Sagittarius except the least luminous objects (Willman 1, Willman et al. 2005; Segue 1, Belokurov et al. 2007; Segue 2, Belokurov et al. 2009; Boötes II, Walsh et al. 2007; Leo V, Belokurov et al. 2008; Pisces I, Watkins et al. 2009; and…”
Milky Way Satellite Census. I. The Observational Selection Function for Milky Way Satellites in DES Y3 and Pan-STARRS DR1 (2019, methodology): “…multi-band optical imaging surveys— combined with automated search algorithms—have greatly increased the known population of Milky Way satellites (Willman et al. 2005a,b; Zucker et al. 2006a,b; Belokurov et al. 2006, 2007, 2008, 2009, 2010; Grillmair 2006, 2009; Sakamoto & Hasegawa 2006; Irwin…”
Gravitational probes of dark matter physics (2017, background): “elopments and restatements of this problem. Importantly, dozens of new satellites of the Milky Way and Andromeda galaxies have been discovered in the past fteen years, notably in the SDSS (e.g., Ref. [286]), PAndAS [287], DES (e.g., Ref. [288]), PanSTARRS (e.g., Ref. [289]), MagLiteS [290], HSC (Hyp”