ner_tags: O O O O O O O O O O O O O O O B-Survey I-Survey I-Survey I-Survey I-Survey B-Survey B-Citation I-Citation I-Citation I-Citation O O O B-Survey I-Survey I-Survey I-Survey B-Survey B-Citation I-Citation I-Citation I-Citation O O O O O O O O B-CelestialObject I-CelestialObject O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject B-CelestialRegion B-Citation I-Citation I-Citation I-Citation O O O O O O O O B-CelestialRegion I-CelestialRegion O O O O O O B-Model I-Model I-Model B-Model B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O B-Citation I-Citation I-Citation O O O O O O O O B-Model O O O O B-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O O O O O O O B-Citation I-Citation O B-Citation O O O O O B-Citation I-Citation O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O O O O O O O O B-Citation I-Citation I-Citation I-Citation O B-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation O B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject B-Citation I-Citation I-Citation I-Citation O B-Citation I-Citation I-Citation I-Citation O O O O O O O B-CelestialObject I-CelestialObject I-CelestialObject I-CelestialObject B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject B-CelestialObject B-Citation I-Citation I-Citation I-Citation O O O B-CelestialObject B-CelestialObject O B-Citation I-Citation I-Citation I-Citation O O O O O O O O B-CelestialObject O O B-CelestialObject I-CelestialObject B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-CelestialRegion I-CelestialRegion B-Citation I-Citation I-Citation I-Citation O O O O O O O O O O O O O O O B-CelestialObject I-CelestialObject I-CelestialObject B-CelestialObject O O O O O O O B-CelestialObject B-Citation I-Citation I-Citation I-Citation O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O O B-CelestialObject I-CelestialObject O O O O O B-Survey O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O O O O O O O O B-CelestialObject O O O O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation I-Citation I-Citation I-Citation O O O O O B-Survey O O
tokens: years. This is due in large part to deep, wide-field imaging surveys such as the Two Micron All Sky Survey (2MASS; Skrutskie et al. 2006 ) and the Sloan Digital Sky Survey (SDSS; York et al. 2000 ), which have enabled large-scale studies of the Milky Way to its outermost radii, and have resulted in the discovery of a variety of new and interesting substructures that lie within the Milky Way’s halo (Ivezić et al. 2012 ). Our understanding of the formation of the Galactic halo has therefore evolved from a simple Eggen, Lynden-Bell, Sandage (ELS; Eggen et al. 1962 ) monolithic collapse model to a much more complex and dynamic structure that is still being shaped by the merging of neighboring smaller galaxies (Searle Zinn 1978 ). This merger process is predicted by hierarchical ΛCDM models of galaxy formation (Steinmetz Navarro 2002 ; Bullock Johnston 2005 ; Font et al. 2011 ). In addition to classical globular clusters (Harris 1996 , 2010 ) and classical dwarf galaxies (Mateo 1998 ), these modern surveys have also discovered many low-luminosity globular clusters (Koposov et al. 2007 ; Balbinot et al. 2013 ), a large number of ultra-faint dwarf galaxies (Willman et al. 2005a , 2005b ; Belokurov et al. 2006a , 2007 ; Zucker et al. 2006 ; Walsh et al. 2007 ), and stellar streams, which are thought to have originated from the tidal disruption of either dwarf galaxies (see, e.g., the Sagittarius stream; Majewski et al. 2003 ; Belokurov et al. 2006b ) or globular clusters (see, e.g., the Palomar 5 tidal tails; Odenkirchen et al. 2001 ). However, there are other structures that cannot be classified by one of these familiar designations. Modern wide-field deep imaging surveys have enabled for the first time the discovery of large-scale, highly diffuse halo structures that can cover up to several hundred square degrees on the sky. Examples of these recently identified extended stellar distributions include the Virgo overdensity (VOD; Jurić et al. 2008 ) and the Hercules–Aquila (HerAq) cloud (Belokurov et al. 2007 ), both located about 20 kpc from the Sun, and the Pisces overdensity (Sesar et al. 2007 ) at a distance of about 85 kpc. These overdensities have generally been identified by noting excesses of a particular stellar tracer in a restricted area of the sky. For example, excess halo stars were first found in the Virgo constellation (Vivas et al. 2001 ), , by using RR Lyrae stars as tracers. This structure is known as the Virgo stellar stream (VSS) and lies about 20 kpc from the Sun (Duffau et al. 2006 ). In a similar location in the sky, Jurić et al. ( 2008 ), working with a large sample of Milky Way disk and halo stars from SDSS, identified a very extended overdensity that stood out from smooth models of the Galaxy’s thin/thick disks and halo at a heliocentric distance of ∼6–20 kpc. This structure is dubbed the VOD and appears to encompass an area as large as 1000 deg 2 . Bonaca et al. ( 2012 ) used a later data release from SDSS and estimated