The dwarf satellite galaxies in the Local Group are generally considered to be hosted in dark matter subhalos that survived the disruptive processes during infall onto their host halos. It has recently been argued that if the majority of satellites entered the Milky Way halo in a group rather than individually, this could explain the spatial and dynamical peculiarities of its satellite distribution. Such groups were identified as dwarf galaxy associations that are found in the nearby Universe. In this paper we address the question whether galaxies in such associations can be the progenitors of the Milky Way satellite galaxies. We find that the dwarf associations are much more extended than would be required to explain the disk-like distribution of the Milky Way and Andromeda satellite galaxies. We further identify a possible minor filamentary structure, perpendicular to the supergalactic plane, in which the dwarf associations are located, that might be related to the direction of infall of a progenitor galaxy of the Milky Way satellites, if they are of tidal origin.
Deep Dive into Did the Milky Way dwarf satellites enter the halo as a group?.
The dwarf satellite galaxies in the Local Group are generally considered to be hosted in dark matter subhalos that survived the disruptive processes during infall onto their host halos. It has recently been argued that if the majority of satellites entered the Milky Way halo in a group rather than individually, this could explain the spatial and dynamical peculiarities of its satellite distribution. Such groups were identified as dwarf galaxy associations that are found in the nearby Universe. In this paper we address the question whether galaxies in such associations can be the progenitors of the Milky Way satellite galaxies. We find that the dwarf associations are much more extended than would be required to explain the disk-like distribution of the Milky Way and Andromeda satellite galaxies. We further identify a possible minor filamentary structure, perpendicular to the supergalactic plane, in which the dwarf associations are located, that might be related to the direction of infal
Dwarf galaxies in the Local Group, and satellite galaxies of the Milky Way (MW) and Andromeda (M31) in particular, are of great importance to understand the physics of structure formation on galaxy scales because they can be studied in unsurpassable detail. Commonly they are associated with cosmological sub-structures that entered the Milky Way halo and it has been inferred, assuming that the dwarf galaxies are in virial equilibrium, that they are dominated by a massive dark matter component though the nature of this component is still disputed (Gilmore et al. 2007). There are, however, some puzzling findings not yet completely explained.
For instance, it had been noticed that the number of observed satellite galaxies in the Local Group is at least an order of magnitude smaller than expected from cold dark matter (CDM) simulations (Moore et al. 1999;Klypin et al. 1999;Diemand et al. 2008). Different scenarios were subsequently proposed to explain the satellite galaxies in the Local Group in the context of being CDM dominated subhalos: Stoehr et al. (2002) argued that only the most massive subhalos were able to form stars and the rest thus remains dark. Libeskind et al. (2005) proposed that current satellite galaxies are those with the most massive progenitors, also found by Strigari et al. (2007) who alternatively found accordance with the earliest forming halos. In contrast, Sales et al. (2007b) argued for low-mass systems being the origin of satellite galaxies, or it has been suggested that observers just overlooked all the hundreds of satellite galaxies surrounding the Milky Way due to their extreme low star densities (Tollerud et al. 2008).
Recently the interesting scenario was put forward that the satellite galaxies did not enter the Milky Way halo individually in a random fashion, but rather in groups. Li & Helmi (2008) argued that if only one or two groups fell into the MW halo this could account for the observed highly anisotropic spatial appearance of MW companions, the disk of satellites (DoS, Metz, Kroupa, & Jerjen 2007). D ‘Onghia & Lake (2008) suggested that groups formed within LMC-like dark matter host-halos and that these hosts are able to produce many more satellites in subhalos than in a MW-like host-halo. They predicted that such groups are visible today and linked them as the dwarf associations found by Tully et al. (2006).
Here we study the properties of the dwarf associations, and compare them to the satellite galaxies of the Milky Way. In Section 2.1 we discuss the membership of satellites in the proposed Magellanic Group. The implications of the extent of observed dwarf associations is addressed in Section 2.2 and their spatial locations are analysed in Section 2.3. We finally discuss the proposed scenario in Section 3 and summarize our conclusions in Section 4.
- Associations of dwarf galaxies 2.1. A Magellanic Group? D’Onghia & Lake (2008) proposed that the majority of the brighter satellite galaxies of the Milky Way originate from a single group of dwarf galaxies in dark matter subhalos with their main component being the LMC that fell late into the Milky Way halo. Indeed, such associations of dwarf galaxies exist within 8 Mpc of the Local Group (Tully et al. 2006).
Specifically, D’Onghia & Lake proposed that the Magellanic Clouds together with the Sagittarius, Ursa Minor, Draco, Sextans, and Leo II dwarf galaxies once belonged to such a group (hereafter DL08 sample). This appears to be a rather ad-hoc selected sample and is based on a mix of different and disagreeing references. Earlier proposed streams of MW satellite galaxies, that are referred to, consist of different objects (LMC, SMC, Dra, UMi, & Scl; Lynden-Bell 1976), and a second different plane as proposed by Kunkel & Demers (LMC, SMC, Dra, UMi, Car, Leo I & II; Kunkel & Demers 1976;Kunkel 1979) is ∼ 40 • off the one proposed by Lynden-Bell. Later, Lynden-Bell associated Fornax, Leo I, Leo II, and Sculptor to the so-called FLS stream, whereas he confided Carina to the LMC group (Lynden-Bell 1982a,b). It is important to note that two of the galaxies in the DL08 sample, Sextans and Sagittarius, were not known until the 90s (Irwin et al. 1990;Ibata et al. 1994), and thereafter Kroupa, Theis, & Boily (2005) and Metz et al. (2007) did not identify an individual stream, but rather highlighted the fact that all satellite galaxies of the MW belong to a virtual plane -the disk of satellites.
In particular one finds that the Sagittarius dwarf galaxy today must be on a quite different orbit than the other galaxies in the proposed ensemble. Based on its location and proper motion measurements one arrives at an orbit that is perpendicular to that of the Magellanic Clouds (Palma et al. 2002;Metz et al. 2008). Furthermore, models describing the Sagittarius Stream suggest rather short orbital time-scales for Sagittarius, t orb < 1 Gyr, small Galactic apocentric distances, ∼ 60 kpc, and that the dwarf must have been on this close
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