A giant stream of metal-rich stars in the halo of the galaxy M31

Abstract

The Milky Way is a cannibal. Recent observations have revealed the detritus of its digestion of the Sagittarius Dwarf and Magellanic Clouds, apparent as streams of gaseous and stellar debris littering the Galactic Halo ? , ? , ? . The analysis of this material has shown that the dark matter distribution within our own Galaxy is nearly spherical ? , and that much of the Galactic Halo is made up of cannibalized satellite galaxies ? , ? . Yet it remains unclear whether such halo substructures really are as ubiquitous and as numerous as predicted by galaxy formation theory ? . Here we report the discovery of a giant stream of metal-rich stars within the halo of the Andromeda galaxy (M31), the Milky Way's nearest large neighbour. The source of this stream could be the dwarf galaxies M32 and NGC 205, close companions of M31, which have possibly lost a substantial amount of stars due to their tidal interactions with their large neighbour. These observations clearly demonstrate that the epoch of galaxy building still continues, albeit at a modest rate, and that substructure in the form of huge, recently-deposited tidal streams, may be a generic feature of galaxy halos.Within the framework of hierarchical structure formation, large spiral galaxies like the Milky Way or Andromeda arose from the merger of many small galaxies and proto-galaxies ? . Later in their evolution, spiral galaxies become the dominant component in such mergers, digesting smaller systems that fall within their sphere of influence. The complete destruction of the victim is usually progressive and may take several orbits. However, the stellar debris from the destroyed dwarf galaxy follows a similar orbital trajectory to the progenitor, which preferentially will have started life far away from the place of its final demise, and so the tidally disrupted matter tends to be deposited over a broad range in distance from the larger galaxy. Over time, with the accumulation of many such mergers, large galaxies develop an extensive stellar and dark matter "halo", the latter being by far the most massive component of the galaxy. Meanwhile, part of the (dissipative) gas component of the smaller galaxies feeds the growth of the disk of the larger galaxy. This is clearly seen in numerical simulations of galaxy formation, which result in galactic halos comprised of clumps of dark matter ? .Is this prediction of 'dotty cosmology' correct, with halos possessing significant substructure, or are the dark and luminous components of halos distributed smoothly, as had previously been suggested ? ?The majority of the Milky Way halo is metal-poor and, to first order, smoothly distributed, however, recent studies have demonstrated that the halo contains non-negligible stellar substructure. Evidence for the phase-space clumping of halo stars has even been found in the Solar Neighbourhood, where ∼ 10% of stars may be associated to a single ancient accretion event ? . The discovery that the Milky Way is surrounded by a giant rosette-like stream originating from the Sagittarius d...

The Future of Stellar Populations Studies in the Milky Way and the Local Group

“…This work has revealed the Giant Southern Stream (Ibata et al 2001), a halo substructure connecting with other filagree noted around the M31 disk (e.g., Ibata et al 2004, Fardal et al 2006Gilbert et al 2007, which makes it a "Sgr stream-equivalent" in our spiral twin sister. Other M31 halo substructure has also been revealed (e.g., Chapman et al 2008) as well as an entire host of newly found M31 dSph satellites (e.g., Martin et al 2006 S. R. Majewski et al 2007, Zucker et al 2007, Irwin et al 2008, McConnachie et al 2008.…”

Resolving the Stellar Outskirts of M31 and M33

“…The faint structures visible by eye in Figure 1 have effective V-band surface brightnesses in the range 29-30 magnitudes per square arcsec. Early versions of our M31 maps have been discussed in Ibata et al (2001), Ferguson et al (2002) and Irwin et al (2005).…”

A Keck/Deimos Survey of Red Giant Branch Stars in the Outskirts of M31

“…Figure 1 shows the distribution of "blue" RGB stars across our survey area. As already apparent from our interim results [8,4], the distribution of giant branch stars at large radius is very far from uniform. Many of the clumpy features visible in the stellar distribution have effective surface brightnesses Σ V ∼ > 28 mag sq.…”

“…deg. (163 contiguous pointings) around M31 with 800-1200 sec exposures in the V and i passbands [8,4,10]. This depth is sufficient to reach V∼ 24.5, i∼ 23.5, and hence probe the top three magnitudes of the red giant branch (RGB).…”

“…While such features have been previously found in our Milky Way, little has been known, until recently, about their frequency in other galaxies. One of the main goals of the Isaac Newton Telescope Wide-Field Camera (INT WFC) survey of M31 [8,4,10] has been to search for signatures of satellite disruption in the outer halo of our nearest giant neighbour.…”