A Panoramic Deep View of the Stellar Halo of Andromeda Galaxy
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The structure of the outer regions of galaxies is a key area in which to look for fossil remnants of the accreted masses from which the galaxies that we see today are thought to be built. The importance of these regions has increased in recent years as cosmological theories of structure formation become more exact in their predictions, and the observational instrumentation required to conduct these detailed analyses becomes more sophisticated.

Currently composed of 165 individual pointings of the Isaac Newton Telescope Wide Field Camera (INT WFC), the M31 halo survey consists of photometry for over 7 million sources, on a photometric system accurate to 2% over ~40 square degrees on the sky, in some places probing the halo of Andromeda out to 6° (~80 kpc). Observations of 800–1000 seconds in the Johnson V (V') and Gunn i (i') passbands are deep enough to detect individual RGB stars down to V'= 0 and Main Sequence stars down to V'=–1. This unique dataset has provided, for the first time, a panoramic deep view of the stellar halo of a giant galaxy thought to be similar to our own Milky Way.

M31 Halo
A multi-colour mosaic of the INT WFC survey of M31, involving 165 individual pointings over 40 square degrees of the sky, which shows the inhomogeneity of this system. Metal-poor/young stars are coded blue whilst metal rich/older stars are coded red. The (colour-dependant) substructure is obvious, and surprising given the pristene nature of the Galactic disk. The dwarf galaxies Andromeda I & III are visible at the bottom left of this figure; the newly discovered dwarf spheroidal, Andromeda IX, is just visible at the top left as a small blue dot. NGC 205 is also visible in this figure, at the right-hand side of the disk This spectacular image shows in amazing detail the wealth of information that the INT is helping to reveal about the structure of this previously invisible region of galaxies. [ JPEG | TIFF ]

Despite exhibiting a near pristene disk, M31’s halo is full of substructure and points to a history of accretion and disruption. The most obvious piece of substructure is the giant stellar stream (visible in the south-east). This extends to near the edge of the survey —a projected distance of some 60 kpc. In fact, by examining the systematic shift in the luminosity function of the stream as a function of galactocentric radius, a length much greater than 100 kpc is found. The similarity of the colour of this feature with the loop of material at the north of the survey suggests it seems likely that the northern feature is an extension of the stream, after it has passed very close to the centre of the potential of M31.

A second large stellar stream candidate has also been identified with the INT WFC photometry. The visible part of this feature is some 15 kpc long. The progenitor of this feature appears to be the satellite galaxy NGC 205. This object has long been known to be tidally perturbed but it is only now that the full extent of its disruption is becoming clear. Considerable amounts of other substructure exists in addition to these streams.

As well as these and many other obvious substructures, the INT WFC is allowing the identification of previously unknown globular clusters in the halo of M31. These include some of the most distant so far discovered, at projected radii of ~80 kpc. So far 14 have been found, including a whole new class of cluster, much sparser than typical globulars. These objects, of which three candidates have currently been identified, are far less concentrated and have larger half-light radii than normal, making their appearance fuzzy and diffuse. The identification and quantification of the globular cluster system provides yet another valuable handle on the accretion history of this giant galaxy.

Globular cluster
An example of a new class of globular cluster around M31, much sparser than typical globular clusters, being discovered by the INT WFC survey. Fourteen new globular clusters have so far been discovered, many at large projected radii.Three of these objects have morphologies similar to the above.The half- light radii of these clusters are significantly larger than normal. [ JPEG | TIFF ]

The other spiral in the Local Group, the Triangulum Galaxy (M33), has also been surveyed with the INT WFC. The structure of this galaxy is striking in comparison to M31: the lack of substructure is immediately obvious. It appears that not all spiral galaxy haloes need look like M31. There is then the question of the M31 dwarf satellite galaxies. The homogeneous nature of the data has allowed accurate and internally self-consistent distances and metallicities to be measured for each of these galaxies. For the first time, the three dimensional spatial distribution of these objects, has reliably probed and revealing that far from being isotropically distributed and unbiased indicators of the potential of Andromeda, there are strong indications that these objects are preferentially located on the near side of Andromeda, towards the Galaxy.

Distribution of satellite galaxies
The distribution of the satellite galaxies of M31, as derived from the INT WFC photometry of these objects. The coordinate system is an M31–centric system. The plane is the plane of the disk of M31, and each cell corresponds to 100 kpc×100 kpc. l is a longitude measured around the disk of M31, such that l=0 is the longitude of the Galaxy. b is a latitude, measured from the disk of M31. Solid lines indicate objects located above the plane of the disk, while dashed lines indicate objects below the plane of the disk. A clear tendency for the satellites to lie on the near side of M31 can be observed, and suggests an intriguing correlation between the M31 satellites and our own Galaxy. [ JPEG | TIFF ]

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Last modified: 13 December 2010