The Andromeda Stream

The existence of a stream of tidally stripped stars from the Sagittarius Dwarf galaxy demonstrates that the Milky Way is still in the process of accreting mass. More recently, an extensive stream of stars has been uncovered in the halo of the Andromeda galaxy (M31), revealing that it too is cannibalizing a small companion. This paper reports the recent observations of this stream, determining it spatial and kinematic properties, and tracing its three-dimensional structure, as well as describing future observations and what we may learn about the Andromeda galaxy from this giant tidal stream.Comment: 3 Pages. Refereed contribution to the 5th Galacto Chemodynamics conference held in Swinburne, July 2003. Accepted for publication in PAS

A Keck/DEIMOS spectroscopic survey of the faint M31 satellites And IX, And XI, And XII, and And XIII

We present the first spectroscopic analysis of the faint M31 satellite galaxies, AndXI and AndXIII, and a reanalysis of existing spectroscopic data for two further faint companions, And IX and AndXII. By combining data obtained using the DEIMOS spectrograph mounted on the Keck II telescope with deep photometry from the Suprime-Cam instrument on Subaru, we have calculated global properties for the dwarfs, such as systemic velocities, metallicites and half-light radii.We find each dwarf to be very metal poor ([Fe/H] -2 both photometrically and spectroscopically, from their stacked spectrum), and as such, they continue to follow the luminosity-metallicity relationship established with brighter dwarfs. We are unable to resolve a dispersion for And XI due to small sample size and low S/N, but we set a one sigma upper limit of sigma-v <5 km/s. For And IX, And XII and And XIII we resolve velocity dispersions of v=4.5 (+3.4,-3.2), 2.6(+5.1,-2.6) and 9.7(+8.9,-4.5) km/s, and derive masses within the half light radii of 6.2(+5.3,-5.1)x10^6 Msun, 2.4 (+6.5,-2.4)x10^6 Msun and 1.1(+1.4,-0.7)x10^7 Msun respectively. We discuss each satellite in the context of the Mateo relations for dwarf spheroidal galaxies, and the Universal halo profiles established for Milky Way dwarfs (Walker et al. 2009). For both galaxies, this sees them fall below the Universal halo profiles of Walker et al. (2009). When combined with the findings of McConnachie & Irwin (2006a), which reveal that the M31 satellites are twice as extended (in terms of both half-light and tidal radii) as their Milky Way counterparts, these results suggest that the satellite population of the Andromeda system could inhabit halos that are significantly different from those of the Milky Way in terms of their central densities (abridged).Comment: 26 pages, 18 figures, MNRAS submitte

The photometric properties of a vast stellar substructure in the outskirts of M33

We have surveyed $\sim40$sq.degrees surrounding M33 with CFHT MegaCam in the g and i filters, as part of the Pan-Andromeda Archaeological Survey. Our observations are deep enough to resolve the top 4mags of the red giant branch population in this galaxy. We have previously shown that the disk of M33 is surrounded by a large, irregular, low-surface brightness substructure. Here, we quantify the stellar populations and structure of this feature using the PAndAS data. We show that the stellar populations of this feature are consistent with an old population with $\sim-1.6$dex and an interquartile range in metallicity of $\sim0.5$dex. We construct a surface brightness map of M33 that traces this feature to $\mu_V\simeq33$mags\,arcsec$^{-2}$. At these low surface brightness levels, the structure extends to projected radii of $\sim40$kpc from the center of M33 in both the north-west and south-east quadrants of the galaxy. Overall, the structure has an "S-shaped" appearance that broadly aligns with the orientation of the HI disk warp. We calculate a lower limit to the integrated luminosity of the structure of $-12.7\pm0.5$mags, comparable to a bright dwarf galaxy such as Fornax or AndII and slightly less than $1\$ of the total luminosity of M33. Further, we show that there is tentative evidence for a distortion in the distribution of young stars near the edge of the HI disk that occurs at similar azimuth to the warp in HI. The data also hint at a low-level, extended stellar component at larger radius that may be a M33 halo component. We revisit studies of M33 and its stellar populations in light of these new results, and we discuss possible formation scenarios for the vast stellar structure. Our favored model is that of the tidal disruption of M33 in its orbit around M31.Comment: Accepted for publication in ApJ. 17 figures. ApJ preprint forma

The PAndAS view of the Andromeda satellite system - I. A Bayesian search for dwarf galaxies using spatial and color-magnitude information

We present a generic algorithm to search for dwarf galaxies in photometric catalogs and apply it to the Pan-Andromeda Archaeological Survey (PAndAS). The algorithm is developed in a Bayesian framework and, contrary to most dwarf-galaxy-search codes, makes use of both the spatial and color-magnitude information of sources in a probabilistic approach. Accounting for the significant contamination from the Milky Way foreground and from the structured stellar halo of the Andromeda galaxy, we recover all known dwarf galaxies in the PAndAS footprint with high significance, even for the least luminous ones. Some Andromeda globular clusters are also recovered and, in one case, discovered. We publish a list of the 143 most significant detections yielded by the algorithm. The combined properties of the 39 most significant isolated detections show hints that at least some of these trace genuine dwarf galaxies, too faint to be individually detected. Follow-up observations by the community are mandatory to establish which are real members of the Andromeda satellite system. The search technique presented here will be used in an upcoming contribution to determine the PAndAS completeness limits for dwarf galaxies. Although here tuned to the search of dwarf galaxies in the PAndAS data, the algorithm can easily be adapted to the search for any localised overdensity whose properties can be modeled reliably in the parameter space of any catalog.Comment: 20 pages, 16 figures, 1 table; accepted for publication in ApJ. High res pdf available at https://www.dropbox.com/s/7zk7pme2wunwkjv/PAndAS_dwarf_galaxies.pd

PAndAS' cubs: discovery of two new dwarf galaxies in the surroundings of the Andromeda and Triangulum galaxies

We present the discovery of two new dwarf galaxies, Andromeda XXI and Andromeda XXII, located in the surroundings of the Andromeda and Triangulum galaxies (M31 and M33). These discoveries stem from the first year data of the Pan-Andromeda Archaeological Survey (PAndAS), a photometric survey of the M31/M33 group conducted with the Megaprime/MegaCam wide-field camera mounted on the Canada-France-Hawaii Telescope. Both satellites appear as spatial overdensities of stars which, when plotted in a color-magnitude diagram, follow metal-poor, [Fe/H]=-1.8, red giant branches at the distance of M31/M33. Andromeda XXI is a moderately bright dwarf galaxy (M_V=-9.9+/-0.6), albeit with low surface brightness, emphasizing again that many relatively luminous M31 satellites still remain to be discovered. It is also a large satellite, with a half-light radius close to 1 kpc, making it the fourth largest Local Group dwarf spheroidal galaxy after the recently discovered Andromeda XIX, Andromeda II and Sagittarius around the Milky Way, and supports the trend that M31 satellites are larger than their Milky Way counterparts. Andromeda XXII is much fainter (M_V=-6.5+/-0.8) and lies a lot closer in projection to M33 than it does to M31 (42 vs. 224 kpc), suggesting that it could be the first Triangulum satellite to be discovered. Although this is a very exciting possibility in the context of a past interaction of M33 with M31 and the fate of its satellite system, a confirmation will have to await a good distance estimate to confirm its physical proximity to M33. Along with the dwarf galaxies found in previous surveys of the M31 surroundings, these two new satellites bring the number of dwarf spheroidal galaxies in this region to 20.Comment: 10 pages, 6 figures, accepted for publication in ApJ; v2: minor typographical correction

Young accreted globular clusters in the outer halo of M31

We report on Gemini/GMOS observations of two newly discovered globular clusters in the outskirts of M31. These objects, PAndAS-7 and PAndAS-8, lie at a galactocentric radius of ~87 kpc and are projected, with separation ~19 kpc, onto a field halo substructure known as the South-West Cloud. We measure radial velocities for the two clusters which confirm that they are almost certainly physically associated with this feature. Colour-magnitude diagrams reveal strikingly short, exclusively red horizontal branches in both PA-7 and PA-8; both also have photometric [Fe/H] = -1.35 +/- 0.15. At this metallicity, the morphology of the horizontal branch is maximally sensitive to age, and we use the distinctive configurations seen in PA-7 and PA-8 to demonstrate that both objects are very likely to be at least 2 Gyr younger than the oldest Milky Way globular clusters. Our observations provide strong evidence for young globular clusters being accreted into the remote outer regions of M31 in a manner entirely consistent with the established picture for the Milky Way, and add credence to the idea that similar processes play a central role in determining the composition of globular cluster systems in large spiral galaxies in general.Comment: 14 pages, 8 figures, accepted for publication in MNRA