Geraint F. Lewis

The remnants of galaxy formation from a panoramic survey of the region around M31.

In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10–100 times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31’s satellites brighter than Mv = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.

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

Recent observations have revealed streams of gas and stars in the halo of the Milky Way that are the debris from interactions between our Galaxy and some of its dwarf companion galaxies; the Sagittarius dwarf galaxy and the Magellanic clouds. Analysis of the material has shown that much of the halo is made up of cannibalized satellite galaxies, and that dark matter is distributed nearly spherically in the Milky Way. It remains unclear, however, whether cannibalized substructures are as common in the haloes of galaxies as predicted by galaxy-formation theory. Here we report the discovery of a giant stream of metal-rich stars within the halo of the nearest large galaxy, M31 (the Andromeda galaxy). The source of this stream could be the dwarf galaxies M32 and NGC205, which are close companions of M31 and which may have lost a substantial number of stars owing to tidal interactions. The results demonstrate that the epoch of galaxy building still continues, albeit at a modest rate, and that tidal streams may be a generic feature of galaxy haloes.

The Haunted Halos of Andromeda and Triangulum: A Panorama of Galaxy Formation in Action*

We present a deep photometric survey of the Andromeda galaxy, conducted with the wide-field cameras of the CFHT and INT telescopes. The surveyed area covers the inner 50kpc of the galaxy and the Southern quadrant out to a projected distance of ∼ 150kpc. A survey extension to M33 at > 200kpc probes the interface between the halos of these two galaxies. This survey is the first systematic panoramic study of this very outermost region of galaxies. We detect a multitude of largescale structures of low surface brightness, including several streams. Significant variations in stellar populations due to intervening stream-like structures are detected in the inner halo along the minor axis. This, together with the fact that the light profile between 0 ◦ .5 < R < 1 ◦ .3 follows the exponential “extended disk”, is particularly important in shedding light on the mixed and sometimes conflicting results reported in previous studies. Two new relatively luminous (MV ∼ −9) dwarf galaxies And XV and XVI are found in the study; And XVI is a particularly interesting specimen being located 270kpc in front of M31, towards the Milky Way. Underlying the many substructures that we have uncovered lies a faint, smooth and extremely extended halo component, reaching out to 150 kpc, whose stellar populations are predominantly metal-poor. This is consistent with recent claims based on spectroscopy of a small sample of stars. We find that the smooth halo component in M31 has a radially-decreasing profile that can be fit with a Hernquist model of immense scale radius ∼ 55kpc, almost a factor

A vast, thin plane of corotating dwarf galaxies orbiting the Andromeda galaxy

Dwarf satellite galaxies are thought to be the remnants of the population of primordial structures that coalesced to form giant galaxies like the Milky Way. It has previously been suspected that dwarf galaxies may not be isotropically distributed around our Galaxy, because several are correlated with streams of H i emission, and may form coplanar groups. These suspicions are supported by recent analyses. It has been claimed that the apparently planar distribution of satellites is not predicted within standard cosmology, and cannot simply represent a memory of past coherent accretion. However, other studies dispute this conclusion. Here we report the existence of a planar subgroup of satellites in the Andromeda galaxy (M 31), comprising about half of the population. The structure is at least 400 kiloparsecs in diameter, but also extremely thin, with a perpendicular scatter of less than 14.1 kiloparsecs. Radial velocity measurements reveal that the satellites in this structure have the same sense of rotation about their host. This shows conclusively that substantial numbers of dwarf satellite galaxies share the same dynamical orbital properties and direction of angular momentum. Intriguingly, the plane we identify is approximately aligned with the pole of the Milky Way’s disk and with the vector between the Milky Way and Andromeda.

A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion on to the Milky Way

We present an analysis of the asymmetries in the population of Galactic M-giant stars present in the 2MASS All Sky catalogue. Several large-scale asymmetries are detected, the most significant of which is a strong elliptical-shaped stellar over-density, close to the Galactic plane at (l = 240 ◦ , b = 8 ◦ ), in the constellation of Canis Major. A small grouping of globular clusters (NGC 1851, NGC 1904, NGC 2298, and NGC 2808), coincident in position and radial velocity, surround this structure, as do a number of open clusters. The population of M-giant stars in this over-density is similar in number to that in the core of the Sagittarius dwarf galaxy. We argue that this object is the likely dwarf galaxy progenitor of the ring-like structure that has recently been found at the edge of the Galactic disk. A numerical study of the tidal disruption of an accreted dwarf galaxy is presented. The simulated debris fits well the extant position, distance and velocity information on the “Galactic Ring”, as well as that of the M-giant overdensities, suggesting that all these structures are the consequence of a single accretion event. The disrupted dwarf galaxy stream orbits close to the Galactic Plane, with a pericentre at approximately the Solar circle, an orbital eccentricity similar to that of stars in the Galactic thick disk, as well as a vertical scale height similar to that of the thick disk. This finding strongly suggests that the Canis Major dwarf galaxy is a building block of the Galactic thick disk, that the thick disk is continually growing, even up to the present time, and that thick disk globular clusters were accreted onto the Milky Way from dwarf galaxies in co-planar orbits.

The Sydney‐AAO Multi‐object Integral field spectrograph

We demonstrate a novel technology that combines the power of the multi-object spectrograph with the spatial multiplex advantage of an integral field spectrograph (IFS). The SydneyAAO (Australian Astronomical Observatory) Multi-object IFS (SAMI) is a prototype widefield system at the Anglo-Australian Telescope (AAT) that allows 13 imaging fibre bundles (‘hexabundles’) to be deployed over a 1-degree diameter field of view. Each hexabundle comprises 61 lightly fused multi-mode fibres with reduced cladding and yields a 75 per cent filling factor. Each fibre core diameter subtends 1.6 arcsec on the sky and each hexabundle has a field of view of 15 arcsec diameter. The fibres are fed to the flexible AAOmega double-beam spectrograph, which can be used at a range of spectral resolutions (R = λ/δλ ≈ 1700–13 000) over the optical spectrum (3700–9500 A). We present the first spectroscopic results obtained with SAMI for a sample of galaxies at z ≈ 0.05. We discuss the prospects of implementing hexabundles at a much higher multiplex over wider fields of view in order to carry out spatially resolved spectroscopic surveys of 10 4 –10 5 galaxies.

Evidence for stellar substructure in the halo and outer disk of M31

Our panoramic imaging survey of M31 with the INT Wide-Field Camera currently maps an area of 25 squaredegrees around our nearest large galactic neighbour. We discuss evidence for spatial density and metallicity (as inferred from color) variations in the distribution of individual red giant branch stars in the halo and outer disk.

A Keck/DEIMOS spectroscopic survey of faint Galactic satellites: Searching for the least massive dwarf galaxies

We present the results of a spectroscopic survey of the recently discovered faint Milky Way satellites Bootes, Ursa Major I, Ursa Major II and Willman 1 (Will). Using the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope, we have obtained samples that contain from ∼ 15 to ∼85 probable members of these satellites for which we derive radial velocities precise to a few km s -1 down to i ∼ 21-22. About half of these stars are observed with a high enough signal-to-noise ratio to estimate their metallicity to within ±0.2 dex. The characteristics of all the observed stars are made available, along with those of the Canes Venatici I dwarf galaxy that have been analysed in a companion paper. From this data set, we show that Ursa Major II is the only object that does not show a clear radial velocity peak. However, the measured systemic radial velocity (υ r = 115 ± 5kms -1 ) is in good agreement with simulations in which this object is the progenitor of the recently discovered Orphan Stream. The three other satellites show velocity dispersions that make them highly dark matter dominated systems (under the usual assumptions of symmetry and virial equilibrium). In particular, we show that despite its small size and faintness, the Will object is not a globular cluster given its metallicity scatter over -2.0 ≤ [Fe/H] ≤ -1.0 and is therefore almost certainly a dwarf galaxy or dwarf galaxy remnant. We measure a radial velocity dispersion of only 4.3 +2.3-1.3 km s -1 around a systemic velocity of -12.3 ± 2.3 km s -1 which implies a mass-to-light ratio of ∼700 and a total mass of ∼5 x 10 5 M O for this satellite, making it the least massive satellite galaxy known to date. Such a low mass could mean that the 10 7 M O limit that had until now never been crossed for Milky Way and Andromeda satellite galaxies may only be an observational limit and that fainter, less massive systems exist within the Local Group. However, more modelling and an extended search for potential extratidal stars are required to rule out the possibility that these systems have not been significantly heated by tidal interaction.

ON THE ACCRETION ORIGIN OF A VAST EXTENDED STELLAR DISK AROUND THE ANDROMEDA GALAXY

We present the discovery of an inhomogenous, low surface brightness, extended disklike structure around the Andromeda galaxy (M31) based on a large kinematic survey of more than 2800 stars with the Keck DEIMOS multiobject spectrograph. The stellar structure spans radii from 15 kpc out to ~40 kpc, with detections out to R ~ 70 kpc. The constituent stars have velocities close to the expected velocity of circular orbits in the plane of the M31 disk and typically have a velocity dispersion of ~30 km s-1. The color range on the upper red giant branch shows a large spread indicative of a population with a significant range of metallicity. The mean metallicity of the population, measured from Ca II equivalent widths, is [Fe/H] = -0.9 ± 0.2. The morphology of the structure is irregular at large radii and shows a wealth of substructures that must be transitory in nature and are almost certainly tidal debris. The presence of these substructures indicates that the global entity was formed by accretion. This extended disk follows smoothly on from the central parts of M31 disk out to ~40 kpc with an exponential density law with a scale length of 5.1 ± 0.1 kpc, which is similar to that of the bright inner disk. However, the population possesses similar kinematic and abundance properties over the entire region where it is detected in the survey. We estimate that the structure accounts for approximately 10% of the total luminosity of the M31 disk, and given the huge scale, contains ~30% of the total disk angular momentum. This finding indicates that at least some galactic stellar disks are vastly larger than previously thought and are formed, at least in their outer regions, primarily by accretion.

One ring to encompass them all: a giant stellar structure that surrounds the Galaxy

We present evidence that the curious stellar population found by the Sloan Digital Sky Survey in the Galactic anticentre direction extends to other distant fields that skirt the plane of the Milky Way. New data, taken with the Isaac Newton Telescope Wide Field Camera, show a similar population, narrowly aligned along the line of sight, but with a galactocentric distance that changes from similar to15 to similar to20 kpc (over similar to100degrees on the sky). Despite being narrowly concentrated along the line of sight, the structure is fairly extended vertically out of the plane of the disc, with a vertical scaleheight of 0.75 +/- 0.04 kpc. This finding suggests that the outer rim of the Galaxy ends in a low surface brightness stellar ring. Presently available data do not allow us to ascertain the origin of the structure. One possibility is that it is the wraith of a satellite galaxy devoured long ago by the Milky Way, although our favoured interpretation is that it is a perturbation of the disc, possibly the result of ancient warps. Assuming that the ring is smooth and axisymmetric, the total stellar mass in the structure may amount to similar to2 x 10(8) M. up to similar to10(9) M(.).