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Stellar halos may hold some of the best preserved fossils of the formation history of galaxies. They are a natural product of the merging processes that probably take place during the assembly of a galaxy, and hence may well be the most ubiquitous component of galaxies, independently of their Hubble type. This review focuses on our current understanding of the spatial structure, the kinematics and chemistry of halo stars in the Milky Way. In recent years, we have experienced a change in paradigm thanks to the discovery of large amounts of substructure, especially in the outer halo. I discuss the implications of the currently available observational constraints and fold them into several possible formation scenarios. Unraveling the formation of the Galactic halo will be possible in the near future through a combination of large wide field photometric and spectroscopic surveys, and especially in the era of Gaia., 46 pages, 16 figures. References updated and some minor changes. Full-resolution version available at http://www.astro.rug.nl/~ahelmi/stellar-halo-review.pdf

It has been recently claimed that the confined structure of the debris from the Sagittarius dwarf implies that the dark-matter halo of our Galaxy should be nearly spherical, in strong contrast with predictions from cold dark-matter (CDM) simulations, where dark halos are found to have typical density axis ratios of 0.6 - 0.8. In this letter, numerical simulations are used to show that the Sagittarius streams discovered thus far are too young dynamically to be sensitive to the shape of the dark halo of the Milky Way. The data presently available are entirely consistent with a Galactic dark-matter halo that could either be oblate or prolate, with minor-to-major density axis ratios c/a as low as 0.6 within the region probed by the orbit of the Sagittarius dwarf., Comment: 6 pages, 5 figures, MNRAS in press. Various changes to the discussion section, typos fixed, 2 new figures and more references added, conclusions unchanged. High-resolution figures available from http://www.astro.rug.nl/~ahelmi/research/flattening.html

We report the discovery of significant stellar substructure in the halo and outer disk of our nearest large galactic neighbour, M31. Our deep panoramic survey with the Isaac Newton Telescope Wide Field Camera currently maps out an area of ~25 square degrees around M31, extending along the semi-major axis to 55 kpc, and is the first to allow an uninterrupted study of the density and color distribution of individual red giant branch stars across a large fraction of the halo of an external spiral galaxy. We find evidence for both spatial density and metallicity (as inferred from colour information) variations, which are often, but not always, correlated. In addition to the previously reported giant stellar stream (Ibata 2001b), the data reveal the presence of significant stellar overdensities at large radii close to the south-western major axis, in the proximity of the very luminous globular cluster G1, and near the north-eastern major axis, coinciding with and extending beyond the previously-known `northern spur'. The most prominent metallicity variations are found in the southern half of the halo, where two large structures with above average metallicites are apparent; one of these coincides with the giant stellar stream while the other corresponds to a much lower-level stellar enhancement. Our findings contrast with, but do not conflict with, past studies of the M31 halo and outer disk which have suggested a rather homogeneous stellar population at large radius: the bulk of our newly-detected substructure lies in the previously-uncharted far outer regions of the galaxy. We discuss the possible origin of the substructure observed and the implications it has for constraining the galaxy assembly process., Comment: 20 pages, 10 figures. Accepted for publication in AJ (Sep 2002). Version with high resolution embedded figures available at http://www.astro.rug.nl/~ferguson/m31.ps.gz

Stellar streams result from the tidal disruption of satellites and star clusters as they orbit a host galaxy, and can be very sensitive probes of the gravitational potential of the host system. We select and study narrow stellar streams formed in a Milky-Way-like dark matter halo of the Aquarius suite of cosmological simulations, to determine if these streams can be used to constrain the present day characteristic parameters of the halo's gravitational potential. We find that orbits integrated in static spherical and triaxial NFW potentials both reproduce the locations and kinematics of the various streams reasonably well. To quantify this further, we determine the best-fit potential parameters by maximizing the amount of clustering of the stream stars in the space of their actions. We show that using our set of Aquarius streams, we recover a mass profile that is consistent with the spherically-averaged dark matter profile of the host halo, although we ignored both triaxiality and time evolution in the fit. This gives us confidence that such methods can be applied to the many streams that will be discovered by the Gaia mission to determine the gravitational potential of our Galaxy., Comment: ApJ subm

We present six simulations of galactic stellar haloes formed by the tidal disruption of accreted dwarf galaxies in a fully cosmological setting. Our model is based on the Aquarius project, a suite of high resolution N-body simulations of individual dark matter haloes. We tag subsets of particles in these simulations with stellar populations predicted by the Galform semi-analytic model. Our method self-consistently tracks the dynamical evolution and disruption of satellites from high redshift. The luminosity function and structural properties of surviving satellites, which agree well with observations, suggest that this technique is appropriate. We find that accreted stellar haloes are assembled between 1, Comment: 24 pages, 19 figures; accepted for publication in MNRAS, minor changes; version with high resolution images at http://www.virgo.dur.ac.uk/aquarius

We have measured the amount of kinematic substructure in the Galactic halo using the final data set from the Spaghetti project, a pencil-beam high latitude sky survey. Our sample contains 101 photometrically selected and spectroscopically confirmed giants with accurate distance, radial velocity and metallicity information. We have developed a new clustering estimator: the "4distance" measure, which when applied to our data set leads to the identification of 1 group and 7 pairs of clumped stars. The group, with 6 members, can confidently be matched to tidal debris of the Sagittarius dwarf galaxy. Two pairs match the properties of known Virgo structures. Using models of the disruption of Sagittarius in Galactic potentials with different degrees of dark halo flattening, we show that this favors a spherical or prolate halo shape, as demonstrated by Newberg et al. (2007) using SDSS data. One additional pair can be linked to older Sagittarius debris. We find that 20% of the stars in the Spaghetti data set are in substructures. From comparison with random data sets we derive a very conservative lower limit of 10% to the amount of substructure in the halo. However, comparison to numerical simulations shows that our results are also consistent with a halo entirely built up from disrupted satellites, provided the dominating features are relatively broad due to early merging or relatively heavy progenitor satellites., Comment: 15 pages, 14 figures. Replaced to match the published version in ApJ; very minor corrections

The Gaia mission is designed as a Galaxy explorer, and will measure simultaneously, in a survey mode, the five or six phase space parameters of all stars brighter than 20th magnitude, as well as providing a description of their astrophysical characteristics. These measurements are obtained by combining an astrometric instrument with micro-arcsecond capabilities, a photometric system giving the magnitudes and colours in 15 bands and a medium resolution spectrograph named the Radial Velocity Spectrometer (RVS). The latter instrument will produce spectra in the 848 to 874 nm wavelength range, with a resolving power R = 11 500, from which radial velocities, rotational velocities, atmospheric parameters and abundances can be derived. A companion paper (Katz et al. 2004) presents the characteristics of the RVS and its performance. This paper details the outstanding scientific impact of this important part of the Gaia satellite on some key open questions in present day astrophysics. The unbiased and simultaneous acquisition of multi-epoch radial velocities and individual abundances of key elements in parallel with the astrometric parameters is essential for the determination of the dynamical state and formation history of our Galaxy. Moreover, for stars brighter than V=15, the resolving power of the RVS will give information about most of the effects which influence the position of a star in the Hertzsprung-Russell diagram, placing unprecedented constraints on the age, internal structure and evolution of stars of all types. Finally, the RVS multi-epoch observations are ideally suited to the identification, classification and characterisation of the many types of double, multiple and variable stars., 33 pages, 11 figures, in press at MNRAS. Figs 1, 3 and 9 included at reduced resolution; available in full resolution at http://www.blackwell-synergy.com/doi/pdf/10.1111/j.1365-2966.2005.09012.x

The confined nature of the debris from the Sagittarius dwarf to a narrow trail on the sky has recently prompted the suggestion that the dark matter halo of our Galaxy should be nearly spherical (Ibata et al. 2001; Majewski et al. 2003). This would seem to be in strong contrast with predictions from cold dark matter (CDM) simulations, where dark halos are found to have typical density axis ratios of 0.6 to 0.8. Here I present numerical simulations of the evolution of a system like the Sagittarius dSph in a set of Galactic potentials with varying degrees of flattening. These simulations show that the Sagittarius streams discovered so far are too young dynamically to be sensitive to the shape of the dark halo of the Milky Way. The data presently available are entirely consistent with a Galactic dark matter halo that could either be oblate or prolate, with density axis ratios c/a that range from 0.6 to 1.6 within the region of the halo probed by the orbit of the Sagittarius dwarf.

We examine the detectability and interpretation of debris trails caused by satellite disruption in external galaxies using semi-analytic approximations for the dependence of streamer length, width and surface brightness on satellite and primary galaxy characteristics. The semi-analytic method is tested successfully against N-body simulations and then applied to three representative astronomical applications. First, we show how streamer properties can be used to estimate mass-to-light ratios $\Upsilon$ and streamer ages of totally disrupted satellites, and apply the method to the stellar arc in NGC 5907. Second, we discuss how the lack of observed tidal debris around a satellite can provide an upper limit on its mass-loss rate, and, as an example, derive the implied limits on mass-loss rates for M32 and NGC 205 around Andromeda. Finally, we point out that a statistical analysis of streamer properties might be applied to test and refine cosmological models of hierarchical galaxy formation, and use the predicted debris from a standard $\Lambda$CDM realization to test the feasibility of such a study. Using the Local Group satellites and the few known examples of debris trails in the Galaxy and in external systems, we estimate that the best current techniques could characterize the brightest ($R < 29$ mag/ arcsec$^{2}$) portions of the youngest (3 dynamical periods) debris streamers. If systematics can be controlled, planned large-aperture telescopes such as CELT and OWL may allow fainter trails to be detected routinely and thus used for statistical studies such as those required for tests of galaxy formation., Comment: 33 pages, 8 figures. Revised version, accepted for publication in ApJ

We characterize substructure in the simulated stellar halos of Cooper et al. (2010) which were formed by the disruption of satellite galaxies within the cosmological N-body simulations of galactic halos of the Aquarius Project. These stellar halos exhibit a wealth of tidal features: broad overdensities and very narrow faint streams akin to those observed around the Milky Way. The substructures are distributed anisotropically on the sky, a characteristic that should become apparent in the next generation of photometric surveys. The normalized RMS of the density of stars on the sky appears to be systematically larger for our halos compared to the value estimated for the Milky Way from main sequence turn-off stars in the Sloan Digital Sky Survey. We show that this is likely to be due in part to contamination by faint QSOs and redder main sequence stars, and might suggest that ~10% of the Milky Way halo stars have formed in-situ., Comment: 5 pages, 5 color figures, accepted on April 8, 2011 with a change in title. A high-resolution version of the paper may be found at: http://www.astro.rug.nl/~ahelmi/aquarius.html