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Milky Way dark matter halo
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Title: Constraints on the shape of the Milky Way dark matter halo from the Sagittarius stream
Authors: Carlos Vera-Ciro, Amina Helmi

We propose a new model for the dark matter halo of the Milky Way that fits the properties of the stellar stream associated with the Sagittarius dwarf galaxy. Our dark halo is oblate with q = 0.9 for r < 10 kpc, and follows the Law & Majewski (2010) model at large radii, the transition between these regimes occurring at ~30 kpc. The outer halo can be made mildly triaxial, with minor-to-major axis ratio (c/a)_\Phi = 0.8 and intermediate-to-major axis ratio (b/a)_\Phi = 0.9, if the effect of the Large Magellanic Cloud is taken into account. Therefore this new model takes into account the flattening induced by the presence of the Galactic disk and is also more consistent with cosmological expectations.

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RE: Milky Way Halo
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Title: The Velocity Anisotropy of Distant Milky Way Halo Stars from Hubble Space Telescope Proper Motions
Authors: A. J. Deason (UCSC), R. P. Van der Marel (STScI), P. Guhathakurta (UCSC), S.T. Sohn (STScI), T.M. Brown (STScI)

Based on long baseline (5-7 years) multi-epoch HST/ACS photometry, used previously to measure the proper motion of M31, we present the proper motions (PMs) of 13 main-sequence Milky Way halo stars. The sample lies at an average distance of r ~24 kpc from the Galactic center, with a root-mean-square spread of 6 kpc. At this distance, the median PM accuracy is 5 km/s. We devise a maximum likelihood routine to determine the tangential velocity ellipsoid of the stellar halo. The velocity second moments in the directions of the Galactic (l,b) system are < vl^2 >^{} = 123 (+29, -23) km/s, and < vb^2 >^{} = 83 (+24, -16) km/s. We combine these results with the known line-of-sight second moment, < vlos^2 >^{} = 105 5 km/s, at this < r > to study the velocity anisotropy of the halo. We find approximate isotropy between the radial and tangential velocity distributions, with anisotropy parameter beta = 0.0 (+0.2, -0.4). Our results suggest that the stellar halo velocity anisotropy out to r ~ 30 kpc is less radially biased than solar neighbourhood measurements. This is opposite to what is expected from violent relaxation, and may indicate the presence of a shell-type structure at r ~ 24 kpc. With additional multi-epoch HST data, the method presented here has the ability to measure the transverse kinematics of the halo for more stars, and to larger distances. This can yield new improved constraints on the stellar halo formation mechanism, and the mass of the Milky Way.

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Stellar Motions in Outer Halo Shed New Light on Milky Way Evolution

Peering deep into the vast stellar halo that envelops our Milky Way galaxy, astronomers using NASA's Hubble Space Telescope have uncovered tantalizing evidence for the possible existence of a shell of stars that are a relic of cannibalism by our Milky Way.
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Title: Kinematics of the stellar halo and the mass distribution of the Milky Way using BHB stars
Authors: Prajwal R. Kafle, Sanjib Sharma, Geraint F. Lewis, Joss Bland-Hawthorn

Here we present a kinematic study of the Galactic halo out to a radius of ~ 60 kpc, using 4664 blue horizontal branch (BHB) stars selected from the SDSS/SEGUE survey, to determine key dynamical properties. Using a maximum likelihood analysis, we determine the velocity dispersion profiles in spherical coordinates (sigma_{r}, sigma_{theta}, sigma_{phi}) and the anisotropy profile (beta). The radial velocity dispersion profile (sigma_{r}) is measured out to a galactocentric radius of r ~ 60 kpc, but due to the lack of proper-motion information, sigma_{theta}, sigma_{phi} and beta could only be derived directly out to r ~25 kpc. From a starting value of beta approx 0.5 in the inner parts (9<r/\kpc<12), the profile falls sharply in the range r approx 13-18 kpc, with a minimum value of beta=-1.2 at r=17 kpc, rising sharply at larger radius. In the outer parts, in the range 25<r/\kpc<56, we predict the profile to be roughly constant with a value of beta approx 0.5. The newly discovered kinematic anomalies are shown not to arise from halo substructures. We also studied the anisotropy profile of simulated stellar halos formed purely by accretion and found that they cannot reproduce the sharp dip seen in the data. From the Jeans equation, we compute the stellar rotation curve (v_{circ}) of the Galaxy out to r ~ 25 kpc. The mass of the Galaxy within r \lesssim 25 kpc is determined to be 2.1 x 10^{11} solar masses, and with a 3-component fit to v_{circ}(r), we determine the virial mass of the Milky Way dark matter halo to be M_{vir} = 0.9 ^{+0.4}_{-0.3} x 10^{12} solar masses (R_{vir} = 249^{+34}_{-31} kpc).

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RE: Milky Way Halo
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Title: Broken and unbroken: the Milky Way and M31 stellar haloes
Authors: A.J. Deason (UCSC), V. Belokurov (Cambridge), N.W. Evans (Cambridge), K.V. Johnston (Columbia)

We use the Bullock & Johnston suite of simulations to study the density profiles of L*-type galaxy stellar haloes. Observations of the Milky Way and M31 stellar haloes show contrasting results: the Milky Way has a 'broken' profile, where the density falls off more rapidly beyond ~ 25 kpc, while M31 has a smooth profile out to 100 kpc with no obvious break. Simulated stellar haloes, built solely by the accretion of dwarf galaxies, also exhibit this behaviour: some haloes have breaks, while others don't. The presence or absence of a break in the stellar halo profile can be related to the accretion history of the galaxy. We find that a break radius is strongly related to the build up of stars at apocentres. We relate these findings to observations, and find that the 'break' in the Milky Way density profile is likely associated with a relatively early (~ 7-10 Gyr ago) and massive accretion event. In contrast, the absence of a break in the M31 stellar halo profile suggests that its accreted satellites have a wide range of apocentres. Hence, it is likely that M31 has had a much more prolonged accretion history than the Milky Way.

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Title: A Baryonic Solution to the Missing Satellites Problem
Authors: Alyson M. Brooks, Michael Kuhlen, Adi Zolotov, Dan Hooper

It has been demonstrated that the inclusion of baryonic physics can alter the dark matter densities in the centers of low-mass galaxies, making the central dark matter slope more shallow than predicted in pure cold dark matter simulations. This flattening of the dark matter profile can occur in the most luminous subhalos around Milky Way-mass galaxies. Zolotov et al. (2012) have suggested a correction to be applied to the central masses of dark matter-only satellites in order to mimic the affect of (1) the flattening of the dark matter cusp due to supernova feedback in luminous satellites, and (2) enhanced tidal stripping due to the presence of a baryonic disk. In this paper, we apply this correction to the z=0 subhalo masses from the high resolution, dark matter-only Via Lactea II (VL2) simulation, and find that the number of massive subhalos is dramatically reduced. After adopting a stellar mass to halo mass relationship for the VL2 halos, and identifying subhalos that are (1) likely to be destroyed by stripping and (2) likely to have star formation suppressed by photo-heating, we find that the number of massive, luminous satellites around a Milky Way-mass galaxy is in agreement with the number of observed satellites around the Milky Way or M31. We conclude that baryonic processes have the potential to solve the missing satellites problem.

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NASA's Chandra Shows Milky Way is Surrounded by Halo of Hot Gas

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NASA's Chandra Shows Milky Way is Surrounded by Halo of Hot Gas

Astronomers have used NASA's Chandra X-ray Observatory to find evidence our Milky Way Galaxy is embedded in an enormous halo of hot gas that extends for hundreds of thousands of light years. The estimated mass of the halo is comparable to the mass of all the stars in the galaxy. If the size and mass of this gas halo is confirmed, it also could be an explanation for what is known as the "missing baryon" problem for the galaxy.
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Title: Two Distant Halo Velocity Groups Discovered by the Palomar Transient Factory
Authors: B. Sesar, J. G. Cohen, D. Levitan, C. J. Grillmair, M. Juric, E. N. Kirby, R. R. Laher, E. O. Ofek, J. A. Surace, S. R. Kulkarni, T. A. Prince

We report the discovery of two new halo velocity groups (Cancer groups A and B) traced by 8 distant RR Lyrae stars and observed by the Palomar Transient Factory (PTF) survey at R.A.~129 deg, Dec~20 deg (l~205 deg, b~32 deg). Located at 92 kpc from the Galactic center (86 kpc from the Sun), these are some of the most distant substructures in the Galactic halo known to date. Follow-up spectroscopic observations with the Palomar Observatory 5.1-m Hale telescope and W. M. Keck Observatory 10-m Keck I telescope indicate that the two groups are moving away from the Galaxy at v_{gsr} = 78.05.6 km/s (Cancer group A) and v_{gsr} = 16.37.1 km/s (Cancer group B). The groups have velocity dispersions of \sigma_{v_{gsr}}=12.45.0 km/s and \sigma_{v_{gsr}}=14.96.2 km/s, and are spatially extended (about several kpc) making it very unlikely that they are bound systems, and are more likely to be debris of tidally disrupted dwarf galaxies or globular clusters. Both groups are metal-poor (median metallicities of [Fe/H] = -1.6 dex and [Fe/H] =-2.1 dex), and have a somewhat uncertain (due to small sample size) metallicity dispersion of ~0.4 dex, suggesting dwarf galaxies as progenitors. Two additional RR Lyrae stars with velocities consistent with those of the Cancer groups have been observed ~25 deg east, suggesting possible extension of the groups in that direction.

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Milky Way Inner Halo
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Title: The Age of the Milky Way Inner Halo
Authors: Jason Kalirai (STScI)

The Milky Way galaxy is observed to have multiple components with distinct properties, such as the bulge, disk, and halo. Unravelling the assembly history of these populations provides a powerful test to the theory of galaxy formation and evolution, but is often restricted due to difficulties in measuring accurate stellar ages for low mass, hydrogen-burning stars. Unlike these progenitors, the "cinders" of stellar evolution, white dwarf stars, are remarkably simple objects and their fundamental properties can be measured with little ambiguity from spectroscopy. Here I report observations of newly formed white dwarf stars in the halo of the Milky Way, and a separate analysis of archival data on white dwarfs in the well-studied 12.5 billion year old globular cluster Messier 4. From this, I measure the mass distribution of the remnants and invert the stellar evolution process to develop a new relation that links this final stellar mass to the mass of their immediate progenitors, and therefore to the age of the parent population. By applying this technique to a small sample of four nearby and kinematically-confirmed halo white dwarfs, I measure the age of local field halo stars to be 11.4 0.7 billion years. This age is directly tied to the globular cluster age scale, on which the oldest clusters formed 13.5 billion years ago. Future (spectroscopic) observations of newly formed white dwarfs in the Milky Way halo can be used to reduce the present uncertainty, and to probe relative differences between the formation time of the last clusters and the inner halo.

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