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Post Info TOPIC: Sagittarius Stream
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Sagittarius stellar stream
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Title: A fork in the Sagittarius trailing debris
Author: Camila Navarrete, Vasily Belokurov, Sergey E. Koposov, Mike Irwin, Márcio Catelan, Sonia Duffau, Andrew J. Drake

We take advantage of the deep and wide coverage of the VST ATLAS survey to study the line-of-sight structure of the Sagittarius stellar stream in the Southern hemisphere, only ~40° away from the progenitor. We use photometrically selected Sub-Giant Branch (SGB) stars to reveal a complex debris morphology of the trailing arm and detect at least two clear peaks in the SGB distance modulus distribution. The separation between the two line-of-sight components is at least 5 kpc at the edge of the VST ATLAS footprint, but appears to change along the stream, which allows us to conclude that these detections correspond to two physically independent stellar structures, rather than a mix of co-distant stellar populations within a single stream. Our discovery of a fork in the Sgr trailing arm is verified using Blue Horizontal Branch stars and our distance measurements are calibrated using RR Lyrae stars from the Catalina Real-Time Transient Survey. Comparing with numerical simulations of the Sgr dwarf disruption, the more distant of the two components in the fork matches perfectly with the track of the trailing debris. However, no obvious counterpart exists in the simulation for the closer line-of-sight component.

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RE: Sagittarius Stream
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Title: Hubble Space Telescope Proper Motions along the Sagittarius Stream: I. Observations and Results for Stars in Four Fields
Author: Sangmo Tony Sohn, Roeland P. van der Marel, Jeffrey L. Carlin, Steven R. Majewski, Nitya Kallivayalil, David R. Law, Jay Anderson, Michael H. Siegel
(Version v2)

We present a multi-epoch Hubble Space Telescope (HST) study of stellar proper motions (PMs) for four fields spanning 200 degrees along the Sagittarius (Sgr) stream: one trailing arm field, one field near the Sgr dwarf spheroidal tidal radius, and two leading arm fields. We determine absolute PMs of dozens of individual stars per field, using established techniques that use distant background galaxies as stationary reference frame. Stream stars are identified based on combined colour-magnitude diagram and PM information. The results are broadly consistent with the few existing PM measurements for the Sgr galaxy and the trailing arm. However, our new results provide the highest PM accuracy for the stream to date, the first PM measurements for the leading arm, and the first PM measurements for individual stream stars; we also serendipitously determine the PM of the globular cluster NGC~6652. In the trailing-arm field, the individual PMs allow us to kinematically separate trailing-arm stars from leading-arm stars that are 360 degrees further ahead in their orbit. Also, in three of our fields we find indications that two distinct kinematical components may exist within the same arm and wrap of the stream. Qualitative comparison of the HST data to the predictions of the Law & Majewski and Penarrubia et al. N-body models show that the PM measurements closely follow the predicted trend with Sgr longitude. This provides a successful consistency check on the PM measurements, as well as on these N-body approaches (which were not tailored to fit any PM data).

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Title: Does the Sagittarius Stream constrain the Milky Way halo to be triaxial?
Authors: Rodrigo Ibata, Geraint F. Lewis, Nicolas F. Martin, Michele Bellazzini, Matteo Correnti

Recent analyses of the stellar stream of the Sagittarius dwarf galaxy have claimed that the kinematics and three-dimensional location of the M-giant stars in this structure constrain the dark matter halo of our Galaxy to possess a triaxial shape that is extremely flattened, being essentially an oblate ellipsoid oriented perpendicular to the Galactic disk. Using a new stream-fitting algorithm, based on a Markov Chain Monte Carlo procedure, we investigate whether this claim remains valid if we allow the density profile of the Milky Way halo greater freedom. We find stream solutions that fit the leading and trailing arms of this structure even in a spherical halo, although this would need a rising Galactic rotation curve at large Galactocentric radius. However, the required rotation curve is not ruled out by current constraints. It appears therefore that for the Milky Way, halo triaxiality, despite its strong theoretical motivation, is not required to explain the Sagittarius stream. This degeneracy between triaxiality and the halo density profile suggests that in future endeavours to model this structure, it will be advantageous to relax the strict analytic density profiles that have been used to date.

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Title: A Pan-STARRS1 View of the Bifurcated Sagittarius Stream
Authors: C. T. Slater, E. F. Bell, E. F. Schlafly, M. Juric, N. F. Martin. H.-W. Rix, E. J. Bernard, W. S. Burgett, K. C. Chambers, D. P. Finkbeiner, B. Goldman, N. Kaiser, E. A. Magnier, E. P. Morganson, P. A. Price, J. L. Tonry

We use data from the Pan-STARRS1 survey to present a panoramic view of the Sagittarius tidal stream in the southern Galactic hemisphere. As a result of the extensive sky coverage of Pan-STARRS1, the southern stream is visible along more than 60 degrees of its orbit, nearly double the length seen by the SDSS. The recently discovered southern bifurcation of the stream is also apparent, with the fainter branch of the stream visible over at least 30 degrees. Using a combination of fitting both the main sequence turn-off and the red clump, we measure the distance to both arms of the stream in the south. We find that the distances to the bright arm of the stream agree very well with the N-body models of Law & Majewski (2010). We also find that the faint arm lies ~5 kpc closer to the Sun than the bright arm, similar to the behaviour seen in the northern hemisphere.

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Title: The Northern wraps of the Sagittarius Stream as traced by Red Clump stars: distances, intrinsic widths and stellar densities
Authors: M. Correnti, M. Bellazzini, R.A. Ibata, F.R. Ferraro, A. Varghese

We trace the tidal Stream of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) using Red Clump stars from the catalogue of the Sloan Digital Sky Survey - Data Release 6, in the range 150° < RA < 220°, corresponding to the range of orbital azimuth 220° < Lambda < 290°. Substructures along the line of sight are identified as significant peaks in the differential star count profiles (SCP) of candidate Red Clump stars. A proper modelling of the SCPs allows us to obtain: (a) <10% accurate, purely differential distances with respect to the main body of Sgr, (b) estimates of the FWHM along the line of sight, and (c) estimates of the local density, for each detected substructure. In the range 255° < Lambda < 290° we cleanly and continuously trace various coherent structures that can be ascribed to the Stream, in particular: the well known northern portion of the leading arm, running from d~43 kpc at Lambda ~ 290° to d ~ 30 kpc at Lambda ~ 255°, and a more nearby coherent series of detections lying at constant distance d ~ 25 kpc, that can be identified with a wrap of the trailing arm. The latter structure, predicted by several models of the disruption of Sgr dSph, was never traced before; comparison with existing models indicates that the difference in distance between these portions of the leading and trailing arms may provide a powerful tool to discriminate between theoretical models assuming different shapes of the Galactic potential. A further, more distant wrap in the same portion of the sky is detected only along a couple of lines of sight.

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