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Register a new userA 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: III. Constraints on the Flattening of the Galactic Halo
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Kathryn V. Johnston
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M giants selected from the Two Micron All Sky Survey (2MASS) have been used to trace streams of tidal debris apparently associated with the Sagittarius dwarf spheroidal galaxy (Sgr) that entirely encircle the Galaxy. While the Sgr M giants are generally aligned with a single great circle on the sky, we measure a difference of 10.4 +- 2.6 degrees between the mean orbital poles of the great circles that best fit debris leading and trailing Sgr, which can be attributed to the precession of Sgrs orbit over the range of phases explored by the data set. Simulations of the destruction of Sgr in potentials containing bulge, disk and halo components best reproduce this level of precession along the same range of orbital phases if the potential contours of the halo are only slightly flattened, with the ratio between the axis length perpendicular to and in the disk in the range q = 0.90-0.95 (corresponding to isodensity contours with q_rho ~ 0.83 - 0.92). Oblate halos are strongly preferred over prolate (q_rho > 1) halos, and flattenings in the potential of q <= 0.85 (q_rho <= 0.75) and q >= 1.05 (q_rho >= 1.1) are ruled out at the 3-sigma level. More extreme values of q <= 0.80 (q_rho <= 0.6) and q >= 1.25 (q_rho >= 1.6) are ruled out at the 7-sigma and 5-sigma levels respectively. These constraints will improve as debris with larger separation in orbital phase can be found.
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We have assembled a large-area spectroscopic survey of giant stars in the Sagittarius (Sgr) dwarf galaxy core. Using medium resolution (R ~15,000), multifiber spectroscopy we have measured velocities of these stars, which extend up to 12 degrees from the galaxys center (3.7 core radii or 0.4 times the King limiting radius). From these high quality spectra we identify 1310 Sgr members out of 2296 stars surveyed distributed across 24 different fields across the Sgr core. Additional slit spectra were obtained of stars bridging from the Sgr core to its trailing tail. Our systematic, large area sample shows no evidence for significant rotation, a result at odds with the ~20 km/s rotation required as an explanation for the bifurcation seen in the Sgr tidal stream; the observed small (<= 4 km/s) velocity trend along primarily the major axis is consistent with models of the projected motion of an extended body on the sky with no need for intrinsic rotation. The Sgr core is found to have a flat velocity dispersion (except for a kinematically colder center point) across its surveyed extent and into its tidal tails, a property that matches the velocity dispersion profiles measured for other Milky Way dwarf spheroidal (dSph) galaxies. We comment on the possible significance of this observed kinematical similarity for the dynamical state of the other classical Milky Way dSphs in light of the fact that Sgr is clearly a strongly tidally disrupted system.
We present the first all-sky view of the Sagittarius (Sgr) dwarf galaxy mapped by M giant star tracers detected in the complete Two Micron All-Sky Survey (2MASS). The main body is fit with a King profile of 30 deg limiting radius, but with a break in the density profile from stars in tidal tails. We argue that much of the observed structure beyond the 224 core radius may be unbound as the satellite undergoes catastrophic disruption. A striking, >150 deg trailing tidal tail extends from the Sgr center and arcs across the South Galactic Hemisphere. A prominent leading debris arm extends from the Sgr center northward of the Galactic plane to an ~40 kpc apoGalacticon, loops towards the North Galactic Cap (NGC) and descends back towards the Galactic plane, foreshortened and covering the NGC. The Sgr tails lie along a well-defined orbital plane that shows little precession, which supports the notion of a nearly spherical Galactic potential. The Sun lies near the path of leading Sgr debris; thus, former Sgr stars may be near or in the solar neighborhood. The number of M giants in the Sgr tails is >15% that within the King limiting radius of the Sgr center. That several gigayear old M giants are so widespread along the Sgr tidal arms not only places limits on the dynamical age of these arms but poses a timing problem that bears on the recent binding energy of the Sgr core and that is naturally explained by recent and catastrophic mass loss. Sgr appears to contribute >75% of the high latitude, halo M giants; no evidence for M giant tidal debris from the Magellanic Clouds is found. Generally good correspondence is found between the M giant, all-sky map of the Sgr system and all previously published detections of potential Sgr debris with the exception of Sgr carbon stars -- which must be subluminous to resolve the discrepancy.
We report on the discovery and chemical abundance analysis of the first CEMP-r/s star detected in the Sagittarius dwarf Spheroidal Galaxy, by means of UVES high resolution spectra. The star, found in the outskirts of Sgr dSph, along the main body major axis, is a moderately metal poor giant (T$_{eff}$=4753 K, log g=1.75, [Fe/H]=-1.55), with [C/Fe]=1.13 placing it in the so-called high-carbon band, and strong s-process and r-process enrichment ([Ba/Fe]=1.4, [Eu/Fe]=1.01). Abundances of 29 elements from C to Dy were obtained. The chemical pattern appears to be best fitted by a scenario where an r-process pollution event pre-enriched the material out of which the star was born as secondary in a binary system whose primary evolved through the AGB phase, providing C and s-process enrichment.
Gravitational interactions between the Large Magellanic Cloud (LMC) and the stellar and dark matter halo of the Milky Way are expected to give rise to disequilibrium phenomena in the outer Milky Way. A local wake is predicted to trail the orbit of the LMC, while a large-scale over-density is predicted to exist across a large area of the northern Galactic hemisphere. Here we present the detection of both the local wake and Northern over-density (hereafter the collective response) in an all-sky star map of the Galaxy based on 1301 stars at 60<R_gal<100 kpc. The location of the wake is in good agreement with an N-body simulation that includes the dynamical effect of the LMC on the Milky Way halo. The density contrast of the wake and collective response are both stronger in the data than in the simulation. The detection of a strong local wake is independent evidence that the Magellanic Clouds are on their first orbit around the Milky Way. The wake traces the path of the LMC, which will provide insight into the orbit of the LMC, which in turn is a sensitive probe of the mass of the LMC and the Milky Way. These data demonstrate that the outer halo is not in dynamical equilibrium, as is often assumed. The morphology and strength of the wake could be used to test the nature of dark matter and gravity.
A pole-count analysis of the infrared 2MASS survey is presented, in order to identify faint stream-like structures within the halo of the Milky Way. Selecting stars with colours consistent with M-giant stars, we find a strong over-density of sources on a stream with pole (l=95,b=13), which corresponds to the pole of the orbit of the Sagittarius dwarf galaxy. This great-circle feature of width ~12 degrees, contains 5% of the late M-giants in the Halo. No other stream-like structures are detected in M-giants in the 2MASS Second Incremental Data Release (2IDR), and in particular, we find no evidence for a stellar component to the Magellanic Stream. This suggests that the present accretion rate of low-mass satellites with a luminous component is very low, and the formation of the luminous component of the Halo must have been essentially complete before the accretion of the Sagittarius dwarf galaxy, more than 3Gyr ago. We also search for great-circle streams using almost all high-latitude (|b|>30) sources in the 2IDR dataset. No narrow great-circle streams of width 0.5-2 degrees were found, though we were only sensitive to relatively nearby (<17kpc) remnants of massive (10^6 Msun) globular clusters. If the Galactic potential is close to being spherical, as some recent observations suggest, the lack of observed great-circle streams is consistent with the presence of dark matter substructures in the Halo. Although alternative explanations cannot be ruled out from our analysis of the 2IDR dataset, future experiments with better statistics have the potential to reveal the heating effect of dark matter substructure on stellar streams.
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