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Register a new userA Diffuse Metal-Poor Component of the Sagittarius Stream Revealed by the H3 Survey
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The tidal disruption of the Sagittarius dwarf galaxy has generated a spectacular stream of stars wrapping around the entire Galaxy. We use data from $Gaia$ and the H3 Stellar Spectroscopic Survey to identify 823 high-quality Sagittarius members based on their angular momenta. The H3 Survey is largely unbiased in metallicity, and so our sample of Sagittarius members is similarly unbiased. Stream stars span a wide range in [Fe/H] from $-0.2$ to $approx -3.0$, with a mean overall metallicity of $langle$[Fe/H]$rangle=-0.99$. We identify a strong metallicity-dependence to the kinematics of the stream members. At [Fe/H]$gt -0.8$ nearly all members belong to the well-known cold ($sigma_v lt 20$ km/s) leading and trailing arms. At intermediate metallicities ($-1.9 lt$[Fe/H]$lt -0.8$) a significant population (24$%$) emerges of stars that are kinematically offset from the cold arms. These stars also appear to have hotter kinematics. At the lowest metallicities ([Fe/H]$lesssim-2$), the majority of stars (69$%$) belong to this kinematically-offset diffuse population. Comparison to simulations suggests that the diffuse component was stripped from the Sagittarius progenitor at earlier epochs, and therefore resided at larger radius on average, compared to the colder metal-rich component. We speculate that this kinematically diffuse, low metallicity, population is the stellar halo of the Sagittarius progenitor system.
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We present the first full six-dimensional panoramic portrait of the Sagittarius stream, obtained by searching for wide stellar streams in the Gaia DR2 dataset with the STREAMFINDER algorithm. We use the kinematic behavior of the sample to devise a selection of Gaia RR Lyrae, providing excellent distance measurements along the stream. The proper motion data are complemented with radial velocities from public surveys. We find that the global morphological and kinematic properties of the Sagittarius stream are still reasonably well reproduced by the simple Law & Majewski (2010) model (LM10), although the model overestimates the leading arm and trailing arm distances by up to $sim 15$%. The sample newly reveals the leading arm of the Sagittarius stream as it passes into very crowded regions of the Galactic disk towards the Galactic Anticenter direction. Fortuitously, this part of the stream is almost exactly at the diametrically opposite location from the Galactic Center to the progenitor, which should allow an assessment of the influence of dynamical friction and self-gravity in a way that is nearly independent of the underlying Galactic potential model.
Using a variety of stellar tracers -- blue horizontal branch stars, main-sequence turn-off stars and red giants -- we follow the path of the Sagittarius (Sgr) stream across the sky in Sloan Digital Sky Survey data. Our study presents new Sgr debris detections, accurate distances and line-of-sight velocities that together help to shed new light on the puzzle of the Sgr tails. For both the leading and the trailing tail, we trace the points of their maximal extent, or apo-centric distances, and find that they lie at $R^L$ = 47.8 $pm$ 0.5 kpc and $R^T$ = 102.5 $pm$ 2.5 kpc respectively. The angular difference between the apo-centres is 93.2 $pm$ 3.5 deg, which is smaller than predicted for logarithmic haloes. Such differential orbital precession can be made consistent with models of the Milky Way in which the dark matter density falls more quickly with radius. However, currently, no existing Sgr disruption simulation can explain the entirety of the observational data. Based on its position and radial velocity, we show that the unusually large globular cluster NGC 2419 can be associated with the Sgr trailing stream. We measure the precession of the orbital plane of the Sgr debris in the Milky Way potential and show that, surprisingly, Sgr debris in the primary (brighter) tails evolves differently to the secondary (fainter) tails, both in the North and the South.
We report the detection, from observations using the James Clerk Maxwell Telescope, of CO J $=$ 3$to$ 2 transition lines in six carbon stars, selected as members of the Galactic Halo and having similar infrared colors. Just one Halo star had been detected in CO before this work. Infrared observations show that these stars are red (J-K $>$3), due to the presence of large dusty circumstellar envelopes. Radiative transfer models indicates that these stars are losing mass with rather large dust mass-loss rates in the range 1--3.3 $times$$10^{-8}$M$_{odot}$yr$^{-1}$, similar to what can be observed in the Galactic disc. We show that two of these stars are effectively in the Halo, one is likely linked to the stream of the Sagittarius Dwarf Spheroidal galaxy (Sgr dSph), and the other three stars certainly belong to the thick disc. The wind expansion velocities of the observed stars are low compared to carbon stars in the thin disc and are lower for the stars in the Halo and the Sgr dSph stream than in the thick disc. We discuss the possibility that the low expansion velocities result from the low metallicity of the Halo carbon stars. This implies that metal-poor carbon stars lose mass at a rate similar to metal-rich carbon stars, but with lower expansion velocities, as predicted by recent theoretical models. This result implies that the current estimates of mass-loss rates from carbon stars in Local Group galaxies will have to be reconsidered.
We present the metallicities and carbon abundances of four newly discovered metal-poor stars with $ -2.2 <$ [Fe/H] $< -1.6$ in the Sagittarius dwarf spheroidal galaxy. These stars were selected as metal-poor member candidates using a combination of public photometry from the SkyMapper Southern Sky Survey and proper motion data from the second data release from the Gaia mission. The SkyMapper filters include a metallicity-sensitive narrow-band $v$ filter centered on the Ca II K line, which we use to identify metal-poor candidates. In tandem, we use proper motion data to remove metal-poor stars that are not velocity members of the Sagittarius dwarf spheroidal galaxy. We find that these two datasets allow for efficient identification of metal-poor members of the Sagittarius dwarf galaxy to follow-up with further spectroscopic study. Two of the stars we present have [Fe/H] $< -2.0$, which adds to the few other such stars currently identified in the Sagittarius dwarf galaxy that are likely not associated with the globular cluster M54, which resides in the nucleus of the system. Our results confirm that there exists a very metal-poor stellar population in the Sagittarius dwarf galaxy. We find that none of our stars can be classified as carbon-enhanced metal-poor stars. Efficiently identifying members of this population will be helpful to further our understanding of the early chemical evolution of the system.
We present 947 radial velocities of RR Lyrae variable stars in four fields located toward the Galactic bulge, observed within the data from the ongoing Bulge RR Lyrae Radial Velocity Assay (BRAVA-RR). We show that these RR Lyrae stars exhibit hot kinematics and null or negligible rotation and are therefore members of a separate population from the bar/pseudobulge that currently dominates the mass and luminosity of the inner Galaxy. Our RR Lyrae stars predate these structures, and have metallicities, kinematics, and spatial distribution that are consistent with a classical bulge, although we cannot yet completely rule out the possibility that they are the metal-poor tail of a more metal rich ([Fe/H] ~ -1 dex) halo-bulge population. The complete catalog of radial velocities for the BRAVA-RR stars is also published electronically.
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