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Register a new userEmpirical Determination of Dark Matter Velocities using Metal-Poor Stars
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The Milky Way dark matter halo is formed from the accretion of smaller subhalos. These sub-units also harbor stars---typically old and metal-poor---that are deposited in the Galactic inner regions by disruption events. In this Letter, we show that the dark matter and metal-poor stars in the Solar neighborhood share similar kinematics due to their common origin. Using the high-resolution Eris simulation, which traces the evolution of both the dark matter and baryons in a realistic Milky-Way analog galaxy, we demonstrate that metal-poor stars are indeed effective tracers for the local, virialized dark matter velocity distribution. The local dark matter velocities can therefore be inferred from observations of the stellar halo made by the Sloan Digital Sky Survey within 4 kpc of the Sun. This empirical distribution differs from the Standard Halo Model in important ways and suggests that the bounds on the spin-independent scattering cross section may be weakened for dark matter masses below $sim$10 GeV. Data from Gaia will allow us to further refine the expected distribution for the smooth dark matter component, and to test for the presence of local substructure.
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The local velocity distribution of dark matter plays an integral role in interpreting the results from direct detection experiments. We previously showed that metal-poor halo stars serve as excellent tracers of the virialized dark matter velocity distribution using a high-resolution hydrodynamic simulation of a Milky Way--like halo. In this paper, we take advantage of the first textit{Gaia} data release, coupled with spectroscopic measurements from the RAdial Velocity Experiment (RAVE), to study the kinematics of stars belonging to the metal-poor halo within an average distance of $sim 5$ kpc of the Sun. We study stars with iron abundances [Fe/H]$ < -1.5$ and $-1.8$ that are located more than $1.5$ kpc from the Galactic plane. Using a Gaussian mixture model analysis, we identify the stars that belong to the halo population, as well as some kinematic outliers. We find that both metallicity samples have similar velocity distributions for the halo component, within uncertainties. Assuming that the stellar halo velocities adequately trace the virialized dark matter, we study the implications for direct detection experiments. The Standard Halo Model, which is typically assumed for dark matter, is discrepant with the empirical distribution by $sim6sigma$ and predicts fewer high-speed particles. As a result, the Standard Halo Model overpredicts the nuclear scattering rate for dark matter masses below $sim 10$ GeV. The kinematic outliers that we identify may potentially be correlated with dark matter substructure, though further study is needed to establish this correspondence.
The dark matter velocity distribution in the Solar neighbourhood is an important astrophysical input which enters in the predicted event rate of dark matter direct detection experiments. It has been recently suggested that the local dark matter velocity distribution can be inferred from that of old or metal-poor stars in the Milky Way. We investigate this potential relation using six high resolution magneto-hydrodynamical simulations of Milky Way-like galaxies of the Auriga project. We do not find any correlation between the velocity distributions of dark matter and old stars in the Solar neighbourhood. Likewise, there are no strong correlations between the local velocity distributions of dark matter and metal-poor stars selected by applying reasonable cuts on metallicity. In some simulated galaxies, extremely metal-poor stars have a velocity distribution that is statistically consistent with that of the dark matter, but the sample of such stars is so small that we cannot draw any strong conclusions.
The Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) is an unbiased, massively multiplexed spectroscopic survey, designed to measure the expansion history of the universe through low-resolution ($Rsim750$) spectra of Lyman-Alpha Emitters. In its search for these galaxies, HETDEX will also observe a few 10$^{5}$ stars. In this paper, we present the first stellar value-added catalog within the internal second data release of the HETDEX Survey (HDR2). The new catalog contains 120,571 low-resolution spectra for 98,736 unique stars between $10 < G < 22$ spread across the HETDEX footprint at relatively high ($bsim60^circ$) Galactic latitudes. With these spectra, we measure radial velocities (RVs) for $sim$42,000 unique FGK-type stars in the catalog and show that the HETDEX spectra are sufficient to constrain these RVs with a 1$sigma$ precision of 28.0 km/s and bias of 3.5 km/s with respect to the LAMOST surveys and 1$sigma$ precision of 27.5 km/s and bias of 14.0 km/s compared to the SEGUE survey. Since these RVs are for faint ($Ggeq16$) stars, they will be complementary to Gaia. Using t-Distributed Stochastic Neighbor Embedding (t-SNE), we also demonstrate that the HETDEX spectra can be used to determine a stars T${rm{eff}}$, and log g and its [Fe/H]. With the t-SNE projection of the FGK-type stars with HETDEX spectra we also identify 416 new candidate metal-poor ([Fe/H] $< -1$~dex) stars for future study. These encouraging results illustrate the utility of future low-resolution stellar spectroscopic surveys.
We present a novel scenario for the formation of carbon-enhanced metal-poor (CEMP) stars. Carbon enhancement at low stellar metallicities is usually considered a consequence of faint or other exotic supernovae. An analytical estimate of cooling times in low-metallicity gas demonstrates a natural bias, which favours the formation of CEMP stars as a consequence of inhomogeneous metal mixing: carbon-rich gas has a shorter cooling time and can form stars prior to a potential nearby pocket of carbon-normal gas, in which star formation is then suppressed due to energetic photons from the carbon-enhanced protostars. We demonstrate that this scenario provides a natural formation mechanism for CEMP stars from carbon-normal supernovae, if inhomogeneous metal mixing provides carbonicity differences of at least one order of magnitude separated by >10pc. In our fiducial (optimistic) model, 8% (83%) of observed CEMP-no stars ([Ba/Fe]<0) can be explained by this formation channel. This new scenario may change our understanding of the first supernovae and thereby our concept of the first stars. Future 3D simulations are required to assess the likelihood of this mechanism to occur in typical high-redshift galaxies.
We report the discovery of 30 stars with extreme space velocities ($>$ 480 km/s) in the Gaia-DR2 archive. These stars are a subset of 1743 stars with high-precision parallax, large tangential velocity ($v_{tan}>$ 300 km/s), and measured line-of-sight velocity in DR2. By tracing the orbits of the stars back in time, we find at least one of them is consistent with having been ejected by the supermassive black hole at the Galactic Center. Another star has an orbit that passed near the Large Magellanic Cloud (LMC) about 200 Myr ago. Unlike previously discovered blue hypervelocity stars, our sample is metal-poor (-1.5 $<$ [Fe/H] $<$ -1.0) and quite old ($>$ 1 Gyr). We discuss possible mechanisms for accelerating old stars to such extreme velocities. The high observed space density of this population, relative to potential acceleration mechanisms, implies that these stars are probably bound to the Milky Way (MW). If they are bound, the discovery of this population would require a local escape speed of around $sim$ 600 km/s and consequently imply a virial mass of $M_{200} sim 1.4 times 10^{12} M_odot$ for the MW.
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