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تسجيل مستخدم جديدJoint constraints on the Galactic dark matter halo and Galactic Centre from hypervelocity stars
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The mass assembly history of the Milky Way can inform both theory of galaxy formation and the underlying cosmological model. Thus, observational constraints on the properties of both its baryonic and dark matter contents are sought. Here we show that hypervelocity stars (HVSs) can in principle provide such constraints. We model the observed velocity distribution of HVSs, produced by tidal break-up of stellar binaries caused by Sgr A*. Considering a Galactic Centre (GC) binary population consistent with that inferred in more observationally accessible regions, a fit to current HVS data with significance level > 5% can only be obtained if the escape velocity from the GC to 50 kpc is $V_G < 850$ km/s, regardless of the enclosed mass distribution. When a NFW matter density profile for the dark matter halo is assumed, haloes with $V_G < 850$ km/s are in agreement with predictions in the $Lambda$CDM model and that a subset of models around $M_{200} sim 0.5-1.5 times 10^{12}$ solar masses and $r_s < 35$ kpc can also reproduce Galactic circular velocity data. HVS data alone cannot currently exclude potentials with $V_G > 850$ km/s. Finally, specific constraints on the halo mass from HVS data are highly dependent on the assumed baryonic mass potentials. This first attempt to simultaneously constrain GC and dark halo properties is primarily hampered by the paucity and quality of data. It nevertheless demonstrates the potential of our method, that may be fully realised with the ESA Gaia mission.
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Hypervelocity stars (HVSs) travel from the Galactic Centre across the dark matter halo of the Milky Way, where they are observed with velocities in excess of the Galactic escape speed. Because of their quasi-radial trajectories, they represent a uniq
ue probe of the still poorly constrained dark matter component of the Galactic potential. In this paper, we present a new method to produce such constraints. Our likelihood is based on the local HVS density obtained by back-propagating the observed phase space position and quantifies the ejection probability along the orbit. To showcase our method, we apply it to simulated Gaia samples of $sim200$ stars in three realistic Galactic potentials with dark matter components parametrized by spheroidal NFW profiles. We find that individual HVSs exhibit a degeneracy in the scale mass-scale radius plane ($M_s-r_s$) and are able to measure only the combination $alpha = M_s/r_s^2$. Likewise, a degeneracy is also present between $alpha$ and the spheroidal axis-ratio $q$. In the absence of observational errors, we show the whole sample can nail down both parameters with {it sub-per cent} precision (about $1%$ and $0.1%$ for $alpha$ and $q$ respectively) with no systematic bias. This remarkable power to constrain deviations from a symmetric halo is a consequence of the Galactocentric origin of HVSs. To compare our results with other probes, we break the degeneracy in the scale parameters and impose a mass-concentration relation. The result is a competitive precision on the virial mass $M_{200}$ of about $10%$.
Hypervelocity stars (HVS) traverse the Galaxy from the central black hole to the outer halo. We show that the Galactic potential within 200 pc acts as a high pass filter preventing low velocity HVS from reaching the halo. To trace the orbits of HVS t
hroughout the Galaxy, we construct two forms of the potential which reasonably represent the observations in the range 5--100,000 pc, a simple spherically symmetric model and a bulge-disk-halo model. We use the Hills mechanism (disruption of binaries by the tidal field of the central black hole) to inject HVS into the Galaxy and compute the observable spatial and velocity distributions of HVS with masses in the range 0.6--4 Msun. These distributions reflect the mass function in the Galactic Center, properties of binaries in the Galactic Center, and aspects of stellar evolution and the injection mechanism. For 0.6--4 Msun main sequence stars, the fraction of unbound HVS and the asymmetry of the velocity distribution for their bound counterparts increases with stellar mass. The density profiles for unbound HVS decline with distance from the Galactic Center approximately as r^{-2} (but are steeper for the most massive stars which evolve off the main sequence during their travel time from the Galactic Center); the density profiles for the bound ejecta decline with distance approximately as r^{-3}. In a survey with a limiting visual magnitude V of 23, the detectability of HVS (unbound or bound) increases with stellar mass.
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