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Register a new userUsing frequency maps to constrain the distribution function of the Milky Way stellar halo
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Monica Valluri
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Resolved surveys of the Milky Ways stellar halo can obtain all 6 phase space coordinates of tens of thousands of individual stars, making it possible to compute their 3-dimensional orbits. Spectral analysis of large numbers of halo orbits can be used to construct frequency maps which are a compact, yet informative representation of their phase space distribution function (DF). Such maps can be used to infer the major types of orbit families that constitute the DF of stellar halo and their relative abundances. The structure of the frequency maps, especially the resonant orbits, reflects the formation history and shape of the dark matter potential and its orientation relative to the disk. The application of frequency analysis to cosmological hydrodynamic simulations of disk galaxies shows that the orbital families occupied by halo stars and dark matter particles are very similar, implying that stellar halo orbits can be used to constrain the DF of the dark matter halo, possibly impacting future direct dark matter detection experiments. An application of these methods to a sample of sim 16,000 Milky Way halo and thick disk stars from the SDSS-SEGUE survey yields a frequency map with strong evidence for resonant trapping of halo stars by the Milky Way disk, in a manner predicted by controlled simulations in which the disk grows adiabatically. The application of frequency analysis methods to current and future phase space data for Milky Way halo stars will provide new insights into the formation history of the dierent components of the Galaxy and the DF of the halo.
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We measure the total stellar halo luminosity using red giant branch (RGB) stars selected from Gaia data release 2. Using slices in magnitude, colour and location on the sky, we decompose RGB stars belonging to the disc and halo by fitting 2-dimensional Gaussians to the Galactic proper motion distributions. The number counts of RGB stars are converted to total stellar halo luminosity using a suite of isochrones weighted by age and metallicity, and by applying a volume correction based on the stellar halo density profile. Our method is tested and calibrated using Galaxia and N-body models. We find a total luminosity (out to 100 kpc) of L_halo = 7.9 +/- 2.0 x 10^8 L_Sun excluding Sgr, and L_halo = 9.4 +/- 2.4 x 10^8 L_Sun including Sgr. These values are appropriate for our adopted stellar halo density profile and metallicity distribution, but additional systematics related to these assumptions are quantified and discussed. Assuming a stellar mass-to-light ratio appropriate for a Kroupa initial mass function (M*/L = 1.5), we estimate a stellar halo mass of M*_halo = 1.4 +/- 0.4 x 10^9 M_Sun. This mass is larger than previous estimates in the literature, but is in good agreement with the emerging picture that the (inner) stellar halo is dominated by one massive dwarf progenitor. Finally, we argue that the combination of a ~10^9 M_Sun mass and an average metallicity of <[Fe/H]> ~ -1.5 for the Galactic halo points to an ancient (~10 Gyr) merger event.
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Halo stars orbit within the potential of the Milky Way and hence their kinematics can be used to understand the underlying mass distribution. However, the inferred mass distribution depends sensitively upon assumptions made on the density and the velocity anisotropy profiles of the tracers. Also, there is a degeneracy between the parameters of the halo and that of the disk or bulge. Here, we decompose the Galaxy into bulge, disk and dark matter halo and then model the kinematic data of the halo BHB and K-giants from the SEGUE. Additionally, we use the gas terminal velocity curve and the Sgr A$^*$ proper motion. With $R_odot = 8.5$kpc, our study reveals that the density of the stellar halo has a break at $17.2^{+1.1}_{-1.0}$ kpc, and an exponential cut-off in the outer parts starting at $97.7^{+15.6}_{-15.8}$kpc. Also, we find the velocity anisotropy is radially biased with $beta_s= 0.4pm{0.2}$ in the outer halo. We measure halo virial mass $M_{text{vir}} = 0.80^{+0.31}_{-0.16} times 10^{12} M_{odot}$, concentration $c=21.1^{+14.8}_{-8.3}$, disk mass of $0.95^{+0.24}_{-0.30}times10^{11} M_{odot}$, disk scale length of $4.9^{+0.4}_{-0.4}$ kpc and bulge mass of $0.91^{+0.31}_{-0.38} times 10^{10} M_{odot}$. The mass of halo is found to be small and this has important consequences. The giant stars reveal that the outermost halo stars have low velocity dispersion interestingly suggesting a truncation of the stellar halo density rather than a small overall mass of the Galaxy. Our estimates of local escape velocity $v_{rm esc} = 550.9^{+32.4}_{-22.1}$ kms$^{-1}$ and dark matter density $rho^{rm DM}_{odot} = 0.0088^{+0.0024}_{-0.0018} M_{odot} {rm pc^{-3}} $ ($0.35^{+0.08}_{-0.07}$ GeV cm$^{-3}$) are in good agreement with recent estimates. Some of the above estimates are depended on the adopted value of $R_odot$ and of outer power-law index of the tracer number density.
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