The radial profile and flattening of the Milky Ways stellar halo to $rm 80~$kpc from the SEGUE K-giant Survey

We characterise the radial density, metallicity and flattening profile of the Milky Ways stellar halo, based on the large sample of 1757 spectroscopically confirmed giant stars from SDSS/SEGUE-2 after excising stars that were algorithmically attributed to apparent halo substructure (including the Sagittarius stream). Compared to BHB stars or RR Lyrae, giants are more readily understood tracers of the overall halo star population, with less bias in age or metallicity. The well-characterized selection function of the sample enables forward modelling of those data, based on ellipsoidal stellar density models, $ u_* (R,z)$, with Einasto profiles and (broken) power laws for their radial dependence, combined with a model for the metallicity gradient and the flattening profile. Among models with constant flattening, these data are reasonably well fit by an Einasto profile of $n=3.1pm 0.5$ with an effective radius $rm r_{eff} = 15pm2~$kpc and a flattening of $q=0.7pm 0.02$; or comparably well by an equally flattened broken power-law, with radial slopes of $alpha_{in}=2.1pm 0.3$ and $alpha_{out}=3.8pm 0.1$, with a break-radius of $r_{break}=18pm1$~kpc; this is largely consistent with earlier work. We find a modest, but significant metallicity gradient within the outer stellar halo, $rm [Fe/H]$ decreasing outward. If we allow for a variable flattening $q = f(r_{GC} )$, we find the distribution of halo giants to be considerably more flattened at small radii, $q({rm 10~kpc})sim 0.57$, compared to $q(>30{rm kpc})sim 0.8$. Remarkably, the data are then very well fit by a single power-law of index $rm sim 4.2pm0.1$ of the variable $r_qequivsqrt{R^2+(z/q(r))^2}$. In this simple and better fitting model, there is a break in flattening at $sim 20$~kpc, instead of a break in the radial density function.

The SEGUE K giant survey II: A Catalog of Distance Determinations for the SEGUE K giants in the Galactic Halo

We present an online catalog of distance determinations for $rm 6036$ K giants, most of which are members of the Milky Ways stellar halo. Their medium-resolution spectra from SDSS/SEGUE are used to derive metallicities and rough gravity estimates, along with radial velocities. Distance moduli are derived from a comparison of each stars apparent magnitude with the absolute magnitude of empirically calibrated color-luminosity fiducials, at the observed $(g-r)_0$ color and spectroscopic [Fe/H]. We employ a probabilistic approach that makes it straightforward to properly propagate the errors in metallicities, magnitudes, and colors into distance uncertainties. We also fold in ${it prior}$ information about the giant-branch luminosity function and the different metallicity distributions of the SEGUE K-giant targeting sub-categories. We show that the metallicity prior plays a small role in the distance estimates, but that neglecting the luminosity prior could lead to a systematic distance modulus bias of up to 0.25 mag, compared to the case of using the luminosity prior. We find a median distance precision of $16%$, with distance estimates most precise for the least metal-poor stars near the tip of the red-giant branch. The precision and accuracy of our distance estimates are validated with observations of globular and open clusters. The stars in our catalog are up to 125 kpc distant from the Galactic center, with 283 stars beyond 50 kpc, forming the largest available spectroscopic sample of distant tracers in the Galactic halo.

Quantifying Kinematic Substructure in the Milky Ways Stellar Halo

We present and analyze the positions, distances, and radial velocities for over 4000 blue horizontal-branch (BHB) stars in the Milky Ways halo, drawn from SDSS DR8. We search for position-velocity substructure in these data, a signature of the hierarchical assembly of the stellar halo. Using a cumulative close pair distribution (CPD) as a statistic in the 4-dimensional space of sky position, distance, and velocity, we quantify the presence of position-velocity substructure at high statistical significance among the BHB stars: pairs of BHB stars that are close in position on the sky tend to have more similar distances and radial velocities compared to a random sampling of these overall distributions. We make analogous mock-observations of 11 numerical halo formation simulations, in which the stellar halo is entirely composed of disrupted satellite debris, and find a level of substructure comparable to that seen in the actually observed BHB star sample. This result quantitatively confirms the hierarchical build-up of the stellar halo through a signature in phase (position-velocity) space. In detail, the structure present in the BHB stars is somewhat less prominent than that seen in most simulated halos, quite possibly because BHB stars represent an older sub-population. BHB stars located beyond 20 kpc from the Galactic center exhibit stronger substructure than at $rm r_{gc} < 20$ kpc.

The Milky Ways Circular Velocity Curve to 60 kpc and an Estimate of the Dark Matter Halo Mass from Kinematics of ~2400 SDSS Blue Horizontal Branch Stars

We derive new constraints on the mass of the Milky Ways dark matter halo, based on a set of halo stars from SDSS as kinematic tracers. Our sample comprises 2401 rigorously selected Blue Horizontal-Branch (BHB) halo stars drawn from SDSS DR-6. To interpret these distributions, we compare them to matched mock observations drawn from two different cosmological galaxy formation simulations designed to resemble the Milky Way, which we presume to have an appropriate orbital distribution of halo stars. We then determine which value of $rm V_{cir}(r)$ brings the observed distribution into agreement with the corresponding distributions from the simulations. This procedure results in an estimate of the Milky Ways circular velocity curve to $sim 60$ kpc, which is found to be slightly falling from the adopted value of $rm 220 km s^{-1}$ at the Suns location, and implies M$(<60 rm kpc) = 4.0pm 0.7times 10^{11}$M$_odot$. The radial dependence of $rm V_{cir}(r)$, derived in statistically independent bins, is found to be consistent with the expectations from an NFW dark matter halo with the established stellar mass components at its center. If we assume an NFW halo profile of characteristic concentration holds, we can use the observations to estimate the virial mass of the Milky Ways dark matter halo, M$_{rm vir}=1.0^{+0.3}_{-0.2} times 10^{12}$M$_odot$, which is lower than many previous estimates. This estimate implies that nearly 40% of the baryons within the virial radius of the Milky Ways dark matter halo reside in the stellar components of our Galaxy. A value for M$_{rm vir}$ of only $sim 1times10^{12}$M$_odot$ also (re-)opens the question of whether all of the Milky Ways satellite galaxies are on bound orbits.