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Register a new userThe accretion origin of the Milky Ways stellar halo
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We have used data from the Sloan Digital Sky Survey (SDSS) Data Release 5 to explore the overall structure and substructure of the stellar halo of the Milky Way using about 4 million color-selected main sequence turn-off stars. We fit oblate and triaxial broken power-law models to the data, and found a `best-fit oblateness of the stellar halo 0.5<c/a<0.8, and halo stellar masses between Galactocentric radii of 1 and 40kpc of (3.7+/-1.2)x10^8 M_sun. The density profile of the stellar halo is approximately r^{-3}; it is possible that the power law slope is shallower inside 20kpc and steeper outside that radius. Yet, we found that all smooth and symmetric models were very poor fits to the distribution of stellar halo stars because the data exhibit a great deal of spatial substructure. We quantified deviations from a smooth oblate/triaxial model using the RMS of the data around the model profile on scales >~100pc, after accounting for the (known) contribution of Poisson uncertainties. The fractional RMS deviation of the actual stellar distribution from any smooth, parameterized halo model is >~40%: hence, the stellar halo is highly structured. We compared the observations with simulations of galactic stellar halos formed entirely from the accretion of satellites in a cosmological context by analysing the simulations in the same way as the data. While the masses, overall profiles, and degree of substructure in the simulated stellar halos show considerable scatter, the properties and degree of substructure in the Milky Ways halo match well the properties of a `typical stellar halo built exclusively out of the debris from disrupted satellite galaxies. Our results therefore point towards a picture in which an important fraction of the Milky Ways stellar halo has been accreted from satellite galaxies.
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We present an analysis of the radial age gradients for the stellar halos of five Milky Way mass-sized systems simulated as part of the Aquarius Project. The halos show a diversity of age trends, reflecting their different assembly histories. Four of the simulated halos possess clear negative age gradients, ranging from approximately -7 to -19 Myr/kpc , shallower than those determined by recent observational studies of the Milky Ways stellar halo. However, when restricting the analysis to the accreted component alone, all of the stellar halos exhibit a steeper negative age gradient with values ranging from $-$8 to $-$32~Myr/kpc, closer to those observed in the Galaxy. Two of the accretion-dominated simulated halos show a large concentration of old stars in the center, in agreement with the Ancient Chronographic Sphere reported observationally. The stellar halo that best reproduces the current observed characteristics of the age distributions of the Galaxy is that formed principally by the accretion of small satellite galaxies. Our findings suggest that the hierarchical clustering scenario can reproduce the MWs halo age distribution if the stellar halo was assembled from accretion and disruption of satellite galaxies with dynamical masses less than ~10^9.5M_sun, and a minimal in situ contribution.
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.
We present a new, high-resolution chronographic (age) map of the Milky Ways halo, based on the inferred ages of ~130,000 field blue horizontal-branch (BHB) stars with photometry from the Sloan Digital Sky Survey. Our map exhibits a strong central concentration of BHB stars with ages greater than 12 Gyr, extending up to ~15 kpc from the Galactic center (reaching close to the solar vicinity), and a decrease in the mean ages of field stars with distance by 1-1.5 Gyr out to ~45-50 kpc, along with an apparent increase of the dispersion of stellar ages, and numerous known (and previously unknown) resolved over-densities and debris streams, including the Sagittarius Stream. These results agree with expectations from modern LambdaCDM cosmological simulations, and support the existence of a dual (inner/outer) halo system, punctuated by the presence of over-densities and debris streams that have not yet completely phase-space mixed.
We developed a new selection method of halo stars in the phase-space distribution defined by the three integrals of motion in an axisymmetric Galactic potential, ($E$, $L_z$, $I_3$), where $I_3$ is the third integral of motion. The method is used to explore the general chemo-dynamical structure of the halo based on stellar samples from SDSS-SEGUE DR7 and DR16-APOGEE, matched with Gaia-DR2. We found, (a) halo stars can be separated from disk stars by selecting over (1) $0 < L_z < 1500$ kpckms, $(2I_3)^{1/2} > 1000$ kpckms (orbital angle $theta_{rm orb}$ $>$ 15-20 deg), and $E < -1.5 times 10^5$ km$^2$ s$^{-2}$, and (2) $L_z < 0$ kpckms. These selection criteria are free from kinematical biases introduced by the simple high-velocity cuts adopted in recent literature; (b) the averaged, or {it coarse-grained}, halo phased-space distribution shows a monotonic exponential decrease with increasing $E$ and $I_3$ like the Michie-Bodenheimer models; (c) the inner stellar halo described in citet{carollo2007,carollo2010} is found to comprise a combination of Gaia Enceladus debris (GE; citealt{helmi2018}), lowest-$E$ stars (likely in-situ stars), as well as metal-poor prograde stars missed by the high velocity cuts selection; (d) the very metal poor outer halo, ([Fe/H] $< -$2.2), exhibits both retrograde and prograde rotation, with an asymmetric $L_z$ distribution towards high retrograde motions, and larger $theta_{rm orb}$ than those possessed by the GE dominated inner halo; (e) the Sgr dSph galaxy could induce a long-range dynamical effect on local halo stars. Implication for the formation of the stellar halo are also discussed.
We investigate the convective stability of two popular types of model of the gas distribution in the hot Galactic halo. We first consider models in which the halo density and temperature decrease exponentially with height above the disk. These halo models were created to account for the fact that, on some sight lines, the halos X-ray emission lines and absorption lines yield different temperatures, implying that the halo is non-isothermal. We show that the hot gas in these exponential models is convectively unstable if $gamma<3/2$, where $gamma$ is the ratio of the temperature and density scale heights. Using published measurements of $gamma$ and its uncertainty, we use Bayes Theorem to infer posterior probability distributions for $gamma$, and hence the probability that the halo is convectively unstable for different sight lines. We find that, if these exponential models are good descriptions of the hot halo gas, at least in the first few kiloparsecs from the plane, the hot halo is reasonably likely to be convectively unstable on two of the three sight lines for which scale height information is available. We also consider more extended models of the halo. While isothermal halo models are convectively stable if the density decreases with distance from the Galaxy, a model of an extended adiabatic halo in hydrostatic equilibrium with the Galaxys dark matter is on the boundary between stability and instability. However, we find that radiative cooling may perturb this model in the direction of convective instability. If the Galactic halo is indeed convectively unstable, this would argue in favor of supernova activity in the Galactic disk contributing to the heating the hot halo gas.
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