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Register a new userDifferential rotation of the halo traced by the K-giant stars
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We use K-giant stars selected from the LAMOST DR5 to study the variation of the rotational velocity of the galactic halo at different space positions. Modelling the rotational velocity distribution with both the halo and disk components, we find that the rotational velocity of the halo population decreases almost linearly with increasing vertical distance to the galactic disk plane, $Z$, at fixed galactocentric radius, $R$. The samples are separated into two parts with $6<R<12$ kpc and $12<R<20$ kpc. We derive that the decreasing rates along $Z$ for the two subsamples are $-3.07pm0.63$ and $-1.89pm0.37$ km s$^{-1}$ kpc$^{-1}$, respectively. Compared with the TNG simulations, we suggest that this trend is probably caused by the interaction between the disk and halo. The results from the simulations show that only the oblate halo can provide a decreasing rotational velocity with an increasing $Z$. This indicates that the Galactic halo is oblate with galactocentric radius $R<20$ kpc. On the other hand, the flaring of the disk component (mainly the thick disk) is clearly traced by this study, with $R$ between 12 and 20 kpc, the disk can vertically extend to $6sim10$ kpc above the disk plane. What is more interesting is that, we find the Gaia-Enceladus-Sausage (GES) component has a significant contribution only in the halo with $R<12$ kpc, i.e. a fraction of 23$-$47%. While in the outer subsample, the contribution is too low to be well constrained.
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We report on the global structure of the Milky Way (MW) stellar halo up to its outer boundary based on the analysis of blue-horizontal branch stars (BHBs). These halo tracers are extracted from the $(g,r,i,z)$ band multi-photometry in the internal data release of the on-going Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) surveyed over $sim550$~deg$^2$ area. In order to select most likely BHBs by removing blue straggler stars (BSs) and other contamination in a statistically significant manner, we have developed and applied an extensive Bayesian method, instead of the simple color cuts adopted in our previous work, where each of the template BHBs and non-BHBs obtained from the available catalogs is represented as a mixture of multiple Gaussian distributions in the color-color diagrams. We found from the candidate BHBs in the range of 18.5<g<23.5 mag that the radial density distribution over a Galactocentric radius of r=36-360 kpc can be approximated as a single power-law profile with an index of $alpha=3.74^{+0.21}_{-0.22}$ or a broken power-law profile with an index of $alpha_{rm in}=2.92^{+0.33}_{-0.33}$ at $r$ below a broken radius of $r_{rm b}=160^{+18}_{-19}$~kpc and a very steep slope of $alpha_{rm out}=15.0^{+3.7}_{-4.5}$ at $r>r_{rm b}$. The latter profile with a prolate shape having an axial ratio of $q=1.72^{+0.44}_{-0.28}$ is most likely and this halo may hold a rather sharp boundary at r=160kpc. The slopes of the halo density profiles are compared with those from the suite of hydrodynamical simulations for the formation of stellar halos. This comparison suggests that the MW stellar halo may consist of the two overlapping components: the in situ. inner halo as probed by RR Lyrae stars showing a relatively steep radial density profile and the ex situ. outer halo with a shallow profile probed by BHBs here, which is made by accretion of small stellar systems.
The galaxy M49 (NGC 4472) is the brightest early-type galaxy in the Virgo Cluster. It is located in Subcluster B and has an unusually blue, metal-poor outer halo. Planetary nebulae (PNe) are excellent tracers of diffuse galaxy and intragroup light. We present a photometric survey of PNe in the galaxys extended halo to characterise its PN population, as well as the surrounding intragroup light (IGL) of the Subcluster B. PNe were identified based on their bright [OIII]5007 AA emission and absence of a broad-band continuum. We identify 738 PNe out to a radius of 155 kpc from M49s centre from which we define a complete sample of 624 PNe within a limiting magnitude of m_5007=28.8. Comparing the PN number density to the broad-band stellar surface brightness profile, we find a variation of the PN-specific frequency (alpha-parameter) with radius. The outer halo beyond 60 kpc has a 3.2 times higher alpha-parameter compared to the main galaxy halo, which is likely due to contribution from the surrounding blue IGL. We use the Planetary Nebulae Luminosity Function (PNLF) as an indicator of distance and stellar population. Its slope, which correlates empirically with galaxy type, varies within the inner halo. In the eastern quadrant of M49, the PNLF slope is shallower, indicating an additional localised, bright PN population following an accretion event, likely that of the dwarf irregular galaxy VCC1249. We also determined a distance modulus of mu = 31.29+/-0.08 for M49, corresponding to a physical distance of 18.1+/-0.6 Mpc, which agrees with a recent surface-brightness fluctuations distance. The PN populations in the outer halo of M49 are consistent with the presence of a main Sersic galaxy halo with a slight (B-V) colour gradient of 10${}^{-4}$ mag/arcsec surrounded by intragroup light with a very blue colour of (B-V)=0.25 and a constant surface brightness mu_V=28.0 mag/arcsec${}^2$.
Rotational light modulation in Kepler photometry of K - A stars is used to estimate the absolute rotational shear. The rotation frequency spread in 2562 carefully selected stars with known rotation periods is measured using time-frequency diagrams. The variation of rotational shear as a function of effective temperature in restricted ranges of rotation period is determined. The shear increases to a maximum in F stars, but decreases somewhat in the A stars. Theoretical models reproduce the temperature variation quite well. The dependence of rotation shear on rotation rate in restricted temperature ranges is also determined. The dependence of the shear on the rotation rate is weak in K and G stars, increases rapidly for F stars and is strongest in A stars. For stars earlier than type K, a discrepancy exists between the predicted and observed variation of shear with rotation rate. There is a strong increase in the fraction of stars with zero frequency spread with increasing effective temperature. The time-frequency diagrams for A stars are no different from those in cool stars, further supporting the presence of spots in stars with radiative envelopes.
We statistically quantify the amount of substructure in the Milky Way stellar halo using a sample of 4568 halo K giant stars at Galactocentric distances ranging over 5-125 kpc. These stars have been selected photometrically and confirmed spectroscopically as K giants from the Sloan Digital Sky Surveys SEGUE project. Using a position-velocity clustering estimator (the 4distance) and a model of a smooth stellar halo, we quantify the amount of substructure in the halo, divided by distance and metallicity. Overall, we find that the halo as a whole is highly structured. We also confirm earlier work using BHB stars which showed that there is an increasing amount of substructure with increasing Galactocentric radius, and additionally find that the amount of substructure in the halo increases with increasing metallicity. Comparing to resampled BHB stars, we find that K giants and BHBs have similar amounts of substructure over equivalent ranges of Galactocentric radius. Using a friends-of-friends algorithm to identify members of individual groups, we find that a large fraction (~33%) of grouped stars are associated with Sgr, and identify stars belonging to other halo star streams: the Orphan Stream, the Cetus Polar Stream, and others, including previously unknown substructures. A large fraction of sample K giants (more than 50%) are not grouped into any substructure. We find also that the Sgr stream strongly dominates groups in the outer halo for all except the most metal-poor stars, and suggest that this is the source of the increase of substructure with Galactocentric radius and metallicity.
We present the stellar density profile of the outer halo of the Galaxy traced over a range of Galactocentric radii from $15< R_{GC} < 220$ kpc by blue horizontal branch (BHB) stars. These stars are identified photometrically using deep $u-$band imaging from the new Canada-France-Imaging-Survey (CFIS) that reaches 24.5 mag. This is combined with $griz$ bands from Pan-STARRS 1 and covers a total of $sim4000$ deg$^2$ of the northern sky. We present a new method to select BHB stars that has low contamination from blue stragglers and high completeness. We use this sample to measure and parameterize the three dimensional density profile of the outer stellar halo. We fit the profile using (i) a simple power-law with a constant flattening (ii) a flattening that varies as a function of Galactocentric radius (iii) a broken power law profile. We find that outer stellar halo traced by the BHB is well modelled by a broken power law with a constant flattening of $q=0.86 pm 0.02$, with an inner slope of $gamma=4.24 pm 0.08$. This is much steeper than the preferred outer profile that has a slope of $beta=3.21pm 0.07$ after a break radius of $r_b=41.4^{+2.5}_{-2.4}$ kpc. The outer profile of the stellar halo trace by BHB stars is shallower than that recently measured using RR Lyrae, a surprising result given the broad similarity of the ages of these stellar populations.
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