The stellar halo of the Milky Way traced by blue horizontal-branch stars in the Subaru Hyper Suprime-Cam Survey
arxiv(2024)
摘要
We select blue-horizontal branch stars (BHBs) from the internal data release
of the Hyper Suprime-Cam Subaru Strategic Program to reveal the global
structure of the Milky Way (MW) stellar halo. The data are distributed over
∼ 1,100 deg^2 area in the range of 18.5. Our sample
can be fitted to either a single power-law profile with an index of
α=4.11^+0.18_-0.18 or a broken power-law profile with an index of
α_ in=3.90^+0.24_-0.30 at r below a broken radius of r_
b=184^+118_-66 kpc and a very steep slope of α_
out=9.1^+6.8_-3.6 at r>r_ b; the statistical difference between
these fitting profiles is small. Both profiles are found to show prolate shapes
having axial ratios of q=1.47^+0.30_-0.33 and 1.56^+0.34_-0.23,
respectively. We also find a signature of the so-called "splashback radius" for
the candidate BHBs, which can reach as large as r ∼ 575 kpc, although it
is still inconclusive owing to rather large distance errors in this faintest
end of the sample. Our results suggest that the MW stellar halo consists of the
two overlapping components: the in situ inner halo showing a relatively
steep radial density profile and the ex situ outer halo with a shallower
profile, being characteristic of a component formed from accretion of small
stellar systems.
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