J/A+A/700/A244 Mapping the Milky Way with Gaia XP spectra. II. (Wu+, 2025)
Mapping the Milky Way with Gaia Bp/Rp spectra. II:
The inner stellar halo traced by a large sample of blue horizontal branch stars.
Wu W., Ye X., Allende Prieto C., Chen Y., Xue X.-X., Zhao G., Zhao J.,
Aguado D.S., Gonzalez Hernandez J.I., Rebolo R.
<Astron. Astrophys. 700, A244 (2025)>
=2025A&A...700A.244W 2025A&A...700A.244W (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Stars, horizontal branch ; Stars, halo ; Optical
Keywords: stars: horizontal-branch - Galaxy: halo - Galaxy: stellar content -
Galaxy: structure
Abstract:
Due to their nearly constant absolute magnitudes and old ages, Blue
Horizontal Branch (BHB) stars are frequently used as standard candles
to study the kinematics and structures of our galaxy. The number of
identified BHB stars has significantly increased due to the advent of
large scale surveys in the last two decades. Recently, Gaia DR3 was
released including a catalog of around 220 million low-resolution
spectra (Bp/Rp, or XP hereafter). These data have great potential for
identifying many interesting stellar objects including BHB stars.
We select BHB stars based on synthetic photometry and the stellar
atmosphere parameters inferred from Gaia Bp/Rp spectra. We generate
the synthetic SDSS broad-band ugr and Pristine narrow-band CaHK
magnitudes from Gaia Bp/Rp data. A photometric selection of BHB
candidates is made in the (u-g, g-r) and (u-CaHK, g-r) color-color
spaces. A spectroscopic selection in Teff-logg space is applied to
remove stars with high surface gravity. The selection function of BHB
stars is obtained by using the Gaia DR3 photometry. A non-parametric
method, which allows the variation of the vertical flattening q with
the Galactic radius, is adopted to explore the density shape of the
stellar halo.
We construct a full-sky BHB catalog from Gaia Bp/Rp spectra and use it
to explore the shape of the inner stellar halo.
We present a catalog of 44552 high latitude (|b|>20°) BHB
candidates chosen with a well-characterized selection function. The
stellar halo traced by these BHB stars is more flattened at smaller
radii (q=0.4 at r∼8kpc), and becomes nearly spherical at larger radii
(q=0.8 at r∼25kpc). Assuming a variable flattening and excluding
several obvious outliers that might be related to the halo
substructures or contaminants, we obtain a smooth and consistent
relationship between r and q, and the density profile is best fitted
with by a single power law with an index alpha=-4.65±0.04.
Description:
Gaia XP spectra provide an unique opportunity to study the Galactic
structure through the standard candles identified from them. We make
an identification of Blue Horizontal Branch stars from Gaia XP spectra
through combined photometric and spectroscopic cuts. The first Catalog
table1.dat includes the 44552 high latitude (|b|>20°) BHB
candidates, while the second catalog table2.dat contains 520 potential
globular clusters members. Users should check the proper motions of
these globular clusters members to remove possible field stars.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablee1.dat 209 44552 High latitude (|b|>20°) BHB candidates
tablee2.dat 50 520 Potential globular clusters members
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See also:
J/A+A/695/A75 : Mapping the Milky Way with Gaia Bp/Rp spectra (Ye+, 2025)
Byte-by-byte Description of file: tablee1.dat
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Bytes Format Units Label Explanations
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1- 19 I19 --- GaiaDR3 Name of the Gaia DR3 source_id
21- 29 F9.3 K TeffXP Effective temperature derived from Gaia XP
spectra with nsc3 model
31- 35 F5.3 [cm/s2] loggXP Surface gravity derived from Gaia XP spectra
with nsc3 model
38- 56 F19.13 pc Dist16 P16 percentiles distributions of the
derived heliocentric distance
58- 76 F19.13 pc Dist50 P50 percentiles distributions of the
derived heliocentric distance
78- 96 F19.13 pc Dist84 P84 percentiles distributions of the
derived heliocentric distance
98-117 F20.18 --- SF ?=- Selection function
119-126 F8.3 K Teffye Effective temperature derived from
Gaia XP spectra with nsc1 model
128-145 F18.16 mag (u-g)0 Dereddened color of u-g
147-169 E23.16 mag (g-r)0 Dereddened color of g-r
170-187 F18.16 mag (u-CaHK)0 Dereddened color of u-CaHK
189-209 F21.19 mag E(B-V) Extinction from a two-dimensional dust
map dustmaps.sfd.SFDQuery
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Byte-by-byte Description of file: tablee2.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Name Name of the globular cluster
11- 29 F19.15 deg RAdeg Right ascension (J2000) of the potential
globular cluster BHB member
31- 50 F20.16 deg DEdeg Declination (J2000) of the potential
globular cluster BHB member
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Acknowledgements:
Wenbo Wu, wbwu(at)nao.cas.cn
References:
Ye et al., Paper I 2025A&A...695A..75Y 2025A&A...695A..75Y, Cat J/A+A/695/A75
(End) Patricia Vannier [CDS] 04-Jul-2025