J/A+A/665/A58 Substructure in stellar halo near the Sun. II. (Ruiz-Lara+, 2022)
Substructure in the stellar halo near the Sun.
II. Characterisation of independent structures.
Ruiz-Lara T., Matsuno T., Lovdal S.S., Helmi A., Dodd E., Koppelman H.H.
<Astron. Astrophys., 665, A58 (2022)>
=2022A&A...665A..58R 2022A&A...665A..58R (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Associations, stellar ; Stars, nearby ;
Space velocities ; Optical
Keywords: Galaxy: structure - Galaxy: halo - Galaxy: kinematics and dynamics -
Galaxy: stellar content
Abstract:
in an accompanying paper, we present a data-driven method for
clustering in 'integrals of motion' space and apply it to a large
sample of nearby halo stars with 6D phase-space information. The
algorithm identified a large number of clusters, many of which could
tentatively be merged into larger groups.
The goal here is to establish the reality of the clusters and groups
through a combined study of their stellar populations (average age,
metallicity, and chemical and dynamical properties) to gain more
insights into the accretion history of the Milky Way.
To this end, we developed a procedure that quantifies the similarity
of clusters based on the Kolmogorov-Smirnov test using their
metallicity distribution functions, and an isochrone fitting method to
determine their average age, which is also used to compare the
distribution of stars in the colour-absolute magnitude diagram. Also
taking into consideration how the clusters are distributed in
integrals of motion space allows us to group clusters into
substructures and to compare substructures with one another.
We find that the 67 clusters identified by our algorithm can be merged
into 12 extended substructures and 8 small clusters that remain as
such. The large substructures include the previously known
Gaia-Enceladus, Helmi streams, Sequoia, and Thamnos 1 and 2. We
identify a few over-densities that can be associated with the hot
thick disc and host a small metal-poor population. Especially notable
is the largest (by number of member stars) substructure in our sample
which, although peaking at the metallicity characteristic of the thick
disc, has a very well populated metal-poor component, and dynamics
intermediate between the hot thick disc and the halo. We also identify
additional debris in the region occupied by Sequoia with clearly
distinct kinematics, likely remnants of three different accretion
events with progenitors of similar masses. Although only a small
subset of the stars in our sample have chemical abundance information,
we are able to identify different trends of [Mg/Fe] versus [Fe/H] for
the various substructures, confirming our dissection of the nearby
halo.
We find that at least 20% of the halo near the Sun is associated to
substructures. When comparing their global properties, we note that
those substructures on retrograde orbits are not only more metal-poor
on average but are also older. We provide a table summarising the
properties of the substructures, as well as a membership list that can
be used for follow-up chemical abundance studies for example.
Description:
Table A.1 shows an overview of the characteristics of the clusters
detected by the algorithm described in Paper I (Lovdal et al.,
2022A&A...665A..57L 2022A&A...665A..57L, Cat. J/A+A/665/A57) after refining them using a
Mahalanobis distance limit of Dij<2.13. We encourage the reader to
refer to Paper I for a thorough characterisation of the original
clusters.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 41 68 Overview of the characteristics of the clusters
analysed in this work
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See also:
J/A+A/665/A57 : Substructure in the stellar halo near the Sun. I.
(Lovdal+, 2022)
Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 2 I2 --- Cluster [1/68] Cluster identification number,
as in Paper I (Lovdal et al.,
2022A&A...665A..57L 2022A&A...665A..57L, Cat. J/A+A/665/A57)
4- 7 I4 --- N* Number of stars included in the analysis
9- 13 F5.2 10+3kpc.km/s Lz Mean angular momentum in the z-direction
15- 18 F4.2 10+3kpc.km/s Lperp Mean of angular momentum perpendicular
component
20- 25 F6.2 10+4km2/s2 E Mean total energy
27- 30 I4 --- N[Fe/H] Number of observation for [Fe/H]
32- 36 F5.2 [-] [Fe/H] ?=- Mean metallicity
38- 41 F4.2 [-] e_[Fe/H] ?=- Standard deviation on [Fe/H]
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History:
From electronic version of the journal
References:
Lovdal et al., Paper I 2022A&A...665A..57L 2022A&A...665A..57L, Cat. J/A+A/665/A57
(End) Patricia Vannier [CDS] 06-Sep-2023