J/A+A/665/A57 Substructure in the stellar halo near the Sun. I. (Lovdal+, 2022)
Substructure in the stellar halo near the Sun.
I. Data-driven clustering in integrals-of-motion space.
Lovdal S.S., Ruiz-Lara T., Koppelman H.H., Matsuno T., Dodd E., Helmi A.
<Astron. Astrophys. 665, A57 (2022)>
=2022A&A...665A..57L 2022A&A...665A..57L (SIMBAD/NED BibCode)
ADC_Keywords: Milky Way ; Associations, stellar ; Stars, nearby ;
Space velocities ; Optical
Keywords: Galaxy: kinematics and dynamics - Galaxy: formation - Galaxy: halo -
solar neighborhood - Galaxy: evolution - methods: data analysis
Abstract:
Merger debris is expected to populate the stellar haloes of galaxies.
In the case of the Milky Way, this debris should be apparent as clumps
in a space defined by the orbital integrals of motion of the stars.
Our aim is to develop a data-driven and statistics-based method for
finding these clumps in integrals-of-motion space for nearby halo
stars and to evaluate their significance robustly. We used data from
Gaia EDR3, extended with radial velocities from ground-based
spectroscopic surveys, to construct a sample of halo stars within
2.5kpc from the Sun. We applied a hierarchical clustering method that
makes exhaustive use of the single linkage algorithm in
three-dimensional space defined by the commonly used integrals of
motion energy E, together with two components of the angular momentum,
Lz and Lperp. To evaluate the statistical significance of the
clusters, we compared the density within an ellipsoidal region centred
on the cluster to that of random sets with similar global dynamical
properties. By selecting the signal at the location of their maximum
statistical significance in the hierarchical tree, we extracted a set
of significant unique clusters. By describing these clusters with
ellipsoids, we estimated the proximity of a star to the cluster centre
using the Mahalanobis distance. Additionally, we applied the HDBSCAN
clustering algorithm in velocity space to each cluster to extract
subgroups representing debris with different orbital phases. Our
procedure identifies 67 highly significant clusters (>3σ),
containing 12% of the sources in our halo set, and 232 subgroups or
individual streams in velocity space. In total, 13.8% of the stars in
our data set can be confidently associated with a significant cluster
based on their Mahalanobis distance. Inspection of the hierarchical
tree describing our data set reveals a complex web of relations
between the significant clusters, suggesting that they can be
tentatively grouped into at least six main large structures, many of
which can be associated with previously identified halo substructures,
and a number of independent substructures. This preliminary conclusion
is further explored in a companion paper, in which we also
characterise the substructures in terms of their stellar populations.
Our method allows us to systematically detect kinematic substructures
in the Galactic stellar halo with a data-driven and interpretable
algorithm. The list of the clusters and the associated star catalogue
are provided in two tables in electronic format.
Description:
Overdensities in integrals-of-motion space of the Milky Way halo,
corresponding to the statistically significant clusters presented in
our paper. We provide the spatial location and extent of the extracted
clusters, together with the halo star catalogue with corresponding
cluster labels, corresponding to Table 1 and Table 2 of our paper.
In Table 1, the suffixes 0-2 correspond to the energy E, perpendicular
momentum Lperp, and angular momentum in z-direction Lz. The quantities
mu and sigma were computed using the original members of the cluster
as assigned by the single linkage process, after scaling the features,
so the indicated values correspond to values in the scaled data space.
In Table 2, the spatial coordinates and velocity components correspond
to a heliocentric cartesian coordinate system.
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 240 68 Spatial location and extent of the extracted
clusters
table2.dat 283 51671 Halo star catalogue with corresponding cluster
labels
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See also:
https://github.com/SofieLovdal/IOM_clustering :
Source code of the clustering algorithm
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 2 I2 --- Label [1/68] Cluster label
4- 22 F19.16 --- signi [3.01/13.17] Statistical significance
according to Eq. 3
24- 27 I4 --- Norig [12/2137] Number of original members
according to the single linkage procedure
29- 32 I4 --- NDcut [9/3032] Number of stars for which this
cluster is the closest, considering at most a
Mahalanobis distance of 2.13
34- 53 F20.17 --- mu0 [-0.95/0.8] Mean energy, E
55- 74 F20.17 --- mu1 [-0.99/0.42] Mean perpendicular momentum,
Lperp
76- 97 F22.19 --- mu2 [-0.64/0.86] Mean angular momentum in
z-direction, Lz
99-120 E22.17 --- sigma00 [0.00/0.03] Variance in E
122-145 E24.17 --- sigma01 [-0.01/0.01] Covariance between E and Lperp
147-170 E24.17 --- sigma02 [-0.01/0.01] Covariance between E and Lz
172-193 E22.17 --- sigma11 [0.00/0.02] Variance in Lperp
195-218 E24.17 --- sigma12 [-0.01/0.01] Covariance between Lperp and Lz
220-240 E21.16 --- sigma22 [0.00/0.01] Variance in Lz
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Byte-by-byte Description of file: table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 19 I19 --- GaiaEDR3 Gaia EDR3 source_id
21- 44 E24.17 --- x [-2.5/2.5] x-coordinate
46- 69 E24.17 --- y [-2.46/2.48] y-coordinate
71- 93 E23.16 --- z [-2.46/2.5] z-coordinate
95-117 F23.18 km/s vx [-571/498] x-component of velocity vector
119-141 F23.18 km/s vy [-779/272] y-component of velocity vector
143-166 F24.19 km/s vz [-525/516] z-component of velocity vector
168-189 F22.14 km2/s2 E [-169824/-69] Energy
191-212 F22.17 kpc.km/s Lperp [0/4287] Perpendicular angular momentum
214-237 F24.18 kpc.km/s Lz [-4504/4588] Angular momentum in z-direction
239-257 F19.16 --- signi Significance of the cluster corresponding
to Labelorig
259-260 I2 --- Labelorig [0/68] Cluster label according to the
original single linkage assignment
262-263 I2 --- LabelDcut [0/68] Closest cluster according to a
Mahalanobis distance of at least 2.13
265-283 F19.17 --- D Mahalanobis distance to the cluster
corresponding to Labelorig
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Acknowledgements:
Sofie Lovdal, s.s.lovdal(at)rug.nl
(End) Patricia Vannier [CDS] 09-May-2022