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Title: THE VELOCITY ANISOTROPY OF DISTANT MILKY WAY HALO STARS FROM HUBBLE SPACE TELESCOPE PROPER MOTIONS

Abstract

Based on long baseline (5-7 years) multi-epoch HST/ACS photometry, used previously to measure the proper motion of M31, we present the proper motions (PMs) of 13 main-sequence Milky Way halo stars. The sample lies at an average distance of r {approx_equal} 24 kpc from the Galactic center, with a root-mean-square spread of 6 kpc. At this distance, the median PM accuracy is 5 km s{sup -1}. We devise a maximum likelihood routine to determine the tangential velocity ellipsoid of the stellar halo. The velocity second moments in the directions of the Galactic (l, b) system are {sup 1/2}= 123{sup +29}{sub -23} km s{sup -1}, and {sup 1/2}= 83{sup +24}{sub -16} km s{sup -1}. We combine these results with the known line-of-sight second moment, {sup 1/2}= 105{+-}5 km s{sup -1}, at this (r) to study the velocity anisotropy of the halo. We find approximate isotropy between the radial and tangential velocity distributions, with anisotropy parameter {beta}= 0.0{sup +0.2}{sub -0.4}. Our results suggest that the stellar halo velocity anisotropy out to r {approx} 30 kpc is less radially biased than solar neighborhood measurements. This is opposite to what is expected from violent relaxation, and may indicate the presence of a shell-type structuremore » at r {approx} 24 kpc. With additional multi-epoch HST data, the method presented here has the ability to measure the transverse kinematics of the halo for more stars, and to larger distances. This can yield new improved constraints on the stellar halo formation mechanism, and the mass of the Milky Way.« less

Authors:
;  [1]; ;
  1. Department of Astronomy and Astrophysics, University of California Santa Cruz, Santa Cruz, CA 95064 (United States)
Publication Date:
OSTI Identifier:
22167577
Resource Type:
Journal Article
Journal Name:
Astrophysical Journal
Additional Journal Information:
Journal Volume: 766; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 0004-637X
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCURACY; ANISOTROPY; DISTANCE; ISOTROPY; LIMITING VALUES; MASS; MAXIMUM-LIKELIHOOD FIT; MILKY WAY; PHOTOMETRY; PROPER MOTION; RELAXATION; STARS; TELESCOPES; VELOCITY

Citation Formats

Deason, A. J., Guhathakurta, P., Van der Marel, R. P., Sohn, S. T., and Brown, T. M., E-mail: alis@ucolick.org. THE VELOCITY ANISOTROPY OF DISTANT MILKY WAY HALO STARS FROM HUBBLE SPACE TELESCOPE PROPER MOTIONS. United States: N. p., 2013. Web. doi:10.1088/0004-637X/766/1/24.
Deason, A. J., Guhathakurta, P., Van der Marel, R. P., Sohn, S. T., & Brown, T. M., E-mail: alis@ucolick.org. THE VELOCITY ANISOTROPY OF DISTANT MILKY WAY HALO STARS FROM HUBBLE SPACE TELESCOPE PROPER MOTIONS. United States. https://doi.org/10.1088/0004-637X/766/1/24
Deason, A. J., Guhathakurta, P., Van der Marel, R. P., Sohn, S. T., and Brown, T. M., E-mail: alis@ucolick.org. Wed . "THE VELOCITY ANISOTROPY OF DISTANT MILKY WAY HALO STARS FROM HUBBLE SPACE TELESCOPE PROPER MOTIONS". United States. https://doi.org/10.1088/0004-637X/766/1/24.
@article{osti_22167577,
title = {THE VELOCITY ANISOTROPY OF DISTANT MILKY WAY HALO STARS FROM HUBBLE SPACE TELESCOPE PROPER MOTIONS},
author = {Deason, A. J. and Guhathakurta, P. and Van der Marel, R. P. and Sohn, S. T. and Brown, T. M., E-mail: alis@ucolick.org},
abstractNote = {Based on long baseline (5-7 years) multi-epoch HST/ACS photometry, used previously to measure the proper motion of M31, we present the proper motions (PMs) of 13 main-sequence Milky Way halo stars. The sample lies at an average distance of r {approx_equal} 24 kpc from the Galactic center, with a root-mean-square spread of 6 kpc. At this distance, the median PM accuracy is 5 km s{sup -1}. We devise a maximum likelihood routine to determine the tangential velocity ellipsoid of the stellar halo. The velocity second moments in the directions of the Galactic (l, b) system are {sup 1/2}= 123{sup +29}{sub -23} km s{sup -1}, and {sup 1/2}= 83{sup +24}{sub -16} km s{sup -1}. We combine these results with the known line-of-sight second moment, {sup 1/2}= 105{+-}5 km s{sup -1}, at this (r) to study the velocity anisotropy of the halo. We find approximate isotropy between the radial and tangential velocity distributions, with anisotropy parameter {beta}= 0.0{sup +0.2}{sub -0.4}. Our results suggest that the stellar halo velocity anisotropy out to r {approx} 30 kpc is less radially biased than solar neighborhood measurements. This is opposite to what is expected from violent relaxation, and may indicate the presence of a shell-type structure at r {approx} 24 kpc. With additional multi-epoch HST data, the method presented here has the ability to measure the transverse kinematics of the halo for more stars, and to larger distances. This can yield new improved constraints on the stellar halo formation mechanism, and the mass of the Milky Way.},
doi = {10.1088/0004-637X/766/1/24},
url = {https://www.osti.gov/biblio/22167577}, journal = {Astrophysical Journal},
issn = {0004-637X},
number = 1,
volume = 766,
place = {United States},
year = {2013},
month = {3}
}