Galactic googly: the rotation-metallicity bias in the inner stellar halo of the Milky Way

Prajwal R. Kafle, Sanjib Sharma, Aaron S. G. Robotham, Raj K. Pradhan, Magda Guglielmo, Luke J. M. Davies, Simon P. Driver

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Abstract

The first and second moments of stellar velocities encode important information about the formation history of the Galactic halo. However, due to the lack of tangential motion and inaccurate distances of the halo stars, the velocity moments in the Galactic halo have largely remained 'known unknowns'. Fortunately, our off-centric position within the Galaxy allows us to estimate these moments in the galactocentric frame using the observed radial velocities of the stars alone. We use these velocities coupled with the hierarchical Bayesian scheme, which allows easy marginalization over the missing data (the proper motion, and uncertainty-free distance and line-of-sight velocity), to measure the velocity dispersions, orbital anisotropy (beta) and streaming motion (v(rot)) of the halo main-sequence turn-off (MSTO) and K-giant (KG) stars in the inner stellar halo (r less than or similar to 15 kpc). We study the metallicity bias in kinematics of the halo stars and observe that the comparatively metal-rich ([Fe/H] > -1.4) and the metal-poor ([Fe/H] sigma(theta) or sigma(phi) and beta similar or equal to 0.5. The apparent metallicity contrariety in the rotation velocity among the halo sub-populations supports the co-existence of multiple populations in the galactic halo that may have formed through distinct formation scenarios, i.e. in situ versus accretion.

Original languageEnglish
Pages (from-to)2959-2971
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Volume470
Issue number3
DOIs
Publication statusPublished - Sep 2017

Cite this

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title = "Galactic googly: the rotation-metallicity bias in the inner stellar halo of the Milky Way",
abstract = "The first and second moments of stellar velocities encode important information about the formation history of the Galactic halo. However, due to the lack of tangential motion and inaccurate distances of the halo stars, the velocity moments in the Galactic halo have largely remained 'known unknowns'. Fortunately, our off-centric position within the Galaxy allows us to estimate these moments in the galactocentric frame using the observed radial velocities of the stars alone. We use these velocities coupled with the hierarchical Bayesian scheme, which allows easy marginalization over the missing data (the proper motion, and uncertainty-free distance and line-of-sight velocity), to measure the velocity dispersions, orbital anisotropy (beta) and streaming motion (v(rot)) of the halo main-sequence turn-off (MSTO) and K-giant (KG) stars in the inner stellar halo (r less than or similar to 15 kpc). We study the metallicity bias in kinematics of the halo stars and observe that the comparatively metal-rich ([Fe/H] > -1.4) and the metal-poor ([Fe/H] sigma(theta) or sigma(phi) and beta similar or equal to 0.5. The apparent metallicity contrariety in the rotation velocity among the halo sub-populations supports the co-existence of multiple populations in the galactic halo that may have formed through distinct formation scenarios, i.e. in situ versus accretion.",
keywords = "Galaxy: formation, Galaxy: general, Galaxy: halo, Galaxy: kinematics and dynamics, BLUE HORIZONTAL-BRANCH, DIGITAL SKY SURVEY, VELOCITY ANISOTROPY, PROPER MOTIONS, K GIANTS, THEORETICAL ISOCHRONES, MASS-DISTRIBUTION, GALAXY CLUSTERS, DARK-MATTER, KINEMATICS",
author = "Kafle, {Prajwal R.} and Sanjib Sharma and Robotham, {Aaron S. G.} and Pradhan, {Raj K.} and Magda Guglielmo and Davies, {Luke J. M.} and Driver, {Simon P.}",
year = "2017",
month = "9",
doi = "10.1093/mnras/stx1394",
language = "English",
volume = "470",
pages = "2959--2971",
journal = "Monthly Notices of the Royal Astronomical Society",
issn = "0035-8711",
publisher = "OXFORD UNIV PRESS UNITED KINGDOM",
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Galactic googly : the rotation-metallicity bias in the inner stellar halo of the Milky Way. / Kafle, Prajwal R.; Sharma, Sanjib; Robotham, Aaron S. G.; Pradhan, Raj K.; Guglielmo, Magda; Davies, Luke J. M.; Driver, Simon P.

In: Monthly Notices of the Royal Astronomical Society, Vol. 470, No. 3, 09.2017, p. 2959-2971.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Galactic googly

T2 - the rotation-metallicity bias in the inner stellar halo of the Milky Way

AU - Kafle, Prajwal R.

AU - Sharma, Sanjib

AU - Robotham, Aaron S. G.

AU - Pradhan, Raj K.

AU - Guglielmo, Magda

AU - Davies, Luke J. M.

AU - Driver, Simon P.

PY - 2017/9

Y1 - 2017/9

N2 - The first and second moments of stellar velocities encode important information about the formation history of the Galactic halo. However, due to the lack of tangential motion and inaccurate distances of the halo stars, the velocity moments in the Galactic halo have largely remained 'known unknowns'. Fortunately, our off-centric position within the Galaxy allows us to estimate these moments in the galactocentric frame using the observed radial velocities of the stars alone. We use these velocities coupled with the hierarchical Bayesian scheme, which allows easy marginalization over the missing data (the proper motion, and uncertainty-free distance and line-of-sight velocity), to measure the velocity dispersions, orbital anisotropy (beta) and streaming motion (v(rot)) of the halo main-sequence turn-off (MSTO) and K-giant (KG) stars in the inner stellar halo (r less than or similar to 15 kpc). We study the metallicity bias in kinematics of the halo stars and observe that the comparatively metal-rich ([Fe/H] > -1.4) and the metal-poor ([Fe/H] sigma(theta) or sigma(phi) and beta similar or equal to 0.5. The apparent metallicity contrariety in the rotation velocity among the halo sub-populations supports the co-existence of multiple populations in the galactic halo that may have formed through distinct formation scenarios, i.e. in situ versus accretion.

AB - The first and second moments of stellar velocities encode important information about the formation history of the Galactic halo. However, due to the lack of tangential motion and inaccurate distances of the halo stars, the velocity moments in the Galactic halo have largely remained 'known unknowns'. Fortunately, our off-centric position within the Galaxy allows us to estimate these moments in the galactocentric frame using the observed radial velocities of the stars alone. We use these velocities coupled with the hierarchical Bayesian scheme, which allows easy marginalization over the missing data (the proper motion, and uncertainty-free distance and line-of-sight velocity), to measure the velocity dispersions, orbital anisotropy (beta) and streaming motion (v(rot)) of the halo main-sequence turn-off (MSTO) and K-giant (KG) stars in the inner stellar halo (r less than or similar to 15 kpc). We study the metallicity bias in kinematics of the halo stars and observe that the comparatively metal-rich ([Fe/H] > -1.4) and the metal-poor ([Fe/H] sigma(theta) or sigma(phi) and beta similar or equal to 0.5. The apparent metallicity contrariety in the rotation velocity among the halo sub-populations supports the co-existence of multiple populations in the galactic halo that may have formed through distinct formation scenarios, i.e. in situ versus accretion.

KW - Galaxy: formation

KW - Galaxy: general

KW - Galaxy: halo

KW - Galaxy: kinematics and dynamics

KW - BLUE HORIZONTAL-BRANCH

KW - DIGITAL SKY SURVEY

KW - VELOCITY ANISOTROPY

KW - PROPER MOTIONS

KW - K GIANTS

KW - THEORETICAL ISOCHRONES

KW - MASS-DISTRIBUTION

KW - GALAXY CLUSTERS

KW - DARK-MATTER

KW - KINEMATICS

U2 - 10.1093/mnras/stx1394

DO - 10.1093/mnras/stx1394

M3 - Article

VL - 470

SP - 2959

EP - 2971

JO - Monthly Notices of the Royal Astronomical Society

JF - Monthly Notices of the Royal Astronomical Society

SN - 0035-8711

IS - 3

ER -