The cold veil of the Milky Way stellar halo

A. J. Deason, V. Belokurov, N. W. Evans, S. E. Koposov, R. J. Cooke, J. Peñarrubia, C. F.P. Laporte, M. Fellhauer, M. G. Walker, Edward W Olszewski

Research output: Contribution to journalArticle

83 Citations (Scopus)

Abstract

We build a sample of distant (D > 80 kpc) stellar halo stars with measured radial velocities. Faint (20 < g < 22) candidate blue horizontal branch (BHB) stars were selected using the deep, but wide, multi-epoch Sloan Digital Sky Survey photometry. Follow-up spectroscopy for these A-type stars was performed using the Very Large Telescope (VLT) FOcal Reducer and low dispersion Spectrograph 2 (FORS2) instrument. We classify stars according to their Balmer line profiles, and find that seven are bona fide BHB stars and 31 are blue stragglers (BS). Owing to the magnitude range of our sample, even the intrinsically fainter BS stars can reach out to D ~ 90 kpc. We complement this sample of A-type stars with intrinsically brighter, intermediate-age, asymptotic giant branch stars. A set of four distant cool carbon stars is compiled from the literature and we perform spectroscopic follow-up on a further four N-type carbon stars using the William Herschel Telescope (WHT) Intermediate dispersion Spectrograph and Imaging System (ISIS) instrument. Altogether, this provides us with the largest sample to date of individual star tracers out to r ~ 150 kpc. We find that the radial velocity dispersion of these tracers falls rapidly at large distances and is surprisingly cold (sr ≈50-60 km s-1) between 100 and 150 kpc. Relating the measured radial velocities to the mass of theMilkyWay requires knowledge of the (unknown) tracer density profile and anisotropy at these distances. Nonetheless, by assuming the stellar halo stars between 50 and 150 kpc have a moderate density fall-off(with power-law slope α < 5) and are on radial orbits (σt 2r 2 < 1), we infer that the mass within 150 kpc is less than 1012M and suggest it probably lies in the range (5-10) × 1011M. We discuss the implications of such a low mass for the Milky Way.

Original languageEnglish (US)
Pages (from-to)2840-2853
Number of pages14
JournalMonthly Notices of the Royal Astronomical Society
Volume425
Issue number4
DOIs
StatePublished - Oct 1 2012

Fingerprint

halos
tracer
blue stars
stars
radial velocity
tracers
horizontal branch stars
carbon stars
carbon
spectrographs
power law
anisotropy
spectroscopy
telescopes
cool stars
asymptotic giant branch stars
profiles
complement
photometry
cold

Keywords

  • Blue stragglers
  • Galaxy: fundamental parameters
  • Galaxy: halo
  • Galaxy: kinematics and dynamics
  • Stars: carbon
  • Stars: horizontal branch

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Deason, A. J., Belokurov, V., Evans, N. W., Koposov, S. E., Cooke, R. J., Peñarrubia, J., ... Olszewski, E. W. (2012). The cold veil of the Milky Way stellar halo. Monthly Notices of the Royal Astronomical Society, 425(4), 2840-2853. https://doi.org/10.1111/j.1365-2966.2012.21639.x

The cold veil of the Milky Way stellar halo. / Deason, A. J.; Belokurov, V.; Evans, N. W.; Koposov, S. E.; Cooke, R. J.; Peñarrubia, J.; Laporte, C. F.P.; Fellhauer, M.; Walker, M. G.; Olszewski, Edward W.

In: Monthly Notices of the Royal Astronomical Society, Vol. 425, No. 4, 01.10.2012, p. 2840-2853.

Research output: Contribution to journalArticle

Deason, AJ, Belokurov, V, Evans, NW, Koposov, SE, Cooke, RJ, Peñarrubia, J, Laporte, CFP, Fellhauer, M, Walker, MG & Olszewski, EW 2012, 'The cold veil of the Milky Way stellar halo', Monthly Notices of the Royal Astronomical Society, vol. 425, no. 4, pp. 2840-2853. https://doi.org/10.1111/j.1365-2966.2012.21639.x
Deason AJ, Belokurov V, Evans NW, Koposov SE, Cooke RJ, Peñarrubia J et al. The cold veil of the Milky Way stellar halo. Monthly Notices of the Royal Astronomical Society. 2012 Oct 1;425(4):2840-2853. https://doi.org/10.1111/j.1365-2966.2012.21639.x
Deason, A. J. ; Belokurov, V. ; Evans, N. W. ; Koposov, S. E. ; Cooke, R. J. ; Peñarrubia, J. ; Laporte, C. F.P. ; Fellhauer, M. ; Walker, M. G. ; Olszewski, Edward W. / The cold veil of the Milky Way stellar halo. In: Monthly Notices of the Royal Astronomical Society. 2012 ; Vol. 425, No. 4. pp. 2840-2853.
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KW - Stars: horizontal branch

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