Stellar halo

The stellar halo of a galaxy refers to the component of its galactic halo containing stars. The halo extends far outside a galaxy's brightest regions and typically contains its oldest and most metal poor stars.

Observation history

Early studies, investigating the shape of the stellar halo of the Milky Way, found some evidence that it may vary with increasing distance from the galaxy.[1] These studies found halos with spherically shaped outer regions and flatter inner regions.[2] Large surveys in the 21st century such as the Sloan Digital Sky Survey have allowed the shape and distribution of the stellar halo to be investigated in much more detail; this data has been used to postulate a triaxial or oblate halo.[3][4] More recent studies have found the halo to be flattened with a broken power law radius dependence; evidence for triaxiality is unclear.[5]

As a result of their faint brightness, observations of stellar halos in distant galaxies have required very long exposure times, the stacking of data from numerous galaxies to obtain averaged properties, or observing only the resolved stellar populations. Individual resolved stars in stellar halos can only be measured in the Milky Way and Andromeda.[6] The furthest stellar halos detected are at a redshift distance of 1.[7]

Structure/properties

In the Lambda-CDM model of the universe, galaxies grow by mergers. Such mergers are the cause of substructure observed in the stellar halo of galaxies; streams of stars from disrupted satellite galaxies are detectable through their coherence in space or velocity; a number of these streams are observable around the Milky Way.[8][9] As a result of the buildup from an assortment of satellite galaxies, variations in properties such as metallicity are present across stellar populations in halos.[10]

Astrophysical simulations of galaxies have predicted that stellar halos should have two components; one inner region dominated by stars which formed within the galaxy, and an outer region primarily composed of stars accreted through merger events. Predictions for these components include different structure and rotation directions.[11] Observational evidence for this dual halo in the Milky Way has been claimed but contested.[12][13]

Milky Way

Studies of the Milky Way galaxy have found that approximately 0.1-1% of its total stellar mass is contained within the stellar halo, and that it extends to over 100 kiloparsecs from the galactic centre.[14]

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See also

References
  1. Kinman, T. D.; Wirtanen, C. A.; Janes, K. A. (July 1966). "An RR Lyrae Star Survey with the Lick 20-INCH Astrograph IV. a Survey of Three Fields Near the North Galactic Pole". The Astrophysical Journal Supplement Series. 13: 379. Bibcode:1966ApJS...13..379K. doi:10.1086/190140.
  2. Hartwick, F. D. A. (1987). "The structure of the Galactic halo". Proceedings of the NATO Advanced Study Institute. 207: 281–290. Bibcode:1987ASIC..207..281H.
  3. Newberg, Heidi Jo; Yanny, Brian (1 October 2006). "The Milky Way's stellar halo - lumpy or triaxial?". Journal of Physics: Conference Series. 47: 195–204. arXiv:astro-ph/0507671. doi:10.1088/1742-6596/47/1/024.
  4. Jurić, Mario; Ivezić, Željko; Brooks, Alyson (February 2008). "The Milky Way Tomography with SDSS. I. Stellar Number Density Distribution". The Astrophysical Journal. 673 (2): 864–914. arXiv:astro-ph/0510520. Bibcode:2008ApJ...673..864J. doi:10.1086/523619.
  5. Deason, A. J.; Belokurov, V.; Evans, N. W. (1 October 2011). "The Milky Way stellar halo out to 40 kpc: squashed, broken but smooth". Monthly Notices of the Royal Astronomical Society. 416 (4): 2903–2915. arXiv:1104.3220. Bibcode:2011MNRAS.416.2903D. doi:10.1111/j.1365-2966.2011.19237.x. Retrieved 27 January 2016.
  6. Monachesi, Antonela; Bell, Eric F.; Radburn-Smith, David J.; Bailin, Jeremy (4 February 2016). "The GHOSTS survey – II. The diversity of halo colour and metallicity profiles of massive disc galaxies". Monthly Notices of the Royal Astronomical Society. 457 (2): 1419–1446. arXiv:1507.06657. Bibcode:2016MNRAS.457.1419M. doi:10.1093/mnras/stv2987. Retrieved 18 February 2016.
  7. Trujillo, Ignacio; Bakos, Judit (2013-05-11). "Stellar haloes of disc galaxies at z ∼ 1". Monthly Notices of the Royal Astronomical Society. 431 (2): 1121–1135. arXiv:1207.7023. Bibcode:2013MNRAS.431.1121T. doi:10.1093/mnras/stt232. ISSN 0035-8711.
  8. Helmi, Amina (22 April 2008). "The stellar halo of the Galaxy". The Astronomy and Astrophysics Review. 15 (3): 145–188. arXiv:0804.0019. Bibcode:2008A&ARv..15..145H. doi:10.1007/s00159-008-0009-6.
  9. Ibata, Rodrigo; Gibson, Brad (April 2007). "The Ghosts of Galaxies Past". Scientific American. 296 (4): 40–45. Bibcode:2007SciAm.296d..40I. doi:10.1038/scientificamerican0407-40. PMID 17479629.
  10. Gilbert, Karoline M.; Kalirai, Jason S.; Guhathakurta, Puragra; Beaton, Rachael L. (10 November 2014). "GLOBAL PROPERTIES OF M31'S STELLAR HALO FROM THE SPLASH SURVEY. II. METALLICITY PROFILE". The Astrophysical Journal. 796 (2): 76. arXiv:1409.3843. Bibcode:2014ApJ...796...76G. doi:10.1088/0004-637X/796/2/76.
  11. McCarthy, I. G.; Font, A. S.; Crain, R. A.; Deason, A. J. (1 March 2012). "Global structure and kinematics of stellar haloes in cosmological hydrodynamic simulations". Monthly Notices of the Royal Astronomical Society. 420 (3): 2245–2262. arXiv:1111.1747. Bibcode:2012MNRAS.420.2245M. doi:10.1111/j.1365-2966.2011.20189.x. Retrieved 18 February 2016.
  12. Beers, Timothy C.; Carollo, Daniela; Ivezić, Željko; An, Deokkeun (10 February 2012). "The Case for the Dual Halo of the Milky Way". The Astrophysical Journal. 746 (1): 34. arXiv:1104.2513. Bibcode:2012ApJ...746...34B. doi:10.1088/0004-637X/746/1/34.
  13. Schönrich, Ralph; Asplund, Martin; Casagrande, Luca (1 May 2014). "Does SEGUE/SDSS indicate a dual Galactic halo?". The Astrophysical Journal. 786 (1): 7. arXiv:1403.0937. Bibcode:2014ApJ...786....7S. doi:10.1088/0004-637X/786/1/7.
  14. Cooper, A. P.; Cole, S.; Frenk, C. S. (1 August 2010). "Galactic stellar haloes in the CDM model". Monthly Notices of the Royal Astronomical Society. 406 (2): 744–766. arXiv:0910.3211. Bibcode:2010MNRAS.406..744C. doi:10.1111/j.1365-2966.2010.16740.x. Retrieved 27 January 2016.
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