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Stars made in outflows may populate the stellar halo of the Milky Way

Yu, Sijie and Bullock, James S. and Wetzel, Andrew and Sanderson, Robyn E. and Graus, Andrew S. and Boylan-Kolchin, Michael and Nierenberg, Anna M. and Grudić, Michael Y. and Hopkins, Philip F. and Kereš, Dušan and Faucher-Giguère, Claude-André (2020) Stars made in outflows may populate the stellar halo of the Milky Way. Monthly Notices of the Royal Astronomical Society, 494 (2). pp. 1539-1559. ISSN 0035-8711. doi:10.1093/mnras/staa522.

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We study stellar-halo formation using six Milky-Way-mass galaxies in FIRE-2 cosmological zoom simulations. We find that 5−40 per cent of the outer (50–300 kpc) stellar halo in each system consists of in-situ stars that were born in outflows from the main galaxy. Outflow stars originate from gas accelerated by superbubble winds, which can be compressed, cool, and form co-moving stars. The majority of these stars remain bound to the halo and fall back with orbital properties similar to the rest of the stellar halo at z = 0. In the outer halo, outflow stars are more spatially homogeneous, metal-rich, and alpha-element-enhanced than the accreted stellar halo. At the solar location, up to ∼10 per cent of our kinematically identified halo stars were born in outflows; the fraction rises to as high as ∼40 per cent for the most metal-rich local halo stars ([Fe/H] >−0.5). Such stars can be retrograde and create features similar to the recently discovered Milky Way ‘Splash’ in phase space. We conclude that the Milky Way stellar halo could contain local counterparts to stars that are observed to form in molecular outflows in distant galaxies. Searches for such a population may provide a new, near-field approach to constraining feedback and outflow physics. A stellar halo contribution from outflows is a phase-reversal of the classic halo formation scenario of Eggen, Lynden-Bell & Sandange, who suggested that halo stars formed in rapidly infalling gas clouds. Stellar outflows may be observable in direct imaging of external galaxies and could provide a source for metal-rich, extreme-velocity stars in the Milky Way.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Yu, Sijie0000-0002-1019-0341
Bullock, James S.0000-0003-4298-5082
Wetzel, Andrew0000-0003-0603-8942
Sanderson, Robyn E.0000-0003-3939-3297
Boylan-Kolchin, Michael0000-0002-9604-343X
Grudić, Michael Y.0000-0002-1655-5604
Hopkins, Philip F.0000-0003-3729-1684
Kereš, Dušan0000-0002-1666-7067
Faucher-Giguère, Claude-André0000-0002-4900-6628
Additional Information:© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2020 February 14. Received 2020 January 31; in original form 2019 December 10. Published: 03 March 2020. This project was developed in part at the 2019 Santa Barbara Gaia Sprint, hosted by the Kavli Institute for Theoretical Physics (KITP) at the University of California, Santa Barbara. This research was supported in part at KITP by the Heising-Simons Foundation and the National Science Foundation (NSF) under grant No. PHY-1748958. SY and JSB were supported by NSF AST-1910346, AST-1518291, HST-AR-14282, and HST-AR-13888. AW received support from NASA, through ATP grant 80NSSC18K1097 and HST grants GO-14734 and AR-15057 from STScI, the Heising-Simons Foundation, and a Hellman Fellowship. ASG is supported by the McDonald Observatory at the University of Texas at Austin, through the Harlan J. Smith fellowship. MBK acknowledges support from NSF grants AST-1517226, AST-1910346, and NSF CAREER grant AST-1752913 and from NASA grants NNX17AG29G and HST-AR-14282, HST-AR-14554, HST-AR-15006, HST-GO-14191, and HST-GO-15658 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. Support for PFH was provided by NSF Collaborative Research Grants 1715847 & 1911233, NSF CAREER grant 1455342, NASA grants 80NSSC18K0562, JPL 1589742. Numerical calculations were run on the Caltech compute cluster ‘Wheeler,’ allocations from XSEDE TG-AST130039 and PRAC NSF.1455342 supported by the NSF, and NASA HEC SMD-16-7592. DK was supported by NSF grant AST-1715101 and the Cottrell Scholar Award from the Research Corporation for Science Advancement. CAFG was supported by NSF through grants AST-1517491, AST-1715216, and NSF CAREER award AST-1652522; by NASA through grant 17-ATP17-0067; and by a Cottrell Scholar Award from the Research Corporation for Science Advancement. This work also made use of ASTROPY ,7 a community-developed core Python package for Astronomy (Astropy Collaboration 2013, 2018), MATPLOTLIB (Hunter 2007), NUMPY (van der Walt, Colbert & Varoquaux 2011), and the NASA Astrophysics Data System.
Group:Astronomy Department, TAPIR
Funding AgencyGrant Number
Heising-Simons FoundationUNSPECIFIED
NASA Hubble FellowshipGO-14734
NASA Hubble FellowshipAR-15057
Space Telescope Science InstituteUNSPECIFIED
Hellman FellowshipUNSPECIFIED
McDonald ObservatoryUNSPECIFIED
University of Texas at AustinUNSPECIFIED
Cottrell Scholar of Research CorporationUNSPECIFIED
Subject Keywords:methods: numerical – galaxies: evolution – galaxies: formation – galaxies: haloes – galaxies: structure
Issue or Number:2
Record Number:CaltechAUTHORS:20200310-095909852
Persistent URL:
Official Citation:Sijie Yu, James S Bullock, Andrew Wetzel, Robyn E Sanderson, Andrew S Graus, Michael Boylan-Kolchin, Anna M Nierenberg, Michael Y Grudić, Philip F Hopkins, Dušan Kereš, Claude-André Faucher-Giguère, Stars made in outflows may populate the stellar halo of the Milky Way, Monthly Notices of the Royal Astronomical Society, Volume 494, Issue 2, May 2020, Pages 1539–1559,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:101817
Deposited By: Tony Diaz
Deposited On:10 Mar 2020 17:49
Last Modified:16 Nov 2021 18:06

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