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Modelling galactic conformity with the colour–halo age relation in the Illustris simulation

Bray, Aaron D. and Pillepich, Annalisa and Sales, Laura V. and Zhu, Emily and Genel, Shy and Rodriguez-Gomez, Vicente and Torrey, Paul and Nelson, Dylan and Vogelsberger, Mark and Springel, Volker and Eisenstein, Daniel J. and Hernquist, Lars (2016) Modelling galactic conformity with the colour–halo age relation in the Illustris simulation. Monthly Notices of the Royal Astronomical Society, 455 (1). pp. 185-198. ISSN 0035-8711.

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Comparisons between observational surveys and galaxy formation models find that dark matter haloes’ mass can largely explain their galaxies’ stellar mass. However, it remains uncertain whether additional environmental variables, known as assembly bias, are necessary to explain other galaxy properties. We use the Illustris simulation to investigate the role of assembly bias in producing galactic conformity by considering 18 000 galaxies with M_(stellar) > 2 × 10^9 M_⊙. We find a significant signal of galactic conformity: out to distances of about 10 Mpc, the mean red fraction of galaxies around redder galaxies is higher than around bluer galaxies at fixed stellar mass. Dark matter haloes exhibit an analogous conformity signal, in which the fraction of haloes formed at earlier times (old haloes) is higher around old haloes than around younger ones at fixed halo mass. A plausible interpretation of galactic conformity is the combination of the halo conformity signal with the galaxy colour–halo age relation: at fixed stellar mass, particularly towards the low-mass end, Illustris’ galaxy colours correlate with halo age, with the reddest galaxies (often satellites) preferentially found in the oldest haloes. We explain the galactic conformity effect with a simple semi-empirical model, assigning stellar mass via halo mass (abundance matching) and galaxy colour via halo age (age matching). Regarding comparison to observations, we conclude that the adopted selection/isolation criteria, projection effects, and stacking techniques can have a significant impact on the measured amplitude of the conformity signal.

Item Type:Article
Related URLs:
URLURL TypeDescription
Pillepich, Annalisa0000-0003-1065-9274
Genel, Shy0000-0002-3185-1540
Rodriguez-Gomez, Vicente0000-0002-9495-0079
Torrey, Paul0000-0002-5653-0786
Vogelsberger, Mark0000-0001-8593-7692
Springel, Volker0000-0001-5976-4599
Hernquist, Lars0000-0001-6950-1629
Additional Information:© 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Accepted 2015 October 2. Received 2015 October 1; in original form 2015 August 24. First published online November 2, 2015. ADB, AP, and SG thank Andrew Hearin for helpful discussions and comments on the manuscript and thank Frank van den Bosch, Jeremy Tinker, Guinevere Kauffmann, Alexie Leauthaud, and Cameron McBride for useful discussions. ADB thanks the OSU CCAPP for hosting its 2014 HOD workshop. AP acknowledges support from the HST grant HST-AR-13897. SG acknowledges support provided by NASA through Hubble Fellowship grant HST-HF2-51341.001-A awarded by the STScI, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. PT acknowledges support from NASA ATP Grant NNX14AH35G. VS acknowledges support through the European Research Council under ERC-StG grant EXAGAL-308037. LH acknowledges support from NASA grant NNX12AC67G and NSF grant AST-1312095. The Illustris-1 simulation was run on the CURIE supercomputer at CEA/France as part of PRACE project RA0844, and the SuperMUC computer at the Leibniz Computing Centre, Germany, as part of GCS-project pr85je. The further simulations were run on the Harvard Odyssey and CfA/ITC clusters, the Ranger and Stampede supercomputers at the Texas Advanced Computing Center through XSEDE, and the Kraken supercomputer at Oak Ridge National Laboratory through XSEDE. The analysis presented in this paper was conducted on the Harvard Odyssey and CfA/ITC clusters.
Funding AgencyGrant Number
NASA Hubble FellowshipHST-HF2-51341.001-A
European Research Council (ERC)EXAGAL-308037
Subject Keywords:galaxies: formation – galaxies: haloes – cosmology: theory – dark matter
Issue or Number:1
Record Number:CaltechAUTHORS:20160204-165111604
Persistent URL:
Official Citation:Aaron D. Bray, Annalisa Pillepich, Laura V. Sales, Emily Zhu, Shy Genel, Vicente Rodriguez-Gomez, Paul Torrey, Dylan Nelson, Mark Vogelsberger, Volker Springel, Daniel J. Eisenstein, and Lars Hernquist Modelling galactic conformity with the colour–halo age relation in the Illustris simulation MNRAS (January 01, 2016) Vol. 455 185-198 doi:10.1093/mnras/stv2316 First published online November 2, 2015
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:64252
Deposited By: George Porter
Deposited On:08 Feb 2016 19:38
Last Modified:09 Mar 2020 13:19

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