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Galaxy and Mass Assembly (GAMA): halo formation times and halo assembly bias on the cosmic web

Tojeiro, Rita; Eardley, Elizabeth; Peacock, John A.; Norberg, Peder; Alpaslan, Mehmet; Driver, Simon P.; Henriques, Bruno; Hopkins, Andrew M.; Kafle, Prajwal R.; Robotham, Aaron S.G.; Thomas, Peter; Tonini, Chiara; Wild, Vivienne

Galaxy and Mass Assembly (GAMA): halo formation times and halo assembly bias on the cosmic web Thumbnail


Rita Tojeiro

Elizabeth Eardley

John A. Peacock

Mehmet Alpaslan

Simon P. Driver

Bruno Henriques

Andrew M. Hopkins

Prajwal R. Kafle

Aaron S.G. Robotham

Peter Thomas

Chiara Tonini

Vivienne Wild


We present evidence for halo assembly bias as a function of geometric environment (GE). By classifying Galaxy and Mass Assembly (GAMA) galaxy groups as residing in voids, sheets, filaments or knots using a tidal tensor method, we find that low-mass haloes that reside in knots are older than haloes of the same mass that reside in voids. This result provides direct support to theories that link strong halo tidal interactions with halo assembly times. The trend with GE is reversed at large halo mass, with haloes in knots being younger than haloes of the same mass in voids. We find a clear signal of halo downsizing – more massive haloes host galaxies that assembled their stars earlier. This overall trend holds independently of GE. We support our analysis with an in-depth exploration of the L-Galaxies semi-analytic model, used here to correlate several galaxy properties with three different definitions of halo formation time. We find a complex relationship between halo formation time and galaxy properties, with significant scatter. We confirm that stellar mass to halo mass ratio, specific star formation rate (SFR) and mass-weighed age are reasonable proxies of halo formation time, especially at low halo masses. Instantaneous SFR is a poor indicator at all halo masses. Using the same semi-analytic model, we create mock spectral observations using complex star formation and chemical enrichment histories, which approximately mimic GAMA’s typical signal-to-noise ratio and wavelength range. We use these mocks to assert how well potential proxies of halo formation time may be recovered from GAMA-like spectroscopic data.


Tojeiro, R., Eardley, E., Peacock, J. A., Norberg, P., Alpaslan, M., Driver, S. P., …Wild, V. (2017). Galaxy and Mass Assembly (GAMA): halo formation times and halo assembly bias on the cosmic web. Monthly Notices of the Royal Astronomical Society, 470(3), 3720-3741.

Journal Article Type Article
Acceptance Date Jun 9, 2017
Online Publication Date Jun 14, 2017
Publication Date Sep 21, 2017
Deposit Date Sep 6, 2017
Publicly Available Date Sep 6, 2017
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 470
Issue 3
Pages 3720-3741
Related Public URLs


Published Journal Article (3.6 Mb)

Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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