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When should we treat galaxies as isolated?

Hopkins, Philip F. and Kereš, Dušan and Ma, Chung-Pei and Quataert, Eliot (2010) When should we treat galaxies as isolated? Monthly Notices of the Royal Astronomical Society, 401 (2). pp. 1131-1140. ISSN 0035-8711.

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Traditionally, secular evolution is defined as evolution of systems where the internal growth of structure and instabilities dominates the growth via external drivers (e.g. accretion and mergers). Most study has focused on ‘isolated’ galaxies, where seed asymmetries may represent realistic cosmological substructure, but subsequent evolution ignores galaxy growth and interactions. Large-scale modes in the disc then grow on a time-scale of the order of a disc rotation period (∼0.1–1 Gyr). If, however, galaxies evolve cosmologically on a shorter time-scale, then it may not be appropriate to consider them ‘isolated’. We outline simple scalings to ask whether, under realistic conditions, the time-scale for secular evolution is shorter than the time-scale for cosmological accretion and mergers. We show that this is the case in a relatively narrow but important range of perturbation amplitudes corresponding to substructure or mode/bar fractional amplitudes δ ∼ 0.01–0.1, the range of most interest for observed strong bars and most pseudo-bulges. At smaller amplitudes δ ≪ 0.1, systems are not isolated: typical discs will grow by accretion at a comparable level over even a single dynamical time. At larger amplitudes δ≫ 0.1, the evolution is no longer secular; the direct gravitational evolution of the seed substructure swamps the internal disc response. We derive criteria for when discs can be well approximated as ‘isolated’ as a function of mass, redshift and disc stability. The relevant parameter space shrinks at higher mass, higher disc stability and higher z as accretion rates increase. The cosmological rate of galaxy evolution also defines a maximum bar/mode lifetime of practical interest, of ∼0.1 t_(Hubble)(z). Longer lived modes will encounter cosmological effects and will decouple from their drivers (if they are driven).

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Hopkins, Philip F.0000-0003-3729-1684
Kereš, Dušan0000-0002-1666-7067
Quataert, Eliot0000-0001-9185-5044
Additional Information:© 2009 The Authors. Journal compilation © 2009 RAS. Accepted 2009 September 11. Received 2009 September 10; in original form 2009 February 12. We thank Lars Hernquist and T. J. Cox for helpful discussions, as well as Simon White and the anonymous referee for suggestions that greatly improved this manuscript. Support for PFH was provided by the Miller Institute for Basic Research in Science, University of California, Berkeley. DK acknowledges the support of the ITC fellowship at the Harvard College Observatory.
Funding AgencyGrant Number
Miller Institute for Basic Research in ScienceUNSPECIFIED
Harvard College ObservatoryUNSPECIFIED
Subject Keywords:galaxies: active, galaxies: evolution, galaxies: formation, galaxies: spiral, cosmology: theory
Issue or Number:2
Record Number:CaltechAUTHORS:20200521-163330227
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Official Citation:Philip F. Hopkins, Dušan Kereš, Chung-Pei Ma, Eliot Quataert, When should we treat galaxies as isolated?, Monthly Notices of the Royal Astronomical Society, Volume 401, Issue 2, January 2010, Pages 1131–1140,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:103395
Deposited By: George Porter
Deposited On:22 May 2020 18:07
Last Modified:22 May 2020 18:07

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