When should we treat galaxies as isolated?
Abstract
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).
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.Attached Files
Published - mnras0401-1131.pdf
Accepted Version - 0909.3306.pdf
Files
Name | Size | Download all |
---|---|---|
md5:a9e0d351393e7784d1bc352f3315b610
|
717.7 kB | Preview Download |
md5:873a0cc8849cd0e8808777c3d5a03691
|
339.6 kB | Preview Download |
Additional details
- Eprint ID
- 103395
- Resolver ID
- CaltechAUTHORS:20200521-163330227
- Miller Institute for Basic Research in Science
- Harvard College Observatory
- Created
-
2020-05-22Created from EPrint's datestamp field
- Updated
-
2021-11-16Created from EPrint's last_modified field