1 We include the SMC, LMC and M33 in this category.
The effects of photoionization on galaxy formation – II. Satellite galaxies in the Local Group
Version of Record online: 6 JUN 2002
Monthly Notices of the Royal Astronomical Society
Volume 333, Issue 1, pages 177–190, June 2002
How to Cite
Benson, A. J., Frenk, C. S., Lacey, C. G., Baugh, C. M. and Cole, S. (2002), The effects of photoionization on galaxy formation – II. Satellite galaxies in the Local Group. Monthly Notices of the Royal Astronomical Society, 333: 177–190. doi: 10.1046/j.1365-8711.2002.05388.x
3 We are able to predict the properties of these very faint galaxies by extrapolating our standard rules for star formation, feedback, etc. to these very small objects. However, it should be kept in mind that these very faint galaxies typically contain only a few thousand solar masses of stars. It is not clear how well our simplified rules for star formation describe reality in such systems, where the entire galaxy is less massive than a single giant molecular cloud.
4 The number density of satellites within the virial radius of a Milky Way-type halo in our model scales roughly as r−2 for r≳10 kpc, with a rapid cut-off towards Rvir, but flattens significantly at smaller radii, since these inner satellites are strongly affected by tidal forces and dynamical friction.
5 For reference, half of the Local Group satellites listed by Mateo (1998), as well as the LMC, SMC and M33, are irregulars and half are spheroidals; for galaxies brighter than MV=-8, the model predicts 78 per cent disc-dominated galaxies and 22 per cent spheroidals. It is likely that the morphological evolution of satellites has been infuenced by dynamical processes not included in our model (see e.g., Mayer et al. 2001).
6 The factor of follows from the assumption of isotropic stellar orbits in an singular isothermal potential, for a stellar distribution with an r−3 density profile, assumptions which may not be relevant to real galaxies. Alternatively, the average line-of-sight velocity dispersion of the satellite's spheroid can be estimated by modelling the spheroid as a King profile (which is often a good fit to observed data) and then solving the Jeans equation (see Paper I, equation 21). Assuming isotropic orbits, we find Vbulge≈1.2σ★ for model satellites, albeit with larger scatter, where Vbulge is the circular velocity of the bulge at the stellar half-mass radius, and σ★ is the stellar line-of-sight velocity dispersion. Adopting this latter value does not significantly change our conclusions for the velocity function.
7 In our calculations, galaxies are modelled with surface density profiles which initially extend to the virial radius of the halo in which they formed. We can therefore define a tidal radius for satellites even if this turns out to be much larger than the visible extent of the galaxy. In practice, when this occurs, the tidal radius plotted should be considered to be tidal radius of the satellite's halo rather than that of the satellite itself.
- Issue online: 6 JUN 2002
- Version of Record online: 6 JUN 2002
- Accepted 2002 February 2. Received 2002 January 30; in original form 2001 August 13
- galaxies: formation;
- intergalactic medium;
- Local Group;
- cosmology: theory
We use a self-consistent model of galaxy formation and the evolution of the intergalactic medium to study the effects of the reionization of the Universe at high redshift on the properties of satellite galaxies like those seen around the Milky Way. Photoionization suppresses the formation of small galaxies, so that surviving satellites are preferentially those that formed before the Universe reionized. As a result, the number of satellites expected today is about an order of magnitude smaller than the number inferred by identifying satellites with subhaloes of the same circular velocity in high-resolution simulations of the dark matter. The resulting satellite population has an abundance similar to that observed in the Local Group, although the distribution of circular velocities differs somewhat from the available data. We explore many other properties of satellite galaxies, including their gas content, metallicity and star formation rate, and find generally good agreement with available data. Our model predicts the existence of many as yet undetected satellites in the Local Group. We quantify their observability in terms of their apparent magnitude and surface brightness, and also in terms of their constituent stars. A near-complete census of the Milky Way's satellites would require imaging to V≈20 and to a surface brightness fainter than 26 V-band magnitudes per square arcsecond. Satellites with integrated luminosity V=15 should contain of order 100 stars brighter than B=26, with central stellar densities of a few tens per square arcminute. Discovery of a large population of faint satellites would provide a strong test of current models of galaxy formation.