Astron. Astrophys. 360, 76-84 (2000)

1. Introduction

Galaxies of any morphological type and luminosity are known to be surrounded by DM halos, whose properties are remarkably universal (Salucci & Persic 1997 and references therein). The presence of DM halos has been detected through a variety of observational methods (Danziger 1997), from rotation curves in spirals (Giraud 2000, Swaters 1999, Persic et al. 1996) to ratios in ellipticals (Bertola et al. 1993, Loewenstein & White III 1999 and references therein). To summarize, the properties of DM halos can be described as follows (Salucci & Persic 1997):

• dark and visible matter are well mixed already inside the luminous region of the galaxy;

• the transition radius between the inner, baryon dominated region, and the outer, DM dominated region, moves inward progressively with decreasing luminosity;

• a halo core radius, comparable with the optical radius, is detected at all luminosities and for all morphologies;

• the luminous mass fraction varies with luminosity in a fashion common to all galaxy types: it is comparable with the cosmological baryon fraction at , but it decreases by about a factor of 100 at ;

• finally, for any Hubble type, the central halo density increases with decreasing luminosity.

Attempts to model the properties of DM halos in a cosmological context with N-body simulations trace back to Dubinski & Carlberg (1991) in the frame of cold dark matter (CDM) theory. The halos were found to be strongly triaxial and to exhibit a power law density profile varying from -1 in the center to -4 in the outskirts (Hernquist 1990 profile). Then, Navarro et al. (1996) (hereafter NFW), found that, independently from the adopted initial perturbation spectrum, the cosmological model and the halo mass, all DM halos possess the same universal density profile, fitted by the formula

where , is the critical density for closure, is a dimensionless characteristic density, and is a scale radius, which defines where the profile shape has a slope of -2. ¹

This profile has a spike in the center of the halo, and differs in its asymptotic behavior from the Hernquist profile, decreasing as far from the halo center. Other simulations, with higher resolution and/or different initial conditions, confirmed the basic features of these findings, but disagreed with respect to some important aspects (Cole & Lacey 1996, Moore et al. 1997, van der Bosh, 1999). In fact, there are claims that the universality of the functional form (1) arises as a direct consequence of the hierarchical merging history of CDM halos (Syer & White (1997), or as a more generic feature of gravitational collapse (Huss et al. 1999). However, recently, the steepness of the central cusp has been found to vary significantly among different realizations, i.e. among halos (Jing & Suto 2000). It now seems likely that, CDM halos follow Eq. (1) with but with large variations of with mass and also at a given halo mass.

On the other hand, a large discrepancy exists between CDM halo predictions and DM observations (Salucci & Persic 1997). Halos around galaxies show a density distribution which is inconsistent with Eq. (1). In particular, they have a density central core larger than the stellar scale-length and their density is:

with , being the effective radius.

The disagreement between theory and observations on the mass distribution, and the existence of global scaling laws that couple the dark and the luminous matter (Persic et al. 1996) prompt the investigation of the past dynamical history of galaxies.

N-body/hydrodynamical simulations are an effective tool to obtain crucial information on the late stages of galaxy formation which is in some sense orthogonal to that we obtain with semi-analitycal methods or that we infer from observations. In fact, such simulations can account for the "physical" interaction between gas and dark matter. Moreover, many relevant physical processes occurring in the baryonic components, like thermal shocks, pressure forces and dissipation are explicitly taken into account.

The layout of the paper is as follow. In Sect. 2 we briefly describe the numerical tool, in Sect. 3 we discuss the initial conditions. In Sects. 4 and 5 we show the evolution of a giant and a dwarf elliptical, respectively. Finally, Sect. 6 summarizes the results.

© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000
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