The Standard View of Galaxy Formation

It is widely accepted that galaxies were formed in a hierarchical collapse, dominated by non-baryonic dark matter. We begin by considering the arguments that support this view.

The evidence that the Universe is dominated by dark matter has been well documented (e.g. Trimble 1987; Carr 1994). It can be summarised by the statement that, on large scales, almost all virialised objects have high mass-to-light ratios. Furthermore, in order to account for primordial nucleosythesis, the baryonic contribution to the closure density must be relatively small (e.g. Walker et al. 1991). Using their determination of the primordial deuterium abundance, Burles & Tytler (1998) find that contribution of baryons to the density parameter is

$\begin{displaymath} \Omega_{\rm b} \simeq 0.019 h^{-2}, \end{displaymath}$

(1)

where h is the Hubble constant in units of 100 km ${\rm s}^{-1}$ $\rm Mpc^{-1}$ . Other determinations generally give a higher deuterium abundance, hence smaller values for

$\Omega_{\rm b}$ (e.g. Webb et al. 1997). The density parameter for all matter, $\Omega_0$ , is still uncertain, but values about 0.3 are consistent with a range of recent cosmological data (e.g. Carlberg et al. 1997; Merchan et al. 1998; Cavaliere, Menci & Tozzi 1998; Donahue et al.1998; Lineweaver 1998; but see Gross et al. 1997; Blanchard & Bartlett 1998). Comparing this to

$\Omega_{\rm b}$ , we see that the bulk of the dark matter must be non-baryonic.

The argument for a hierarchical collapse is well supported by both theory and observations. It would require a highly contrived distribution of density fluctuations to avoid a collapse hierarchy by forming all current structures in single collapses. The much simpler alternative is a hierarchical collapse. There is also considerable evidence for continuing hierarchical collapse, such as mergers between galaxies (Schweizer 1986) and between clusters of galaxies (Forman & Jones 1982).

While dark matter dominates, it is baryons that form all visible structure in the Universe. Clusters of galaxies are the largest virialised objects, so the baryon fractions in clusters are most likely to be representative of the Universe as a whole. Any warm dark matter (e.g. Gross et al. 1997) contributes more to the mass of the larger objects, so that the baryon fraction may decrease with mass. If so, the baryon fraction in clusters would be lower than average, but there are no obvious plausible mechanisms to make the baryon fraction in clusters higher than average.

Most determinations of the gas fraction in clusters (e.g. White, Jones & Forman 1997) apply to a region within about 1 Mpc of the cluster centre, but it has been noted on many occasions that the gas fraction is a rising function of radius. In a recent determination, Ettori, Fabian & White (1998) find that the gas fraction within 0.85 r₂₀₀ of the centre of the Perseus cluster is about 30 percent (for H₀ = 50 km ${\rm s}^{-1}$ $\rm Mpc^{-1}$ ; cluster gas fraction scales as h^-3/2). Here, r₂₀₀ is the radius within which the mean density of the cluster is 200 times the background density. Although it possible to contrive to make the numbers less certain (e.g.Gunn & Thomas (1996), the mass of the X-ray emitting gas in clusters is quite well determined, so that the main source of uncertainty in the gas fraction is the total mass of the cluster. Although there has been some dispute about the accuracy of X-ray mass determinations (principally on small scales), they give results that agree with other methods on large scales (e.g. Evrard 1997). In order for such high gas fractions to be consistent with the baryon density limit from primordial nucleosynthesis (1), the density parameter, $\Omega_0$ , must be low (White et al. 1993).

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Contents Page: Volume 16, Number 1

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The Standard View of Galaxy Formation