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Gas and Galaxy Formation Publ. Astron. Soc. Aust., 1999, 16, 3–7. Gas and Galaxy Formation P. E. J. Nulsen Department of Engineering Physics, University of Wollongong, Wollongong, NSW 2522, Australia [email protected] Received 1998 November 2, accepted 1999 February 1 Abstract: The theory of galaxy formation is reviewed briey. From the evidence of clusters today, the primordial gas fraction was 20% or more. Thus, while the Universe is dominated by dark matter, gas plays an appreciable role in galaxy formation. Collapses of dwarf protogalaxies produce predominantly cold gas. It is argued that, in such cold collapses, the collapsed gas is largely self-gravitating. As a result, gas processes play a critical role in determining the visible structure of galaxies. Keywords: galaxies: formation 1 Introduction their determination of the primordial deuterium We start with a brief and highly selective review abundance, Burles & Tytler (1998) found that the of the theory of galaxy formation, with a focus on contribution of baryons to the density parameter is the importance of gas processes. It is then argued ' 2 that most of the gas in dwarf protogalaxies will be b 0 019h , (1) self-gravitating, so that gas processes play a major role in determining the visible structure of dwarf where h is the Hubble constant in units of 100 galaxies. km s1 Mpc1. Other determinations generally give Section 2 discusses the arguments for a dark a higher deuterium abundance, hence smaller values matter dominated hierarchical collapse. While the for b (e.g. Webb et al. 1997). The density dominant form of matter is dark, it is argued that parameter for all matter, 0, is still uncertain, but the primordial gas fraction must be at least 20% values about 03 are consistent with a range of to account for the high gas fractions in clusters. recent cosmological data (e.g. Carlberg et al. 1997; Section 3 outlines why fully numerical models for Merchan et al. 1998; Cavaliere, Menci & Tozzi galaxy formation may not be the most reliable. It 1998; Donahue et al. 1998; Lineweaver 1998; but is argued in Section 4 that the early collapse of, at see Gross et al. 1998; Blanchard & Bartlett 1998). least, the larger protodwarf galaxies leads to major Comparing this to b, we see that the bulk of the starbursts and outows. Finally, in Section 5, it is dark matter must be non-baryonic. shown that the disks of most galaxies are largely The argument for a hierarchical collapse is well self-gravitating. In particular, this means that we supported by both theory and observations. It would should not expect gravitating matter in the region require a highly contrived distribution of density of the disk to be well-tted by the universal dark uctuations to avoid a collapse hierarchy by forming matter prole of Navarro, Frenk & White (1997 — all current structures in single collapses. The much NFW), which models collisionless galaxy halos. simpler alternative is a hierarchical collapse. There is also considerable evidence for continuing hierarchical 2 Standard View of Galaxy Formation collapse, such as mergers between galaxies (Schweizer It is widely accepted that galaxies were formed in 1986) and between clusters of galaxies (Forman & a hierarchical collapse, dominated by non-baryonic Jones 1982). dark matter. We begin by considering the arguments While dark matter dominates, it is baryons that that support this view. form all visible structure in the Universe. Clusters The evidence that the Universe is dominated by of galaxies are the largest virialised objects, so the dark matter has been well documented (e.g. Trimble baryon fractions in clusters are most likely to be 1987; Carr 1994). It can be summarised by the representative of the Universe as a whole. Any warm statement that, on large scales, almost all virialised dark matter (e.g. Gross et al. 1998) contributes objects have high mass-to-light ratios. Furthermore, more to the mass of the larger objects, so that in order to account for primordial nucleosythesis, the the baryon fraction may decrease with mass. If baryonic contribution to the closure density must so, the baryon fraction in clusters would be lower be relatively small (e.g. Walker et al. 1991). Using than average, but there are no obvious plausible qAstronomical Society of Australia 1999 1323-3580/99/010003$05.00 4 P. E. J. Nulsen mechanisms to make the baryon fraction in clusters gas than they should. This eect is greatest when higher than average. the cooling time of the shocked gas is comparable Most determinations of the gas fraction in clusters to the dynamical time, that is, in the collapse of (e.g. White, Jones & Forman 1997) apply to a region ‘normal’ protogalaxies (Rees & Ostriker 1977). within about 1 Mpc of the cluster centre, but it has A popular alternative to the full numerical models been noted on many occasions that the gas fraction is are the semi-analytical models (White & Frenk 1991; a rising function of radius. In a recent determination, Kaumann, White & Guiderdoni 1993; Baugh, Cole Ettori, Fabian & White (1998) found that the gas & Frenk 1996; Nulsen & Fabian 1997; Somerville fraction within 085r200 of the centre of the Perseus & Primack 1998). In these models the collapse 1 1 cluster is about 30% (for H0 =50kms Mpc ; hierarchy is usually simulated using some form 3/2 cluster gas fraction scales as h ). Here, r200 is the of Press–Schechter theory and the outcomes of radius within which the mean density of the cluster individual collapses are determined from heuristic is 200 times the background density. Although it arguments. Because they require relatively little possible to contrive to make the numbers less certain computation, the semi-analytical models make it e.g. Gunn & Thomas (1996), the mass of the X-ray easy to test large ranges of parameters (e.g. to test emitting gas in clusters is quite well determined, models of star formation). In principle, a galaxy so that the main source of uncertainty in the gas formation model should account for everything we fraction is the total mass of the cluster. Although see, but in practice, the models try to account for there has been some dispute about the accuracy the overall properties of galaxies. Semi-analytical of X-ray mass determinations (principally on small models have had some success, but in many respects scales), they give results that agree with other their treatment of collapses is based weakly on methods on large scales (e.g. Evrard 1997). In order physical models. Furthermore, dierent collapse for such high gas fractions to be consistent with the models can account for many of the same data. It baryon density limit from primordial nucleosynthesis is likely to be some time before we can say that (1), the density parameter 0 must be low (White we have a denitive model for the details of galaxy et al. 1993). formation. 3 Numerical Simulations of Galaxy Formation 4 Dwarf (Cold) Galaxy Formation On the face of it, the combined numerical N -body Since dwarf galaxies are an important topic, the and hydrodynamic simulations of galaxy formation rest of my remarks are addressed to their formation. implement physical collapse models most accurately Although it needs modication to allow for dark (e.g. Thomas et al. 1998) and so ought to give matter, the argument of Rees & Ostriker (1977) is the most reliable results. Unfortunately, they are still essentially valid. It shows that shock heating hampered by a number of problems. First, there during the collapse of small galaxies is transient is our ignorance of what controls star formation. at best. Thus, the collapse of a small protogalaxy Since the stars endow galaxies with many of their results in cold gas within a dark matter halo. I will observed properties, the process of star formation use the term ‘dwarf’ loosely to refer to any system is critical to galaxy formation. Yet, we do not resulting from such a cold collapse. know what governs the rate of star formation or the The collapse of the dark matter associated with initial mass function. This makes the handling of the dwarf is dissipation-less and leads to a halo star formation equally uncertain in all models, so of the form proposed by Navarro, Frenk & White that numerical accuracy is not the advantage that (1997). This may be modied by gas processes, as it might be. discussed below. Its only signicant function for the The other problems of the full numerical simu- current purpose is that it provides a potential that lations are largely artifacts of limits on computing connes the cold gas, squeezing it to high density. resources which limit the resolution of the simula- Consider the early collapse of a halo with a total 10 tions (e.g. Weinberg, Hernquist & Katz 1997). Ways mass of 10 M10 M. Using standard arguments have been found around most such problems, but from semi-analytical models for galaxy formation they raise the issue that some problems have yet (e.g. Nulsen, Barcons & Fabian 1998), such a halo to be identied. An example of a problem that is would have a velocity dispersion of about not widely known is the excessive radiative cooling 1 1 ' 3 3 1 in shocks in numerical simulations (Maguire 1996). 46M10t9 km s , (2) This arises because the numerically simulated shocks 9 are orders of magnitude thicker than the real ones, if it collapsed at 10 t9 yr. If the gas fraction in the keeping gas at intermediate temperatures, where collapse is 02 (Section 2), then the protogalactic the cooling rate is high, for much longer than in halo will contain about 2 109 M of cold gas.
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