8.5 The Rôle of Dwarfs in Hierarchical Structure Formation
What rôle do metal-poor dwarf galaxies play in the hierarchical build-up of galaxies and structure in the early Universe. Observations suggest high merger frequencies at high redshift, but what is merging? Certainly not just normal spirals but more likely dwarfs that may or may not have counterparts in the local Universe.
If galaxies form by the cooling and condensation of gas within dark matter halos, the ability to detect them depends on the sizes of the pieces out of which galaxies are assembled and on the time interval during which this takes place. There have been suggestions that the principal galaxy building blocks are to be found throughout the redshift range of about z = 2 to 5. In any case, such young galaxies would, in their earliest phase, be expected to have strong emission lines but faint continua and would hence fall far below the magnitude threshold of current Lyman break surveys. In fact there is a trend for local galaxies (see for instance the Terlevich et al. 1991, catalogue of H II galaxies, and Fig. 12) with very strong emission lines to have fainter magnitudes. Moreover, if galaxies are more inclined to be in groups and clusters at the time of formation they offer an interesting challenge for existing CDM models. HST images obtained by Pascarelle et al. (1998) have revealed emitting objects in sheets or clustered in ways that fit the theoretical picture of Rauch et al. (1997). Observations suggest high merger frequencies at high redshift, but again the nature of the merging blocks (masses and metallicity) remains to be investigated.
Have today's giant galaxies (big spirals and ellipticals) ever been metal-poor? The intuitive answer would of course be - yes. But, if massive galaxies are built up by successive merger of small galaxies, it is possible that they were already rather metal rich when reaching a total mass comparable to that of an L* galaxy. In that case, the only metal-poor galaxies at any redshift will be low mass galaxies, difficult to study at cosmological distances.
A long standing problem in galaxy formation models is the so called overcooling problem (White and Rees 1978), leading to an overproduction of dwarf galaxies at the expense of massive galaxies, in disagreement with observations of the luminosity function of galaxies. This problem still remains in current galaxy formation models (cf. Somerville and Primack 1998). Various mechanisms, have been proposed such as SN feedback and a high UV background keeping dwarf sized clouds ionised, preventing them to cool (Efstathiou 1992). The problem may be somewhat relaxed if most dwarfs have merged into giants or have been disrupted. If some low mass galaxies formed before the Universe was re-ionised and subsequently experience a blow-out of their ISM, they could represent population III objects, responsible for cosmic metal enrichment at very early epochs (see Ferrara and Tolstoy 1999 and references therein).