Merger model

  To circumvent the metal production problem and the absence of a high-redshift tail in redshift distributions, others [\protect\astronciteBroadhurst et al.1992][\protect\astronciteRoccaVolmerange and Guiderdoni1990] have suggested a high merger rate for the faint blue galaxies to reduce their z0.4 comoving density to the present-day value. A consequence of this strong merger model is that most metals would end up in L galaxies, where they are found today. However, this picture suffers from several problems.

Only roughly 15 by light of the galaxies at z =0.35 can merge into ellipticals, otherwise there would be too much blue light in local elliptical galaxies [\protect\astronciteDalcanton1993]. Only 415 by mass of the galaxies at z=0.35 can be removed by merging into spiral galaxies. More extensive merging could violate the constraints on the thinness of spiral disks[\protect\astronciteDalcanton1993][\protect\astronciteTòth and Ostriker1992]. This constraint is based on the assumption that the energy deposited in disks by mergers goes into random motion (``disk heating''). However, if spiral disks are more resilient and the energy goes into exciting coherent modes in disks as suggested by others [\protect\astronciteHuang and Carlberg1996], then present-day galaxies could have accreted satellites with up to 25-30 of their mass without detectable thickening of their disks.

Faint galaxies are surprisingly weakly clustered [\protect\astronciteEfstathiou et al.1991]. Their clustering amplitude, a factor of 2 lower than local normal galaxies, is similar to that of local starbursting and HII galaxies [\protect\astronciteInfante and Pritchet1995]. Merger rates required to remove the excess galaxies are uncomfortably large as a typical galaxy at z = 0.3 must merge with 15-30 of all its neighbors within 750 h kpc [\protect\astronciteBernstein et al.1994]. In other, milder merger scenarios [\protect\astronciteCarlberg and Charlot1992], galaxies undergo extensive merging between z = 1 and z = 0.5. At z = 1, the characteristic galaxy mass is 25 of the z = 0 value. Interactions at z< 0.5 mainly brighten and blue galaxies with little change in comoving density.