There have been very few studies focusing on bulges at high redshift, and only one addressing the comparison between bulges in field and cluster environments.
Ellis et al. (2001) analyzed a sample of early-type and spiral galaxies from the northern and southern Hubble Deep Fields. They compared the central (inner 5%) colors of spirals with clearly visible bulges with the integrated colors of ellipticals in their sample up to a redshift of ~1. They found that both ellipticals and bulges exhibit a dispersion in their colors at a given redshift, but that their distributions are different: a smaller fraction of ellipticals is blue. It seems that there is an almost total absence of bulges as red as those predicted by the model of a passive evolution, while this scenario provides on the contrary a good description for the majority of the early-type population. The authors concluded that the optical-luminosity-weighted ages of bulges, to a redshift of at least 0.6, are less than those of the reddest ellipticals. At even higher redshifts some bulges are found to be as red as ellipticals though, suggesting that some kind of rejuvenation is at play at intermediate redshift.
Koo et al. (2005a) presented a sample of ellipticals and bulges from the DEEP Groth Strip Survey with redshift between ~0.7 and ~1. This time, the images are decomposed into bulge and disk components by fitting a de Vaucouleurs light profile for the former and an exponential one for the latter. They find that red bulges (85% of them) are nearly as red as, or redder than, the integrated color of either local early-type or distant cluster galaxies. The color-magnitude relations have similarly shallow slope and small scatter. Blue bulges are among the least-luminous ones, and are of similarly low surface brightness to local bulges of similar size. The authors consider that they cannot be genuine proto-bulges and are instead mostly residing in morphologically peculiar galaxies. Interestingly, in most red objects, they detect emission lines indicative of continuous star formation, albeit at a low level.
As stated by Koo et al. themselves, the fact that Ellis et al.'s sample encompasses faint bulges, whereas their sample is restricted to luminous ones, together with the very different ways of deriving the bulge colors, might be at the origin of the contradictory results of the two works.
Koo et al. (2005b) presented an analysis of luminous bulges (MB < 19.5) in a cluster and in the field at redshift ~0.8. They demonstrated that the rest-frame colors, slope, and dispersion of the color-magnitude relation of cluster and field bulges are nearly the same. This also means that they are no larger than in samples at lower redshift. This is in sharp contrast with some theoretical expectations of an increasing fraction of recent star formation with redshift and/or a longer timescale of formation in the field than in clusters. However, here again the consequence of the selection of luminous bulges must be investigated.
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