Detection of massive forming galaxies at redshifts z>l

Abstract

THE complex problem of when and how galaxies formed has only recently become susceptible to direct attack. It has been known for some time that the excess of blue galaxies counted at faint magnitudes^1á¤-5 implies that a considerable fraction of the massive-star formation in the Universe occurred at redshift z < 3 (refs 6, 7); but surprisingly, spectroscopic studies^8á¤-11 of galaxies down to a B (blue) magnitude of 24 have found little sign of the expected high-z progenitors of current massive galaxies—finding instead mostly small blue galaxies at modest redshifts (z ≈ 0.3). This unexpected population has diverted attention from the possibility that early massive galaxies in the process of star formation might also be found in the faint blue excess. From spectroscopic observations deep enough to encompass a large population of field galaxies (that is, those not associated with any cluster) at z > 1, we show here that in fact these forming galaxies are present in substantial numbers at B≈24, and that the era from redshifts 1 to 2 was clearly an important period of galaxy formation.