Published online by Cambridge University Press: 10 November 2010
Observations of faint blue extended objects with the Hubble Space Telescope by Dressier et al. (1993) are discussed. It is argued that the objects may be regions of violent star formation in unseen galaxies at z = 0.4.
On a 6-hr Wide Field Camera exposure with the Hubble Space Telescope of the rich cluster CL 0939+4713, Dressier et al. (1993) have found an apparent group of 15–30 faint extended objects with magnitudes 22 < r < 25. The objects are typically 1″ in angular size with bright central regions only a few tenths of an arcsecond in size, and are distributed over a region 40″ × 20″. The size and appearance of individual objects, their blue colors (known from broad-band ground-based imaging), and their clustering suggest, as Dressier et al. (1993) speculate, that they could be associated with each other, and – most importantly – that they are considerably more distant than the cluster CL 0939+4713 at z = 0.40. Furthermore, they assume that these objects could be physically associated with an unresolved, extremely blue object with the spectrum of a QSO at z = 2.055. If so, it means that at least a part of the objects may represent the early stage of formation of galaxies.
Should this association of the objects with QSO be confirmed, it would have lead to the conclusion that the luminous parts of these objects have physical diameters of about 1–3 kpc.