3.2. The small angular sizes of the faintest galaxies
On first seeing the HDF image (in a national newspaper) I was struck by the amount of blank sky it contained. Together with the flat count slope, an important secondary conclusion arising from this simple observation is the small angular sizes of the faintest sources.
This is a newer result mainly due to HST but one whose interpretation is perhaps less straightforward (see Ferguson, this volume). Most of the early ground-based photographic and CCD data was taken in what is now considered to be mediocre seeing. Astronomers at the NTT and CFHT showed the importance of improving the dome seeing and demonstrated that most of the accessible faint blue galaxies brighter than the count slope 'break point' (B < 25) were resolved (Giraud 1992, Colless et al. 1994). The first deep Keck images (Smail et al. 1995) and HST Medium Deep Survey images (Roche et al. 1996) suggested half-light radii of < 0.3 arcsec at fainter limits. HDF has extended this trend to considerably fainter limits (Fig. 2).
Figure 2. Correct intensity-weighted first moment radius versus apparent magnitude with population median and standard deviations for sources in the HDF.
The result has a simple interpretation in the context of hierarchical galaxy formation since, beyond z = 1, a small angular size corresponds to a physically small source ( 2-3 h-1 kpc) regardless of the cosmological model. On the other hand, surface brightness losses and effects due to band shifting may conspire to reduce extended sources to apparently point-like HII regions at high redshift. Coaddition of representative cases suggests this is unlikely to be the case. However, NICMOS images may give a more representative indication of the physical sizes involved.