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.