330 Highlights in modern observational cosmology
Figure 11.7.Illustration of the Lyman break (‘drop-out’) technique in the HDF-N from
Dickinson (1998). Top panel: model spectrum of a star-forming galaxy atz = 3 .0.
Its flat UV continuum (infνunits) is truncated by the 912A Lyman limit, which is ̊
redshifted between the U and B filters of the WFPC2 camera aboard the HST. Intervening
neutral hydrogen along the light of sight further suppresses the continuum blueward of Lyα
(1216A). ̊ Bottom: HDF-N galaxy, spectroscopically confirmed atz= 2 .8, as observed in
the four WFPC2 bandpasses. Its flux is constant in V and I, it dims in B and completely
vanishes in the U-band image.
Follow-up spectroscopy with the Keck telescope has confirmed that objects
selected in this fashion were indeed star-forming galaxies at 2. z. 3. 5
(Steidelet al1996). The same technique can be applied to search for higher
redshifts galaxies/AGN, for example, objects atz&4, the so-called ‘B drop-outs’
(Steidelet al1999), although it becomes much harder as they become fainter
(R>24) and more rare. To date, approximately 900 galaxies have measured
with a spectroscopic redshift atz 3 ± 0 .5 and approximately 50 at 4.z.5.
By exploring relatively large volumes atz∼3, these studies have taught us much
about the star formation density (see section 11.3.5) and large-scale structure (e.g.