2.4 & IMPLICATIONS FOR GALAXY
FORMATION
3.1. Deep F410M surveys for faint
Ly
emitting candidates at
z 2.4
Pascarelle et al. (1996b;
P96b) report evidence in support of the
'bottom-up' hypothesis,
in the form of the detection of 18 significant compact emitters in deep
HST images in the medium band filter F410M - likely
Ly at z
2.4 (see
Section 3.2) - with spectroscopic
confirmation thus far for eight of these subgalactic-sized clumps at
z 2.391 ± 0.004
with the MMT and the KPNO 4 m. Deeper Keck spectroscopic follow-up of
the faintest
z 2.4 candidates is
presented by Armus et al. (1997;
A97). These may be
time building
blocks from which present-day galaxies were made. In this section, we
provide further
evidence that galaxies, or time small star-forming subsystems from which
they formed,
tended to exist in small groups or proto-structures in the early Universe.
The initial discovery of a group of z
2.4 candidates was made from
ground-based
photometry with a medium-band (150 Å-wide) filter centered at 4130
Å (Ly at z
2.39)
in the field surrounding the weak radio galaxy 53W002 at z =
2.390 (W91),
yielding two
other candidates at the same redshift
(P96a). Fortuitously, the
existence of a nearly
identical medium-band filter on HST (F410M, centered at 4100 Å or
Ly at z
2.4)
allowed the same observations to be conducted with WFPC2 in Cycle 5 at
much higher
sensitivity and spatial resolution than could be achieved from the
ground. The HST is
able to achieve nearly the full point-source sensitivity gain, which is
lacking in ground-based
data, because of the characteristically small sizes of the z
2.4 candidates (Figs. 1
& 5a). These z
2.4
candidates are labeled in the color Plate of
P96b, which covers
about 2.5 x 2.5 arcmin of the sky or ~ 0.7 x 0.7 Mpc at z
2.39.
To confirm these results, two long parallel observations were made with
WFPC2 using
the F410M, F450W, & F814W filters in Cycle 6 (out of three fields
scheduled thus far).
The first parallel field was at 21h-5°, and the second
at 16h+82°, each consisting of
seven orbits (about 4 x 2400s in F410M, 2 x 1500s in F450W, and ~ 800s in F814W
for color information). Because of the more limited number of orbits,
proper image averaging and CR-rejection is crucial, and was done with a
custom-written IDL routine to deal specifically with a small number (n
4) of low-signal
images (Co97).
Photometry was done as in
W91,
utilizing user-input apertures and a sky box that is interpolated
underneath the object aperture by fitting a sloped plane to the pixels
unaffected by
faint neighbors (P96b).
Approximately 40 objects were detected simultaneously in the
three filters in each parallel field, and 115 in the deeper 53W002
field. Each photometric
"curve of growth" was individually examined, and apertures were adjusted
to give the
best possible total magnitudes, while maintaining the same size aperture
in the three
filters for each object. Compared to the interactive package of
O96,
sky-estimates were
produced consistent within 0.07% and aperture magnitudes grown-to-total
consistent
within 0.05 mag (with an rms 0.22
mag for each algorithm).
We believe that the discovery of this z
2.39 group is not
likely strongly biased by
targeting the weak radio galaxy 53W002, because the field galaxy counts
in this region
are consistent with those in several other randomly selected fields
(Ca95,
D95b,
O96),
and the surface density of radio sources at this flux level (~ 5 mJy at
8.4 GHz) is high
enough that one such source would be found in every few WFPC2 fields
(W93).
Despite
its steep spectral index, 53W002 is ~ 30 x weaker than most of the
luminous 3CR sources,
which are known to cluster strongly, and therefore its presence is
likely to have less of an effect on the surroundings.