3.2. Details of the WFPC2 F410M search for
Ly
emitting z
2.4 candidates
The HST data are presented in the color-color diagram of
Fig. 4, which
shows photometry
for 115 objects detected simultaneously in the Cycle 5 broad-band WFPC2
filters V606
and B450, and in the medium-band filter F410M. Results
from the two Cycle 6 HST
F410M fields are also plotted in Fig. 4. (For
consistency with the Cycle 5 F410M field,
the (B450-V606) colors were
interpolated from the (B450-I814)
colors using the effective
central wavelengths of the three broad-band filters involved, following
Ho95b). As with
the ground-based data, the F410M passband is contained entirely inside
the B450 filter,
so that proper continuum subtraction is possible. At z
2.39, these bands sample the
emitted wavelengths of 1200-2100 Å, where the spectra of young
star-forming galaxies are relatively flat
(W91).
The solid line in
Fig. 4 shows the expected relation for objects
with a flat power-law spectrum
(F
-) across these three adjacent
filters. Indeed
most of the general field objects are distributed around it. There is a
group of 18 objects
which are located 
2
below this line, showing that
they have significant emission in
F410M, which is likely Ly
emission at z 2.39, as
argued by
P96b and
below. Eight of
these z 2.4 candidates
are spectroscopically confirmed with the MMT and the KPNO
4m
(P96a,
P96b,
Co97,
K97),
out of ten objects for which spectra of high enough quality
were obtained, indicating this method of finding compact z
2.4 candidates is at least
70% reliable. Apart from
the three brighter z
2.4 objects, which were easily seen in
ground-based photometry
(P96a)
and contained weak active galactic nuclei (AGN), most
of the sample has Ly emission
with average restframe equivalent widths (estimated from
the F410M photometry) of only 40-50 Å - more typical of ionization
arising purely from star formation (cf.
S96a,
S96b).
|
Figure 4. (F410M-B450)
versus (B450-V606) color-color
diagram from 51 WFPC2 orbits on
both the Cycle 5 53W002 field (circles,
P96b)
and the two Cycle 6 parallel F410M fields (squares,
P97).
The solid line shows the expected relation for a featureless
power-law (F |
Two open circles in the region below the lower
2 error boundary populated by the
z 2.4 candidates were
spectroscopically confirmed not to be at z
2.39 but are stars
(M stars can turn out to be apparent F410M-emitters given their
complicated spectra
around 4000 Å). Another ~ 250 objects were detected in
V606 and B450 (with
B450 
26.0 mag), but not in the F410M image. They have
2 lower limits in
(F410M-B450)
-0.2 mag and are not plotted here. For 26.0
B450
27.5 mag, the
B450-band sample
is 90% complete
(O96),
but the underexposed F410M image does not provide useful
upper limits for such faint objects. The Cycle 6 fields are shallower,
and have fewer objects, but have similar fractions of lower limits in
F410M-B450
( 0.0 mag). These
limits are indicated by the dotted arrows at the right axis of
Fig. 4. Both the observed
and the restframe ultra-violet reddening vector expected at z
2.39 are indicated by
the arrows, and suggest that the few reddest candidates may have a
visual absorption of
AV 2-3
mag. Most of the compact z 2.4
Ly emitting candidates, however, have
rather blue colors and are likely not very reddened internally.
The differential volume element at z
2.39 is ~ 20x larger than
at z 0.097, making
it less likely that F410M traces [O II] at z
0.097 than
Ly
at z 2.4. Any other
emission lines between 1216 Å and 3727 Å strong enough to be
detected (e.g., C-IV or
Mg II) arise from objects known to have much lower surface densities
(e.g., powerful
luminous radio galaxies or QSOs). No other strong emission lines exist
in this range for star-forming objects
(K96),
implying that most of the 18 significant candidates are
likely at z 2.39. The
reliability of the 18 z 2.39
candidates (Fig. 4) is further
strengthened by the fact that three of the spectroscopically confirmed
members are at the lower boundary in the
(Ly410-B450)
color of the general field population.