DEEP GALAXY COUNTS
Counting galaxies as a function of
apparent magnitude is one of the classical cosmological tests and an important
probe of both the geometry and the evolutionary history of the Universe.
Previously, accurate counts had been possible from Schmidt photograghs
to about B=24mag. Now CCD detectors offer a factor of 10-20 improvement
in sensitivity, reaching far fainter magnitude limits. Important constraints
can now be placed on allowable combinations of q0
and evolution. Already the sheer number of galaxies seen is uncomfortably
large for a high q0 Universe.
Of particular interest is the
possible existence of a turn-over in the differential number count at B=27mag
which, if real, could indicate a redshift cut-off. This could be to do
to galaxies having strong Lyman limit systems and hence being invisible
in the B band above z=4; alternatively a low redshift of galaxy formation
could be the cause.
Researchers used the INT to obtain
a series of CCD exposures totalling 24 hours on one field; stacking these
has yielded the deepest B band image ever taken. About 3000 objects were
detected in an area 3.5 x 5.5 arcminutes with a magnitude limit of B=28mag.
The turn-over at B=27mag is as yet unconfirmed, though detailed corrections
for coincidence (merging of faint galaxy images) are not yet complete.
| More
information
ING facilities involved:
-
Isaac Newton Telescope
using the prime focus CCD camera
Some references:
-
Metcalfe, N. et al,
1991 "Galaxy number counts - II CCD observations to B=25 mag", MNRAS,
249,
498
-
Metcalfe, N. et al,
1991, "Ultra-deep INT CCD imaging of the faintest galaxies", GEMINI Newsletter
Royal Greenwich Obs., 34, 12
-
Metcalfe, N. et al.,1995,
"Galaxy number counts - III Deep CCD observations to B=27.5mag",MNRAS,
273,
257
|
|
|
