This is a discretionary time proposal to follow up on our open time project
of CAM and PHOT imaging of high-z radio galaxies. These objects are the
progenitors of present-day massive ellipticals, and their stellar populations
are believed to have formed in a strong burst at high z. We have been very
succesful with one object, the z=3.5 radio galaxy 6C1909+722, detecting it
with PHOT at 160 micron and with CAM at 12 micron. As a confirmation, we have
also detected it with SCUBA at the JCMT at 850 micron. The rest-frame far-IR
emission has a total luminosity of about 5e13 Lsun, corresponding to a star
formation rate of 5000 Msun/yr if powered by star formation. Perhaps even more
exciting is our detection of the source with CAM at 12 microns. This
corresponds to rest frame 2.6 micron and thus measures a combination of hot
dust emission and photospheric emission from an evolved stellar population,
that can only be disentagled with further data ISOCAM data, at shorter
wavelengths. This is the purpose of the present request. We request 3 hours of
CAM time to observe this object with LW2 (7 micron), to a 5 sigma limit of
0.07 mJy. This corresponds to rest frame 1.6 micron and thus samples ONLY an
evolved stellar population (if present), NOT any hot dust. Non-detection at
this sensitivity level is HIGHLY significant and will show the ABSENCE of
an evolved underlying population, confirming that the object is a truly
primeval galaxy, cuurently forming its stellar population in an immense burst.
On the other hand, a detection will allow us to measure the mass of an evolved
population, and if a significant old population is present, will push its
formation redshift back to very high z. Either way this project will have
fundamental implications fro the formation of massive galaxies, and will be
a unique contribution of ISO to this field.