  We experimentally and theoretically study the formation of dressed states
emerging from strong collective coupling of the narrow intercombination line of
Yb atoms to a single mode of a high-finesse optical cavity. By permanently
trapping and cooling the Yb atoms during their interaction with the cavity, we
gain continuous experimental access to the dressed states. This allows us to
detect both their field and their atomic properties, by simultaneously
measuring the steady-state cavity transmission and free-space fluorescence. By
varying the cavity and probe frequencies, we observe coupled atom-cavity states
with atom number-dependent splitting, the hallmark of collective strong
coupling of the atoms with the single cavity mode. We find additional
fluorescence output at atomic resonance, which we explain by the effects of
dephasing and inhomogeneous broadening. We compare our experimental results
with a theoretical model and find good qualitative agreement.
