  We prove a factorization of completely bounded maps from a $C^*$-algebra $A$
(or an exact operator space $E\subset A$) to $\ell_2$ equipped with the
operator space structure of $(C,R)_\theta$ ($0<\theta<1$) obtained by complex
interpolation between the column and row Hilbert spaces. More precisely, if $F$
denotes $\ell_2$ equipped with the operator space structure of $(C,R)_\theta$,
then $u: A \to F$ is completely bounded iff there are states $f,g$ on $A$ and
$C>0$ such that \[ \forall a\in A\quad \|ua\|^2\le C
f(a^*a)^{1-\theta}g(aa^*)^{\theta}.\] This extends the case $\theta=1/2$
treated in a recent paper with Shlyakhtenko. The constants we obtain tend to 1
when $\theta \to 0$ or $\theta\to 1$. We use analogues of "free Gaussian"
families in non semifinite von Neumann algebras. As an application, we obtain
that, if $0<\theta<1$, $(C,R)_\theta$ does not embed completely isomorphically
into the predual of a semifinite von Neumann algebra. Moreover, we characterize
the subspaces $S\subset R\oplus C$ such that the dual operator space $S^*$
embeds (completely isomorphically) into $M_*$ for some semifinite von neumann
algebra $M$: the only possibilities are $S=R$, $S=C$, $S=R\cap C$ and direct
sums built out of these three spaces. We also discuss when $S\subset R\oplus C$
is injective, and give a simpler proof of a result due to Oikhberg on this
question. In the appendix, we present a proof of Junge's theorem that $OH$
embeds completely isomorphically into a non-commutative $L_1$-space. The main
idea is similar to Junge's, but we base the argument on complex interpolation
and Shlyakhtenko's generalized circular systems (or ``generalized free
Gaussian"), that somewhat unifies Junge's ideas with those of our work with
Shlyakhtenko.
