Let $S$ be a compact complex manifold of dimension $2$ and assume that there exists a two-dimensional family of curves on $S$. Is it true then that the algebraic dimension of $S$ is $2$, i.e. that $S$ is Moishezon (and hence projective)?


By a two-dimensional family of curves I mean a complex analytic subset $X \subset S \times T$ where $T$ is some compact complex space of dimension 2, and such that fibres $X\_t$ are complex spaces of dimension 1 (possibly singular and reducible), and such that $X\_t$ and $X\_s$ are distinct for $t \neq s$. One can assume that the projection of $X$ on $T$ is flat or smooth, if it helps.



Families of curves on compact complex surfaces and algebraicity
The answer is *yes* under the assumption that *the general element of the mobile part of the family is irreducible*, and a proof goes at follows.


Write your analytic family of curves as $$\{X\_t\} = Z + \{M\_t\},$$
where $Z$ is the fixed part (i.e., the maximal effective divisor contained in any member of the family) and $\{M\_t\}$ is a $2$-dimensional analytic family with at most isolated base points (the mobile part).


Then, calling $M$ the cohomology class of $M\_t$, which is by definition independent on $t$, we claim that $M^2 >0$. In fact, given any irreducible $M\_t$ and a general point $p$ on it, our assumption on the dimension implies that we can find an irreducible $M\_s$, with $s \neq t$, such that $p \in M\_s$, hence $p \in M\_t \cap M\_s$ and this proves our claim. 


Now we are done, since a smooth, compact, complex surface is projective if and only if there exists on it a divisor with strictly positive self-intersection, see Barth-Peters-Van de Ven, *Compact Complex Surfaces*, Chapter 4, Theorem 5.2.


**Remark.** If the general element of the mobile part $\{M\_t\}$ is not irreducible, the result is in general false. For instance, take a surface $S$ with algebraic dimension $1$, and consider the fibration $f \colon S \to \mathbb{P}^1$ given by the essentially unique meromorphic function on $S$ (if necessary, blow-up some point on $S$ in order to make it a holomorphic map). Writing $F\_{u}$ for the fibre over $u \in \mathbb{P}^1$, the linear system $|F\_0 + F\_{\infty}|$ provides a $2$-dimensional family of curves on $S$, whose members are all disjoint unions of two fibres.


