We perform a linear dynamical stability analysis of a general hydrodynamic
model of chemotactic aggregation [Chavanis & Sire, Physica A, in press (2007)].
Specifically, we study the stability of an infinite and homogeneous
distribution of cells against "chemotactic collapse". We discuss the analogy
between the chemotactic collapse of biological populations and the
gravitational collapse (Jeans instability) of self-gravitating systems. Our
hydrodynamic model involves a pressure force which can take into account
several effects like anomalous diffusion or the fact that the organisms cannot
interpenetrate. We also take into account the degradation of the chemical which
leads to a shielding of the interaction like for a Yukawa potential. Finally,
our hydrodynamic model involves a friction force which quantifies the
importance of inertial effects. In the strong friction limit, we obtain a
generalized Keller-Segel model similar to the generalized Smoluchowski-Poisson
system describing self-gravitating Langevin particles. For small frictions, we
obtain a hydrodynamic model of chemotaxis similar to the Euler-Poisson system
describing a self-gravitating barotropic gas. We show that an infinite and
homogeneous distribution of cells is unstable against chemotactic collapse when
the "velocity of sound" in the medium is smaller than a critical value. We
study in detail the linear development of the instability and determine the
range of unstable wavelengths, the growth rate of the unstable modes and the
damping rate, or the pulsation frequency, of the stable modes as a function of
the friction parameter and shielding length. For specific equations of state,
we express the stability criterion in terms of the density of cells