Momentum transfer by an internal source of ionizing radiation - an important feedback process during galaxy formation ?!

The role of momentum transfer (``radiation pressure'') due to an internal source of ionizing radiation for the formation of baryonic structures is investigated. Fully-ionized self-gravitating gaseous objects can be radiation-pressure supported on a characteristic length scale $D_{\rm rp} \sim 100 \pc - 3 \kpc$. On smaller scales momentum transfer due to ionizing radiation will be the dominant force and for spherical collapsing objects of mass $\la 10^{10}\Msol$ a complete bounce is possible. A population of massive stars and/or accretion onto a central compact object are natural sources of ionizing radiation in newly-forming baryonic structures. Radiation pressure is therefore likely to play an important role for the dynamical and thermal evolution of the intergalactic medium and the mass-to-light ratio of small galaxies. The effect will be especially important for hierarchical cosmogonies where galactic structures build up by merging of smaller objects. Radiation pressure due to ionizing radiation might furthermore be responsible for substructures of size $D_{\rm rp}$ in the stellar component of large galaxies.