Star Formation in Satellite Galaxies

We present narrowband observations of the Hα emission in a sample of 31 satellites orbiting isolated giant spiral galaxies. The sample studied spans the range -19 mag < MB < -15 mag. Hα emission was detected in all the spiral and irregular objects with fluxes in the range × 10-14 ergs cm-2 s-1. The average and maximum values for the current star formation rates are 0.68 and 3.66 M⊙ yr-1, respectively. Maps of the spatial distribution of ionized gas are presented. The star-forming regions show a rich structure in which frequently discrete complexes are imposed over more diffuse structures. In general, the current star formation rates are smaller than the mean values in the past, obtained from the stellar content; this probably indicates a declining rate with time in the generation of new stars. However, the reserve of gas is enough to continue fueling the current levels of star formation activity for at least another Hubble time. Four of the objects (NGC 2718b, NGC 4541e, and NGC 5965a1 and NGC 5965a2) with higher current star formation rates show clear signs of interaction with close companions of comparable brightness at projected distances of 25, 20, and 2 kpc, respectively. The only two galaxies in our sample that do not show star formation activity are members of these interacting systems, and it is unclear if this is a consequence of intrinsic properties (both are early Hubble types) or if it is related to possible disruption of the external parts due to the interaction. In the case of the pair NGC 2718a-2718b there are indications of gas transport between the galaxies.