Sample of minor merger of galaxies : II. Modelling HII region properties

Abstract

Context. Studies of interacting galaxies have shown that dwarf galaxies may be produced in tidal tails during collisions. Numerical simulations suggest that the total detachment of the tidal dwarf requires the companion’s mass to be comparable to or higher than that of the parent galaxy. This would imply that minor mergers (interaction between unequal mass galaxies) might not form tidal dwarf galaxies (TDGs). Aims. The objectives of the paper is to analyse the main properties of a sample of 116 HII regions previously selected from Hα images of 11 minor mergers of galaxies and discuss the nature of these regions comparing their properties with those observed in HII regions in normal and isolated galaxies and TDG candidates. Methods. Individual parameters such as: blue absolute magnitude, Mв, Hα luminosity, L(Hα), star formation rate, SFR(Hα) and mass, M/Mʘ of these regions were derived and compared with those of the HII regions in normal isolated galaxies, as well as in tidal dwarf candidates. Results. A full 62% of the HII regions are brighter than Mв ≤ −15 mag, which is an upper limit for giant HII complexes observed in the spiral arms of Sa and Sb galaxies. Five regions are as bright as the tidal dwarf candidates in the tidal tail of Arp 105. It was also found that 43 regions have Hα luminosities brighter than 10⁴° erg sˉ¹, which is the upper luminosity limit for HII regions in normal Sc galaxies. The estimated stellar masses of 32 HII regions are 1 × 10⁶ < M/Mʘ < 3 × 10⁷. These values are comparable to those of young massive clusters (YMC). Conclusions. In our sample we have detected 9 HII regions presenting Hα luminosities which correspond to star formation rates higher than those found in normal HII regions in spiral galaxies. These regions are associated with very massive clusters (1× 10⁸ M ) typical of TDGs.