Sample of minor merger of galaxies : optical CCD surface photometry and HII region properties

Abstract

We present the results of the B, V and I photometry of eleven southern minor mergers. The total apparent B magnitude, integrated B − V and V − I colours were measured. We built B, V, and I equivalent profiles for each galaxy and decomposed them into bulge and disk components when possible. From Hα+N[II] images we have estimated the basic photometric parameters of the HII regions, such as position, size, B − V and V − I colours, Hα+[NII] luminosity and EW(Hα + [NII]) equivalent width. Primary components have blue absolute magnitudes in the range −22 < Mв < −18, with a peak at Mв = −22. The magnitudes of the secondary components are in the range −22 < Mв < −16 with a maximum at Mв = −19. Most pairs have ΔMв ~ 2, which means that in luminosity the primary galaxy is on average about 6 times brighter than the secondary. We found a linear correlation between the luminosity ratios of the components and their ratio of major diameters, leading to mass ratios between 0.04 < Msecondary/M primary < 0.2, suggesting indeed that our sample is formed by minor mergers. On average the galaxies have colours bluer than those of isolated galaxies with the same morphological type. Most of the HII regions and evolved star-forming regions of the sample were formed between 3.6 to 13.7 Myr ago with an average of (6.3 ± 0.7) Myr. The HII region properties, luminosity, sizes and ages are similar in both components. The HII regions have log (Hα+[NII]) luminosity between 38.6 and 41.7. The HII region luminosity function for the whole sample fits a power law of index α = –1.33. The linear correlation between the luminosity L(Hα+[NII]) and the size of the HII regions has slope of 2.12 ± 0.06. We found that the disk of the primary component is more luminous than those of Lu’s sample, while the disk of the secondary is smaller and fainter. A plot of the disk parameters does not change with colour. This indicates that the different stellar populations in the disks were affected in the same way.