The VIMOS VLT Deep Survey. The contribution of minor mergers to the growth of LB ≳ LB* galaxies since z ~ 1 from spectroscopically identified pairs

Aims: The role of minor galaxy mergers in galaxy evolution, and in particular to mass assembly, remains an open question. In this work we measure the merger fraction, fm, of LB ⪆ L*B galaxies in the VVDS-Deep spectroscopic survey, and study its dependence on the B-band luminosity ratio the pair galaxies, mu ≡ LB,2/LB,1, focusing on minor mergers with 1/10 <= mu < 1/4, and on the rest-frame NUV - r colour of the principal galaxies. Methods: We use spectroscopic pairs with redshift z ≲ 1 in the VVDS-Deep survey to define kinematical close pairs as those galaxies with a separation on the sky plane 5 h-1 { kpc < rp <= r_p^max} and a relative velocity Deltav <= 500 km s-1 in redshift space. We vary rp^max from 30 h-1 kpc to 100 h-1 kpc. We study fm in two redshift intervals and for several values of mu, from 1/2 to 1/10. We take mu >= 1/4 and 1/10 <= mu < 1/4 as major and minor mergers. Results: The merger fraction increases with z and its dependence on mu is well described by a power-law function, fm ( >= mu) ∝ mus. The value of s evolves from s = -0.60 ± 0.08 at z = 0.8 to s = -1.02 ± 0.13 at z = 0.5. The fraction of minor mergers for bright galaxies shows little evolution with redshift as a power-law (1 + z)m with index m = -0.4 ± 0.7 for the merger fraction and m = -0.5 ± 0.7 for the merger rate, in contrast with the increase in the major merger fraction (m = 1.3 ± 0.5) and rate (m = 1.3 ± 0.6) for the same galaxies. We split our principal galaxies in red and blue, finding that i) fm is higher for red galaxies at every mu, ii) fm^red does not evolve with z, with s = -0.79 ± 0.12 at 0.2 < z < 0.95, and iii) fm^blue evolves dramatically: the major merger fraction of blue galaxies decreases by a factor of three with cosmic time, while the minor merger fraction of blue galaxies is roughly constant. Conclusions: Our results show that the mass of normal LB ⪆ LB* galaxies has grown by about 25% since z 1 because of the combined effects of minor and major mergers. The relative contribution of the mass growth by merging is 25% due to minor mergers and 75% due to major mergers. The relative effect of merging is more important for red than for blue galaxies, with red galaxies subject to 0.5 minor and 0.7 major mergers since z 1, which leads to a mass growth of 40% and a size increase by a factor of 2. Our results also suggest that, for blue galaxies, minor mergers likely lead to early-type spirals rather than elliptical galaxies. These results show that minor merging is a significant but not dominant mechanism contributing to the mass growth of galaxies in the last 8 Gyr.