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, f(m), of L-B greater than or similar to L-B* galaxies in the VVDS-Deep spectroscopic survey, and study its dependence on the B-band luminosity ratio the pair galaxies, mu L-B,L-2/L-B,L-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 greater than or similar to 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 < r(p) <= r(p)(max) and a relative velocity Delta v <= 500 km s(-1) in redshift space. We vary r(p)(max) from 30 h(-1) kpc to 100 h(-1) kpc. We study f(m) 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, f(m) (>=mu) proportional to mu(s). 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) f(m) is higher for red galaxies at every mu, ii) f red m does not evolve with z, with s = -0.79 +/- 0.12 at 0.2 < z < 0.95, and iii) f blue m 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 L-B greater than or similar to L-B* galaxies has grown by about 25% since z similar to 1 because of the combined effects of minor and major mergers. The relative contribution of the mass growth by merging is similar to 25% due to minor mergers and similar to 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 similar to 1, which leads to a mass growth of similar to 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 similar to 8 Gyr.