The VIMOS VLT Deep Survey

The contribution of minor mergers to the growth of L_B $\ga$} ≳ L_B^* galaxies since z  ~  1 from spectroscopically identified pairs^⋆

¹ Laboratoire d’Astrophysique de Marseille, Pôle de l’Étoile, Site de Château-Gombert, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France
e-mail: carlos.lopez@oamp.fr
² Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
³ INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁴ Laboratoire d’Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 Av. E. Belin, 31400, France
⁵ IASF-INAF, via Bassini 15, 20133 Milano, Italy
⁶ INAF-Osservatorio Astronomico di Brera, via Brera 28, 20021 Milan, Italy
⁷ Institut d’Astrophysique de Paris, UMR 7095, 98bis Bd Arago, 75014 Paris, France
⁸ Observatoire de Paris, LERMA, 61 avenue de l’Observatoire, 75014 Paris, France
⁹ The Andrzej Soltan Institute for Nuclear Studies, ul. Hoza 69, 00-681 Warszawa, Poland
¹⁰ Astronomical Observatory of the Jagiellonian University, ul Orla 171, 30-244 Kraków, Poland
¹¹ Center for Theoretical Physics PAS, Al. Lotnikow 32/46, 02-668 Warsaw, Poland

Received: 29 September 2010
Accepted: 17 February 2011

Abstract

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 ${\mathit{L}}_{\mathit{B}}\gtrsim {\mathit{L}}_{\mathit{B}}^{\mathrm{\ast }}$ galaxies in the VVDS-Deep spectroscopic survey, and study its dependence on the B-band luminosity ratio the pair galaxies, μ ≡ L_B,2/L_B,1, focusing on minor mergers with 1/10 ≤ μ < 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 $\mathrm{5} {\mathit{h}}^{-1} \mathrm{kpc}\mathit{<}{\mathit{r}}_{\mathrm{p}}\mathrm{\le }{\mathit{r}}_{\mathrm{p}}^{\max }$ and a relative velocity Δv ≤ 500 km s^-1 in redshift space. We vary ${\mathit{r}}_{\mathrm{p}}^{\max }$ from 30   h^-1 kpc to 100   h^-1 kpc. We study f_m in two redshift intervals and for several values of μ, from 1/2 to 1/10. We take μ ≥ 1/4 and 1/10 ≤ μ < 1/4 as major and minor mergers.

Results. The merger fraction increases with z and its dependence on μ is well described by a power-law function, f_m   ( ≥ μ) ∝ μ^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 μ, ii) ${\mathit{f}}_{\mathrm{m}}^{\mathrm{red}}$ does not evolve with z, with s =  −0.79 ± 0.12 at 0.2 < z < 0.95, and iii) ${\mathit{f}}_{\mathrm{m}}^{\mathrm{blue}}$ 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 ${\mathit{L}}_{\mathit{B}}\gtrsim {\mathit{L}}_{\mathit{B}}^{\mathrm{\ast }}$ 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.