The dilution peak, metallicity evolution, and dating of galaxy interactions and mergers

¹ SMC-ISC-CNR & Physics Department, Univ. Roma “La Sapienza”, Pl. Aldo Moro 2, 00185 Rome, Italy e-mail: montuorm@roma1.infn.it
² Observatoire de Paris, section de Meudon, GEPI, 5 Place Jules Jannsen, 92195 Meudon, France
³ Observatoire de Paris, LERMA, CNRS, UPMC, 61 Avenue de l'Observatoire, 75014 Paris, France

Received: 22 February 2010
Accepted: 18 May 2010

Abstract

Strong inflows of gas from the outer disk to the inner kiloparsecs are induced during the interaction of disk galaxies. This inflow of relatively low-metallicity gas dilutes the metallicity of the circumnuclear gas. This process is critical for the galaxy evolution. We have investigated several aspects of the process as the timing and duration of the dilution and its correlation with the induced star formation. We analysed major (1:1) gas-rich interactions and mergers, spanning a range of initial orbital characteristics. Star formation and metal enrichment from SNe are included in our model. Our results show that the strongest trend is between the star formation rate and the dilution of the metals in the nuclear region; i.e., the more intense the central burst of star formation, the more the gas is diluted. This trend comes from strong inflows of relatively metal-poor gas from the outer regions of both disks, which fuels the intense star formation and lowers the overall metallicity for a time. The strong inflows happen on timescales of about 10⁸ years or less (i.e., on an internal dynamical time of the disk in the simulations), and the most intense star formation and lowest gas phase metallicities are seen generally after the first pericentre passage. As the star formation proceeds and the merger advances, the dilution reduces and enrichment becomes dominant – ultimately increasing the metallicity of the circumnuclear gas to a level higher than the initial metallicities of the merging galaxies. The “fly-bys” – pairs that interact but do not merge – also cause some dilution. We even see some dilution early in the merger or in the “fly-bys” and thus do not observe a strong trend between the nuclear metallicities and separation in our simulations until the merger is well advanced. We also analyse the O and Fe enrichment of the ISM, and show that the evolution of the α/Fe ratios, as well as the dilution of the central gas metallicity, can be used as a clock for dating the interaction.