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

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 10 8 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.

The dust content of high-zsubmillimeter galaxies revealed byHerschel

“…Montuori et al 2010). More generally, if the discrepancy between gas and dust metal mass also applies to other populations of high-z starburst galaxies (e.g.…”

Characteristics of thick disks formed through minor mergers: stellar excesses and scale lengths

“…Because mergers can cause strong gas inflows (e.g., Montuori et al 2010), we would also expect that the properties of the merger-induced thick disk relate somehow to the bulge growth. Although not discussed in detail in this study, we indeed find that the bulge component does grow during minor mergers, thus slowly shifting the galaxies towards earlier Hubble types, and that the effects of minor mergers are cumulative in thickening the disk.…”

On the properties of the interstellar medium in extremely metal-poor blue compact dwarf galaxies

“…Whereas most low-luminosity BCDs show a smooth morphology in their LSB host, luminous (M B < ∼ −19 mag) ones often show tidal distortions on deep images (e.g., Bergvall & Östlin 2002;Lagos et al 2007), suggesting strong interactions or merging as the primary triggering agent of their starburst activity (e.g., Taylor et al 1995;Pustilnik et al 2001a;Brosch et al 2004;Ekta et al 2008). If SF activity in HS 2236+1344 is due to a recent or ongoing merger, as suggested by Moiseev et al (2010), a flatter metallicity distribution may be evolved through transport and mixing of metals (e.g., Rupke et al 2010;Montuori et al 2010) following tidal tail formation. Thus far, in most studies on chemical abundances in SF dwarf galaxies (e.g., Lee & Skillman 2004;Kehrig et al 2008;Lagos et al 2009Lagos et al , 2012 there are no clear evidences of abundance variations.…”