Galaxy And Mass Assembly (GAMA): galaxy close pairs, mergers and the future fate of stellar mass

Robotham, A. S. G., Driver, S. P., Davies, L. J. M., Hopkins, A. M., Baldry, I. K., Agius, N. K., Bauer, A. E., Bland-Hawthorn, J., Brough, S. et al (2014) Galaxy And Mass Assembly (GAMA): galaxy close pairs, mergers and the future fate of stellar mass. Monthly Notices of the Royal Astronomical Society, 444 (4). pp. 3986-4008. ISSN 0035-8711

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We use a highly complete subset of the Galaxy And Mass Assembly II (GAMA-II) redshift sample to fully describe the stellar mass dependence of close pairs and mergers between 108 and 1012 M⊙. Using the analytic form of this fit we investigate the total stellar mass accreting on to more massive galaxies across all mass ratios. Depending on how conservatively we select our robust merging systems, the fraction of mass merging on to more massive companions is 2.0–5.6 per cent. Using the GAMA-II data we see no significant evidence for a change in the close pair fraction between redshift z = 0.05 and 0.2. However, we find a systematically higher fraction of galaxies in similar mass close pairs compared to published results over a similar redshift baseline. Using a compendium of data and the function γM = A(1 + z)m to predict the major close pair fraction, we find fitting parameters of A = 0.021 ± 0.001 and m = 1.53 ± 0.08, which represents a higher low-redshift normalization and shallower power-law slope than recent literature values. We find that the relative importance of in situ star formation versus galaxy merging is inversely correlated, with star formation dominating the addition of stellar material below M∗ and merger accretion events dominating beyond M∗. We find mergers have a measurable impact on the whole extent of the galaxy stellar mass function (GSMF), manifest as a deepening of the ‘dip’ in the GSMF over the next ∼Gyr and an increase in M∗ by as much as 0.01–0.05 dex.

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