Measurement of non‐axisymmetry in centres of advanced mergers of galaxies

We measure the non‐axisymmetry in the luminosity distribution in the inner few kpc of the remnants of advanced mergers of galaxies with a view to understand the relaxation in the central regions. For this, we analyse the images from the Two‐Micron All‐Sky Survey (2MASS) archival data for a selected sample of 12 merging galaxies, which show signs of interaction but have a single nucleus. The central regions are fitted by elliptical isophotes whose centres are allowed to vary to get the best fit. The centres of isophotes show a striking sloshing pattern with a spatial variation of up to 20–30 per cent within the central 1 kpc. This indicates mass asymmetry and a dynamically unrelaxed behaviour in the central region. Next, we Fourier‐analyse the galaxy images while keeping the centre constant and measure the deviation from axisymmetry in terms of the fractional Fourier amplitudes ( A 1 , A 2 , etc.) as a function of radius. All the mergers show a high value of lopsidedness (up to A 1 ∼ 0.2) in the central 5 kpc. The m = 2 asymmetry is even stronger, with values of A 2 up to ∼0.3, and in three cases these are shown to represent bars. The corresponding values denoting non‐axisymmetry in inner regions of a control sample of eight non‐merger galaxies are found to be several times smaller. Surprisingly, this central asymmetry is seen even in mergers where the outer regions have relaxed into a smooth elliptical‐like r 1/4 profile or a spiral‐like exponential profile. Thus, the central asymmetry is long lived, estimated to be ∼1 Gyr, and hence lasts for over 100 local dynamical time‐scales. These central asymmetries are expected to play a key role in the future dynamical evolution of the central region of a merger, and can help in feeding of a central active galactic nucleus.