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

Abstract

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 Fourieranalyse 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.

NGC 5523: An isolated product of soft galaxy mergers?

“…Figure 6 also displays isophotal definitions of the inner and outer disks. As shown in Table 5, the brightness minimum and the nucleated bulge (brightness maximum) are each associated with distinct physical centers, similar to the effects observed in interacting galaxies (Schweizer (1996);Jog & Maybhate (2006)). However, asymmetric galaxies are also found in low density environments with no direct indications for recent interactions (e.g., Matthews & Gallagher (1997), Matthews & Gallagher (2002)).…”

Diffuse X-Ray-emitting Gas in Major Mergers

“…Only one nucleus is seen in the K band (Chitre & Jog 2002). A bar-like structure is detected in the near-infrared (Jog & Maybhate 2006). Although it is not an elliptical, we classify Arp 163 as a stage 7 merger because it has no tails.…”

The Bulgeless Seyfert/Liner Galaxy NGC 3367: Disk, Bar, Lopsidedness, and Environment

“…The possibility of a gas rich minor merger although not discarded is constrained to current very low mass satellites or toward the past 3-4 Gyr. For comparison, typical fractional lopsidedness amplitude A 1 within the central 5 kpc from Two Micron All Sky Survey (2MASS) images of advanced mergers of galaxies (Jog & Maybhate 2006) go from ∼0.12 to 0.2, higher than the values found in the inner regions of NGC 3367. Note that those advanced mergers were selected as having merged into a single nucleus but still show indications of interactions, including visible tidal tails, contrary to what is observed in NGC 3367.…”