Radial and 2D Colour Properties of E+A Galaxies

We investigate the radial colour gradient and two-dimensional (2D) colour properties of 22 E+A galaxies with 5.5Å < Hδ EW < 8.5Å and 49 normal early-type galaxies as a control sample at a redshift of < 0.2 in the Second Data Release of the Sloan Digital Sky Survey. We found that a substantial number of E+A galaxies exhibit positive slopes of radial colour gradient (bluer gradients toward the centre) which are seldom seen in normal early-type galaxies. We found irregular ‘Colour Morphologies’ – asymmetrical and clumpy patterns – at the centre of g−r and r−i 2D colourmaps of E+A galaxies with positive slopes of colour gradient. Kolomogorov-Smirnov two-sample tests show that g−r and r−i colour gradient distributions of E+A galaxies differ from those of early-type galaxies with a more than 99.99% significance level. We also found a tight correlation between radial colour gradients and colours, and between radial colour gradients and 4000Å break in the E+A sample; E+A galaxies which exhibit bluer colour or weaker D4000 tend to have positive slopes of radial colour gradient. We compared the GISSEL model and E+A’s observational quantities, Hδ EW, D4000 and u−g colour, and found that almost all our E+A galaxies are situated along a single evolution track. Therefore, these results are interpreted as E+A galaxies evolving from Hδ EW ∼ 8Å to Hδ EW ∼ 5Å, with colour gradients changing from positive to negative, and with the irregular 2D colourmap becoming smoother, during a time scale of ∼300 Myr. Our results favor the hypothesis that E+A galaxies are post-starburst galaxies caused by merger/interaction, having undergone a centralized violent starburst.