Radial and 2D colour properties of E+A galaxies

Abstract

We investigated the radial colour gradient and two-dimensional (2D) colour properties of 22 E+A galaxies with 5.5 < Hdelta equivalent width(EW) < 8.5\AA 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 colour maps 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 per cent significance level. We also found a tight correlation between radial colour gradients and colours, and between radial colour gradients and the 4000-\AA 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 observational quantities, Hdelta EW, D4000 and u-g colour, and found that almost all our E+A galaxies are situated along a single evolutionary track. Therefore, these results are interpreted as E+A galaxies evolving from Hdelta EW ~ 8 \AA to Hdelta EW ~ 5 \AA, with colour gradients changing from positive to negative, and with the irregular 2D colour map becoming smoother, during a time-scale of ~300 Myr. Our results favour the hypothesis that E+A galaxies are post-starburst galaxies caused by merger/interaction, having undergone a centralized violent starburst.