Sersic Properties of Disc Galaxy Mergers

Abstract

Sersic parameters characterising the density profiles of remnants formed in collision-less disc galaxy mergers are obtained; no bulge is included in our simulations. For the luminous component we find that the Sersic index is n ~ (1.5,5.3) with <n> ~ 3 +/- 1 and an effective radius of R_e ~ (1.6,12.9) kpc with <R_e> ~ 5 +/- 3 kpc. A strong correlation of n with the central projected density I_0 is found [n ~ I_0^(-0.14)] which is consistent with observations. No positive linear correlation between the size (R_e) and structure (n) of our remnants is found; we do not advocate the existence of this. The photometric plane (PHP) of the luminous component [n ~ R_e^(0.05) I_0^(0.15)] agrees well, within the uncertainties and the assumption of a constant mass-to-light ratio, with those observationally determined for ellipticals. We found that the surface defined by Sersic parameters (n, R_e, mu_0) in log-space is not a true plane, but a pseudo-plane with a small curvature at low values of n owed to intrinsic properties of the Sersic model. The dark haloes of the remnants have a 3-dimensional Sersic index of <n> ~ 4 +/- 0.5 that are smaller than the ones obtained for dark haloes in LCDM cosmologies <n> ~ 6 +/- 1. A tight dark Sersic ``plane'' (DSP) is also defined by the parameters of the remnants haloes with n \~ r_e^(0.07) rho_0^(0.10). We conclude that collision-less merger remnants of pure disc galaxies have Sersic properties and correlations consistent with those of observed in early-type galaxies and local remnants. It seems that a ``primordial'' bulge in spirals is not a necessary condition to form bona fide ellipticals on grounds of the Sersic properties of remnants.