Magnitude and size evolution of bulgeless galaxies

Abstract

We examine the magnitude and size evolution of bulgeless (discs with no-bulge or pseudo-bulge) galaxies up to z ∼ 0.9 in rest-frame B band. Their evolution is compared to that of normal-discs (or discs with classical bulge). The study is done for luminous sources (M_B ≤ -20) in two equal-volume redshift bins (0.4 ≤ z < 0.77 and 0.77 ≤ z < 1.0) and a local range (0.02 ≤ z < 0.05). The mean surface-brightness, overline{μ }_B, from z_mean = 0.89 to z_mean = 0.04, shows a dimming of 0.79 mag arcsec^-2 for bulgeless galaxies and 1.16 mag arcsec^-2 for normal-discs. The characteristic magnitude, M_B^*, shows an increase of 0.55 mag for bulgeless galaxies and 0.95 mag for normal-discs. Both dimming and faintness observed since z ∼ 0.9 is more pronounced for the normal-discs by ∼0.4 mag. The size distribution is log-normal and both bulgeless and normal-discs show a slight increase in the mean value, Δ overline{log (R_e)} ∼ 0.11, from z_mean = 0.89 to z_mean = 0.04. The proportion of bulgeless galaxies in the full disc sample undergoes a considerable decline with decrease in redshift. This along with the larger dimming and faintness seen for normal-discs suggests that some fraction of the bulgeless sources switch to the normal-disc morphology with time. To ascertain the validity of studying morphology in the optical, the properties of the galaxies observed in both rest-frame B and I band are compared. The common sample is more luminous in the I band but the sizes are larger in the B band for more than 74 per cent of the sources. The variation in the Sérsic-index values of the galaxies in the two rest-bands is minor enough to have any effect on the morphological classification.