Abstract:
Cosmological simulations predict more classical bulges than their observational counterparts in the local Universe. Here, we
quantify evolution of the bulges since z = 0.1 using photometric parameters of nearly 39 000 unbarred disc galaxies from SDSS
DR7 which are well represented by two components. We adopted a combination of the Sersic ´ index and Kormendy relation to
separate classical bulges and disc-like pseudo-bulges. We found that the fraction of pseudo-bulges (classical bulges) smoothly
increases (decreases) as the Universe gets older. In the history of the Universe, there comes a point (z ≈ 0.016) when classical
bulges and pseudo-bulges become equal in number. The fraction of pseudo-bulges rises with increasing bulge to disc half-light
radius ratio until Re/Rhlr ≈ 0.6 suggesting that a concentrated disc is the most favourable place for pseudo-bulge formation.
The mean ellipticity of pseudo-bulges is always greater than that of classical bulges and it decreases with decreasing redshift,
indicating that the bulges tend to be more axisymmetric with evolution. Also, the massive bulges are progressing towards
axisymmetry at steeper rate than the low-mass bulges. There is no tight correlation of bulge Sersic ´ index evolution with other
photometric properties of the galaxy. Using the sample of multicomponent fitting of S4G data and N-body galaxy models, we have
verified that our results are consistent or even more pronounced with multicomponent fitting and high-resolution photometry.
