Growth of disc-like pseudo-bulges in SDSS DR7 since z = 0.1

TL;DR

This study examines the evolution of pseudo-bulges and classical bulges in nearly 39,000 SDSS DR7 disc galaxies since z=0.1, revealing a shift towards pseudo-bulge dominance and increased axisymmetry over cosmic time.

Contribution

It provides the first large-scale observational analysis of bulge evolution since z=0.1, using combined photometric criteria to distinguish bulge types and track their evolution.

Findings

Pseudo-bulges increase in fraction over time, surpassing classical bulges around z=0.016.

Pseudo-bulges are more elongated and become more axisymmetric with decreasing redshift.

Massive bulges evolve faster towards axisymmetry than low-mass bulges.

Abstract

Cosmological simulations predict more classical bulges than their observational counterpart 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 S\'ersic 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 ($z \approx 0.016$) 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 R$_{\rm e}$/R$_{\rm hlr} \approx 0.6$ suggesting 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 S\'ersic index evolution with other photometric properties of the galaxy. Using the sample of multi-component fitting of $S^4G$ data and $N-$body galaxy models, we have verified that our results are consistent or even more pronounced with multi-component fitting and high-resolution photometry.