The Demographics of galactic bulges in the SDSS database

TL;DR

This study provides a comprehensive database of bulge-disk decompositions for over 14,000 SDSS galaxies, revealing that bulge properties vary systematically with bulge-to-total ratio and show signs of recent star formation, indicating diverse formation histories.

Contribution

The paper introduces a detailed, large-scale database of bulge-disk decompositions and analyzes bulge properties across a broad galaxy sample, highlighting systematic variations and signs of recent star formation.

Findings

Bulge colors are mostly red, except in low B/T galaxies.

Bulges follow similar scaling relations to ellipticals, with deviations in low B/T cases.

Low B/T bulges show signs of recent star formation with young stellar populations.

Abstract

We present a new database of our two-dimensional bulge-disk decompositions for 14,233 galaxies drawn from SDSS DR12 in order to examine the properties of bulges residing in the local universe ($0.005 < z < 0.05$). We performed decompositions in $g$ and $r$ bands by utilizing the {\sc{galfit}} software. The bulge colors and bulge-to-total ratios are found to be sensitive to the details in the decomposition technique, and hence we hereby provide full details of our method. The $g-r$ colors of bulges derived are almost constantly red regardless of bulge size except for the bulges in the low bulge-to-total ratio galaxies ($B/T_{\rm r} \lesssim 0.3$). Bulges exhibit similar scaling relations to those followed by elliptical galaxies, but the bulges in galaxies with lower bulge-to-total ratios clearly show a gradually larger departure in slope from the elliptical galaxy sequence. The scatters around the scaling relations are also larger for the bulges in galaxies with lower bulge-to-total ratios. Both the departure in slopes and larger scatters are likely originated from the presence of young stars. The bulges in galaxies with low bulge-to-total ratios show signs of a frosting of young stars so substantial that their luminosity-weighted Balmer-line ages are as small as 1\,Gyr in some cases. While bulges seem largely similar in optical properties to elliptical galaxies, they do show clear and systematic departures as a function of bulge-to-total ratio. The stellar properties and perhaps associated formation processes of bulges seem much more diverse than those of elliptical galaxies.