Structural properties of disk galaxies: The intrinsic equatorial ellipticity of bulges

TL;DR

This study analyzes the intrinsic shapes of bulges in disk galaxies, revealing most are triaxial and providing insights into their formation scenarios using a new decomposition algorithm.

Contribution

Introduces GASP2D, a new algorithm for two-dimensional bulge-disk decomposition, and derives the intrinsic ellipticity distribution of bulges in a large galaxy sample.

Findings

80% of bulges are triaxial ellipsoids

Mean axial ratio in the equatorial plane is 0.85

No significant dependence on galaxy morphology or luminosity

Abstract

The structural parameters of a magnitude-limited sample of 148 unbarred S0-Sb galaxies were derived to study the correlations between bulge and disk parameters as well as the probability distribution function (PDF) of the intrinsic equatorial ellipticity of bulges. A new algorithm (GASP2D) was used to perform the bidimensional bulge-disk decomposition of the J-band galaxy images extracted from the archive of the 2MASS survey. The PDF of intrinsic ellipticities was derived from the distribution of the observed ellipticities of the bulges and misalignments between the the bulges and disks. About 80% of the observed bulges are not oblate but triaxial ellipsoids. Their mean axial ratio in the equatorial plane is <B/A>=0.85. There is not significant dependence of their PDF on morphology, light concentration or luminosity. This has to be explained by the different scenarios of bulge formation.