[Paper Review] Erratum: Structural properties of disk galaxies. I. The intrinsic equatorial ellipticity of bulges

J. Méndez‐Abreu, J. A. L. Aguerri+2 more|arXiv (Cornell University)|Oct 29, 2007

Astronomy and Astrophysical Research3 references12 citations

TL;DR

This study uses a novel 2D photometric decomposition algorithm (GASP2D) on J-band Two Micron All Sky Survey images to derive the intrinsic equatorial ellipticity of bulges in 148 unbarred S0–Sb galaxies. It finds that ~80% of these bulges are triaxial ellipsoids with a mean equatorial axial ratio ⟨B/A⟩ = 0.85, indicating they formed via mergers or secular evolution rather than oblate collapse.

ABSTRACT

(Abridged) A variety of formation scenarios was proposed to explain the diversity of properties observed in bulges. Studying their intrinsic shape can help in constraining the dominant mechanism at the epochs of their assembly. The structural parameters of a magnitude-limited sample of 148 unbarred S0--Sb galaxies were derived in order to study the correlations between bulges and disks as well as the probability distribution function (PDF) of the intrinsic equatorial ellipticity of bulges. It is presented a new fitting algorithm (GASP2D) to perform the two-dimensional photometric decomposition of galaxy surface-brightness distribution. This was assumed to be the sum of the contribution of a bulge and disk component characterized by elliptical and concentric isophotes with constant (but possibly different) ellipticity and position angles. Bulge and disk parameters of the sample galaxies were derived from the J-band images which were available in the Two Micron All Sky Survey. The PDF of the equatorial ellipticity of the bulges was derived from the distribution of the observed ellipticities of bulges and misalignments between bulges and disks. Strong correlations between the bulge and disk parameters were found. About 80% of bulges in unbarred lenticular and early-to-intermediate spiral galaxies 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, and luminosity. The interplay between bulge and disk parameters favors scenarios in which bulges assembled from mergers and/or grew over long times through disk secular evolution. But all these mechanisms have to be tested against the derived distribution of bulge intrinsic ellipticities.

Motivation & Objective

• To constrain the dominant formation mechanisms of galactic bulges by measuring their intrinsic 3D shapes.
• To address the limitation of previous studies that rely on observed ellipticities, which are biased by projection effects and misalignments.
• To test whether bulge formation is dominated by rapid dissipative collapse, mergers, or secular evolution through their intrinsic shape.
• To derive a probability distribution function (PDF) of intrinsic equatorial ellipticity that is robust against observational biases.
• To assess the impact of nuclear bars on the inferred shape of bulges, given their potential to mimic triaxiality.

Proposed method

• Developed a new 2D photometric decomposition algorithm (GASP2D) to model galaxy surface brightness as the sum of Sérsic bulge and exponential disk components with constant ellipticity and position angle.
• Applied GASP2D to J-band images from the Two Micron All Sky Survey (2MASS) to minimize dust and young star contamination.
• Used a Levenberg-Marquardt fitting algorithm to robustly estimate structural parameters (e.g., effective radius, Sérsic index, scale length).
• Derived the intrinsic equatorial ellipticity PDF by statistically correcting observed ellipticities and position angle misalignments between bulge and disk using Monte Carlo simulations.
• Assessed the impact of nuclear bars by simulating 30% and 100% nuclear bar fractions and comparing resulting PDFs.
• Validated results against fundamental plane and FJ/PP relations to ensure consistency with known elliptical galaxy properties.

Experimental results

Research questions

• RQ1 What is the intrinsic equatorial ellipticity distribution of bulges in unbarred S0–Sb galaxies, and how does it constrain formation mechanisms?
• RQ2 To what extent do observed ellipticities and bulge-disk misalignments bias the inferred 3D shape of bulges?
• RQ3 Is the observed triaxiality of bulges consistent with formation via mergers, secular evolution, or early dissipative collapse?
• RQ4 How does the presence of nuclear bars affect the inferred intrinsic shape of bulges?
• RQ5 Does the intrinsic ellipticity of bulges depend on morphology, luminosity, or light concentration?

Key findings

• Approximately 80% of bulges in unbarred lenticular and early-to-intermediate spiral galaxies are not oblate but triaxial ellipsoids, indicating non-spherical formation processes.
• The mean axial ratio in the equatorial plane is ⟨B/A⟩ = 0.85, corresponding to an intrinsic ellipticity ⟨E⟩ = 0.15, with no significant dependence on morphology, luminosity, or light concentration.
• The derived probability distribution function (PDF) of intrinsic equatorial ellipticity is robust and independent of the possible presence of nuclear bars, even when assuming a 30% nuclear bar fraction.
• Bulges follow the same fundamental plane, FJ, and PP relations as elliptical galaxies, supporting a common formation mechanism for bulges and ellipticals.
• Strong correlations between bulge and disk parameters—such as increasing disk scale length with bulge effective radius and velocity dispersion—support secular evolution as a key formation pathway.
• The results are consistent with numerical simulations of hierarchical merging and disk secular evolution, but cannot distinguish between these mechanisms alone; intrinsic shape is a critical discriminant.