Intrinsic Axis Ratio Distribution of Early-type Galaxies From Sloan Digital Sky Survey

TL;DR

This study analyzes the intrinsic shapes of early-type galaxies using SDSS data, revealing their three-dimensional axis ratio distributions and how these vary with luminosity, while considering classification biases.

Contribution

It introduces a method to de-project observed axis ratios into 3D types using Gaussian distributions, providing new insights into galaxy shapes and their luminosity dependence.

Findings

Oblate, prolate, and triaxial galaxy fractions are approximately 29%, 26%, and 45%.

Luminous galaxies tend to be more triaxial, less luminous ones more oblate.

Galaxy shape distributions are unaffected by local density variations.

Abstract

Using Sloan Digital Sky Survey Data Release 5, we have investigated the intrinsic axis ratio distribution (ARD) for early-type galaxies. We have constructed a volume-limited sample of 3,922 visually-inspected early-type galaxies at $0.05 \leq z \leq 0.06$ carefully considering sampling biases caused by the galaxy isophotal size and luminosity. We attempt to de-project the observed ARD into three-dimensional types (oblate, prolate, and triaxial), which are classified in terms of triaxiality. We confirm that no linear combination of $randomly$-distributed axis ratios of the three types can reproduce the observed ARD. However, using Gaussian intrinsic distributions, we have found reasonable fits to the data with preferred mean axis ratios for oblate, prolate, and triaxial (triaxials in two axis ratios), $\mu_o=0.44, \mu_p=0.72, \mu_{t,\beta}=0.92, \mu_{t,\gamma}=0.78$ where the fractions of oblate, prolate and triaxial types are \textrm{O:P:T}=0.29^{\pm0.09}:0.26^{\pm0.11}:0.45^{\pm0.13}$. We have also found that the luminous sample ($-23.3 < M_r \leq -21.2$) tends to have more triaxials than the less luminous ($-21.2 < M_r <-19.3$) sample does. Oblate is relatively more abundant among the less luminous galaxies. Interestingly, the preferences of axis ratios for triaxial types in the two luminosity classes are remarkably similar. We have not found any significant influence of the local galaxy number density on ARD. We show that the results can be seriously affected by the details in the data selection and type classification scheme. Caveats and implications on galaxy formation are discussed.