The Intrinsic Shape of Galaxies in SDSS/Galaxy Zoo

TL;DR

This study models the 3D shapes of SDSS galaxies using morphological data from Galaxy Zoo, revealing how intrinsic shapes and dust extinction vary with galaxy properties, and providing refined estimates of galaxy shapes.

Contribution

It introduces a non-parametric approach to infer intrinsic galaxy shapes from axis ratio distributions, accounting for dust extinction and removing spiral interlopers for more accurate results.

Findings

Spiral galaxies have lower dust extinction than previously estimated.

Elliptical galaxies are more spherical when brighter and larger.

Disc thickness increases with luminosity and size.

Abstract

By modelling the axis ratio distribution of SDSS DR8 galaxies we find the intrinsic 3D shapes of spirals and ellipticals. We use morphological information from the Galaxy Zoo project and assume a non-parametric distribution intrinsic of shapes, while taking into account dust extinction. We measure the dust extinction of the full sample of spiral galaxies and find a smaller value than previous estimations, with an edge-on extinction of $E_0 = 0.284^{+0.015}_{-0.026}$ in the SDSS r band. We also find that the distribution of minor to major axis ratio has a mean value of $ 0.267 \pm 0.009 $, slightly larger than previous estimates mainly due to the lower extinction used; the same affects the circularity of galactic discs, which are found to be less round in shape than in previous studies, with a mean ellipticity of $0.215 \pm 0.013$. For elliptical galaxies, we find that the minor to major axis ratio, with a mean value of $0.584 \pm 0.006$, is larger than previous estimations due to the removal of spiral interlopers present in samples with morphological information from photometric profiles. These interlopers are removed when selecting ellipticals using Galaxy Zoo data. We find that the intrinsic shapes of galaxies and their dust extinction vary with absolute magnitude, colour and physical size. We find that bright elliptical galaxies are more spherical than faint ones, a trend that is also present with galaxy size, and that there is no dependence of elliptical galaxy shape with colour. For spiral galaxies we find that the reddest ones have higher dust extinction as expected, due to the fact that this reddening is mainly due to dust. We also find that the thickness of discs increases with luminosity and size, and that brighter, smaller and redder galaxies have less round discs.