Galaxy Zoo: Morphological Classifications for 120,000 Galaxies in HST Legacy Imaging

TL;DR

This paper presents a large dataset of galaxy morphologies from Hubble images, using crowdsourced classifications, including a new calibration method for different luminosities and redshifts, enabling extensive studies of galaxy evolution.

Contribution

It introduces a comprehensive morphological catalog for nearly 120,000 galaxies with a novel calibration technique using artificially-redshifted images.

Findings

Largest dataset of galaxy morphologies to date for z~1

Includes detailed classifications like bulge/disk, spiral structure, and bars

Provides raw and calibrated vote fractions for large-scale galaxy evolution studies

Abstract

We present the data release paper for the Galaxy Zoo: Hubble (GZH) project. This is the third phase in a large effort to measure reliable, detailed morphologies of galaxies by using crowdsourced visual classifications of colour composite images. Images in GZH were selected from various publicly-released Hubble Space Telescope Legacy programs conducted with the Advanced Camera for Surveys, with filters that probe the rest-frame optical emission from galaxies out to $z \sim 1$. The bulk of the sample is selected to have $m_{I814W} < 23.5$,but goes as faint as $m_{I814W} < 26.8$ for deep images combined over 5 epochs. The median redshift of the combined samples is $z = 0.9 \pm 0.6$, with a tail extending out to $z \sim 4$. The GZH morphological data include measurements of both bulge- and disk-dominated galaxies, details on spiral disk structure that relate to the Hubble type, bar identification, and numerous measurements of clump identification and geometry. This paper also describes a new method for calibrating morphologies for galaxies of different luminosities and at different redshifts by using artificially-redshifted galaxy images as a baseline. The GZH catalogue contains both raw and calibrated morphological vote fractions for 119,849 galaxies, providing the largest dataset to date suitable for large-scale studies of galaxy evolution out to $z \sim 1$.