PHAT Stellar Cluster Survey. II. Andromeda Project Cluster Catalog

TL;DR

This paper presents a comprehensive catalog of 2,753 stellar clusters in Andromeda, created through citizen science classifications, high-resolution imaging, and extensive photometric analysis, significantly expanding known clusters in the galaxy.

Contribution

The study introduces a robust, crowd-sourced method for identifying and cataloging star clusters in Andromeda using Hubble data, increasing the known cluster count by a factor of six.

Findings

Catalog includes 2,753 clusters and 2,270 background galaxies.

The catalog is 50% complete down to ~500 solar masses for young clusters.

High-resolution imaging enables detailed photometric measurements across six filters.

Abstract

We construct a stellar cluster catalog for the Panchromatic Hubble Andromeda Treasury (PHAT) survey using image classifications collected from the Andromeda Project citizen science website. We identify 2,753 clusters and 2,270 background galaxies within ~0.5 deg$^2$ of PHAT imaging searched, or ~400 kpc$^2$ in deprojected area at the distance of the Andromeda galaxy (M31). These identifications result from 1.82 million classifications of ~20,000 individual images (totaling ~7 gigapixels) by tens of thousands of volunteers. We show that our crowd-sourced approach, which collects >80 classifications per image, provides a robust, repeatable method of cluster identification. The high spatial resolution Hubble Space Telescope images resolve individual stars in each cluster and are instrumental in the factor of ~6 increase in the number of clusters known within the survey footprint. We measure integrated photometry in six filter passbands, ranging from the near-UV to the near-IR. PHAT clusters span a range of ~8 magnitudes in F475W (g-band) luminosity, equivalent to ~4 decades in cluster mass. We perform catalog completeness analysis using >3000 synthetic cluster simulations to determine robust detection limits and demonstrate that the catalog is 50% complete down to ~500 solar masses for ages <100 Myr. We include catalogs of clusters, background galaxies, remaining unselected candidates, and synthetic cluster simulations, making all information publicly available to the community. The catalog published here serves as the definitive base data product for PHAT cluster science, providing a census of star clusters in an L$^*$ spiral galaxy with unmatched sensitivity and quality.