PHANGS-HST: New Methods for Star Cluster Identification in Nearby Galaxies

Abstract

We present an innovative and widely applicable approach for the detection and classification of stellar clusters, developed for the PHANGS-HST Treasury Program, an 𝑁𝑈𝑉-to-𝐼 band imaging campaign of 38 spiral galaxies. Our pipeline first generates a unified master source list for stars and candidate clusters, to enable a self-consistent inventory of all star formation products. To distinguish cluster candidates from stars, we introduce the Multiple Concentration Index (MCI) parameter, and measure inner and outer MCIs to probe morphology in more detail than with a single, standard concentration index (CI). We improve upon cluster candidate selection, jointly basing our criteria on expectations for MCI derived from synthetic cluster populations and published cluster catalogues, yielding model and empirical selection regions (respectively). Selection purity (confirmed clusters versus candidates, assessed via humanbased classification) is high (up to 70%) for moderately luminous sources in the empirical selection region, and somewhat lower overall (outside the region or fainter). The number of candidates rises steeply with decreasing luminosity, but pipeline-integrated Machine Learning (ML) classification prevents this from being problematic. We quantify the performance of our PHANGS-HST methods in comparison to LEGUS for a sample of four galaxies in common to both surveys, finding overall agreement with 50-75% of human verified star clusters appearing in both catalogues, but also subtle differences attributable to specific choices adopted by each project. The PHANGS-HST ML-classified Class 1 or 2 catalogues reach ∼ 1 magnitude fainter, ∼ 2× lower stellar mass, and are 2−5× larger in number, than attained in the human classified samples.

A panchromatic view of star cluster formation in a simulated dwarf galaxy starburst

“…Stars can be excluded from the cluster catalogue as having very centrally concentrated flux (low CI), while background galaxies are characterized by extended emission (high CI). Recently, Thilker et al (2021) introduced an updated formulation for the concentration index which better accounts for the differences in the radial light profiles between PSF-sized stars and extended star clusters.…”

PHANGS-MUSE: the HII region luminosity function of local star-forming galaxies

“…With its 19 nearby star-forming galaxies and the detection of tens of thousands of H ii regions at a spatial resolution of about 70 pc, the PHANGS-MUSE data are ideal to study the H ii region LF, marking a turning point in terms of statistics and ability to deblend/resolve H ii regions. Furthermore, the availability of PHANGS-ALMA (Leroy et al 2021) and PHANGS-HST (Lee et al 2021) observations opens up the possibility to compare the H ii region LFs to the MFs of giant molecular clouds (GMCs) and young stellar regions (Wei et al 2020;Rosolowsky et al 2021;Thilker et al 2021;Whitmore et al 2021) and to investigate how the demographics of substructure changes over the course of the star formation and feedback process.…”