The resolved stellar populations in the LEGUS galaxies

TL;DR

This study presents detailed photometric catalogs and analyzes the distribution of massive stars in 50 nearby galaxies, revealing that many stars form in unbound systems rather than clusters, with implications for star formation theories.

Contribution

It provides comprehensive photometric data for 50 galaxies and demonstrates that star formation in unbound systems is significant, challenging existing cluster evolution models.

Findings

The fraction of massive stars in clusters remains constant across different star formation rates.

An excess of mass in unbound systems suggests many stars form outside bound clusters.

Star formation timescales in unbound structures are comparable or longer than 10 Myr.

Abstract

The Legacy ExtraGalactic UV Survey (LEGUS) is a multiwavelength Cycle 21 Treasury program on the Hubble Space Telescope. It studied 50 nearby star-forming galaxies in five bands from the near UV to the I-band, combining new Wide Field Camera 3 observations with archival Advanced Camera for Surveys data. LEGUS was designed to investigate how star formation occurs and develops on both small and large scales, and how it relates to the galactic environments. In this paper we present the photometric catalogs for all the apparently single stars identified in the 50 LEGUS galaxies. Photometric catalogs and mosaicked images for all filters are available for download. We present optical and near UV color-magnitude diagrams for all the galaxies. For each galaxy we derived the distance from the tip of the red giant branch. We then used the NUV color-magnitude diagrams to identify stars more massive than 14 Mo, and compared their number with the number of massive stars expected from the GALEX FUV luminosity. Our analysis shows that the fraction of massive stars forming in star clusters and stellar associations is about constant with the star formation rate. This lack of a relation suggests that the time scale for evaporation of unbound structures is comparable or longer than 10 Myr. At low star formation rates this translates to an excess of mass in clustered environments as compared to model predictions of cluster evolution, suggesting that a significant fraction of stars form in unbound systems.