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Hα Morphologies of Star Clusters: HII region evolution timescales in 16 LEGUS galaxies

Published onJun 01, 2020
Hα Morphologies of Star Clusters: HII region evolution timescales in 16 LEGUS galaxies

The analysis of star clusters in tandem with the morphology of their HII regions is integral to understanding the physical processes that clear a cluster’s natal gas, and can also provide insight into the accuracy of the SED-fitting that describes the star clusters. We report on results expanding upon the work of Hannon et al. (2019), in which Hubble Space Telescope (HST) imaging from LEGUS (Legacy ExtraGalactic Ultraviolet Survey) was used to study young (<10 Myr) star clusters in 3 nearby spiral galaxies. By now including all LEGUS fields which have cluster catalogs and Hα imaging (21 fields across 16 galaxies), we have tripled the previous study’s sample size to include nearly 2000 young (<10Myr) star clusters for a more robust analysis. In these studies, star clusters are classified by their Hα morphology (concentrated, partially exposed, no-emission) and whether they have neighboring clusters (which could affect the clearing timescales). We find consistency between this study and Hannon et al. 2019: 1) the ratio of young star clusters in each class (25%, 17%, and 58% for concentrated, partially exposed, and no-emission Hα morphologies, respectively) are all within 2% of each other. 2) the relative cluster ages of each Hα class remain intact (medians of 2 Myr, 3 Myr, and 5 Myr for young clusters with concentrated, partially exposed and no-emission Hα morphologies, respectively), and though the median ages of the first two classes are slightly younger in this study (each by 1 Myr), the two distributions are very similar and imply a short (<1 Myr) clearing timescale. 3) Clusters with concentrated Hα and those with no emission share similar E(B-V) values, which has been shown to be a result of stochastic sampling effects for lower-mass clusters, where the median mass of clusters in each morphological class is ~1000 M. A key addition in this study is that Hα morphology classifications have been made for all star clusters (~3800 in total) regardless of SED-age, which affords us greater opportunity to examine anomalous cases as well as the quality of the SED-fitting in general. This examination reveals, for example, that very young (<3 Myr) clusters classified as having no Hα emission are, in fact, much older than their SED best-fit age suggests.

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