H α morphologies of star clusters: a LEGUS study of H II region evolution time-scales and stochasticity in low-mass clusters

December 2019 • 2019MNRAS.490.4648H

Authors • Hannon, Stephen • Lee, Janice C. • Whitmore, B. C. • Chandar, R. • Adamo, A. • Mobasher, B. • Aloisi, A. • Calzetti, D. • Cignoni, M. • Cook, D. O. • Dale, D. • Deger, S. • Della Bruna, L. • Elmegreen, D. M. • Gouliermis, D. A. • Grasha, K. • Grebel, E. K. • Herrero, A. • Hunter, D. A. • Johnson, K. E. • Kennicutt, R. • Kim, H. • Sacchi, E. • Smith, L. • Thilker, D. • Turner, J. • Walterbos, R. A. M. • Wofford, A.

Abstract • The morphology of H II regions around young star clusters provides insight into the time-scales and physical processes that clear a cluster's natal gas. We study ∼700 young clusters (≤10 Myr) in three nearby spiral galaxies (NGC 7793, NGC 4395, and NGC 1313) using Hubble Space Telescope (HST) imaging from LEGUS (Legacy ExtraGalactic Ultraviolet Survey). Clusters are classified by their H α morphology (concentrated, partially exposed, no-emission) and whether they have neighbouring clusters (which could affect the clearing time-scales). Through visual inspection of the HST images, and analysis of ages, reddenings, and stellar masses from spectral energy distributions fitting, together with the (U- B), (V - I) colours, we find (1) the median ages indicate a progression from concentrated (∼3 Myr), to partially exposed (∼4 Myr), to no H α emission (>5 Myr), consistent with the expected temporal evolution of H II regions and previous results. However, (2) similarities in the age distributions for clusters with concentrated and partially exposed H α morphologies imply a short time-scale for gas clearing (≲1 Myr). Also, (3) our cluster sample's median mass is ∼1000 M_⊙, and a significant fraction (∼ 20{{ per cent}}) contain one or more bright red sources (presumably supergiants), which can mimic reddening effects. Finally, (4) the median E(B - V) values for clusters with concentrated H α and those without H α emission appear to be more similar than expected (∼0.18 versus ∼0.14, respectively), but when accounting for stochastic effects, clusters without H α emission are less reddened. To mitigate stochastic effects, we experiment with synthesizing more massive clusters by stacking fluxes of clusters within each H α morphological class. Composite isolated clusters also reveal a colour and age progression for H α morphological classes, consistent with analysis of the individual clusters.

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