The morphology of H II regions around young star clusters provides

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.

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

Cignoni, M.;


The morphology of H II regions around young star clusters provides
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.
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