H α morphologies of star clusters: a LEGUS study of H ii region evolution time-scales and stochasticity in low-mass clusters. Issue 4 (12th October 2019)

• Record Type: Journal Article
• 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
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 490:Issue 4(2019)
• Page Start: 4648
• Page End: 4665
• Publication Date: 2019-10-12
• Subjects: ISM: H ii regions -- ISM: evolution -- galaxies: star clusters -- galaxies: ISM; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz2820
• Languages: English
• ISSNs: 0035-8711

Abstract

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 ($\sim 20{{\ \rm per\ cent}}$ ) 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.