HST FUV Spectroscopy of Super Star Cluster A in the Green Pea Analog Mrk 71: Revealing the Presence of Very Massive Stars

Linda J. Smith; M. S. Oey; Svea Hernandez; Jenna Ryon; Claus Leitherer; Stephane Charlot; Gustavo Bruzual; Daniela Calzetti; You-Hua Chu; Matthew J. Hayes; Bethan L. James; Anne. E. Jaskot; Goeran Oestlin

arXiv · 2310.03413

HST FUV Spectroscopy of Super Star Cluster A in the Green Pea Analog Mrk 71: Revealing the Presence of Very Massive Stars

Linda J. Smith·M. S. Oey·Svea Hernandez·Jenna Ryon·Claus Leitherer·Stephane Charlot·Gustavo Bruzual·Daniela Calzetti·You-Hua Chu·Matthew J. Hayes·Bethan L. James·Anne. E. Jaskot·Goeran Oestlin

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Abstract

Mrk 71 is a low metallicity (Z = 0.16 Z_sun) starburst region in the local dwarf galaxy NGC 2366, hosting two super star clusters (SSCs A and B), and is recognized as a Green Pea (GP) analog with SSC A responsible for the GP properties. We present STIS and FOS far-ultraviolet (FUV) spectra of the embedded SSC Mrk 71-A obtained with the Hubble Space Telescope (HST). The STIS FUV spectrum shows the characteristic features of very massive stars (VMS, masses > 100 M_sun) and we derive an age of 1+/-1 Myr by comparison with the Charlot & Bruzual suite of spectral population synthesis models with upper mass limits of 300 and 600 M_sun. We compare the STIS spectrum with all known SSC spectra exhibiting VMS signatures: NGC 5253-5, R136a, NGC 3125-A1 and the z = 2.37 Sunburst cluster. We find that the cluster mass-loss rates and wind velocities, as characterized by the C IV P Cygni profiles and the He II emission line strengths, are very similar over Z = 0.16 to 0.4 Z_sun. This agrees with predictions that the optically thick winds of VMS will be enhanced near the Eddington limit and show little metallicity dependence. We find very strong damped Lyman-alpha absorption with log N(H I) = 22.2 cm-2 associated with Mrk 71-A. We discuss the natal environment of this young SSC in terms of radiatively-driven winds, catastrophic cooling and recent models where the cluster is surrounded by highly pressurized clouds with large neutral columns.

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Linda J. Smith, M. S. Oey, Svea Hernandez, Jenna Ryon, Claus Leitherer, Stephane Charlot, Gustavo Bruzual, Daniela Calzetti, You-Hua Chu, Matthew J. Hayes, Bethan L. James, Anne. E. Jaskot, Goeran Oestlin. 2023-10-05. HST FUV Spectroscopy of Super Star Cluster A in the Green Pea Analog Mrk 71: Revealing the Presence of Very Massive Stars. https://arxiv.org/abs/2310.03413

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