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Physical Properties of II Zw 40's Super Star Cluster and Nebula: New Insights and Puzzles from UV Spectroscopy

Leitherer, Claus and Byler, Nell and Lee, Janice C. and Levesque, Emily M. (2018) Physical Properties of II Zw 40's Super Star Cluster and Nebula: New Insights and Puzzles from UV Spectroscopy. Astrophysical Journal, 865 (1). Art. No. 55. ISSN 1538-4357. doi:10.3847/1538-4357/aada84.

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We analyze far-ultraviolet spectra and ancillary data of the super star cluster SSC-N and its surrounding H ii region in the nearby dwarf galaxy II Zw 40. From the ultraviolet spectrum, we derive a low internal reddening of E(B − V) = 0.07 ± 0.03, a mass of (9.1 ± 1.0) × 10^5 M_⊙, a bolometric luminosity of (1.1 ± 0.1) × 10^9 L_⊙, a number of ionizing photons of (6 ± 2) × 10^(52) s^(−1), and an age of (2.8 ± 0.1) Myr. These parameters agree with the values derived from optical and radio data, indicating no significant obscured star formation, absorption of photons by dust, or photon leakage. SSC-N and its nebulosity are an order of magnitude more massive and luminous than 30 Doradus and its ionizing cluster. Photoionization modeling suggests a high ionization parameter and a C/O ratio where C is between primary and secondary. We calculate diagnostic emission-line ratios and compare SSC-N to local star-forming galaxies. The SSC-N nebula does not coincide with the locus defined by local galaxies. Rather, it coincides with the location of "Green Pea" galaxies, objects that are often considered nearby analogs of the galaxies reionizing the universe. Most stellar features are well reproduced by synthetic spectra. However, the SSC-N cluster has strong, broad, stellar He ii λ1640 emission that cannot be reproduced, suggesting a deficit of He-enhanced stars with massive winds in the models. We discuss possible sources for the broad He ii emission, including very massive stars and/or enhanced mixing processes.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Leitherer, Claus0000-0003-2685-4488
Byler, Nell0000-0002-7392-3637
Lee, Janice C.0000-0002-2278-9407
Additional Information:© 2018. The American Astronomical Society. Received 2018 June 25; revised 2018 August 10; accepted 2018 August 13; published 2018 September 20. Support for this work has been provided by NASA through grant number GO-14102 from the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. This research has made extensive use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Subject Keywords:galaxies: ISM; galaxies: starburst; galaxies: stellar content; H ii regions; ultraviolet: galaxies
Issue or Number:1
Record Number:CaltechAUTHORS:20180920-144412109
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Official Citation:Claus Leitherer et al 2018 ApJ 865 55
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:89812
Deposited By: George Porter
Deposited On:20 Sep 2018 22:10
Last Modified:16 Nov 2021 00:38

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