Physical Properties of II Zw 40's Super Star Cluster and Nebula: New Insights and Puzzles from UV Spectroscopy
Date
2018
Authors
Leitherer, Claus
Byler, Nell
Lee, Janice
Levesque, Emily
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IOP Publishing
Abstract
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) × 105 Me, a bolometric luminosity of (1.1 ± 0.1) × 109 Le, a
number of ionizing photons of (6 ± 2) × 1052 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.
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Source
The Astrophysical Journal
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Journal article
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2037-12-31
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