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Abstract Observations of high-redshift galaxies ( z > 5) have shown that these galaxies have extreme emission lines with equivalent widths much larger than their local star-forming counterparts. Extreme emission line galaxies (EELGs) in the nearby universe are likely analogs to galaxies during the Epoch of Reionization and provide nearby laboratories to understand the physical processes important to the early universe. We use Hubble Space Telescope/Cosmic Origins Spectrograph and Large Binocular Telescope/Multi-Object Double Spectrographs spectra to study two nearby EELGs, J104457 and J141851. The far-UV spectra indicate that these two galaxies contain stellar populations with ages ≲10 Myr and metallicities ≤0.15 Z ⊙ . We use photoionization modeling to compare emission lines from models of single-age bursts of star formation to observed emission lines and find that the single-age bursts do not reproduce high-ionization lines including [O iii ] or very-high-ionization lines like He ii or O iv ]. Photoionization modeling using the stellar populations fit from the UV continuum similarly is not capable of reproducing the very-high-energy emission lines. We add a blackbody to the stellar populations fit from the UV continuum to model the necessary high-energy photons to reproduce the very-high-ionization lines of He ii and O iv ]. We find that we need a blackbody of 80,000 K and ∼45%–55% of the luminosity from the blackbody and young stellar population to reproduce the very-high-ionization lines while simultaneously reproducing the low-, intermediate-, and high-ionization emission lines. Our self-consistent model of the ionizing spectra of two nearby EELGs indicates the presence of a previously unaccounted-for source of hard ionizing photons in reionization analogs.
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HST UV Spectroscopy of the Dwarf Starburst Galaxy Pox 186
Abstract Studying the galaxies responsible for reionization is often conducted through local reionization-era analogs; however, many of these local analogs are too massive to be representative of the low-mass star-forming galaxies that are thought to play a dominant role in reionization. The local, low-mass dwarf starburst galaxy Pox 186 is one such system with physical conditions representative of a reionization-era starburst galaxy. We present deep ultraviolet (UV) spectroscopy of Pox 186 to study its stellar population and ionization conditions and to compare these conditions to other local starburst galaxies. The new Cosmic Origins Spectrograph data are combined with archival observations to cover ∼1150–2000 Å and allow for an assessment of Pox 186's stellar population, the relative enrichment of C and O, and the escape of ionizing photons. We detect significant Lyαand low-ionization state absorption features, indicative of previously undetected neutral gas in Pox 186. The C/O relative abundance, log(C/O) = −0.62 ± 0.02, is consistent with other low-metallicity dwarf galaxies and suggests a comparable star formation history in these systems. We compare UV line ratios in Pox 186 to those of dwarf galaxies and photoionization models, and we find excellent agreement for the ratios utilizing the intense Ciii], Oiii], and double-peaked Civlines. However, the UV and optical Heiiemission is faint and distinguishes Pox 186 from other local starburst dwarf galaxies. We explore mechanisms that could produce faint Heii, which have implications for the low-mass reionization-era galaxies that may have similar ionization conditions.
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- Award ID(s):
- 2009894
- PAR ID:
- 10528671
- Publisher / Repository:
- American Astronomical Society
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 955
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 112
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.3847/1538-4357/acf294
