Extreme Star-Forming Galaxies: Local laboratories to constrain models of ionizing sources in the reionization epoch
Abstract
The rest-UV emission line spectra of star-forming galaxies (SFGs) at z>6 serves as a primary tool to reveal the nature of ionizing sources that are responsible for the cosmic reionization. Low-metallicity star-forming dwarf galaxies at z<0.3, show similar strong UV emission lines (eg; C IV, He II, O III], and C III]), and are being used to calibrate the UV diagnostics that can be applied to study galaxies in the reionization epoch. However, the high equivalent widths (EW > 20 Angstrom) seen for UV emission lines in z>6 galaxies, have not been observed in any of the local analogs. Such high EWs pose a challenge to the photoionization models, requiring the lowest metallicities, youngest stellar ages, and highest ionization parameters as model inputs to produce the required hard ionizing spectrum, and extreme emission lines. We propose to obtain HST COS (G140L) and STIS (G230L) spectra of a unique sample of 5 SFGs, recently identified as having extreme [OIII]/[OII] ratios (>22) with evidence for hard ionizing radiation from young, massive stars with ages < 3 Myrs. The photoionization models predict high nebular C IV and C III] EWs (> 15 Angstrom), comparable to galaxies in the reionization epoch. Our goal is to (1) compare the fluxes and EWs of the UV emission lines to the model predictions and infer the ISM conditions, (2) calibrate the UV diagnostics that predict potential Lyman continuum escape, and (3) obtain constraints on the ionizing sources (very massive stars, WR stars, or binaries) based on the hardness of the ionizing radiation. The results from our analysis of local analogs will offer crucial insights into the stellar populations and ISM conditions in extreme starbursts at high redshifts that are responsible for reionization.
- Publication:
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HST Proposal
- Pub Date:
- May 2020
- Bibcode:
- 2020hst..prop16213R