THE ORIGIN AND OPTICAL DEPTH OF IONIZING RADIATION IN THE “GREEN PEA” GALAXIES
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Although Lyman-continuum (LyC) radiation from star-forming galaxies likely drove the reionization of the universe, observations of star-forming galaxies at low redshift generally indicate low LyC escape fractions. However, the extreme [O iii]/[O ii] ratios of the z = 0.1–0.3 Green Pea galaxies may be due to high escape fractions of ionizing radiation. To analyze the LyC optical depths and ionizing sources of these rare, compact starbursts, we compare nebular photoionization and stellar population models with observed emission lines in the Peas' Sloan Digital Sky Survey (SDSS) spectra. We focus on the six most extreme Green Peas, the galaxies with the highest [O iii]/[O ii] ratios and the best candidates for escaping ionizing radiation. The Balmer line equivalent widths and He i λ3819 emission in the extreme Peas support young ages of 3–5 Myr, and He ii λ4686 emission in five extreme Peas signals the presence of hard ionizing sources. Ionization by active galactic nuclei or high-mass X-ray binaries is inconsistent with the Peas' line ratios and ages. Although stacked spectra reveal no Wolf–Rayet (WR) features, we tentatively detect WR features in the SDSS spectra of three extreme Peas. Based on the Peas' ages and line ratios, we find that WR stars, chemically homogeneous O stars, or shocks could produce the observed He ii emission. If hot stars are responsible, then the Peas' optical depths are ambiguous. However, accounting for emission from shocks lowers the inferred optical depth and suggests that the Peas may be optically thin. The Peas' ages likely optimize the escape of LyC radiation; they are old enough for supernovae and stellar winds to reshape the interstellar medium, but young enough to possess large numbers of UV-luminous O or WR stars.
Subfields: Astronomy and Astrophysics, Instrumentation
View Paper: https://www.semanticscholar.org/paper/00736a1372b9cca15a0489db551b6b7a1b2bb91d
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Citations in the corpus
- STRONG NEBULAR LINE RATIOS IN THE SPECTRA of z ∼ 2–3 STAR FORMING GALAXIES: FIRST RESULTS FROM KBSS-MOSFIRE (2014, methodology): “First, we note that our simple photoionization models [more sophisticated, but less general, models were presented by Jaskot & Oey (2013) and Amorín et al. (2012) in interpret- ing their data] presented in section 4 above are able to reproduce both the position of the GPs in the BPT plane (left…”
- Using Lyman-α to detect galaxies that leak Lyman continuum (2014, background): “…sight, aligned by chance with an outflow axis, or peculiar objects, with extremely high ionisation parameters (e.g. blue compact galaxies, green peas, extremely strong emission line galaxies; Cardamone et al. 2009; Izotov et al. 2011; Jaskot & Oey 2013), may present these characteristics, however.”
- J1154+2443: a low-redshift compact star-forming galaxy with a 46 per cent leakage of Lyman continuum photons (2017, background): “It was recently argued that low-mass compact galaxies at low redshifts z < 1 with very active star formation may be promising candidates for escaping ionizing radiation (Jaskot & Oey 2013; Stasińska et al. 2015).”
- Lyman-α spectral properties of five newly discovered Lyman continuum emitters (2016, methodology): “Comparison of Lyα and [O iii]/[O ii] diagnostics for LyC escape As discussed above, high [O iii]/[O ii] ratios have been proposed to trace density-bounded H ii regions in galaxies (Jaskot & Oey 2013; Nakajima & Ouchi 2014).”
- An extreme [O III] emitter at z = 3.2: a low metallicity Lyman continuum source (2015, background): “The extreme [O iii]/Hβ ratio would lead to consider it as an AGN (Lamareille et al. 2004; Juneau et al. 2014), but this ratio can also be explained by extreme ISM physical conditions (Jaskot & Oey 2013; Nakajima & Ouchi 2014).”
- Ionization state of inter-stellar medium in galaxies: evolution, SFR-M * -Z dependence, and ionizing photon escape (2013, background): “A similar suggestion of the high fesc is inferred for GPs (Jaskot & Oey 2013).”
- Spectroscopy of superluminous supernova host galaxies. A preference of hydrogen-poor events for extreme emission line galaxies (2014, background): “3) suggest high effective temperatures in their environments (probably >50000 K; Jaskot & Oey 2013; Steidel et al. 2014).”
- Detection of high Lyman continuum leakage from four low-redshift compact star-forming galaxies (2016, background): “Jaskot & Oey (2013) and Nakajima & Ouchi (2014) have proposed that extreme [O iii]λ5007/[O ii]λ3727 ratios may be due to high escape fractions of ionizing photons and have argued that this could be the case for some “Green Pea” galaxies at z ∼ 0.1−0.3 or in z ∼ 2 − 3 LAEs.”
- Low-redshift Lyman continuum leaking galaxies with high [O iii]/[O ii] ratios (2018, background): “A fair fraction of compact SFGs is characterised by high line ratios O32 = [O iii]λ5007/[O ii]λ3727 > ∼ 5 (Stasińska et al. 2015) indicating that they may contain density-bounded H ii regions and be potential LyC leakers as suggested e.g. by Jaskot & Oey (2013) and Nakajima & Ouchi (2014).”