Chemodynamics of green pea galaxies – I. Outflows and turbulence driving the escape of ionizing photons and chemical enrichment

Abstract

We investigate the ionised gas kinematics, physical properties and chemical abundances of SDSS J142947, a Green Pea galaxy at redshift z∼ 0.17 with strong, doublepeak Lyα emission and indirect evidence of Lyman continuum (LyC) leakage. Using high-dispersion spectroscopy, we perform a multi-component analysis of emission-line profiles. Our model consistently fits all lines as a narrow component with intrinsic velocity dispersion σ ∼ 40 km s −1 , and two broader blue-shifted components with σ ∼ 90 km s −1 and ∼ 250 km s −1 . We find electron densities and temperatures, ionisation conditions, and direct O/H and N/O abundances for each component. A highly ionised, metal-poor, young and compact starburst dominates narrow emission, showing evidence of hard radiation fields and elevated N/O. The blue-shifted broader components are consistent with highly turbulent, possibly clumpy ionised gas at the base of a strong photoionised outflow, which accounts for 50% of the integrated emission-line fluxes. The outflow is dense and metal-enriched compared to the H ii regions, with expansion velocities larger than those obtained from UV interstellar absorption lines under standard assumptions. Some of these metals may be able to escape, with outflows loading factors comparable to those found in high-z galaxies of similar SFR/Area. Our findings depict a two-stage starburst picture; hard radiation fields from young star clusters illuminate a turbulent and clumpy ISM that has been eroded by SNe feedback. While UV data suggest an extended Lyα halo with high average H i column density, LyC photons could only escape from SDSS J142947 through low H i density channels or filaments in the ISM approaching density-bounded conditions, traced by outflowing gas.

The Low-redshift Lyman-continuum Survey: [S II]-deficiency and the leakage of ionizing radiation

“…These objects are defined to be compact, very massive, and young objects located at or near the galactic nucleus (Heckman et al 2011;Borthakur et al 2014). Similar findings have likewise been reported in Wang et al (2019); Hogarth et al (2020).…”

Emission-line Wings Driven by Lyman Continuum in the Green Pea Analog Mrk 71

“…Mrk71/NGC 2366 is a critical nearby analog of the Green Peas. Some of the most extreme GPs also show broad, symmetric, emission-line wings, although with lower maximum velocities (Amorín et al 2012;Hogarth et al 2020). Our result that this feature is driven by LyC/Lyα suggests a possible similar process in these objects, consistent with their relatively high LyC/Lyα transparency.…”

“…Adiabatic feedback, by contrast, is more likely to produce multiple nebular components associated with shells, as seen in, e.g., 30 Doradus (Chu & Kennicutt 1994), which shows nebular line splitting and asymmetry. Indeed, many GP line profiles are better fit by multiple Gaussian components (Amorín et al 2012;Hogarth et al 2020;Bosch et al 2019), which appear to be unresolved, individual star-forming complexes with outflows driven by stellar winds and SNe.…”

Reconciling escape fractions and observed line emission in Lyman-continuum-leaking galaxies

“…The galaxy is ionizationbounded but LyC photons can escape through channels, which are matter-bounded. Such holes could be carved in the ISM by supernovae explosions or could result from hydrodynamical fragmentation effects induced by turbulence or stellar feedback (e.g., Kakiichi & Gronke 2019;Hogarth et al 2020). We expect such mechanisms to efficiently clear out channels, either producing low-density channels or almost emptied holes with small clumps of matter remaining within.…”