Chemodynamics of Green Pea galaxies -- I. Outflows and turbulence
  driving the escape of ionising photons and chemical enrichment
  We investigate the ionised gas kinematics, physical properties and chemical
abundances of SDSS$J$142947, a Green Pea galaxy at redshift z$\sim$0.17 with
strong, double-peak Ly$\alpha$ 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 $\sigma
\sim$40 km s$^{-1}$, and two broader blue-shifted components with $\sigma
\sim$90 km s$^{-1}$ and $\sim$250 km s$^{-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 $\gtrsim$50% of the integrated emission-line fluxes. The outflow
is dense and metal-enriched compared to the HII regions, with expansion
velocities larger than those obtained from UV interstellar absorption lines
under standard assumptions. Some of these metals may be able to escape, with
outflows loading factors comparable to those found in high-$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
Ly$\alpha$ halo with high average HI column density, LyC photons could only
escape from SDSS$J$142947 through low HI density channels or filaments in the
ISM approaching density-bounded conditions, traced by outflowing gas.
