Mrk 71/NGC 2366: The nearest Green Pea analog
  We present the remarkable discovery that the dwarf irregular galaxy NGC 2366
is an excellent analog of the Green Pea (GP) galaxies, which are characterized
by extremely high ionization parameters. The similarities are driven
predominantly by the giant H II region Markarian 71 (Mrk 71). We compare the
system with GPs in terms of morphology, excitation properties, specific
star-formation rate, kinematics, absorption of low-ionization species,
reddening, and chemical abundance, and find consistencies throughout. Since
extreme GPs are associated with both candidate and confirmed Lyman continuum
(LyC) emitters, Mrk 71/NGC 2366 is thus also a good candidate for LyC escape.
The spatially resolved data for this object show a superbubble blowout
generated by mechanical feedback from one of its two super star clusters
(SSCs), Knot B, while the extreme ionization properties are driven by the <1
Myr-old, enshrouded SSC Knot A, which has ~ 10 times higher ionizing
luminosity. Very massive stars (> 100 Msun) may be present in this remarkable
object. Ionization-parameter mapping indicates the blowout region is optically
thin in the LyC, and the general properties also suggest LyC escape in the line
of sight. Mrk 71/NGC 2366 does differ from GPs in that it is 1 - 2 orders of
magnitude less luminous. The presence of this faint GP analog and candidate LyC
emitter (LCE) so close to us suggests that LCEs may be numerous and
commonplace, and therefore could significantly contribute to the cosmic
ionizing budget. Mrk 71/NGC 2366 offers an unprecedentedly detailed look at the
viscera of a candidate LCE, and could clarify the mechanisms of LyC escape.
