Mrk 71/NGC 2366: The Nearest Green Pea Analog

Abstract

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 M ) 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.

Dense CO in Mrk 71-A: Superwind Suppressed in a Young Super Star Cluster

“…Our prediction was dramatically confirmed when Izotov et al (2016a, b) detected Lyman continuum emission from all five of their targeted Green Peas, thereby instantly doubling the number of confirmed local LCEs. The properties of Mrk 71 are quantitatively consistent with those of Green Peas and, like those objects, Mrk 71 shows a variety of features that are consistent with optically thin Lyman continuum (Micheva et al 2017 System temperatures were between 90 and 240 K, and the precipitable water vapor ∼2-3 mm. Data reduction, calibration and imaging were performed with CLIC and MAPPING software of GILDAS 1 , using standard procedures.…”

“…Mrk 71-B is an exposed cluster with mass 1.5 × 10 4 M (Micheva et al 2017) and age of 3 − 5 Myr constrained by the presence of classical WR stars (Drissen et al 2000). Mrk 71-A, however, strongly dominates the complex, producing about ten times more ionizing photons, implying an SSC stellar mass of M = 1.4 × 10 5 M (Micheva et al 2017;Drissen et al 2000). To date, Mrk 71-A has only been detected as a compact, extreme excitation H ii region.…”

Kinematics and Optical Depth in the Green Peas: Suppressed Superwinds in Candidate LyC Emitters*

“…The recently identified, local GP analog Mrk 71-A (Micheva et al 2017) appears to be a similarly young starburst without evident mechanical feedback. Mrk 71-A may be only ∼1 Myr-old, and CO kinematics demonstrate the absence of an energy-driven superwind from the 10 5 M ⊙ SSC .…”

Magnifying the Early Episodes of Star Formation: Super Star Clusters at Cosmological Distances*

“…This could be the case along some directions (not necessarily along the line of sight) in which the opacity to ionizing photons is low. Interestingly, similar transverse cavities possibly optically thin in the Lyman continuum have been observed in the local super star clusters, where expanding shells of gas (superbubbles) and blow-out regions can be produced by substantial mechanical feedback generated by massive super star clusters (e.g., Mrk71, James et al 2016;Micheva et al 2017).…”

“…The detection of nebular high-ionization emission lines has underscored the considerable contribution of hot and massive stars to the transparency of the medium in low-luminosity (L L*  ) systems (Vanzella et al 2016a), as expected in scenarios in which stellar winds and supernova explosions blow cavities in the interstellar medium (e.g., Jaskot & Oey 2013;Micheva et al 2017). In fact, the rest-frame ultraviolet and optical line ratios observed in z 0  , z 2 3 = -and z 6 > galaxies suggest a possible evolution of the ionizing radiation field, such that the presence of hot stars seems more common at high redshift, with blackbody mean effective temperatures on the order of 50.000-60.000 K at z 2 3 = - (Fosbury et al 2003;Steidel et al 2014;Holden et al 2016), or even hotter at z 6 > (Stark et al 2015;Mainali et al 2016).…”