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# CO(2-1) Observations of Mrk 71

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We report the detection of CO(J=2-1) coincident with the super star cluster (SSC) Mrk 71-A in the nearby Green Pea analog galaxy, NGC 2366. Our NOEMA observations reveal a compact, ~7 pc, molecular cloud whose mass (10^5 M_sun) is similar to that of the SSC, consistent with a high star-formation efficiency, on the order of 0.5. There are two, spati...

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## Citations

... Possibly due to the interaction, the size-linewidth relation deviates from that in the disk of the Milky Way. Finally, Mrk 71, also known as NGC 2363 and located at a distance of 3.4 Mpc [83], contains two Super Star Clusters (SSCs). The clumpy CO condensations observed include a component, exhibiting two velocity features, that coincide in projection with the SSC Mrk 71-A and may undergo momentum-driven feedback. ...
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Dwarf galaxies are by far the most numerous galaxies in the Universe, showing properties that are quite different from those of their larger and more luminous cousins. This review focuses on the physical and chemical properties of the interstellar medium of those dwarfs that are known to host significant amounts of gas and dust. The neutral and ionized gas components and the impact of the dust will be discussed, as well as first indications for the existence of active nuclei in these sources. Cosmological implications are also addressed, considering the primordial helium abundance and the similarity of local Green Pea galaxies with young, sometimes proto-galactic sources in the early Universe.
... Possibly due to the interaction, the size-linewidth relation deviates from that in the disk of the Milky Way. Finally, Mrk 71, also known as NGC 2363 and located at a distance of 3.4 Mpc [83], contains two Super Star Clusters (SSCs). The clumpy CO condensations observed include a component, exhibiting two velocity features, that coincide in projection with the SSC Mrk 71-A and may undergo momentum-driven feedback. ...
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Dwarf galaxies are by far the most numerous galaxies in the Universe, showing properties that are quite different from those of their larger and more luminous cousins. This review focuses on the physical and chemical properties of the interstellar medium of those dwarfs that are known to host significant amounts of gas and dust. The neutral and ionized gas components and the impact of the dust will be discussed, as well as first indications for the existence of active nuclei in these sources. Cosmological implications are also addressed, considering the primordial helium abundance and the similarity of local Green Pea galaxies with young, sometimes protogalactic sources in the early Universe.
... Catastrophic cooling conditions have been suggested to be present in some of these extreme starbursts (e.g., Silich et al. 2020;Jaskot et al. 2019;Oey et al. 2017) based on kinematic and other evidence. If this is indeed the case, then our results would imply that mostly likely the heating efficiency in these systems is significantly reduced relative to the fiducial scalings for stellar wind velocities (Section 4). ...
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... For each cloud radius, we run five simulations with different random-phase turbulent initial conditions. Our parameter space spans nearly 3 orders of magnitude in initial density, enabling us to investigate the regulation of SF by stellar winds for conditions ranging from typical Milky Way GMCs (Heyer & Dame 2015;Evans et al. 2021) to the birth clouds of SSCs (Johnson et al. 2015;Oey et al. 2017;Turner et al. 2017;Leroy et al. 2018;Emig et al. 2020). Each simulation is run (at least) until only 5% of the cold gas mass remains on the grid, corresponding to a median duration over the turbulent realizations of 14.8, 6.14, 3.49, and 1.75 Myr for models R20-R2.5, respectively. ...
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Stellar winds contain enough energy to easily disrupt the parent cloud surrounding a nascent star cluster, and for this reason they have long been considered candidates for regulating star formation. However, direct observations suggest most wind power is lost, and Lancaster et al. recently proposed that this is due to efficient mixing and cooling processes. Here we simulate star formation with wind feedback in turbulent, self-gravitating clouds, extending our previous work. Our simulations cover clouds with an initial surface density of 10 ² –10 ⁴ M ⊙ pc ⁻² and show that star formation and residual gas dispersal are complete within two to eight initial cloud freefall times. The “efficiently cooled” model for stellar wind bubble evolution predicts that enough energy is lost for the bubbles to become momentum-driven; we find that this is satisfied in our simulations. We also find that wind energy losses from turbulent, radiative mixing layers dominate losses by “cloud leakage” over the timescales relevant for star formation. We show that the net star formation efficiency (SFE) in our simulations can be explained by theories that apply wind momentum to disperse cloud gas, allowing for highly inhomogeneous internal cloud structure. For very dense clouds, the SFE is similar to those observed in extreme star-forming environments. Finally, we find that, while self-pollution by wind material is insignificant in cloud conditions with moderate density (only ≲10 ⁻⁴ of the stellar mass originated in winds), our simulations with conditions more typical of a super star cluster have star particles that form with as much as 1% of their mass in wind material.
... For each cloud radius we run five simulations with different random-phase turbulent initial conditions. Our parameter space spans nearly 3 orders of magnitude in initial density, enabling us to investigate the regulation of star formation by stellar winds for conditions ranging from typical Milky Way GMCs (Heyer & Dame 2015;Evans et al. 2021) to the birth clouds of SSCs (Johnson et al. 2015;Oey et al. 2017;Turner et al. 2017;Leroy et al. 2018;Emig et al. 2020). Each simulation is run (at least) until only 5% of the cold gas mass remains on the grid, corresponding to a median duration over the turbulent realizations of 14.8, 6.14, 3.49, and 1.75 Myrs for model R20-R2.5, respectively. ...
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Stellar winds contain enough energy to easily disrupt the parent cloud surrounding a nascent star cluster, and for this reason have been considered candidates for regulating star formation. However, direct observations suggest most wind power is lost, and Lancaster21a,b recently proposed that this is due to efficient mixing and cooling processes. Here, we simulate star formation with wind feedback in turbulent, self-gravitating clouds, extending our previous work. Our simulations cover clouds with initial surface density $10^2-10^4$ $M_{\odot} \, {\rm pc}^{-2}$, and show that star formation and residual gas dispersal is complete within 2 - 8 initial cloud free-fall times. The "Efficiently Cooled" model for stellar wind bubble evolution predicts enough energy is lost for the bubbles to become momentum-driven, we find this is satisfied in our simulations. We also find that wind energy losses from turbulent, radiative mixing layers dominate losses by "cloud leakage" over the timescales relevant for star formation. We show that the net star formation efficiency (SFE) in our simulations can be explained by theories that apply wind momentum to disperse cloud gas, allowing for highly inhomogeneous internal cloud structure. For very dense clouds, the SFE is similar to those observed in extreme star-forming environments. Finally, we find that, while self-pollution by wind material is insignificant in cloud conditions with moderate density (only $\lesssim 10^{-4}$ of the stellar mass originated in winds), our simulations with conditions more typical of a super star cluster have star particles that form with as much as 1\% of their mass in wind material.
... This scenario apparently applies in several M82 SSCs (Smith et al. 2006;Westmoquette et al. 2014), NGC 5253 (e.g., Silich et al. 2020), and a few extreme GPs . Oey et al. (2017) detect 10 5 M of molecular gas within 7 pc of Knot A that shows momentum-conserving expansion, implying that the SSC has failed to clear its environment, unlike its older neighbor, Knot B (Figure 1a). ...
... Other mechanisms also depend on energy-driven feedback (e.g., Tenorio-Tagle et al. 1997). However, adiabatic feedback from stellar winds and supernovae do not seem to explain the Mrk71 wings (GD94, Roy et al. 1992;Oey et al. 2017). ...
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We propose that the origin of faint, broad emission-line wings in the Green Pea (GP) analog Mrk 71 is a clumpy, LyC and/or Ly$\alpha$-driven superwind. Our spatially-resolved analysis of Gemini-N/GMOS-IFU observations shows that these line wings with terminal velocity $>3000~\rm{km~s^{-1}}$ originate from the super star cluster (SSC) Knot A, and propagate to large radii. The object's observed ionization parameter and stellar surface density are close to their theoretical maxima, and radiation pressure dominates over gas pressure. Together with a lack of evidence for supernova feedback, these imply a radiation-dominated environment. We demonstrate that a clumpy, radiation-driven superwind from Knot A is a viable model for generating the extreme velocities, and in particular, that Lyman continuum and/or Ly$\alpha$ opacity must be responsible. We find that the Mrk 71 broad wings are best fitted with power laws, as are those of a representative extreme GP and a luminous blue variable star, albeit with different slopes. This suggests that they may share a common wind-acceleration mechanism. We propose that high-velocity, power-law wings may be a distinctive signature of radiation feedback, and of radiatively-driven winds, in particular.
... Catastrophic cooling conditions have been suggested to be present in some of these extreme starbursts (e.g., Silich et al. 2020;Jaskot et al. 2019;Oey et al. 2017) based on kinematic and other evidence. If this is indeed the case, then our results would imply that mostly likely the heating efficiency in these systems is significantly reduced relative to the fiducial scalings for stellar wind velocities (Section 4). ...
Preprint
Full-text available
Superwinds and superbubbles driven by mechanical feedback from super star clusters (SSCs) are common features in many star-forming galaxies. While the adiabatic fluid model can well describe the dynamics of superwinds, several observations of starburst galaxies revealed the presence of compact regions with suppressed superwinds and strongly radiative cooling, i.e., catastrophic cooling. In the present study, we employ the non-equilibrium atomic chemistry and cooling package MAIHEM, built on the FLASH hydrodynamics code, to generate a grid of models investigating the dependence of cooling modes on the metallicity, SSC outflow parameters, and ambient density. While gas metallicity plays a substantial role, catastrophic cooling is more sensitive to high mass-loading and reduced kinetic heating efficiency. Our hydrodynamic simulations indicate that the presence of a hot superbubble does not necessarily imply an adiabatic outflow, and vice versa. Using CLOUDY photoionization models, we predict UV and optical line emission for both adiabatic and catastrophic cooling outflows, for radiation-bounded and partially density-bounded models. Although the line ratios predicted by our radiation-bounded models agree well with observations of star-forming galaxies, they do not provide diagnostics that unambiguously distinguish the parameter space of catastrophically cooling flows. Comparison with observations suggests a small degree of density bounding, non-equilibrium ionization, and/or observational bias toward the central outflow regions.
... This suggests that winds could potentially be the dominant feedback process in higher density environments. In particular, our theory may be able to explain several aspects of the extremely compact molecular clouds seen to be forming super star clusters in nearby galaxies (Johnson et al. 2015;Oey et al. 2017;Turner et al. 2017;Leroy et al. 2018;Emig et al. 2020). As super star clusters/young massive clusters (e.g. ...
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Winds from massive stars have velocities of 1000 km/s or more, and produce hot, high pressure gas when they shock. We develop a theory for the evolution of bubbles driven by the collective winds from star clusters early in their lifetimes, which involves interaction with the turbulent, dense interstellar medium of the surrounding natal molecular cloud. A key feature is the fractal nature of the hot bubble's surface. The large area of this interface with surrounding denser gas strongly enhances energy losses from the hot interior, enabled by turbulent mixing and subsequent cooling at temperatures T = 10^4-10^5 K where radiation is maximally efficient. Due to the extreme cooling, the bubble radius scales differently (R ~ t^1/2) from the classical Weaver77 solution, and has expansion velocity and momentum lower by factors of 10-10^2 at given R, with pressure lower by factors of 10^2 - 10^3. Our theory explains the weak X-ray emission and low shell expansion velocities of observed sources. We discuss further implications of our theory for observations of the hot bubbles and cooled expanding shells created by stellar winds, and for predictions of feedback-regulated star formation in a range of environments. In a companion paper, we validate our theory with a suite of hydrodynamic simulations.
... However, the solutions with strong radiative cooling demonstrate a departure from the adiabatic solutions, the so-called catastrophic cooling (Silich et al. 2004). Observations of several starburst galaxies suggest the presence of catastrophic cooling in stellar clusters (Smith et al. 2006;Oey et al. 2017;Turner et al. 2017). ...
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Mechanical feedback from young massive stars in super star clusters contributes to the formation of superwinds and superbubbles in star-forming regions. We conduct hydrodynamic simulations using the non-equilibrium ionization package MAIHEM to explore how outflow velocity, mass-loading, metallicity, and ambient density can affect the occurrence of catastrophic cooling. To predict optical and UV emission lines, we apply the photoionization code CLOUDY to the physical conditions predicted by our hydrodynamic simulations. Our results could be useful for characterizing catastrophic cooling in starburst regions like the nearby Green Peas and distant star-forming galaxies.
... The most extreme star forming environments can lead to massive (M * > 10 5 M ) and compact (r ∼ 1 pc) so-called "super" star clusters (SSCs; e.g., Portegies Zwart et al. 2010). Because they are often deeply embedded, observations of young SSCs in the process of forming are rare (Herrera & Boulanger 2017;Oey et al. 2017;Turner et al. 2017;Leroy et al. 2018;Emig et al. 2020). Observations and simulations both indicate that SSCs should have high SFEs (e.g., Skinner & Ostriker 2015;Oey et al. 2017;Turner et al. 2017;Krumholz et al. 2019, L. Lancaster et al., in prep.). ...
... Because they are often deeply embedded, observations of young SSCs in the process of forming are rare (Herrera & Boulanger 2017;Oey et al. 2017;Turner et al. 2017;Leroy et al. 2018;Emig et al. 2020). Observations and simulations both indicate that SSCs should have high SFEs (e.g., Skinner & Ostriker 2015;Oey et al. 2017;Turner et al. 2017;Krumholz et al. 2019, L. Lancaster et al., in prep.). Given these high SFEs, by what process(es) do these SSCs disperse their natal gas? ...
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Young massive clusters play an important role in the evolution of their host galaxies, and feedback from the high-mass stars in these clusters can have profound effects on the surrounding interstellar medium. The nuclear starburst in the nearby galaxy NGC253 at a distance of 3.5 Mpc is a key laboratory in which to study star formation in an extreme environment. Previous high resolution (1.9 pc) dust continuum observations from ALMA discovered 14 compact, massive super star clusters (SSCs) still in formation. We present here ALMA data at 350 GHz with 28 milliarcsecond (0.5 pc) resolution. We detect blueshifted absorption and redshifted emission (P-Cygni profiles) towards three of these SSCs in multiple lines, including CS 7$-$6 and H$^{13}$CN 4$-$3, which represents direct evidence for previously unobserved outflows. The mass contained in these outflows is a significant fraction of the cluster gas masses, which suggests we are witnessing a short but important phase. Further evidence of this is the finding of a molecular shell around the only SSC visible at near-IR wavelengths. We model the P-Cygni line profiles to constrain the outflow geometry, finding that the outflows must be nearly spherical. Through a comparison of the outflow properties with predictions from simulations, we find that none of the available mechanisms completely explains the observations, although dust-reprocessed radiation pressure and O star stellar winds are the most likely candidates. The observed outflows will have a very substantial effect on the clusters' evolution and star formation efficiency.