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CO(2-1) Observations of Mrk 71 

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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... Such wind velocities are observed and readily achievable in star-forming galaxies across a broad swath of cosmic time (e.g., Steidel et al. 2010;Heckman et al. 2011Heckman et al. , 2015 and can be attributed to outflows, stellar winds, and/or supernovae. Furthermore, winds in this velocity range are observed in cases of nitrogen excess at > 10 (e.g., GHZ2, Castellano et al. 2024) and at lower redshift (e.g., in Mrk 996 and the Sunburst arc James et al. 2009;Rivera-Thorsen et al. 2017;Vanzella et al. 2020;Mainali et al. 2022) and can occur even when feedback is suppressed (e.g., in Mrk 71, Oey et al. 2017). Interestingly, CLASSY galaxy J1253-3012 (SHOC391) exhibits N iv] and N iii] in the shock regime in Figure 5 as well as wind velocities between 90 and 140 km s −1 (e.g., Xu et al. 2022), making it an ideal candidate for contributions from low-velocity shocks. ...
... The northern knot of I Zw 18 contains a substantial wind of ionized gas with velocities of 60-80 km s −1 (e.g., Petrosian et al. 1997;Arroyo-Polonio et al. 2024), capable of driving low-velocity shocks consistent with its C iii], C iv, He ii flux ratios. I Zw 18 also exhibits a small amount of ionized gas in a superwind reaching as high as 3 × 10 3 km s −1 (Arroyo-Polonio et al. 2024), which may indicate suppressed stellar feedback such as in Mrk 71 (e.g., Oey et al. 2017). SBS0335-052 E exhibits similar low-velocity shocks of ∼ 100 km s −1 in its older starforming knots and much slower winds of 30-40 km s −1 in its youngest star-forming knots (e.g., Izotov et al. 2006). ...
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The new era of galaxy evolution studies hearkened in by JWST has led to the discovery of z > 5 galaxies exhibiting excess nitrogen with log(N/O)~1 dex or more than expected from log(N/O) vs 12+log(O/H) trends in the local Universe. A variety of novel enrichment pathways have been presented to explain the apparent nitrogen excess, invoking a wide range of processes from very massive stars to stripped binaries to fine-tuned star-formation histories. However, understanding the excitation mechanism responsible for the observed nebular emission is necessary to accurately infer chemical abundances. As of yet, the ionization sources of these galaxies have not been thoroughly explored, with radiative shocks left out of the picture. We present a suite of homogeneous excitation models for star-forming galaxies, active galactic nuclei, and radiative shocks, with which we explore possible explanations for the apparent nitrogen excess. We propose new BPT-style diagnostics to classify galaxies at z > 5, finding that, when combined with O iii] 1660,66 and He ii 1640, N iii] 1747-54 / C iii] 1907,09 best selects shock-dominated galaxies while N iv] 1483,86 / C iii] 1907,09 best distinguishes between active black holes and star forming galaxies. From our diagnostics, we find that slow/intermediate radiative shocks (v = 75-150 km/s) are most consistent with observed UV emission line flux ratios in nitrogen-bright galaxies. Accounting for the effects of shocks can bring nitrogen estimates into better agreement with abundance patterns observed in the local Universe and may be attributable to Wolf Rayet populations actively enriching these galaxies with nitrogen and possibly driving winds responsible for these shocks.
... Ochsendorf et al. 2017).A subsequent study byHirschauer et al. (2020), which investigates new methods for establishing color cuts through the implementation of kernel density estimate techniques, employed the same archival JHK UKIRT and Spitzer IRAC and MIPS data to confirm the large presence of YSOs in Spitzer I. Furthermore, they postulated that this region may be a proto-SSC (M *  10 5 M e ). SSCs represent an extreme mode of star formation where the stellar surface densities exceed those of H II regions and OB associations(Nayak et al. 2019) by orders of magnitude, and the star formation efficiencies are high (see, e.g.,Turner et al. 2015;Oey et al. 2017). Within extragalactic members of the Local Group, the R136 cluster in the LMC's 30 Doradus region is the best-studied SSC (see, e.g.,Chevance et al. 2020;Wong et al. 2022). ...
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We present an imaging survey of the Spitzer I star-forming region in NGC 6822 conducted with the NIRCam and MIRI instruments on board JWST. Located at a distance of 490 kpc, NGC 6822 is the nearest non-interacting low-metallicity (∼0.2 Z ⊙ ) dwarf galaxy. It hosts some of the brightest known H ii regions in the local universe, including recently discovered sites of highly embedded active star formation. Of these, Spitzer I is the youngest and most active, and houses 90 color-selected candidate young stellar objects (YSOs) identified from Spitzer Space Telescope observations. We revisit the YSO population of Spitzer I with these new JWST observations. By analyzing color–magnitude diagrams constructed with NIRCam and MIRI data, we establish color selection criteria and construct spectral energy distributions to identify candidate YSOs and characterize the full population of young stars, from the most embedded phase to the more evolved stages. In this way, we have identified 140 YSOs in Spitzer I. Comparing to previous Spitzer studies of the NGC 6822 YSO population, we find that the YSOs we identify are fainter and less massive, indicating that the improved resolution of JWST allows us to resolve previously blended sources into multiple objects.
... Such pressures correspond to GMCs with surface densities (Σ  Σ crit = 10 3 M ☉ pc −2 )-2-3 orders of magnitude higher than typical of GMCs in the local Universe-which are the likely sites of so-called super star cluster formation (e.g., McCrady et al. 2005;Portegies Zwart et al. 2010;Turner et al. 2015;Smith et al. 2020); observational estimates seem to be consistent with a high value of ò * for these conditions (Turner et al. 2017;Emig et al. 2020;Rico-Villas et al. 2020;Smith et al. 2020;Costa et al. 2021;He et al. 2022;McKinney et al. 2023;Sun et al. 2024). The environments that host these conditions are relatively rare in the local Universelimited to scenarios such as nuclear starbursts (e.g., Leroy et al. 2018;Emig et al. 2020;Levy et al. 2021), merging luminous infrared (IR)-bright galaxies (e.g., Johnson et al. 2015;Finn et al. 2019;Inami et al. 2022), and localized starbursts in dwarf galaxies (e.g., Ochsendorf et al. 2017;Oey et al. 2017;Turner et al. 2017). On the other hand, the higher densities, gas fractions, merger rates, and accretion rates of galaxies at higher redshift suggest that high-pressure conditions are more commonly realized at these epochs; indeed, conditions observed in dusty starburst galaxies (Casey et al. 2014), prequiescent massive compact galaxies (Diamond-Stanic et al. 2012;Rupke et al. 2019), and proto-globular-cluster candidates resolved via gravitational lensing (Vanzella et al. 2022a(Vanzella et al. , 2022bPascale et al. 2023) reflect these conditions. ...
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... Komarova et al. (2021) suggest that these features centered on Mrk 71-A originate in a dense, clumpy, Lyman continuum or Lyα-driven superwind. Oey et al. (2017) present CO(J = 2-1) observations of Mrk 71-A and detect a compact (∼7 pc) molecular cloud with a mass of 10 5 M e , which is similar to the mass of the SSC itself and implies a high star formation efficiency of 50%. These observations suggest that stellar winds are not effective in fully clearing the natal gas at an age of 1 Myr and may be suppressed by catastrophic cooling due to the high densities in young compact SSCs (e.g., Silich et al. 2004;Silich & Tenorio-Tagle 2017. ...
... Neutral hydrogen is detected along the line of sight and shown to be associated with Mrk 71-A with a high column density of N(H I) =10 22.2 cm −2 but little dust (Section 4.1). Oey et al. (2017) detect a compact CO cloud with a size of 7 pc and mass of 10 5 M e coincident with Mrk 71-A. The presence of high-density neutral and molecular gas colocated with the SSC is consistent with the findings of Oey et al. (2023), who use FUV nebular C IV imaging of Mrk 71-A to study the mechanical feedback. ...
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Mrk 71 is a low-metallicity ( Z = 0.16 Z ☉ ) starburst region in the local dwarf galaxy NGC 2366, hosting two super star clusters (SSCs A and B), and it is recognized as a Green Pea (GP) analog with SSC A responsible for the GP properties. We present STIS and FOS far-ultraviolet (FUV) spectra of the embedded SSC Mrk 71-A obtained with the Hubble Space Telescope. The STIS FUV spectrum shows the characteristic features of very massive stars (VMS; masses >100 M ⊙ ) and we derive an age of 1 ± 1 Myr by comparison with the Charlot & Bruzual suite of spectral population synthesis models with upper mass limits of 300 and 600 M ⊙ . We compare the STIS spectrum with all known SSC spectra exhibiting VMS signatures: NGC 5253-5, R136a, NGC 3125-A1, and the z = 2.37 Sunburst cluster. We find that the cluster mass-loss rates and wind velocities, as characterized by the C iv P Cygni profiles and the He ii emission line strengths, are very similar over Z = 0.16–0.4 Z ☉ . This agrees with predictions that the optically thick winds of VMS will be enhanced near the Eddington limit and show little metallicity dependence. We find very strong damped Ly α absorption with N (H i ) =10 22.2 cm ⁻² associated with Mrk 71-A. We discuss the natal environment of this young SSC in terms of radiatively driven winds, catastrophic cooling, and recent models where the cluster is surrounded by highly pressurized clouds with large neutral columns.
... This leads to an increase in the star formation efficiency, star formation rate, and timescale for star formation in a given region (e.g., Shima et al. 2017). It is consistent with the higher star formation efficiencies seen in super star clusters (e.g., Turner et al. 2015;Herrera & Boulanger 2017;Oey et al. 2017), and it has been linked to the formation of multiple stellar populations found in globular clusters (Krause et al. 2012;Lochhaas & Thompson 2017;Silich & Tenorio-Tagle 2018). ...
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The classical model of massive-star mechanical feedback is based on effects at solar metallicity ( Z ⊙ ), yet feedback parameters are very different at low metallicity. Metal-poor stellar winds are much weaker, and more massive supernova progenitors likely collapse directly to black holes without exploding. Thus, for ∼0.4 Z ⊙ we find reductions in the total integrated mechanical energy and momentum of ∼40% and 75%, respectively, compared to values classically expected at solar metallicity. But in particular, these changes effectively delay the onset of mechanical feedback until ages of ∼10 Myr. Feedback from high-mass X-ray binaries could slightly increase mechanical luminosity between ages 5 and 10 Myr, but it is stochastic and unlikely to be significant on this timescale. Stellar dynamical mechanisms remove most massive stars from clusters well before 10 Myr, which would further promote this effect; this process is exacerbated by gas retention implied by weak feedback. Delayed mechanical feedback implies that radiation feedback therefore dominates at early ages, which is consistent with the observed absence of superwinds in some extreme starbursts. This scenario may lead to higher star formation efficiencies, multiple stellar populations in clusters, and higher Lyman continuum escape. This could explain the giant star-forming complexes in metal-poor galaxies and the small sizes of OB superbubble shells relative to their inferred ages. It could also drive modest effects on galactic chemical evolution, including on oxygen abundances. Thus, delayed low-metallicity mechanical feedback may have broad implications, including for early cosmic epochs.
... A subsequent study by Hirschauer et al. (2020), which investigates new methods for establishing color cuts through implementation of kernel density estimate techniques, employed the same archival Spitzer IRAC and MIPS data to confirm the large presence of YSOs in Spitzer I. Furthermore, they postulated that this region may be a proto-SSC (M * ≳ 10 5 M ⊙ ). SSCs represent an extreme mode of star formation where the stellar surface densities exceed those of HII regions and OB associations (Nayak et al. 2019) by orders of magnitude and the star formation efficiencies are high (see, e.g., Turner et al. 2015;Oey et al. 2017). Within extragalactic members of the Local Group, the R136 cluster in the LMC's 30 Doradus region is the best-studied SSC (see, e.g., Chevance et al. 2020;Wong et al. 2022). ...
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... This suggests that radiation pressure is a strong candidate for driving these outflows. Similarly, our results seem to suggest that a potential outflow of molecular gas observed in NGC 2366, coincident with the Mrk 71-A SSC ( v out ∼ 11 km s −1 ; Oey et al. 2017 ) is likely driven by radiation pressure. ...
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We use three-dimensional radiation hydrodynamic (RHD) simulations to study the formation of massive star clusters under the combined effects of direct ultraviolet (UV) and dust-reprocessed infrared (IR) radiation pressure. We explore a broad range of mass surface density Σ ∼ 102–105 M⊙ pc−2, spanning values typical of weakly star-forming galaxies to extreme systems such as clouds forming super-star clusters, where radiation pressure is expected to be the dominant feedback mechanism. We find that star formation can only be regulated by radiation pressure for Σ ≲ 103 M⊙ pc−2, but that clouds with Σ ≲ 105 M⊙ pc−2 become super-Eddington once high star formation efficiencies (80 %\sim 80~{{\%}}) are reached, and therefore launch the remaining gas in a steady outflow. These outflows achieve mass-weighted radial velocities of ∼15 – 30 km s−1, which is ∼0.5 – 2.0 times the cloud escape speed. This suggests that radiation pressure is a strong candidate to explain recently observed molecular outflows found in young super-star clusters in nearby starburst galaxies. We quantify the relative importance of UV and IR radiation pressure in different regimes, and deduce that both are equally important for Σ ∼ 103 M⊙ pc−2, whereas clouds with higher (lower) density are increasingly dominated by the IR (UV) component. Comparison with control runs without either the UV or IR bands suggests that the outflows are primarily driven by the impulse provided by the UV component, while IR radiation has the effect of rendering a larger fraction of gas super-Eddington, and thereby increasing the outflow mass flux by a factor of ∼2.
... The center of NGC 253 offers a rich starburst environment in which Leroy et al. (2018) observe an overall efficiency of star formation of ∼50% in SSCs and an efficiency per freefall time that is similar to the central kiloparsec of IRAS08 (when using a Milky Way α CO ), ò ff ∼ 10%. Similar results are found in the SSC Mrk 71A (Oey et al. 2017). Indeed, Otí-Floranes et al. ...
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We report on the internal distribution of star formation efficiency in IRAS 08339+6517 (hereafter IRAS08), using ∼200 pc resolution CO(2 − 1) observations from NOEMA. The molecular gas depletion time changes by 2 orders-of-magnitude from disk-like values in the outer parts to less than 10 ⁸ yr inside the half-light radius. This translates to a star formation efficiency per freefall time that also changes by 2 orders-of-magnitude, reaching 50%–100%, different than local spiral galaxies and the typical assumption of constant, low star formation efficiencies. Our target is a compact, massive disk galaxy that has a star formation rate 10× above the z = 0 main sequence; Toomre Q ≈ 0.5−0.7 and high gas velocity dispersion ( σ mol ≈ 25 km s ⁻¹ ). We find that IRAS08 is similar to other rotating, starburst galaxies from the literature in the resolved Σ SFR ∝ Σ mol N relation. By combining resolved literature studies we find that the distance from the main sequence is a strong indicator of the Kennicutt-Schmidt power-law slope, with slopes of N ≈ 1.6 for starbursts from 100 to 10 ⁴ M ⊙ pc ⁻² . Our target is consistent with a scenario in which violent disk instabilities drive rapid inflows of gas. It has low values of Toomre- Q , and also at all radii, the inflow timescale of the gas is less than the depletion time, which is consistent with the flat metallicity gradients in IRAS08. We consider these results in light of popular star formation theories; in general observations of IRAS08 find the most tension with theories in which star formation efficiency is a constant. Our results argue for the need of high-spatial-resolution CO observations for a larger number of similar targets.
... 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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Full-text available
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.