Super Star Clusters Research Articles

Super star clusters and their multiple stellar populations

AbstractWe present a scenario for the formation of super star clusters (with masses larger than 105 M⊙) in which multiple generations of star formation will occur. We stress that the gas left over (∼50%) from first generation (1G) star formation should be retained in such massive clusters (thanks to their deep potential wells, with escape speeds larger than 10 km/s) and be available for a second or even third generation of stars, with the basic HeCNONaMgAl chemical anomalies observed in globular clusters, the latter assumed to be the descendents of these super star clusters. One new feature of this model is the role of C+ cooling of the dense warm trapped neutral or ionized gas which defines a characteristic temperature of ∼100 K, leading to a second generation (2G) of stars with a top-heavy IMF (M > 5 M⊙). The ashes of the 2G very massive stars (VMS, M > 100 M⊙) sampled in this IMF quickly pollute and dilute the left-over pristine gas with their slow winds (that cannot escape the cluster), while the majority of massive stars develop fast winds (that actually can escape from the cluster). Meanwhile, much of the remaining dense T = 100 K gas contracts gravitationally in the massive cluster and may reach densities of the order of 109 cm−3, in which case the Jeans mass drops to about 0.2 M⊙ and leads to a substantial low-mass pre-MS 3G population (most likely on a very short timescale). In this way, we may solve both the mass budget and the excess Helium problem in proto-globular clusters, while also explaining the Na-O and Mg-Al anti-correlations resulting from hot H-burning of very massive stars at 45MK and 75MK, respectively.

IFU investigation of possible Lyman continuum escape from Mrk 71/NGC 2366

Context. Mrk 71/NGC 2366 is the closest green pea (GP) analog and candidate Lyman Continuum (LyC) emitter. Recently, 11 LyC-leaking GPs have been detected through direct observations of the ionizing continuum, making this the most abundant class of confirmed LyC-emitters at any redshift. High resolution, multiwavelength studies of GPs can lead to an understanding of the method(s), through which LyC escapes from these galaxies. Aims. The proximity of Mrk 71/NCG 2366 offers unprecedented detail on the inner workings of a GP analog, and enables us to identify the mechanisms of LyC escape. Methods. We used 5825–7650 Å integral field unit PMAS observations to study the kinematics and physical conditions in Mrk 71. An electron density map was obtained from the [S II] ratio. A fortuitous second order contamination by the [O II] λ3727 doublet enabled the construction of an electron temperature map. Resolved maps of sound speed, thermal broadening, “true” velocity dispersion, and Mach number were obtained and compared to the high resolution magnetohydrodynamic SImulating the LifeCycle of molecular Clouds (SILCC) simulations. Results. Two regions of increased velocity dispersion indicative of outflows are detected to the north and south of the super star cluster, knot B, with redshifted and blueshifted velocities, respectively. We confirm the presence of a faint broad kinematical component, which is seemingly decoupled from the outflow regions, and is fainter and narrower than previously reported in the literature. Within uncertainties, the low- and high-ionization gasses move together. Outside of the core of Mrk 71, an increase in Mach numbers is detected, implying a decrease in gas density. Simulations suggest this drop in density can be as high as ∼4 dex, down to almost optically thin levels, which would imply a nonzero LyC escape fraction along the outflows even when assuming all of the detected H I gas is located in front of Mrk 71 in the line of sight. Conclusions. Our results strongly indicate that kinematical feedback is an important ingredient for LyC leakage in GPs.

Does radiative feedback make faint z > 6 galaxies look small?

Recent observations of lensed sources have shown that the faintest ($M_{\mathrm{UV}} \approx -15\,\mathrm{mag}$) galaxies observed at z=6-8 appear to be extremely compact. Some of them have inferred sizes of less than 40 pc for stellar masses between $10^6$ and $10^7\,\mathrm{M}_{\odot}$, comparable to individual super star clusters or star cluster complexes at low redshift. High-redshift, low-mass galaxies are expected to show a clumpy, irregular morphology and if star clusters form in each of these well-separated clumps, the observed galaxy size would be much larger than the size of an individual star forming region. As supernova explosions impact the galaxy with a minimum delay time that exceeds the time required to form a massive star cluster, other processes are required to explain the absence of additional massive star forming regions. In this work we investigate whether the radiation of a young massive star cluster can suppress the formation of other detectable clusters within the same galaxy already before supernova feedback can affect the galaxy. We find that in low-mass ($M_{200} \lesssim 10^{10}\,\mathrm{M}_{\odot}$) haloes, the radiation from a compact star forming region with an initial mass of $10^{7}\,\mathrm{M}_{\odot}$ can keep gas clumps with Jeans masses larger than $\approx 10^{7}\,\mathrm{M}_{\odot}$ warm and ionized throughout the galaxy. In this picture, the small intrinsic sizes measured in the faintest $z=6-8$ galaxies are a natural consequence of the strong radiation field that stabilises massive gas clumps. A prediction of this mechanism is that the escape fraction for ionizing radiation is high for the extremely compact, high-z sources.

Forming Super Star Clusters in the Central Starburst of NGC 253

Abstract NGC 253 hosts the nearest nuclear starburst. Previous observations show a region rich in molecular gas, with dense clouds associated with recent star formation. We used the Atacama Large Submillimeter/Millimeter Array (ALMA) to image the 350 GHz dust continuum and molecular line emission from this region at 2 pc resolution. Our observations reveal ∼14 bright, compact (∼2–3 pc FWHM) knots of dust emission. Most of these sources are likely to be forming super star clusters (SSCs) based on their inferred dynamical and gas masses, association with 36 GHz radio continuum emission, and coincidence with line emission tracing dense, excited gas. One source coincides with a known SSC, but the rest remain invisible in Hubble near-infrared (IR) imaging. Our observations imply that gas still constitutes a large fraction of the overall mass in these sources. Their high brightness temperature at 350 GHz also implies a large optical depth near the peak of the IR spectral energy distribution. As a result, these sources may have large IR photospheres, and the IR radiation force likely exceeds L/c. Still, their moderate observed velocity dispersions suggest that feedback from radiation, winds, and supernovae are not yet disrupting most sources. This mode of star formation appears to produce a large fraction of stars in the burst. We argue for a scenario in which this phase lasts ∼1 Myr, after which the clusters shed their natal cocoons but continue to produce ionizing photons. The strong feedback that drives the observed cold gas and X-ray outflows likely occurs after the clusters emerge from this early phase.

Massive star cluster formation under the microscope atz = 6

We report on a superdense star-forming region with an effective radius (R_e) smaller than 13 pc identified at z=6.143 and showing a star-formation rate density \Sigma_SFR~1000 Msun/yr/kpc2 (or conservatively >300 Msun/yr/kpc2). Such a dense region is detected with S/N>40 hosted by a dwarf extending over 440 pc, dubbed D1 (Vanzella et al. 2017b). D1 is magnified by a factor 17.4+/-5.0 behind the Hubble Frontier Field galaxy cluster MACS~J0416 and elongated tangentially by a factor 13.2+/-4.0 (including the systematic errors). The lens model accurately reproduces the positions of the confirmed multiple images with a r.m.s. of 0.35", and the tangential stretch is well depicted by a giant multiply-imaged Lya arc. D1 is part of an interacting star-forming complex extending over 800 pc. The SED-fitting, the very blue ultraviolet slope (\beta ~ -2.5, F(\lambda) ~ \lambda^\beta) and the prominent Lya emission of the stellar complex imply that very young (< 10-100 Myr), moderately dust-attenuated (E(B-V)<0.15) stellar populations are present and organised in dense subcomponents. We argue that D1 (with a stellar mass of 2 x 10^7 Msun) might contain a young massive star cluster of M < 10^6 Msun and Muv~-15.6 (or m_uv=31.1), confined within a region of 13 pc, and not dissimilar from some local super star clusters (SSCs). The ultraviolet appearance of D1 is also consistent with a simulated local dwarf hosting a SSC placed at z=6 and lensed back to the observer. This compact system fits into some popular globular cluster formation scenarios. We show that future high spatial resolution imaging (e.g., E-ELT/MAORY-MICADO and VLT/MAVIS) will allow us to spatially resolve light profiles of 2-8 pc.

The curious case of the companion: evidence for cold accretion onto a dwarf satellite near the isolated elliptical NGC 7796

Context. The isolated elliptical (IE) NGC 7796 is accompanied by an interesting early-type dwarf galaxy, named NGC 7796-DW1. It exhibits a tidal tail, very boxy isophotes, and multiple nuclei or regions (A, B, and C) that are bluer than the bulk population of the galaxy, indicating a younger age. These properties are suggestive of a dwarf–dwarf merger remnant. Aims. Dwarf–dwarf mergers are poorly understood, but may have a high importance for dwarf galaxy evolution. We want to investigate the properties of the dwarf galaxy and its components to find more evidence for a dwarf–dwarf merger or for alternative formation scenarios. Methods. We use the Multi-Unit Spectroscopic Explorer (MUSE) at the VLT to investigate NGC 7796-DW1. We extract characteristic spectra to which we apply the STARLIGHT population synthesis software to obtain ages and metallicities of the various population components of the galaxy. This permits us to isolate the emission lines for which fluxes and flux ratios can be measured and to which strong-line diagnostic tools can be applied. Results. The galaxy’s main body is old and metal-poor. A surprising result is the extended line emission in the galaxy, forming a ring-like structure with a projected diameter of 2.2 kpc. The line ratios fall into the regime of HII-regions, although OB-stellar populations cannot be identified by spectral signatures. The low Hα surface brightnesses indicate unresolved star-forming substructures, which means that broad-band colours are not reliable age or metallicity indicators. Nucleus A is a relatively old (7 Gyr or older) and metalpoor super star cluster, most probably the nucleus of the dwarf, now displaced. The star-forming regions B and C show younger and distinctly more metal-rich components. The emission line ratios of regions B and C indicate an almost solar oxygen abundance, if compared with radiation models of HII regions. Oxygen abundances from empirical calibrations point to only half-solar. The ring-like Hα-structure does not exhibit signs of rotation or orbital movements. Conclusions. NGC 7796-DW1 occupies a particular role in the group of transition-type galaxies with respect to its origin and current evolutionary state, being the companion of an IE. The dwarf–dwarf merger scenario is excluded because of the missing metal-rich merger component. A viable alternative is gas accretion from a reservoir of cold, metal-rich gas. NGC 7796 has to provide this gas within its X-ray bright halo. As illustrated by NGC 7796-DW1, cold accretion may be a general solution to the problem of extended star formation histories in transition dwarf galaxies.

Diagnostics of a nuclear starburst: water and methanol masers

We test models of starburst driven outflows using observations of the 22.2 GHz H$_2$O and 36.2 GHz class I CH$_3$OH maser lines. We have observed the starburst galaxy NGC 253 using the Karl G. Jansky Very Large Array. We present evidence for entrainment of star-forming dense-molecular gas in the outflow of NGC 253. We also show that H$_2$O masers are associated with forming super star clusters and not with supernova remnants. We detect four new 36 GHz CH$_3$OH masers in the central kpc and show possible evidence for a star-formation origin of two class I CH$_3$OH masers. Such high resolution observations are essential for understanding the origin of these masers.

Super star cluster feedback driving ionization, shocks and outflows in the halo of the nearby starburst ESO 338-IG04

Context. Stellar feedback strongly affects the interstellar medium (ISM) of galaxies. Stellar feedback in the first galaxies likely plays a major role in enabling the escape of LyC photons, which contribute to the re-ionization of the Universe. Nearby starburst galaxies serve as local analogues allowing for a spatially resolved assessment of the feedback processes in these galaxies. Aims.We aim to characterize the feedback effects from the star clusters in the local high-redshift analogue ESO 338-IG04 on the ISM and compare the results with the properties of the most massive clusters. Methods. We used high quality VLT/MUSE optical integral field data to derive the physical properties of the ISM such as ionization, density, shocks, and performed new fitting of the spectral energy distributions of the brightest clusters in ESO 338-IG04 from HST imaging. Results.We find that ESO 338-IG04 has a large ionized halo which we detect to a distance of 9 kpc. We identify four Wolf-Rayet (WR) clusters based on the blue and red WR bump. We follow previously identified ionization cones and find that the ionization of the halo increases with distance. Analysis of the galaxy kinematics shows two complex outflows driven by the numerous young clusters in the galaxy. We find a ring of shocked emission traced by an enhanced [O I]/Hα ratio surrounding the starburst and at the end of the outflow. Finally we detect nitrogen enriched gas associated with the outflow, likely caused by the WR stars in the massive star clusters. Conclusions. Photoionization dominates the central starburst and sets the ionization structure of the entire halo, resulting in a density bounded halo, facilitating the escape of LyC photons. Outside the central starburst, shocks triggered by an expanding super bubble become important. The shocks at the end of the outflow suggest interaction between the hot outflowing material and the more quiescent halo gas.

Physical Properties of II Zw 40's Super Star Cluster and Nebula: New Insights and Puzzles from UV Spectroscopy

Abstract We analyze far-ultraviolet spectra and ancillary data of the super star cluster SSC-N and its surrounding H ii region in the nearby dwarf galaxy II Zw 40. From the ultraviolet spectrum, we derive a low internal reddening of E(B − V) = 0.07 ± 0.03, a mass of (9.1 ± 1.0) × 105 M ⊙, a bolometric luminosity of (1.1 ± 0.1) × 109 L ⊙, a number of ionizing photons of (6 ± 2) × 1052 s−1, and an age of (2.8 ± 0.1) Myr. These parameters agree with the values derived from optical and radio data, indicating no significant obscured star formation, absorption of photons by dust, or photon leakage. SSC-N and its nebulosity are an order of magnitude more massive and luminous than 30 Doradus and its ionizing cluster. Photoionization modeling suggests a high ionization parameter and a C/O ratio where C is between primary and secondary. We calculate diagnostic emission-line ratios and compare SSC-N to local star-forming galaxies. The SSC-N nebula does not coincide with the locus defined by local galaxies. Rather, it coincides with the location of “Green Pea” galaxies, objects that are often considered nearby analogs of the galaxies reionizing the universe. Most stellar features are well reproduced by synthetic spectra. However, the SSC-N cluster has strong, broad, stellar He ii λ1640 emission that cannot be reproduced, suggesting a deficit of He-enhanced stars with massive winds in the models. We discuss possible sources for the broad He ii emission, including very massive stars and/or enhanced mixing processes.

ALMA Observations toward the Starburst Dwarf Galaxy NGC 5253. I. Molecular Cloud Properties and Scaling Relations

Abstract We present high-spatial-resolution (∼0.″2, or ∼3 pc) CO(2–1) observations of the nearest young starburst dwarf galaxy, NGC 5253, taken with the Atacama Large Millimeter/submillimeter Array. We have identified 118 molecular clouds with average values of 4.3 pc in radius and 2.2 km s−1 in velocity dispersion, which comprise the molecular cloud complexes observed previously with ∼100 pc resolution. We derive for the first time in this galaxy the I(CO)–N(H2) conversion factor, cm−2(K km s−1)−1, based on the virial method. The line width and mass-to-size relations of the resolved molecular clouds present an offset on average toward higher line widths and masses with respect to quiescent regions in other nearby spiral galaxies and our Galaxy. The offset in the scaling relation reaches its maximum in regions close to the central starburst, where velocity dispersions are ∼0.5 dex higher and gas mass surface densities are as high as . These central clouds are gravitationally bound despite the high internal pressure. A spatial comparison with star clusters found in the literature enables us to identify six clouds that are associated with young star clusters. Furthermore, the star formation efficiencies (SFEs) of some of these clouds exceed those found in star-cluster-forming clouds within our Galaxy. We conclude that once a super star cluster is formed, the parent molecular clouds are rapidly dispersed by the destructive stellar feedback, which results in such a high SFE in the central starburst of NGC 5253.

Major impact from a minor merger

Context. Minor mergers are important processes contributing significantly to how galaxies evolve across the age of the Universe. Their impact on the growth of supermassive black holes and star formation is profound – about half of the star formation activity in the local Universe is the result of minor mergers. Aims. The detailed study of dense molecular gas in galaxies provides an important test of the validity of the relation between star formation rate and HCN luminosity on different galactic scales – from whole galaxies to giant molecular clouds in their molecular gas-rich centers. Methods. We use observations of HCN and HCO+ 1−0 with NOEMA and of CO3−2 with the SMA to study the properties of the dense molecular gas in the Medusa merger (NGC 4194) at 1′′ resolution. In particular, we compare the distribution of these dense gas tracers with CO2−1 high-resolution maps in the Medusa merger. To characterize gas properties, we calculate the brightness temperature ratios between the three tracers and use them in conjunction with a non-local thermodynamic equilibrium (non-LTE) radiative line transfer model. Results. The gas represented by HCN and HCO+ 1−0, and CO3−2 does not occupy the same structures as the less dense gas associated with the lower-J CO emission. Interestingly, the only emission from dense gas is detected in a 200 pc region within the “Eye of the Medusa”, an asymmetric 500 pc off-nuclear concentration of molecular gas. Surprisingly, no HCN or HCO+ is detected for the extended starburst of the Medusa merger. Additionally, there are only small amounts of HCN or HCO+ associated with the active galactic nucleus. The CO3−2/2−1 brightness temperature ratio inside “the Eye” is ~2.5 – the highest ratio found so far – implying optically thin CO emission. The CO2−1/HCN 1−0 (~9.8) and CO2−1/HCO+ 1−0 (~7.9) ratios show that the dense gas filling factor must be relatively high in the central region, consistent with the elevated CO3−1/2−1 ratio. Conclusions. The line ratios reveal an extreme, fragmented molecular cloud population inside the Eye with large bulk temperatures (T &gt; 300 K) and high gas densities (n(H2) &gt; 104 cm-3). This is very different from the cool, self-gravitating structures of giant molecular clouds normally found in the disks of galaxies. The Eye of the Medusa is found at an interface between a large-scale minor axis inflow and the central region of the Medusa. Hence, the extreme conditions inside the Eye may be the result of the radiative and mechanical feedback from a deeply embedded, young and massive super star cluster formed due to the gas pile-up at the intersection. Alternatively, shocks from the inflowing gas entering the central region of the Medusa may be strong enough to shock and fragment the gas. For both scenarios, however, it appears that the HCN and HCO+ dense gas tracers are not probing star formation, but instead a post-starburst and/or shocked ISM that is too hot and fragmented to form newstars. Thus, caution is advised in taking the detection of emission from dense gas tracers as evidence of ongoing or imminent star formation.

Ionized Gas Motions and the Structure of Feedback near a Forming Globular Cluster in NGC 5253

We observed Brackett $\alpha$ 4.05$\mu$m emission towards the supernebula in NGC 5253 with NIRSPEC on Keck II in adaptive optics mode, NIRSPAO, to probe feedback from its exciting embedded super star cluster (SSC). NIRSPEC's Slit-Viewing Camera was simultaneously used to image the K-band continuum at $\sim$$0.1''$ resolution. We register the IR continuum with HST imaging, and find that the visible clusters are offset from the K-band peak, which coincides with the Br $\alpha$ peak of the supernebula and its associated molecular cloud. The spectra of the supernebula exhibit Br $\alpha$ emission with a strong, narrow core. The linewidths are 65-76 km s$^{-1}$, FWHM, comparable to those around individual ultra-compact HII regions within our Galaxy. A weak, broad (FWHM$\simeq$150-175 km s$^{-1}$) component is detected on the base of the line, which could trace a population of sources with high-velocity winds. The core velocity of Br $\alpha$ emission shifts by +13 km s$^{-1}$ from NE to SW across the supernebula, possibly indicating a bipolar outflow from an embedded object, or linked to a foreground redshifted gas filament. The results can be explained if the supernebula comprises thousands of ionized wind regions around individual massive stars, stalled in their expansion due to critical radiative cooling and unable to merge to drive a coherent cluster wind. Based on the absence of an outflow with large mass loss, we conclude that feedback is currently ineffective at dispersing gas, and the SSC retains enriched material out of which it may continue to form stars.

Discovery of a Molecular Collision Front in Interacting Galaxies NGC 4567/4568 with ALMA

Abstract We present results of 12CO(J = 1–0) imaging observations of NGC 4567/4568, a galaxy pair in a close encounter, with the Atacama Large Millimeter/Submillimeter Array (ALMA). For the first time, we find clear evidence of a molecular collision front with a velocity dispersion that is 16.8 ± 1.4 km s−1 at the overlapping region, owing to high spatial and velocity resolution. By integrating over the velocity width that corresponds to the molecular collision front, we find a long filamentary structure with a size of 1800 pc × 350 pc at the collision front. This filamentary molecular structure spatially coincides with a dark lane seen in the R-band image. We find four molecular clouds in the filament, each with a radius of 30 pc and mass of 106 M ⊙; the radii matching a typical value for giant molecular clouds (GMCs) and the masses corresponding to those between GMCs and giant molecular associations (GMAs). All four clouds are gravitationally bound. The molecular filamentary structure and its physical conditions are similar to the structure expected via numerical simulation. The filament could be a progenitor of super star clusters.

Radiation pressure in super star cluster formation

The physics of star formation at its extreme, in the nuclei of the densest and the most massive star clusters in the universe - potential massive black hole nurseries - has for decades eluded scrutiny. Spectroscopy of these systems has been scarce, whereas theoretical arguments suggest that radiation pressure on dust grains somehow inhibits star formation. Here, we harness an accelerated Monte Carlo radiation transport scheme to report a radiation hydrodynamical simulation of super star cluster formation in turbulent clouds. We find that radiation pressure reduces the global star formation efficiency by 30-35%, and the star formation rate by 15-50%, both relative to a radiation-free control run. Overall, radiation pressure does not terminate the gas supply for star formation and the final stellar mass of the most massive cluster is $\sim 1.3\times10^6\,M_\odot$. The limited impact as compared to in idealized theoretical models is attributed to a radiation-matter anti-correlation in the supersonically turbulent, gravitationally collapsing medium. In isolated regions outside massive clusters, where the gas distribution is less disturbed, radiation pressure is more effective in limiting star formation. The resulting stellar density at the cluster core is $\ge 10^{8}\,M_{\odot}\,\textrm{pc}^{-3}$, with stellar velocity dispersion $\gtrsim 70\,\text{km}\,\text{s}^{-1}$. We conclude that the super star cluster nucleus is propitious to the formation of very massive stars via dynamical core collapse and stellar merging. We speculate that the very massive star may avoid the claimed catastrophic mass loss by continuing to accrete dense gas condensing from a gravitationally-confined ionized phase.

Prodigious and Continuous Formation of Super Star Clusters from Cooled Intracluster Gas

AbstractGlobular clusters (GCs) — compact and massive star clusters found ubiquitously around galaxies — are believed to be ancient relics (ages ≳ 10 Gyr) from the early formative phase of galaxies, although their physical origin remains widely debated. The most numerous GC populations are hosted by giant elliptical galaxies, where they can exhibit a broad dispersion in colour interpreted as a wide spread in metallicity. Here, we show that many thousands of similarly compact and massive super star clusters have formed at an approximately steady rate over, at least, the past ~1 Gyr around the nearby giant elliptical galaxy, NGC 1275, at the centre of the Perseus cluster. The number distribution of these young star clusters appears to exhibit a similar dependence in luminosity and mass as the even more numerous but older GCs around NGC 1275. In just a few Gyr, these super star clusters will evolve to become indistinguishable in broadband optical colours from the older GCs, and their spread in age add to the dispersion in colour of these GCs. The spatial distribution of the super star clusters resembles the filamentary network of multiphase gas in the cluster core, implying that they formed from molecular gas amassed from cooling of the hot intracluster gas. The sustained formation of super star clusters from cooled intracluster gas constitutes a previously unrecognised but prodigious source of GCs over cosmic timescales, and contributes to both their enormous numbers and broad colour dispersion in giant elliptical galaxies.

Radiation pressure in galactic discs: stability, turbulence, and winds in the single-scattering limit

The radiation force on dust grains may be dynamically important in driving turbulence and outflows in rapidly star-forming galaxies. Recent studies focus on the highly optically-thick limit relevant to the densest ultra-luminous galaxies and super star clusters, where reprocessed infrared photons provide the dominant source of electromagnetic momentum. However, even among starburst galaxies, the great majority instead lie in the so-called "single-scattering" limit, where the system is optically-thick to the incident starlight, but optically-thin to the re-radiated infrared. In this paper we present a stability analysis and multidimensional radiation-hydrodynamic simulations exploring the stability and dynamics of isothermal dusty gas columns in this regime. We describe our algorithm for full angle-dependent radiation transport based on the discontinuous Galerkin finite element method. For a range of near-Eddington fluxes, we show that the medium is unstable, producing convective-like motions in a turbulent atmosphere with a scale height significantly inflated compared to the gas pressure scale height and mass-weighted turbulent energy densities of $\sim 0.01-0.1$ of the midplane radiation energy density, corresponding to mass-weighted velocity dispersions of Mach number $\sim 0.5-2$. Extrapolation of our results to optical depths of $10^3$ implies maximum turbulent Mach numbers of $\sim20$. Comparing our results to galaxy-averaged observations, and subject to the approximations of our calculations, we find that radiation pressure does not contribute significantly to the effective supersonic pressure support in star-forming disks, which in general are substantially sub-Eddington. We further examine the time-averaged vertical density profiles in dynamical equilibrium and comment on implications for radiation-pressure-driven galactic winds.

Enhanced dust emissivity power-law index along the western H α filament of NGC 1569

We used a data set from AKARI and Herschel images at wavelengths from 7 $\mu$m to 500 $\mu$m to catch the evidence of dust processing in galactic winds in NGC 1569. Images show a diffuse infrared (IR) emission extending from the galactic disk into the halo region. The most prominent filamentary structure seen in the diffuse IR emission is spatially in good agreement with the western H$\alpha$ filament (western arm). The spatial distribution of the $F_\mathrm{350}/F_\mathrm{500}$ map shows high values in regions around the super-star clusters (SSCs) and towards the western arm, which are not found in the $F_\mathrm{250}/F_\mathrm{350}$ map. The color-color diagram of $F_\mathrm{250}/F_\mathrm{350}$-$F_\mathrm{350}/F_\mathrm{500}$ indicates high values of the emissivity power-law index ($\beta_\mathrm{c}$) of the cold dust component in those regions. From a spectral decomposition analysis on a pixel-by-pixel basis, a $\beta_\mathrm{c}$ map shows values ranging from $\sim1$ to $\sim2$ over the whole galaxy. In particular, high $\beta_\mathrm{c}$ values of $\sim2$ are only observed in the regions indicated by the color-color diagram. Since the average cold dust temperature in NGC 1569 is $\sim30$ K, $\beta_\mathrm{c}<2.0$ in the far-IR and sub-mm region theoretically suggests emission from amorphous grains, while $\beta_\mathrm{c}=2.0$ suggests that from crystal grains. Given that the enhanced $\beta_\mathrm{c}$ regions are spatially confined by the HI ridge that is considered to be a birthplace of the SSCs, the spatial coincidences may indicate that dust grains around the SSCs are grains of relatively high crystallinity injected by massive stars originating from starburst activities and that those grains are blown away along the HI ridge and thus the western arm.

The Association of Molecular Gas and Natal Super Star Clusters in Henize 2–10

Abstract We present ALMA observations of the dwarf starburst galaxy He 2–10 in combination with previous SMA CO observations to probe the molecular environments of natal super star clusters (SSCs). These observations include the HCO+(1-0), HCN(1-0), HNC(1-0), and CCH(1-0) molecular lines, as well as 88 GHz continuum with a spatial resolution of . After correcting for the contribution from free–free emission to the 88 GHz continuum flux density (∼60% of the 88 GHz emission), we derive a total gas mass for He 2–10 of M ⊙, roughly 5%–20% of the dynamical mass. Based on a principle component analysis, HCO+ is found to be the best “general” tracer of molecular emission. The line widths and luminosities of the CO emission suggests that the molecular clouds could either be as small as ∼8 pc, or alternately have enhanced line widths. The CO emission and 88 GHz continuum are anti-correlated, suggesting that either the dust and molecular gas are not cospatial, which could reflect that the 88 GHz continuum is dominated by free–free emission. The CO and CCH emission are also relatively anti-correlated, which is consistent with the CCH being photo-enhanced, and/or the CO being dissociated in the regions near the natal SSCs. The molecular line ratios of regions containing the natal star clusters are different from the line ratios observed for regions elsewhere in the galaxy. In particular, the regions with thermal radio emission all have , and the HCO+/CO ratio appears to be correlated with the evolutionary stage of the clusters.

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

Abstract We report the detection of CO( ) coincident with the super star cluster (SSC) Mrk 71-A in the nearby Green Pea analog galaxy, NGC 2366. Our observations with the Northern Extended Millimeter Array reveal a compact, ∼7 pc, molecular cloud whose mass ( ) is similar to that of the SSC, consistent with a high star formation efficiency, on the order of 0.5. There are two spatially distinct components separated by 11 . If expanding, these could be due to momentum-driven stellar wind feedback. Alternatively, we may be seeing remnants of the infalling, colliding clouds responsible for triggering the SSC formation. The kinematics are also consistent with a virialized system. These extreme, high-density, star-forming conditions inhibit energy-driven feedback; the co-spatial existence of a massive, molecular cloud with the SSC supports this scenario, and we quantitatively confirm that any wind-driven feedback in Mrk 71-A is momentum-driven, rather than energy-driven. Since Mrk 71-A is a candidate Lyman continuum emitter, this implies that energy-driven superwinds may not be a necessary condition for the escape of ionizing radiation. In addition, the detection of nebular continuum emission yields an accurate astrometric position for the Mrk 71-A. We also detect four other massive molecular clouds in this giant star-forming complex.

The star-forming complex LMC-N79 as a future rival to 30 Doradus

Within the early Universe, ‘extreme’ star formation may have been the norm rather than the exception 1,2 . Super star clusters (with masses greater than 105 solar masses) are thought to be the modern-day analogues of globular clusters, relics of a cosmic time (redshift z ≳ 2) when the Universe was filled with vigorously star-forming systems 3 . The giant H ii region 30 Doradus in the Large Magellanic Cloud is often regarded as a benchmark for studies of extreme star formation 4 . Here, we report the discovery of a massive embedded star-forming complex spanning about 500 pc in the unexplored southwest region of the Large Magellanic Cloud, which manifests itself as a younger, embedded twin of 30 Doradus. Previously known as N79, this region has a star-formation efficiency greater than that of 30 Doradus, by a factor of about 2, as measured over the past ~0.5 Myr. Moreover, at the heart of N79 lies the most luminous infrared compact source discovered with large-scale infrared surveys of the Large Magellanic Cloud and Milky Way, possibly a precursor to the central super star cluster of 30 Doradus, R136. The discovery of a nearby candidate super star cluster may provide invaluable information to understand how extreme star formation proceeds in the current and high-redshift Universe. The relatively unexplored southwestern region of the Large Magellanic Cloud is host to a massive, embedded star-forming complex that rivals the star-forming efficiency of 30 Doradus. Its most luminous object could be a super star cluster in formation.