Low Star Formation Efficiency Research Articles

A Comprehensive Metallicity Analysis of J0332−3557: Establishing a z ∼ 4 Anchor for Direct Gas Metallicity and C/O Abundance Investigations

We provide one of the most comprehensive metallicity studies at z ∼ 4 by analyzing the UV/optical Hubble Space Telescope photometry and rest-frame Very Large Telescope (VLT)/FORS2 UV and VLT/XSHOOTER optical spectra of J0332−3557, a gravitationally lensed galaxy magnified by a factor of 20. With a 5σ detection of the auroral O iii] λ1666 line, we are able to derive a direct gas metallicity estimate for our target. We find Z gas =12+log(O/H)=8.26±0.06 , which is compatible with an increase of both the gas fraction and the outflow metal loading factor from z ∼ 0 to z ∼ 4. J0332−3557 is the most metal-rich individual galaxy at z ∼ 4 for which the C/O ratio has been measured. We derive a low log(C/O) = −1.02 ± 0.2, which suggests that J0332−3557 is in the early stages of interstellar medium carbon enrichment driven mostly by massive stars. The low C/O abundance also indicates that J0332−3557 is characterized by a low star formation efficiency, higher yields of oxygen, and longer burst duration. We find that EWC III]1906,9 is as low as ∼3 Å, and the main drivers of the low EWC III]1906,9 are the higher gas metallicity and the low C/O abundance. J0332−3557 is characterized by one diffuse and two more compact regions ∼1 kpc in size. We find that the carbon emission mostly originates in the compact knots. Our study on J0332−3557 serves as an anchor for studies investigating the evolution of metallicity and C/O abundance across different redshifts.

Feedback-free starbursts at cosmic dawn. Observable predictions for JWST

We extend the analysis of a physical model within the standard cosmology that robustly predicts a high star-formation efficiency (SFE) in massive galaxies at cosmic dawn due to feedback-free starbursts (FFBs). This model implies an excess of bright galaxies at $z 10$ compared to the standard models based on the low SFE at later epochs, an excess that is indicated by JWST observations. Here we provide observable predictions of galaxy properties based on the analytic FFB scenario. These can be compared with simulations and JWST observations. We use the model to approximate the SFE as a function of redshift and mass, assuming a maximum SFE of $ max 1$ in the FFB regime. From this, we derive the evolution of the galaxy mass and luminosity functions as well as the cosmological evolution of stellar and star-formation densities. We then predict the star-formation history (SFH), galaxy sizes, outflows, gas fractions, metallicities, and dust attenuation, all as functions of mass and redshift in the FFB regime. The major distinguishing feature of the model is the occurrence of FFBs above a mass threshold that declines with redshift. The luminosities and star formation rates in bright galaxies are predicted to be in excess of extrapolations of standard empirical models and standard cosmological simulations, an excess that grows from $z 9$ to higher redshifts. The FFB phase of $ is predicted to show a characteristic SFH that fluctuates on a timescale of $ The stellar systems are compact ($ at $z 10$ and declining with $z$). The galactic gas consists of a steady wind driven by supernovae from earlier generations, with high outflow velocities FWHM low gas fractions ($<\!0.1$), low metallicities ($ and low dust attenuation UV 0.5$ at $z 10$ and declining with $z$). We make tentative comparisons with current JWST observations for initial insights, anticipating more complete and reliable datasets for detailed quantitative comparisons in the future. The FFB predictions are also offered in digital form.

WISDOM project XX. – Strong shear tearing molecular clouds apart in NGC 524

ABSTRACT Early-type galaxies (ETGs) are known to harbour dense spheroids of stars but scarce star formation (SF). Approximately a quarter of these galaxies have rich molecular gas reservoirs yet do not form stars efficiently. We study here the ETG NGC 524, with strong shear suspected to result in a smooth molecular gas disc and low star-formation efficiency (SFE). We present new spatially resolved observations of the 12CO(2-1)-emitting cold molecular gas from the Atacama Large Millimeter/sub-millimeter Array (ALMA) and of the warm ionized-gas emission lines from SITELLE at the Canada–France–Hawaii Telescope. Although constrained by the resolution of the ALMA observations (≈37 pc), we identify only 52 GMCs with radii ranging from 30 to 140 pc, a low mean molecular gas mass surface density 〈Σgas〉 ≈ 125 M⊙ pc−2 and a high mean virial parameter 〈αobs, vir〉 ≈ 5.3. We measure spatially resolved molecular gas depletion times (τdep ≡ 1/SFE) with a spatial resolution of ≈100 pc within a galactocentric distance of 1.5 kpc. The global depletion time is ≈2.0 Gyr but τdep increases towards the galaxy centre, with a maximum τdep, max ≈ 5.2 Gyr. However, no pure H ii region is identified in NGC 524 using ionized-gas emission-line ratio diagnostics, so the τdep inferred are in fact lower limits. Measuring the GMC properties and dynamical states, we conclude that shear is the dominant mechanism shaping the molecular gas properties and regulating SF in NGC 524. This is supported by analogous analyses of the GMCs in a simulated ETG similar to NGC 524.

Strong outflows and inefficient star formation in the reionization-era ultrafaint dwarf galaxy Eridanus ii

ABSTRACT We present novel constraints on the underlying galaxy formation physics (e.g. mass-loading factor, star formation history, and metal retention) at z ≳ 7 for the low-mass (M* ∼ 105 M⊙) Local Group ultrafaint dwarf galaxy (UFD) Eridanus ii (Eri ii). Using a hierarchical Bayesian framework, we apply a one-zone chemical evolution model to Eri ii’s CaHK-based photometric metallicity distribution function (MDF; [Fe/H]) and find that the evolution of Eri ii is well characterized by a short, exponentially declining star formation history ($\tau _\text{SFH}=0.39\pm _{0.13}^{0.18}$ Gyr), a low star formation efficiency ($\tau _\text{SFE}=27.56\pm _{12.92}^{25.14}$ Gyr), and a large mass-loading factor ($\eta =194.53\pm _{42.67}^{33.37}$). Our results are consistent with Eri ii forming the majority of its stars before the end of reionization. The large mass-loading factor implies strong outflows in the early history of Eri ii and is in good agreement with theoretical predictions for the mass scaling of galactic winds. It also results in the ejection of >90 per cent of the metals produced in Eri ii. We make predictions for the distribution of [Mg/Fe]–[Fe/H] in Eri ii as well as the prevalence of ultra metal-poor stars, both of which can be tested by future chemical abundance measurements. Spectroscopic follow-up of the highest metallicity stars in Eri ii ([Fe/H] > −2) will greatly improve model constraints. Our new framework can readily be applied to all UFDs throughout the Local Group, providing new insights into the underlying physics governing the evolution of the faintest galaxies in the reionization era.

Quantifying the Tension between Cosmological Models and JWST Red Candidate Massive Galaxies

We develop a Python tool to estimate the tail distribution of the number of dark matter halos beyond a mass threshold and in a given volume in a light-cone. The code is based on the extended Press–Schechter model and is computationally efficient, typically taking a few seconds on a personal laptop for a given set of cosmological parameters. The high efficiency of the code allows a quick estimation of the tension between cosmological models and the red candidate massive galaxies released by the James Webb Space Telescope, as well as scanning the theory space with the Markov Chain Monte Carlo method. As an example application, we use the tool to study the cosmological implication of the candidate galaxies presented in Labbé et al. The standard Λ cold dark matter (ΛCDM) model is well consistent with the data if the star formation efficiency can reach ∼0.3 at high redshift. For a low star formation efficiency ϵ ∼ 0.1, the ΛCDM model is disfavored at ∼2σ–3σ confidence level.

Cosmological Constraints on Neutrino Masses in Light of JWST Red and Massive Candidate Galaxies

The overabundance of the red and massive candidate galaxies observed by the James Webb Space Telescope (JWST) implies efficient structure formation or large star formation efficiency at high redshift z ∼ 10. In the scenario of a low or moderate star formation efficiency, because massive neutrinos tend to suppress the growth of structure of the universe, the JWST observation tightens the upper bound of the neutrino masses. Assuming Λ cold dark matter cosmology and a star formation efficiency ∈[0.05, 0.3] (flat prior), we perform joint analyses of Planck+JWST and Planck+BAO+JWST, and obtain improved constraints ∑m ν < 0.196 eV and ∑m ν < 0.111 eV at 95% confidence level, respectively. Based on the above assumptions, the inverted mass ordering, which implies ∑m ν ≥ 0.1eV, is excluded by Planck+BAO+JWST at 92.7% confidence level.

The ALMaQUEST Survey. XII. Dense Molecular Gas as Traced by HCN and HCO+ in Green Valley Galaxies

Abstract We present Atacama Large Millimeter/submillimeter Array (ALMA) observations of two dense gas tracers, HCN (1−0) and HCO+ (1-0) for three galaxies in the green valley and two galaxies on the star-forming main sequence with comparable molecular gas fractions as traced by the CO (1−0) emissions, selected from the ALMaQUEST survey. We investigate whether the deficit of molecular gas star formation efficiency (SFEmol) that leads to the low specific star formation rate (sSFR) in these green valley galaxies is due to a lack of dense gas (characterized by the dense gas fraction f dense) or the low star formation efficiency of dense gas (SFEdense). We find that SFEmol as traced by the CO emissions, when considering both star-forming and retired spaxels together, is tightly correlated with SFEdense and depends only weakly on f dense. The sSFR on kiloparsec scales is primarily driven by SFEmol and SFEdense, followed by the dependence on f mol, and is least correlated with f dense or the dense-gas-to-stellar mass ratio (R dense). When compared with other works in the literature, we find that our green valley sample shows lower global SFEmol and lower SFEdense while exhibiting similar dense gas fractions when compared to star-forming and starburst galaxies. We conclude that the star formation of the three green valley galaxies with a normal abundance of molecular gas is suppressed, mainly due to the reduced SFEdense rather than the lack of dense gas.

Detection of CO(1−0) Emission at the Tips of the Tidal Tail in the Antennae Galaxies

The tip of the tidal tail, resulting from an encounter between galaxies, features gas concentrations and some star-forming regions, such as tidal dwarf galaxies (TDGs). This region provides a unique laboratory for examining the star formation process in a dynamical environment distinct from that of disk galaxies. Using the Nobeyama 45 m telescope, we conducted 12CO(1−0) position-switching observations at the tips of the southern tidal tail in the Antennae galaxies. We detected CO emission not only from the two star-forming TDG candidates but also in regions with no significant star formation. Adopting a Galactic CO-to-H2 conversion factor without helium correction, the H2 gas surface density is ∼5–12 M ⊙ pc−2. In most regions, the molecular-to-atomic gas ratio is around unity (0.6–1.9), but we find a region with a high ratio with a 3σ lower limit of >7.2. The star formation efficiency (SFE) of molecular gas is notably low (<0.15 Gyr−1), indicating less active star formation than in both nearby disk galaxies (∼0.5–1.0 Gyr−1) and other TDGs previously observed. Including previous observations, the molecular gas SFEs vary widely among TDGs/tidal tails, from 10−2 to 10 Gyr−1, demonstrating significant variations in star formation activity. Potential factors contributing to the low SFE in the Antennae tail tips include extensive tides and/or the young age of the tail.

The H i-rich ultra-diffuse galaxies follow the extended Schmidt law

ABSTRACT The ${\rm H\, {\small I}}$-rich ultra-diffuse galaxies (HUDGs) offer a unique case for studies of star formation laws as they host low star formation efficiency and low-metallicity environments where gas is predominantly atomic. We collect a sample of six HUDGs in the field and investigate their location in the extended Schmidt law ($\Sigma _{\text{SFR }} \propto \left(\Sigma _{\text{star}}^{0.5} \Sigma _{\text{gas}}\right)^{1.09}$). They are consistent with this relationship well (with deviations of only 1.1σ). Furthermore, we find that HUDGs follow the tight correlation between the hydrostatic pressure in the galaxy mid-plane and the quantity on the x-axis ($\rm log(\Sigma _{star}^{0.5}\Sigma _{gas})$) of the extended Schmidt law. This result indicates that these HUDGs can be self-regulated systems that reach the dynamical and thermal equilibrium. In this framework, the stellar gravity compresses the disc vertically and counteracts the gas pressure in the galaxy mid-plane to regulate the star formation as suggested by some theoretical models.

H i content of selected mid-infrared bright, starburst blue compact dwarf galaxies

ABSTRACT We report measurements of H i content in 11 nearby, actively star-forming, blue compact dwarf galaxies (BCDs) from 21 cm observations with the Arecibo telescope. These BCDs, selected by their red (W2[4.6 $\mu$m]−W3[12 $\mu$m]&amp;gt;3.8 mag) and bright mid-infrared (MIR) emission (W4[22 $\mu$m]&amp;lt;7.6 mag), have high specific star formation rates (median sSFR ∼10−7.8 yr−1), similar to high redshift galaxies. H i emission was detected in six sources. We analyse our new detections in the context of previous H i observations of 218 dwarf irregulars (dIs) and BCDs in the literature. The MH i–M* relation resulting from our observations confirms the dominating fraction of H i gas among baryons in galaxies with lower stellar masses. This Arecibo BCD sample has significantly lower median H i depletion time-scales (τH i ∼ 0.3 Gyr) than other dIs/BCDs (∼ 6.3 Gyr) in the literature. The majority of the sources (10/11) in the Arecibo sample are very red in W1[3.4 $\mu$m]−W2[4.6 $\mu$m] colour (&amp;gt;0.8 mag) implying the presence of warm dust. We investigate the relation of τHI with stellar mass (M*) and sSFR. We find that τH i is significantly anticorrelated with M* for higher sSFR (&amp;gt;10−8.5 yr−1) and with sSFR for higher stellar mass ($\gt 10^{7.5}\, {\rm M}_{\odot }$) dwarf galaxies. The high sSFR for the BCDs in the Arecibo observed sample is mainly due to their high atomic gas star formation efficiency (SFE) or low τH i. The low τH i or high SFE in these sources is possibly due to runaway star formation in compact and dense super star clusters.

CO-CHANGES – I. IRAM 30-m CO observations of molecular gas in the sombrero galaxy

ABSTRACT Molecular gas plays a critical role in explaining the quiescence of star formation (SF) in massive isolated spiral galaxies, which could be a result of either the low molecular gas content and/or the low SF efficiency. We present IRAM 30-m observations of the CO lines in the Sombrero galaxy (NGC 4594), the most massive spiral at $d\lesssim 30\rm ~Mpc$. We detect at least one of the three CO lines covered by our observations in all 13 observed positions located at the galactic nucleus and along an $\sim 25\rm ~kpc$-diameter dusty ring. The total extrapolated molecular gas mass of the galaxy is $M_{\rm H_2}\approx 4\times 10^{8}\rm ~M_\odot$. The measured maximum CO gas rotation velocity of $\approx 381\rm ~km~s^{-1}$ suggests that NGC 4594 locates in a dark matter halo with a mass $M_{\rm 200}\gtrsim 10^{13}\rm ~M_\odot$. Comparing to other galaxy samples, NGC 4594 is extremely gas poor and SF inactive, but the SF efficiency is apparently not inconsistent with that predicted by the Kennicutt–Schmidt law, so there is no evidence of enhanced SF quenching in this extremely massive spiral with a huge bulge. We also calculate the predicted gas supply rate from various sources to replenish the cold gas consumed in SF, and find that the galaxy must experience a starburst stage at high redshift, then the leftover or recycled gas provides SF fuels to maintain the gradual growth of the galactic disc at a gentle rate.

The extended ‘stellar halo’ of the Ursa Minor dwarf galaxy

ABSTRACT Stellar candidates in the Ursa Minor (UMi) dwarf galaxy have been found using a new Bayesian algorithm applied to Gaia EDR3 data. Five of these targets are located in the extreme outskirts of UMi, from ∼5 to 12 elliptical half-light radii (rh), where rh(UMi) = 17.32 ± 0.11 arcmin, and have been observed with the high-resolution Gemini Remote Access to CFHT ESPaDOnS Spectrograph at the Gemini North telescope. Precise radial velocities (σRV &amp;lt; 2 km s−1) and metallicities ($\sigma _{\rm {{\rm [Fe/H]}}}\ \lt\ 0.2$ dex) confirm their memberships of UMi. Detailed analysis of the brightest and outermost star (Target 1, at ∼12rh), yields precision chemical abundances for the α (Mg, Ca, and Ti), odd-Z (Na, K, and Sc), Fe-peak (Fe, Ni, and Cr), and neutron-capture (Ba) elements. With data from the literature and APOGEE data release 17, we find the chemical patterns in UMi are consistent with an outside-in star-formation history that includes yields from core-collapse supernovae, asymptotic giant branch stars, and Type Ia supernovae. Evidence for a knee in the [α/Fe] ratios near [Fe/H] ∼ −2.1 indicates a low star-formation efficiency similar to that in other dwarf galaxies. Detailed analysis of the surface number density profile shows evidence that UMi’s outskirts have been populated by tidal effects, likely as a result of completing multiple orbits around the Galaxy.

Global survey of star clusters in the Milky Way

Aims. We built Galactic open star cluster mass functions (CMFs) for different age sub-samples and spatial locations in the wider solar neighbourhood. Here, we present a simple cluster formation and evolution model to reproduce the main features of the CMFs. Methods. We used an unbiased working sample of 2227 clusters of the Milky Way Star Cluster (MWSC) catalogue, which occupy the heliocentric cylinders with magnitude-dependent completeness radii of 1–5 kpc. The MWSC survey provides an extended set of open star cluster parameters, including tidal radii, distances, and ages. From an analytic three-component Galaxy model, we derived tidal masses of clusters with a typical accuracy of about 70%. Our simple model includes a two-section cluster initial mass function, constant cluster formation rate, supervirial phase after a sudden expulsion of the remaining gas, and cluster mass loss due to stellar evolution and the clusters’ gradual destruction in the Galactic tidal field. The dynamical evolution model is based on previous N-body simulations. Results. The obtained tidal masses have been added to the MWSC catalogue. A general CMF (GCMF), built for all cluster ages around the Sun, has a bell-like shape and extends over four decades in mass. The high-mass slope found for tidal mass log mt/M⊙ ≥ 2.3 is equal to 1.14 ± 0.07. The CMFs for different age groups show the same high-mass slopes, while the low-mass slope is nearly flat for the youngest sub-sample (clusters younger than 20 Myr) and about −0.7 for the others. The inner and outer sub-samples covering Galactocentric radii R = 4.2–8.1 kpc and 8.9–13.5 kpc, respectively, are consistent with the GCMF, once the exponential decline of the Galactic disc density is taken into account. The model suggests star formation with low efficiency of 15–20%, where only 10% of stars remain bound in a cluster after gas expulsion and subsequent violent relaxation. The cluster formation rate required to reproduce the observed distributions in age and mass is about 0.4 M⊙ pc−2 Gyr−1. Conclusions. The obtained high-mass slope of the GCMF for the wide neighbourhood of the Sun is similar to slopes determined earlier in nearby galaxies for more luminous clusters with log m/M⊙ &gt; 3.8. The MWSC catalogue supports models with a low star-formation efficiency, where 90% of stars are lost quickly after gas expulsion. The obtained cluster formation rate corresponds to open clusters’ contribution to the stellar content of the thin disc at the level of 30%.

Early evolution and three-dimensional structure of embedded star clusters

ABSTRACT We perform simulations of star cluster formation to investigate the morphological evolution of embedded star clusters in the earliest stages of their evolution. We conduct our simulations with Torch, which uses the Amuse framework to couple state-of-the-art stellar dynamics to star formation, radiation, stellar winds, and hydrodynamics in Flash. We simulate a suite of 104 M⊙ clouds at 0.0683 pc resolution for ∼2 Myr after the onset of star formation, with virial parameters α = 0.8, 2.0, 4.0 and different random samplings of the stellar initial mass function and prescriptions for primordial binaries. Our simulations result in a population of embedded clusters with realistic morphologies (sizes, densities, and ellipticities) that reproduce the known trend of clouds with higher initial α having lower star formation efficiencies. Our key results are as follows: (1) Cluster mass growth is not monotonic, and clusters can lose up to half of their mass while they are embedded. (2) Cluster morphology is not correlated with cluster mass and changes over ∼0.01 Myr time-scales. (3) The morphology of an embedded cluster is not indicative of its long-term evolution but only of its recent history: radius and ellipticity increase sharply when a cluster accretes stars. (4) The dynamical evolution of very young embedded clusters with masses ≲1000 M⊙ is dominated by the overall gravitational potential of the star-forming region rather than by internal dynamical processes such as two- or few-body relaxation.

Statistical Study of the Star Formation Efficiency in Bars: Is Star Formation Suppressed in Gas-rich Bars?

The dependence of the star formation efficiency (SFE) on galactic structures—especially whether the SFE in the bar region is lower than those in other regions—has recently been debated. We report the SFEs of 18 nearby gas-rich massive star-forming barred galaxies with large apparent bar major axes (≧75″). We statistically measure the SFE by distinguishing the center, the bar end, and the bar regions for the first time. The molecular gas surface density is derived from archival CO(1–0) and/or CO(2–1) data by assuming a constant CO-to-H2 conversion factor (α CO), and the star formation rate surface density is derived from a linear combination of far-UV and mid-IR intensities. The angular resolution is 15″, which corresponds to 0.3–1.8 kpc. We find that the ratio of the SFE in the bar to that in the disk was systematically lower than unity (typically 0.6–0.8), which means that the star formation in the bar is systematically suppressed. Our results are inconsistent with similar recent statistical studies, which have reported that the SFE tends to be independent of galactic structures. This inconsistency can be attributed to the differences in the definitions of the bar region, the spatial resolutions, the α CO, and the sample galaxies. Furthermore, we find a negative correlation between the SFE and the velocity width of the CO spectrum, which is consistent with the idea that the large dynamical effects—such as strong shocks, large shears, and fast cloud–cloud collisions caused by the noncircular motion of the bar—result in a low SFE.

Ultra-diffuse Galaxies as Extreme Star-forming Environments. I. Mapping Star Formation in H i-rich UDGs

Ultra-diffuse galaxies (UDGs) are both extreme products of galaxy evolution and extreme environments in which to test our understanding of star formation. In this work, we contrast the spatially resolved star formation activity of a sample of 22 H i-selected UDGs and 35 low-mass galaxies from the NASA Sloan Atlas (NSA) catalog within 120 Mpc. We employ a new joint spectral energy distribution fitting method to compute star formation rate and stellar mass surface density maps that leverage the high spatial resolution optical imaging data of the Hyper Suprime-Cam Subaru Strategic Program and the UV coverage of the Galaxy Evolution Explorer, along with H i radial profiles estimated from a subset of galaxies that have spatially resolved H i maps. We find that UDGs have low star formation efficiencies as a function of their atomic gas down to scales of 500 pc. We additionally find that the stellar mass-weighted sizes of our UDG sample are unremarkable when considered as a function of their H i mass—their stellar sizes are comparable to NSA dwarfs at fixed H i mass. This is a natural result in the picture where UDGs are forming stars normally, but at low efficiencies. We compare our results to predictions from contemporary models of galaxy formation, and find in particular that our observations are difficult to reproduce in models where UDGs undergo stellar expansion due to vigorous star formation feedback should bursty star formation be required down to z = 0.

Hyper-Eddington black hole growth in star-forming molecular clouds and galactic nuclei: can it happen?

ABSTRACT Formation of supermassive black holes (BHs) remains a theoretical challenge. In many models, especially beginning from stellar relic ‘seeds,’ this requires sustained super-Eddington accretion. While studies have shown BHs can violate the Eddington limit on accretion disc scales given sufficient ‘fuelling’ from larger scales, what remains unclear is whether or not BHs can actually capture sufficient gas from their surrounding interstellar medium (ISM). We explore this in a suite of multiphysics high-resolution simulations of BH growth in magnetized, star-forming dense gas complexes including dynamical stellar feedback from radiation, stellar mass-loss, and supernovae, exploring populations of seeds with masses $\sim 1\!-\!10^{4}\, \mathrm{M}_{\odot }$. In this initial study, we neglect feedback from the BHs: so this sets a strong upper limit to the accretion rates seeds can sustain. We show that stellar feedback plays a key role. Complexes with gravitational pressure/surface density below $\sim 10^{3}\, \mathrm{M}_{\odot }\, {\rm pc^{-2}}$ are disrupted with low star formation efficiencies so provide poor environments for BH growth. But in denser cloud complexes, early stellar feedback does not rapidly destroy the clouds but does generate strong shocks and dense clumps, allowing $\sim 1{{\ \rm per\ cent}}$ of randomly initialized seeds to encounter a dense clump with low relative velocity and produce runaway, hyper-Eddington accretion (growing by orders of magnitude). Remarkably, mass growth under these conditions is almost independent of initial BH mass, allowing rapid intermediate-mass black hole (IMBH) formation even for stellar-mass seeds. This defines a necessary (but perhaps not sufficient) set of criteria for runaway BH growth: we provide analytic estimates for the probability of runaway growth under different ISM conditions.

Ultra-diffuse Galaxies as Extreme Star-forming Environments. II. Star Formation and Pressure Balance in H i-rich UDGs

In addition to occupying the extreme, diffuse tail of the dwarf galaxy population, ultra-diffuse galaxies (UDGs) are themselves a key laboratory in which to study star formation in extreme low-density environments. In the second paper of this series, we compare the spatially resolved star formation activity of 22 H i-selected UDGs and 21 “normal” dwarf galaxies within 120 Mpc to predictions within the pressure-regulated, feedback-modulated (PRFM) theory of star formation. To do so, we employ a joint spectral energy distribution fitting method that allows us to estimate star formation rate and stellar mass surface density from UV-optical imaging. We find that the PRFM framework extends successfully to the UDG regime—although the UDGs in our sample show unusually low star formation rate surface densities given their H i content, this low star formation efficiency can be naturally explained by the diffuse structure of the UDGs. In fact, when cast in the PRFM framework, the relationship between midplane pressure and star formation in the UDG sample is in good agreement not only with the “normal” dwarf reference sample, but also with measurements from more massive galaxies. Our results suggest that despite their low star formation efficiencies, the H i-rich UDGs need not be forming stars in an exotic manner. We also find that the UDGs are likely H2 poor compared even to the overall dwarf population.

Accelerating Early Massive Galaxy Formation with Primordial Black Holes

Recent observations with JWST have identified several bright galaxy candidates at z ≳ 10, some of which appear unusually massive (up to ∼1011 M ⊙). Such early formation of massive galaxies is difficult to reconcile with standard ΛCDM predictions, demanding a very high star formation efficiency (SFE), possibly even in excess of the cosmic baryon mass budget in collapsed structures. With an idealized analysis based on linear perturbation theory and the Press–Schechter formalism, we show that the observed massive galaxy candidates can be explained with lower SFE than required in ΛCDM if structure formation is accelerated/seeded by massive (≳109 M ⊙) primordial black holes (PBHs) that make a up a small fraction (∼10−6–10−3) of dark matter, considering existing empirical constraints on PBH parameters. We also discuss the potential observational signatures of PBH cosmologies in the JWST era. More work needs to be done to fully evaluate the viability of such PBH models to explain observations of the high-z Universe.

Star cluster formation in clouds with externally driven turbulence

ABSTRACT Star clusters are known to be formed in turbulent molecular clouds. How turbulence is driven in molecular clouds and what effect this has on star formation is still unclear. We compare a simulation setup with turbulent driving everywhere in a periodic box with a setup where turbulence is only driven around the outside of the box. We analyse the resulting gas distribution, kinematics, and the population of stars that are formed from the cloud. Both setups successfully produce a turbulent velocity field with a power-law structure function, the externally driven cloud has a more central, monolithic, clump, while the fully driven cloud has many smaller, more dispersed, clumps. The star formation follows the cloud morphology producing large clusters, with high star-forming efficiency in the externally driven simulations and sparse individual star formation with much lower star formation efficiency in the fully driven case. We conclude that the externally driven method, which resembles a Global Hierarchical Collapse (GHC) scenario, produces star clusters that more closely match with observations.