High Star Formation Efficiency Research Articles

Witnessing an extreme, highly efficient galaxy formation mode with resolved Lyman-α and Lyman-continuum emission

J1316+2614 at z = 3.613 is the UV-brightest (MUV = −24.7) and strongest Lyman continuum-emitting (fescLyC ≈ 90%) star-forming galaxy known; it also shows signatures of inflowing gas from its blue-dominated Lyα profile. We present high-resolution imaging with the Hubble Space Telescope (HST) and the Very Large Telescope (VLT) of the LyC, Lyα, rest-UV, and optical emission of J1316+2614. Detailed analysis of the LyC and UV light distributions reveals compact yet resolved profiles, with LyC and UV morphologies showing identical half-light radii of reff ≃ 220 pc. The continuum-subtracted Lyα emission, obtained with the HST ramp-filter FR551N, reveals an extended filamentary structure of ≃6.0 kpc oriented south to north with only residual flux within the stellar core, suggesting a Lyα ‘hole’. Our spectral energy distribution analysis shows that J1316+2614 is characterised by a young (5.7 ± 1.0 Myr), nearly un-obscured stellar population with a high star-formation rate (SFR = 898 ± 181 M⊙ yr−1) and a stellar mass of M⋆young = (4.8 ± 0.3) × 109 M⊙. Additionally, the spectral energy distribution analysis supports the absence of an underlying old stellar population (M⋆old ≤ 2.8 × 109 M⊙, 3σ). J1316+2614 presents remarkably high SFR and stellar mass surface densities of log(Σ SFR[M⊙ yr−1 kpc−2]) = 3.47 ± 0.11 and log(ΣM⋆[M⊙pc−2]) = 4.20 ± 0.06, respectively, which are among the highest observed in star-forming galaxies and are more typically observed in local young massive star clusters and globular clusters. Our findings indicate that J1316+2614 is a powerful, young, and compact starburst that is leaking a significant amount of LyC photons due to a lack of gas and dust within the starburst. We explored the conditions for gas expulsion using a simple energetic balance and find that, given the strong binding force in J1316+2614, a high star-formation efficiency (ϵSF ≥ 0.7) is necessary to explain the removal of gas and its exposed nature. Our results thus suggest a close link between high ϵSF and high fescLyC. This high efficiency can also naturally explain the remarkably high SFR, UV luminosity, and efficient mass growth of J1316+2614, which acquired at least 62% of its mass in the last 6 Myr. J1316+2614 may exemplify an intense, feedback-free starburst with a high ϵSF, similar to those proposed for UV-bright galaxies at high redshifts.

Chemical evolution of a young super star cluster at the Sunburst Arc

ABSTRACT Recent observations of high-redshift galaxies have revealed starburst galaxies with excessive amounts of nitrogen, well above that expected in standard evolutionary models. The Sunburst Arc galaxy, particularly its young and massive star cluster, represents the closest ($z=2.4$) and brightest of these as a strongly lensed object. In this work, we study the chemical history of this star cluster to determine the origin of the elevated gas-phase nitrogen using a chemical evolution model. Our model includes the enrichment of OB stars through stellar winds and core-collapse supernovae assuming that massive stars ($M\gt 25$ $\mathrm{ M}_\odot$) collapse directly into black holes at the end of their lives. We fit the model parameters to the observed chemical abundances of the Sunburst Arc cluster: O/H, C/O, and N/O. We find that the observed chemical abundances can be explained by models featuring intense star formation events, characterized by rapid gas accretion and high star formation efficiencies. Additionally, the stellar population contributing to the gas enrichment must exclude Wolf–Rayet stars. These conditions might be present in other nitrogen-rich objects as their similar chemical abundances suggest a common history. As previous studies have proposed the presence of Wolf–Rayet stars in the new nitrogen-rich objects, further research using chemodynamic modeling is necessary to ascertain the true nature of these objects.

Star Formation by Supernova Implosion

Recent hydrodynamical simulations of the late stages of supernova remnant (SNR) evolution have revealed that as they merge with the ambient medium, SNRs implode, leading to the formation of dense clouds in their center. While being highly chemically enriched by their host SNR, these clouds appear to have similar properties as giant molecular clouds, which are believed to be the main site of star formation. Here, we develop a simple model in order to estimate the efficiency of the star formation that might be triggered by the implosion of SNRs. We separately consider two cases: cyclic star formation, maintained by the episodic driving of feedback from new generations of stars, and a single burst of star formation, triggered by a single explosion. We find that in the cyclic case, star formation is inefficient, with implosion-triggered star formation contributing a few percent of the observed star formation efficiency per freefall timescale. In the single-burst case, higher star formation efficiencies can be obtained. However, while the implosion-triggered process might not contribute much to the overall star formation, due to the high chemical enrichment of the birth clouds, it can explain the formation of a significant fraction of metal-rich stars.

Spectroscopic Confirmation of an Ultra-Massive Galaxy in a Protocluster at z ∼ 4.9

We present spectroscopic confirmation of an ultra-massive galaxy (UMG) with log(M⋆/M⊙)=10.98±0.07 at zspec=4.8947 in the Extended Groth Strip (EGS), based on deep observations of Ly α emission with Keck/DEIMOS. The ultra-massive galaxy (UMG-28740) is the most massive member in one of the most significant overdensities in the EGS, with four additional photometric members with log(M⋆/M⊙)>10.5 within Rproj∼1 cMpc. Spectral energy distribution (SED) fitting using a large suite of star formation histories and two sets of high-quality photometry from ground- and space-based facilities consistently estimates the mass of this object to be log(M⋆/M⊙)∼11 with a small standard deviation between measurements ( σ=0.07). While the best-fit SED models agree on stellar mass, we find discrepancies in the estimated star formation rate for UMG-28740, resulting in either a star-forming or quiescent system. 𝐽𝑊𝑆𝑇/NIRCam photometry of UMG-28740 strongly favors a quiescent scenario, demonstrating the need for high-quality mid-IR observations. Assuming the galaxy to be quiescent, UMG-28740 formed the bulk of its stars at z>10 and is quenching at z∼8, resulting in a high star formation efficiency at high redshift ( ϵ∼0.2 at z∼5 and ϵ≳1 at z≳8). As the most massive galaxy in its protocluster environment, UMG-28740 is a unique example of the impossibly early galaxy problem.

Understanding stellar populations in thin and thick discs of edge-on galaxies with MUSE – I. The case of the reignited S0 galaxy ESO 544-27

ABSTRACT Edge-on galaxies act as the best laboratories to understand the origin of thin and thick discs in galaxies. Measurement of spatially resolved stellar population properties in such galaxies, particularly age, metallicity, and [α/Fe], are crucial to understanding the formation and evolution of disc galaxies. Such measurements are made possible from stellar population model fits to deep integral field spectroscopic (IFU) observations of resolved galaxies. We utilize archival MUSE IFU observations of the edge-on galaxy ESO 544-27 to uncover the formation history of its thin and thick discs through its stellar populations. We find the thin disc of the galaxy is dominated by an old (>9 Gyr) low [α/Fe] metal-rich stellar population. Its outer thick disc is dominated by an old (>9 Gyr) high [α/Fe] metal-rich component that should have formed with higher star formation efficiency than the Milky Way thick disc. We thus find [α/Fe] dichotomy in ESO 544-27 with its thin and thick discs dominated by low and high [α/Fe] stellar populations, respectively. However, we also find a metal-rich younger (<2 Gyr old) stellar population in ESO 544-27. The galaxy was nearly quenched until its star formation was reignited recently first in the outer and inner thick disc (∼1 Gyr ago) and then in the thin disc (∼600 Myr ago). We thus find that both the low [α/Fe] thin and high [α/Fe] thick discs of ESO 544-27 are inhabited primarily by similarly old metal-rich stellar populations, a contrast to that of other galaxies with known thin and thick disc stellar population properties.

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.

The Interplay between the Initial Mass Function and Star Formation Efficiency through Radiative Feedback at High Stellar Surface Densities

The observed rest-UV luminosity function at cosmic dawn (z ∼ 8–14) measured by JWST revealed an excess of UV-luminous galaxies relative to many prelaunch theoretical predictions. A high star formation efficiency (SFE) and a top-heavy initial mass function (IMF) are among the mechanisms proposed for explaining this excess. Although a top-heavy IMF has been proposed for its ability to increase the light-to-mass ratio (ΨUV), the resulting enhanced radiative pressure from young stars could decrease the SFE, potentially driving galaxy luminosities back down. In this Letter, we use idealized radiation hydrodynamic simulations of star cluster formation to explore the effects of a top-heavy IMF on the SFE of clouds typical of the high-pressure conditions found at these redshifts. We find that the SFE in star clusters with solar-neighborhood-like dust abundance decreases with increasingly top-heavy IMFs—by ∼20% for an increase of a factor of 4 in ΨUV and by 50% for a factor of ∼10 in ΨUV. However, we find that an expected decrease in the dust-to-gas ratio (∼0.01 × solar) at these redshifts can completely compensate for the enhanced light output. This leads to a (cloud-scale; ∼10 pc) SFE that is ≳70% even for a factor of 10 increase in ΨUV, implying that highly efficient star formation is unavoidable for high surface density and low-metallicity conditions. Our results suggest that a top-heavy IMF, if present, likely coexists with efficient star formation in these galaxies.

Physical Properties of Hyperluminous, Dust-obscured Quasars at z ∼ 3: Multiwavelength Spectral Energy Distribution Analysis and Cold Gas Content Revealed by ALMA

We present a UV to millimeter spectral energy distribution (SED) analysis of 16 hyperluminous, dust-obscured quasars at z ∼ 3, selected by the Wide-field Infrared Survey Explorer. We aim to investigate the physical properties of these quasars, with a focus on their molecular gas content. We decompose the SEDs into three components: stellar, cold dust, and active galactic nucleus (AGN). By doing so, we are able to derive and analyze the relevant properties of each component. We determine the molecular gas mass from CO line emission based on Atacama Large Millimeter/submillimeter Array (ALMA) observations. By including ALMA observations in the multiwavelength SED analysis, we derive the molecular gas fractions, gas depletion timescales, and star formation efficiencies (SFEs). Their sample median and 16th–84th quartile ranges are fgas∼0.33−0.17+0.33 , t depl ∼ 39−28+85 Myr, and SFE ∼ 297−195+659 K km s−1 pc−2, respectively. Compared to main-sequence galaxies, they have lower molecular gas content and higher SFEs, similar to quasars in the literature. This suggests that the gas in these quasars is rapidly depleted, likely as the result of intense starburst activity and AGN feedback. The observed correlations between these properties and the AGN luminosities further support this scenario. Additionally, we infer the black hole to stellar mass ratio and black hole mass growth rate, which indicate significant central black hole mass assembly over short timescales. Our results are consistent with the scenario that our sample represents a short transition phase toward unobscured quasars.

Pure Spectroscopic Constraints on UV Luminosity Functions and Cosmic Star Formation History from 25 Galaxies at z spec = 8.61–13.20 Confirmed with JWST/NIRSpec

We present pure spectroscopic constraints on the UV luminosity functions and cosmic star formation rate (SFR) densities from 25 galaxies at z spec = 8.61–13.20. By reducing the JWST/NIRSpec spectra taken in multiple programs of Early Release Observation, Early Release Science, General Observer, and Director’s Discretionary Time observations with our analysis technique, we independently confirm 16 galaxies at z spec = 8.61–11.40, including new redshift determinations, and a bright interloper at z spec = 4.91 that was claimed as a photometric candidate at z ∼ 16. In conjunction with nine galaxies at redshifts up to z spec = 13.20 in the literature, we make a sample of 25 spectroscopically confirmed galaxies in total and carefully derive the best estimates and lower limits of the UV luminosity functions. These UV luminosity function constraints are consistent with the previous photometric estimates within the uncertainties and indicate mild redshift evolution toward z ∼ 12, showing tensions with some theoretical models of rapid evolution. With these spectroscopic constraints, we obtain firm lower limits of the cosmic SFR densities and spectroscopically confirm a high SFR density at z ∼ 12 beyond the constant star formation efficiency models, which supports earlier claims from the photometric studies. While there are no spectroscopically confirmed galaxies with very large stellar masses violating the ΛCDM model due to the removal of the bright interloper, we confirm star-forming galaxies at z spec = 11–13 with stellar masses much higher than model predictions. Our results indicate possibilities of high star formation efficiency (>5%), a hidden active galactic nucleus, a top-heavy initial mass function (possibly with Population III), and large scatter/variance. Having these successful and unsuccessful spectroscopy results, we suggest observational strategies for efficiently removing low-redshift interlopers for future JWST programs.

Implications of the Stellar Mass Density of High-z Massive Galaxies from JWST on Warm Dark Matter

A significant excess of the stellar mass density at high redshift has been discovered from the early data release of James Webb Space Telescope (JWST), and it may require a high star formation efficiency. However, this will lead to large number density of ionizing photons in the epoch of reionization (EoR), so that the reionization history will be changed, which can arise tension with the current EoR observations. Warm dark matter (WDM), via the free streaming effect, can suppress the formation of small-scale structure as well as low-mass galaxies. This provides an effective way to decrease the ionizing photons when considering a large star formation efficiency in high-z massive galaxies without altering the cosmic reionization history. On the other hand, the constraints on the properties of WDM can be derived from the JWST observations. In this work, we study WDM as a possible solution to reconcile the JWST stellar mass density of high-z massive galaxies and reionization history. We find that, the JWST high-z comoving cumulative stellar mass density alone has no significant preference for either CDM or WDM model. But using the observational data of other stellar mass density measurements and reionization history, we obtain that the WDM particle mass with keV and star formation efficiency parameter in 2σ confidence level can match both the JWST high-z comoving cumulative stellar mass density and the reionization history.

JWST Constraints on the UV Luminosity Density at Cosmic Dawn: Implications for 21 cm Cosmology

An unprecedented array of new observational capabilities are starting to yield key constraints on models of the epoch of first light in the Universe. In this Letter we discuss the implications of the UV radiation background at cosmic dawn inferred by recent JWST observations for radio experiments aimed at detecting the redshifted 21 cm hyperfine transition of diffuse neutral hydrogen. Under the basic assumption that the 21 cm signal is activated by the Lyα photon field produced by metal-poor stellar systems, we show that a detection at the low frequencies of the EDGES and SARAS3 experiments may be expected from a simple extrapolation of the declining UV luminosity density inferred at z ≲ 14 from JWST early galaxy data. Accounting for an early radiation excess above the cosmic microwave background suggests a shallower or flat evolution to simultaneously reproduce low- and high-z current UV luminosity density constraints, which cannot be entirely ruled out, given the large uncertainties from cosmic variance and the faint-end slope of the galaxy luminosity function at cosmic dawn. Our findings raise the intriguing possibility that a high star formation efficiency at early times may trigger the onset of intense Lyα emission at redshift z ≲ 20 and produce a cosmic 21 cm absorption signal 200 Myr after the Big Bang.

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]>3.8 mag) and bright mid-infrared (MIR) emission (W4[22 $\mu$m]<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 (>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 (>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.

The IR Compactness of Dusty Galaxies Sets Star Formation and Dust Properties at z ∼ 0–2

The surface densities of gas, dust, and stars provide a window into the physics of star formation that, until the advent of high-resolution far-IR/submillimeter observations, has been historically difficult to assess among dusty galaxies. To study the link between IR surface densities and dust properties, we leverage the Atacama Large Millimetre/Submillimetre Array archive to measure the extent of cold dust emission in 15 z ∼ 2 IR-selected galaxies selected on the basis of having available mid-IR spectroscopy from Spitzer. We use the mid-IR spectra to constrain the relative balance between dust heating from star formation and active galactic nuclei (AGNs), and to measure emission from polycylic aromatic hydrocarbons (PAHs), small dust grains that play a key role in the photoelectric heating of gas. In general, we find that dust-obscured star formation at high IR surface densities exhibits similar properties at low and high redshift, namely, local luminous IR galaxies (LIRGs) have comparable PAH luminosity to total dust mass ratios as high-z galaxies, and star formation at z ∼ 0–2 is more efficient at high IR surface densities despite the fact that our sample of high-z galaxies is closer to the main sequence than local LIRGs. High star formation efficiencies are coincident with a decline in the PAH-to-IR luminosity ratio reminiscent of the deficit observed in far-IR fine-structure lines. Changes in the gas and dust conditions arising from high star formation surface densities might help drive the star formation efficiency up. This could help explain the high efficiencies needed to reconcile star formation and gas volume densities in dusty galaxies at cosmic noon.

The inefficiency of stellar feedback in driving galactic outflows in massive galaxies at high redshift

ABSTRACT Recent observations indicate that galactic outflows are ubiquitous in high-redshift (high-z) galaxies, including normal star-forming galaxies, quasar hosts, and dusty star-forming galaxies (DSFGs). However, the impact of outflows on the evolution of their hosts is still an open question. Here, we analyse the star-formation histories and galactic outflow properties of galaxies in massive haloes ($10^{12}\, {\rm M}_{\odot }\ \lt\ M_{\rm vir}\ \lt\ 5\times 10^{12}\, {\rm M}_{\odot }$) at z ≳ 5.5 in three zoom-in cosmological simulations from the MassiveFIRE suite, as part of the Feedback In Realistic Environments (FIRE) project. The simulations were run with the FIRE-2 model, which does not include feedback from active galactic nuclei. The simulated galaxies resemble z > 4 DSFGs, with star-formation rates of $\sim\!{1000}\ {\rm M}_{\odot }\, \rm yr^{-1}$ and molecular gas masses of Mmol ∼ 1010 M⊙. However, the simulated galaxies are characterized by higher circular velocities than those observed in high-z DSFGs. The mass loading factors from stellar feedback are of the order of ∼0.1, implying that stellar feedback is inefficient in driving galactic outflows and gas is consumed by star formation on much shorter time-scales than it is expelled from the interstellar medium. We also find that stellar feedback is highly inefficient in self-regulating star formation in this regime, with an average integrated star formation efficiency (SFE) per dynamical time of 30 per cent. Finally, compared with FIRE-2 galaxies hosted in similarly massive haloes at lower redshift, we find lower mass loading factors and higher SFEs in the high-z sample. We argue that both effects originate from the higher total and gas surface densities that characterize high-z massive systems.

Primordial power spectrum in light of JWST observations of high redshift galaxies

ABSTRACT Early data releases of JWST have revealed several high redshift massive galaxy candidates by photometry, and some of them have been confirmed spectroscopically. We study their implications on the primordial power spectrum. In the first part, we use the CEERS photometric survey data, along with respective spectroscopic updates, to compute the cumulative comoving stellar mass density. We find that a very high star formation efficiency (unlikely in various theoretical scenarios) is required to explain these observations within Lambda cold dark matter (ΛCDM) cosmology. We show that the tension can be eased if the primordial power spectrum has a blue tilt. In the second part, we study spectroscopically confirmed galaxies reported in the JADES survey to investigate their implications on a red-tilted primordial power spectrum. We estimate the star formation efficiency from an earlier observation at similar redshift by Spitzer, and find that the star formation efficiency is an order of magnitude smaller than required to explain the CEERS photometric observations mentioned earlier. Using the estimated star formation efficiency, we find the strongest constraints on the red tilt of the power spectrum over some scales. Our study shows that JWST will be an excellent probe of the power spectrum and can lead to novel discoveries.

Atomic Hydrogen Scaling Relations at z ≈ 0.35

The atomic hydrogen (H i) properties of star-forming galaxies in the local universe are known to correlate with other galaxy properties via the “H i scaling relations.” The redshift evolution of these relations serves as an important constraint on models of galaxy evolution. However, until recently, there were no estimates of the H i scaling relations at cosmological distances. Using data from a deep Giant Metrewave Radio Telescope H i 21 cm survey of the Extended Groth Strip, and the technique of spectral-line stacking, we determine the scaling relation between the H i mass and the stellar mass for star-forming galaxies at z ≈ 0.35. We use this measurement, along with the main-sequence relation in galaxies, to infer the dependence of the H i depletion timescale of these galaxies on their stellar mass. We find that massive star-forming galaxies at z ≈ 0.35, with stellar mass M * ≳ 109.5 M ⊙, are H i-poor compared to local star-forming galaxies of a similar stellar mass. However, their characteristic H i depletion time is lower by a factor of ≈5 than that of their local analogs, indicating a higher star formation efficiency at intermediate redshifts (similar to that at z ≈ 1). While our results are based on a relatively small cosmic volume and could thus be affected by cosmic variance, the short characteristic H i depletion timescales (≲3 Gyr) of massive star-forming galaxies at z ≈ 0.35 indicate that they must have acquired a significant amount of neutral gas through accretion from the circumgalactic medium over the past 4 Gyr, to avoid quenching of their star formation activity.

Efficient formation of massive galaxies at cosmic dawn by feedback-free starbursts

ABSTRACT JWST observations indicate a surprising excess of luminous galaxies at z ∼ 10 and above, consistent with efficient conversion of the accreted gas into stars, unlike the suppression of star formation by feedback at later times. We show that the high densities and low metallicities at this epoch guarantee a high star formation efficiency (SFE) in the most massive dark-matter haloes. Feedback-free starbursts (FFBs) occur when the free-fall time is shorter than ∼$1\, {\rm Myr}$, below the time for low-metallicity massive stars to develop winds and supernovae. This corresponds to a characteristic density of ∼$3\!\times \!10^3\, {\rm cm}^{-3}$. A comparable threshold density permits a starburst by allowing cooling to star-forming temperatures in a free-fall time. The galaxies within ∼1011 M⊙ haloes at z ∼ 10 are expected to have FFB densities. The halo masses allow efficient gas supply by cold streams in a halo crossing time ∼$80\, {\rm Myr}$. The FFBs gradually turn all the accreted gas into stars in clusters of ∼104–7 M⊙ within galaxies that are rotating discs or shells. The starbursting clouds are insensitive to radiative feedback and are shielded against feedback from earlier stars. We predict high SFE above thresholds in redshift and halo mass, where the density is $10^{3\!-\!4}\, {\rm cm}^{-3}$. The z ∼ 10 haloes of ∼1010.8 M⊙ are predicted to host galaxies of ∼1010 M⊙ with star formation rate ∼$65\,\mathrm{ M}_\odot \, {\rm yr}^{-1}$, blue colours, and sub-kpc sizes. The metallicity is ≤0.1 Z⊙ with little dust, gas, outflows, and hot circumgalactic gas, allowing a top-heavy initial mass function but not requiring it. The compact galaxies with thousands of young FFB clusters may have implications on reionization, black hole growth, and globular clusters.

Fuzzy Dark Matter as a Solution to Reconcile the Stellar Mass Density of High-z Massive Galaxies and Reionization History

The JWST early release data show unexpected high stellar mass densities of massive galaxies at 7 < z < 11. A high star formation efficiency is probably needed to explain this. However, such a high star formation efficiency would greatly increase the number of ionizing photons, which would be in serious conflict with current cosmic microwave background (CMB) and other measurements of cosmic reionization history. To solve this problem, we explore fuzzy dark matter (FDM), which is composed of ultra-light scalar particles, e.g., ultra-light axions, and calculate its halo mass function and stellar mass density for different axion masses. We find that a FDM model with m a ≃ 5 × 10−23 eV and a possible uncertainty range ∼3 × 10−23–10−22 eV can effectively suppress the formation of small halos and galaxies, so that with higher star formation efficiency both the JWST data at z ∼ 8 and the reionization history measurements from optical depth of CMB scattering and ionization fraction can be simultaneously matched. We also find that the JWST data at z ∼ 10 are still too high to fit in this scenario. We note that the estimated mean redshift of the sample may have large uncertainty, that it can be as low as z ∼ 9 depending on adopted spectral energy distribution templates and photometric-redshift code. In addition, warm dark matter with ∼keV mass can also be an alternative choice, since it should have similar effects on halo formation as FDM.

Outflows driven by direct and reprocessed radiation pressure in massive star clusters

ABSTRACT We use 3D 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–$10^5 \, \mathrm{M}_{\odot } \, \mathrm{pc}^{-2}$, 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 $\Sigma \lesssim 10^3 \, \mathrm{M}_{\odot } \, \mathrm{pc}^{-2}$, but that clouds with $\Sigma \lesssim 10^5 \, \mathrm{M}_{\odot } \, \mathrm{pc}^{-2}$ become super-Eddington once high star formation efficiencies (${\sim}80~{{\ \rm per\ cent}}$) are reached, and therefore launch the remaining gas in a steady outflow. These outflows achieve mass-weighted radial velocities of ∼15–$30\, \mathrm{km} \, \mathrm{s}^{-1}$, 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 $\Sigma \sim 10^3 \, \mathrm{M}_{\odot } \, \mathrm{pc}^{-2}$, 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.

Probing the rapid formation of black holes and their Galaxy hosts in QSOs

ABSTRACTUsing the modelling code x-cigale, we reproduced the spectral energy distributions (SEDs) of 1359 SDSS quasi-stellar objects (QSOs) within the redshift range 0 &amp;lt; z &amp;lt; 4, for which we have NIR/MIR fluxes with the highest quality and spectral data characterizing their supermassive black hole (SMBHs). Consistent with a rapid formation of the host galaxies, the star formation histories (SFHs) have small e-folding, at most 750 Myr using an SFH function for spiral or 1000 Myr using one for elliptical. Above z ∼ 1.6, the two solutions are degenerate, the SEDs being dominated by the active galactic nucleus (AGN) continuum and high star formation rates (SFRs), typical of starburst galaxies, while at lower redshifts the starburst nature of the host, independent from its morphology, is better reproduced by a spiral SFH. In general, the SFR increases with the redshift, the mass of the bulge, the AGN luminosity, and Eddington ratio, suggesting there is no evidence of AGN quenching of star formation. Comparing the specific BH accretion rate (BHAR) with specific SFR, all the QSOs at any redshift trace a linear sequence below the Eddington luminosity, in parallel and above the one-to-one relation, implying that QSOs are in a special phase of evolution during which the growth in mass of their SMBH is more rapid than the growth in mass of their galaxy hosts. This particular phase is consistent with a scenario where the galaxy hosts of QSOs in the past grew in mass more rapidly than their SMBHs, suggesting that a high star formation efficiency during their formation was responsible in limiting their masses.