High-redshift Massive Galaxies Research Articles

Evidence for a Shallow Evolution in the Volume Densities of Massive Galaxies at z = 4–8 from CEERS

We analyze the evolution of massive (log10[M ⋆/M ⊙] > 10) galaxies at z ∼ 4–8 selected from JWST Cosmic Evolution Early Release Survey (CEERS). We infer the physical properties of all galaxies in the CEERS NIRCam imaging through spectral energy distribution (SED) fitting with dense basis to select a sample of high-redshift massive galaxies. Where available we include constraints from additional CEERS observing modes, including 18 sources with MIRI photometric coverage, and 28 sources with spectroscopic confirmations from NIRSpec or NIRCam WFSS. We sample the recovered posteriors in stellar mass from SED fitting to infer the volume densities of massive galaxies across cosmic time, taking into consideration the potential for sample contamination by active galactic nuclei. We find that the evolving abundance of massive galaxies tracks expectations based on a constant baryon conversion efficiency in dark matter halos for z ∼ 4–8. At higher redshifts, we observe an excess abundance of massive galaxies relative to this simple model, resulting in a shallower decline of observed volume densities of massive galaxies. These higher abundances can be explained by modest changes to star formation physics and/or the efficiencies with which star formation occurs in massive dark matter halos, and are not in tension with modern cosmology.

High-redshift Merger Model for Low-frequency Gravitational Wave Background

In 2023, the Pulsar Timing Array Collaborations announced the discovery of a gravitational wave background (GWB), predominantly attributed to supermassive black hole binary (SMBHB) mergers. However, the detected GWB is several times stronger than the default value expected from galactic observations at low and moderate redshifts. Recent findings by the James Webb Space Telescope have unveiled a substantial number of massive, high-redshift galaxies, suggesting more massive SMBHB mergers at these early epochs. Motivated by these findings, we propose an “early merger” model that complements the standard merger statistics by incorporating these early, massive galaxies. We compare the early and standard “late merger” models, which assume peak merger rates in the local Universe, and match both merger models to the currently detected GWB. Our analysis shows that the early merger model has a significantly lower detection probability for single binaries and predicts a ∼30% likelihood that the first detectable single source will be highly redshifted and remarkably massive with rapid frequency evolution. In contrast, the late merger model predicts a nearly monochromatic first source at low redshift. The future confirmation of an enhanced population of massive high-redshift galaxies and the detection of fast-evolving binaries would strongly support the early merger model, offering significant insights into the evolution of galaxies and SMBHs.

Exploring the dark energy equation of state with JWST

Observations from the James Webb Space Telescope (JWST) have unveiled several galaxies with stellar masses M∗≳1010M⊙\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_*\\gtrsim 10^{10} M_\\odot$$\\end{document} at redshifts 7.4≲z≲9.1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$7.4\\lesssim z\\lesssim 9.1$$\\end{document}. These remarkable findings indicate an unexpectedly high stellar mass density, which contradicts the prediction of the ΛCDM\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Lambda \\rm{CDM}$$\\end{document} model. Our study utilizes the Chevallier–Polarski–Linder (CPL) parameterization, one of the dynamic dark energy models, to probe the role of dark energy on shaping galaxy formation. By considering varying star formation efficiencies within this framework, our analysis demonstrates that in a universe with a higher proportion of dark energy, more massive galaxies are formed at high redshifts, given a fixed perturbation amplitude observed today. These intriguing results highlight the promising prospect of revealing the nature of dark energy by analyzing the high-redshift massive galaxies.

Rapidly growing primordial black holes as seeds of the massive high-redshift JWST Galaxies

Rapidly growing primordial black holes as seeds of the massive high-redshift JWST Galaxies

ALMA 1.1 mm Observations of a Conservative Sample of High-redshift Massive Quiescent Galaxies in SHELA

We present a sample of 30 massive (log(M */M ⊙) > 11) z = 3–5 quiescent galaxies selected from the Spitzer-HETDEX Exploratory Large Area (SHELA) Survey and observed at 1.1 mm with Atacama Large Millimeter/submillimeter Array (ALMA) Band 6 observations. These ALMA observations would detect even modest levels of dust-obscured star formation, on the order of ∼20 M ⊙ yr−1 at z ∼ 4 at the 1σ level, allowing us to quantify the amount of contamination from dusty star-forming sources in our quiescent sample. Starting with a parent sample of candidate massive quiescent galaxies from the Stevans et al. v1 SHELA catalog, we use the Bayesian Bagpipes spectral energy distribution fitting code to derive robust stellar masses (M *) and star formation rates (SFRs) for these sources, and select a conservative sample of 36 candidate massive (M * > 1011 M ⊙) quiescent galaxies, with specific SFRs >2σ below the Salmon et al. star-forming main sequence at z ∼ 4. Based on the ALMA imaging, six of these candidate quiescent galaxies show the presence of significant dust-obscured star formation, and thus were removed from our final sample. This implies a ∼17% contamination rate from dusty star-forming galaxies with our selection criteria using the v1 SHELA catalog. This conservatively selected quiescent galaxy sample at z = 3–5 will provide excellent targets for future observations to constrain better how massive galaxies can both grow and shut down their star formation in a relatively short period.

Exploring the environment, magnetic fields, and feedback effects of massive high-redshift galaxies with [C II

Context. Massive galaxies are expected to grow through different transformative evolutionary phases. High-redshift starburst galaxies and quasars are thought to be such phases and thus provide insight into galaxy evolution. Several physical mechanisms are predicted to play an important role in driving these phases; for example, interaction with companion galaxies, active galactic nuclei feedback, and possibly magnetic fields. Aims. Our aim is to characterize the physical properties and the environment of the submillimeter galaxy AzTEC-3 at z = 5.3 and the lensed quasar BRI 0952−0115 at z = 4.4, and to set a limit on the polarization properties of the two sources. We intend to place these two sources in the broader context of galaxy evolution, specifically star formation and mass growth through cosmic time. Methods. We used full polarization, sub-arcsecond-resolution, ALMA band-7 observations of both BRI 0952−0115 and AzTEC-3. We detect [C II] (2P3/2−2P1/2) line emission towards both BRI 0952−0115 and AzTEC-3, along with companions in each field. We present an updated gravitational lensing model for BRI 0952−0115 for correction of gravitational magnification. Results. We present infrared luminosities, star-formation rates, and [C II] line to infrared luminosity ratios for each source. The [C II] emission line profile for both BRI 0952−0115 and AzTEC-3 exhibit a broad, complex morphology, indicating the possible presence of outflows. We present evidence of a “gas bridge” between AzTEC-3 and a companion source. Modified blackbody spectral energy distribution fitting is used to analyze the properties of [C II] detected companion sources in the field of both the submillimeter galaxy and the quasar. We investigated the possible role of the detected companions in outflow signatures. Using a simple dynamical mass estimate for the sources, we suggest that both systems are undergoing minor or major mergers. No polarization is detected for the [C II], placing an upper limit below that of theoretical predictions. Conclusions. Our results show that high-velocity wings are detected, indicating possible signs of massive outflows; however, the presence of companion galaxies can affect the final interpretation. Furthermore, the results provide additional evidence in support of the hypothesis that massive galaxies form in overdense regions, growing through minor or major mergers with companion sources. Finally, strong, ordered magnetic fields are unlikely to exist at the kiloparsec scale in the two studied sources.

First Light And Reionisation Epoch Simulations (FLARES) – III. The properties of massive dusty galaxies at cosmic dawn

ABSTRACT Using the First Light And Reionisation Epoch Simulations (flares), we explore the dust-driven properties of massive high-redshift galaxies at z ∈ [5, 10]. By post-processing, the galaxy sample using the radiative transfer code skirt we obtain the full spectral energy distribution. We explore the resultant luminosity functions, IRX–β relations as well as the luminosity-weighted dust temperatures in the Epoch of Reionisation (EoR). We find that most of our results are in agreement with the current set of observations, but underpredict the number densities of bright IR galaxies, which are extremely biased towards the most overdense regions. We see that the flares IRX–β relation (for 5 ≤ z ≤ 8) pre-dominantly follows the local starburst relation. The IRX shows an increase with stellar mass, plateauing at the high-mass end (∼1010 M⊙) and shows no evolution in the median normalisation with redshift. We also look at the dependence of the peak dust temperature (Tpeak) on various galaxy properties including the stellar mass, IR luminosity, and sSFR, finding the correlation to be strongest with sSFR. The luminosity-weighted dust temperatures increase towards higher redshifts, with the slope of the Tpeak–redshift relation showing a higher slope than the lower redshift relations obtained from previous observational and theoretical works. The results from flares, which are able to provide a better statistical sample of high-redshift galaxies compared to other simulations, provides a distinct vantage point for the high-redshift Universe.

Evolution of perturbation and power spectrum in a two-component ultralight axionic universe

The evolution of cosmic perturbations in a two-component ultralight axionic universe is investigated. We present the first spectral computation of perturbations in multi-component universes. A particular case composed of light extreme axions and free massive particles offers a possibility for the formation of very high-redshift massive galaxies, which are typically required to host massive early quasars. Our computation retains the information of perturbed velocities for individual axion components, opening a new avenue for setting up initial conditions for future axion dark matter simulations.

BIRTH of the COSMOS field: primordial and evolved density reconstructions during cosmic high noon

ABSTRACT This work presents the first comprehensive study of structure formation at the peak epoch of cosmic star formation over 1.4 ≤ z ≤ 3.6 in the Cosmic Evolution Survey (COSMOS) field, including the most massive high-redshift galaxy proto-clusters at that era. We apply the extended COSMIC BIRTH algorithm to account for a multitracer and multisurvey Bayesian analysis at Lagrangian initial cosmic times. Combining the data of five different spectroscopic redshift surveys (zCOSMOS-deep, VUDS, MOSDEF, ZFIRE, and FMOS–COSMOS), we show that the corresponding unbiased primordial density fields can be inferred, if a proper survey completeness computation from the parent photometric catalogues, and a precise treatment of the non-linear and non-local evolution on the light-cone is taken into account, including (i) gravitational matter displacements, (ii) peculiar velocities, and (iii) galaxy bias. The reconstructions reveal a holistic view on the known proto-clusters in the COSMOS field and the growth of the cosmic web towards lower redshifts. The inferred distant dark matter density fields concurrently with other probes like tomographic reconstructions of the intergalactic medium will explore the interplay of gas and dark matter and are ideally suited to study structure formation at high redshifts in the light of upcoming deep surveys.

Measuring dynamical masses from gas kinematics in simulated high-redshift galaxies

ABSTRACT Advances in instrumentation have recently extended detailed measurements of gas kinematics to large samples of high-redshift galaxies. Relative to most nearby, thin disc galaxies, in which gas rotation accurately traces the gravitational potential, the interstellar medium (ISM) of $z$ ≳ 1 galaxies is typically more dynamic and exhibits elevated turbulence. If not properly modelled, these effects can strongly bias dynamical mass measurements. We use high-resolution FIRE-2 cosmological zoom-in simulations to analyse the physical effects that must be considered to correctly infer dynamical masses from gas kinematics. Our analysis covers a range of galaxy properties from low-redshift Milky-Way-mass galaxies to massive high-redshift galaxies (M⋆ > 1011 M⊙ at $z$ = 1). Selecting only snapshots where a disc is present, we calculate the rotational profile $\bar{v}_\phi (r)$ of the cool ($10^{3.5}\,\lt {\it T}\lt 10^{4.5}~\rm {K}$) gas and compare it to the circular velocity $v_{\rm c}=\sqrt{GM_{\rm enc}/r}$. In the simulated galaxies, the gas rotation traces the circular velocity at intermediate radii, but the two quantities diverge significantly in the centre and in the outer disc. Our simulations appear to over-predict observed rotational velocities in the centres of massive galaxies (likely from a lack of black hole feedback), so we focus on larger radii. Gradients in the turbulent pressure at these radii can provide additional radial support and bias dynamical mass measurements low by up to 40 per cent. In both the interior and exterior, the gas’ motion can be significantly non-circular due to e.g. bars, satellites, and inflows/outflows. We discuss the accuracy of commonly used analytic models for pressure gradients (or ‘asymmetric drift’) in the ISM of high-redshift galaxies.

The hidden satellites of massive galaxies and quasars at high redshift

ABSTRACT Using cosmological, radiation-hydrodynamic simulations targeting a rare ${\approx}2 \times 10^{12} \, \rm {\rm M}_{\odot }$ halo at $z = 6$, we show that the number counts and internal properties of satellite galaxies within the massive halo are sensitively regulated by a combination of local stellar radiative feedback and strong tidal forces. Radiative feedback operates before the first supernova explosions erupt and results in less tightly bound galaxies. Satellites are therefore more vulnerable to tidal stripping when they accrete on to the main progenitor and are tidally disrupted on a significantly shorter time-scale. Consequently, the number of satellites with $M_{\rm \star } \gt 10^{7} \, \rm {\rm M}_{\odot }$ within the parent system’s virial radius drops by up to $60 \, {\rm per\, cent}$ with respect to an identical simulation performed without stellar radiative feedback. Radiative feedback also impacts the central galaxy, whose effective radius increases by a factor ≲3 due to the presence of a more extended and diffuse stellar component. We suggest that the number of satellites in the vicinity of massive high-redshift galaxies is an indication of the strength of stellar radiative feedback and can be anomalously low in the extreme cosmic environments of high-redshift quasars.

High-redshift Massive Quiescent Galaxies Are as Flat as Star-forming Galaxies: The Flattening of Galaxies and the Correlation with Structural Properties in CANDELS/3D-HST

Abstract We investigate the median flattening of galaxies at 0.2 < z < 4.0 in all five CANDELS/3D-HST fields via the apparent axis ratio, q. We separate the sample into bins of the redshift, stellar mass, Sérsic index, size, and UVJ-determined star-forming state to discover the most important drivers of the median q (q med). Quiescent galaxies at z < 1 and M * > 1011 M ⊙ are rounder than those at lower masses, consistent with the hypothesis that they have grown significantly through dry merging. The massive quiescent galaxies at higher redshifts become flatter and are as flat as star-forming massive galaxies at 2.5 < z < 3.5, consistent with formation through direct transformations or wet mergers. We find that in quiescent galaxies, correlations with q med and M *, and z and r e are driven by the evolution in the Sérsic index (n), consistent with the growing accumulation of minor mergers at lower redshifts. Interestingly, n does not drive these trends fully in star-forming galaxies. Instead, the strongest predictor of q in star-forming galaxies is the effective radius, where larger galaxies are flatter. Our findings suggest that q med is the tracing bulge-to-total ratio, which would explain why smaller/more massive star-forming galaxies are rounder than their extended/less massive analogs, although it is unclear why the Sérsic index correlates more weakly with flattening for star-forming galaxies than for quiescent galaxies.

The combined effect of AGN and supernovae feedback in launching massive molecular outflows in high-redshift galaxies

We have recently improved our model of active galactic nucleus (AGN) by attaching the supermassive black hole (SMBH) to a massive nuclear star cluster (NSC). Here we study the effects of this new model in massive, gas-rich galaxies with several simulations of different feedback recipes with the hydrodynamics code RAMSES. These simulations are compared to a reference simulation without any feedback, in which the cooling halo gas is quickly consumed in a burst of star formation. In the presence of strong supernovae (SN) feedback, we observe the formation of a galactic fountain that regulates star formation over a longer period, but without halting it. If only AGN feedback is considered, as soon as the SMBH reaches a critical mass, strong outflows of hot gas are launched and prevent the cooling halo gas from reaching the disk, thus efficiently halting star formation, leading to the so-called "quenching". If both feedback mechanisms act in tandem, we observe a non-linear coupling, in the sense that the dense gas in the supernovae-powered galactic fountain is propelled by the hot outflow powered by the AGN at much larger radii than without AGN. We argue that these particular outflows are able to unbind dense gas from the galactic halo, thanks to the combined effect of SN and AGN feedback. We speculate that this mechanism occurs at the end of the fast growing phase of SMBH, and is at the origin of the dense molecular outflows observed in many massive high-redshift galaxies.

Evolution of Galactic Outflows at Revealed with SDSS, DEEP2, and Keck Spectra

Abstract We conduct a systematic study of galactic outflows in star-forming galaxies at based on the absorption lines of optical spectra taken from SDSS DR7, DEEP2 DR4, and Keck (Erb et al.). We carefully make stacked spectra of homogeneous galaxy samples with similar stellar mass distributions at and perform the multicomponent fitting of model absorption lines and stellar continua to the stacked spectra. We obtain the maximum ( ) and central ( ) outflow velocities and estimate the mass loading factors (η), a ratio of the mass outflow rate to the star formation rate (SFR). Investigating the redshift evolution of the outflow velocities measured with the absorption lines whose depths and ionization energies are similar (Na i D and Mg i at ; Mg ii and C ii at ), we identify, for the first time, that the average value of ( ) significantly increases by 0.05–0.3 dex from to 2 at a given SFR. Moreover, we find that the value of η increases from to 2 by at a given halo circular velocity , albeit with a potential systematics caused by model parameter choices. The redshift evolution of ( ) and η is consistent with the galaxy-size evolution and the local velocity–SFR surface density relation and explained by high gas fractions in high-redshift massive galaxies, which is supported by recent radio observations. We obtain a scaling relation of for in our galaxies that agrees with the momentum-driven outflow model ( ) within the uncertainties.

Cluster mass calibration at high redshift: HST weak lensing analysis of 13 distant galaxy clusters from the South Pole Telescope Sunyaev–Zel'dovich Survey

We present an HST/ACS weak gravitational lensing analysis of 13 massive high-redshift (z_median=0.88) galaxy clusters discovered in the South Pole Telescope (SPT) Sunyaev-Zel'dovich Survey. This study is part of a larger campaign that aims to robustly calibrate mass-observable scaling relations over a wide range in redshift to enable improved cosmological constraints from the SPT cluster sample. We introduce new strategies to ensure that systematics in the lensing analysis do not degrade constraints on cluster scaling relations significantly. First, we efficiently remove cluster members from the source sample by selecting very blue galaxies in V-I colour. Our estimate of the source redshift distribution is based on CANDELS data, where we carefully mimic the source selection criteria of the cluster fields. We apply a statistical correction for systematic photometric redshift errors as derived from Hubble Ultra Deep Field data and verified through spatial cross-correlations. We account for the impact of lensing magnification on the source redshift distribution, finding that this is particularly relevant for shallower surveys. Finally, we account for biases in the mass modelling caused by miscentring and uncertainties in the concentration-mass relation using simulations. In combination with temperature estimates from Chandra we constrain the normalisation of the mass-temperature scaling relation ln(E(z) M_500c/10^14 M_sun)=A+1.5 ln(kT/7.2keV) to A=1.81^{+0.24}_{-0.14}(stat.) +/- 0.09(sys.), consistent with self-similar redshift evolution when compared to lower redshift samples. Additionally, the lensing data constrain the average concentration of the clusters to c_200c=5.6^{+3.7}_{-1.8}.

Stellar populations, stellar masses and the formation of galaxy bulges and discs at z < 3 in CANDELS

We present a multi-component structural analysis of the internal structure of $1074$ high redshift massive galaxies at $1<z<3$ from the CANDELS HST Survey. In particular we examine galaxies best-fit by two structural components, and thus likely forming discs and bulges. We examine the stellar mass, star formation rates, and colours of both the inner `bulge' and outer `disc' components for these systems using SED information from the resolved ACS+WFC3 HST imaging. We find that the majority of both inner and outer components lie in the star-forming region of UVJ space ($68$ and $90$ per cent respectively). However, the inner portions, or the likely forming bulges, are dominated by dusty star formation. Furthermore, we show that the outer components of these systems have a higher star formation rate than their inner regions, and the ratio of star formation rate between `disc' and `bulge' increases at lower redshifts. Despite the higher star formation rate of the outer component, the stellar mass ratio of inner to outer component remains constant through this epoch. This suggests that there is mass transfer from the outer to inner components for typical two component forming systems, thus building bulges from disks. Finally, using Chandra data we find that the presence of an AGN is more common in both $1$-component spheroid-like galaxies and $2$-component systems ($13\pm3$ and $11\pm2$ per cent) than in $1$-component disc-like galaxies ($3\pm1$ per cent), demonstrating that the formation of a central inner-component likely triggers the formation of central massive black holes in these galaxies.

A model for the origin of bursty star formation in galaxies

We propose a simple analytic model to understand when star formation is time-steady versus bursty in galaxies. Recent models explain the observed Kennicutt-Schmidt relation between star formation rate and gas surface densities in galaxies as resulting from a balance between stellar feedback and gravity. We argue that bursty star formation occurs when such an equilibrium cannot be stably sustained, and identify two regimes in which galaxy-scale star formation should be bursty: i) at high redshift (z>~1) for galaxies of all masses, and ii) at low masses (depending on gas fraction) for galaxies at any redshift. At high redshift, characteristic galactic dynamical timescales become too short for supernova feedback to effectively respond to gravitational collapse in galactic discs (an effect recently identified for galactic nuclei), whereas in dwarf galaxies star formation occurs in too few bright star-forming regions to effectively average out. Burstiness is also enhanced at high redshift owing to elevated gas fractions in the early Universe. Our model can thus explain the bursty star formation rates predicted in these regimes by recent high-resolution galaxy formation simulations, as well as the bursty star formation histories observationally-inferred in both local dwarf and high-redshift galaxies. In our model, bursty star formation is associated with particularly strong spatio-temporal clustering of supernovae. Such clustering can promote the formation of galactic winds and our model may thus also explain the much higher wind mass loading factors inferred in high-redshift massive galaxies relative to their z~0 counterparts.

CLUSTERING OF EXTREMELY RED OBJECTS IN THE SUBARU GTO 2DEG2 FIELD

We study the angular correlation function of bright (<TEX>$K_s{\leq}19.5$</TEX>) Extremely Red Objects (EROs) selected in the Subaru GTO 2<TEX>$deg^2$</TEX> field. By applying the color selection criteria of <TEX>$R-K_s$</TEX> > 5.0, 5.5, and 6.0, we identify 9055, 4270, and 1777 EROs, respectively. The number density is consistent with similar studies on the optical - NIR color selected red galaxies. The angular correlation functions are derived for EROs with different limiting magnitude and different <TEX>$R-K_s$</TEX> color cut. When we assume that the angular correlation function <TEX>$w({\theta})$</TEX> follows a form of a power-law (i.e., <TEX>$w({\theta})=A{\theta}^{-{\delta}}$</TEX>), the value of the amplitude A was larger for brighter EROs compared to the fainter EROs. The result suggests that the brighter, thus more massive high-redshift galaxies, are clustered more strongly compared to the less massive galaxies. Assuming that EROs have redshift distribution centered at <z> ~ 1.1 with <TEX>${\sigma}_z=0.15$</TEX>, the spatial correlation length <TEX>$r_0$</TEX> of the EROs estimated from the observed angular correlation function ranges <TEX>${\sim}6-10h^{-1}Mpc$</TEX>. A comparison with the clustering of dark matter halos in numerical simulation suggests that the EROs are located in most massive dark matter halos and could be progenitors of <TEX>$L_{\ast}$</TEX> elliptical galaxies.

Planck’s dusty GEMS

We study the properties of the foreground galaxy of the Ruby, the brightest gravitationally lensed high-redshift galaxy on the sub-millimeter sky as probed by the Planck satellite, and part of our sample of Planck’s dusty GEMS. The Ruby consists of an Einstein ring of 1.4′′ diameter at z = 3.005 observed with ALMA at 0.1′′ resolution, centered on a faint, red, massive lensing galaxy seen with HST/WFC3, which itself has an exceptionally high redshift, z = 1.525 ± 0.001, as confirmed with VLT/X-shooter spectroscopy. Here we focus on the properties of the lens and the lensing model obtained with LENSTOOL. The rest-frame optical morphology of this system is strongly dominated by the lens, while the Ruby itself is highly obscured, and contributes less than 10% to the photometry out to the K band. The foreground galaxy has a lensing mass of (3.70 ± 0.35) × 1011M⊙. Magnification factors are between 7 and 38 for individual clumps forming two image families along the Einstein ring. We present a decomposition of the foreground and background sources in the WFC3 images, and stellar population synthesis modeling with a range of star-formation histories for Chabrier and Salpeter initial mass functions (IMFs). Only the stellar mass range obtained with the latter agrees well with the lensing mass. This is consistent with the bottom-heavy IMFs of massive high-redshift galaxies expected from detailed studies of the stellar masses and mass profiles of their low-redshift descendants, and from models of turbulent gas fragmentation. This may be the first direct constraint on the IMF in a lens at z = 1.5, which is not a cluster central galaxy.

Strongly baryon-dominated disk galaxies at the peak of galaxy formation ten billion years ago.

In the cold dark matter cosmology, the baryonic components of galaxies-stars and gas-are thought to be mixed with and embedded in non-baryonic and non-relativistic dark matter, which dominates the total mass of the galaxy and its dark-matter halo. In the local (low-redshift) Universe, the mass of dark matter within a galactic disk increases with disk radius, becoming appreciable and then dominant in the outer, baryonic regions of the disks of star-forming galaxies. This results in rotation velocities of the visible matter within the disk that are constant or increasing with disk radius-a hallmark of the dark-matter model. Comparisons between the dynamical mass, inferred from these velocities in rotational equilibrium, and the sum of the stellar and cold-gas mass at the peak epoch of galaxy formation ten billion years ago, inferred from ancillary data, suggest high baryon fractions in the inner, star-forming regions of the disks. Although this implied baryon fraction may be larger than in the local Universe, the systematic uncertainties (owing to the chosen stellar initial-mass function and the calibration of gas masses) render such comparisons inconclusive in terms of the mass of dark matter. Here we report rotation curves (showing rotation velocity as a function of disk radius) for the outer disks of six massive star-forming galaxies, and find that the rotation velocities are not constant, but decrease with radius. We propose that this trend arises because of a combination of two main factors: first, a large fraction of the massive high-redshift galaxy population was strongly baryon-dominated, with dark matter playing a smaller part than in the local Universe; and second, the large velocity dispersion in high-redshift disks introduces a substantial pressure term that leads to a decrease in rotation velocity with increasing radius. The effect of both factors appears to increase with redshift. Qualitatively, the observations suggest that baryons in the early (high-redshift) Universe efficiently condensed at the centres of dark-matter haloes when gas fractions were high and dark matter was less concentrated.