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The rotation velocities in the outer disks of six massive star-forming galaxies are shown to decrease with disk radius, owing to high baryonic mass fractions and large velocity dispersions. Early galaxies not so dark In the cold dark matter cosmology, the baryonic components of galaxies (stars and gas) are thought to be mixed with non-baryonic and non-relativistic dark matter, which dominates the total mass. In the local Universe, dark matter dominates the outer, baryonic regions of the disks of star-forming galaxies, leading to rotation velocities of the visible matter within the disk that are constant or increase with disk radius—an essential feature of the dark-matter model. Reinhard Genzel et al . now report rotation curves for the outer disks of six massive, high-redshift star-forming galaxies and find that the rotation velocities decrease as radius increases. They propose a combination of two causes. First, these high-redshift galaxies were strongly baryon dominated, with dark matter playing a smaller part than in the local Universe and, second, the radial pressure gradient observed in the disks slows the rotation velocity as radius increases. The effect of both factors appears to increase with redshift. 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 1 . 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 2 . 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 3 , 4 , 5 , 6 . 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 7 . 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.

Most present-day galaxies with stellar masses ≥1011 solar masses show no ongoing star formation and are dense spheroids. Ten billion years ago, similarly massive galaxies were typically forming stars at rates of hundreds solar masses per year. It is debated how star formation ceased, on which time scales, and how this \"quenching\" relates to the emergence of dense spheroids. We measured stellar mass and star-formation rate surface density distributions in star-forming galaxies at redshift 2.2 with ∼1-kiloparsec resolution. We find that, in the most massive galaxies, star formation is quenched from the inside out, on time scales less than 1 billion years in the inner regions, up to a few billion years in the outer disks. These galaxies sustain high star-formation activity at large radii, while hosting fully grown and already quenched bulges in their cores.

We analyze Hα or CO rotation curves extending out to several galaxy effective radii for 100 massive, large, star-forming disk galaxies (SFGs) across the peak of cosmic galaxy star formation (z ∼ 0.6–2.5), more than doubling the previous sample presented by Genzel et al. and Price et al. The observations were taken with SINFONI and KMOS integral-field spectrographs at the ESO-Very Large Telescope, LUCI-LBT, NOEMA-IRAM, and Atacama Large Millimeter/submillimeter Array. We fit the major-axis kinematics with beam-convolved, forward models of turbulent rotating disks with bulges embedded in dark matter (DM) halos, including the effects of pressure support. The fraction of dark to total matter within the disk effective radius (R e ∼ 5 kpc), f DM(R e) = V 2 DM(R e)/V 2 circ(R e) decreases with redshift: at z ∼ 1 (z ∼ 2) the median DM fraction is 0.38 ± 0.23 (0.27 ± 0.18), and a third (half) of all galaxies are maximal disks with f DM(R e) < 0.28. DM fractions correlate inversely with the baryonic surface density, and the low DM fractions can be explained with a flattened, or cored, inner DM density distribution. At z ∼ 2, there is ≈40% less DM mass on average within R e compared to expected values based on cosmological stellar-mass–halo-mass relations. The DM deficit is more evident at high star formation rate surface densities (≳2.5 M ⊙ yr−1 kpc2) and galaxies with massive bulges (≥1010 M ⊙). A combination of stellar or active galactic nucleus feedback, and/or heating due to dynamical friction, may drive the DM from cuspy into cored mass distributions, pointing to an efficient buildup of massive bulges and central black holes at z ∼ 2 SFGs.

We report high-quality Hα/CO imaging spectroscopy of nine massive (log median stellar mass = 10.65 M ⊙) disk galaxies on the star-forming main sequence (henceforth SFGs), near the peak of cosmic galaxy evolution (z ∼ 1.1–2.5), taken with the ESO Very Large Telescope, IRAM-NOEMA, and Atacama Large Millimeter/submillimeter Array. We fit the major axis position–velocity cuts with beam-convolved, forward models with a bulge, a turbulent rotating disk, and a dark matter (DM) halo. We include priors for stellar and molecular gas masses, optical light effective radii and inclinations, and DM masses from our previous rotation curve analysis of these galaxies. We then subtract the inferred 2D model-galaxy velocity and velocity dispersion maps from those of the observed galaxies. We investigate whether the residual velocity and velocity dispersion maps show indications for radial flows. We also carry out kinemetry, a model-independent tool for detecting radial flows. We find that all nine galaxies exhibit significant nontangential flows. In six SFGs, the inflow velocities (v r ∼ 30–90 km s−1, 10%–30% of the rotational component) are along the minor axis of these galaxies. In two cases the inflow appears to be off the minor axis. The magnitudes of the radial motions are in broad agreement with the expectations from analytic models of gravitationally unstable, gas-rich disks. Gravitational torques due to clump and bar formation, or spiral arms, drive gas rapidly inward and result in the formation of central disks and large bulges. If this interpretation is correct, our observations imply that gas is transported into the central regions on ∼10 dynamical timescales.

The orbital distribution of the S-star cluster surrounding the supermassive black hole in the center of the Milky Way is analyzed. A tight dependence of the pericenter distance r p on orbital eccentricity e ⋆ is found, log(rp)∼(1−e⋆) , which cannot be explained simply by a random distribution of semimajor axis and eccentricities. No stars are found in the region with high e ⋆ and large log(rp) or in the region with low e ⋆ and small log(rp) . Although the sample is still small, the G-clouds show a very similar distribution. The likelihood P(log(rp),(1−e⋆)) to determine the orbital parameters of S-stars is determined. P is very small for stars with large e ⋆ and large log(rp) . S-stars might exist in this region. To determine their orbital parameters, one however needs observations over a longer time period. On the other hand, if stars would exist in the region of low log(rp) and small e ⋆, their orbital parameters should by now have been determined. That this region is unpopulated therefore indicates that no S-stars exist with these orbital characteristics, providing constraints for their formation. We call this region, defined by log(rp/AU)<1.57+2.6(1−e⋆) , the zone of avoidance. Finally, it is shown that the observed frequency of eccentricities and pericenter distances is consistent with a random sampling of log(rp) and e ⋆ if one takes into account the fact that no stars exist in the zone of avoidance and that orbital parameters cannot yet be determined for stars with large r p and large e ⋆.

The broadening of atomic emission lines by high-velocity motion of gas near accreting supermassive black holes is an observational hallmark of quasars 1 . Observations of broad emission lines could potentially constrain the mechanism for transporting gas inwards through accretion disks or outwards through winds 2 . The size of regions for which broad emission lines are observed (broad-line regions) has been estimated by measuring the delay in light travel time between the variable brightness of the accretion disk  continuum and the emission lines 3 —a method known as reverberation mapping. In some models the emission lines arise from a continuous outflow 4 , whereas in others they arise from orbiting gas clouds 5 . Directly imaging such regions has not hitherto been possible because of their small angular size (less than 10 −4 arcseconds 3 , 6 ). Here we report a spatial offset (with a spatial resolution of 10 −5 arcseconds, or about 0.03 parsecs for a distance of 550 million parsecs) between the red and blue photo-centres of the broad Paschen-α line of the quasar 3C 273 perpendicular to the direction of its radio jet. This spatial offset corresponds to a gradient in the velocity of the gas and thus implies that the gas is orbiting the central supermassive black hole. The data are well fitted by a broad-line-region model of a thick disk of gravitationally bound material orbiting a black hole of 3 × 10 8 solar masses. We infer a disk radius of 150 light days; a radius of 100–400 light days was found previously using reverberation mapping 7 – 9 . The rotation axis of the disk aligns in inclination and position angle with the radio jet. Our results support the methods that are often used to estimate the masses of accreting supermassive black holes and to study their evolution over cosmic time. High-angular-resolution observations of the quasar 3C 273 reveal that it has a relatively small but thick disk, viewed nearly face-on, in which material is orbiting the central supermassive black hole.

Gas supply to the stars Star formation requires the presence of cold molecular gas, which makes up only a small fraction of the total mass of the Milky Way and nearby galaxies where only a few new stars are formed per year. To establish whether the rapid star formation occurring in distant massive galaxies reflects a greater supply of cold gas or a more efficient process of star formation, gas content was surveyed in massive-star-forming galaxies at two cosmic epochs — at redshifts of approximately 1.2 and 2.3, when the Universe was 40% and 24% of its current age. The results reveal that distant star-forming galaxies were indeed gas rich and that the star-formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy mass is three to ten times higher in distant galaxies than in today's massive spiral galaxies. Stars form from cold molecular interstellar gas, which is relatively rare in the local Universe, such that galaxies like the Milky Way form only a few new stars per year. However, typical massive galaxies in the distant Universe formed stars much more rapidly, suggesting that young galaxies were more rich in molecular gas. The results of a survey of molecular gas in samples of typical massive star-forming galaxies when the Universe was 40% and 24% of its current age now reveal that distant star-forming galaxies were indeed gas rich. Stars form from cold molecular interstellar gas. As this is relatively rare in the local Universe, galaxies like the Milky Way form only a few new stars per year. Typical massive galaxies in the distant Universe formed stars an order of magnitude more rapidly 1 , 2 . Unless star formation was significantly more efficient, this difference suggests that young galaxies were much more molecular-gas rich. Molecular gas observations in the distant Universe have so far largely been restricted to very luminous, rare objects, including mergers and quasars 3 , 4 , 5 , and accordingly we do not yet have a clear idea about the gas content of more normal (albeit massive) galaxies. Here we report the results of a survey of molecular gas in samples of typical massive-star-forming galaxies at mean redshifts < z > of about 1.2 and 2.3, when the Universe was respectively 40% and 24% of its current age. Our measurements reveal that distant star forming galaxies were indeed gas rich, and that the star formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy baryonic mass at z = 2.3 and z = 1.2 is respectively about 44% and 34%, three to ten times higher than in today’s massive spiral galaxies 6 . The slow decrease between z  ≈ 2 and z  ≈ 1 probably requires a mechanism of semi-continuous replenishment of fresh gas to the young galaxies.

We present a high-resolution kinematic study of the massive main-sequence star-forming galaxy (SFG) SDSS J090122.37+181432.3 (J0901) at z = 2.259, using ∼0.″36 Atacama Large Millimeter/submillimeter Array CO(3–2) and ∼0.″1–0.″5 SINFONI/VLT Hα observations. J0901 is a rare, strongly lensed but otherwise normal massive ( log(M⋆/M⊙)∼11 ) main-sequence SFG, offering a unique opportunity to study a typical massive SFG under the microscope of lensing. Through forward dynamical modeling incorporating lensing deflection, we fit the CO and Hα kinematics in the image plane out to about one disk effective radius (R e ∼ 4 kpc) at an ∼600 pc delensed physical resolution along the kinematic major axis. Our results show high intrinsic dispersions of the cold molecular and warm ionized gas (σ 0,mol. ∼ 40 km s−1 and σ 0,ion. ∼ 66 km s−1) that remain constant out to R e; a moderately low dark matter fraction (f DM ∼ 0.3–0.4) within R e; and a centrally peaked Toomre Q parameter—agreeing well with the previously established σ 0 versus z, f DM versus Σbaryon, and Q's radial trends using large-sample non-lensed main-sequence SFGs. Our data further reveal a high stellar mass concentration within ∼1–2 kpc with little molecular gas, and a clumpy molecular gas ring-like structure at R ∼ 2–4 kpc, in line with the inside-out quenching scenario. Our further analysis indicates that J0901 had assembled half of its stellar mass only ∼400 Myr before its observed cosmic time, and the cold gas ring and dense central stellar component are consistent with signposts of a recent wet compaction event of a highly turbulent disk found in recent simulations.

A unique galaxy at z = 2.2, zC406690, has a striking clumpy large-scale ring structure that persists from rest-frame UV to near-infrared, yet has an ordered rotation and lies on the star formation main sequence. We combine new JWST/NIRCam and Atacama Large Millimeter/submillimeter Array (ALMA) band 4 observations, together with previous Very Large Telescope/SINFONI integral field spectroscopy and Hubble Space Telescope imaging to reexamine its nature. The high-resolution Hα kinematics is best fitted if the mass is distributed within a ring with total mass Mring ≈ 2 × 1010 M⊙ and radius Rring = 4.6 kpc, together with a central undetected mass component (e.g., a “bulge”) with a dynamical mass of Mbulge = 8 × 1010 M⊙. We also consider a purely flux-emitting ring superposed over a faint exponential disk, or a highly “cuspy” dark matter halo, both disfavored against a massive ring model. The low-resolution CO(4–3) line and 142 GHz continuum emission imply total molecular and dust gas masses of Mmol,gas = 7.1 × 1010 M⊙ and Mdust = 3 × 108 M⊙, respectively, over the entire galaxy, giving a dust-to-gas ratio of 0.7%. We estimate that roughly half the gas and dust mass lie inside the ring, and that ∼10% of the total dust is in a foreground screen that attenuates the stellar light of the bulge in the rest-frame UV to near-infrared. Sensitive high-resolution ALMA observations will be essential to confirm this scenario and study the gas and dust distribution.

A gas cloud three times the mass of Earth is observed falling towards Sagittarius A*, the supermassive black hole at the centre of our Galaxy. The attractions of the Galactic Centre The radio source Sgr A* in Sagittarius is thought to be the site of a supermassive black hole lying at the centre of the Milky Way. A study of stellar orbits has identified an object moving towards Sgr A* at a speed of 1,700 kilometres per second. Its low temperature and spectral properties suggest that it is a dusty cloud of ionized gas, three times the mass of Earth, in the process of falling into the black hole. Models predict that as the cloud gets closer to the black hole, X-ray emissions will become much brighter, and a giant radiation flare may be emitted in a few years if the cloud breaks up and feeds gas into the black hole. Measurements of stellar orbits 1 , 2 , 3 provide compelling evidence 4 , 5 that the compact radio source Sagittarius A* at the Galactic Centre is a black hole four million times the mass of the Sun. With the exception of modest X-ray and infrared flares 6 , 7 , Sgr A* is surprisingly faint, suggesting that the accretion rate and radiation efficiency near the event horizon are currently very low 3 , 8 . Here we report the presence of a dense gas cloud approximately three times the mass of Earth that is falling into the accretion zone of Sgr A*. Our observations tightly constrain the cloud’s orbit to be highly eccentric, with an innermost radius of approach of only ∼3,100 times the event horizon that will be reached in 2013. Over the past three years the cloud has begun to disrupt, probably mainly through tidal shearing arising from the black hole’s gravitational force. The cloud’s dynamic evolution and radiation in the next few years will probe the properties of the accretion flow and the feeding processes of the supermassive black hole. The kilo-electronvolt X-ray emission of Sgr A* may brighten significantly when the cloud reaches pericentre. There may also be a giant radiation flare several years from now if the cloud breaks up and its fragments feed gas into the central accretion zone.