We report the discovery of a remarkable concentration of massive galaxies with extended X-ray emission at z spec = 2.506, which contains 11 massive (M * 10 11 M ) galaxies in the central 80kpc region (11.6σ overdensity). We have spectroscopically confirmed 17 member galaxies with 11 from CO and the remaining ones from Hα. The X-ray luminosity, stellar mass content and velocity dispersion all point to a collapsed, cluster-sized dark matter halo with mass M 200c = 10 13.9±0.2 M , making it the most distant X-ray-detected cluster known to date. Unlike other clusters discovered so far, this structure is dominated by star-forming galaxies (SFGs) in the core with only 2 out of the 11 massive galaxies classified as quiescent. The star formation rate (SFR) in the 80kpc core reaches ∼3400 M yr −1 with a gas depletion time of ∼ 200 Myr, suggesting that we caught this cluster in rapid build-up of a dense core. The high SFR is driven by both a high abundance of SFGs and a higher starburst fraction (∼ 25%, compared to 3%-5% in the field). The presence of both a collapsed, cluster-sized halo and a predominant population of massive SFGs suggests that this structure could represent an important transition phase between protoclusters and mature clusters. It provides evidence that the main phase of massive galaxy passivization will take place after galaxies accrete onto the cluster, providing new insights into massive cluster formation at early epochs. The large integrated stellar mass at such high redshift challenges our understanding of massive cluster formation.

We present ALMA Band 9 observations of the [C II]158µ m emission for a sample of 10 main-sequence galaxies at redshift z∼2, with typical stellar masses (log M /M ∼10.0-10.9) and star formation rates (∼35-115 M yr −1 ). Given the strong and well understood evolution of the interstellar medium from the present to z = 2, we investigate the behaviour of the [C II] emission and empirically identify its primary driver. We detect [C II] from six galaxies (four secure, two tentative) and estimate ensemble averages including non detections. The [C II]-to-infrared luminosity ratio (L [C II] /L IR ) of our sample is similar to that of local main-sequence galaxies (∼ 2 × 10 −3 ), and ∼ 10 times higher than that of starbursts. The [C II] emission has an average spatial extent of 4 -7 kpc, consistent with the optical size. Complementing our sample with literature data, we find that the [C II] luminosity correlates with galaxies' molecular gas mass, with a mean absolute deviation of 0.2 dex and without evident systematics: the [C II]-to-H 2 conversion factor (α [C II] ∼ 30 M /L ) is largely independent of galaxies' depletion time, metallicity, and redshift. [C II] seems therefore a convenient tracer to estimate galaxies' molecular gas content regardless of their starburst or main-sequence nature, and extending to metal-poor galaxies at low-and high-redshifts. The dearth of [C II] emission reported for z > 6-7 galaxies might suggest either a high star formation efficiency or a small fraction of UV light from star formation reprocessed by dust.

We present the first results of an ALMA survey of the lower fine structure line of atomic carbon [C I]( 3 P 1 − 3 P 0 ) in far infrared-selected galaxies on the main sequence at z ∼ 1.2 in the COSMOS field. We compare our sample with a comprehensive compilation of data available in literature for local and high-redshift starbursting systems and quasars. We show that the [C I]( 3 P 1 → 3 P 0 ) luminosity correlates on global scales with the infrared luminosity L IR similarly to low-J CO transitions. We report a systematic variation of L [C I] 3 P1 − 3 P0 /L IR as a function of the galaxy type, with the ratio being larger for main-sequence galaxies than for starbursts and sub-millimeter galaxies at fixed L IR . The L [C I] 3 P1 − 3 P0 /L CO(2−1) and M [C I] /M dust mass ratios are similar for main-sequence galaxies and for local and high-redshift starbursts within a 0.2 dex intrinsic scatter, suggesting that [C I] is a good tracer of molecular gas mass as CO and dust. We derive a fraction of f [C I] = M [C I] /M C ∼ 3 − 13% of the total carbon mass in the atomic neutral phase. Moreover, we estimate the neutral atomic carbon abundance, the fundamental ingredient to calibrate [C I] as a gas tracer, by comparing L [C I] 3 P1 − 3 P0 and available gas masses from CO lines and dust emission. We find lower [C I] abundances in mainsequence galaxies than in starbursting systems and sub-millimeter galaxies, as a consequence of the canonical α CO and gas-to-dust conversion factors. This argues against the application to different galaxy populations of a universal standard [C I] abundance derived from highly biased samples.

We investigate the physical conditions of ionized gas in high-z star-forming galaxies using diagnostic diagrams based on the rest-frame optical emission lines. The sample consists of 701 galaxies with an Hα detection at 1.4 z 1.7, from the FMOS-COSMOS survey, that represent the normal starforming population over the stellar mass range 10 9.6 M * /M ⊙ 10 11.6 with those at M * > 10 11 M ⊙ being well sampled. We confirm an offset of the average location of star-forming galaxies in the BPT diagram ([O iii]/Hβ vs. [N ii]/Hα), primarily towards higher [O iii]/Hβ, compared with local galaxies. Based on the [S ii] ratio, we measure an electron density (n e = 220 +170 −130 cm −3 ), that is higher than that of local galaxies. Based on comparisons to theoretical models, we argue that changes in emission-line ratios, including the offset in the BPT diagram, are caused by a higher ionization parameter both at fixed stellar mass and at fixed metallicity with additional contributions from a higher gas density and possibly a hardening of the ionizing radiation field. Ionization due to AGNs is ruled out as assessed with Chandra. As a consequence, we revisit the mass-metallicity relation using [N ii]/Hα and a new calibration including [N ii]/[S ii] as recently introduced by Dopita et al. Consistent with our previous results, the most massive galaxies (M * 10 11 M ⊙ ) are fully enriched, while those at lower masses have metallicities lower than local galaxies. Finally, we demonstrate that the stellar masses, metallicities and star formation rates of the FMOS sample are well fit with a physically-motivated model for the chemical evolution of star-forming galaxies.

We report two secure (z = 3.775, 4.012) and one tentative (z ≈ 3.767) spectroscopic confirmations of massive and quiescent galaxies close to their quenching epoch through K-band observations with Keck/MOSFIRE and VLT/X-Shooter. The stellar continuum emission, the absence of strong nebular emission lines and the lack of significant far-infrared detections confirm the passive nature of these objects, disfavoring the alternative solution of low-redshift dusty star-forming interlopers. We derive stellar masses of log(M /M ) ∼ 11 and ongoing star formation rates placing these galaxies 1 − 2 dex below the main sequence at their redshifts. The adopted parametrization of the star formation history suggests that these sources experienced a strong ( SFR ∼ 1200 − 3500 M yr −1 ) and short (∼ 50 Myr) burst of star formation, peaking ∼ 150 − 500 Myr before the time of observation, all properties reminiscent of the characteristics of sub-millimeter galaxies (SMGs) at z > 4. We investigate this connection by comparing the comoving number densities and the properties of these two populations. We find a fair agreement only with the deepest sub-mm surveys detecting not only the most extreme 2 Valentino et al.starbursts, but also more normal galaxies. We support these findings by further exploring the Illustris-TNG cosmological simulation, retrieving populations of both fully quenched massive galaxies at z ∼ 3 − 4 and SMGs at z ∼ 4 − 5, with number densities and properties in broad agreement with the observations at z ∼ 3, but in increasing tension at higher redshift. Nevertheless, as suggested by the observations, not all the progenitors of quiescent galaxies at these redshifts shine as bright SMGs in their past and, similarly, not all bright SMGs quench by z ∼ 3, both fractions depending on the threshold assumed to define the SMGs themselves. This cautions against the blind application of the assumption of a univocal connection between the two populations at high redshift.

ALMA measurements for 93 Herschel -selected galaxies at 1.1 z 1.7 in COSMOS reveal a sizable (> 29%) population with compact star formation (SF) sizes, lying on average > ×3.6 below the optical stellar mass (M )-size relation of disks. This sample widely spans the star-forming Main Sequence (MS), having 10 8 M 10 11.5 M and 20 SF R 680 M yr −1 . The 32 size measurements and 61 upper limits are measured on ALMA images that combine observations of CO(5-4), CO(4-3), CO(2-1) and λ obs ∼ 1.1−1.3 mm continuum, all tracing the star-forming molecular gas. These compact galaxies have instead normally extended K band sizes, suggesting strong specific SF R gradients. Compact galaxies comprise the 50 ± 18% of MS galaxies at M > 10 11 M . This is not expected in standard bi-modal scenarios where MS galaxies are mostly steadily-growing extended disks. We suggest that compact MS objects are early post-starburst galaxies in which the merger-driven boost of SF has subsided. They retain their compact SF size until either further gas accretion restores pre-merger galaxy-wide SF, or until becoming quenched. The fraction of merger-affected SF inside the MS seems thus larger than anticipated and might reach ∼ 50% at the highest M . The presence of large galaxies above the MS demonstrates an overall poor correlation between galaxy SF size and specific SF R.

Early type galaxies (ETG) contain most of the stars present in the local Universe and, above a stellar mass of ∼ 5 × 10 10 M , vastly outnumber spiral galaxies like the Milky Way. These massive spheroidal galaxies have, in the present day, very little gas or dust in proportion to their mass 1 , and their stellar populations have been evolving passively for over 10 billion years. The physical mechanisms that led to the termination of star formation in these galaxies and depletion of their interstellar medium remain largely conjectural. In particular, there are currently no direct measurements of the amount of residual gas that might be still present in newly quiescent spheroids at high redshift 2 . Here we show that quiescent ETGs at z ∼ 1.8, close to their epoch of quenching, contained at least 2 orders of magnitude more dust at fixed stellar mass than local ETGs. This implies the presence of substantial amounts of gas (5 − 10%), which was however consumed less efficiently than in more active galaxies, probably due to their spheroidal morphology, and consistently with our simulations. This lower star formation efficiency, and an extended hot gas halo possibly maintained by persistent feedback from an active galactic nucleus (AGN), combine to keep ETGs mostly passive throughout cosmic time.The presence of quiescent galaxies, with very low relative star formation rates (SFR), has been established up to z ∼ 3 3,4 . Their existence a mere 2 Gyr after the Big Bang implies that, in at least some regions of the Universe, the processes responsible for the cessation of star formation were already very efficient. The termination of star formation in ETGs is usually attributed to the removal of cool gas reservoirs (e.g., by stellar or quasar feedback 5 ) and/or by the suppression of gas infall and cooling (e.g., by virial shocks or AGN feedback 6,7 ). Alternatively, the growth of bulges and stellar spheroids is thought to stabilise gas reservoirs, making star formation inefficient compared to disk galaxies 8,9 . If the latter plays an important role in galaxy quenching, we might expect substantial reservoirs of untapped gas to exist in galaxies that have recently turned quiescent. Detecting this residual gas at high redshift, close to the epoch of quenching for massive quiescent galaxies, is however very challenging 2 and all attempts have so far been unsuccessful. 11) and keeping only objects that are individually undetected at observed mid-infrared (MIR) wavelengths (Methods). These criteria ensure that the sample contains only the least star-forming galaxies at z = 1.4 − 2.5, with clear early-type morphologies (as implied by a high median Sérsic index n ∼ 3.5; Methods). We extract cutouts centred at the position of each galaxy from the 24µm (MIR), 100 − 500µm (FIR), 0.85 − 1.1 mm (sub-millimetre; sub-mm), 10 and 20 cm (radio) observations of COS-MOS and perform a median stacking analysis at each wavelength. After correcting for the contribution from satellite galaxies and unassociated neighbours in the line of sight (M...

We report Atacama Large Millimeter Array observations of the neutral atomic carbon transitions [C i] and multiple CO lines in a sample of ∼30 main-sequence galaxies at , including novel information on [C i] and CO for 7 of such normal objects. We complement our observations with a collection of >200 galaxies with coverage of similar transitions, spanning the z = 0–4 redshift interval and a variety of ambient conditions from local to high-redshift starbursts. We find systematic variations in the [C i]/IR and [C i]/high-J upper (J upper = 7) CO luminosity ratios among the various samples. We interpret these differences as increased dense molecular gas fractions and star formation efficiencies in the strongest high-redshift starbursts with respect to normal main-sequence galaxies. We further report constant  / ratios across the galaxy populations and redshifts, suggesting that gas temperatures T exc traced by [C i] do not strongly vary. We find only a mild correlation with T dust and that, generally, T exc ≲ T dust. We fit the line ratios with classical photodissociation region models, retrieving consistently larger densities and intensities of the UV radiation fields in submillimeter galaxies than in main-sequence and local objects. However, these simple models fall short in representing the complexity of a multiphase interstellar medium and should be treated with caution. Finally, we compare our observations with the Santa Cruz semi-analytical model of galaxy evolution, recently extended to simulate submillimeter emission. While we confirm the success in reproducing the CO lines, we find systematically larger [C i] luminosities at fixed IR luminosity than predicted theoretically. This highlights the necessity of improving our understanding of the mechanisms regulating the [C i] emission on galactic scales. We release our data compilation to the community.