Low-Luminosity Members of the R Coronae Australis Infrared Cluster

Passive early-type galaxies dominate cluster cores at z ≲ 1.5. At higher redshift, cluster core galaxies are observed to have on-going star-formation, which is fueled by cold molecular gas. We measured the molecular gas reservoir of the central region around the radio-loud active galactic nucleus (AGN) in the cluster CARLA J1103 + 3449 at z = 1.44 using NOEMA. The AGN synchrotron emission dominates the continuum emission at 94.48 GHz, and we measured its flux at the AGN position and at the position of two radio jets. Combining our measurements with published results over the range 4.71–94.5 GHz, and assuming Ssynch ∝ ν−α, we obtain a flat spectral index of α = 0.14 ± 0.03 for the AGN core emission, and a steeper index of α = 1.43 ± 0.04 and α = 1.15 ± 0.04 at positions close to the western and eastern lobes, respectively. The total spectral index is α = 0.92 ± 0.02 over the range 73.8 MHz–94.5 GHz. We detect two CO(2–1) emission lines, both blueshifted with respect to the AGN. Their emission corresponds to two regions, ~17 kpc southeast and ~14 kpc southwest of the AGN, not associated with galaxies. In these two regions, we find a total massive molecular gas reservoir of Mgastot = 3.9 ± 0.4 × 1010 M⊙, which dominates (≳60%) the central total molecular gas reservoir. These results can be explained by massive cool gas flows in the center of the cluster. The AGN early-type host is not yet quenched; its star formation rate is consistent with being on the main sequence of star-forming galaxies in the field (star formation rate ~30–140 M⊙ yr−1), and the cluster core molecular gas reservoir is expected to feed the AGN and the host star formation before quiescence. The other confirmed cluster members show star formation rates at ~2σ below the field main sequence at similar redshifts and do not have molecular gas masses larger than galaxies of similar stellar mass in the field.

We explore how the presence of detectable molecular gas depends on the inferred star formation histories (SFHs) in eight massive, quiescent galaxies at z ∼ 0.7. Half of the sample have clear detections of molecular gas, traced by CO(2–1). We find that the molecular gas content is unrelated to the rate of star formation decline prior to the most recent 1 Gyr, suggesting that the gas reservoirs are not left over from their primary star formation epoch. However, the recent SFHs of CO-detected galaxies demonstrate evidence for secondary bursts of star formation in their last Gyr. The fraction of stellar mass formed in these secondary bursts ranges from f burst ≈ 0.3%–6% and ended between t end-burst ≈ 0–330 Myr ago. The CO-detected galaxies form a higher fraction of mass in the last Gyr () compared to the CO-undetected galaxies (). The galaxies with gas reservoirs have enhanced late-time star formation, highlighting this as a contributing factor to the observed heterogeneity in the gas reservoirs in high-redshift quiescent galaxies. We find that the amount of gas and star formation driven by these secondary bursts are inconsistent with that expected from dry minor mergers, and instead are likely driven by recently accreted gas, i.e., gas-rich minor mergers. This conclusion would not have been made based on SFRUV+IR measurements alone, highlighting the power of detailed SFH modeling in the interpretation of gas reservoirs. Larger samples are needed to understand the frequency of low-level rejuvenation among quiescent galaxies at intermediate redshifts, and to what extent this drives the diversity of molecular gas reservoirs.

Brightest cluster galaxies (BCGs) at the centers of clusters are among the most massive galaxies in the Universe. Their star formation history and stellar mass assembly are highly debated. Recent studies suggest the presence of an emerging population of intermediate-zstar-forming and gas-rich BCGs, whose molecular gas reservoirs that feed star formation might be impacted by strong environmental processing. We have selected three of the most strongly star-formingz ∼ 0.4 BCGs in the equatorial field of the Kilo-Degree Survey (KiDS) and observed them with the IRAM 30 m telescope in the first three CO transitions. We found clear double-horn CO(1→0) and CO(3→2) emission for the KiDS 1433 BCG, yielding a large molecular gas reservoir withMH2 = (5.9 ± 1.2)×1010 M⊙and a high gas-to-stellar mass ratioMH2/M⋆ = (0.32−0.10+0.12). We thus increase the still limited sample of distant BCGs with detections in multiple CO transitions. The double-horn emission for the KiDS 1433 BCG implies a low gas concentration, while a modeling of the spectra yields an extended molecular gas reservoir, with a characteristic radius of ∼(5−7) kpc, which is reminiscent of the mature extended-disk phase that is observed in some local BCGs. For the remaining two BCGs, we are able to set robust upper limits ofMH2/M⋆ < 0.07 and < 0.23, which are among the lowest for distant BCGs. We then combined our observations with available stellar, star formation, and dust properties of the targeted BCGs, and compared them with a sample of ∼100 distant cluster galaxies, including additional intermediate-zBCGs, with observations in CO from the literature. Altogether, our analysis shows that the molecular gas properties of star-forming BCGs are heterogeneous. On the one hand, gas-rich BCGs show extended gas reservoirs that sustain the significant star formation activity, but the efficiency is low, which is reminiscent of recent gas infall. On the other hand, the existence of similarly star forming but gas-poor BCGs suggests that gas depletion precedes star formation quenching.

We report the serendipitous discovery of an overdensity of CO emitters in an X-ray-identified cluster (Log10 M halo/M ⊙ ∼ 13.6 at z = 1.3188) using ALMA. We present spectroscopic confirmation of six new cluster members exhibiting CO(2–1) emission, adding to two existing optical/IR spectroscopic members undetected in CO. This is the lowest-mass cluster to date at z > 1 with molecular gas measurements, bridging the observational gap between galaxies in the more extreme, well-studied clusters (Log10 M halo/M ⊙ ≳ 14) and those in group or field environments at cosmic noon. The CO sources are concentrated on the sky (within ∼1 arcmin diameter) and phase space analysis indicates the gas resides in galaxies already within the cluster environment. We find that CO sources sit in similar phase space as CO-rich galaxies in more massive clusters at similar redshifts (have similar accretion histories) while maintaining field-like molecular gas reservoirs, compared to scaling relations. This work presents the deepest CO survey to date in a galaxy cluster at z > 1, uncovering gas reservoirs down to M H 2 > 1.6 × 10 10 M ⊙ (5σ at 50% primary beam). Our deep limits rule out the presence of gas content in excess of the field scaling relations; however, combined with literature CO detections, cluster gas fractions in general appear systematically high, on the upper envelope or above the field. This study is the first demonstration that low-mass clusters at z ∼ 1–2 can host overdensities of CO emitters with surviving gas reservoirs, in line with the prediction that quenching is delayed after first infall while galaxies consume the gas bound to the disk.

Comparison of the ISM properties of a wide range of metal poor galaxies with normal metal-rich galaxies reveals striking differences. We find that the combination of the low dust abundance and the active star formation results in a very porous ISM filled with hard photons, heating the dust in dwarf galaxies to overall higher temperatures than their metal-rich counterparts. This results in photodissociation of molecular clouds to greater depths, leaving relatively large PDR envelopes and difficult-to-detect CO cores. From detailed modeling of the low-metallicity ISM, we find significant fractions of CO-dark H2 - a reservoir of molecular gas not traced by CO, but present in the [CII] and [CI]-emitting envelopes. Self-consistent analyses of the neutral and ionized gas diagnostics along with the dust SED is the necessary way forward in uncovering the multiphase structure of galaxies.

This paper presents the results of 475h of interferometric observations with the Australia Telescope Compact Array towards the Spiderweb protocluster at z=2.16. We search for large, extended molecular gas reservoirs among 46 previously detected CO(1−0) emitters, employing a customised method we developed. Based on the CO emission images and position–velocity diagrams, as well as the ranking of sources using a binary weighting of six different criteria, we have identified 14 robust and 7 tentative candidates that exhibit large extended molecular gas reservoirs. These extended reservoirs are defined as having sizes greater than 40 kpc or supergalactic scale. This result suggests a high frequency of extended gas reservoirs, comprising at least 30 percent of our CO-selected sample. An environmental study of the candidates is carried out based on Nth nearest neighbour and we find that the large molecular gas reservoirs tend to exist in denser regions. The spatial distribution of our candidates is mainly centred on the core region of the Spiderweb protocluster. The performance and adaptability of our method are discussed. We found 13 (potentially) extended gas reservoirs located in eight galaxy (proto)clusters from the literature. We noticed that large extended molecular gas reservoirs surrounding (normal) star-forming galaxies in protoclusters are rare. This may be attributable to the lack of observations low-J CO transitions and the lack of quantitative analyses of molecular gas morphologies. The large gas reservoirs in the Spiderweb protocluster are potential sources of the intracluster medium seen in low redshift Virgo- or Coma-like galaxy clusters.

Molecular hydrogen (H2) is an important component of galaxies because it fuels star formation and the accretion of gas onto active galactic nuclei (AGN), the two processes that can generate the large infrared luminosities of gas-rich galaxies. Observations of spectral-line emission from the tracer molecule carbon monoxide (CO) are used to probe the properties of this gas. But the lines that have been studied in the local Universe-mostly the lower rotational transitions of J = 1 --> 0 and J = 2 --> 1-have hitherto been unobservable in high-redshift galaxies. Instead, higher transitions have been used, although the densities and temperatures required to excite these higher transitions may not be reached by much of the gas. As a result, past observations may have underestimated the total amount of molecular gas by a substantial amount. Here we report the discovery of large amounts of low-excitation molecular gas around the infrared-luminous quasar APM08279+5255 at redshift z = 3.91, using the two lowest excitation lines of 12 CO (J = 1 --> 0 and J = 2 --> 1). The maps confirm the presence of hot and dense gas near the nucleus, and reveal an extended reservoir of molecular gas with low excitation that is 10 to 100 times more massive than the gas traced by the higher-excitation observations. This raises the possibility that significant amounts of low-excitation molecular gas may exist in the environments of high-redshift (z > 3) galaxies.

We present CO (2−1) observations of 72 galaxies in the nearby, disturbed Antlia Cluster with the Atacama Pathfinder Experiment telescope. The galaxies in our sample are selected to span a wide range of stellar masses (108 M ⊙ ≲ M ⋆ ≲ 1010 M ⊙) and star formation rates (0.0005 M ⊙ yr−1 < SFR < 0.3 M ⊙ yr−1). Reaching a depth of 23 mJy in 50 km s−1 channels, we report a total CO detection rate of 37.5% and a CO detection rate of 86% for sources within 1 dex of the main sequence. We compare our sample with a similar sample of galaxies in the field, finding that, for a fixed stellar mass and SFR, galaxies in the Antlia Cluster have comparable molecular gas reservoirs to field galaxies. We find that ∼41% (11/27) of our CO detections display non-Gaussian CO (2−1) emission-line profiles, and a number of these sources display evidence of quenching in their optical images. We also find that the majority of our sample lies either just below or far below the main sequence of field galaxies, further hinting at potential ongoing quenching. We conclude that the Antlia Cluster represents an intermediate environment between fields and dense clusters, where the gentler intracluster medium (ICM) allows the cluster members to retain their reservoirs of molecular gas, but in which the disturbed ICM is just beginning to influence the member galaxies, resulting in high SFRs and possible ongoing quenching.

In around ≈25% of early-type galaxies (ETGs) UV emission from young stellar populations is present. Molecular gas reservoirs have been detected in these systems (e.g. Young et al. (2011), providing the fuel for this residual star-formation. The environment in which this molecular gas is found is quite different than that in spiral galaxies however, with harsher radiation fields, deeper potentials and high metallicity and alpha-element abundances. Here we report on one element of our multi-faceted programme to understand the similarities and differences between the gas reservoirs in spirals and ETGs. We use spatially resolved observations from the CARMA mm-wave interferometer to investigate the size of the molecular reservoirs in the the CO-rich ATLAS3D ETGs (survey described in Alatalo et al. 2012, submitted). We find that the molecular gas extent is smaller in absolute terms in ETGs than in late-type galaxies, but that the size distributions are similar once scaled by the galaxies optical/stellar characteristic scale-lengths (Fig 1, left). Amongst ETGs, we find that the extent of the molecular gas is independent of the kinematic misalignment, despite the many reasons why misaligned gas might have a smaller extent. The extent of the molecular gas does depend on environment, with Virgo cluster ETGs having less extended molecular gas reservoirs (Fig 1, right). Whatever the cause, this further emphases that cluster ETGs follow different evolutionary pathways from those in the field. Full details of this work will be presented in Davis et al. (2012), submitted.

Starburst galaxies at the peak of cosmic star formation are among the most extreme star-forming engines in the Universe, producing stars over about 100 million years (ref. 2). The star-formation rates of these galaxies, which exceed 100 solar masses per year, require large reservoirs of cold molecular gas to be delivered to their cores, despite strong feedback from stars or active galactic nuclei. Consequently, starburst galaxies are ideal for studying the interplay between this feedback and the growth of a galaxy. The methylidyne cation, CH+, is a most useful molecule for such studies because it cannot form in cold gas without suprathermal energy input, so its presence indicates dissipation of mechanical energy or strong ultraviolet irradiation. Here we report the detection of CH+ (J = 1-0) emission and absorption lines in the spectra of six lensed starburst galaxies at redshifts near 2.5. This line has such a high critical density for excitation that it is emitted only in very dense gas, and is absorbed in low-density gas. We find that the CH+ emission lines, which are broader than 1,000 kilometres per second, originate in dense shock waves powered by hot galactic winds. The CH+ absorption lines reveal highly turbulent reservoirs of cool (about 100 kelvin), low-density gas, extending far (more than 10 kiloparsecs) outside the starburst galaxies (which have radii of less than 1 kiloparsec). We show that the galactic winds sustain turbulence in the 10-kiloparsec-scale environments of the galaxies, processing these environments into multiphase, gravitationally bound reservoirs. However, the mass outflow rates are found to be insufficient to balance the star-formation rates. Another mass input is therefore required for these reservoirs, which could be provided by ongoing mergers or cold-stream accretion. Our results suggest that galactic feedback, coupled jointly to turbulence and gravity, extends the starburst phase of a galaxy instead of quenching it.

We use high-resolution data from the millimetre-Wave Interferometric Survey of Dark Object Masses (WISDOM) project to investigate the connection between circumnuclear gas reservoirs and nuclear activity in a sample of nearby galaxies. Our sample spans a wide range of nuclear activity types including radio galaxies, Seyfert galaxies, low-luminosity active galactic nuclei (AGN) and inactive galaxies. We use measurements of nuclear millimetre continuum emission along with other archival tracers of AGN accretion/activity to investigate previous claims that at, circumnuclear scales (<100 pc), these should correlate with the mass of the cold molecular gas. We find that the molecular gas mass does not correlate with any tracer of nuclear activity. This suggests the level of nuclear activity cannot solely be regulated by the amount of cold gas around the supermassive black hole (SMBH). This indicates that AGN fuelling, that drives gas from the large-scale galaxy to the nuclear regions, is not a ubiquitous process and may vary between AGN type, with time-scale variations likely to be very important. By studying the structure of the central molecular gas reservoirs, we find our galaxies have a range of nuclear molecular gas concentrations. This could indicate that some of our galaxies may have had their circumnuclear regions impacted by AGN feedback, even though they currently have low nuclear activity. Alternatively, the nuclear molecular gas concentrations in our galaxies could instead be set by secular processes.

Context. Probing the molecular gas reservoirs of z ≳ 6 quasar (QSO) host galaxies is fundamental to understanding the coevolution of star formation and black hole growth in these extreme systems. Yet, there is still an inhomogeneous coverage of molecular gas tracers for z ≳ 6 QSO hosts. Aims. To measure the average excitation and mass of the molecular gas reservoirs in the brightest z > 6.5 QSO hosts, we combined new observations of CO(2–1) emission with existing observations of CO(6–5), CO(7–6), [C I] (2–1), [C II] 158 μm, and dust-continuum emission. Methods. We reduced and analysed observations of CO(2–1), taken with the Karl G. Jansky Very Large Array, in three z = 6.5 − 6.9 QSO hosts – the highest redshift observations of CO(2–1) to date. By combining these with the nine z = 5.7 − 6.4 QSO hosts for which CO(2–1) emission has already been observed, we studied the spread in molecular gas masses and CO excitation of z ≳ 6 QSOs. Results. Two of our three QSOs, P036+03 and J0305–3150, were not detected in CO(2–1), implying more highly excited CO than in the well-studied z = 6.4 QSO J1148+5251. However, we detected CO(2–1) emission at 5.1σ for our highest-redshift target, J2348–3054, yielding a molecular gas mass of (1.2 ± 0.2)×1010 M⊙, assuming αCO = 0.8 (K km s−1 pc2)−1 and r2, 1 = 1. This molecular gas mass is equivalent to the lower limit on the dynamical mass measured previously from resolved [C II] 158 μm observations, implying that there is little mass in stars or neutral gas within the [C II]-emitting region and that a low CO-to-H2 conversion factor is applicable. On average, these z ≳ 6 QSO hosts have far higher CO(6–5)-, CO(7–6)-, and [C II] 158 μm versus CO(2–1) line ratios than the local gas-rich and IR-luminous galaxies that host active galactic nuclei, but with a large range of values, implying some variation in their interstellar medium conditions. We derived a mean CO(6–5)-to-CO(1–0) line luminosity ratio of r6, 1 = 0.9 ± 0.2. Conclusions. Our new CO(2–1) observations show that even at 780 Myr after the Big Bang, QSO host galaxies can already have molecular gas masses of 1010 M⊙, consistent with a picture in which these z ≳ 6 QSOs reside in massive starbursts that are coevolving with the accreting supermassive black holes. Their high gas versus dynamical masses and extremely high line excitation imply the presence of extremely dense and warm molecular gas reservoirs illuminated by strong interstellar radiation fields.

We present recent Chandra X-ray observations of the RX J0821.0+0752 galaxy cluster, in addition to ALMA observations of the CO(1–0) and CO(3–2) line emission tracing the molecular gas in its central galaxy. All of the CO line emission, originating from a molecular gas reservoir, is located several kiloparsecs away from the nucleus of the central galaxy. The cold gas is concentrated into two main clumps surrounded by a diffuse envelope. They form a wide filament coincident with a plume of bright X-ray emission emanating from the cluster core. This plume encompasses a putative X-ray cavity that is only large enough to have uplifted a small percent of the molecular gas. Unlike other brightest cluster galaxies, stimulated cooling, where X-ray cavities lift low-entropy cluster gas until it becomes thermally unstable, cannot have produced the observed gas reservoir. Instead, the molecular gas has likely formed as a result of sloshing motions in the intracluster medium induced by a nearby galaxy. Sloshing can emulate uplift by dislodging gas from the galactic center. This gas has the shortest cooling time, so it will condense if disrupted for long enough.

We present Atacama Large Millimeter/submillimeter Array (ALMA) CO(2–1) spectroscopy of six massive (log10 / > 11.3) quiescent galaxies at z ∼ 1.5. These data represent the largest sample using CO emission to trace molecular gas in quiescent galaxies above z > 1, achieving an average 3σ sensitivity of ∼ 1010 . We detect one galaxy at 4σ significance and place upper limits on the molecular gas reservoirs of the other five, finding molecular gas mass fractions (3σ upper limits). This is 1–2 orders of magnitude lower than coeval star-forming galaxies at similar stellar mass, and comparable to galaxies at z = 0 with similarly low specific star formation rate (sSFR). This indicates that their molecular gas reservoirs were rapidly and efficiently used up or destroyed, and that gas fractions are uniformly low (<6%) despite the structural diversity of our sample. The implied rapid depletion time of molecular gas ( < 0.6 Gyr) disagrees with extrapolations of empirical scaling relations to low sSFR. We find that our low gas fractions are instead in agreement with predictions from both the recent simba cosmological simulation, and from analytical “bathtub” models for gas accretion onto galaxies in massive dark matter halos (log at z = 0). Such high mass halos reach a critical mass of log by z ∼ 4 that halt the accretion of baryons early in the universe. Our data are consistent with a simple picture where galaxies truncate accretion and then consume the existing gas at or faster than typical main-sequence rates. Alternatively, we cannot rule out that these galaxies reside in lower mass halos, and low gas fractions may instead reflect either stronger feedback, or more efficient gas consumption.

In order to understand galaxy evolution through cosmic times it is critical to derive the properties of the molecular gas content of galaxies, i.e. the material out of which stars ultimately form. The last decade has seen rapid progress in this area, with the detection of massive molecular gas reservoirs at high redshifts in submillimeter-selected galaxies and quasars. In the latter case, molecular gas reservoirs have been quantified out to redshifts z>6, i.e. towards the end of cosmic reionization when the universe was less than one Gyr old. The recent discovery of molecular gas in more normal galaxies have extended these studies from the most extreme objects in the universe (SFR~1000M_sun/yr; quasars and submillimeter galaxies) to more 'normal' starforming systems at redshifts 1.5-2.5 (with SFR~100M_sun/yr). However, detecting the molecular gas reservoirs of high-redshift galaxies that only have moderate star formation rates (~<10 M_sun/yr, similar to the faint galaxies seen in the Hubble Ultra Deep Field) will likely have to await the completion of ALMA.