Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales 1-3 . The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas 4 . Recent theory 5-7 and simulations 8-10 instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds -a departure from the 'hot mode' accretion model -although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z = 0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma [11][12][13] . Under the right conditions, thermal instabilities can precipitate from this hot gas, producing a rain of cold clouds that fall toward the galaxy's centre 14 , sustaining star formation amid a kiloparsec-scale molecular nebula that inhabits its core 15 . The observations show that these cold clouds also fuel black hole accretion, revealing 'shadows' cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole in the galaxy centre, which serves as a bright backlight. Corroborating evidence from prior observations 16 of warmer atomic gas at extremely high spatial resolution 17 , along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it. We observed the Abell 2597 Brightest Cluster Galaxy (Fig. 1) with the Atacama Large Millimeter/submillimeter Array (ALMA), enabling us to create a three-dimensional map of both the location and motions of cold gas at uniquely high sensitivity and spatial resolution. The ALMA receivers were sensitive to emission from the J = 2 − 1 rotational line of the carbon monoxide (CO) molecule. CO(2-1) emission is used as a tracer of cold (∼ 10 − 30 K) molecular hydrogen, which is vastly more abundant, but not directly observable at these low temperatures.The continuum-subtracted CO(2-1) images (Fig. 2) reveal that the filamentary emission line nebula that spans the galaxy's innermost ∼ 30 kpc (Fig. 1b) consists not only of warm ionised gas [18][19][20] , but cold molecular gas as well. In projection, the optical emission line nebula is cospatial and morphologically matched with CO(2-1) emission detected at a significance between > ∼ 3σ (in the outer filaments) and > ∼ 20σ (in the nuclear region) above the background noise level. The warm ionised nebula is therefore likely to have a substantial molecular component, consistent with results for other similar galaxies 21 . The total measured CO(2-1) line flux corresponds to a molecular hydrogen gas mass of M H 2 = (1.8 ± 0.2) × 10 9...
We present ALMA and MUSE observations of the Brightest Cluster Galaxy in Abell 2597, a nearby (z = 0.0821) cool core cluster of galaxies. The data map the kinematics of a three billion solar mass filamentary nebula that spans the innermost 30 kpc of the galaxy's core. Its warm ionized and cold molecular components are both cospatial and comoving, consistent with the hypothesis that the optical nebula traces the warm envelopes of many cold molecular clouds that drift in the velocity field of the hot X-ray atmosphere. The clouds are not in dynamical equilibrium, and instead show evidence for inflow toward the central supermassive black hole, outflow along the jets it launches, and uplift by the buoyant hot bubbles those jets inflate. The entire scenario is therefore consistent with a galaxy-spanning "fountain", wherein cold gas clouds drain into the black hole accretion reservoir, powering jets and bubbles that uplift a cooling plume of low-entropy multiphase gas, which may stimulate additional cooling and accretion as part of a self-regulating feedback loop. All velocities are below the escape speed from the galaxy, and so these clouds should rain back toward the galaxy center from which they came, keeping the fountain long-lived. The data are consistent with major predictions of chaotic cold accretion, precipitation, and stimulated feedback models, and may trace processes fundamental to galaxy evolution at effectively all mass scales.
Context. The study of galaxy luminosity functions (LFs) in different environments provides powerful constraints on the physics of galaxy evolution. The infrared (IR) LF is a particularly useful tool since it is directly related to the distribution of galaxy star-formation rates (SFRs). Aims. We aim to determine the galaxy IR LF as a function of the environment in a supercluster at redshift 0.23 to shed light on the processes driving galaxy evolution in and around clusters. Methods. We base our analysis on multi-wavelength data, which include optical, near-IR, and mid-to far-IR photometry, as well as redshifts from optical spectroscopy. We identify 467 supercluster members in a sample of 24-μm-selected galaxies, on the basis of their spectroscopic (153) and photometric (314) redshifts. IR luminosities and stellar masses are determined for supercluster members via spectral energy distribution fitting. Galaxies with active galactic nuclei are identified by a variety of methods and excluded from the sample. SFRs are obtained for the 432 remaining galaxies from their IR luminosities via the Kennicutt relation. Results. We determine the IR LF of the whole supercluster as well as the IR LFs of three different regions in the supercluster: the cluster core, a large-scale filament, and the cluster outskirts (excluding the filament). A comparison of the IR LFs of the three regions, normalized by the average number densities of r-band selected normal galaxies, shows that the filament (respectively, the core) contains the highest (respectively, the lowest) fraction of IR-emitting galaxies at all levels of IR luminosities, and the highest (respectively, the lowest) total SFR normalized by optical galaxy richness. Luminous IR galaxies (LIRGs) are almost absent in the core region. The relation between galaxy specific SFRs and stellar masses does not depend on the environment, and it indicates that most supercluster LIRGs are rather massive galaxies with relatively low specific SFRs. A comparison with previous IR LF determinations from the literature confirms that the mass-normalized total SFR in clusters increases with redshift, but more rapidly than previously suggested for redshifts < ∼ 0.4. Conclusions. The IR LF shows an environmental dependence that is not simply related to the local galaxy density. The filament, an intermediate-density region in the A1763 supercluster, contains the highest fraction of IR-emitting galaxies. We interpret our findings within a possible scenario for the evolution of galaxies in and around clusters.
We identify close companions of brightest cluster galaxies (BCGs) for the purpose of quantifying the rate at which these galaxies grow via mergers. By exploiting deep photometric data from the Canada–France–Hawaii Telescope Legacy Survey (CFHTLS), we probe the number of companions per BCG (Nc) with luminosity ratios down to those corresponding to potential minor mergers of 20:1. We also measure the average luminosity in companions per galaxy (Lc). We find that Nc and Lc rise steeply with luminosity ratio for both the BCGs, and a control sample of other bright, red, cluster galaxies. The trend for BCGs rises more steeply, resulting in a larger number of close companions. For companions within 50 kpc of a BCG, Nc = 1.38 ± 0.14 and Lc = 2.14 ± 0.31 × 1010 L⊙, and for companions within 50 kpc of a luminosity matched control sample of non‐BCGs, Nc = 0.87 ± 0.08 and Lc = 1.48 ± 0.20 × 1010 L⊙. This suggests that the BCGs are likely to undergo more mergers compared to otherwise comparable luminous galaxies. Additionally, compared to a local sample of luminous red galaxies, the more distant sample presented in this study (with redshifts between 0.15 and 0.39) shows a higher Nc, suggesting that the younger and smaller BCGs are still undergoing hierarchical formation. Using the Millennium Simulations we model and estimate the level of contamination due to unrelated cluster galaxies. The contamination by interloping galaxies is 50 per cent within projected separations of 50 kpc, but within 30 kpc, 60 per cent of identified companions are real physical companions. We conclude that the luminosity of bound merger candidates down to luminosity ratios of 20:1 could be adding as much as 10 per cent to the mass of a typical BCG over 0.5 Gyr at redshifts of z ∼ 0.3.
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