The Astropy Project supports and fosters the development of open-source and openly developed Python packages that provide commonly needed functionality to the astronomical community. A key element of the Astropy Project is the core package astropy, which serves as the foundation for more specialized projects and packages. In this article, we provide an overview of the organization of the Astropy project and summarize key features in the core package, as of the recent major release, version 2.0. We then describe the project infrastructure designed to facilitate and support development for a broader ecosystem of interoperable packages. We conclude with a future outlook of planned new features and directions for the broader Astropy Project.

We propose a novel method to constrain turbulence and bulk motions in massive galaxies, galaxy groups and clusters, exploring both simulations and observations. As emerged in the recent picture of the top-down multiphase condensation, the hot gaseous halos are tightly linked to all other phases in terms of cospatiality and thermodynamics. While hot halos (∼ 10 7 K) are perturbed by subsonic turbulence, warm (∼ 10 4 K) ionized and neutral filaments condense out of the turbulent eddies. The peaks condense into cold molecular clouds (< 100 K) raining in the core via chaotic cold accretion (CCA). We show all phases are tightly linked in terms of the ensemble (wide-aperture) velocity dispersion along the line of sight. The correlation arises in complementary long-term AGN feedback simulations and high-resolution CCA runs, and is corroborated by the combined Hitomi and new Integral Field Unit measurements in Perseus cluster. The ensemble multiphase gas distributions (from UV to radio band) are characterized by substantial spectral line broadening (σ v,los ≈ 100 -200 km s −1 ) with mild line shift. On the other hand, pencil-beam detections (as HI absorption against the AGN backlight) sample the small-scale clouds displaying smaller broadening and significant line shift up to several 100 km s −1 (for those falling toward the AGN), with increased scatter due to the turbulence intermittency. We present new ensemble σ v,los of the warm Hα+[NII] gas in 72 observed cluster/group cores: the constraints are consistent with the simulations and can be used as robust proxies for the turbulent velocities, in particular for the challenging hot plasma (otherwise requiring extremely long X-ray exposures). Finally, we show the physically motivated criterion C ≡ t cool /t eddy ≈ 1 best traces the condensation extent region and presence of multiphase gas in observed clusters and groups. The ensemble method can be applied to many available spectroscopic datasets and can substantially advance our understanding of multiphase halos in light of the next-generation multiwavelength missions.

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...

Multi-phase filamentary structures around Brightest Cluster Galaxies (BCG) are likely a key step of AGN-feedback. We observed molecular gas in 3 cool cluster cores: Centaurus, Abell S1101, and RXJ1539.5 and gathered ALMA (Atacama Large Millimeter/submillimeter Array) and MUSE (Multi Unit Spectroscopic Explorer) data for 12 other clusters. Those observations show clumpy, massive and long, 3-25 kpc, molecular filaments, preferentially located around the radio bubbles inflated by the AGN (Active Galactic Nucleus). Two objects show nuclear molecular disks. The optical nebula is certainly tracing the warm envelopes of cold molecular filaments. Surprisingly, the radial profile of the Hα/CO flux ratio is roughly constant for most of the objects, suggesting that (i) between 1.2 to 7 times more cold gas could be present and (ii) local processes must be responsible for the excitation. Projected velocities are between 100-400 km s −1 , with disturbed kinematics and sometimes coherent gradients. This is likely due to the mixing in projection of several thin (as yet) unresolved filaments. The velocity fields may be stirred by turbulence induced by bubbles, jets or merger-induced sloshing. Velocity and dispersions are low, below the escape velocity. Cold clouds should eventually fall back and fuel the AGN. We compare the filament's radial extent, r fil , with the region where the X-ray gas can become thermally unstable. The filaments are always inside the low-entropy and short cooling time region, where t cool /t ff <20 (9 of 13 sources). The range t cool /t ff , 8-23 at r fil , is likely due to (i) a more complex gravitational potential affecting the free-fall time t ff (sloshing, mergers. . . ); (ii) the presence of inhomogeneities or uplifted gas in the ICM, affecting the cooling time t cool . For some of the sources, r fil lies where the ratio of the cooling time to the eddy-turnover time, t cool /t eddy , is approximately unity.

We report the discovery that the known "changing look" AGN Mrk 1018 has changed spectral type for a second time. New VLT-MUSE data taken in 2015 as part of the Close AGN Reference Survey (CARS) shows that the AGN has returned to its original Seyfert 1.9 classification. The CARS sample is selected to contain only bright type 1 AGN, but Mrk 1018's broad emission lines and continuum, typical of type 1 AGN, have almost entirely disappeared. We use spectral fitting of the MUSE spectrum and previously available spectra to determine the drop in broad line flux and the Balmer decrement. We find that the broad line flux has decreased by a factor of 4.75 ± 0.5 in Hα since an SDSS spectrum was taken in 2000. The Balmer decrement has not changed significantly implying no enhanced reddening with time, but the remaining broad lines are more asymmetric than those present in the type 1 phase. We posit that the change is due to an intrinsic drop in flux from the accretion disk rather than variable extinction or a tidal disruption event.

We carry out a comprehensive Bayesian correlation analysis between hot halos and direct masses of supermassive black holes (SMBHs), by retrieving the X-ray plasma properties (temperature, luminosity, density, pressure, masses) over galactic to cluster scales for 85 diverse systems. We find new key scalings, with the tightest relation being the M • − T x , followed by M • − L x . The tighter scatter (down to 0.2 dex) and stronger correlation coefficient of all the X-ray halo scalings compared with the optical counterparts (as the M • − σ e ) suggest that plasma halos play a more central role than stars in tracing and growing SMBHs (especially those that are ultramassive). Moreover, M • correlates better with the gas mass than dark matter mass. We show the important role of the environment, morphology, and relic galaxies/coronae, as well as the main departures from virialization/self-similarity via the optical/X-ray fundamental planes. We test the three major channels for SMBH growth: hot/Bondilike models have inconsistent anti-correlation with X-ray halos and too low feeding; cosmological simulations find SMBH mergers as sub-dominant over most of the cosmic time and too rare to induce a central-limit-theorem effect; the scalings are consistent with chaotic cold accretion (CCA), the rain of matter condensing out of the turbulent X-ray halos that sustains a long-term self-regulated feedback loop. The new correlations are major observational constraints for models of SMBH feeding/feedback in galaxies, groups, and clusters (e.g., to test cosmological hydrodynamical simulations), and enable the study of SMBHs not only through X-rays, but also via the Sunyaev-Zel'dovich effect (Compton parameter), lensing (total masses), and cosmology (gas fractions).

We present ALMA observations of the CO(1-0) and CO(3-2) line emission tracing filaments of cold molecular gas in the central galaxy of the cluster PKS 0745-191. The total molecular gas mass of 4.6±0.3×10 9 M , assuming a Galactic X CO factor, is divided roughly equally between three filaments each extending radially 3 − 5 kpc from the galaxy centre. The emission peak is located in the SE filament ∼ 1 arcsec (2 kpc) from the nucleus. The velocities of the molecular clouds in the filaments are low, lying within ±100 km s −1 of the galaxy's systemic velocity. Their FWHMs are less than 150 km s −1 , which is significantly below the stellar velocity dispersion. Although the molecular mass of each filament is comparable to a rich spiral galaxy, such low velocities show that the filaments are transient and the clouds would disperse on < 10 7 yr timescales unless supported, likely by the indirect effect of magnetic fields. The velocity structure is inconsistent with a merger origin or gravitational free-fall of cooling gas in this massive central galaxy. If the molecular clouds originated in gas cooling even a few kpc from their current locations their velocities would exceed those observed. Instead, the projection of the N and SE filaments underneath X-ray cavities suggests they formed in the updraft behind bubbles buoyantly rising through the cluster atmosphere. Direct uplift of the dense gas by the radio bubbles appears to require an implausibly high coupling efficiency. The filaments are coincident with low temperature X-ray gas, bright optical line emission and dust lanes indicating that the molecular gas could have formed from lifted warmer gas that cooled in situ.

We present a study of 107 galaxies, groups, and clusters spanning ∼3 orders of magnitude in mass, ∼5 orders of magnitude in central galaxy star formation rate (SFR), ∼4 orders of magnitude in the classical cooling rate (Ṁ cool ≡ M gas (r < r cool )/t cool ) of the intracluster medium (ICM), and ∼5 orders of magnitude in the central black hole accretion rate. For each system in this sample we measure the ICM cooling rate,Ṁ cool , using archival Chandra X-ray data and acquire the SFR and systematic uncertainty in the SFR by combining over 330 estimates from dozens of literature sources. With these data, we estimate the efficiency with which the ICM cools and forms stars, finding ǫ cool ≡ SF R/Ṁ cool = 1.4 ± 0.4% for systems withṀ cool > 30 M ⊙ yr −1 . For these systems, we measure a slope in the SFR-Ṁ cool relation greater than unity, suggesting that the systems with the strongest cool cores are also cooling more efficiently. We propose that this may be related to, on average, higher black hole accretion rates in the strongest cool cores, which could influence the total amount (saturating near the Eddington rate) and dominant mode (mechanical vs radiative) of feedback. For systems withṀ cool < 30 M ⊙ yr −1 , we find that the SFR andṀ cool are uncorrelated, and show that this is consistent with star formation being fueled at a low (but dominant) level by recycled ISM gas in these systems. We find an intrinsic log-normal scatter in SFR at fixedṀ cool of 0.52 ± 0.06 dex (1σ RMS), suggesting that cooling is tightly self-regulated over very long timescales, but can vary dramatically on short timescales. There is weak evidence that this scatter may be related to the feedback mechanism, with the scatter being minimized (∼0.4 dex) for systems for which the mechanical feedback power is within a factor of two of the cooling luminosity.