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Multi-phase filamentary structures around brightest cluster galaxies (BCG) are likely a key step of AGN-feedback. We observed molecular gas in three cool cluster cores, namely 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. 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 H α /CO flux ratio is roughly constant for most of the objects, suggesting that (i) between 1.2 and 6 times more cold gas could be present and (ii) local processes must be responsible for the excitation. Projected velocities are between 100 and 400 km s −1 , with disturbed kinematics and sometimes coherent gradients. This is likely due to the mixing in projection of several thin (and 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 radial extent of the filaments, 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 of t cool / t ff of 8−23 at r fil , is likely due to (i) a more complex gravitational potential affecting the free-fall time t ff (sloshing, mergers, etc.) and (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.
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Testing the Limits of AGN Feedback and the Onset of Thermal Instability in the Most Rapidly Star-forming Brightest Cluster Galaxies
Abstract We present new, deep, narrow- and broadband Hubble Space Telescope observations of seven of the most star-forming brightest cluster galaxies (BCGs). Continuum-subtracted [O II ] maps reveal the detailed, complex structure of warm ( T ∼ 10 4 K) ionized gas filaments in these BCGs, allowing us to measure spatially resolved star formation rates (SFRs) of ∼60–600 M ⊙ yr −1 . We compare the SFRs in these systems and others from the literature to their intracluster medium cooling rates ( M ̇ cool ), measured from archival Chandra X-ray data, finding a best-fit relation of log ( SFR ) = ( 1.66 ± 0.17 ) log ( M ̇ cool ) + (−3.22 ± 0.38) with an intrinsic scatter of 0.39 ± 0.09 dex. This steeper-than-unity slope implies an increasingly efficient conversion of hot ( T ∼ 10 7 K) gas into young stars with increasing M ̇ cool , or conversely a gradual decrease in the effectiveness of AGN feedback in the strongest cool cores. We also seek to understand the physical extent of these multiphase filaments that we observe in cluster cores. We show, for the first time, that the average extent of the multiphase gas is always smaller than the radii at which the cooling time reaches 1 Gyr, the t cool / t ff profile flattens, and that X-ray cavities are observed. This implies a close connection between the multiphase filaments, the thermodynamics of the cooling core, and the dynamics of X-ray bubbles. Interestingly, we find a one-to-one correlation between the average extent of cool multiphase filaments and the radius at which the cooling time reaches 0.5 Gyr, which may be indicative of a universal condensation timescale in cluster cores.
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- Award ID(s):
- 1751096
- PAR ID:
- 10439159
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 940
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 140
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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ABSTRACT Radiative cooling and active galactic nucleus heating are thought to form a feedback loop that regulates the evolution of low-redshift cool-core galaxy clusters. Numerical simulations suggest that the formation of multiphase gas in the cluster core imposes a floor on the ratio of cooling time (tcool) to free-fall time (tff) at min(tcool/tff) ≈ 10. Observations of galaxy clusters show evidence for such a floor, and usually the cluster cores with min(tcool/tff) ≲ 30 contain abundant multiphase gas. However, there are important outliers. One of them is Abell 2029 (A2029), a massive galaxy cluster (M200 ≳ 1015 M⊙) with min(tcool/tff) ∼ 20, but little apparent multiphase gas. In this paper, we present high-resolution 3D hydrodynamic adaptive mesh refinement simulations of a cluster similar to A2029 and study how it evolves over a period of 1–2 Gyr. Those simulations suggest that A2029 self-regulates without producing multiphase gas because the mass of its central black hole ($${\sim} 5 \times 10^{10} \, \mathrm{ M}_\odot$$) is great enough for Bondi accretion of hot ambient gas to produce enough feedback energy to compensate for radiative cooling.more » « less
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null (Ed.)Context. The excitation of the filamentary gas structures surrounding giant elliptical galaxies at the center of cool-core clusters, also known as brightest cluster galaxies (BCGs), is key to our understanding of active galactic nucleus (AGN) feedback, and of the impact of environmental and local effects on star formation. Aims. We investigate the contribution of thermal radiation from the cooling flow surrounding BCGs to the excitation of the filaments. We explore the effects of small levels of extra heating (turbulence), and of metallicity, on the optical and infrared lines. Methods. Using the C LOUDY code, we modeled the photoionization and photodissociation of a slab of gas of optical depth A V ≤ 30 mag at constant pressure in order to calculate self-consistently all of the gas phases, from ionized gas to molecular gas. The ionizing source is the extreme ultraviolet (EUV) and soft X-ray radiation emitted by the cooling gas. We tested these models comparing their predictions to the rich multi-wavelength observations from optical to submillimeter, now achieved in cool core clusters. Results. Such models of self-irradiated clouds, when reaching sufficiently large A V , lead to a cloud structure with ionized, atomic, and molecular gas phases. These models reproduce most of the multi-wavelength spectra observed in the nebulae surrounding the BCGs, not only the low-ionization nuclear emission region like optical diagnostics, [O III ] λ 5007 Å/H β , [N II ] λ 6583 Å/H α , and ([S II ] λ 6716 Å+[S II ] λ 6731 Å)/H α , but also the infrared emission lines from the atomic gas. [O I ] λ 6300 Å/H α , instead, is overestimated across the full parameter space, except for very low A V . The modeled ro-vibrational H 2 lines also match observations, which indicates that near- and mid-infrared H 2 lines are mostly excited by collisions between H 2 molecules and secondary electrons produced naturally inside the cloud by the interaction between the X-rays and the cold gas in the filament. However, there is still some tension between ionized and molecular line tracers (i.e., CO), which requires optimization of the cloud structure and the density of the molecular zone. The limited range of parameters over which predictions match observations allows us to constrain, in spite of degeneracies in the parameter space, the intensity of X-ray radiation bathing filaments, as well as some of their physical properties like A V or the level of turbulent heating rate. Conclusions. The reprocessing of the EUV and X-ray radiation from the plasma cooling is an important powering source of line emission from filaments surrounding BCGs. C LOUDY self-irradiated X-ray excitation models coupled with a small level of turbulent heating manage to simultaneously reproduce a large number of optical-to-infrared line ratios when all the gas phases (from ionized to molecular) are modeled self-consistently. Releasing some of the simplifications of our model, like the constant pressure, or adding the radiation fields from the AGN and stars, as well as a combination of matter- and radiation-bounded cloud distribution, should improve the predictions of line emission from the different gas phases.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.3847/1538-4357/ac9790
