ALMA observations of massive molecular gas filaments encasing radio bubbles in the Phoenix cluster (1611.00017v2)

Abstract: We report new ALMA observations of the CO(3-2) line emission from the $2.1\pm0.3\times10^{10}\rm\thinspace M_{\odot}$ molecular gas reservoir in the central galaxy of the Phoenix cluster. The cold molecular gas is fuelling a vigorous starburst at a rate of $500-800\rm\thinspace M_{\odot}\rm\; yr^{-1}$ and powerful black hole activity in the form of both intense quasar radiation and radio jets. The radio jets have inflated huge bubbles filled with relativistic plasma into the hot, X-ray atmospheres surrounding the host galaxy. The ALMA observations show that extended filaments of molecular gas, each $10-20\rm\; kpc$ long with a mass of several billion solar masses, are located along the peripheries of the radio bubbles. The smooth velocity gradients and narrow line widths along each filament reveal massive, ordered molecular gas flows around each bubble, which are inconsistent with gravitational free-fall. The molecular clouds have been lifted directly by the radio bubbles, or formed via thermal instabilities induced in low entropy gas lifted in the updraft of the bubbles. These new data provide compelling evidence for close coupling between the radio bubbles and the cold gas, which is essential to explain the self-regulation of feedback. The very feedback mechanism that heats hot atmospheres and suppresses star formation may also paradoxically stimulate production of the cold gas required to sustain feedback in massive galaxies.

ALMA Observations of Molecular Gas Filaments and Radio Bubbles in the Phoenix Cluster

The paper "ALMA observations of massive molecular gas filaments encasing radio bubbles in the Phoenix cluster" presents a detailed examination of the molecular gas structures within the central galaxy of the Phoenix cluster using new observations from the Atacama Large Millimeter/submillimeter Array (ALMA). The Phoenix cluster is noted for hosting a vigorous starburst and powerful black hole activity, both fueled by an extensive molecular gas reservoir. The paper focuses on how this molecular gas, in the form of massive filaments, interacts with radio bubbles created by AGN feedback.

Observational Findings

The ALMA data reveal that the central galaxy in the Phoenix cluster possesses a molecular gas mass of approximately $2.1 \times 10^{10} M_\odot$, with the molecular gas extending into large filaments encasing radio bubbles. These filaments are several billion solar masses each and are characterized by smooth velocity gradients and narrow line widths, indicative of ordered gas flows. Importantly, these structures are extensively placed around the peripheries of radio bubbles, suggesting a possible interaction mechanism between the molecular gas and AGN-mediated feedback processes.

Implications of Molecular Gas Dynamics

The paper posits that the molecular gas filaments either originate from gas lifted by the radio bubbles or formed through thermal instabilities in the updraft of such bubbles. The resulting dynamics are crucial for understanding the self-regulation of feedback in the cluster. The velocities observed in these molecular structures, especially at large radii from the central galaxy, suggest that the gas may eventually decouple from the hot intracluster medium, a process fueled by buoyant, radio-mode AGN feedback.

One of the key implications is that while the AGN feedback heats the surrounding gas and suppresses star formation, it may paradoxically stimulate the production and regulation of cold molecular gas, essential for sustaining and fueling star formation and AGN activities in massive galaxies. This balance of feedback is vital for explaining the evolutionary trajectories of massive galaxies from active, star-forming systems to more quiescent ellipticals.

Theoretical and Practical Perspectives

The paper highlights the critical link between molecular gas dynamics and radio bubble evolution within the cluster’s core. This connection is essential to advance theoretical models of AGN feedback and galaxy evolution. Such feedback mechanisms have been hypothesized to truncate galaxy growth by coupling efficiently across a range of spatial scales. The Phoenix cluster observations provide a profound observational basis for these theoretical frameworks, demonstrating that radio-mode feedback can drive large-scale gas flows and contribute to a feedback-regulated balance in star formation rates.

Future Directions

Future work will need to focus on refining the understanding of the molecular gas’s role in AGN feedback. Specifically, observations with higher resolution and a broader sample of clusters could help clarify how widespread this feedback mechanism is and to what extent cold gas and AGN-driven outflows shape galaxy evolution over cosmic time. The apparent inefficiency in gravitational free-fall, revealed by the low intrinsic velocities, calls for an exploration of additional physical processes, such as magnetic tension or interaction with pre-existing structures, that might be at play.

In summary, the paper provides compelling observational evidence that links the molecular gas morphology and kinematics in the Phoenix cluster with AGN-driven feedback, offering insights essential for the theoretical modeling of galaxy formation and evolution.