Cold Gas in Galaxy Cluster Cores

Abstract

We present a survey of 73 galaxy groups and clusters aimed at determining the role played by the cold gas in the feedback process. We use optical Integral Field Spectroscopy from the VIsible Multi Object Spectrograph in conjunction with high spatial resolution X-ray observations from the Chandra X-ray Observatory and extensive multi-wavelength observations to study the interactions of the cold gas with the Intra Cluster Medium and the Brightest Cluster Galaxy. The wealth of information provided by these observations has allowed us to study the kinematic structure of the cold gas in the largest sample of cluster cores to date. We use this information to shed new light three key questions i) Are all line emitting systems highly disturbed? ii) What role does the Brightest Cluster Galaxy play in the cooling of gas from the ICM? iii) What role does the cold gas play in the feedback process and what is its relationship to the fueling of the AGN?. The analysis of the full survey immediately suggests that the line emitting gas in the majority of cluster cores isn’t highly disturbed. While several clusters do show a disturbed Hα morphology (13/73) the majority appeared uniform and quiescent (45/73). Similarly the velocity structure of the ionised gas within most systems also appears to be very ordered and interestingly appears decoupled from the stellar
kinematics of the brightest cluster galaxy. Several of the more disturbed systems do show evidence of an interaction with another cluster member, however, this is not
common to all disturbed systems. One of the most interesting discoveries to come out of the sample analysis is the identification of a small sub sample of objects which show an offset between the brightest cluster galaxy and the bulk of the ionised gas. Comparison to X-ray
observations indicates that the cluster core is spatially coincident with the ionised gas emission and not the galaxy. These objects provided an opportunity to study the
role played by the BCG in the cooling of gas from the ICM. The kinematics of these objects suggests that these offsets should be transient and short lived. Despite this we identify a substantial mass of cold molecular gas (∼109 M⊙ ) associated with the offset emission, and not the BCG, for one object. This mass is consistent with the upper limit of the cooling during the lifetime of the offset suggesting that the cooling continues to occur in the core despite being offset from the BCG. This result clearly
indicates that while the BCG may typically reside at the centre of most clusters it is not required for cooling gas to condense from the ICM.
Finally, we address the ordered velocity structure of the ionised gas in BCG in much greater detail. As a pilot project we study the kinematics of Hydra-A using detailed multi-wavelength observations and identify the presence of a disk of cold gas which has an axis of rotation parallel to the axis along which the AGN is inflating cavities into the ICM. Comparison to the energetics of the cavities suggests that the mass of cold gas in the disk is sufcient to fuel a future outburst of comparable magnitude. Noting the similar velocity structure in many of the clusters in our sample we apply a similar analysis to the full sample. We use kinemetry and determine that ≈62% of the sample have ionised gas kinematics which are consistent with a rotating disk. A linear trend is identified between the peak rotational amplitude of these disks and the linewidth of CO observations which suggests that the large mass of cold molecular gas present in the cores of clusters shares the kinematic structure of the ionised gas. Finally, we compare the axis of rotation of all disks with radio and X-ray observations and identify a clear alignment between the jets/cavities and the disks axis of rotation. The prevalence of these disks and their alignment with the axis along which the AGN is injecting mechanical energy into the ICM clearly suggests that they play an important role in the feedback process and provides a link between the gas cooling on kpc scales and the fueling of the black hole at the centre of the BCG.