On The Origin of Cold Molecular Gas In Giant Elliptical Galaxies

Abstract

We present an analysis of 55 giant elliptical galaxies residing in the centre of clusters and groups to investigate the origin of H₂ molecular gas centrally located in these galaxies. Using Chandra X-ray data and IRAM 30m CO line surveys, we report a sharp threshold for the detection of molecular gas when the central cooling time or entropy index of the hot atmosphere falls below ~1 Gyr or ~35 keV cm², respectively. This shows a direct relationship between the molecular gas and the local state of the intracluster gas, which is found to always exceed the amount of detected molecular gas within the field of view of the telescope. These findings point to thermal instability in the hot atmosphere as the origin of the observed molecular gas. We compare our results to the predictions of precipitation models where thermal instabilities are triggered when the minimum of the cooling to free-fall time ratio (tcool/tff) is less than some constant. For our sample, we find this constant to be ~25. We find these models to be inconsistent with observations as the effects of continual AGN outbursts on the ICM do not lead to the large fluctuations in central gas density and cooling time implied in these models. Consequently, we find that the cooling time threshold forecasts CO emission more reliably than the min(tcool/tff) criterion as applied in this study. We conclude with a discussion of the stimulated feedback model that includes the important physics of thermal instability and the uplift of low entropy gas by buoyantly rising X-ray cavities supported by a growing number of ALMA observations.