A ${}^{13}$CO Detection in a Massive Molecular Gas Flow in Abell 1795's Central Galaxy

Abstract: We present new ALMA ${}<sup>{13}$CO(2-1) observations of the central galaxy in the cool-core cluster Abell 1795, targeting the origin of extended molecular filaments. The filaments extend for 5--7 kpc and are closely drawn up around large radio bubbles inflated by jets from the central active galactic nucleus (AGN). Smooth velocity gradients along their lengths are consistent with entrained flows lifted by the expanding bubbles. Here we detect the northern molecular filament in the low-opacity molecular tracer ${}<sup>{13}$CO(2-1), which can provide a more reliable measure of the molecular gas mass and conditions than ${}<sup>{12}$CO. The velocity structure in ${}<sup>{13}$CO(2-1) is consistent with that of the ${}<sup>{12}$CO(2-1) emission. Using a range of plausible excitation temperatures and covering fractions, we thereby determine a filament molecular gas mass of (0.1--2) $\times \ 10⁹ \ \text{M}<em>\odot$, or down to an order of magnitude below previous estimates. This implies a CO-to-H$_2$ conversion factor of $α</em>\text{CO} =$ 0.4--5 $\ \text{M}_\odot$ (K km s$<sup>{-1}$ pc$<sup>2)<sup>{-1}$ in the filament, or $\sim$ 0.1--1 times the standard assumption of the Milky Way value. Direct entrainment by the N radio bubble is now plausible, as it can provide sufficient mechanical energy and buoyant support for the molecular gas flow. Similarly, measured X-ray cooling rates are also now consistent with in-situ condensation of uplifted, low-entropy hot gas. Both formation mechanisms require multiple generations of bubbles at higher filament masses.