Dynamics of Molecular Gas in the Central Region of the Quasar I$\,$Zwicky$\,$1

Abstract

We present a study of the molecular gas distribution and kinematics in the cicumnuclear region (radii $\lesssim 2\,$kpc) of the $z\approx0.061$ quasar I$\,$Zwicky$\,$1 using a collection of available Atacama Large Millimeter/submillimeter Array (ALMA) observations of the carbon monoxide (CO) emission. With an angular resolution of $\sim0.36''$ (corresponding to $\sim\,400\,\rm pc$), the host galaxy sub-structures including the nuclear molecular gas disk, spiral arms, and a compact bar-like component are resolved. We analyzed the gas kinematics based on the CO image cube and obtained the rotation curve and radial distribution of velocity dispersion. The velocity dispersion is about $30\,\rm km\,s^{-1}$ in the outer CO disk region and rises up to $\gtrsim 100\,\rm km\,s^{-1}$ at radius $\lesssim 1\,$kpc, suggesting that the central region of disk is dynamically hot. We constrain the CO-to-$\rm H_2$ conversion factor, $α_{\rm CO}$, by modeling the cold gas disk dynamics. We find that, with prior knowledge about the stellar and dark matter components, the $α_{\rm CO}$ value in the circumnuclear region of this quasar host galaxy is $1.55_{-0.49}^{+0.47}\,M_\odot\,\left(\rm K\,km\,s^{-1}\,pc^2\right)^{-1}$, which is between the value reported in ultra-luminous infrared galaxies and in the Milky-Way. The central 1$\,$kpc region of this quasar host galaxy has significant star formation activity, which can be identified as a nuclear starburst. We further investigate the high velocity dispersion in the central region. We find that the ISM turbulent pressure derived from the gas velocity dispersion is in equilibrium with the weight of the ISM. This argues against extra power from AGN feedback that significantly affects the kinematics of the cold molecular gas.