The multi-phase winds of Markarian 231: from the hot, nuclear, ultra-fast wind to the galaxy-scale, molecular outflow

Abstract

Mrk 231 is a nearby ultra-luminous IR galaxy exhibiting a kpc-scale, multi-phase AGN-driven outflow. This galaxy represents the best target to investigate in detail the morphology and energetics of powerful outflows, as well as their still poorly-understood expansion mechanism and impact on the host galaxy. In this work, we present the best sensitivity and angular resolution maps of the molecular disk and outflow of Mrk 231, as traced by CO(2−1) and (3−2) observations obtained with the IRAM/PdBI. In addition, we analyze archival deep Chandra and NuSTAR X-ray observations. We use this unprecedented combination of multi-wavelength data sets to constrain the physical properties of both the molecular disk and outflow, the presence of a highly-ionized ultra-fast nuclear wind, and their connection. The molecular CO(2−1) outflow has a size of ~1 kpc, and extends in all directions around the nucleus, being more prominent along the south-west to north-east direction, suggesting a wide-angle biconical geometry. The maximum projected velocity of the outflow is nearly constant out to ~1 kpc, thus implying that the density of the outflowing material must decrease from the nucleus outwards as ~r-2. This suggests that either a large part of the gas leaves the flow during its expansion or that the bulk of the outflow has not yet reached out to ~1 kpc, thus implying a limit on its age of ~1 Myr. Mapping the mass and energy rates of the molecular outflow yields OF = [500−1000] M⊙ yr-1 and Ėkin,OF = [7−10] × 10^(43) erg s^(-1). The total kinetic energy of the outflow is Ekin,OF is of the same order of the total energy of the molecular disk, Edisk. Remarkably, our analysis of the X-ray data reveals a nuclear ultra-fast outflow (UFO) with velocity −20 000 km s^(-1), UFO = [0.3−2.1] M_⊙ yr^(-1), and momentum load UFO/ rad = [0.2−1.6]. We find Ėkin,UFO ~ Ėkin,OF as predicted for outflows undergoing an energy conserving expansion. This suggests that most of the UFO kinetic energy is transferred to mechanical energy of the kpc-scale outflow, strongly supporting that the energy released during accretion of matter onto super-massive black holes is the ultimate driver of giant massive outflows. The momentum flux OF derived for the large scale outflows in Mrk 231 enables us to estimate a momentum boost OF/ UFO ≈ [30−60]. The ratios Ėkin,UFO/Lbol,AGN = [1−5] % and Ėkin,OF/Lbol,AGN = [1−3] % agree with the requirements of the most popular models of AGN feedback.