Evidence for A Hot Wind from High-resolution X-ray Spectroscopic Observation of the Low-luminosity Active Galactic Nucleus in NGC 7213

Abstract

Super-massive black holes (SMBHs) spend most of their lifetime accreting at a rate well below the Eddington limit, manifesting themselves as low-luminosity active galactic nuclei (LLAGNs). The prevalence of a hot wind from LLAGNs is a generic prediction by theories and numerical simulations of black hole accretion and is recently becoming a crucial ingredient of AGN kinetic feedback in cosmological simulations of galaxy evolution. However, direct observational evidence for this hot wind is still scarce. In this work, we identify significant Fe XXVI Ly$\alpha$ and Fe XXV K$\alpha$ emission lines from high-resolution Chandra grating spectra of the LLAGN in NGC\,7213, a nearby Sa galaxy hosting a $\sim10^8\rm~M_\odot$ SMBH, confirming previous work. We find that these lines exhibit a blueshifted line-of-sight velocity of $\sim1100\rm~km s^{-1}$ and a high XXVI Ly$\alpha$ to XXV K$\alpha$ flux ratio implying for a $\sim16$ keV hot plasma. By confronting these spectral features with synthetic X-ray spectra based on our custom magnetohydrodynamical simulations, we find that the high-velocity, hot plasma is naturally explained by the putative hot wind driven by the hot accretion flow powering this LLAGN. Alternative plausible origins of this hot plasma, including stellar activities, AGN photoionization and the hot accretion flow itself, are quantitatively disfavored. The inferred kinetic energy and momentum carried by the wind can serve as strong feedback to the environment. We compare NGC\,7213 to M81*, in which strong evidence for a hot wind was recently presented, and discuss implications on the universality and detectability of hot winds from LLAGNs.