Is the Bardeen-Petterson effect responsible for the warping and precession in NGC 4258?

Abstract

Strong evidence for the presence of a warped Keplerian accretion disc in NGC 4258 (M 106) has been inferred from the kinematics of water masers detected at subparsec scales. Assuming a power-law accretion disc and using constraints on the disc parameters derived from observational data, we have analysed the relativistic Bardeen‐Petterson effect driven by a Kerr black hole as the potential physical mechanism responsible for the disc warping. We found that the Bardeen‐Petterson radius is comparable to or smaller than the inner radius of the maser disc (independent of the allowed value for the black hole spin parameter). Numerical simulations for a wide range of physical conditions have shown that the evolution of a misaligned disc due to the Bardeen‐Petterson torques usually produces an inner flat disc and a warped transition region with a smooth gradient in the tilt and twist angles. Since this structure is similar to that seen in NGC 4258, we propose that the Bardeen‐Petterson effect may be responsible for the disc warping in this galaxy. We estimated the time-scale necessary for the disc inside of the Bardeen‐Petterson radius to align with the black hole’s equator, as a function of the black hole spin. Our results show that the Bardeen‐Petterson effect can align the disc within a few billion years in the case of NGC 4258. Finally, we show that if the observed curvature of the outer anomalous arms in the galactic disc of NGC 4258 is associated with the precession of its radio jet/counterjet, then the Bardeen‐Petterson effect can provide the required precession period.