External gas accretion provides a fresh gas supply to the active S0 galaxy NGC 5077

¹ DARK, Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen, Denmark
e-mail: sandra.raimundo@nbi.ku.dk
² Department of Physics and Astronomy, University of California, Los Angeles, CA 90095, USA
³ Department of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK

Received: 27 December 2020
Accepted: 2 March 2021

Abstract

In early-type galaxies, externally accreted gas is thought to be the main source of gas replenishment at late times. We use MUSE integral field spectroscopy data to study the active S0 galaxy NGC 5077, which is known to have disturbed dynamics that are indicative of a past external interaction. We confirm the presence of a stellar kinematically distinct core with a diameter of 2.8 kpc that is counter-rotating with respect to the main stellar body of the galaxy. We find that the counter-rotating core consists of an old stellar population that is not significantly different from the rest of the galaxy. The ionised gas is strongly warped and extends out to 6.5 kpc in the polar direction and in a filamentary structure. The gas dynamics is complex, with significant changes in the position angle as a function of radius. The ionised gas line ratios are consistent with LINER excitation by the active galactic nucleus, both in the nucleus and at kiloparsec scales. We discover a nuclear outflow with projected velocity V ∼ 400 km s⁻¹, consistent with a hollow outflow cone intersecting the plane of the sky. The properties of the misaligned gas match predictions from numerical simulations of misaligned gas infall after a gas-rich merger. The warp and change in the gas orientation as a function of radius are consistent with gas relaxation due to stellar torques; these are stronger at small radii where the gas aligns faster than in the outer regions, driving gas to the nucleus. The stellar and gas dynamics indicate that NGC 5077 has had at least two external interactions, one that resulted in the formation of the counter-rotating core, followed by the second, late-time external gas accretion. NGC 5077 illustrates the importance of external interactions in the replenishment of the galaxy gas reservoir and the nuclear gas content available for black hole fuelling.