17 March 2021

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

The whirling disc of NGC 4526
The whirling disc of NGC 4526 Credit: ESA/Hubble & NASA

Transfer of gas between galaxies is a common event in the Universe. For galaxies with a typical low amount of native gas, such as elliptical and lenticular galaxies, gas that is accreted from outside the galaxy will be a major source of gas replenishment in the last Gigayears of the history of the Universe. Gas replenishment can be important not only for star formation but also to fuel the supermassive black hole at the centre of the galaxy.


In this paper accepted for publication in Astronomy & Astrophysics, we study the lenticular galaxy NGC 5077 using state-of-the-art observations with the MUSE instrument on the Very Large Telescope. This instrument obtains an image of the galaxy and a spectrum for each pixel in the image, resulting in a 3D data cube (https://en.wikipedia.org/wiki/Multi-unit_spectroscopic_explorer).


Numerical simulations predict that in the case of gas accretion after a gas-rich major merger (merger of two galaxies with comparable masses), elliptical or lenticular galaxies can show a warped misaligned gas distribution with infall of gas to the nucleus. This paper presents the first observational evidence of this theoretical prediction. 


By analysing the stellar and gas dynamics we find that NGC 5077 has had at least two external gas accretion events in its past. The most likely scenario for NGC 5077 is that several Gyr ago it went through a major merger with another galaxy (i.e. the first gas accretion event). Most of the major merger gas was promptly removed temporarily or permanently from the galaxy by supernova driven outflows.  Late time accretion of misaligned leftover gas from the merger or the accretion of small gas-rich satellites, provided fresh gas to the galaxy (i.e. the second gas accretion event). When the accretion rate of gas decreased, the process of gas relaxation due to the stellar gravitational torques started to dominate, causing a significant warp to the gas, with the smaller radii gas aligning first, and resulting in the configuration that we currently observe with MUSE and that has been predicted by simulations. It is expected that in the future, the gas will continue to lose angular momentum due to the gravitational torques from the stars, and will inflow to the nucleus to replenish the gas reservoir available for the feeding of the black hole.


The results from this paper highlight the importance of external accretion events for the gas supply in elliptical and lenticular galaxies and to the replenishment of the supermassive black hole gas reservoir.