Origin of the spectacular tidal shells of galaxy NGC474

Abstract

Context. The lenticular galaxy NGC 474 hosts a rich system of tidal shells and streams, some of which are exceptionally bright. Two teams recently presented spectroscopic observations of the brightest shells. These were the first shell spectra ever observed in integrated starlight. The authors studied the stellar populations of the shell, of the center of the galaxy and of its globular clusters. The precise formation scenario for the tidal features of this prominent galaxy however still remained unclear.Aims. Here, we add further clues on their formation from the radii of the shells, and we present a scenario for the formation of the tidal features that seems to be unique and explaining all available data. Methods. Shell radii are analyzed with the shell identification method, and we run self-consistent simulations of the formation of the tidal features. We consider Newtonian as well as MOND gravity. Results. Observations suggest that the tidal features originate from the accretion of a spiral galaxy. The shell identification method yields that the merging galaxies collided first 1.3 Gyr ago and then again 0.9 Gyr ago, thereby forming the shells in two generations. This would also explain the young ages of stellar populations in the center of the galaxy and the young age of the globular clusters. The analytic models of shell propagation, that underlie the shell identification method, are verified by a simulation. The simulations reproduce well the observed morphology of the tidal features. The accreted spiral likely reached NGC 474 nearly radially, in the plane of the sky, from the south, its rotation axis pointing toward us. It should have had a stellar mass of around 1/6 of NGC 474, i.e. 10 9.8 M . It seems that all tidal features in the galaxy originate from one merger.

Characterization of low surface brightness structures in annotated deep images

“…Most of the shells have a radius smaller than 40 kpc with a few extending to 80 kpc. For instance the shells of the prototypical galaxy NGC0474 range from 10 to 50 kpc Bílek et al (2021). We do not observe shells beyond 80 kpc, which seems Obviously the comparison is not straightforward as the shell orientation (not well constrained from our annotations of real systems) and differences in surface brightness need to be taken into account.…”

“…Concentricity -From numerical simulations, shells are usually formed as concentric structures (e.g., Pop et al 2018;Ebrová et al 2021;Bílek et al 2021), a prediction we can directly test with our observations. To compute the concentricity, we assume that the curved line defining the shell is a circular arc, and we compute its center 18 .…”

“…Hence, the study of the position of a shell relatively to the center of its primary galaxy is important, as comparisons with simulations can give us hints about the merger that triggered the shells, as well as its age (e.g., Ebrová et al 2021;Bílek et al 2021). Shell radii are useful also for investigating the gravitational fields of galaxies (e.g., Hernquist & Quinn 1987;Bílek et al 2013).…”

“…They have the main advantage of providing the ground truth when interpreting the results. Indeed, it is possible to track the merger trees of LSB hosting galaxies and their 3D information to follow the LSB structures wrapping around galaxies (e.g., Hendel & Johnston 2015;Pop et al 2018;Mancillas et al 2019;Ebrová et al 2021;Bílek et al 2021).…”

Characterization of Low Surface Brightness structures in annotated deep images

“…They have the main advantage of providing the ground truth when interpreting the results. Indeed, it is possible to track the merger trees of LSB hosting galaxies and their 3D information to follow the LSB structures wrapping around galaxies (e.g., Hendel & Johnston 2015;Pop et al 2018;Mancillas et al 2019;Ebrová et al 2021;Bílek et al 2021).…”