Origin of the spectacular tidal shells of galaxy NGC 474

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 still remained unclear, however. 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 can explain all available data. Methods. Shell radii were analyzed with the shell identification method, and we ran self-consistent simulations of the formation of the tidal features. We considered Newtonian as well as MOND gravity. Results. Observations suggest that the tidal features originate from the accretion of a spiral galaxy. According to the shell identification method, the merging galaxies first collided 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 the observed morphology of the tidal features well. The accreted spiral likely reached NGC 474 on the plane of the sky nearly radially from the south, its rotation axis pointing toward us. It probably had a stellar mass of about one-sixth of NGC 474, that is, 109.8 M⊙. Apparently, all tidal features in the galaxy originate from one merger.

From starburst to quenching: merger-driven evolution of the star formation regimes in a shell galaxy

“…Due to its numerous prominent shells, NGC 474 is a prototypical system of a shell galaxy, often put in the spotlight when studying the formation of shells and shell galaxies. The recent simulation of Bílek et al (2022) confirms that a shell morphology similar to that of NGC 474 could be explained by a radial interaction. However, besides constraints on its formation history, recent works have also reported some oddities.…”

“…Pinpointing the origin of these clusters, and assessing the star formation activity in shell galaxies in general, calls for numerical simulations that can reproduce the physical conditions for star and cluster formation in these type of systems. However, the simulation of Bílek et al (2022) does not include gas dynamics. New simulations are therefore required to address the question of the origin(s) of clusters in outer shells, and more generally when and where star formation occurs in shell galaxies.…”

From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity

“…Some of these techniques were recently applied to the shell galaxy NGC 474 [519]. Their N-body MOND simulation of it agrees well with the shell positions and even with the velocity dispersion of the shell where a comparison was made.…”

A Stream Come True -- Connecting tidal tails, shells, streams, and planes with galaxy kinematics and formation history

“…Shells appear as circular arcs centered around a galaxy (top panel of Figure 1). They have sharp outer borders and are believed to form through the radial infall and disruption of massive satellites (e.g., Quinn 1984;Pop et al 2018;Karademir et al 2019;Bílek et al 2022). Streams are thin slivers of stars around the host galaxy (middle panel of Figure 1) that are formed through the tidal disruption of an infalling galaxy on a circular orbit (e.g., Karademir et al 2019).…”