Globular Clusters in NGC 4839 Falling into Coma: Evidence for the Second Infall?

TL;DR

This study uses wide-field imaging to analyze globular cluster distributions around NGC 4839, providing evidence that it has undergone a second infall into the Coma cluster, supporting a specific dynamical scenario.

Contribution

It presents new observational evidence of globular cluster distribution differences supporting the second infall scenario for NGC 4839.

Findings

NGC 4839's GC system is more compact than NGC 4816's.

The radial density profile of NGC 4839's GCs shows an abrupt drop at ~80 kpc.

The effective radius of NGC 4839's GC system is about one-third that of NGC 4816's.

Abstract

NGC 4839 is the brightest galaxy (cD) of the NGC 4839 group at $R\approx 1$ Mpc in the south-west of the Coma cluster, which is known to be falling into Coma. However, it has been controversial whether it is in the first phase of infall or in the second phase of infall after passing the Coma center. We present a wide field study of globular clusters (GCs) in NGC 4839 and its environment based on Hyper Suprime-Cam $gr$ images in the Subaru archive. We compare the GC system of NGC 4839 with that of NGC 4816, which is the brightest member (S0) of the nearby group and lies at a similar distance in the west from the Coma center. Interestingly the spatial distribution of the GCs in NGC 4839 is significantly more compact than that of the GCs in NGC 4816. In addition, the radial number density profile of the GCs in NGC 4839 shows an abrupt drop at $R_{N4839}\approx 80$ kpc, while that of the GCs in NGC 4816 shows a continuous slow decline even in the outer region at $80<R_{N4816}<500$ kpc. The effective radius of the NGC 4839 GC system is about three times smaller than that of the NGC 4816 GC system. This striking difference can be explained if NGC 4839 lost a significant fraction of the GCs in its outskirt when it passed through Coma. This supports strongly the second infall scenario where the NGC 4839 passed the Coma center about 1.6 Gyr ago, and began the second infall after reaching the apocenter in the south-west recently.