A Trail of the Invisible: Blue Globular Clusters Trace the Radial Density Distribution of the Dark Matter -- Case Study of NGC 4278

TL;DR

Deep optical observations of NGC 4278 reveal that blue globular clusters effectively trace the galaxy's dark matter distribution, providing a new method for studying dark matter in galaxies where X-ray data is insufficient.

Contribution

This study demonstrates that blue globular clusters can serve as reliable tracers of dark matter distribution in galaxies lacking strong X-ray signals.

Findings

Blue GCs trace the total mass density profile over large radii.

Red GCs correlate with the stellar mass density.

Galaxy appears relaxed with no fine substructure out to 70 kpc.

Abstract

We present new, deep optical observations of the early-type galaxy NGC 4278, which is located in a small loose group. We find that the galaxy lacks fine substructure, i.e., it appears relaxed, out to a radius of $\sim$70 kpc. Our $g$- and $i$-band surface brightness profiles are uniform down to our deepest levels of $\sim$28 mag arcsec$^{-2}$. This spans an extremely large radial range of more than 14 half-mass radii. Combined with archival globular cluster (GC) number density maps and a new analysis of the total mass distribution obtained from archival Chandra X-ray data, we find that the red GC subpopulation traces well the stellar mass density profile from 2.4 out to even 14 half-mass radii, while the blue GC subpopulation traces the total mass density profile of the galaxy over a large radial range. Our results reinforce the scenario that red GCs form mostly in-situ along with the stellar component of the galaxy, while the blue GCs are more closely aligned with the total mass distribution in the halo and were accreted along with halo matter. We conclude that for galaxies where the X-ray emission from the hot halo is too faint to be properly observable and as such is not available to measure the dark matter profile, the blue GC population can be used to trace this dark matter component out to large radii.