Evidence for Two Phases of Galaxy Formation from Radial Trends in the Globular Cluster System of NGC 1407

TL;DR

This study investigates the radial metallicity gradients of globular clusters in NGC 1407, providing evidence for a two-phase galaxy formation process involving early in-situ formation and later accretion.

Contribution

It offers observational support for the two-phase galaxy formation model by analyzing globular cluster metallicity gradients in NGC 1407 and comparing with other galaxies.

Findings

Steep metallicity gradients within 5-8.5 R_e for both GC subpopulations

Constant metallicity at larger radii beyond 8.5 R_e

Similar radial trends observed in other galaxies like M87 and the Milky Way

Abstract

Here we present the colours of individual globular clusters (GCs) around the massive elliptical galaxy NGC 1407 out to a projected galactocentric radius of 140 kpc or 17 galaxy effective radii (R$_e$). Such data are a proxy for the halo metallicity. We find steep, and similar, metallicity gradients of ~ -0.4 dex per dex for both the blue (metal-poor) and red (metal-rich) GC subpopulations within 5-8.5 R_e (40-70 kpc). At larger radii the mean GC colours (metallicity) are constant. A similar behaviour is seen in a wide-field study of M87's GC system, and in our own Galaxy. We interpret these radial metallicity trends to indicate an inner region formed by early in-situ dissipative processes and an outer halo formed by ongoing accretion of low mass galaxies and their GCs. These results provide observational support for the model of galaxy formation whereby massive galaxies form inside-out in two phases. We have also searched the literature for other massive early-type galaxies with reported GC metallicity gradients in their inner regions. No obvious correlation with galaxy mass or environment is found but the sample is currently small.