The Survey of Centaurus A's Baryonic Structures (SCABS) – II. The extended globular cluster system of NGC 5128 and its nearby environment

Abstract

Wide-field $u'g'r'i'z'$ Dark Energy Camera observations centred on the giant elliptical galaxy NGC5128 covering $\sim21deg^2$ are used to compile a new catalogue of $\sim3200$ globular clusters (GCs). We report 2404 new candidates, including the vast majority within $\sim140$kpc of NGC5128. We find evidence for a transition at a galactocentric radius of $R_{\rm gc}\approx55$kpc from GCs intrinsic to NGC5128 to those likely to have been accreted from dwarf galaxies or that may transition to the intra-group medium of the Centaurus A galaxy group. We fit power-law surface number density profiles of the form $\Sigma_{N, R_{\rm gc}}\propto R_{\rm gc}^\Gamma$ and find that inside the transition radius, the red GCs are more centrally concentrated than the blue, with $\Gamma_{\rm inner,red}\approx-1.78$ and $\Gamma_{\rm inner,blue}\approx-1.40$. Outside this region both profiles flatten, more dramatically for the red GCs ($\Gamma_{\rm outer,red}\approx-0.33$) compared to the blue ($\Gamma_{\rm outer,blue}\approx-0.61$), although the former is more likely to suffer contamination by background sources. The median $(g'\!-\!z')_0\!=\!1.27$mag colour of the inner red population is consistent with arising from the amalgamation of two giant galaxies each less luminous than present-day NGC5128. Both in- and out-ward of the transition radius, we find the fraction of blue GCs to dominate over the red GCs, indicating a lively history of minor-mergers. Assuming the blue GCs to originate primarily in dwarf galaxies, we model the population required to explain them, while remaining consistent with NGC5128's present-day spheroid luminosity. We find that several dozen dwarfs of luminosities $L_{dw,V}\simeq10^{6-9.3}L_{V,\odot}$, following a Schechter luminosity function with a faint-end slope of $-1.50\leq\alpha\leq-1.25$ is favoured, many of which may have already been disrupted in NGC5128's tidal field.