LeMMINGs. Multi-wavelength constraints on the co-existence of nuclear star clusters and AGN in nucleated galaxies

Abstract

The relation between nuclear star clusters (NSCs) and the growth of the central SMBHs, as well as their connection to the properties of the host galaxies, is crucial for understanding the evolution of galaxies. Recent observations have revealed that about 10 per cent of nucleated galaxies host hybrid nuclei, consisting of both NSCs and accreting SMBHs that power active galactic nuclei (AGN). Motivated by the potential of the recently published multi-wavelength data sets from LeMMINGs survey, here we present the most thorough investigation to date of the incidence of hybrid nuclei in a large sample of 100 nearby nucleated galaxies (10 E, 25 S0, 63 S, and 2 Irr), covering a wide range in stellar mass (M∗,gal∼108.7−1012 Msun). We identify the nuclei and derive their properties by performing detailed 1D and 2D multi-component decompositions of the optical and near-infrared HST stellar light distributions of the galaxies using Sérsic and core-Sérsic models. Our AGN diagnostics are based on homogeneously derived nuclear 1.5 GHz e-MERLIN radio, Chandra X-ray (0.3--10 keV) and optical emission-line data. We determine the nucleation fraction (fnuc) as the relative incidence of nuclei across the LeMMINGs HST sample and find fnuc= 100/149 (= 67 ± 7 per cent), confirming previous work, with a peak value of 49/56~(= 88±13 per cent) at bulge masses M∗,bulge∼109.4- 1010.8 Msun. We identify 30 nucleated LeMMINGs galaxies that are optically active, radio-detected and X-ray luminous (LX>1039 erg s−1). This indicates that our nucleated sample has a lower limit ∼ 30 per cent occupancy of hybrid nuclei, which is a function of M∗,bulge and M∗,gal. We find that hybrid nuclei have a number density of (1.5±0.4)×10−5 Mpc−3.