The build-up of nuclear stellar cusps in extreme starburst galaxies and major mergers

Abstract

Nuclear stellar cusps are defined as central excess light component in the stellar light profiles of galaxies and are suggested to be stellar relics of intense compact starbursts in the central ∼100–500 pc region of gas-rich major mergers. Here, we probe the build-up of nuclear cusps during the actual starburst phase for a complete sample of luminous infrared galaxy (LIRG) systems (85 LIRGs, with 11.4 < log [LIR/L⊙] < 12.5) in the Great Observatories All-sky LIRG Survey sample. Cusp properties are derived via 2D fitting of the nuclear stellar light imaged in the near-infrared (NIR) by the Hubble Space Telescope and have been combined with mid-infrared (IR) diagnostics for active galactic nucleus (AGN)/starburst characterization. We find that nuclear stellar cusps are resolved in 76 per cent of LIRGs (merger and non-interacting galaxies). The cusp strength and luminosity increase with far-IR luminosity (excluding AGN) and merger stage, confirming theoretical models that starburst activity is associated with the build-up of nuclear stellar cusps. Evidence for ultracompact nuclear starbursts is found in ∼13 per cent of LIRGs, which have a strong unresolved central NIR light component but no significant contribution of an AGN. The nuclear NIR surface density (measured within 1 kpc radius) increases by a factor of ∼5 towards late merger stages. A careful comparison to local early-type galaxies with comparable masses reveals (a) that local (U)LIRGs have a significantly larger cusp fraction and (b) that the majority of the cusp LIRGs have host galaxy luminosities (H band) similar to core ellipticals which are roughly one order in magnitude larger than those for cusp ellipticals.