Nuclear stellar cusps are resolved in 76 per cent of (U)LIRGs while the remaining 24 per cent have flat nuclear profiles (core galaxies) with nuclear slopes <0.3. The cusp strength and luminosity increase with far-IR luminosity (excluding AGN), confirming models that recent starburst activity is associated with the build-up of stellar cusps. In particular, all galaxies above log [LIR/L☉] = 11.9 have cusp luminosities of (Lcusp ≥ 1010 L☉) with an average value Lcusp/L☉ = [3.8 ± 1.9] × 1010, which is roughly five times larger than for lower IR luminous galaxies (log [LIR/L☉ ~11.5]). An average increase in cusp luminosity ΔLcusp ~2.5 × 1010 L☉ is found as a function of far-IR luminosity and merger stage, which corresponds to a build-up of stellar mass ΔMcusp of (7 ± 3.5) × 109M☉. Most of the cusp build-up seems to occur within a few 100Myr during the final coalescence of the nuclei. If we assume that most of the cusp mass is built up during one starburst, a time-scale of ~60 ± 30Myr would be required based on the average SFR in (U)LIRGs.
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Left: Histogram of the cusp slope (γ) of our sample of interacting (blue) and non-interacting LIRGs (red), indicating a dichotomy between core (γ ~ 0) and cusp galaxies (γ > 0.3). Right: The cusp luminosity (bottom) as a function of IR luminosity. For core galaxies (arrow down marker) the upper luminosities limits are plotted (based on their uncertainty in γ which is typically ±0.1). The red solid (dashed) line shows the interpolated mean (median) cusp luminosity Lcusp as a function of IR luminosity. Note that galaxies with dominant AGN contribution to the LIR (EQW PAH 6.2μm < 0.27, diamond marker) have been excluded in the mean values.
The nuclear stellar surface-brightness profiles of local (U)LIRGs are very different from those of present day's early-type galaxies with comparable masses. Our comparison between (U)LIRGs and a large sample of local elliptical galaxies (within the same host galaxy mass range) reveals (a) a significant larger cusp/core ratio in (U)LIRGs (≥3.2:1) than in ellipticals (≤0.7:1) and (b) (U)LIRGs do not follow the cusp to core dependency of ellipticals as a function of host galaxy mass (MH-γ diagram) and clearly represent a distinct population. The majority of the cusp (U)LIRGs have on average host luminosities similar to core ellipticals, which is one order in magnitude larger than for cusp ellipticals. Moreover, gas-rich late-stage mergers show no indications of a cusp destruction by BH-binary scouring during the coalescence of their nuclei. These results suggest that the progenitors of present day's massive ellipticals must have either expelled most of their gas during a brief episode in cosmic time and destroyed their cusps in subsequent gas-poor remerger events, or that SF was quickly shut down before gas could efficiently accumulate in the very centre to build up a cusp (e.g. via feedback from supernovae, hot stars and AGN due to higher gas fractions at high-z). The reason why there are almost no cusp LIRGs observed at smaller masses could be possibly explained by the fact that galaxies in the early Universe had significantly larger gas fractions, eventually enabling them to reach critical gas pressure rates and instabilities to trigger central starbursts at much smaller host galaxy masses.
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Left: Histogram of the Nuker gamma parameter for LIRGs (top panel) and early-type galaxies (bottom panel; sample from Lauer et al. (2007) within the same total luminosity range (-25.5 < MH < -22.5). Late-stage mergers are indicated as filled histogram in the top panel. Right: Overview of the Nuker gamma parameter as a function of the total absolute H-band magnitude (2MASS). While early-type galaxies (red, plus marker; Lauer et al. 2007) show a strong dependence of the presence of cusp and cores as a function of host magnitude, LIRGs (blue, circle marker) do not follow this trend and represent a distinct population. Late-stage mergers are indicated with a filled circle.
With increasing cusp strength and luminosity along the merger process, the nuclear structure becomes more compact which is observed as an increasing cusp surface density (factor of 5-10) and total nuclear surface density within a radius of 1kpc (factor of 4-5). This result is in agreement with the significant increase of the bulge surface density shown in Haan et al. (2011) , suggesting that both components of a merger remnant, one formed through violent relaxation of progenitor stars (dissipationless, spheroid) and the other through gas dissipation (cusp), become more compact towards the late merger stage due to gas infall, which might be a possible explanation for the dramatic increase in bulge compactness in high-redshift studies (e.g. CANDELS).
The cusp properties as a function of merger stage for all sources with γ > 0.1. Top left: the cusp slope (γ), top right: the cusp luminosity LCusp, mid left: the total NIR luminosity within 1kpc radius (excluding the central unresolved PSF component), mid right: the cusp surface density Lcusp/(π Rcusp2), bottom left: the total IR (LIR) luminosity of the galaxy, and bottom right: the luminosity of the central unresolved component (PSF). The number of sources in each merger stage bin is given at the bottom. The median values (with the standard error of the mean as blue error bar)are indicated with blue diamond markers and connected with a solid line.
Comparison of the nuclear structure in four cusp LIRGs observed in the HST H-band to corresponding HST Paα emission line images reveals that the Paα emission has various morphologies (nuclear star-forming rings, nuclear minispiral and concentrated emission), suggesting that there is apparently no single morphological SF feature associated with the build-up of cusps. However, all four galaxies have a strong Paα emission component with roughly the same size as the cusp, which demonstrates that the current SF is still linked up with the build-up of the cusp.
Evidence for ultracompact nuclear starbursts is found in ~13 per cent of LIRGs, which have a strong unresolved central light component but no significant evidence of an AGN.