Dissipation and Extra Light in Galactic Nuclei. I. Gas‐Rich Merger Remnants

Abstract

We study the origin and properties of "extra" or "excess" central light in the surface brightness profiles of remnants of gas-rich mergers. By combining a large set of hydrodynamical simulations with data on observed mergers that span a broad range of profiles at various masses and degrees of relaxation, we show how to robustly separate the physically meaningful extra light -i.e. the stellar population formed in a compact central starburst during a gas-rich merger -from the outer profile established by violent relaxation acting on stars already present in the progenitor galaxies prior to the final stages of the merger. This separation is sensitive to the treatment of the profile, and we demonstrate that certain fitting procedures can yield physically misleading results. We show that our method reliably recovers the younger starburst population, and examine how the properties and mass of this component scale with e.g. the mass, gas content, and other aspects of the progenitors. We consider the time evolution of the profiles in different bands, and estimate the biases introduced by observational studies at different phases and wavelengths. We show that, when appropriately quantified, extra light is ubiquitous in both observed and simulated gas-rich merger remnants, with sufficient mass (∼ 3 − 30% of the stellar mass) to explain the apparent discrepancy in the maximum phase-space densities of ellipticals and their progenitor spirals. The nature of this central component provides a powerful new constraint on the formation histories of observed systems and can inform both our studies of their progenitors and our understanding of the global kinematics and structure of spheroids.

Subaru and e-Merlin observations of NGC 3718

“…and studied 2MASS and kinematic data of NGC 3718, for which they find elliptical-like 18 We agree with Hopkins et al (2008) on the danger of doing three component fits. Our fit is indeed very sensitive to the adopted lower and upper limits.…”

“…Kormendy & Ho (2013), Hopkins et al (2008Hopkins et al ( , 2009a, instead of the observed core. The suggested absence of sufficient amounts of nuclear gas, however, lead naturally to the limited star formation observed.…”

“…As the cold gas experiences tidal torques due to the violence of a major merger, it is channeled towards the central region, triggering SMBH growth, SMBH feedback, and an intense nuclear starburst (Hopkins et al 2008). The nuclear starburst forms the extra light component in these objects, which is observationally recognizable as a break towards a steeper inner surface brightness profile when compared to an inward extrapolation of the outer profile's Sersic fit (e.g.…”

Fossil groups origins

“…In particular, the low value of n unveil the kind of mergers that have formed the galaxy. In order to get a similar light profile it is required a very gas rich merger for the progenitors (∼80% gas rich; see Hopkins et al 2008). Figure 10 shows the velocity distribution of the 78 member galaxies.…”

The fate of magnetic fields in colliding galaxies

“…Moreover, burst of star formation driven by the interaction (see e.g. Naab et al, 2006, Hopkins et al, 2008 and references therein) could continously seed new magnetic field, thus intensifying the amplification of magnetic fields even more.…”