Core Galaxies and SMBHs

I investigate the relations between central super-massive black holes (SMBH) and partially-depleted galaxy cores, and use them as tools to study the processes that lead to the formation of massive galaxies.

What are stellar depleted cores? — 
 Luminous early type galaxies (MB < -20.5 mag) are typically characterized by the presence of a “core” (i.e. a flattening of their inner radial light profile). In the most favored scenario, the core is shaped by a binary black hole (BH )system which eject stars via three-body interactions, before collapsing into the SMBH (Begelman et al. 1980). Scaling relations between the mass of the SMBH (M) and the core parameters (e.g. Lauer et al. 2007) support this hypothesis, although they are affected by significant scatter.

WFPC2 captures a binary SMBH kicking stars out of the bulge - Paolo Bonfini

WFPC2 captures a binary SMBH kicking stars out of the bulge – Paolo Bonfini

▸ Characterizing cores in massive galaxies
To perform photometric studies of core galaxies I created GALFIT-CORSAIR (Bonfini P. 2014): a modification of the 2D fitting software GALFIT (Peng et al. 2010) which includes the core-Sérsic model (Graham et al. 2003). GALFIT-CORSAIR has been applied to investigate the alleged greatest core ever found in a galaxy (i.e. Holm 15A), finding that the galaxy is actually core-less (Bonfini P. et al, 2015 ApJ, 807, 136B). In the case of the brightest cluster galaxy (BCG) of the Abell 2261 we instead found support for the alternative “stalled infalling perturber” core-formation scenario (Bonfini P. & Graham W.A. 2016, ApJ, 829, 81B).

▸ Evolution of cores
However, the most striking result of my research was to reveal how cores develop after the last merger which shaped the main stellar body of the galaxy,  in parallel with the relaxation of the potential. To show this, in Bonfini P. et al. 2018 we calibrated core prominence against the morphological “fine structures”. Fine structures include all those signatures which trace galaxy-galaxy interactions, such as shells, tidal tails, stellar streams, etc. (a compendium can be found on the MATLAS webpage), and they can therefore be used as a proxy of time elapsed from the last merger. The figure below shows that the size of the core (horizontal axis) increases with decreasing fine structure (vertical axis), i.e. it increases with time due to the action of the SMBH binary.

sigma-core

Reproduced from Bonfini et al. (2017). Left — An example of galaxy with significant morphological fine structure. Center — Fine structure significance (Σ) as a function of relative mass deficit (M_def/M_sph; indicating the stellar mass removed by the central SMBH). The gray arrows show the proposed evolutionary track. Right — Example of galaxy surface brightness profile (solid red curve) displaying a stellar depleted core, which manifests as an inner “flattening”. The “missing light” is defined as the difference between the observed core-Sérsic profile (solid red curve) and the Sérsic profile (dashed green curve) extrapolated inwards from the outer radii.

▸ Cores and dust
We are trying to address a problem which has been for too long “swept under the carpet”: can cores be mimicked by simple dust absorption? In Bonfini P. et al. (2018b), we showed that indeed this might be the case in some objects. We prove it by fitting dust-absorbed 2D models (using GALFIT and SKIRT) to real galaxy images. For this study, we used NGC 4494, a galaxy known to host a clear-cut dust ring, and NGC 4552, which was thought to host a core. Our study reveals instead that the light deficiency at the center of NGC 4552 is due to a central dust ring, and not a core.

dust_rings

Reproduced from Bonfini et al. (2018,).  Results of our fitting pipeline for NGC 4552 (top), and NGC 4494 (bottom). Left — Original Hubble images. Center — Best-fit dust-absorbed model generated through GALFIT+SKIRT. Right — Data minus model residuals.

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