Black Hole Mass vs Stellar
Mass scaling relation
It is well established that most galaxies host at their nuclear regions supermassive black holes that grow over time by devouring matter from their immediate environment. During such “active” phases large amounts of energy are generated thereby making the central regions of galaxies shine bright – a characteristic property of the astrophysical objects called Active Galactic Nuclei (in short AGN). The energy generated as supermassive black holes grow, can also affect their host galaxies by e.g. heating or pushing away their gaseous material and therefore impacting their evolutionary path. Understanding the interplay between black holes and galaxies remains a central theme in current astrophysical research.
One important observational relation that provides valuable information on how black holes and their galaxies affect each other is the one that connects the mass of the black hole and the mass of the stars in the host galaxy. Different observational experiments have been devised over the years to constrain this correlation and to understand how tight it is or whether it changes when the Universe was younger.
X-ray Excess variance as a function of AGN luminosity
X-ray Excess variance of AGN at X-rays as a function of their mean X-ray luminosity measured in the 0.5-7keV energy band. The Excess Variance is a measure of how much the X-ray light emitted by AGN flickers with time. Higher values correspond to larger fluctuations around the mean brightness. The X-ray luminosity measures how much radiation the AGN produces, i.e. how powerful it is. The data points are measurements of the X-ray variability (taken from Paolillo et al. 2017). The pink shaded region is our model fit to these observations.
Black Hole Mass versus Stellar Mass scaling relation
Black Hole Mass (y-axis) vs Stellar Mass (x-axis) scaling relation. The pink shaded region is the correlation inferred by fitting the model to the variability observations on the left panel. The lines show independent constraints of this correlation. A detailed discussion can be found in Georgakakis et al. (2021).
In our work published in Georgakakis et al. (2021) we propose a new independent method for measuring the Black-Hole vs Stellar Mass correlation by exploiting the fact that the light emitted by AGN varies with time, i.e. it flickers. It is known that this flickering depends on both the mass of the black hole but also on the rate at which matter falls onto it. More massive black holes, for example, show smaller variations of their brightness with time. This opens the potential to measure black hole masses for individual AGN by analysing the variations of their brightness as a function of time. In our work we extend this argument from individual objects to AGN populations: by modeling the observed variability properties of large AGN samples, it is possible to constrain the Black-Hole vs Stellar Mass correlation of the population.
Our study presents the modeling methodology that enables the interpretation of the variability at X-ray wavelengths of large populations of AGN. One of the inputs of the model is the Black Hole Mass vs Stellar Mass scaling relation. Fitting the model to observational data therefore provides a handle on this relation.
The application of this approach is demonstrated in the left panel of the figure. It shows observational measurements of how much the X-ray light of AGN flickers as a function of their (mean) luminosity. The plot shows that more luminous AGN, i.e. those that produce more radiation at X-rays, tend to flicker less, i.e. show lower brightness fluctuations at X-ray wavelengths. The shaded region in that figure is the model fit to the observations. The right panel shows the predictions of the model for the Black Hole Mass vs Stellar Mass relation based on the X-ray variability measurements shown in the left panel.
