A non-parametric estimate of mass ‘scoured’ in galaxy cores

Abstract

We present a simple estimate of the mass 'deficits' in cored spheroids, as a function of galaxy mass and radius within the galaxy. Previous attempts to measure such deficits depended on fitting some functional form to the profile at large radii extrapolating inwards; this is sensitive to the assumed functional form and does not allow for variation in nuclear profile shapes. For example, we show that literally interpreting the residuals from a single/cored Sersic function fit as implied 'deficit' can be misleading. Instead, we take advantage of larger data sets to directly construct stellar mass profiles of observed systems and measure the stellar mass enclosed in a series of physical radii (M(*)(< R)), for samples of cusp and core spheroids at the same stellar mass. We show that there is a significant (model-independent) bimodality in this distribution of central structure for this sample at small radii. We non-parametrically measure the median offset between core and cusp populations (the 'deficit' delta M(*)(< R)). We can then construct the scoured mass profile as a function of radius, without reference to any assumed functional form. The mass deficit rises in power-law fashion (delta M(*)(< R) proportional to R1.3-1.8) from a significant but small mass at R less than or similar to 10 pc, to asymptote to a maximum similar to 0.5-2 M(BH) at similar to 100 pc, where M(BH) is the mass of the central, supermassive black hole (BH) hosted by the spheroid. At larger radii there is no statistically significant separation between populations; the upper limit to the cumulative scoured mass at similar to kpc is similar to 2-4 M(BH). This does not depend strongly on stellar mass. The dispersion in M(*)(< R) appears larger in the core population, possibly reflecting the fact that core scouring increases the scatter in central profile shapes. We measure this broadening effect as a function of radius. The relatively low mass deficits inferred, and characteristic radii, are in good agreement with models of 'scouring' from BH binary systems.