The Supermassive Black Hole Mass - Spheroid Stellar Mass Relation for S\'ersic and Core-S\'ersic Galaxies

Abstract

We have examined the relationship between supermassive black hole mass (M$_{BH}$) and the stellar mass of the host spheroid (M$_{sph,*}$) for a sample of 75 nearby galaxies. To derive the spheroid stellar masses we used improved 2MASS K$_s$-band photometry from the ARCHANGEL photometry pipeline. Dividing our sample into core-S\'ersic and S\'ersic galaxies, we find that they are described by very different M$_{BH}$-M$_{sph,*}$ relations. For core-S\'ersic galaxies - which are typically massive and luminous, with M$_{BH} \gtrsim 2x10^8 M_\odot$ - we find M$_{BH} \propto M_{sph,*}^{(0.97 \pm 0.14)}$, consistent with other literature relations. However, for the S\'ersic galaxies - with typically lower masses, M$_{sph,*} \lesssim 3x10^10 M_\odot$ - we find M$_BH \propto M_{sph,*}^{(2.22 \pm 0.58)}$, a dramatically steeper slope that differs by more than 2 standard deviations. This relation confirms that, for S\'ersic galaxies, M$_{BH}$ is not a constant fraction of M$_{sph,*}$. S\'ersic galaxies can grow via the accretion of gas which fuels both star formation and the central black hole, as well as through merging. Their black hole grows significantly more rapidly than their host spheroid, prior to growth by dry merging events that produce core-S\'ersic galaxies, where the black hole and spheroid grow in lock step. We have additionally compared our S\'ersic M$_{BH}$-M$_{sph,*}$ relation with the corresponding relation for nuclear star clusters, confirming that the two classes of central massive object follow significantly different scaling relations.