The Spitzer/IRAC view of black hole-bulge scaling relations

We present a mid-infrared investigation of the scaling relations between supermassive black hole masses (M BH ) and the structural parameters of the host spheroids in local galaxies. This work is based on 2D bulge-disc decompositions of Spitzer/IRAC 3.6 m images of 57 galaxies with M BH estimates. We first verify the accuracy of our decomposition by examining the Fundamental Plane (FP) of spheroids at 3.6 m. Our estimates of effective radii (R e ) and average surface brightnesses, combined with velocity dispersions from the literature, define a FP relation consistent with previous determinations but doubling the observed range in R e . None of our galaxies is an outlier of the FP, demonstrating the accuracy of our bulge-disc decomposition which also allows us to independently identify pseudo-bulges in our sample. We calibrate M/L at 3.6 m by using the tight M dyn -L bul relation (0.1 dex intrinsic dispersion) and find that no colour corrections are required to estimate the stellar mass. The 3.6 m luminosity is thus the best tracer of stellar mass yet studied. We then explore the connection between M BH and bulge structural parameters (luminosity, mass, effective radius). We find tight correlations of M BH with both 3.6 m bulge luminosity and dynamical mass (M BH /M dyn 1/1000), with intrinsic dispersions of 0.35 dex, similar to the M BH - relation. Our results are consistent with previous determinations at shorter wavelengths. By using our calibrated M/L, we rescale M BH -L bul to obtain the M BH -M relation, which can be used as the local reference for high-z studies which probe the cosmic evolution of M BH -galaxy relations and where the stellar mass is inferred directly from luminosity measurements. The analysis of pseudo-bulges shows that four out of nine lie on the scaling relations within the observed scatter, while those with small M BH are significantly displaced. We explore the different origins for such behaviour while considering the possibility of nuclear morphological components not reproduced by our 2D decomposition.