Stellar cusp and warm dust at the heart of NGC1068

Abstract

Establishing precisely how stars and interstellar medium distribute within the central 100 pc area around an AGN, down to the pc scale, is key for understanding how the very final transfer of matter from kpc scale to the sub-parsec size of the accretion disc is achieved. Using AO-assisted (SPHERE-VLT) near-IR images in H and Ks and narrow-band of the Seyfert 2 galaxy NGC1068 we analyse the radial distribution of brightness in the central r < 100 pc area down to the pc scale. The median-averaged radial profiles are adjusted by a cusp (power-law) plus a central point-source. A simple radiative transfer model is used to interpret the data. We find that the fit of profiles beyond 10pc is done quite precisely at Ks by a cusp of exponent -2.0 plus a central point-source and by a cusp of exponent -1.2 at H. The difference between H and Ks can be explained by differential extinction, provided that the distribution of dust is itself cuspy as r^-1. However, the required stellar density follows a r^-4 cusp, much steeper than any other cusp theoretically predicted and the mass of the cluster is unreasonable, even introducing a segregation in the stellar population with an excess of giant stars inward. A much more acceptable solution is found with a K profile dominated by warm dust emission, while the H profile corresponds to a stellar cusp. NGC1068 is shown to satisfy a relationship between half-light radius, cusp luminosity and exponent which suggests that the cusp is the remnant of a recent starbust. We identify the central point-like source with the very hot dust at the internal wall of the putative torus and derive an intrinsic luminosity that requires an overall extinction AK ~ 8, a value consistent with predictions by several models.

arXiv comment20 pages. Revised version after referee's comments. Recommended for publication in A&A, after revision (this version)

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