Extragalactic observations with Adaptive Optics: Polarisation in Active galactic Nuclei and Study of Super Stellar Clusters
Creators: name: Grosset, Lucas
Publisher: name: Zenodo
Year: 2019
Subjects: Galaxies: Seyfert
polarimetry,
Radiative transfer
Near infrared
high angular resolution
adaptive optics
super stellar clusters

description: The unified model for AGN stats that Seyfert 1 or 2 types are the same type of object harbouring a luminous accretion disk surrounded by a thick torus, but seen under different viewing angles. This model has been successfully tested for many years by several observers, bringing some evolutions to the initial model, but on the torus, one of the most important pieces, we still lack information because of its limited extension and of the high contrast required. We took advantage of SPHERE’s extreme contrast and high angular resolution to propose a near infrared (NIR) polarimetric observation of the archetypal Seyfert 2 galaxy NGC 1068 in H and Ks broad bands, revealing a clear double hourglass shape of centro-symmetric pattern and a central pattern diverging from these. We continued these measurements in narrow bands (in the NIR) and at shorter wavelengths (R band) in order to study the wavelength dependency of the measured polarisation, which we are currently analysing. If the features are wavelength-dependent, this will bring new constraints on the properties of scatterers.

Polarimetry is a powerful tool as it gives access to more information than spectro- scopy or imaging alone. In particular, indications on the geometry of the scatterers, the orientation of the magnetic fields or the physical conditions of matter can be re- vealed thanks to the additional parameters measured (the polarisation degree and the polarisation position angle for linear polarisation). The counterpart of polarimetric measurements is that analyses of data is not straightforward. The use of numerical simulations, and especially radiative transfer codes is necessary to fully understand the observations.  The first part  describes a radiative transfer code, MontAGN, with the aim of reproducing the features that we observed, thus including full processing of polarisation. This code allowed us to bring some constraining results on the optical depth of the structures. The torus’ optical depth was constrained to 20 in the Ks band as a minimum. Our investigations on the densities in the ionisation cone are consistent with densities of 2.0 × 109 m−3, in the range of previously estimated values for this AGN.

The last part of the PhD work was dedicated to SSC (also known as young massive clusters). These clusters correspond to the more massive example of star forming clusters, often about 10 Myr and still embedded in a dusty shell. These clusters exhibit extreme observed star formation rates. We took advantage of an instrumental run of the multi-object AO demonstrator CANARY instrument, installed at the Wil- liam Hershel Telescope in 2013 to obtain images in H and Ks bands at higher resolution than previously achievable. Data on the galaxy IRAS 21101+5810 were reduced and analysed, constraining the age of the clusters between 10 and 100 Myr and the ex- tinction to about AV ≈ 3. Photometry was obtained thanks to a new algorithm to estimate the galaxy background, bringing improvements on the fitting of the clusters’ luminosity distributions.