Embedded star formation in the extended narrow line region of Centaurus A: Extreme mixing observed by MUSE

¹ ASTRON, the Netherlands Institute for Radio Astronomy, PO 2, 7990 AA Dwingeloo, The Netherlands
e-mail: santoro@astro.rug.nl
² Kapteyn Astronomical Institute, University of Groningen, PO 800, 9700 AV, Groningen, The Netherlands
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA, Leiden, The Netherlands
⁴ Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK

Received: 19 February 2016
Accepted: 12 April 2016

Abstract

We present a detailed study of the complex ionization structure in a small (~250 pc) extended narrow line region (ENLR) cloud near Centaurus A using the Multi Unit Spectroscopic Explorer. This cloud is located in the so-called outer filament of ionized gas (about 15 kpc from the nucleus) where jet-induced star formation has been suggested to occur by different studies. We find that, despite the small size, a mixture of ionization mechanisms is operating, resulting in considerable complexity in the spatial ionization structure. The area includes two H ii regions where star formation is occurring and another location where star formation must have ceased very recently. Interestingly, the extreme Balmer decrement of one of the star forming regions (Hα/Hβ_obs ~ 6) indicates that it is still heavily embedded in its natal cocoon of gas and dust. At all three locations a continuum counterpart is found with spectra matching those of O/B stars local to Centaurus A. The H ii regions are embedded in a larger gas complex which is photoionized by the radiation of the central active galactic nucleus (AGN), but the O/B stars affect the spatial ionization pattern in the ENLR cloud very locally. In particular, in the surroundings of the youngest star forming region, we can isolate a tight mixing sequence in the diagnostic diagram going from gas with ionization due to a pure stellar continuum to gas only photoionized by the AGN. These results emphasize the complexity and the mixture of processes occurring in star forming regions under the influence of an AGN radiation. This is relevant for our understanding of AGN-induced star formation suggested to occur in a number of objects, including this region of Centaurus A. They also illustrate that these young stars influence the gas over only a limited region.