LLAMA: Stellar populations in the nuclei of ultra-hard X-ray-selected AGN and matched inactive galaxies^⋆

¹ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: burtscher@strw.leidenuniv.nl
² Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
³ Departamento de Astronomia, Universidade Federal do Rio Grande do Sul, IF, CP 15051, 91501-970 Porto Alegre, RS, Brazil
⁴ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁵ Department of Physics & Astronomy, University of Alaska Anchorage, Anchorage, AK 99508-4664, USA
⁶ Institute of Astronomy and Astrophysics, Academia Sinica, 11F of AS/NTU Astronomy-Mathematics Building, No.1, Sec. 4, Roosevelt Rd, Taipei 10617, Taiwan
⁷ Departamento de Física/CCNE, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil
⁸ OmegaLambdaTec GmbH, Lichtenbergstr. 8, 85748 Garching, Germany
⁹ Space Sciences, Technologies, and Astrophysics Research Institute, Université de Liège, 4000 Sart Tilman, Belgium
¹⁰ Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, MD 20742, USA

Received: 17 February 2021
Accepted: 11 May 2021

Abstract

The relation between nuclear (≲50 pc) star formation and nuclear galactic activity is still elusive; theoretical models predict a link between the two, but it is unclear whether active galactic nuclei (AGNs) should appear at the same time, before, or after nuclear star formation activity. We present a study of this relation in a complete, volume-limited sample of nine of the most luminous (log L_{14 − 195 keV} > 10^42.5 erg s⁻¹) local AGNs (the LLAMA sample), including a sample of 18 inactive control galaxies (six star-forming; 12 passive) that are matched by Hubble type, stellar mass (9.5 ≲ log M_⋆/M_⊙ ≲ 10.5), inclination, and distance. This allows us to calibrate our methods on the control sample and perform a differential analysis between the AGN and control samples. We performed stellar population synthesis on VLT/X-shooter spectra in an aperture corresponding to a physical radius of ≈150 pc. We find young (≲30 Myr) stellar populations in seven out of nine AGNs and in four out of six star-forming control galaxies. In the non-star-forming control population, in contrast, only two out of 12 galaxies show such a population. We further show that these young populations are not indicative of ongoing star formation, providing evidence for models that see AGN activity as a consequence of nuclear star formation. Based on the similar nuclear star formation histories of AGNs and star-forming control galaxies, we speculate that the latter may turn into the former for some fraction of their time. Under this assumption, and making use of the volume completeness of our sample, we infer that the AGN phase lasts for about 5% of the nuclear starburst phase.