The ALPINE-ALMA [CII] survey: Double stellar population and active galactic nucleus activity in a galaxy at z ∼ 5.5

¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
e-mail: luigi.barchiesi2@unibo.it
² INAF-OAS, Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ Department of Astronomy, University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁴ SISSA, Via Bonomea 265, 34136 Trieste, Italy
⁵ IPAC, California Institute of Technology 1200 E California Boulevard, Pasadena, CA, 91125 USA
⁶ Aix-Marseille Univ., CNRS, LAM, Marseille, France
⁷ Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁸ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁹ Institut d’Astrophysique de Paris, Sorbonne Université, CNRS, UMR7095, 75014 Paris, France
¹⁰ Waseda Research Institute for Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo, 169-8555 Japan
¹¹ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo, Japan
¹² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
¹³ Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran Bretaña 1111, Valparaíso, Chile
¹⁴ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH UK
¹⁵ National Centre for Nuclear Research, ul. Pasteura 7, 02-093 Warsaw, Poland
¹⁶ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
¹⁷ The Caltech Optical Observatories, California Institute of Technology, Pasadena, CA, 91125 USA

Received: 30 August 2022
Accepted: 29 November 2022

Abstract

Context. GDS J033218.92-275302.7 (here GS-14) is a z ∼ 5.5 galaxy with unusual UV spectral features that have been interpreted as signatures of either a double stellar population or of an active galactic nucleus (AGN). GS-14 was detected in [C II] 158 μm as part of the ALPINE survey and was found to be the galaxy with the lowest molecular gas fraction (f_mol = M_molgas/(M_molgas + M_*)∼0.1) of that sample.

Aims. We exploited the multiwavelength coverage of GS-14 to investigate the properties and the origin of its emission.

Methods. We fit the UV-to-near-IR spectral energy distribution (SED) with a single and double stellar population and/or AGN component. We analyzed the latest release of the VIMOS spectrum, which shows highly ionized emission lines (O VI1032 Å, N V1240 Å, and N IV] 1483,1486 Å). The line equivalent widths and line ratios were compared with those observed in galaxies and AGN, as well as with the predictions from radiation transfer models for star-forming galaxies, AGN, and shocks.

Results. The SED-fitting provides a total stellar mass of M_* = (4 ± 1)×10¹⁰ M_⊙, an age of the main stellar population of ∼ 670 Myr, and a recent (∼ 8 Myr) short burst of star formation (SF) of ∼90 M_⊙ yr⁻¹. We do not find a significant contribution from an AGN, although we do not have any coverage in the mid-IR, where the dust emission of the AGN would peak. The N V line has a characteristic P Cygni profile. Fitting it with stellar models suggests a ∼ 3 Myr old population of stars with a mass of ∼5 × 10⁷ M_⊙, consistent with a second component of young stars, as found in the SED-fitting analysis. The N V profile also shows evidence for an additional component of nebular emission. The comparison of the line ratios (N IV]/N V and O VI/N V) with theoretical models allows us to associate the emission with SF or AGN, but the strong radiation field required to ionize the O VI is more commonly related to AGN activity.

Conclusions. Studying GS-14, we found evidence for an old and already evolved stellar population at z ∼ 5.5, and we show that the galaxy is experiencing a second short burst of SF. In addition, GS-14 carries signatures of obscured AGN activity. The AGN could be responsible for the short depletion time of this galaxy, thus making GS-14 one of the two ALPINE sources with indications of an active nucleus and an interesting target for future follow-ups to understand the connection between SF and AGN activity.