A unique distant submillimeter galaxy with an X-ray-obscured radio-luminous active galactic nucleus
1 Department of Physics, Anhui Normal University Wuhu Anhui 241000 PR China
2 CAS Key Laboratory for Researches in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China Hefei Anhui 230026 PR China
3 School of Astronomy and Space Science, University of Science and Technology of China Hefei 230026 PR China
4 Max Planck Institute for Astronomy Königstuhl 17 69117 Heidelberg Germany
5 Laboratoire AIM-Paris-Saclay, CEA/DSM/Irfu – CNRS – Université Paris Diderot, CEA-Saclay 91191 Gif-sur-Yvette France
6 Institute of Astronomy, The University of Tokyo, 2-21-1 Osawa, Mitaka Tokyo 181-0015 Japan
7 Yunnan Observatories, Chinese Academy of Sciences Kunming 650011 PR China
8 Academia Sinica Institute of Astronomy and Astrophysics PO Box 23-141 Taipei 10617 Taiwan
9 Max-Planck-Institut für extraterrestrische Physik Giessenbachstrasse 1 85748 Garching, bei München Germany
10 Purple Mountain Observatory, Chinese Academy of Sciences Nanjing 210008 PR China
11 The Australian National University, Mt Stromlo Observatory Cotter Rd Weston Creek ACT 2611 Australia
Accepted: 25 August 2018
Aims. We present a multiwavelength study of an atypical submillimeter galaxy, GH500.30, in the GOODS-North field, with the aim to understand its physical properties of stellar and dust emission, as well as the central active galactic nucleus (AGN) activity. Although it is shown that the source is likely an extremely dusty galaxy at high redshift, its exact position of submillimeter emission is unknown.
Methods. We use NOEMA observation at 1.2 mm with subarcsecond resolution to resolve the dust emission, and precisely localize the counterparts at other wavelengths, which allows us to better constrain its stellar and dust spectral energy distribution (SED) as well as redshift. We carry out the new near-infrared (NIR) photometry of GH500.30 observed with HST, and perform panchromatic SED modelling from ultraviolet (UV)/optical to submillimeter. We derive the photometric redshift using both NIR and far-infrared (FIR) SED modeling, and place constraints on the stellar and dust properties such as stellar mass, age, dust attenuation, IR luminosity, and star-formation rate (SFR). The AGN properties are inferred from the X-ray spectral analysis and radio observations, and its contribution to the total IR luminosity is estimated from the broadband SED fittings using MAGPHYS.
Results. With the new NOEMA interferometric imaging, we confirm that the source is a unique dusty galaxy. It has no obvious counterpart in the optical and even NIR images observed with HST at λ ≲ 1.4 μm. Photometric-redshift analyses from both stellar and dust SED suggest it to likely be at z ≳ 4, though a lower redshift at z ≳ 3.1 cannot be fully ruled out (at 90% confidence interval). Explaining its unusual optical-to-NIR properties requires an old stellar population (∼0.67 Gyr), coexisting with a very dusty ongoing starburst component. The latter is contributing to the FIR emission, with its rest-frame UV and optical light being largely obscured along our line of sight. If the observed fluxes at the rest-frame optical/NIR wavelengths were mainly contributed by old stars, a total stellar mass of ∼3.5 × 1011 M⊙ would be obtained. An X-ray spectral analysis suggests that this galaxy harbors a heavily obscured AGN with NH = 3.3+2.0−1.7 × 1023 cm−2 and an intrinsic 2–10 keV luminosity of Lx ∼ 2.6 × 1044 erg s−1, which places this object among distant type 2 quasars. The radio emission of the source is extremely bright, which is an order of magnitude higher than the star-formation-powered emission, making it one of the most distant radio-luminous dusty galaxies.
Conclusions. The combined characteristics of the galaxy suggest that the source appears to have been caught in a rare but critical transition stage in the evolution of submillimeter galaxies, where we are witnessing the birth of a young AGN and possibly the earliest stage of its jet formation and feedback.
Key words: galaxies: starburst / galaxies: high-redshift / galaxies: active / submillimeter: galaxies / X-rays: galaxies
© ESO 2018