ALMA reveals a warm and compact starburst around a heavily obscured supermassive black hole at z = 4.75

¹ INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: roberto.gilli@oabo.inaf.it
² Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁵ Istituto di Radioastronomia & Italian ALMA Regional Centre, Via P. Gobetti 101, 40129 Bologna, Italy
⁶ Laboratoire AIM, CEA/DSM-CNRS-Université Paris Diderot, Irfu/Service d’Astrophysique, CEA Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France
⁷ NOAO, 950 North Cherry Avenue, 85719 Tucson, USA
⁸ ICREA and Institut de Ciències del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), Martí i Franquès 1, 08028 Barcelona, Spain
⁹ Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK
¹⁰ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
¹¹ UPMC-CNRS, UMR7095, Institut d’Astrophysique de Paris, 75014 Paris, France
¹² ESO, Karl-Schwarzschild-Str 2, 85748 Garching bei München, Germany

Received: 22 October 2013
Accepted: 4 January 2013

Abstract

We report ALMA Cycle 0 observations at 1.3 mm of LESS J033229.4-275619 (XID403), an ultraluminous infrared galaxy at z = 4.75 in the Chandra Deep Field South hosting a Compton-thick QSO. The source is not resolved in our data at a resolution of ~0.75 arcsec, placing an upper-limit of 2.5 kpc to the half-light radius of the continuum emission from heated-dust. After deconvolving for the beam size, however, we found a ~3σ indication of an intrinsic source size of 0.27 ± 0.08 arcsec (Gaussian FWHM), which would correspond to r_half ~ 0.9 ± 0.3 kpc. We build the far-infrared SED of XID403 by combining datapoints from both ALMA and Herschel and fit it with a modified blackbody spectrum. For the first time, we measure the dust temperature T_d = 58.5 ± 5.3 K in this system, which is comparable to what has been observed in other high-z submillimeter galaxies. The measured star formation rate is SFR = 1020 ± 150 M_⊙ yr^-1, in agreement with previous estimates at lower S/N. Based on the measured SFR and source size, we constrain the SFR surface density to be Σ_SFR > 26M_⊙ yr^-1 kpc^-2 (~200M_⊙ yr^-1 kpc^-2 for r_half ~ 0.9 kpc). The compactness of this starburst is comparable to what has been observed in other local and high-z starburst galaxies. If the gas mass measured from previous [CII] and CO(2–1) observations at low resolution is confined within the same dust region, assuming r_half ~ 0.9 ± 0.3 kpc, this would produce a column density of N_H ~ 0.3−1.1 × 10²⁴ cm^-2 towards the central SMBH, similar to the column density of ≈1.4 × 10²⁴ cm^-2 measured from the X-rays. Then, in principle, if both gas and dust were confined on sub-kpc scales, this would be sufficient to produce the observed X-ray column density without any need of a pc-scale absorber (e.g. the torus postulated by Unified Models). We speculate that the high compactness of star formation, together with the presence of a powerful AGN, likely produce an outflowing wind. This would be consistent with the ~350 km s^-1 velocity shift observed between the Lyα emission and the submm lines ([CII], CO(2–1), [NII]) and with the highly-ionized Fe emission line at ~6.9 keV rest-frame tentatively observed in the X-ray spectrum. Finally, our observations show that, besides the mass, star formation rate and gas depletion timescale, XID403 has also the right size to be one of the progenitors of the compact quiescent massive galaxies seen at z ~ 3.