Volume 628, August 2019
|Number of page(s)||12|
|Published online||15 August 2019|
The gentle monster PDS 456
Kiloparsec-scale molecular outflow and its implications for QSO feedback
INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Roma, Italy
2 INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
3 Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, PI, Italy
4 Dipartimento di Fisica e Astronomia, Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
5 INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
6 INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
7 INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
8 European Southern Observatory, Karl-Schwarzschild-str. 2, 85748 Garching bei München, Germany
9 Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Avenue, Cambridge CB3 0HE, UK
10 Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
11 Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
Accepted: 13 June 2019
We report on the first ALMA observation of the CO(3−2) and rest-frame ∼340 GHz continuum emission in PDS 456, which is the most luminous, radio-quiet QSO in the local Universe (z ≃ 0.18), with a bolometric luminosity LBol ∼ 1047 erg s−1. ALMA angular resolution allowed us to map scales as small as ∼700 pc. The molecular gas reservoir traced by the core of the very bright CO(3−2) emission line is distributed in a compact rotating disk, with a size of ∼1.3 kpc, seen close to face-on (i ∼ 25 deg). Fast CO(3−2) emission in the velocity range v ∈ [ − 1000, 500] km s−1 is also present. Specifically, we detect several blue-shifted clumps out to ∼5 kpc from the nucleus, in addition to a compact (R ≲ 1.2 kpc), broad emission component. These components reveal a galaxy-wide molecular outflow, with a total mass Mmolout ∼ 2.5 × 108 M⊙ (for an αCO = 0.8 M⊙ (K km s−1 pc2)−1) and a mass outflow rate Ṁmol ∼ 290 M⊙ yr−1. The corresponding depletion time is τdep ∼ 8 Myr, shorter than the rate at which the molecular gas is converted into stars, indicating that the detected outflow is potentially able to quench star-formation in the host. The momentum flux of the molecular outflow normalised to the radiative momentum output (i.e. LBol/c) is ≲1, comparable to that of the X-ray ultra-fast outflow (UFO) detected in PDS 456. This is at odds with the expectations for an energy-conserving expansion suggested for most of the large-scale outflows detected in low-luminosity AGNs so far. We suggest three possible scenarios that may explain this observation: (i) in very luminous AGNs such as our target the molecular gas phase is tracing only a fraction of the total outflowing mass; (ii) a small coupling between the shocked gas by the UFO and the host-galaxy interstellar medium (ISM); and (iii) AGN radiation pressure may be playing an important role in driving the outflow.
Key words: galaxies: active / quasars: individual: PDS 456 / galaxies: evolution / quasars: emission lines / techniques: imaging spectroscopy / techniques: interferometric
© ESO 2019
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