Extended ionised and clumpy gas in a normal galaxy at z = 7.1 revealed by ALMA

¹ Cavendish Laboratory, University of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK
e-mail: sc888@mrao.cam.ac.uk
² Kavli Institute for Cosmology, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
³ Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
⁴ Nordita, KTH Royal Institute of Technology and Stockholm University Roslagstullsbacken 23, 106 91 Stockholm, Sweden
⁵ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy
⁶ INAF–Bologna Astronomical Observatory, via Ranzani 1, 40127 Bologna, Italy
⁷ Square Kilometre Array Organization, Jodrell Bank Observatory, Lower Withington, Macclesfield, Cheshire SK11 9DL, UK

Received: 27 December 2016
Accepted: 10 April 2017

Abstract

We present new ALMA observations of the [O iii]88 μm line and high angular resolution observations of the [C ii]158 μm line in a normal star forming galaxy at z = 7.1. Previous [C ii] observations of this galaxy had detected [C ii] emission consistent with the Lyα redshift but spatially slightly offset relative to the optical (UV-rest frame) emission. The new [C ii] observations reveal that the [C ii] emission is partly clumpy and partly diffuse on scales larger than about 1 kpc. [O iii] emission is also detected at high significance, offset relative to the optical counterpart in the same direction as the [C ii] clumps, but mostly not overlapping with the bulk of the [C ii] emission. The offset between different emission components (optical/UV and different far-IR tracers) is similar to that which is observed in much more powerful starbursts at high redshift. We show that the [O iii] emitting clump cannot be explained in terms of diffuse gas excited by the UV radiation emitted by the optical galaxy, but it requires excitation by in-situ (slightly dust obscured) star formation, at a rate of about 7 M_⊙ yr^-1. Within 20 kpc from the optical galaxy the ALMA data reveal two additional [O iii] emitting systems, which must be star forming companions. We discuss that the complex properties revealed by ALMA in the z ~ 7.1 galaxy are consistent with expectations by recent models and cosmological simulations, in which differential dust extinction, differential excitation and different metal enrichment levels, associated with different subsystems assembling a galaxy, are responsible for the various appearance of the system when observed with distinct tracers.