Structure and kinematics of a massive galaxy at z ∼ 7

¹ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
e-mail: ana.posses@mail.udp.cl
² Las Campanas Observatory, Carnegie Institution of Washington, Casilla 601, La Serena, Chile
³ Cavendish Laboratory/Kavli Institute for Cosmology, University of Cambridge, 19 J. J. Thomson Ave., Cambridge CB3 0HE, UK
⁴ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁵ Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
⁶ Joint ALMA Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
⁷ Institute of Astrophysics, Foundation for Research and Technology-Hellas (FORTH), Heraklion 70013, Greece
⁸ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus
⁹ Departamento de Astronomía, Universidad de Concepción, Barrio Universitario, Concepción, Chile
¹⁰ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
¹¹ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK

Received: 22 February 2022
Accepted: 22 June 2022

Abstract

Context. Observations of the rest-frame UV emission of high-redshift galaxies suggest that the early stages of galaxy formation involve disturbed structures. Imaging the cold interstellar medium (ISM) can provide a unique view of the kinematics associated with the assembly of galaxies.

Aims. In this paper, we analyze the spatial distribution and kinematics of the cold ionized gas of the normal star-forming galaxy COS-2987030247 at z = 6.8076, based on new high-resolution observations of the [C II] 158 μm line emission obtained with the Atacama Large Millimeter/submillimeter Array (ALMA).

Methods. The analysis of these observations allowed us to: compare the spatial distribution and extension of the [C II] and rest-frame UV emission, model the [C II] line data-cube using the ^3DBAROLO code, and measure the [C II] luminosity and star formation rate (SFR) surface densities in the galaxy subregions.

Results. The system is found to be composed of a main central source, a fainter north extension, and candidate [C II] companions located 10-kpc away. We find similar rest-frame UV and [C II] spatial distributions, suggesting that the [C II] emission emerges from the star-forming regions. The agreement between the UV and [C II] surface brightness radial profiles rules out diffuse, extended [C II] emission (often called a [C II] halo) in the main galaxy component. The [C II] velocity map reveals a velocity gradient in the north-south direction, suggesting ordered motion, as commonly found in rotating-disk galaxies. However, higher resolution observations would be needed to rule out a compact merger scenario. Our model indicates an almost face-on galaxy (i ∼ 20°), with a average rotational velocity of 86 ± 16 km s⁻¹ and a low average velocity dispersion, σ < 30 km s⁻¹. This result implies a dispersion lower than the expected value from observations and semi-analytic models of high redshift galaxies. Furthermore, our measurements indicate that COS-2987030247 and its individual regions systematically lie within the local L_[CII]-SFR relationship, yet slightly below the local Σ_[CII]-Σ_UV relation.

Conclusions. We argue that COS-2987030247 is a candidate rotating disk experiencing a short period of stability which will possibly become perturbed at later times by accreting sources.