Identification of large equivalent width dusty galaxies at 4 < z < 6 from sub-millimetre colours

¹ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
e-mail: denis.burgarella@lam.fr
² Centro de Astronomía (CITEVA), Universidad de Antofagasta, Avenida Angamos 601, Antofagasta, Chile
³ Division of Particle and Astrophysical Science, Graduate School of Science, Nagoya University, Aichi 464-8602, Japan
⁴ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan
⁵ Department of Physics, School of Advanced Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan
⁶ Waseda Research Institute for Science and Engineering, Faculty of Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan

Received: 13 July 2022
Accepted: 10 November 2022

Abstract

Context. Infrared (IR), sub-millimetre (sub-mm), and millimetre (mm) databases contain a huge quantity of high-quality data. However, a large part of these data are photometric, and they are thought not to be useful to derive quantitative information on the nebular emission of galaxies.

Aims. The aim of this project is first to identify galaxies at z ≳ 4–6 and in the epoch of reionisation from their sub-millimetre colours. We also aim to show that the colours can be used to try and derive physical constraints from photometric bands when accounting for the contribution from the IR fine structure lines to these photometric bands.

Methods. We modelled the flux of IR fine structure lines with CLOUDY and added them to the dust continuum emission with CIGALE. Including (or not) emission lines in the simulated spectral energy distribution (SED) modifies the broad-band emission and colours.

Results. The introduction of the lines allows us to identify strong star forming galaxies at z ≳ 4–6 from the [log₁₀(PSW_250μm)/(PMW_350μm) versus log₁₀(LABOCA_870μm)/(PLW_500μm)] colour-colour diagram. By comparing the relevant models to each observed galaxy colour, we are able to roughly estimate the fluxes of the lines and the associated nebular parameters. This method allows us to identify a double sequence in a plot built from the ionisation parameter and the gas metallicity.

Conclusions. The HII and photodissociation region fine structure lines are an essential part of the SEDs. It is important to add them when modelling the spectra, especially at z ≳ 4–6, where their equivalent widths can be large. Conversely, we show that we can extract some information on strong-IR fine structure lines and on the physical parameters related to the nebular emission from IR colour-colour diagrams.