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

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(PSW250μm)/(PMW350μm) versus log10(LABOCA870μm)/(PLW500μm)] 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.

CEERS Key Paper. V. Galaxies at 4 < z < 9 Are Bluer than They Appear–Characterizing Galaxy Stellar Populations from Rest-frame ∼1 μm Imaging

“…There are observations that z > 2 galaxies have a higher incidence of "extreme" emission lines with rest-frame EWs up to ≈1000 Å (e.g., van der Wel et al 2011;Tang et al 2019;Tran et al 2020;Boyett et al 2022;Matthee et al 2022;Pérez-González et al 2023;Sun et al 2022), consistent with inferences made from the >3 μm colors of z > 6 galaxies (Smit et al 2015;Roberts-Borsani et al 2016;Castellano et al 2017;Hutchison et al 2019;Endsley et al 2021). Because the EW scales with redshift as (1 + z), this implies these lines have a stronger impact for high-redshift galaxies for bandpasses of fixed wavelength width (e.g., Papovich et al 2001;Burgarella et al 2023).…”

Modeling the spectral energy distribution of starburst galaxies

“…In the far-infared (FIR) to submillimeter (submm) wavelength ranges, metal fine-structure lines such as [OIII]52µm, [OI]63µm, and [OIII]88µm lines can therefore be used to trace the gas metallicity (Nagao et al 2011) and star formation in galaxies (Schaerer et al 2020), or to trace the physical conditions in the galactic interstellar medium (ISM). They may also contribute substantially to broadband fluxes (Burgarella et al 2023).…”