Abstract
The rest-frame far infrared (FIR) fine structure lines [CII]158µm, [OIII]88µm, [NII]122µm and [NIII]57µm are excellent tools for probing the physical conditions of high-redshift galaxies, as they trace different properties of the gas in the multi-phase interstellar medium (ISM). At z > 6, the FIR lines are redshifted into sub-millimeter wavelengths, which are observable with ground-based facilities such as ALMA, and they have the further advantage of being unaffected by dust extinction. FIR line ratios, such as [OIII]88µm/[CII]158µm and [OIII]88µm/[NII]122µm have shown to be promising for tracing the ionization state and metallicity of the ISM. At z > 6, the [OIII]88µm/[CII]158µm luminosity ratio is observed to be peculiarly higher compared to the ratio measured in galaxies at z ∼ 0 with comparable star formation rates (SFR). The origin of such higher [OIII]88µm/[CII]158µm luminosity ratios at high redshift is still debated, with several contributing factors proposed, such as a lower C/O abundance ratio, lower gas phase metallicity and higher ionization parameters. Cosmological simulations have been used to reproduce these high-z results, taking into account factors such as enhanced SFRs due to starbursts and core-collapse supernova enrichment. However, no model has so far been able to successfully reproduce the enhanced line ratios as well as the [CII]158µm-SFR and [OIII]88µm-SFR relations observed at high z. In this work, we model the [CII]158µm, [OIII]88µm, [NII]122µm and [NIII]57µm emission lines of Ponos: a highresolution (mgas = 883.4 M⊙) cosmological zoom-in simulation of a z = 6.5 progenitor of a massive local galaxy, where the simulation has been post-processed with radiative transfer (RT) using Kramses-RT. To model the FIR emission lines we use the 1D photoionization code Cloudy, where we change the carbon, nitrogen and oxygen abundances based on constraints of the C/O and N/O abundance ratios from observations of high-redshift galaxies and local high-redshift analogs. With this, we aim to make the emission line modelling more representative of the ISM properties at z > 6. Our modelling results in [OIII]88µm/[CII]158µm luminosity ratios that are a factor of ∼5 higher than other models that assume solar abundances. We also find a better agreement with z > 6 observational constraints for the L[CII] − SFR and L[OIII] − SFR relations. This shows how a lower C/O abundance ratio, motivated by theoretical and observational constraints, is crucial for explaining the observed high [OIII]88µm/[CII]158µm ratio. We propose that, in addition to chemistry, extreme ionization parameters could potentially contribute to enhancing the [OIII]88µm/[CII]158µm ratios even further. In addition to enhanced [OIII]88µm/[CII]158µm ratios, recent observations at z ∼6−10 have estimated unexpectedly high N/O abundance ratios. We explored this in our models, and found iv that the super-solar N/O abundances yielded [OIII]88µm/[NII]122µm ratios that agreed with semi-analytical models assuming much lower ionization parameters than estimated for Ponos. By assuming sub-solar N/O abundance ratios from local metal-poor dwarfs galaxies observations, our models yield [OIII]88µm/[NII]122µm luminosity ratios that is in better agreement with, but still lower than, semi-analytical models which estimate ionization parameters close to what we predict for Ponos. We need more sensitive observations of the FIR and sub-millimeter emission lines at z > 6, especially [OIII], [NII] and [NIII], with available estimates of the C/O and N/O abundance ratios, to further constrain the high-redshift FIR line ratio
Supervisors:
Associate Professor Claudia Cicone, Institute of Theoretical Astrophysics, UiO
Doctoral Research Fellow Alice Schimek, Institute of Theoretical Astrophysics, UiO
Intern. assessor: Professor David F. Mota, Institute of Theoretical Astrophysics, UiO
Extern. assessor: Dr. Martin Pierre Rey, department of Physics, University of Oxford,