Research Blog
Welcome to my Research Blog.
This is mostly meant to document what I am working on for myself, and to communicate with my colleagues. It is likely filled with errors!
This project is maintained by ndrakos
This post will summarize my initial plans for including reionization modelling with the DREaM galaxy catalog. A lot of the background in this post is from Robertson 2021.
The production of ionizing photons from galaxies can be expressed as:
\[\dot{n}_{\rm ion} = f_{\rm esc} \xi_{\rm ion} \rho_{\rm UV}\]where \(\dot{n}_{\rm ion}\) is the comoving ionizing emissivity, \(f_{\rm esc}\) is the escape fraction of Lyman continuum (LyC) photons, \(\xi_{\rm ion}\) is the ionizing photon production efficiency, and \(\rho_{\rm UV}\) is the comoving UV luminosity density.
\(\rho_{\rm UV}\) can be calculated to from the abundances and luminosities of the galaxies in DREaM, and was already done in the original DREaM paper.
\(\xi_{\rm ion}\) should depend on age, metallicity, SMF and binarity. It can be measured directly from the galaxy SED, as outlined in Naidu et. al 2020.
Usually \(\xi_{\rm ion}\) is defined in terms of the rate of ionizing photons, \(N(H^0)\), per unit UV luminosity, measured at 1500 Angstroms. \(\xi_{\rm ion}\) can be calculated by integrating the flux produced below the Lyman limit to get \(N(H^0)\), and then normalizing by the SED-flux at 1500 Angstroms:
\[\xi_{\rm ion} = \frac{N(H^0)}{L_{1500}} [\rm s^{-1} erg^{-1} s^{-1} Hz^{-1}]\]The LyC escape fraction, \(f_{\rm esc}\) is the fraction of photons that escape galaxies to ionize intergalactic hydrogen. This is the least constrained quantity out of \(f_{\rm esc}\), \(\xi_{\rm ion}\) and \(\rho_{\rm UV}\)
Naidu et. al 2020 fits a model in which \(f_{\rm esc} = a \Sigma^{b}_{\rm SFR}\) (see their equation 8). For now I will use this model.
Reionization can be expressed as the volume-filling fraction of ionized gas:
\[\frac{ dQ_{\rm HII} }{ dt} = \frac{ \dot{n}_{\rm ion} } {\langle n_H \rangle} - \frac{Q}{\bar{t}_{\rm rec}}\]\(\dot{n}_{\rm ion}\): can be calculated as described above
\(\langle n_H \rangle = X_p \Omega_b \rho_c\) is the co-moving density of hydrogen, where \(X_p\) is the primordial mass-fraction of hydrogen. Should just be able to calculate from \(\Lambda\)CDM parameters.
\(t_{\rm rec}\) is the recombination time of ionized hydrogen in the IGM, and is given by Eq 5 in Naidu et al. 2020. There are some assumptions they make when calculating this parameter.
I am going to begin by calculating the above values, and making sure they are reasonable. However, I want to consider variations to these models. Here are some notable ones:
\(\xi_{\rm ion}\) should be dependent on the exact SED modelling. For now I will use the SED modelling I had originally used in DREaM, but I need to look more into how much variations on this could influence results.
The Naidu et. al 2020 model is not firmly established. I might look at other models (e.g. constant fesc), and calculate how well various surveys can constrain this scenario.
The value for \(t_{\rm rec}\) depends on things like the inhomogenity of the IGM. I will explore some of the assumptions here in more detail.