Research

Research Interests

My main research interests are the formation of galaxies in the early Universe and their role in cosmic reionization. In the hierarchical scenario of structure formation, galaxies formed within dark matter haloes, and therefore provide observable probes for dark matter structure formation. With the help of powerful computer clusters, the formation and evolution of dark matter structures can be simulated using large-scale cosmological N-body/hydrodynamical simulations. Semi-analytic models implemented upon dark matter merger trees from cosmological simulations have become an efficient way to study the formation and evolution of galaxies.

Star-forming galaxies are generally considered to be the main energy source for ionizing the intergalactic medium (IGM) during the epoch of reionization (EoR). However, due to extinction by dust in the interstellar medium, only a small fraction of UV photons can escape from galaxies (e.g. with the escape fraction f_esc ≲ 0.2) and ionize the IGM. Calculations show that ionizing photons (emitted into the IGM) from detected galaxies may not be sufficient to maintain reionization at redshift z∼6. We therefore need to investigate the abundance and the ionizing photon emissivity of faint galaxies below current detection limits (e.g. M_UV∼−17 at z∼6). In particular, the following questions related to galaxy formation during the EoR have motivated my PhD research:

• How does the faint-end slope of the galaxy UV luminosity function behave below current detection limits?
• How much contribution do undetected faint galaxies make to reionization?
• How do extremely faint and very high-z galaxies form and evolve?
• How can we model the sizes of protogalaxies and compare to observed sizes?

Current Work

My PhD research is a part of the DRAGONS (Dark-ages Reionization And Galaxy-formation Observables from Numerical Simulations) project ― a collaboration between researchers at Melbourne, Swinburne and SNS-Pisa. DRAGONS combines N-body simulations, hydrodynamical simulations and semi-analytic models to simulate observations of high-z galaxies and the morphology and structure of cosmic reionization.

The N-body simulations and the galaxy formation model used in my work are specially designed for studying galaxy formation during the epoch of reionization (z>6). Our N-body simulations (Poole et al. 2016; Angel et al. 2016) have a high mass resolution allowing us to model galaxies within low-mass haloes with virial temperatures close to the hydrogen cooling limits of 10⁴ K. The simulations have a high snapshot cadence of 11 Myr, which resolves the dynamical time of galactic discs at z>6 and the main-sequence life time of massive stars. Our galaxy formation model (Mutch et al. 2016a) implements a delayed supernova feedback scheme and includes a temporally and spatially coupled UV background feedback.

I have developed a code that calculates spectral energy distributions (SEDs) and multi-band photometric luminosities for model galaxies. This is accomplished by integrating the star-formation histories of galaxies with template SEDs from stellar population synthesis models. Lyα absorption and dust extinction are also included. The calculated photometric luminosities and colours can then be directly compared with high-z observations.

Using the luminosities and other intrinsic properties calculated from our semi-analytic models and N-body simulations, I have obtained a number of key results about galaxy observables during reionization, including:

• Our model suggests the slope of the UV luminosity function remains steep below current detection limits until at least ∼−14 mag, indicating a large number of unobserved faint galaxies at z > 6.
• Undetected faint galaxies emit more UV photons than brighter galaxies at z > 7. These faint galaxies will contribute significantly to the reionization background.
• The luminosity dependence of halo mass, and other intrinsic galactic properties such as star-formation rate and stellar mass, extends down to until ∼−14 mag, but may break at even fainter luminosities.
• The size evolution of galaxies during reionization are shown to be an important observational probe for galaxy formation physics, allowing us to discriminate between various star formation and feedback models..

My work in-progress is studying the chemical evolution of high-z galaxies. The metallicity of galaxies is related to the dust abundance and the escape fraction, f_esc, of ionizing photons. The value of f_esc during the EoR is important, because it determines the total emissivity of galaxies and the required abundance of undetected galaxies. Due to the strong IGM absorption, the measurement of metallicities of high-z galaxies is difficult, and f_esc for high-z galaxies is usually inferred from their local analogues. However, this extrapolation is not secure and needs to be systematically investigated. Some simulations suggested that f_esc may decrease with dark matter halo mass and cosmic time. We want to study the stellar mass--metallicity relation of high-z galaxies and its evolution using semi-analytic models.

Please see Dark-ages Reionization And Galaxy-formation Observables from Numerical Simulations (DRAGONS) project for more information.