Formation of the first galaxies and
the connection to reionization

One of the greatest challenges facing astrophysicist today is the formation of the first stars, black holes and galaxies and how they interacted with the surrounding gas, thereby ionizing the universe. This reionization process apparently began several hundred million years after the Big Bang and finished some 600 million years later. We are now entering a gold era, in which many new telescopes and instruments become available to study the first generation of stars and galaxies. In particular, we are looking forward to work with the James Webb Space Telescope (JWST), the Extremely Large Telescope (ELT), and the Square Kilometre Array (SKA). We will use these facilities to study the stellar population in the first galaxies and how these galaxies impact the surrounding gas. One of the key goal of our research is to synergistically develop and use theoretical and observational tools, which will help us to constrain the physics of the formation of the first stars, black holes and galaxies as well as to draw inspiration for new observational enterprises.

Empirical model for the formation and evolution of galaxies

To shed new light on the evolution of galaxies during the epoch of reionization, we have developed an empirical framework that relates the growth of galaxies to the growth of the host dark matter halos (Tacchella et al. 2013, Tacchella et al. 2018). In particular, the star-formation rates (SFRs) of halos is directly proportional to the accretion rate of their dark matter halos. The only free parameter of the model is the star-formation efficiency, which describes how efficiently gas is converted into stars, and encapsulates the complicated baryonic processes such as gas cooling, star formation, and various feedback processes into a single parameter. The star-formation efficiency is empirically calibrated with the UV luminosity function of galaxies at redshift z=4.

This simple framework is then able to reproduce key observational constraints of the galaxy population at high redshifts, such as the evolution of the cosmic star-formation rate density, UV luminosity function, and stellar mass function. Our study highlights that the primary driver of galaxy evolution across cosmic time is the buildup of dark matter halos, without the need to invoke a redshift-dependent efficiency in converting gas into stars. In particular, the increase in the cosmic star-formation rate density at high redshifts can be explained by the increase in the number density of halos that host star-forming galaxies. This model has been used as a benchmark in many observational and theoretical works. Specifically, the figure on the right shows the prediction of the stellar-to-halo mass relation, highlighting that current model predictions vary widely. Future observations of our group will help constraining this relation and therefore constrain models of galaxy formation.

Which sources reionize the universe? The case for massive galaxies.

With Harvard graduate student Rohan Naidu, we have investigate what current constraints of the neutral fraction in the universe in the universe imply for escape of ionizing photons of galaxies (Naidu, Tacchella, et al. 2020). Using the Tacchella et al. 2018 galaxy model, we find that bright galaxies need to play in important role in driving reionization, since only those galaxies can lead to a rather late, but rapid reionization history (see figure on the right). We propose a physically motivated model relating the escape fraction of galaxies to the star-formation rate surface density. Since the star-formation rate surface densities of galaxies falls by ~2.5 dex between z=8 and z=0, our model explains the humble upper limits on escape fractions at lower redshifts and its required evolution to escape fractions of 20% at z>6. Within this model, strikingly, <5% of galaxies with stellar masses of > 1e8 account for 80% of the reionization budget. We predict that bright Lyman-Continuum leakers (like COLA1) will become increasingly common toward z∼6 and that the drivers of reionization do not lie hidden across the faint end of the luminosity function but are already known to us.

What can we learn about dark matter from the first generation of galaxies?

With Harvard under-graduate student Diana Khimey, we have studies how the future observations by the James Webb Space Telescope (JWST) can constrain the nature of dark matter. In a Warm Dark Matter universe the formation of the first structure is expected to be delayed relative to a Cold Dark Matter universe. In Khimey, Bose & Tacchella 2021, we use a semi-empirical model of galaxy formation (Tacchella et al. 2018) to investigate the extent to which uncertainties in the implementation of baryonic physics may be degenerate with the predictions of two different models of dark matter: Cold Dark Matter (CDM) and a 7 keV sterile neutrino, which behaves as Warm Dark Matter (WDM). Our models are calibrated to the observed UV luminosity function at 𝑧 = 4 using two separate dust attenuation prescriptions, which manifest as high and low star formation efficiency in low mass halos. We find that while at fixed star formation efficiency, there are marked differences in the abundance of faint galaxies in the two dark matter models at high-z, these differences are mimicked easily by varying the star-formation efficiency in the same dark matter model (see figure on the left). Our results suggest that JWST will likely be more informative in constraining baryonic processes operating in high-z galaxies than it will be in constraining the nature of dark matter.

THESAN project: radiation-magneto-hydrodynamic simulations of the Epoch of Reionization

We are collaborating with Rahul Kannan (Harvard), Enrico Garaldi (MPA) and Aaron Smith (Harvard) on the THESAN project, which is a new radiation-magneto-hydrodynamic simulations of galaxies in the Epoch of Reionization. This numerical simulation simultaneously models the large scale statistical properties of the intergalactic medium during reionization and the resolved characteristics of the galaxies responsible for it. Please contact us if you are interested in working on these simulations.