FirstLight III: Rest-frame UV-optical spectral energy distributions of simulated galaxies at cosmic dawn

Using the FirstLight database of 300 zoom-in cosmological simulations we provide rest-frame UV-optical spectral energy distributions of galaxies with complex star-formation histories that are coupled to the non-uniform gas accretion history of galactic halos during cosmic dawn. The population at any redshift is very diverse ranging from starbursts to quiescent galaxies even at a fixed stellar mass. This drives a redshift-dependent relation between UV luminosity and stellar mass with a large scatter, driven by the specific star formation rate. The UV slope and the production efficiency of Lyman continuum photons have high values, consistent with dust-corrected observations. This indicates young stellar populations with low metallicities. The FirstLight simulations make predictions on the rest-frame UV-optical absolute magnitudes, colors and optical emission lines of galaxies at z=6-12 that will be observed for the first time with JWST and the next generation of telescopes in the coming decade.Comment: 10 pages+appendix , 11 figures. Accepted at MNRAS. The FirstLight database is available at http://www.ita.uni-heidelberg.de/~ceverino/FirstLight/index.htm

On column density thresholds and the star formation rate

We present the results of a numerical study designed to address the question of whether there is a column density threshold for star formation within molecular clouds. We have simulated a large number of different clouds, with volume and column densities spanning a wide range of different values, using a state-of-the-art model for the coupled chemical, thermal and dynamical evolution of the gas. We show that star formation is only possible in regions where the mean (area-averaged) column density exceeds $10^{21} \: {\rm cm^{-2}}$. Within the clouds, we also show that there is a good correlation between the mass of gas above a K-band extinction $A_{\rm K} = 0.8$ and the star formation rate (SFR), in agreement with recent observational work. Previously, this relationship has been explained in terms of a correlation between the SFR and the mass in dense gas. However, we find that this correlation is weaker and more time-dependent than that between the SFR and the column density. In support of previous studies, we argue that dust shielding is the key process: the true correlation is one between the SFR and the mass in cold, well-shielded gas, and the latter correlates better with the column density than the volume density.Comment: 21 pages and 12 figures. Accepted for publication in MNRA

Is atomic carbon a good tracer of molecular gas in metal-poor galaxies?

Carbon monoxide (CO) is widely used as a tracer of molecular hydrogen (H2) in metal-rich galaxies, but is known to become ineffective in low metallicity dwarf galaxies. Atomic carbon has been suggested as a superior tracer of H2 in these metal-poor systems, but its suitability remains unproven. To help us to assess how well atomic carbon traces H2 at low metallicity, we have performed a series of numerical simulations of turbulent molecular clouds that cover a wide range of different metallicities. Our simulations demonstrate that in star-forming clouds, the conversion factor between [CI] emission and H2 mass, $X_{\rm CI}$, scales approximately as $X_{\rm CI} \propto Z^{-1}$. We recover a similar scaling for the CO-to-H2 conversion factor, $X_{\rm CO}$, but find that at this point in the evolution of the clouds, $X_{\rm CO}$ is consistently smaller than $X_{\rm CI}$, by a factor of a few or more. We have also examined how $X_{\rm CI}$ and $X_{\rm CO}$ evolve with time. We find that $X_{\rm CI}$ does not vary strongly with time, demonstrating that atomic carbon remains a good tracer of H2 in metal-poor systems even at times significantly before the onset of star formation. On the other hand, $X_{\rm CO}$ varies very strongly with time in metal-poor clouds, showing that CO does not trace H2 well in starless clouds at low metallicity.Comment: 16 pages, 9 figures. Updated to match the version accepted by MNRAS. The main change from the previous version is a new sub-section (3.6) discussing the possible impact of freeze-out and other processes not included in our numerical simulation

Does the CO-to-H2 conversion factor depend on the star formation rate?

We present a series of numerical simulations that explore how the `X-factor', $X_{CO}$ -- the conversion factor between the observed integrated CO emission and the column density of molecular hydrogen -- varies with the environmental conditions in which a molecular cloud is placed. Our investigation is centred around two environmental conditions in particular: the cosmic ray ionisation rate (CRIR) and the strength of the interstellar radiation field (ISRF). Since both these properties of the interstellar medium have their origins in massive stars, we make the assumption in this paper that both the strength of the ISRF and the CRIR scale linearly with the local star formation rate (SFR). The cloud modelling in this study first involves running numerical simulations that capture the cloud dynamics, as well as the time-dependent chemistry, and ISM heating and cooling. These simulations are then post-processed with a line radiative transfer code to create synthetic 12CO (1-0) emission maps from which $X_{CO}$ can be calculated. We find that for 1e4 solar mass virialised clouds with mean density 100 cm$^{-3}$, $X_{CO}$ is only weakly dependent on the local SFR, varying by a factor of a few over two orders of magnitude in SFR. In contrast, we find that for similar clouds but with masses of 1e5 solar masses, the X-factor will vary by an order of magnitude over the same range in SFR, implying that extra-galactic star formation laws should be viewed with caution. However, for denser ($10^4$ cm$^{-3}$), super-virial clouds such as those found at the centre of the Milky Way, the X-factor is once again independent of the local SFR.Comment: 16 pages, 5 figures. Accepted by MNRA

Radiative Feedback and the First Stars

Numerical simulations suggest that the very first stars to form do so within cool gas in small protogalaxies. These protogalaxies have low virial temperatures, and cooling within them is dominated by molecular hydrogen, H₂. This is easily destroyed by ultraviolet radiation from newly-formed stars, and this ‘radiative feedback’ may play an important role in regulating star formation in the early universe.To study the effects of radiative feedback, requires an accurate chemical model. Examples currently in the literature assume the gas to be optically thin, and give erroneous results in optically thick gas. In chapter 2 of this thesis I develop a chemical model that correctly treats the photochemistry of optically thick primordial gas. I also discuss the approximations that remain, and estimate the accuracy of the model.In chapter 3, I examine the role of radiative feedback on small scales. Using a simple protogalactic model, I determine the growth timescales and final sizes of H₂ regions within a protogalaxy and discuss the effect of photoionizing radiation on dense clumps of gas. I also examine the effects of photodissociation, and present a simple method for estimating the photodissociation timescale in optically thick gas. I find that radiative feedback occurs rapidly in diffuse protogalactic gas, but that dense clumps can resist its effects and survive to form stars.On larger scales, previous work has shown that early star formation rapidly builds up a soft UV background that can suppress star formation within the smallest protogalaxies. However, this work does not include the effects of X-rays produced by sources associated with star formation, which will catalyze H₂ formation and reduce the effects of photodissociation. In chapter 4, I examine the importance of these X-rays by self-consistently modelling the growth of the X-ray and UV backgrounds together with their effects on gas within protogalaxies. An important result is the determination of Tent, the critical virial temperature above which protogalaxies can cool. The evolution of Tcrit is presented for various X-ray source models, and compared to a model with no X-ray background, allowing the effects of X-ray feedback to be assessed

The Abundance of Molecular Hydrogen and its Correlation with Midplane Pressure in Galaxies: Non-Equilibrium, Turbulent, Chemical Models

Observations of spiral galaxies show a strong linear correlation between the ratio of molecular to atomic hydrogen surface density R_mol and midplane pressure. To explain this, we simulate three-dimensional, magnetized turbulence, including simplified treatments of non-equilibrium chemistry and the propagation of dissociating radiation, to follow the formation of H_2 from cold atomic gas. The formation time scale for H_2 is sufficiently long that equilibrium is not reached within the 20-30 Myr lifetimes of molecular clouds. The equilibrium balance between radiative dissociation and H_2 formation on dust grains fails to predict the time-dependent molecular fractions we find. A simple, time-dependent model of H_2 formation can reproduce the gross behavior, although turbulent density perturbations increase molecular fractions by a factor of few above it. In contradiction to equilibrium models, radiative dissociation of molecules plays little role in our model for diffuse radiation fields with strengths less than ten times that of the solar neighborhood, because of the effective self-shielding of H_2. The observed correlation of R_mol with pressure corresponds to a correlation with local gas density if the effective temperature in the cold neutral medium of galactic disks is roughly constant. We indeed find such a correlation of R_mol with density. If we examine the value of R_mol in our local models after a free-fall time at their average density, as expected for models of molecular cloud formation by large-scale gravitational instability, our models reproduce the observed correlation over more than an order of magnitude range in density.Comment: 24 pages, 4 figures, accepted for publication in Astrophys. J, changes include addition of models with higher radiation fields and substantial clarification of the narrativ