Jason Jaacks

The Evolution of the Galaxy Rest-Frame Ultraviolet Luminosity Function Over the First Two Billion Years

We present a robust measurement and analysis of the rest-frame ultraviolet (UV) luminosity functions at z = 4-8. We use deep Hubble Space Telescope imaging over the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey/GOODS fields, the Hubble Ultra Deep Field, and the Hubble Frontier Field deep parallel observations near the Abell 2744 and MACS J0416.1-2403 clusters. The combination of these surveys provides an effective volume of 0.6-1.2 x 10(6) Mpc(3) over this epoch, allowing us to perform a robust search for faint (M-UV = -18) and bright (Muv \textless -21) high-redshift galaxies. We select candidate galaxies using a well-tested photometric redshift technique with careful screening of contaminants, finding a sample of 7446 candidate galaxies at 3.5 \textless z \textless 8.5, with \textgreater1000 galaxies at z approximate to 6-8. We measure both a stepwise luminosity function for candidate galaxies in our redshift samples, and a Schechter function, using a Markov Chain Monte Carlo analysis to measure robust uncertainties. At the faint end, our UV luminosity functions agree with previous studies, yet we find a higher abundance of UV-bright candidate galaxies at z \textgreater= 6. Our best-fit value of the characteristic magnitude MN is consistent with -21 at z \textgreater=, 5, which is different than that inferred based on previous trends at lower redshift, and brighter at similar to 2 sigma significance than previous measures at z = 6 and 7. At z = 8, a single power law provides an equally good fit to the UV luminosity function, while at z = 6 and 7 an exponential cutoff at the bright end is moderately preferred. We compare our luminosity functions to semi-analytical models, and find that the lack of evolution in M-UV(*) is consistent with models where the impact of dust attenuation on the bright end of the luminosity function decreases at higher redshift, although a decreasing impact of feedback may also be possible. We measure the evolution of the cosmic star-formation rate (SFR) density by integrating our observed luminosity functions to M-UV = -17, correcting for dust attenuation, and find that the SFR density declines proportionally to (1 +z)(-4.3 +/- 0 5) at z \textgreater 4, which is consistent with observations at z \textgreater= 9. Our observed luminosity functions are consistent with a reionization history that starts at z greater than or similar to 10, completes at z \textgreater 6, and reaches a midpoint (xH = 0.5) at 6.7 \textless z \textless9.4. Finally, using a constant cumulative number density selection and an empirically derived rising star-formation history, our observations predict that the abundance of bright z = 9 galaxies is likely higher than previous constraints, although consistent with recent estimates of bright z similar to 10 galaxies.

IMPACT OF H2-BASED STAR FORMATION MODEL ON THE Z ≥ 6 LUMINOSITY FUNCTION AND THE IONIZING PHOTON BUDGET FOR REIONIZATION

We present the results of a numerical study examining the effect of H2-based star formation (SF) model on the rest-frame UV luminosity function and star formation rate function (SFRF) of z ≥ 6 galaxies, and the implications for reionization. Using cosmological hydrodynamical simulations outfitted with an H2-SF model, we find good agreement with our previous results (non-H2 SF model) and observations at Muv ≤ −18. However at Muv > −18, we find that the LF deviates from both our previous work and current observational extrapolations, producing significantly fewer low luminosity galaxies and exhibiting additional turnover at the faint end. We constrain the redshift evolution of this turnover point using a modified Schechter function that includes additional terms to quantify the turnover magnitude (M t uv ) and subsequent slope (�). We find that M t uv evolves from M t uv = −17.33

MOLECULAR HYDROGEN REGULATED STAR FORMATION IN COSMOLOGICAL SMOOTHED PARTICLE HYDRODYNAMICS SIMULATIONS

Some observations have shown that star formation (SF) correlates tightly with the presence of molecular hydrogen (H2); therefore, it is important to investigate its implication on galaxy formation in a cosmological context. In the present work, we implement a sub-grid model (hereafter H2-SF model) that tracks the H2 mass fraction within our cosmological smoothed particle hydrodynamics code GADGET-3 by using an equilibrium analytic model of Krumholz et al. This model allows us to regulate the SF in our simulation by the local abundance of H2 rather than the total cold gas density, which naturally introduces the dependence of SF on metallicity. We investigate the implications of the H2-SF model on galaxy population properties, such as the stellar-to-halo mass ratio (SHMR), baryon fraction, cosmic star formation rate density (SFRD), galaxy specific SFR, galaxy stellar mass functions (GSMF), and Kennicutt-Schmidt (KS) relationship. The advantage of our work over the previous ones is having a large sample of simulated galaxies in a cosmological volume from high redshift to z = 0. We find that low-mass halos with M DM < 1010.5 M ☉ are less efficient in producing stars in the H2-SF model at z ≥ 6, which brings the simulations into better agreement with the observational estimates of the SHMR and GSMF at the low-mass end. This is particularly evident by a reduction in the number of low-mass galaxies at M ≤ 108 M ☉ in the GSMF. The overall SFRD is also reduced at high z in the H2 run, which results in slightly higher SFRD at low redshift due to more abundant gas available for SF at later times. This new H2 model is able to reproduce the empirical KS relationship at z = 0 naturally, without the need for setting its normalization by hand, and overall it seems to have more advantages than the previous pressure-based SF model.

Duty cycle and the increasing star formation history of z ≥ 6 galaxies

We examine the duty cycle (DC) and the star formation history (SFH) for high-redshift galaxies at z ≥ 6 using cosmological hydrodynamic simulations. We find that, even though individual galaxies have bursty SFH, the averaged SFH between z ∼ 15 and 6 can be characterized well by either an exponentially increasing functional form with characteristic time-scales of 70–200 Myr for galaxies with stellar masses Ms ∼ 106 to >1010 M⊙, respectively, or a simple power-law form which exhibits similar mass-dependent time-scales. Using the SFH of individual galaxies, we measure the DC of star formation (DC SFH), i.e. the fraction of time a galaxy of a particular mass spends above a star formation rate (SFR) threshold which would make it observable to the Hubble Space Telescope (HST) during a given epoch. We also examine the fraction of galaxies at a given redshift that are brighter than a rest-frame ultraviolet magnitude (Muv ∼ −18), which is sufficient to make them observable (). We find that both DC SFH and make a sharp transition from zero (for galaxies with ) to unity (for ). The measured DC is also manifested in the intrinsic scatter in the Ms - SFR relationship (∼1 dex) and Ms - Muv relationship (ΔMuv ∼ ±1 mag). We provide analytic fits to the DC as a function of Ms using a sigmoid function which can be used to correct for catalogue incompleteness. We consider the effects of DC to the observational estimate of galaxy stellar mass functions (GSMF) and the SFR density (SFRD), and find that it results in much shallower low-mass end slopes of the GSMF and a reduction of ≳70 per cent of our intrinsic SFRD, making our simulation results more compatible with observational estimates.

Baseline metal enrichment from Population III star formation in cosmological volume simulations

We utilize the hydrodynamic and N-body code {\small GIZMO} coupled with our newly developed sub-grid Population~III (Pop~III) Legacy model, designed specifically for cosmological volume simulations, to study the baseline metal enrichment from Pop~III star formation at

CONNECTING the DOTS: TRACKING GALAXY EVOLUTION USING CONSTANT CUMULATIVE NUMBER DENSITY at 3 ≤ z ≤ 7

z>7

Dust extinction in the first galaxies

. In this idealized numerical experiment, we only consider Pop~III star formation. We find that our model Pop~III star formation rate density (SFRD), which peaks at

High-z galaxy formation, reionization of the universe, and the impact of H2-based star formation model

\sim 10^{-3}\ {\rm M_\odot yr^{-1} Mpc^{-1}}

Steep faint‐end slopes of galaxy mass and luminosity functions at z≥ 6 and the implications for reionization

near

Legacy of star formation in the pre-reionization universe

z\sim10