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Register a new userThe Galaxy UV Luminosity Function Before the Epoch of Reionization
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We present a model for the evolution of the galaxy ultraviolet (UV) luminosity function (LF) across cosmic time where star formation is linked to the assembly of dark matter halos under the assumption of a mass dependent, but redshift independent, efficiency. We introduce a new self-consistent treatment of the halo star formation history, which allows us to make predictions at $z>10$ (lookback time $lesssim500$ Myr), when growth is rapid. With a calibration at a single redshift to set the stellar-to-halo mass ratio, and no further degrees of freedom, our model captures the evolution of the UV LF over all available observations ($0lesssim zlesssim10$). The significant drop in luminosity density of currently detectable galaxies beyond $zsim8$ is explained by a shift of star formation toward less massive, fainter galaxies. Assuming that star formation proceeds down to atomic cooling halos, we derive a reionization optical depth $tau = 0.056^{+0.007}_{-0.010}$, fully consistent with the latest Planck measurement, implying that the universe is fully reionized at $z=7.84^{+0.65}_{-0.98}$. In addition, our model naturally produces smoothly rising star formation histories for galaxies with $Llesssim L_*$ in agreement with observations and hydrodynamical simulations. Before the epoch of reionization at $z>10$ we predict the LF to remain well-described by a Schechter function, but with an increasingly steep faint-end slope ($alphasim-3.5$ at $zsim16$). Finally, we construct forecasts for surveys with JWST~and WFIRST and predict that galaxies out to $zsim14$ will be observed. Galaxies at $z>15$ will likely be accessible to JWST and WFIRST only through the assistance of strong lensing magnification.
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Ultra-faint galaxies are hosted by small dark matter halos with shallow gravitational potential wells, hence their star formation activity is more sensitive to feedback effects. The shape of the faint-end of the high-$z$ galaxy luminosity function (LF) contains important information on star formation and its interaction with the reionization process during the Epoch of Reionization (EoR). High-$z$ galaxies with $M_{rm UV}gtrsim-17$ have only recently become accessible thanks to the Frontier Fields (FFs) survey combining deep {it HST} imaging and the gravitational lensing effect. In this paper we investigate the faint-end of the LF at redshift $>$5 using the data of FFs clusters Abell 2744 (A2744), MACSJ0416.1-2403 (M0416), MACSJ0717.5+3745 (M0717) and MACSJ1149.5+2223 (M1149). We analyze both an empirical and a physically-motivated LF model to obtain constraints on a possible turn-over of LF at faint magnitudes. In the empirical model the LF drops fast when the absolute UV magnitude $M_{rm UV}$ is much larger than a turn-over absolute UV magnitude $M_{rm UV}^{rm T}$. We obtain $M_{rm UV}^{rm T}gtrsim-14.6 $ (15.2) at 1 (2) $sigma$ confidence level (C.L.) for $zsim6$. In the physically-motivated analytical model, star formation in halos with circular velocity below $v_c^*$ is fully quenched if these halos are located in ionized regions. Using updated lensing models and new additional FFs data, we re-analyze previous constraints on $v_c^*$ and $f_{rm esc}$ presented by Castellano et al. 2016a (C16a) using a smaller dataset. We obtain new constraints on $v_c^*lesssim 59$ km s$^{-1}$ and $f_{rm esc}lesssim 56%$ (both at 2$sigma$ C.L.) and conclude that there is no turn-over detected so far from the analyzed FFs data. Forthcoming {it JWST} observations will be key to tight these constraints further.
The time frame in which hydrogen reionization occurred is highly uncertain, but can be constrained by observations of Lyman-alpha (Ly$alpha$) emission from distant sources. Neutral hydrogen in the intergalactic medium (IGM) attenuates Ly$alpha$~photons emitted by galaxies. As reionization progressed the IGM opacity decreased, increasing Ly$alpha$~visibility. The galaxy Ly$alpha$~luminosity function (LF) is thus a useful tool to constrain the timeline of reionization. In this work, we model the Ly$alpha$~LF as a function of redshift, $z=5-10$, and average IGM neutral hydrogen fraction, $overline{x}_textsc{hi}$. We combine the Ly$alpha$~luminosity probability distribution obtained from inhomogeneous reionization simulations with a model for the UV LF to model the Ly$alpha$~LF. As the neutral fraction increases, the average number density of Ly$alpha$~emitting galaxies decreases, and are less luminous, though for $overline{x}_textsc{hi} lesssim 0.4$ there is only a small decrease of the Ly$alpha$~LF. We use our model to infer the IGM neutral fraction at $z=6.6, 7.0, 7.3$ from observed Ly$alpha$~LFs. We conclude that there is a significant increase in the neutral fraction with increasing redshift: $overline{x}_textsc{hi}(z=6.6)=0.08^{+ 0.08}_{- 0.05}, , overline{x}_textsc{hi}(z=7.0)=0.28 pm 0.05$ and $overline{x}_textsc{hi}(z=7.3)=0.83^{+ 0.06}_{- 0.07}$. We predict trends in the Ly$alpha$~luminosity density and Schechter parameters as a function of redshift and the neutral fraction. We find that the Ly$alpha$~luminosity density decreases as the universe becomes more neutral. Furthermore, as the neutral fraction increases, the faint-end slope of the Ly$alpha$~LF steepens, and the characteristic Ly$alpha$~luminosity shifts to lower values, concluding that the evolving shape of the Ly$alpha$~LF -- not just its integral -- is an important tool to study reionization.
Determinations of the UV luminosity function of AGN at high redshifts are important for constraining the AGN contribution to reionization and understanding the growth of supermassive black holes. Recent inferences of the luminosity function suffer from inconsistencies arising from inhomogeneous selection and analysis of AGN data. We address this problem by constructing a sample of more than 80,000 colour-selected AGN from redshift z=0 to 7.5. While this sample is composed of multiple data sets with spectroscopic redshifts and completeness estimates, we homogenise these data sets to identical cosmologies, intrinsic AGN spectra, and magnitude systems. Using this sample, we derive the AGN UV luminosity function from redshift z=0 to 7.5. The luminosity function has a double power law form at all redshifts. The break magnitude $M_*$ of the AGN luminosity function shows a steep brightening from $M_*sim -24$ at z=0.7 to $M_*sim -29$ at z=6. The faint-end slope $beta$ significantly steepens from $-1.7$ at $z<2.2$ to $-2.4$ at $zsimeq 6$. In spite of this steepening, the contribution of AGN to the hydrogen photoionization rate at $zsim 6$ is subdominant (< 3%), although it can be non-negligible (~10%) if these luminosity functions hold down to $M_{1450}=-18$. Under reasonable assumptions, AGN can reionize HeII by redshift z=2.9. At low redshifts (z<0.5), AGN can produce about half of the hydrogen photoionization rate inferred from the statistics of HI absorption lines in the IGM. Our global analysis of the luminosity function also reveals important systematic errors in the data, particularly at z=2.2--3.5, which need to be addressed and incorporated in the AGN selection function in future in order to improve our results. We make various fitting functions, luminosity function analysis codes, and homogenised AGN data publicly available.
If the dark matter (DM) were composed of axions, then structure formation in the Universe would be suppressed below the axion Jeans scale. Using an analytic model for the halo mass function of a mixed DM model with axions and cold dark matter, combined with the abundance-matching technique, we construct the UV-luminosity function. Axions suppress high-$z$ galaxy formation and the UV-luminosity function is truncated at a faintest limiting magnitude. From the UV-luminosity function, we predict the reionization history of the universe and find that axion DM causes reionization to occur at lower redshift. We search for evidence of axions using the Hubble Ultra Deep Field UV-luminosity function in the redshift range $z=6$-$10$, and the optical depth to reionization, $tau$, as measured from cosmic microwave background polarization. All probes we consider consistently exclude $m_alesssim 10^{-23}text{ eV}$ from contributing more than half of the DM, with our strongest constraint ruling this model out at more than $8sigma$ significance. In conservative models of reionization a dominant component of DM with $m_a=10^{-22}text{ eV}$ is in $3sigma$ tension with the measured value of $tau$, putting pressure on an axion solution to the cusp-core problem. Tension is reduced to $2sigma$ for the axion contributing only half of the DM. A future measurement of the UV-luminosity function in the range $z=10$-$13$ by JWST would provide further evidence for or against $m_a=10^{-22}text{ eV}$. Probing still higher masses of $m_a=10^{-21}text{ eV}$ will be possible using future measurements of the kinetic Sunyaev-Zeldovich effect by Advanced ACTPol to constrain the time and duration of reionization.
We present the first predictions for the $L_{rm [CII]}$ - SFR relation and [CII] luminosity function (LF) in the Epoch of Reionization (EoR) based on cosmological hydrodynamics simulations using the SIMBA suite plus radiative transfer calculations via SIGAME. The sample consists of 11,137 galaxies covering halo mass $log M_{rm halo}in$[9, 12.4] $M_odot$, star formation rate SFR$in$[0.01, 330] $M_odot$ yr$^{-1}$, and metallicity $<Z_{rm gas}>_{rm SFR}in$[0.1, 0.9] $Z_odot$. The simulated $L_{rm [CII]}$-SFR relation is consistent with the range observed, but with a spread of $simeq$0.3 dex at the high end of SFR ($>$100 $M_odot$ yr$^{-1}$) and $simeq$0.6 dex at the lower end, and there is tension between our predictions and the values of $L_{rm [CII]}$ above 10$^{8.5}$ $L_odot$ observed in some galaxies reported in the literature. The scatter in the $L_{rm [CII]}$-SFR relation is mostly driven by galaxy properties, such that at a given SFR, galaxies with higher molecular gas mass and metallicity have higher $L_{rm [CII]}$. The [CII] LF predicted by SIMBA is consistent with the upper limits placed by the only existing untargeted flux-limited [CII] survey at the EoR (ASPECS) and those predicted by semi-analytic models. We compare our results with existing models and discuss differences responsible for the discrepant slopes in the $L_{rm [CII]}$-SFR relatiion.
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