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Register a new userThe imprint of cosmic reionisation on the luminosity function of galaxies
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Sownak Bose
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The (re)ionisation of hydrogen in the early universe has a profound effect on the formation of the first galaxies: by raising the gas temperature and pressure, it prevents gas from cooling into small haloes thus affecting the abundance of present-day small galaxies. Using the Galform semi-analytic model of galaxy formation, we show that two key aspects of the reionisation process -- when reionisation takes place and the characteristic scale below which it suppresses galaxy formation -- are imprinted in the luminosity function of dwarf galaxies. We focus on the luminosity function of satellites of galaxies like the Milky Way and the LMC, which is easier to measure than the luminosity function of the dwarf population as a whole. Our results show that the details of these two characteristic properties of reionisation determine the shape of the luminosity distribution of satellites in a unique way, and is largely independent of the other details of the galaxy formation model. Our models generically predict a bimodality in the distribution of satellites as a function of luminosity: a population of faint satellites and population of bright satellites separated by a valley forged by reionisation. We show that this bimodal distribution is present at high statistical significance in the combined satellite luminosity function of the Milky Way and M31. We make predictions for the expected number of satellites around LMC-mass dwarfs where the bimodality may also be measurable in future observational programmes. Our preferred model predicts a total of $26 pm 10$ (68 per cent confidence) satellites brighter than ${rm M}_V=0$ in LMC-mass systems.
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We investigate whether the satellite luminosity function (LF) of primary galaxies identified in the Sloan Digital Sky Survey (SDSS) depends on whether the host galaxy is in a filament or not. Isolated primary galaxies are identified in the SDSS spectroscopic sample while potential satellites (that are up to 4 magnitudes fainter than their hosts) are searched for in the much deeper photometric sample. Filaments are constructed from the galaxy distribution by the Bisous process. Isolated primary galaxies are divided into two subsamples: those in filaments and those not in filaments. We examine the stacked mean satellite LF of both the filament and non-filament sample and find that, on average, the satellite LFs of galaxies in filaments is significantly higher than those of galaxies not in filaments. The filamentary environment can increases the abundance of the brightest satellites ($M_mathrm{sat.} < M_mathrm{prim.} + 2.0$), by a factor of $sim 2$ compared with non-filament isolated galaxies. This result is independent of primary galaxy magnitude although the satellite LF of galaxies in the faintest magnitude bin, is too noisy to determine if such a dependence exists. Since our filaments are extracted from a spectroscopic flux-limited sample, we consider the possibility that the difference in satellite LF is due to a redshift, colour or environmental bias, finding these to be insufficient to explain our result. The dependence of the satellite LF on the cosmic web suggests that the filamentary environment may have a strong effect on the efficiency of galaxy formation.
Small galaxies are thought to be the main contributors to the ionising budget of the Universe before reionisation was complete. There have been a number of numerical studies trying to quantify their ionising efficiency through the escape fraction $f_{esc}$. While there is a clear trend that $f_{esc}$ is higher for smaller haloes, there is a large scatter in the distribution of $f_{esc}$ for a single halo mass. We propose that this is due to the intrinsic burstiness of star formation in low mass galaxies. We performed high resolution radiative hydrodynamics simulations with Ramses-RT to model the evolution of three galaxies and their ionising efficiency. We found that the variability of $f_{esc}$ follows that of the star formation rate. We then discuss the consequences of this variability on the observability of such galaxies by JWST.
We present a new technique to estimate the evolution of the very faint end of the UV luminosity function (LF) out to $zsim5$. Measured star formation histories (SFHs) from the fossil record of Local Group galaxies are used to reconstruct the LF down to M$_{UV}sim-5$ at $zsim5$ and M$_{UV}sim-1.5$ at $z<1$. Such faint limits are well beyond the current observational limits and are likely to remain beyond the limits of next generation facilities. The reconstructed LFs, when combined with direct measurements of the LFs at higher luminosity, are well-fit by a standard Schechter function with no evidence of a break to the faintest limits probed by this technique. The derived faint end slope, $alpha$, steepens from $approx-1.2$ at $z<1$ to $approx-1.6$ at $4<z<5$. We test the effects of burstiness in the SFHs and find the recovered LFs to be only modestly affected. Incompleteness corrections for the faintest Local Group galaxies and the (unlikely) possibility of significant luminosity-dependent destruction of dwarf galaxies between high redshift and the present epoch are important uncertainties. These and other uncertainties can be mitigated with more detailed modeling and future observations. The reconstructed faint end LF from the fossil record can therefore be a powerful and complementary probe of the high redshift faint galaxies believed to play a key role in the reionization of the Universe.
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.
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.
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