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Register a new userBeacons into the Cosmic Dark Ages: Boosted transmission of Ly$alpha$ from UV bright galaxies at $z gtrsim 7$
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Recent detections of Lyman alpha (Ly$alpha$) emission from $z>7.5$ galaxies were somewhat unexpected given a dearth of previous non-detections in this era when the intergalactic medium (IGM) is still highly neutral. But these detections were from UV bright galaxies, which preferentially live in overdensities which reionize early, and have significantly Doppler-shifted Ly$alpha$ line profiles emerging from their interstellar media (ISM), making them less affected by the global IGM state. Using a combination of reionization simulations and empirical ISM models we show, as a result of these two effects, UV bright galaxies in overdensities have $>2times$ higher transmission through the $zsim7$ IGM than typical field galaxies, and this boosted transmission is enhanced as the neutral fraction increases. The boosted transmission is not sufficient to explain the observed high Ly$alpha$ fraction of $M_mathrm{UV} lesssim -22$ galaxies (Stark et al. 2017), suggesting Ly$alpha$ emitted by these galaxies must be stronger than expected due to enhanced production and/or selection effects. Despite the bias of UV bright galaxies to reside in overdensities we show Ly$alpha$ observations of such galaxies can accurately measure the global neutral hydrogen fraction, particularly when Ly$alpha$ from UV faint galaxies is extinguished, making them ideal candidates for spectroscopic follow-up into the cosmic Dark Ages.
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The highly neutral inter-galactic medium (IGM) during the Epoch of Reionization (EoR) is expected to suppress Ly$alpha$ emission with damping-wing absorption, causing nearly no Ly$alpha$ detection from star-forming galaxies at $z{sim}8$. However, spectroscopic observations of the 4 brightest galaxies (${rm H}_{160}{sim}25$ mag) at these redshifts do reveal prominent Ly$alpha$ line, suggesting locally ionised IGM. In this paper, we explore the Ly$alpha$ IGM transmission and environment of bright galaxies during the EoR using the Meraxes semi-analytic model. We find brighter galaxies to be less affected by damping-wing absorption as they are effective at ionizing surrounding neutral hydrogen. Specifically, the brightest sources (${rm H}_{160}{lesssim}25.5$ mag) lie in the largest ionized regions in our simulation, and have low attenuation of their Ly$alpha$ from the IGM (optical depth ${<}1$). Fainter galaxies (25.5 mag${<}{rm H}_{160}{<}27.5$ mag) have transmission that depends on UV luminosity, leading to a lower incidence of Ly$alpha$ detection at fainter magnitudes. This luminosity-dependent attenuation explains why Ly$alpha$ has only been observed in the brightest galaxies at $z{sim}8$. Follow-up observations have revealed counterparts in the vicinity of these confirmed $z{sim}8$ Ly$alpha$ emitters. The environments of our modelled analogues agree with these observations in the number of nearby galaxies, which is a good indicator of whether Ly$alpha$ can be detected among fainter galaxies. At the current observational limit, galaxies with ${ge}2$--5 neighbours within $2{times}2$ are ${sim}2$--3 times more likely to show Ly$alpha$ emission. JWST will discover an order of magnitude more neighbours, revealing ${gtrsim}50$ galaxies in the largest ionizing bubbles and facilitating direct study of reionization morphology.
Spectroscopic confirmation of galaxies at z~7 and above has been extremely difficult, owing to a drop in intensity of Ly-alpha emission in comparison with samples at z~6. This crucial finding could potentially signal the ending of cosmic reionization. However it is based on small datasets, often incomplete and heterogeneous in nature. We introduce a flexible Bayesian framework, useful to interpret such evidence. Within this framework, we implement two simple phenomenological models: a smooth one, where the distribution of Ly-alpha is attenuated by a factor es with respect to z~6; a patchy one where a fraction ep is absorbed/non-emitted while the rest is unabsorbed. From a compilation of 39 observed z~7 galaxies we find es=0.69+-0.12 and ep=0.66+-0.16. The models can be used to compute fractions of emitters above any equivalent width W. For W>25AA, we find X^{25}_{z=7}=0.37+-0.11 (0.14+-0.06) for galaxies fainter (brighter) than M_{UV}=-20.25 for the patchy model, consistent with previous work, but with smaller uncertainties by virtue of our full use of the data. At z~8 we combine new deep (5-sigma flux limit 10^{-17}ergs^{-1}cm^{-2}) Keck-NIRSPEC observations of a bright Y-dropout identified by our BoRG Survey, with those of three objects from the literature and find that the inference is inconclusive. We compute predictions for future near-infrared spectroscopic surveys and show that it is challenging but feasible to constrain the distribution of Ly-alpha emitters at z~8 and distinguish between models.
We present a new investigation of the intergalactic medium (IGM) near the end of reionization using dark gaps in the Lyman-alpha (Ly$alpha$) forest. Using spectra of 55 QSOs at $z_{rm em}>5.5$, including new data from the XQR-30 VLT Large Programme, we identify gaps in the Ly$alpha$ forest where the transmission averaged over 1 comoving $h^{-1},{rm Mpc}$ bins falls below 5%. Nine ultra-long ($L > 80~h^{-1},{rm Mpc}$) dark gaps are identified at $z<6$. In addition, we quantify the fraction of QSO spectra exhibiting gaps longer than $30~h^{-1},{rm Mpc}$, $F_{30}$, as a function of redshift. We measure $F_{30} simeq 0.9$, 0.6, and 0.15 at $z = 6.0$, 5.8, and 5.6, respectively, with the last of these long dark gaps persisting down to $z simeq 5.3$. Comparing our results with predictions from hydrodynamical simulations, we find that the data are consistent with models wherein reionization extends significantly below redshift six. Models wherein the IGM is essentially fully reionized that retain large-scale fluctuations in the ionizing UV background at $z lesssim 6$ are also potentially consistent with the data. Overall, our results suggest that signature of reionization in the form of islands of neutral hydrogen and/or large-scale fluctuations in the ionizing background remain present in the IGM until at least $z simeq 5.3$.
The Ly-alpha luminosity function (LF) of high-redshift Ly-alpha emitters (LAEs) is one of the few observables of the re-ionization epoch accessible to date with 8-10 m class telescopes. The evolution with redshift allows one to constrain the evolution of LAEs and their role in re-ionizing the Universe at the end of the Dark Ages. We have performed a narrow-band imaging program at 1.06 microns at the CFHT, targeting Ly-alpha emitters at redshift z ~ 7.7 in the CFHT-LS D1 field. From these observations we have derived a photometric sample of 7 LAE candidates at z ~ 7.7. We derive luminosity functions for the full sample of seven objects and for sub-samples of four objects. If the brightest objects in our sample are real, we infer a luminosity function which would be difficult to reconcile with previous work at lower redshift. More definitive conclusions will require spectroscopic confirmation.
We compare a sample of five high-resolution, high S/N Ly$alpha$ forest spectra of bright $6<z lesssim 6.5$ QSOs aimed at spectrally resolving the last remaining transmission spikes at $z>5$ with those obtained from mock absorption spectra from the Sherwood and Sherwood-Relics suites of hydrodynamical simulations of the intergalactic medium (IGM). We use a profile fitting procedure for the inverted transmitted flux, $1-F$, similar to the widely used Voigt profile fitting of the transmitted flux $F$ at lower redshifts, to characterise the transmission spikes that probe predominately underdense regions of the IGM. We are able to reproduce the width and height distributions of the transmission spikes, both with optically thin simulations of the post-reionization Universe using a homogeneous UV background and full radiative transfer simulations of a late reionization model. We find that the width of the fitted components of the simulated transmission spikes is very sensitive to the instantaneous temperature of the reionized IGM. The internal structures of the spikes are more prominant in low temeperature models of the IGM. The width distribution of the observed transmission spikes, which require high spectral resolution ($leq $ 8 km/s) to be resolved, is reproduced for optically thin simulations with a temperature at mean density of $T_0= (11000 pm 1600,10500pm 2100,12000 pm 2200)$ K at $z= (5.4,5.6,5.8)$. This is weakly dependent on the slope of the temperature-density relation, which is favoured to be moderately steeper than isothermal. In the inhomogeneous, late reionization, full radiative transfer simulations where islands of neutral hydrogen persist to $zsim5.3$, the width distribution of the observed transmission spikes is consistent with the range of $T_0$ caused by spatial fluctuations in the temperature-density relation.
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