Intergalactic Medium Transmission Research Articles

A SPectroscopic Survey of Biased Halos In the Reionization Era (ASPIRE): JWST Supports Earlier Reionization around [O iii] Emitters

Understanding when and how reionization happened is crucial for studying the early structure formation and the properties of the first galaxies in the Universe. At z > 5.5, the observed intergalactic medium (IGM) optical depth shows a significant scatter, indicating an inhomogeneous reionization process. However, the nature of the inhomogeneous reionization remains debated. A SPectroscopic survey of biased halos In the Reionization Era (ASPIRE) is a JWST Cycle 1 program that has spectroscopically identified >400 [O iii] emitters in 25 quasar fields at z > 6.5. Combined with deep ground-based optical spectroscopy of ASPIRE quasars, the ASPIRE program provides the current largest sample for IGM-galaxy connection studies during cosmic reionization. We present the first results of IGM effective optical depth measurements around [O iii] emitters using 14 ASPIRE quasar fields. We find the IGM transmission is tightly related to reionization era galaxies to the extent that a significant excess of Lyα transmission exists around [O iii] emitters. We measure the stacked IGM effective optical depth of IGM patches associated with [O iii] emitters and find they reach the same IGM effective optical depth at least d z ∼ 0.1 ahead of those IGM patches where no [O iii] emitters are detected, supporting earlier reionization around [O iii] emitters. Our results indicate an enhancement in IGM Lyα transmission around [O iii] emitters at scales beyond 25 h −1 cMpc, consistent with the predicted topology of reionization from fluctuating UV background models.

Lyman Continuum Emission from Active Galactic Nuclei at 2.3 ≲ z ≲ 3.7 in the UVCANDELS Fields

We present the results of our search for Lyman continuum (LyC)-emitting (weak) active galactic nuclei (AGN) at redshifts 2.3 ≲ z ≲ 4.9 from Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) F275W observations in the Ultraviolet Imaging of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (UVCANDELS) fields. We also include LyC emission from AGN using HST WFC3 F225W, F275W, and F336W imaging found in Early Release Science (ERS) and Hubble Deep UV Legacy Survey data. We performed exhaustive queries of the Vizier database to locate AGN with high-quality spectroscopic redshifts. In total, we found 51 AGN that met our criteria within the UVCANDELS and ERS footprints. Out of these 51, we find 12 AGN that had ≥4σ detected LyC flux in the WFC3/UVIS images. Using a wide variety of space-based plus ground-based data, ranging from X-ray to radio wavelengths, we fit the multiwavelength photometric data of each AGN to a CIGALE spectral energy distribution (SED) using AGN models and correlate various SED parameters to the LyC flux. Kolmogorov–Smirnov tests of the SED parameter distributions for the LyC-detected and nondetected AGN showed they are likely not distinct samples. However, we find that the X-ray luminosity, star formation onset age, and disk luminosity show strong correlations relative to their emitted LyC flux. We also find strong correlations of the LyC flux to several dust parameters, i.e., polar and toroidal dust emission and 6 μm luminosity, and anticorrelations with metallicity and A FUV. We simulate the LyC escape fraction (f esc) using the CIGALE and intergalactic medium transmission models for the LyC-detected AGN and find an average f esc ≃ 18%, weighted by uncertainties. We stack the LyC fluxes of subsamples of AGN according to the wavelength continuum region in which they are detected and find no significant distinctions in their LyC emission, although our submillimeter-detected F336W sample (3.15 < z < 3.71) shows the brightest stacked LyC flux. These findings indicate that LyC production and escape in AGN are more complicated than the simple assumption of thermal emission and a 100% escape fraction. Further testing of AGN models with larger samples than presented here is needed.

EIGER. I. A Large Sample of [O iii]-emitting Galaxies at 5.3 < z < 6.9 and Direct Evidence for Local Reionization by Galaxies

We present a first sample of 117 [O iii] λλ4960, 5008–selected star-forming galaxies at 5.33 < z < 6.93 detected in JWST/NIRCam 3.5 μm slitless spectroscopy of a field centered on the hyperluminous quasar SDSS J0100+2802, obtained as part of the Emission-line galaxies and Intergalactic Gas in the Epoch of Reionization (EIGER) survey. Three prominent galaxy overdensities are observed, one of them at the redshift of the quasar. Galaxies are found within 200 pkpc and 105 km s−1 of four known metal absorption-line systems. We focus on the role of the galaxies in ionizing the intergalactic medium (IGM) during the later stages of cosmic reionization and construct the mean Lyα and Lyβ transmission as a function of distance from the galaxies. At the lowest redshifts in our study, 5.3 < z < 5.7, the IGM transmission rises monotonically with distance from the galaxies, as seen previously at lower redshifts. In contrast, at 5.7 < z < 6.14, the transmission of both Lyα and Lyβ first increases with distance but then peaks at a distance of 5 cMpc before declining. Finally, in the region 6.15 < z < 6.26, where the additional ionizing radiation from the quasar dominates, the monotonic increase in transmission with distance is reestablished. This result is interpreted to represent evidence that the transmission of the IGM at z ∼ 5.9 toward J0100+2802 results from the “local” ionizing radiation of galaxies that dominates over the much-reduced cosmic background.

Photometric IGM tomography with Subaru/HSC: the large-scale structure of Lyαemitters and IGM transmission in the COSMOS field atz˜ 5

ABSTRACTWe present a novel technique called ‘photometric IGM tomography’ to map the intergalactic medium (IGM) at z ≃ 4.9 in the COSMOS field. It utilizes deep narrow-band (NB) imaging to photometrically detect faint Ly α forest transmission in background galaxies across the Subaru/Hyper-Suprime Cam (HSC)’s $1.8\rm \, sq.\, deg$ field of view and locate Ly α emitters (LAEs) in the same cosmic volume. Using ultra-deep HSC images and Bayesian spectral energy distribution fitting, we measure the Ly α forest transmission at z ≃ 4.9 along a large number (140) of background galaxies selected from the DEIMOS10k spectroscopic catalogue at 4.98 &amp;lt; z &amp;lt; 5.89 and the SILVERRUSH LAEs at z ≃ 5.7. We photometrically measure the mean Ly α forest transmission and achieve a result consistent with previous measurements based on quasar spectra. We also measure the angular LAE-Ly α forest cross-correlation and Ly α forest autocorrelation functions and place an observational constraint on the large-scale fluctuations of the IGM around LAEs at z ≃ 4.9. Finally, we present the reconstructed 2D tomographic map of the IGM, co-spatial with the large-scale structure of LAEs, at a transverse resolution of $11 \, h^{-1}\rm cMpc$ across $140\, h^{-1}\rm cMpc$ in the COSMOS field at z ≃ 4.9. We discuss the observational requirements and the potential applications of this new technique for understanding the sources of reionization, quasar radiative history, and galaxy–IGM correlations across z ∼ 3–6. Our results represent the first proof-of-concept of photometric IGM tomography, offering a new route to examining early galaxy evolution in the context of the large-scale cosmic web from the epoch of reionization to cosmic noon.

CLEAR: Boosted Lyα Transmission of the Intergalactic Medium in UV-bright Galaxies

Reionization is an inhomogeneous process, thought to begin in small ionized bubbles of the intergalactic medium (IGM) around overdense regions of galaxies. Recent Lyα studies during the epoch of reionization show evidence that ionized bubbles formed earlier around brighter galaxies, suggesting higher IGM transmission of Lyα from these galaxies. We investigate this problem using IR slitless spectroscopy from the Hubble Space Telescope (HST) Wide-Field Camera 3 (WFC3) G102 grism observations of 148 galaxies selected via photometric redshifts at 6.0 < z < 8.2. These galaxies have spectra extracted from the CANDELS Lyα Emission at Reionization (CLEAR) survey. We combine the CLEAR data for 275 galaxies with the Keck Deep Imaging Multi-Object Spectrograph and MOSFIRE data set from the Texas Spectroscopic Search for Lyα Emission at the End of Reionization Survey. We constrain the Lyα equivalent width (EW) distribution at 6.0 < z < 8.2, which is described by an exponential form, , with the characteristic e-folding scale width (W 0). We confirm a significant drop in the Lyα strength (i.e., W 0) at z > 6. Furthermore, we compare the redshift evolution of W 0 between galaxies at different UV luminosities. UV-bright (M UV < −21 [i.e., L UV > L*]) galaxies show weaker evolution with a decrease of 0.4 ( ± 0.2) dex in W 0 at z > 6, while UV-faint (M UV > −21 [L UV < L*]) galaxies exhibit a significant drop of 0.7–0.8 (±0.2) dex in W 0 from z < 6 to z > 6. If the change in W 0 is proportional to the change in the IGM transmission for Lyα photons, then this is evidence that the transmission is “boosted” around UV-brighter galaxies, suggesting that reionization proceeds faster in regions around such galaxies.

A cautionary tale of Ly C escape fraction estimates from high-redshift galaxies

ABSTRACTMeasuring the escape fraction, fesc, of ionizing, Lyman continuum (Ly C) radiation is key to our understanding of the process of cosmic reionization. In this paper, we provide a methodology for recovering the posterior probability distribution of the Ly C escape fraction, $f_{\rm esc}^{\rm PDF}$, considering both the observational uncertainties and ensembles of simulated transmission functions through the intergalactic medium (IGM). We present an example of this method applied to a VUDS galaxy at z = 3.64 and find $f_{\rm esc}^{\rm PDF}$ = 0.51$^{+0.33}_{-0.34}$ and compare this to the values computed assuming averaged IGM transmission with and without consideration of detection bias along average sightlines yielding $f_{\rm esc}^{\langle T \rangle }$ = 1.40$^{+0.80}_{-0.42}$ and $f_{\rm esc}^{\rm bias}$ = 0.82$^{+0.33}_{-0.16}$. Our results highlight the limitations of methods assuming average, smooth transmission functions. We also present MOSFIRE data for a sample of seven Ly C candidates selected based on photometric redshifts at z &amp;gt; 3.4, but find that all seven have overestimated photometric redshifts by Δz ∼ 0.2 making them unsuitable for Ly C measurements. This results likely due to a bias induced by our selection criteria.

Dark-ages reionization and galaxy formation simulation XX. The Ly α IGM transmission properties and environment of bright galaxies during the epoch of reionization

ABSTRACT The highly neutral intergalactic medium (IGM) during the epoch of reionization (EoR) is expected to suppress Ly α emission with damping-wing absorption, causing nearly no Ly α detection from star-forming galaxies at z ∼ 8. However, spectroscopic observations of the four brightest galaxies (H160 ∼ 25 mag) at these redshifts do reveal prominent Ly α line, suggesting locally ionized IGM. In this paper, we explore the Ly α 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 (H160 ≲ 25.5 mag) lie in the largest ionized regions in our simulation, and have low attenuation of their Ly α from the IGM (optical depth &amp;lt;1). Fainter galaxies (25.5 mag &amp;lt; H160 &amp;lt; 27.5 mag) have transmission that depends on UV luminosity, leading to a lower incidence of Ly α detection at fainter magnitudes. This luminosity-dependent attenuation explains why Ly α has only been observed in the brightest galaxies at z ∼ 8. Follow-up observations have revealed counterparts in the vicinity of these confirmed z ∼ 8 Ly α 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 α can be detected among fainter galaxies. At the current observational limit, galaxies with ≥2–5 neighbours within 2 arcmin × 2 arcmin are ∼2–3 times more likely to show Ly α emission. JWST will discover an order of magnitude more neighbours, revealing ≳50 galaxies in the largest ionizing bubbles and facilitating direct study of reionization morphology.

CDFS-6664: A Candidate of Lyman-continuum Emission at z ∼ 3.8 Detected by the Hubble Deep UV Legacy Survey

Abstract We report the detection of Lyman continuum (LyC) emission from the galaxy, CDFS-6664, at z = 3.797 in a sample of Lyman break galaxies with detected [O iii] emission lines. The LyC emission is detected with a significance ∼5σ in the F336W band of the Hubble Deep UV Legacy Survey, corresponding to the 650–770 Å rest frame. The light centroid of the LyC emission is offset from the galaxy center by about 0.″2 (1.4 pkpc). The Hubble deep images at longer wavelengths show that the emission is unlikely provided by low-redshift interlopers. The photometric and spectroscopic data show that the possible contribution of an active galactic nucleus is quite low. Fitting the spectral energy distribution of this source to stellar population synthesis models, we find that the galaxy is young (∼50 Myr) and actively forming stars with a rate of 52.1 ± 4.9 M ⊙ yr−1. The significant star formation and the spatially offset LyC emission support a scenario where the ionizing photons escape from the low-density cavities in the ISM excavated by massive young stars. From the nebular model, we estimate the escape fraction of LyC photons to be 38% ± 7% and the corresponding intergalactic medium (IGM) transmission to be 60%, which deviates more than 3σ from the average transmission. The unusually high IGM transmission of LyC photons in CDFS-6664 may be related to a foreground type-2 quasar, CDF-202, at z = 3.7, with a projected separation of 1.′2 only. The quasar may have photoevaporated optically thick absorbers and enhance the transmission on the sightline of CDFS-6664.

Lyman-α transmission properties of the intergalactic medium in the CoDaII simulation

ABSTRACT The decline in abundance of Lyman-α (Lyα) emitting galaxies at z ≳ 6 is a powerful and commonly used probe to constrain the progress of cosmic reionization. We use the CoDaII simulation, which is a radiation hydrodynamic simulation featuring a box of ∼94 comoving Mpc side length, to compute the Lyα transmission properties of the intergalactic medium (IGM) at z ∼ 5.8 to 7. Our results mainly confirm previous studies, i.e. we find a declining Lyα transmission with redshift and a large sightline-to-sightline variation. However, motivated by the recent discovery of blue Lyα peaks at high redshift, we also analyse the IGM transmission on the blue side, which shows a rapid decline at z ≳ 6 of the blue transmission. This low transmission can be attributed not only to the presence of neutral regions but also to the residual neutral hydrogen within ionized regions, for which a density even as low as $n_{\rm HI}\sim 10^{-9}\, \mathrm{cm}^{-3}$ (sometimes combined with kinematic effects) leads to a significantly reduced visibility. Still, we find that $\sim 1{{\ \rm per\ cent}}$ of sightlines towards M1600AB ∼ −21 galaxies at z ∼ 7 are transparent enough to allow a transmission of a blue Lyα peak. We discuss our results in the context of the interpretation of observations.

Searching for the shadows of giants – II. The effect of local ionization on the Ly α absorption signatures of protoclusters at redshift z ∼ 2.4

ABSTRACT Local variations in the intergalactic medium (IGM) neutral hydrogen fraction will affect the Ly α absorption signature of protoclusters identified in tomographic surveys. Using the IllustrisTNG simulations, we investigate how the AGN proximity effect and hot, collisionally ionized gas arising from gravitational infall and black hole feedback changes the Ly α absorption associated with $M_{z=0}\simeq 10^{14}\, {\rm M}_\odot$ protoclusters at z ≃ 2.4. We find that protocluster galaxy overdensities exhibit a weak anticorrelation with Ly α transmission in IGM transmission maps, but local H$\, \rm \scriptstyle I$ ionization enhancements due to hot $T\gt 10^{6}\rm \, K$ gas or nearby AGN can disrupt this relationship within individual protoclusters. On average, however, we find that strong reductions in the IGM neutral fraction are limited to within $\lesssim 5h^{-1}\, \textrm {cMpc}$ of the dark matter haloes. Local ionization enhancements will therefore have a minimal impact on the completeness of protocluster identification in tomographic surveys if smoothing Ly α transmission maps over scales of $\sim 4 h^{-1}\, \textrm {cMpc}$, as is typically done in observations. However, if calibrating the relationship between the matter density and Ly α transmission in tomographic maps using simple analytical models for the Ly α forest opacity, the presence of hot gas around haloes can still result in systematically lower estimates of Mz = 0 for the most massive protoclusters.

Less and more IGM-transmitted galaxies from z ∼ 2.7 to z ∼ 6 from VANDELS and VUDS

Aims. Our aim is to analyze the variance of the intergalactic medium (IGM) transmission by studying this parameter in the rest-frame UV spectra of a large sample of high-redshift galaxies. Methods. We made use of the VIMOS Ultra Deep Survey and the VANDELS public survey to gain insight into the far UV spectrum of 2.7 &lt; z &lt; 6 galaxies. Using the SPARTAN fitting software, we estimated the IGM toward individual galaxies and then divided them into two sub-samples characterized by a transmission above or below the theoretical prescription. We created average spectra of combined VUDS and VANDELS data for each set of galaxies in seven redshift bins. Results. The resulting spectra clearly exhibit the variance of the IGM transmission that can be seen directly from high-redshift galaxy observations. Computing the optical depth based on the IGM transmission, we find an excellent agreement with results for quasi-stellar objects. In addition, our measurements appear to suggest that there is a large dispersion of redshift where a complete Gunn-Peterson Trough occurs, depending on the line of sight.

The role of galaxies and AGN in reionizing the IGM – III. IGM–galaxy cross-correlations at z ∼ 6 from eight quasar fields with DEIMOS and MUSE

ABSTRACT We present improved results of the measurement of the correlation between galaxies and the intergalactic medium transmission at the end of reionization. We have gathered a sample of 13 spectroscopically confirmed Lyman-break galaxies (LBGs) and 21 Lyman-α emitters (LAEs) at angular separations 20 arcsec ≲ θ ≲ 10 arcmin (∼0.1–4 pMpc at z ∼ 6) from the sightlines to eight background z ≳ 6 quasars. We report for the first time the detection of an excess of Lyman-α transmission spikes at ∼10–60 cMpc from LAEs (3.2σ) and LBGs (1.9σ). We interpret the data with an improved model of the galaxy–Lyman-α transmission and two-point cross-correlations, which includes the enhanced photoionization due to clustered faint sources, enhanced gas densities around the central bright objects and spatial variations of the mean free path. The observed LAE(LBG)–Lyman-α transmission spike two-point cross-correlation function (2PCCF) constrains the luminosity-averaged escape fraction of all galaxies contributing to reionization to $\langle f_{\rm esc} \rangle _{M_{\rm UV}\lt -12} = 0.14_{-0.05}^{+0.28}\, (0.23_{-0.12}^{+0.46})$. We investigate if the 2PCCF measurement can determine whether bright or faint galaxies are the dominant contributors to reionization. Our results show that a contribution from faint galaxies ($M_{\rm UV} \gt -20 \, (2\sigma)$) is necessary to reproduce the observed 2PCCF and that reionization might be driven by different sub-populations around LBGs and LAEs at z ∼ 6.

The intergalactic medium transmission towards z ≳ 4 galaxies with VANDELS and the impact of dust attenuation

Aims. Our aim is to estimate the intergalactic medium (IGM) transmission towards UV-selected star-forming galaxies at z ≳ 4 and study the effect of the dust attenuation on these measurements. Methods. The UV spectrum of high-redshift galaxies is a combination of their intrinsic emission and the effect of the IGM absorption along their line of sight. Using data coming from the unprecedentedly deep spectroscopy from the VANDELS ESO public survey carried out with the VIMOS instrument, we compute both the dust extinction and the mean transmission of the IGM as well as its scatter from a set of 281 galaxies at z &gt; 3.87. Because of a degeneracy between the dust content of the galaxy and the IGM, we first estimate the stellar dust extinction parameter E(B − V) and study the result as a function of the dust prescription. Using these measurements as constraint for the spectral fit we estimate the IGM transmission Tr(Lyα). Both photometric and spectroscopic spectral energy distribution fits are performed using the SPectroscopy And photometRy fiTting tool for Astronomical aNalysis which is able to fit the spectral continuum of the galaxies as well as photometric data. Results. Using the classical Calzetti attenuation law we find that E(B − V) goes from 0.11 at z = 3.99 to 0.08 at z = 5.15. These results are in very close agreement with published measurements. We estimate the IGM transmission and find that the transmission is decreasing with increasing redshift from Tr(Lyα) = 0.53 at z = 3.99 to 0.28 at z = 5.15. We also find a large standard deviation around the average transmission that is more than 0.1 at every redshift. Our results are in very good agreement with both previous measurements from AGN studies and with theoretical models.

VLT/FORS2 view at z ~ 6: Lyman-α emitter fraction and galaxy physical properties at the edge of the epoch of cosmic reionization

The fraction of Lyman-α emitters (LAEs) among the galaxy population has been found to increase from z ~ 0 to z ~ 6 and drop dramatically at z> 6. This drop has been interpreted as an effect of an increasingly neutral intergalactic medium (IGM) with increasing redshift, while a Lyman continuum escape fraction evolving with redshift and/or a sudden change of galaxy physical properties can also contribute to the decreasing LAE fraction. We report the result of a large VLT/FORS2 program aiming to confirm spectroscopically a large galaxy sample at z ≥ 6 that has been selected in several independent fields through the Lyman break technique. Combining those data with archival data, we create a large and homogeneous sample of z ~ 6 galaxies (N = 127), complete in terms of Lyα detection at > 95% for Lyα equivalent width EW(Lyα) ≥ 25 Å. We use this sample to derive a new measurement of the LAE fraction at z ~ 6 and derive the physical properties of these galaxies through spectral energy distribution (SED) fitting. We find a median LAE fraction at z ~ 6 lower than in previous studies, while our sample exhibits typical properties for z ~ 6 galaxies in terms of UV luminosity and UVβ slope. The comparison of galaxy physical properties between LAEs and non-LAEs is comparable to results at lower redshift: LAEs with the largest EW(Lyα) exhibit bluer UV slopes, are slightly less massive and less star-forming. The main difference between LAEs and non-LAEs is that the latter are significantly dustier. Using predictions of our SED fitting code accounting for nebular emission, we find an effective Lyα escape fraction fesceff(Lyα) = 0.23-0.17+0.36 remarkably consistent with the value derived by comparing UV luminosity function with Lyα luminosity function. We conclude that the drop in the LAE fraction from z ~ 6 to z> 6 is less dramatic than previously found and the effect of an increasing IGM neutral fraction is possibly observed at 5 <z< 6. The processes driving the escape of Lyα photons at z ~ 6 are similar to those at lower redshifts and based on our derived fesceff(Lyα), we find that the IGM has a relatively small impact on Lyα photon visibility at z ~ 6, with a lower limit for the IGM transmission to Lyα photons, TIGM ≳ 0.20, likely due to the presence of outflows.

A New Constraint on Reionization from the Evolution of the Lyα Luminosity Function at z ∼ 6–7 Probed by a Deep Census of z = 7.0 Lyα Emitter Candidates to 0.3L* †

Abstract We detect 20 z = 7.0 Lyα emitter (LAE) candidates to L(Lyα) ∼ 2 × 1042 erg s−1 or 0.3 L z = 7 * and in a volume of 6.1 × 105 Mpc3 in the Subaru Deep Field and the Subaru/XMM-Newton Deep Survey field by 82 hr and 37 hr of Subaru Suprime-Cam narrowband NB973 and reddest optical y-band imaging. We compare their Lyα and UV luminosity functions (LFs) and densities and Lyα equivalent widths (EWs) to those of z = 5.7, 6.6, and 7.3 LAEs from previous Suprime-Cam surveys. The Lyα LF (density) rapidly declines by a factor of ×1.5 (1.9) in L(Lyα) at z = 5.7–6.6 (160 Myr), ×1.5 (1.6) at z = 6.6–7.0 (60 Myr) at the faint end, and ×2.0 (3.8) at z = 7.0–7.3 (40 Myr). Also, in addition to the systematic decrease in EW at z = 5.7–6.6 previously found, two-thirds of the z = 7.0 LAEs detected in the UV continuum exhibit lower EWs than the z = 6.6 ones. Moreover, while the UV LF and density do not evolve at z = 5.7–6.6, they modestly decline at z = 6.6–7.0, implying galaxy evolution contributing to the decline of the Lyα LF. Comparison of the z = 7.0 Lyα LF to the one predicted by an LAE evolution model further reveals that galaxy evolution alone cannot explain all of the decline of the Lyα LF. If we attribute the discrepancy to Lyα attenuation by neutral hydrogen, the intergalactic medium transmission of Lyα photons at z = 7.0 would be T Ly α IGM ≤ 0.6 – 0.7 . It is lower (higher) than the T Ly α IGM at z = 6.6 (7.3) derived by previous studies, suggesting rapid increase in neutral fraction at z &gt; 6.

VIMOS Ultra-Deep Survey (VUDS): IGM transmission towards galaxies with 2.5 &lt; z &lt; 5.5 and the colour selection of high-redshift galaxies

The observed UV rest-frame spectra of distant galaxies are the result of their intrinsic emission combined with absorption along the line of sight produced by the inter-galactic medium (IGM). Here we analyse the evolution of the mean IGM transmission Tr(Lyα) and its dispersion along the line of sight for 2127 galaxies with 2.5 &lt; z &lt; 5.5 in the VIMOS Ultra Deep Survey (VUDS). We fitted model spectra combined with a range of IGM transmission to the galaxy spectra using the spectral fitting algorithm GOSSIP+. We used these fits to derive the mean IGM transmission towards each galaxy for several redshift slices from z = 2.5 to z = 5.5. We found that the mean IGM transmission defined as Tr(Lyα) = e− τ (with τ as the HI optical depth) is 79%, 69%, 59%, 55%, and 46% at redshifts 2.75, 3.22, 3.70, 4.23, and 4.77, respectively. We compared these results to measurements obtained from quasar lines of sight and found that the IGM transmission towards galaxies is in excellent agreement with quasar values up to redshift z ~ 4. We found tentative evidence for a higher IGM transmission at z ≥ 4 compared to results from QSOs, but a degeneracy between dust extinction and IGM prevents us from firmly concluding whether the internal dust extinction for star-forming galaxies at z &gt; 4 takes a mean value significantly in excess of E(B−V) &gt; 0.15. Most importantly, we found a large dispersion of IGM transmission along the lines of sight towards distant galaxies with 68% of the distribution within 10 to 17% of the median value in δz = 0.5 bins, similar to what is found on the lines of sight towards QSOs. We demonstrate that taking this broad range of IGM transmission into account is important when selecting high-redshift galaxies based on their colour properties (e.g. LBG or photometric redshiftselection) because failing to do so causes a significant incompleteness in selecting high-redshift galaxy populations. We finally discuss the observed IGM properties and speculate that the broad range of observed transmissions might be the result of cosmic variance and clustering along lines of sight. This clearly shows that the sources that cause this extinction need to be more completely modelled.

Can the intergalactic medium cause a rapid drop in Lyα emission at z &gt; 6?

The large cross-section of the Lyα line makes it a sensitive probe of the ionization state of the intergalactic medium (IGM). Here, we present the most complete study to date of the IGM Lyα opacity, and its application to the redshift evolution of the ‘Lyα fraction’, i.e. the fraction of colour-selected galaxies with a detectable Lyα emission line. We use a tiered approach, which combines large-scale seminumeric simulations of reionization with moderate-scale hydrodynamic simulations of the ionized IGM. This allows us to simultaneously account for evolution in both: (i) the opacity from an incomplete (patchy) reionization, parametrized by the filling factor of ionized regions, QH ii; and (ii) the opacity from self-shielded systems in the ionized IGM, parametrized by the average photoionization rate inside H ii regions, 〈Γ12〉H ii. In contrast to recent empirical models, attenuation from patchy reionization has a unimodal distribution along different sightlines, while attenuation from self-shielded systems is more bimodal. We quantify the average IGM transmission in our (QH ii, 〈Γ12〉H ii) parameter space, which can easily be used to interpret new data sets. Our new, improved models highly disfavour an evolution in 〈Γ12〉H ii as the sole driver of a large change in IGM opacity. Using current observations, we predict that the Lyα fraction cannot drop by more than a factor of ≈2 with IGM attenuation alone, even for H ii filling factors as low as QH ii ≳ 0.1. Larger changes in the Lyα fraction could result from a co-evolution with galaxy properties. Marginalizing over 〈Γ12〉H ii, we find that current observations constrain QH ii(z ≈ 7) ≤ 0.6, at a 68 per cent confidence level (CL). However, all of our parameter space is consistent with observations at 95 per cent CL, highlighting the need for larger observational samples at z ≥ 6.

On the use of Lyα emitters as probes of reionization

We use numerical simulations to study the effects of the patchiness of a partly reionized intergalactic medium (IGM) on the observability of Lyα emitters (LAEs) at high redshifts (z ≳ 6). We present a new model that divides the Lyα radiative transfer into a (circum)galactic and an extragalactic (IGM) part, and investigate how the choice of intrinsic line model affects the IGM transmission results. We use our model to study the impact of neutral hydrogen on statistical observables such as the Lyα rest-frame equivalent width (REW) distribution, the LAE luminosity function and the two-point correlation function. We find that if the observed changes in LAE luminosity functions and equivalent width distributions between z ∼ 6 and 7 are to be explained by an increased IGM neutral fraction alone, we require an extremely late and rapid reionization scenario, where the Universe was ∼40 per cent ionized at z = 7, ∼50 per cent ionized at z = 6.5 and ∼100 per cent ionized at z = 6. This is in conflict with other observations, suggesting that intrinsic LAE evolution at z ≳ 6 cannot be completely neglected. We show how the two-point correlation function can provide more robust constraints once future observations obtain larger LAE samples, and provide predictions for the sample sizes needed to tell different reionization scenarios apart.

Cosmic evolution of the C iv in high-resolution hydrodynamic simulations

We investigate the properties of triply ionized carbon (C iv) in the intergalactic medium (IGM) using a set of high resolution and large box size cosmological hydrodynamic simulations of a Lambda cold dark matter (ΛCDM) model. We rely on a modification of the publicly available TreeSPH code gadget-2 that self-consistently follows the metal enrichment mechanism by means of a detailed chemical evolution model. We focus on several numerical implementations of galactic feedback: galactic winds in the energy-driven and momentum-driven prescriptions, galactic winds hydrodynamically coupled to the surrounding gas and active galactic nuclei (AGNs) powered by gas accretion on to massive black holes. Furthermore, our results are compared to a run in which galactic feedback is not present and we also explore different initial stellar mass function. After having addressed some global properties of the simulated volume like the impact on the star formation rate and the content in carbon and C iv, we extract mock IGM transmission spectra in neutral hydrogen (H i) and C iv and perform Voigt profile fitting. The results are then compared with high-resolution quasar (QSO) spectra obtained with the Ultraviolet Echelle Spectrograph (UVES) at the Very Large Telescope (VLT) and the High Resolution Echelle Spectrograph (HIRES) at Keck. We find that feedback has little impact on statistics related to the neutral hydrogen, while C iv is more affected by galactic winds and/or AGN feedback. The feedback schemes investigated have a different strength and redshift evolution with a general tendency for AGN feedback to be more effective at lower redshift than galactic winds. When the same analysis is performed over observed and simulated C iv lines, we find a reasonably good agreement between data and simulations over the column density range NC IV= 1012.5-15 cm−2. Also the C iv linewidth distribution appears to be in agreement with the observed values, while the H i Doppler parameters, bH I, are in general too large, showing that the diffuse cosmic web is heated more than what is inferred by observations. The simulation without feedback fails in reproducing the C iv systems at high column densities at all redshift, while the AGN feedback case agrees with observations only at z < 3, when this form of feedback is particularly effective. We also present scatter plots in the b–N and in the NC IV–NH I planes, showing that there is rough agreement between observations and simulations only when feedback is taken into account. Although it seems difficult to constrain the nature and the strength of galactic feedback using the present framework and find a unique model that fits all the observations, these simulations offer the perspective of understanding the galaxy–IGM interplay and how metals produced by stars can reach the low-density IGM.

Luminosity functions of Ly emitting galaxies and cosmic reionization of hydrogen

Recent observations imply that the observed number counts of Lyα emitters (LAEs) evolved significantly between z= 5.7 and 6.5. It has been suggested that this was due to a rapid evolution in the ionization state, and hence transmission of the intergalactic medium (IGM) which caused Lyα flux from z= 6.5 galaxies to be more strongly suppressed. In this paper, we consider the joint evolution of the Lyα and the ultraviolet luminosity functions (LFs) and show that the IGM transmission evolved between z= 6.5 and 5.7 by a factor 1.1 < R < 1.8 (95 per cent confidence level). This result is insensitive to the underlying model of the Lyα LF (as well as cosmic variance). Using a model for IGM transmission, we find that the evolution of the mean IGM density through cosmic expansion alone may result in a value for the ratio of transmissions as high as R= 1.3. Thus, the existing LFs do not provide evidence for overlap. Furthermore, the constraint R < 1.8 suggests that the Universe at z= 6.5 was more than half-ionized by volume, that is, xi,V > 0.5.