The expected detection of dust emission from high-redshift Lyman α galaxies

Recent results have shown that a substantial fraction of high-redshift Lyman α (Lyα) galaxies contain considerable amounts of dust. This implies that Lyα galaxies are not primordial, as has been thought in the past. However, this dust has not been directly detected in emission; rather it has been inferred based on extinction estimates from rest-frame ultraviolet (UV) and optical observations. This can be tricky, as both dust and old stars redden galactic spectra at the wavelengths used to infer dust. Measuring dust emission directly from these galaxies is thus a more accurate way to estimate the total dust mass, giving us real physical information on the stellar populations and interstellar medium enrichment. New generation instruments, such as the Atacama Large Millimeter Array and Sub-Millimeter Array, should be able to detect dust emission from some of these galaxies in the submillimeter. Using measurements of the UV spectral slopes, we derive far-infrared flux predictions for of a sample of  23 z ≥ 4  Lyα galaxies. We find that in only a few hours, we can detect dust emission from 39 ± 22 per cent of our Lyα galaxies. Comparing these results to those found from a sample of 21 Lyman break galaxies (LBGs), we find that LBGs are on average 60 per cent more likely to be detected than Lyα galaxies, implying that they are more dusty, and thus indicating an evolutionary difference between these objects. These observations will provide better constraints on dust in these galaxies than those derived from their UV and optical fluxes alone. Undeniable proof of dust in these galaxies could explain the larger than expected Lyα equivalent widths seen in many Lyα galaxies today.     

Understanding the redshift evolution of the luminosity functions of Lyman α emitters

We present a semi-analytical model of star formation which explains simultaneously the observed ultraviolet (UV) luminosity function (LF) of high-redshift Lyman break galaxies (LBGs) and LFs of Lyman α emitters. We consider both models that use the Press–Schechter (PS) and Sheth–Tormen (ST) halo mass functions to calculate the abundances of dark matter haloes. The Lyman α LFs at   z ≲ 4  are well reproduced with only ≲10 per cent of the LBGs emitting Lyman α lines with rest equivalent width greater than the limiting equivalent width of the narrow band surveys. However, the observed LF at   z > 5  can be reproduced only when we assume that nearly all LBGs are Lyman α emitters. Thus, it appears that  4 < z < 5  marks the epoch when a clear change occurs in the physical properties of the high-redshift galaxies. As Lyman α escape depends on dust and gas kinematics of the interstellar medium (ISM), this could mean that on an average the ISM at   z > 5  could be less dusty, more clumpy and having more complex velocity field. All of these will enable easier escape of the Lyman α photons. At   z > 5  , the observed Lyman α LF are well reproduced with the evolution in the halo mass function along with very minor evolution in the physical properties of high-redshift galaxies. In particular, up to   z = 6.5  , we do not see the effect of evolving intergalactic medium opacity on the Lyman α escape from these galaxies.     

The escape of ionizing photons from supernova-dominated primordial galaxies

In order to assess the contribution of Lyman break galaxies (LBGs) and Lyman α emitters (LAEs) at redshifts  3 < z < 7  to the ionization of intergalactic medium (IGM), we investigate the escape fractions of ionizing photons from supernova-dominated primordial galaxies by solving the three-dimensional (3D) radiative transfer. The model galaxy is employed from an ultra-high-resolution chemodynamic simulation of a primordial galaxy by Mori & Umemura, which well reproduces the observed properties of LAEs and LBGs. The total mass of model galaxy is  10¹¹ M_⊙  . We solve not only photoionization but also collisional ionization by shocks. In addition, according to the chemical enrichment, we incorporate the effect of dust extinction, taking the size distributions of dust into account. As a result, we find that dust extinction reduces the escape fractions by a factor of 1.5–8.5 in the LAE phase and by a factor of 2.5–11 in the LBG phase, while the collisional ionization by shocks increases the escape fractions by a factor of  ≈2  . The resultant escape fractions are 0.07–0.47 in the LAE phase and 0.06–0.17 in the LBG phase. These results are well concordant with the recent estimations derived from the flux density ratio at 1500 to 900 Å of LAEs and LBGs. Combining the resultant escape fractions with the luminosity functions of LAEs and LBGs, we find that high- z LAEs and LBGs can ionize the IGM at   z = 3–5  . However, ionizing radiation from LAEs as well as LBGs falls short of ionizing the IGM at   z > 6  . That implies that additional ionization sources may be required at   z > 6  .     

Winds and infalling gas in Lyman break galaxies

A model for gas outflows is proposed which simultaneously explains the correlations between the (i) equivalent widths of low-ionization and Lyα lines, (ii) outflow velocity, and (iii) star formation rate observed in Lyman break galaxies (LBGs). Our interpretation implies that LBGs host short-lived (30 ± 5 Myr) starburst episodes observed at different evolutionary phases. Initially, the starburst powers a hot wind bound by a denser cold shell, which after ≈5 Myr becomes dynamically unstable and fragments; afterwards the fragment evolution is approximately ballistic while the hot bubble continues to expand. As the fragments are gravitationally decelerated, their screening ability of the starlight decreases as the ultraviolet (UV) starburst luminosity progressively dims. LBG observations sample all these evolutionary phases. Finally, the fragments fall back on to the galaxy after ≈60 Myr. This phase cannot be easily probed as it occurs when the starburst UV luminosity has already largely faded; however, galaxies dimmer in the UV than LBGs should show infalling gas.     

ZEN2: a narrow J-band search for z∼ 9 Lyα emitting galaxies directed towards three lensing clusters

We present the results of a continuing survey to detect Lyα emitting galaxies at redshifts   z ∼ 9  : the ' z equals nine' (ZEN) survey. We have obtained deep VLT Infrared Spectrometer and Array Camera observations in the narrow J -band filter NB119 directed towards three massive lensing clusters: Abell clusters 1689, 1835 and 114. The foreground clusters provide a magnified view of the distant Universe and permit a sensitive test for the presence of very high redshift galaxies. We search for   z ∼ 9 Lyα  emitting galaxies displaying a significant narrow-band excess relative to accompanying J -band observations that remain undetected in Hubble Space Telescope ( HST )/Advanced Camera for Surveys (ACS) optical images of each field. No sources consistent with this criterion are detected above the unlensed 90 per cent point-source flux limit of the narrow-band image,   F _NB= 3.7 × 10⁻¹⁸ erg s⁻¹ cm⁻²  . To date, the total coverage of the ZEN survey has sampled a volume at   z ∼ 9  of approximately 1700 comoving Mpc³ to a Lyα emission luminosity of  10⁴³ erg s⁻¹  . We conclude by considering the prospects for detecting   z ∼ 9 Lyα  emitting galaxies in light of both observed galaxy properties at   z < 7  and simulated populations at   z > 7  .     

An 80-kpc Lyα halo around a high-redshift type-2 quasi-stellar object

We announce the discovery of an extended emission-line region associated with a high-redshift type-2 quasi-stellar object (QSO). The halo, which was discovered in our new wide-field narrow-band survey, resides at   z = 2.85  in the Spitzer First Look Survey region and is extended over ∼80 kpc. Deep very long baseline interferometry (VLBI) observations imply that approximately 50 per cent of the radio emission is extended on scales >200 pc. The inferred active galactic nuclei (AGN) luminosity is sufficient to ionize the extended halo, and the optical emission is consistent with being triggered coevally with the radio source. The Lyα halo is as luminous as those found around high-redshift radio galaxies; however, the active nucleus is several orders of magnitude less luminous at radio wavelengths than those Fanarof–Riley type II (FRIIs) more commonly associated with extended emission-line regions. AMS05 appears to be a high-redshift analogue to the radio-quiet quasar E1821+643 which is core dominated, but which also exhibits extended Fanarof–Riley type I (FRI)-like structure and contains an optically powerful AGN. We also find evidence for more quiescent kinematics in the Lyα emission line in the outer regions of the halo, reminiscent of the haloes around the more powerful FRIIs. The optical to mid-infrared spectral energy distribution is well described by a combination of an obscured QSO  ( L _bol∼ 3.4 ± 0.2 × 10¹³ L_⊙)  and a 1.4 Gyr old simple stellar population with mass  ∼3.9 ± 0.3 × 10¹¹ M_⊙  .     

The GLARE Survey – II. Faint z≈ 6 Lyα line emitters in the HUDF

The galaxy population at   z ≈ 6  has been the subject of intense study in recent years, culminating in the Hubble Ultra Deep Field (HUDF) – the deepest imaging survey yet. A large number of high-redshift galaxy candidates have been identified within the HUDF, but until now analysis of their properties has been hampered by the difficulty of obtaining spectroscopic redshifts for these faint galaxies. Our 'Gemini Lyman-Alpha at Reionization Era' (GLARE) project has been designed to undertake spectroscopic follow-up of faint  ( z ' < 28.5)  i '-drop galaxies at   z ≈ 6  in the HUDF. In a previous paper we presented preliminary results from the first 7.5 h of data from GLARE. In this paper we detail the complete survey. We have now obtained 36 h of spectroscopy on a single GMOS slitmask from Gemini-South, with a spectral resolution of  λ/Δλ_FWHM≈ 1000  . We identify five strong Lyα emitters at   z > 5.5  , and a further nine possible line emitters with detections at lower significance. We also place tight constraints on the equivalent width of Lyα emission for a further ten i '-drop galaxies and examine the equivalent width distribution of this faint spectroscopic sample of   z ≈ 6  galaxies. We find that the fraction of galaxies with little or no emission is similar to that at   z ≈ 3  , but that the   z ≈ 6  population has a tail of sources with high rest-frame equivalent widths. Possible explanations for this effect include a tendency towards stronger line emission in faint sources, which may arise from extreme youth or low metallicity in the Lyman-break population at high redshift, or possibly a top-heavy initial mass function.     

The properties of Lyα emitting galaxies in hierarchical galaxy formation models

We present detailed predictions for the properties of Lyα-emitting galaxies in the framework of the Λ cold dark matter cosmology, calculated using the semi-analytical galaxy formation model galform . We explore a model that assumes a top-heavy initial mass function in starbursts and that has previously been shown to explain the sub-millimetre number counts and the luminosity function of Lyman-break galaxies at high redshift. We show that this model, with the simple assumption that a fixed fraction of Lyα photons escape from each galaxy, is remarkably successful at explaining the observed luminosity function of Lyα emitters (LAEs) over the redshift range  3 < z < 6.6  . We also examine the distribution of Lyα equivalent widths and the broad-band continuum magnitudes of emitters, which are in good agreement with the available observations. We look more deeply into the nature of LAEs, presenting predictions for fundamental properties such as the stellar mass and radius of the emitting galaxy and the mass of the host dark matter halo. The model predicts that the clustering of LAEs at high redshifts should be strongly biased relative to the dark matter, in agreement with observational estimates. We also present predictions for the luminosity function of LAEs at   z > 7  , a redshift range that is starting to be be probed by near-infrared surveys and using new instruments such as the Dark Ages Z Lyman Explorer (DAzLE).     

A spectroscopic study of IRAS F10214 + 4724

The z  = 2.286  IRAS galaxy F10214 + 4724 remains one of the most luminous galaxies in the Universe, despite its gravitational lens magnification. We present optical and near-infrared spectra of F10214 + 4724, with clear evidence for three distinct components: lines of width ∼ 1000 km s⁻¹ from a Seyfert 2 nucleus; ≲ 200 km s⁻¹ lines which are likely to be associated with star formation; and a broad (∼ 4000 km s⁻¹) C  III ] 1909-Å emission line which is blueshifted by ∼ 1000 km s⁻¹ with respect to the Seyfert 2 lines. Our study of the Seyfert 2 component leads to several new results. (i) From the double-peaked structure in the Lyα line, and the lack of Lyβ, we argue that the Lyα photons have emerged through a neutral column of N _H ∼ 2.5 × 10²⁵ m⁻², possibly located within the AGN narrow-line region, as proposed for several high-redshift radio galaxies. (ii) The resonant O  VI 1032, 1036-Å doublet (previously identified as Lyβ) is in an optically thick (1:1) ratio. At face value this implies an extreme density ( n _e ∼ 10¹⁷ m⁻³) more typical of broad-line region clouds. However, we attribute this instead to the damping wings of Lyβ from the resonant absorption. (iii) A tentative detection of He  II 1086 suggests little extinction in the rest frame ultraviolet.     

Ly+ and ultraviolet emission from high-redshift gamma-ray burst hosts: to what extent do gamma-ray bursts trace star formation?

We report the result of a search for Lyα emission from the host galaxies of the gamma-ray bursts  (GRBs) 030226 ( z = 1.986), 021004 ( z = 2.335)  and  020124 ( z = 3.198)  . We find that the host galaxy of GRB 021004 is an extended (around 8 kpc) strong Lyα emitter with a rest-frame equivalent width (EW) of 68⁺¹²₋₁₁Å, and a star formation rate of  10.6 ± 2.0 M_⊙ yr⁻¹  . We do not detect the hosts of GRB 030226 and GRB 020124, but the upper limits on their Lyα fluxes do not rule out large rest-frame EWs. In the fields of GRB 021004 and GRB 030226 we find seven and five other galaxies, respectively, with excess emission in the narrow-band filter. These galaxies are candidate Lyα-emitting galaxies in the environment of the host galaxies. We have also compiled a list of all   z ≳ 2  GRB hosts, and demonstrate that a scenario where they trace star formation in an unbiased way is compatible with current observational constraints. Fitting the   z = 3  luminosity function (LF) under this assumption results in a characteristic luminosity of   R *= 24.6  and a faint-end slope of  α=−1.55  , consistent with the LF measured for Lyman-break galaxies.     

Hubble Space Telescope imaging survey of sub-mJy star-forming galaxies – I. Morphologies at z ∼ 0.2

We present the first results of our Hubble Space Telescope HST WFPC2 F814W snapshot imaging survey, targeting virtually all sub-mJy decimetric radio-selected star-forming galaxies. The radio selection at ∼1 GHz is free from extinction effects and the radio luminosities are largely unaffected by AGN contamination, making these galaxies ideal tracers of the cosmic star formation history. A subsample of four targets is presented here, selected at 1.4 GHz from the spectroscopically homogenous and complete samples of Benn et al. and Hopkins et al. The redshifts are confined to a narrow range around z ∼0.2, to avoid differential evolution, with a radio luminosity close to L ∗ where the galaxies dominate the comoving volume-averaged star formation rate. We find clearly disturbed morphologies resembling those of ultraluminous infrared galaxies, indicating that galaxy interactions may be the dominant mechanism for triggering star formation at these epochs. The morphologies are also clearly different from those of coeval quasars and radio galaxies, as found in star-forming galaxies selected at other wavelengths. This may prove challenging for models that propose direct causal links between AGN evolution and the cosmic star formation history at these epochs. The asymmetries are typically much larger than seen in the Canada–France Redshift Survey at similar redshifts, optical luminosities and H α -derived star formation rates, indicating the possible existence of an obscuration-related morphological bias in such samples.     

On the polarization of Hα scattered by neutral hydrogen in active galactic nuclei

Raman scattering by atomic hydrogen converts the UV continuum around Lyβ into optical continuum around Hα, and the basic atomic physics has been discussed in several works on symbiotic stars. We propose that the same process may operate in active galactic nuclei (AGN) and calculate the linear polarization of the broad emission lines Raman-scattered by high-column neutral hydrogen component. The conversion efficiency of the Raman scattering process is discussed and the expected scattered flux is computed using the spectral energy distribution of an AGN given by a typical power law. The high-column H  i component in AGN is suggested by many observations, encompassing the radio through UV and X-ray ranges.   When neutral hydrogen component with a column density ∼10²² cm⁻² is present around the active nucleus, it is found that the scattered Hα is characterized by a very broad width ∼20 000 km s⁻¹ and that the strength of the polarized flux is comparable to that of the electron-scattered flux expected from a conventional unified model of narrow-line AGN. The width of the scattered flux is mainly determined by the column density of the neutral scatterers where the total scattering optical depth becomes of order unity. The asymmetry in the Raman scattering cross-section around Lyβ introduces red asymmetric polarized profiles around Hα. The effects of the blended Lyβ and O  vi 1034 doublet are also investigated.   We briefly discuss the spectropolarimetric observations performed on the Seyfert galaxy IRAS 110548-1131 and the narrow line radio galaxy Cyg A. Several predictions regarding the scattering by the high-column neutral hydrogen component in AGN are discussed.     

The probability distribution function of the Lyman α transmitted flux from a sample of Sloan Digital Sky Survey quasars

We present a measurement of the probability distribution function (PDF) of the transmitted flux in the Lyman α (Lyα) forest from a sample of 3492 quasars included in the Sloan Digital Sky Survey data release 3 (SDSS DR3). Our intention is to investigate the sensitivity of the Lyα flux PDF as measured from low-resolution and low signal-to-noise ratio data to a number of systematic errors such as uncertainties in the mean flux, continuum and noise estimate. The quasar continuum is described by the superposition of a power law and emission lines. We perform a power-law continuum fitting on a spectrum-by-spectrum basis, and obtain an average continuum slope of  α_ν= 0.59 ± 0.36  in the redshift range  2.5 < z < 3.5  . We take into account that the variation in the continuum indices increases the mean flux by 3 and 7 per cent at   z = 3  and 2.4, respectively, as compared to the values inferred with a single (mean) continuum slope. We compare our measurements to the PDF obtained with mock lognormal spectra, whose statistical properties have been constrained to match the observed Lyα flux PDF and power spectrum of high-resolution data. Using our power-law continuum fitting and the SDSS pipeline noise estimate yields a poor agreement between the observed and mock PDFs. Allowing for a break in the continuum slope and, more importantly, for residual scatter in the continuum level substantially improves the agreement. A decrease of ∼10–15 per cent in the mean quasar continuum with a typical rms variance at the 20 per cent level can account for the data, provided that the noise excess correction is no larger than ≲10 per cent.     

On contamination and completeness in z≥ 5 Lyman-break galaxy surveys

A large population of z > 5 Lyman-break galaxies has been identified in recent years. However, the high-redshift galaxies selected by different surveys are subject to a variety of selection effects – some overt, others more subtle. We present an analysis of sample completeness and contamination issues in high-redshift surveys, focusing on surveys at z ≈ 5 and using a spectroscopically confirmed low-redshift sample from the DEEP2 survey in order to characterize contaminant galaxies. We find that most surveys underestimate their contamination from highly clustered galaxies at z ≈ 1 and stars. We consider the consequences of this for both the rest-frame ultraviolet luminosity function and the clustering signal from z ≈ 5 galaxies. We also find that sources with moderate strength Lyman α emission lines can be omitted from dropout surveys due to their blue colours, again effecting the derived luminosity functions. We discuss the points of comparison between different samples, and the applicability of survey-specific results to the population at z > 5 in general.     

Lyman break galaxies at z= 4–6 in cosmological smoothed particle hydrodynamics simulations

We perform a spectrophotometric analysis of galaxies at redshifts z = 4–6 in cosmological smoothed particle hydrodynamics simulations of a Λ cold dark matter universe. Our models include radiative cooling and heating by a uniform ultraviolet (UV) background, star formation, supernova feedback, and a phenomenological model for galactic winds. Analysing a series of simulations of varying box size and particle number allows us to isolate the impact of numerical resolution on our results. Specifically, we determine the luminosity functions in B , V , R , i ' and z ' filters, and compare the results with observational surveys of Lyman break galaxies (LBGs) performed with the Subaru telescope and the Hubble Space Telescope . We find that the simulated galaxies have UV colours consistent with observations and fall in the expected region of the colour–colour diagrams used by the Subaru group. The stellar masses of the most massive galaxies in our largest simulation increase their stellar mass from   M _★∼ 10¹¹ M_⊙  at z = 6 to   M _★∼ 10^11.7 M_⊙  at z = 3. Assuming a uniform extinction of E ( B − V ) = 0.15, we also find reasonable agreement between simulations and observations in the space density of UV bright galaxies at z = 3–6, down to the magnitude limit of each survey. For the same moderate extinction level of E ( B − V ) ∼ 0.15, the simulated luminosity functions match observational data, but have a steep faint-end slope with α∼−2.0. We discuss the implications of the steep faint-end slope found in the simulations. Our results confirm the generic conclusion from earlier numerical studies that UV bright LBGs at z ≥ 3 are the most massive galaxies with E ( B − V ) ∼ 0.15 at each epoch.     

Are galaxies with active galactic nuclei a transition population?

We present the results of an analysis of a well-selected sample of galaxies with active and inactive galactic nuclei from the Sloan Digital Sky Survey, in the range  0.01 < z < 0.16  . The SDSS galaxy catalogue was split into two classes of active galaxies, Type 2 active galactic nuclei (AGN) and composites, and one set of inactive, star-forming/passive galaxies. For each active galaxy, two inactive control galaxies were selected by matching redshift, absolute magnitude, inclination, and radius. The sample of inactive galaxies naturally divides into a red and a blue sequence, while the vast majority of AGN hosts occur along the red sequence. In terms of Hα equivalent width (EW), the population of composite galaxies peaks in the valley between the two modes, suggesting a transition population. However, this effect is not observed in other properties such as the colour–magnitude space or colour–concentration plane. Active galaxies are seen to be generally bulge-dominated systems, but with enhanced Hα emission compared to inactive red-sequence galaxies. AGN and composites also occur in less dense environments than inactive red-sequence galaxies, implying that the fuelling of AGN is more restricted in high-density environments. These results are therefore inconsistent with theories in which AGN host galaxies are a 'transition' population. We also introduce a systematic 3D spectroscopic imaging survey, to quantify and compare the gaseous and stellar kinematics of a well-selected, distance-limited sample of up to 20 nearby Seyfert galaxies, and 20 inactive control galaxies with well-matched optical properties. The survey aims to search for dynamical triggers of nuclear activity and address outstanding controversies in optical/infrared imaging surveys.     

The ultraviolet excess in nearby powerful radio galaxies: evidence for a young stellar component

We present William Herschel Telescope spectropolarimetry observations of a complete RA-limited sample of nine low-redshift  (0.05< z <0.2)  3CR radio sources in order to investigate the nature of the ultraviolet (UV) excess in nearby powerful radio galaxies. Of the nine galaxies studied in detail from this sample, we find that four show a measurable UV excess following nebular continuum subtraction, but none of the sources shows significant polarization in the UV. One of the radio galaxies with a UV excess – 3C 184.1 – shows evidence for broad permitted lines and hence direct active galactic nucleus (AGN) light. In the remaining three galaxies we argue that the most likely contributor to the UV excess is a young stellar component. For these three galaxies we find that the best-fitting model for the optical/UV continuum consists of a combination of an old stellar population  (10–15 Gyr  old elliptical galaxy) plus a reddened young stellar population  (0.05–2 Gyr)  . The reddened young stellar component typically accounts for half of the total flux at 4780 Å, following nebular continuum subtraction, and   E ( B - V )  values of between 0.2 and 0.7 mag are required. However, for the majority of sources in our sample (six out of nine), continuum modelling provides no evidence for a significant young stellar component in the nuclear regions of the host galaxies. Our results are discussed in the context of far-infrared evidence for star formation activity.     

XMM–Newton surveys of the Canada–France Redshift Survey Fields – III. The environments of X-ray selected active galactic nuclei at 0.4 < z < 0.6

The environmental properties of a sample of 31 hard X-ray selected active galactic nuclei (AGN) are investigated, from scales of 500 kpc down to 30 kpc, and are compared to a control sample of inactive galaxies. All the AGN lie in the redshift range  0.4 < z < 0.6  . The accretion luminosity density of the Universe peaks close to this redshift range, and the AGN in the sample have X-ray luminosities close to the knee in the hard X-ray luminosity function, making them representative of the population that dominated this important phase of energy conversion.
Using both the spatial clustering amplitude and near-neighbour counts, it is found that the AGN have environments that are indistinguishable from normal, inactive galaxies over the same redshift range and with similar optical properties. Typically, the environments are of subcluster richness, in contrast to similar studies of high- z quasars, which are often found in clusters with comparable richness to the Abell   R ≥ 0  clusters.
It is suggested that minor mergers with low-mass companions are a likely candidate for the mechanism by which these modest luminosity AGN are fuelled.     

ΛCDM predictions for galaxy protoclusters – I. The relation between galaxies, protoclusters and quasars at z∼ 6

Motivated by recent observational studies of the environment of   z ∼ 6  QSOs, we have used the Millennium Run (MR) simulations to construct a very large  (∼4°× 4°)  mock redshift survey of star-forming galaxies at   z ∼ 6  . We use this simulated survey to study the relation between density enhancements in the distribution of i ₇₇₅-dropouts and Lyα emitters, and their relation to the most massive haloes and protocluster regions at   z ∼ 6  . Our simulation predicts significant variations in surface density across the sky with some voids and filaments extending over scales of 1°, much larger than probed by current surveys. Approximately one-third of all   z ∼ 6  haloes hosting i -dropouts brighter than   z = 26.5  mag  (≈ M *_{UV, z =6})  become part of   z = 0  galaxy clusters. i -dropouts associated with protocluster regions are found in regions where the surface density is enhanced on scales ranging from a few to several tens of arcminutes on the sky. We analyse two structures of i -dropouts and Lyα emitters observed with the Subaru Telescope and show that these structures must be the seeds of massive clusters in formation. In striking contrast, six   z ∼ 6  QSO fields observed with Hubble Space Telescope show no significant enhancements in their i ₇₇₅-dropout number counts. With the present data, we cannot rule out the QSOs being hosted by the most massive haloes. However, neither can we confirm this widely used assumption. We conclude by giving detailed recommendations for the interpretation and planning of observations by current and future ground- and space-based instruments that will shed new light on questions related to the large-scale structure at   z ∼ 6  .     

Spectroscopy of the optical Einstein ring 0047 – 2808

We present optical and near-infrared spectroscopic observations of the optical Einstein ring 0047 – 2808. We detect both [O III ] lines λλ4959, 5007 near ∼ 2.3 μm, confirming the redshift of the lensed source as z  = 3.595. The Lyα line is redshifted relative to the [O III ] line by 140 ± 20 km s⁻¹. Similar velocity shifts have been seen in nearby starburst galaxies. The [O III ] line is very narrow, 130 km s⁻¹ FWHM. If the ring is the image of the centre of a galaxy, the one-dimensional stellar velocity dispersion σ = 55 km s⁻¹ is considerably smaller than the value predicted by Baugh et al. for the somewhat brighter Lyman-break galaxies. The Lyα line is significantly broader than the [O III ] line, probably due to resonant scattering. The stellar central velocity dispersion of the early-type deflector galaxy at z  = 0.485 is 250 ± 30 km s⁻¹. This value is in good agreement both with the value predicted from the radius of the Einstein ring (and a singular isothermal sphere model for the deflector), and with the value estimated from the D _n −σ relation.