ALBERT

Description: In the low-redshift Universe, the most powerful radio sources are often associated with gas-rich galaxy mergers or interactions. We here present evidence for an advanced, gas-rich (‘wet’) merger associated with a powerful radio galaxy at a redshift of z  ~ 2. This radio galaxy, MRC 0152-209, is the most infrared-luminous high-redshift radio galaxy known in the Southern hemisphere. Using the Australia Telescope Compact Array, we obtained high-resolution CO(1–0) data of cold molecular gas, which we complement with Hubble Space Telescope ( HST )/ Wide Field Planetary Camera 2 ( WFPC 2) imaging and William Herschel Telescope long-slit spectroscopy. We find that, while roughly M H2  ~ 2 10 10  M of molecular gas coincides with the central host galaxy, another M H2  ~ 3 10 10  M is spread across a total extent of ~60 kpc. Most of this widespread CO(1–0) appears to follow prominent tidal features visible in the rest-frame near-UV HST / WFPC 2 imaging. Lyα emission shows an excess over He II, but a deficiency over L IR , which is likely the result of photoionization by enhanced but very obscured star formation that was triggered by the merger. In terms of feedback, the radio source is aligned with widespread CO(1–0) emission, which suggests that there is a physical link between the propagating radio jets and the presence of cold molecular gas on scales of the galaxy's halo. Its optical appearance, combined with the transformational stage at which we witness the evolution of MRC 0152-209, leads us to adopt the name ‘Dragonfly Galaxy’.

Description: We present the results of observations taken with the X-shooter spectrograph devoted to the study of quasars at z  ~ 6. This paper focuses on the properties of metals at high redshift traced, in particular, by the C iv doublet absorption systems. Six objects were observed with resolutions ~=27 and 34 km s –1 in the visual, and 37.5 and 53.5 km s –1 in the near-infrared. We detected 102 C iv lines in the range: 4.35 〈  z  〈 6.2 of which 27 are above z  ~ 5. Thanks to the characteristics of resolution and spectral coverage of X-shooter, we could also detect 25 Si iv doublets associated with the C iv at z   5. The column density distribution function of the C iv line sample is observed to evolve in redshift for z   5.3, with respect to the normalization defined by low-redshift (1.5 〈  z  〈 4) C iv lines. This behaviour is reflected in the redshift evolution of the C iv cosmic mass density, $\Omega _{\rm C\,{\small {IV}}}$ , of lines with column density in the range $13.4 〈 \log N(\mathrm{C\,\small {IV}}) 〈 15$ , which is consistent with a drop of a factor of ~2 for z   5.3. Considering only the stronger C iv lines ( $13.8 〈 \log N(\mathrm{C\,\small {IV}}) 〈 15$ ), $\Omega _{\rm C\,{\small {IV}}}$ gently rises by a factor of ~10 between z  ~= 6.2 and z  ~= 1.5 with a possible flattening towards z  ~ 0. The increase is well fitted by a power law: $\Omega _{\rm C\,{\small {IV}}} = (2\pm 1)\times 10^{-8} [(1+z)/4]^{-3.1\pm 0.1}$ . An insight into the properties of the C iv absorbers and their evolution with redshift is obtained by comparing the observed column densities of associated C iv , Si iv and C ii absorptions with the output of a set of cloudy photoionization models. As already claimed by cosmological simulations, we find that C iv is a good tracer of the metallicity in the low-density intergalactic medium (IGM) gas at z ~ 5-6 while at z  ~ 3 it arises in gas with overdensity  ~ 100.

Description: We present a CO(1–0) survey for cold molecular gas in a representative sample of 13 high- z radio galaxies (HzRGs) at 1.4 〈 z 〈 2.8, using the Australia Telescope Compact Array. We detect CO(1–0) emission associated with five sources: MRC 0114-211, MRC 0152-209, MRC 0156-252, MRC 1138-262 and MRC 2048-272. The CO(1–0) luminosities are in the range $L^{\prime }_{\rm CO} \sim (5\hbox{--}9) \times 10^{10}$ K km s –1 pc 2 . For MRC 0152-209 and MRC 1138-262, part of the CO(1–0) emission coincides with the radio galaxy, while part is spread on scales of tens of kpc and likely associated with galaxy mergers. The molecular gas mass derived for these two systems is M H2  ~ 6 10 10 M ( M H2 / $L^{\prime }_{\rm CO}$  = 0.8). For the remaining three CO-detected sources, the CO(1–0) emission is located in the halo (~50-kpc) environment. These three HzRGs are among the fainter far-IR emitters in our sample, suggesting that similar reservoirs of cold molecular halo gas may have been missed in earlier studies due to pre-selection of IR-bright sources. In all three cases, the CO(1–0) is aligned along the radio axis and found beyond the brightest radio hotspot, in a region devoid of 4.5 μm emission in Spitzer imaging. The CO(1–0) profiles are broad, with velocity widths of ~1000–3600 km s –1 . We discuss several possible scenarios to explain these halo reservoirs of CO(1–0). Following these results, we complement our CO(1–0) study with detections of extended CO from the literature and find at marginal statistical significance (95 per cent level) that CO in HzRGs is preferentially aligned towards the radio jet axis. For the eight sources in which we do not detect CO(1–0), we set realistic upper limits of $L^{\prime }_{\rm CO} \sim 3\hbox{--}4 \times 10^{10}$ K km s –1 pc 2 . Our survey reveals a CO(1–0) detection rate of 38 per cent, allowing us to compare the CO(1–0) content of HzRGs with that of other types of high- z galaxies.

Description: The rapid decline in the number of strong Lyα emitting galaxiesat z 〉 6 provides evidence for neutral hydrogen in the intergalactic medium, but is difficult to explain with plausible models for reionization. We demonstrate that the observed reduction in Lyα flux from galaxies at z 〉 6 can be explained by evolution in the escape fraction of ionizing photons, f esc . We find that the median observed drop in the fraction of galaxies showing strong Lyα emission, as well as the observed evolution of the Lyα luminosity function both follow from a small increase in f esc of f esc ~ 0.1 from f esc ~ 0.6 at z ~ 6. This high escape fraction may be at odds with current constraints on the ionizing photon escape fraction, which favour smaller values of f esc 20 per cent. However, models that invoke a redshift evolution of f esc that is consistent with these constraints can suppress the z ~ 7 Lyα flux to the observed level, if they also include a small evolution in global neutral fraction of x H i ~ 0.2. Thus, an evolving escape fraction of ionizing photons can be a plausible part of the explanation for evolution in the Lyα emission of high-redshift galaxies. More generally, our analysis also shows that the drop in the Lyα fraction is quantitatively consistent with the observed evolution in the Lyα luminosity functions of Lyα emitters.

Description: 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, Q H  ii ; and (ii) the opacity from self-shielded systems in the ionized IGM, parametrized by the average photoionization rate inside H  ii regions, 〈 12 〉 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 ( Q H  ii , 〈 12 〉 H  ii ) parameter space, which can easily be used to interpret new data sets. Our new, improved models highly disfavour an evolution in 〈 12 〉 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 Q H  ii   0.1. Larger changes in the Lyα fraction could result from a co-evolution with galaxy properties. Marginalizing over 〈 12 〉 H  ii , we find that current observations constrain Q H  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.

Description: We present multicolour Hubble Space Telescope images of the powerful z  = 2.4 radio galaxy MRC 0406–244 and model its complex morphology with several components including a host galaxy, a point source and extended nebular and continuum emission. We suggest that the main progenitor of this radio galaxy was a normal, albeit massive ( M * ~ 10 11 M ), star-forming galaxy. The optical stellar disc of the host galaxy is smooth and well described by a Sérsic profile, which argues against a recent major merger; however, there is also a point-source component which may be the remnant of a minor merger. The half-light radius of the optical disc is constrained to lie in the range 3.5 to 8.2 kpc, which is of similar size to coeval star-forming galaxies. Biconical shells of nebular emission and ultraviolet-bright continuum extend out from the host galaxy along the radio jet axis, which is also the minor axis of the host galaxy. The origin of the continuum emission is uncertain, but it is most likely to be young stars or dust-scattered light from the active galactic nucleus, and it is possible that stars are forming from this material at a rate of $200^{+1420}_{-110}$ ${\rm M}_{\odot }\,{\rm yr}^{-1}\,$ .

Description: The high-redshift radio galaxy MRC 1138–262 (‘Spiderweb Galaxy’; z  = 2.16) is one of the most massive systems in the early Universe and surrounded by a dense ‘web’ of proto-cluster galaxies. Using the Australia Telescope Compact Array, we detected CO(1–0) emission from cold molecular gas – the raw ingredient for star formation – across the Spiderweb Galaxy. We infer a molecular gas mass of M H2  = 6  x 10 10 M (for M H2 / L ' CO  = 0.8). While the bulk of the molecular gas coincides with the central radio galaxy, there are indications that a substantial fraction of this gas is associated with satellite galaxies or spread across the intergalactic medium on scales of tens of kpc. In addition, we tentatively detect CO(1–0) in the star-forming proto-cluster galaxy HAE 229, 250 kpc to the West. Our observations are consistent with the fact that the Spiderweb Galaxy is building up its stellar mass through a massive burst of widespread star formation. At maximum star formation efficiency, the molecular gas will be able to sustain the current star formation rate (SFR 1400 M  yr –1 , as traced by Seymour et al.) for about 40 Myr. This is similar to the estimated typical lifetime of a major starburst event in infrared luminous merger systems.

Description: We report new deep observations obtained with the Atacama Large Millimetre Array (ALMA) aimed at investigating the [C ii ]158 μm line and continuum emission in three spectroscopically confirmed Lyman break galaxies at 6.8 〈  z  ≤ 7.1, i.e. well within the re-ionization epoch. With star formation rates of SFR ~ 5–15M yr – 1 these systems are much more representative of the high- z galaxy population than other systems targeted in the past by millimetre observations. For the galaxy with the deepest observation we detect [C ii ] emission at redshift z  = 7.107, fully consistent with the Ly α redshift, but spatially offset by 0.7 arcsec (4 kpc) from the optical emission. At the location of the optical emission, tracing both the Ly α line and the far-UV continuum, no [C ii ] emission is detected in any of the three galaxies, with 3 upper limits significantly lower than the [C ii ] emission observed in lower redshift galaxies. These results suggest that molecular clouds in the central parts of primordial galaxies are rapidly disrupted by stellar feedback. As a result, [C ii ] emission mostly arises from more external accreting/satellite clumps of neutral gas. These findings are in agreement with recent models of galaxy formation. Thermal far-infrared continuum is not detected in any of the three galaxies. However, the upper limits on the infrared-to-UV emission ratio do not exceed those derived in metal- and dust-poor galaxies.

Description: In the low-redshift Universe, the most powerful radio sources are often associated with gas-rich galaxy mergers or interactions. We here present evidence for an advanced, gas-rich (‘wet’) merger associated with a powerful radio galaxy at a redshift of z  ~ 2. This radio galaxy, MRC 0152-209, is the most infrared-luminous high-redshift radio galaxy known in the Southern hemisphere. Using the Australia Telescope Compact Array, we obtained high-resolution CO(1–0) data of cold molecular gas, which we complement with Hubble Space Telescope ( HST )/ Wide Field Planetary Camera 2 ( WFPC 2) imaging and William Herschel Telescope long-slit spectroscopy. We find that, while roughly M H2  ~ 2 10 10  M of molecular gas coincides with the central host galaxy, another M H2  ~ 3 10 10  M is spread across a total extent of ~60 kpc. Most of this widespread CO(1–0) appears to follow prominent tidal features visible in the rest-frame near-UV HST / WFPC 2 imaging. Lyα emission shows an excess over He II, but a deficiency over L IR , which is likely the result of photoionization by enhanced but very obscured star formation that was triggered by the merger. In terms of feedback, the radio source is aligned with widespread CO(1–0) emission, which suggests that there is a physical link between the propagating radio jets and the presence of cold molecular gas on scales of the galaxy's halo. Its optical appearance, combined with the transformational stage at which we witness the evolution of MRC 0152-209, leads us to adopt the name ‘Dragonfly Galaxy’.

Description: VANDELS is a uniquely deep spectroscopic survey of high-redshift galaxies with the VIMOS spectrograph on ESO’s Very Large Telescope (VLT). The survey has obtained ultradeep optical (0.48 〈 λ 〈 1.0 |$mu $|m) spectroscopy of ≃2100 galaxies within the redshift interval 1.0 ≤ z ≤ 7.0, over a total area of ≃0.2 deg2 centred on the CANDELS Ultra Deep Survey and Chandra Deep Field South fields. Based on accurate photometric redshift pre-selection, 85 per cent of the galaxies targeted by VANDELS were selected to be at z ≥ 3. Exploiting the red sensitivity of the refurbished VIMOS spectrograph, the fundamental aim of the survey is to provide the high-signal-to-noise ratio spectra necessary to measure key physical properties such as stellar population ages, masses, metallicities, and outflow velocities from detailed absorption-line studies. Using integration times calculated to produce an approximately constant signal-to-noise ratio (20 〈 tint〈 80 h), the VANDELS survey targeted: (a) bright star-forming galaxies at 2.4 ≤ z ≤ 5.5, (b) massive quiescent galaxies at 1.0 ≤ z ≤ 2.5, (c) fainter star-forming galaxies at 3.0 ≤ z ≤ 7.0, and (d) X-ray/Spitzer-selected active galactic nuclei and Herschel-detected galaxies. By targeting two extragalactic survey fields with superb multiwavelength imaging data, VANDELS will produce a unique legacy data set for exploring the physics underpinning high-redshift galaxy evolution. In this paper, we provide an overview of the VANDELS survey designed to support the science exploitation of the first ESO public data release, focusing on the scientific motivation, survey design, and target selection.