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Aims. The properties of dust attenuation at rest-frame UV wavelengths are inferred from very high-quality FORS 2 spectra of 78 galaxies from the GMASS survey at redshift $1 < z < 2.5$. These objects complement a previously investigated sample of 108 UV-luminous, intermediate-mass (i.e., with stellar masses ~$10^{10}{-}10^{11}~M_{\sun}$) galaxies at similar redshifts, selected from the FDF spectroscopic survey, the K20 survey, and the GDDS. Detection of the broad absorption feature centred on about 2175 Å (“UV bump”) implies that the average UV extinction curve of a galaxy more closely resembles that of the Milky Way (MW) or the Large Magellanic Cloud (LMC), and differs from that of the Small Magellanic Cloud (SMC).

We present the results of a large program conducted with the Very Large Telescope and augmented by observations with the Keck telescope to search for forming clusters of galaxies near powerful radio galaxies at $2.0 < z < 5.2$. Besides MRC 1138-262 at $z = 2.16$, the radio galaxy observed in our pilot program, we obtained narrow- and broad-band images of eight radio galaxies and their surroundings. The imaging was used to select candidate Lyαemitting galaxies in ~$3\times3$Mpc2areas near the radio galaxies. A total of 300 candidate emitters were found with a rest-frame Lyαequivalent width of EW$_0 > 15$Å and significance $\Sigma \equiv {\it EW}_0/\Delta {\it EW}_0 > 3$. Follow-up spectroscopy was performed on 152 candidates in seven of the radio galaxy fields. Of these, 139 were confirmed to be Lyαemitters, four were low redshift interlopers and nine were non-detections. With the adopted criteria the success rate is $139/152 = 91$%. In addition, 14 objects with EW$_0 < 15$and/or $\Sigma < 3$were confirmed to be Lyαemitters. Combined with the 15 Lyαemitters near MRC 1138-262, we have determined Lyαredshifts for 168 objects near eight radio galaxies.
At least six of our eight fields are overdense in Lyαemitters by a factor 3-5 as compared to the field density of Lyαemitters at similar redshifts, although the statistics in our highest redshift field ($z = 5.2$) are poor. Also, the emitters show significant clustering in velocity space. In the overdense fields, the width of the velocity distributions of the emitters is a factor 2-5 smaller than the width of the narrow-band filters. Taken together, we conclude that we have discovered six forming clusters of galaxies (protoclusters). We estimate that roughly 75% of powerful ($L_\mathrm{2.7\,GHz} > 10^{33}$erg s-1Hz-1sr-1) high redshift radio galaxies reside in a protocluster. The protoclusters have sizes of at least 1.75 Mpc, which is consistent with the structure sizes found by other groups. By using the volume occupied by the overdensities and assuming a bias parameter of $b=3{-}6$, we estimate that the protoclusters have masses in the range $2{-}9 \times 10^{14}$$M_{\odot}$. These protoclusters are likely to be progenitors of present-day (massive) clusters of galaxies. For the first time, we have been able to estimate the velocity dispersion of cluster progenitors from $z\sim5$to ~2. The velocity dispersion of the emitters increases with cosmic time, in agreement with the dark matter velocity dispersion in numerical simulations of forming massive clusters.

We present broad-band imaging with the Subaru Telescope of a $25'$$\times$$25'$field surrounding the radio galaxy TN J1338–1942 at redshift $z = 4.1$. The field contains excesses of Lyman-αemitters (LAEs) and Lyman break galaxies (LBGs) identified with a protocluster surrounding the radio galaxy. Our new wide-field images provide information about the boundary of the protocluster and its surroundings. There are 874 candidate LBGs within our field, having redshifts in the range $z = 3.5{-}4.5$. An examination of the brightest of these (with $i' < 25.0$) shows that the most prominent concentration coincides with the previously discovered protocluster. The diameter of this galaxy overdensity corresponds to ~2 Mpc at $z = 4$, consistent with the previous estimation using LAEs. Several other concentrations of LBGs are observed in the field, some of which may well be physically connected with the $z = 4.1$protocluster. The observed structure in the smoothed LBG distribution can be explained as the projection of large-scale structure, within the redshift range $z = 3.5{-}4.5$, comprising compact overdensities and prominent larger voids. If the $5{-}8$observed compact overdensities are associated with protoclusters, the observed protocluster volume density is ~5 $\times$10-6Mpc-3, similar to the volume density of rich clusters in the local Universe.

Observations of the radio galaxy MRC 0316–257 at $z = 3.13$and the surrounding field are presented. Using narrow- and broad-band imaging obtained with the VLT, 77 candidate Lyαemitters with a rest-frame equivalent width of >15 Å were selected in a ∼7´$\,\times\,7$´ field around the radio galaxy. Spectroscopy of 40 candidate emitters resulted in the discovery of 33 emission line galaxies of which 31 are Lyαemitters with redshifts similar to that of the radio galaxy, while the remaining two galaxies turned out to be [$\ion{O}{ii}$] emitters. The Lyαprofiles have widths (FWHM) in the range of 120–800 km s-1, with a median of 260 km s-1. Where the signal-to-noise was large enough, the Lyαprofiles were found to be asymmetric, with apparent absorption troughs blueward of the profile peaks, indicative of absorption along the line of sight of an $\ion{H}{i}$mass of at least $2 \times 10^2 {-} 5 \times 10^4$M$_{\odot}$. Besides that of the radio galaxy and one of the emitters that is a QSO, the continuum of the emitters is faint, with luminosities ranging from 1.3 L*to <0.03 L*. The colors of the confirmed emitters are, on average, very blue. The median UV continuum slope is $\beta = -1.76$, bluer than the average slope of LBGs with Lyαemission ($\beta \sim -1.09$). A large fraction of the confirmed emitters (∼2/3) have colors consistent with that of dust-free starburst galaxies. Observations with the Advanced Camera for Surveys on the Hubble Space Telescopeshow that the emitters that were detected in the ACS image have a range of different morphologies. Four Lyαemitters (∼25%) were unresolved with upper limits on their half light radii of $r_{\rm h} < 0.6-1.3$kpc, three objects (∼19%) show multiple clumps of emission, as does the radio galaxy, and the rest (∼56%) are single, resolved objects with $r_{\rm h} < 1.5$kpc. A comparison with the sizes of Lyman break galaxies at $z \sim 3$suggests that the Lyαemitters are on average smaller than LBGs. The average star formation rate of the Lyαemitters is 2.6 M$_{\odot}\;{\rm yr}^{-1}$as measured by the Lyαemission line or <3.9 M$_{\odot}\;{\rm yr}^{-1}$as measured by the UV continuum. The properties of the Lyαgalaxies (faint, blue and small) are consistent with young star forming galaxies which are still nearly dust free.
The volume density of Lyαemitting galaxies in the field around MRC 0316–257 is a factor of $3.3^{+0.5}_{-0.4}$larger compared with the density of field Lyαemitters at that redshift. The velocity distribution of the spectroscopically confirmed emitters has a dispersion of 640 km s-1, corresponding to a FWHMof 1510 km s-1, which is substantially smaller than the width of the narrow-band filter (${\it FWHM} \sim 3500$km s-1). The peak of the velocity distribution is located within 200 km s-1of the redshift of the radio galaxy. We conclude that the confirmed Lyαemitters are members of a protocluster of galaxies at $z \sim 3.13$. The size of the protocluster is unconstrained and is larger than $3.3 \times 3.3$Mpc2. The mass of this structure is estimated to be >$3{-}6 \times 10^{14}$M$_{\odot}$and could be the progenitor of a cluster of galaxies similar to e.g. the Virgo cluster.

In this paper we present deep VLT multi wavelength imaging observations of the field around the radio galaxy PKS 1138-262aimed at detecting and studying a (potential) proto-cluster centered at this radio source. PKS 1138-262is a massive galaxy at $z = 2.16$, located in a dense environment as indicated by optical, X-ray and radio observations. We had already found an over-density of Lyαemitting galaxies in this field, consistent with a proto-cluster structure associated with the radio galaxy. In addition, we find 40 candidate Hαemitters that have nominal rest frame equivalent width >25 Å within 1.8 Mpc and 2000 km s-1of the radio galaxy. Furthermore, we find 44 objects with $I - K > 4.3$. This number of extremely red objects (EROs) is about twice the number found in blank field ERO surveys, suggesting that some EROs in this field are part of the proto-cluster. The density of Hαemitters and extremely red objects increases towards the radio galaxy, indicating a physical association. From comparisons with other Kband, ERO, Hαand Lyαsurveys, we conclude that PKS 1138-262is located in a density peak which will evolve into a cluster of galaxies.

Radio galaxy PKS 1138-262is a massive galaxy at $z = 2.16$, located in a dense environment. We have found an overdensity of Lyαemitting galaxies in this field, consistent with a proto-cluster structure associated with the radio galaxy. Recently, we have discovered forty candidate Hαemitters by their excess near infrared narrow band flux. Here, we present infrared spectroscopy of nine of the brightest candidate Hαemitters. All these candidates show an emission line at the expected wavelength. The identification of three of these lines with Hαis confirmed by accompanying [$\ion{N}{ii}$] emission. The spectra of the other candidates are consistent with Hαemission at $z \sim 2.15$, one being a QSO as indicated by the broadness of its emission line. The velocity dispersion of the emitters (360 km s-1) is significantly smaller than that of the narrow band filter used for their selection (1600 km s-1). We therefore conclude that the emitters are associated with the radio galaxy. The star formation rates (SFRs) deduced from the Hαflux are in the range 6-44 $M_\odot$yr-1and the SFRdensity observed is 5-10 times higher than in the HDF-N at $z = 2.23$. The properties of the narrow emission lines indicate that the emitters are powered by star formation and contain very young (<100 Myr) stellar populations with moderately high metallicities.

We present the results of narrow-band and broad-band imaging with the Very Large Telescope of the field surrounding the radio galaxy TN J0924–2201 at $z = 5.2$. Fourteen candidate Lyαemitters with a rest-frame equivalent width of >20 Å were detected. Spectroscopy of 8 of these objects showed that 6 have redshifts similar to that of the radio galaxy. The density of emitters at the redshift of the radio galaxy is estimated to be a factor 1.5–6.2 higher than in the field, and comparable to the density of Lyαemitters in radio galaxy protoclusters at $z = 4.1, 3.1$and 2.2. The Lyαemitters near TN J0924–2201 could therefore be part of a structure that will evolve into a massive cluster. These observations confirm that substantial clustering of Lyαemitters occurs at $z > 5$and support the idea that radio galaxies pinpoint high density regions in the early Universe.

We report the discovery of a Lyman αemitting galaxy at $z = 6.518$. The single line was found in the 43 arcmin2VLT/FORS field by slitless spectroscopy limited to the atmospheric window at $\lambda \sim 9100$Å (sampling Lyαat $6.4 < z < 6.6$). Its counterpart is undetected in a deep Iband image and the line has an asymmetric appearance in a deeper follow-up spectrum. There are no plausible line identifications except for Lyαwith a flux of $1.9\times 10^{-17}$and rest frame equivalent width >80 Å. The lower limit to the star formation rate density at $z = 6.5$derived from our complete sample is $5\times10^{-4}~M_\odot$yr-1Mpc-3, consistent with measurements in the Subaru Deep Field and Hubble Ultra Deep Field but approximately ten times higher than in the Large Area Lyman Alpha survey. This Lyαemitter is among the very small sample of highest redshift galaxies known.

We present results from a Chandra X-ray Observatorystudy of the field X-ray source population in the vicinity of the radio galaxy MRC 1138-262. Many serendipitous X-ray sources are detected in an area of $8'\times8'$around the radio source and 90% are identified in our deep VLT images. The space density of such sources is higher than expected on the basis of the statistics of ROSATand Chandradeep surveys. The most likely explanation is in terms of a concentration of AGN associated with the protocluster at $z = 2.16$which was found around the radio galaxy in previous studies. Two sources have a confirmed spectroscopic redshift close to that of the radio galaxy, and for three more sources other observations suggest that they are associated with the protocluster. Four of these five X-ray sources form, together with the radio galaxy, a filament in the plane of the sky. The direction of the filament is similar to that of the radio source axis, the large scale distribution of the other protocluster members, the 150 kpc-sized emission-line halo and the extended X-ray emission associated with the radio galaxy.
The majority of optically identified X-ray sources in this field have properties consistent with type I AGN, a few could be soft, low luminosity galaxies, one is probably an obscured (type II) AGN and one is a star. These statistics are consistent with the results of deep X-ray surveys.

This letter presents a numerical study of the evolution of an emission line cloud of initial density 10 cm-3, temperature 104K, and size 200 pc, being overtaken by a strong shock wave. Whereas previous simple models proposed that such a cloud would either be completely destroyed, or simply shrink in size, our results show a different and more complex behaviour: due to rapid cooling, the cloud breaks up into many small and dense fragments, which can survive for a long time. We show that such rapid cooling behaviour is expected for a wide range of cloud and shock properties. This process applies to the evolution of emission line clouds being overtaken by the cocoon of a radio jet. The resulting small clouds would be Jeans unstable, and form stars. Our results thus give theoretical credibility to the process of jet induced star formation, one of the explanations for the alignment of the optical/UV and radio axis observed in high redshift radio galaxies.

We present spectroscopic observations of the rest-frame UV line emission around radio galaxy MRC 2104-242 at $z=2.49$, obtained with FORS1 on VLT Antu. The morphology of the halo is dominated by two spatially resolved regions. Lyαis extended by >12´´along the radio axis, C $\rm IV$and He $\rm II$are extended by ∼8´´. The overall spectrum is typical for that of high redshift radio galaxies. The most striking spatial variation is that N $\rm V $is present in the spectrum of the region associated with the center of the galaxy hosting the radio source, the northern region, while absent in the southern region. Assuming that the gas is photoionized by a hidden quasar, the difference in N $\rm V $emission can be explained by a metallicity gradient within the halo, with the northern region having a metallicity of $Z\approx1.5 Z_{\odot}$and $Z\leq0.4 Z_{\odot}$for the southern region. This is consistent with a scenario in which the gas is associated with a massive cooling flow or originates from the debris of the merging of two or more galaxies.

A& A, 367, L5-L8 (2001), DOI: 10.1051/0004-6361:20010041 The letter has been misprinted. Following this page the letter will be printed again with its original page numbers.