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Abstract. 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 Lyα emitters 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 Lyα emitters in radio galaxy protoclusters at z = 4.1, 3.1 and 2.2. The Lyα emitters 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 Lyα emitters occurs at z > 5 and support the idea that radio galaxies pinpoint high density regions in the early Universe.

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We present high resolution X-ray observations of the narrow line radio galaxy PKS 1138−262 at z = 2.156 with the ACIS-S detector on the Chandra observatory. These observations show that the X-ray emission from 1138-262 is dominated by emission from the active galactic nucleus (AGN) with a 2 to 10 keV luminosity of 4 × 10 45 erg s −1 . The relative X-ray and radio properties of the AGN in 1138-262 are similar to those seen for the AGN in the archetype powerful radio galaxy Cygnus A.Between 10% and 25% (depending on energy) of the X-ray emission from 1138-262 is spatially extended on scales of 10 ′′ to 20 ′′ . The extended X-ray emission is elongated, with a major axis aligned with that of the radio source.While the X-ray and radio emissions are elongated on similar scales and position angles, there is no one-to-one correspondence between the radio and X-ray features in the source. The most likely origin for the extended X-ray emission in 1138-262 is thermal emission from shocked gas, although we cannot rule-out a contribution from inverse Compton emission. If the emission is from hot gas, the gas density is 0.05 cm −3 and the gas mass is 2.5 × 10 12 M ⊙ . The pressure in this hot gas is adequate to confine the radio emitting plasma and the optical line emitting gas. We set an upper limit of 1.5 × 10 44 erg s −1 to the (rest frame) 2 to 10 keV luminosity of any normal cluster atmosphere associated with 1138-262.No emission was detected from any of the Lyα emitting galaxies in the (proto-) cluster around 1138-262, outside of the Lyα halo of 1138-262 itself, to a (rest frame) 2 to 10 keV luminosity limit of 1.2 × 10 43 erg s −1 . Emission was detected from a z = 2.183 QSO located 2 ′ west of 1138-262 with a luminosity of 1.8 × 10 44 erg s −1 .

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Abstract. We review millimeter interferometric phase variations caused by variations in the precipitable water vapor content of the troposphere, and we discuss techniques proposed to correct for these variations. We present observations with the Very Large Array (VLA) at 22 and 43 GHz designed to test these techniques. We find that both the fast switching and paired array calibration techniques are effective at reducing tropospheric phase noise for radio interferometers. In both cases, the residual rms phase fluctuations after correction are independent of baseline length for b > b eff-These techniques allow for diffraction-limited imaging of faint sources on arbitrarily long baselines at millimeter wavelengths. We consider the technique of tropospheric phase correction using a measurement of the precipitable water vapor content of the troposphere via a radiometric measurement of the brightness temperature of the atmosphere. Required sensitivities range from 20 mK at 90 GHz to 1 K at 185 GHz for the millimeter array (MMA) and to 120 mK for the VLA at 22 GHz. The minimum gain stability requirement is 200 at 185 GHz at the MMA, assuming that the astronomical receivers are used for radiometry. This increases to 2000 for an uncooled system. The stability requirement is 450 for the cooled system at the VLA at 22 GHz. To perform absolute radiometric phase corrections also requires knowledge of the tropospheric parameters and models to an accuracy of a few percent. It may be possible to perform an "empirically calibrated" radiometric phase correction, in which the relationship between fluctuations in brightness temperature differences and fluctuations in interferometric phases is calibrated by observing a strong celestial calibrator at regular intervals. A number of questions remain concerning this technique, including the following: (1) Over what timescale and distance will this technique allow for radiometric phase corrections when switching between the source and the calibrator? (2) How often will calibration of the r• ms -qbrm s relationship be required?

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