Garret Cotter

Abstract:

In a companion Letter, Jones et al. report the discovery of a cosmic microwave background decrement, indicative of a distant cluster with mass ∼1015 M⊙, toward the quasar pair PC 1643+4631A, B (z = 3.79, 3.83, separation 1980). To search for the cluster responsible, we have obtained R-, J-, and K-band images of the field and have also carried out optical spectroscopy of selected objects in it. No such cluster is evident in these images. Assuming that the cluster causing the decrement is similar to massive clusters already known, our magnitude limits imply that it must lie at about or beyond z = 1. This provides independent support for the X-ray-based distance argument of Jones et al. The cluster must gravitationally lens objects behind it; for a cluster z around 1-2, the Einstein ring radius for sources at z ≈ 3.8 is ∼100″. Simple modeling, producing simultaneously the Sunyaev-Zeldovich effect and the lensing, shows that the source positions of quasars A and B lie within 1100 of each other and may indeed be coincident. The two quasar spectra are found to be remarkably similar apart from their 1% redshift difference. Assuming that A and B are images of a single quasar, we present a possible explanation of this difference.

Abstract:

We have obtained redshifts and infrared images for a sample of faint B2/6C radio sources, the fluxes of which are about six times fainter than those of sources in the bright 3C sample. We now have unambiguous redshifts for 90 per cent of the sources, making this the first faint radio sample with such complete redshift information. We find that the infrared Hubble diagrams (K versus z) of the 3C sample and the B2/6C sample are similar at a low redshift (z < 0.6), but by a redshift of z∼1 the 6C/B2 galaxies are on average ≃0.6 mag fainter in the K-band than are 3C galaxies at the same redshift. This suggests that the bright K magnitudes of 3C galaxies at z∼1 are not the result of stellar evolution, but of a correlation between infrared and radio luminosity. We also find that the infrared structures of B2/6C galaxies at z∼1 are less clearly aligned with their radio structures than are 3C galaxies at this redshift, implying that the strength of the alignment effect depends on radio luminosity. Finally, above a redshift of 2 we find that the dispersion in the K-z relation of the B2/6C sample is ≃2.7 times greater than that at low redshift, a result which is expected if, at these redshifts, we are beginning to penetrate into the epoch in which radio galaxies formed. © 1997 RAS.

Abstract:

In a 15 GHz Ryle Telescope observation of PC 1643+4631A, B, a pair of quasars at redshifts z = 3.79 and 3.83 separated by 198″ on the sky, we find a decrement in the cosmic microwave background (CMB) of -380 ± 64 μJy in a 110″ × 175″ beam. Assuming this to be a Sunyaev-Zeldovich effect due to an intervening cluster, the minimum magnitude of the central temperature decrement is 560 μK. A serendipitous ROSAT observation shows that there is no X-ray-luminous cluster in the direction of the decrement at z < 1. The implied gas mass is ≳2 × 1014 M⊙ (assuming a temperature of ∼5 keV), indicating a total mass of more than 1015 M⊙. This result demonstrates the existence of a massive system too distant to be detected by its emission, but which can be found via its imprint on the CMB. © 1997. The American Astronomical Society. All rights reserved.