Professor Andrew Bunker

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:

THE most distant astronomical objects observed are quasars at redshifts of z ≃ 4.9 (ref. 1), corresponding to a time when the Universe was less than a billion years old. This leaves little time during which the quasars and their host galaxies could form. In principle, the evolutionary state of the host galaxies can be probed by determining how many stars have formed, but this task is not straightforward because light from the quasar itself overwhelms any accompanying starlight. High-redshift radio galaxies-the likely progenitors of luminous elliptical galaxies-provide better targets for such studies, as optical emissions from their active nuclei are observed to be faint. Here we report the discovery of a radio galaxy (6C0140 + 326) at z = 4.41 which shows no evidence for either a stellar continuum or an unobscured quasar nucleus. We conclude that the galaxy associated with the radio source is neither fully formed nor obviously in the process of forming stars. This implies that at least some giant elliptical galaxies are still immature at z ≃ 4.5, and that if the intense bursts of star formation thought to produce the bulk of their stellar populations occur during the radio-bright phase, these star-forming regions are obscured by dust and gas.