Polarized light in high-redshift radio galaxies
HIGH-redshift radio galaxies are characterized by ultraviolet excesses1,2 associated with elongated continuum and emission-line structures aligned along their radio axes3,4. Commonly, these properties are interpreted in terms of jet-induced star formation. However, observations of a blue, polarized continuum knot in the low-redshift radio galaxy PKS2152 — 69 have suggested an alternative beaming/scattering model5,6. To test this model we have obtained imaging-polarimetry observations of two high-redshift radio galaxies: 3C277.2 (z = 0.766) and 3C368 (z = 1.132). We find that both sources are strongly linearly polarized at ultraviolet rest wavelengths, with E-vectors within 25° of perpendicular to the axis of the extended continuum and radio structures. This implies that a large proportion of the ultraviolet light in these objects does not come directly from stars and we suggest it is scattered radiation from the nucleus. These observations also have important implications for beaming models of the active nuclei and for our understanding of the evolution of the host galaxies.
Evidence that the z = 3.4 radio galaxy B2 0902+34 may be a protogalaxy
THE monochromatic luminosities of high-redshift (z > 3) radio galaxies rise steeply between wavelengths of about 2,000 and 5,000 Å, to form a characteristic 'red bump'1–6. It is usually assumed that this bump arises from the photospheric emission of red, post-main-sequence stars. For a sufficient number of stars of this type to have evolved, however, these galaxies must be at least 0.4–2 Gyr old; yet z=3 corresponds to only 1.7 Gyr after the Big Bang (assuming a Hubble constant of 50 km s−1 Mpc−1 and that Ω0 = 1), bringing the larger age estimates uncomfortably close to the beginning of the Universe. Here we show that, at least in the case of the high-redshift radio galaxy B2 0902 + 34, the basic assumption is incorrect: the red bump is caused not by photospheric emission from post-main-sequence stars, but by the presence of bright emission lines from doubly ionized oxygen. Both the spec-trum and the luminosity of the underlying continuum suggest that B2 0902 + 34 is a galaxy observed during its initial burst of star formation.
AXIOMATISABILITY OF WEAKLY FLAT, FLAT, AND PROJECTIVE S-ACTS
ABSTRACT This paper gives necessary and sufficient conditions on a monoid S that the class of flat left S-acts be (first order) axiomatisable. It presents also a streamlined version of the result of Stepanova that the class of projective left S-acts is axiomatisable if and only if the strongly flat left S-acts are axiomatisable and S is left perfect.