5.3 Masquerading PGs

Before looking at what a null detection of the starlight from PGs might mean it is worthwhile to consider if the problem lies in misunderstanding the nature of PGs and whether it is possible that PGs are masquerading as extragalactic objects of a type not yet considered: some galaxies exhibit poweful radio emission, resulting from a relativistic jet eminating from the core of the galaxy in which a black hole is believed to reside. This radio emission can be detected out to great distances and some of these objects are believed to be the site of large amounts star-formation: the source 3C 326.1 at z = 1.8 is forming stars at the rate of 300 M yr^-1 (McCarthy et al. 1987); while the source called 4C 41.17 at z = 3.8 emits 10¹³ L of starlight (where 1 L 3.8 x 10²⁶ W) and contains enormous amonts of dust (Dunlop et al. 1994). Even though these objects are a powerful probe of the general characteristics of galaxies at high redshift, their strong radio signature isolates them as a special class of galaxy - too extreme to be the ancestors of more normal spirals and ellipticals.

Quasi stellar objects (QSOs) are at least candidates on the grounds that they occupy a similar redshift range to that expected of PGs - the furthest QSO has a redshift close to 5 - however, the comparison cannot be taken further than that; QSOs are powered by material accretion onto a black hole and their optical/near-infrared emission is dominated by the output from this process rather than starlight. What QSOs do tell us is that the high abundance of heavy elements, inferred from the absorption lines in their spectra, indicates significant chemical enrichment of the Universe along random lines-of-sight before z 4. The lack of a strong resemblance between other high-redshift objects such as QSOs and the expected characteristics of PGs forces us to conclude that PGs are not being mistaken for other extragalactic sources in large numbers and it is necessary to look for other reasons for their non-detection.