Testing the angular-size versus redshift relation with compact radio sources

Tight cosmological constraints from the angular-size/redshift relation for ultra-compact radio sources

“…In support of statement (5) Figure (2) includes the dashed curve showing a high-redshift cut-off according to the Dabrowski et al (1995) model, with luminosity 2 × 10 24 Watts/Hz, separation 130 pc and Lorentz factor 10. An alternatively explanation would be a down-turn in L(z).…”

“…The saving grace for cosmic radio sources is that their luminosities are known to increase rapidly with increasing z (Dunlop & Peacock 1990), which luminosity evolution allows sources of larger angular size to remain within the flux-limited sample; this effect is enhanced for compact sources, which typically have flat spectra, by the term (1 + z) (1+α) in equation (2) below. For typical source parameters and a flux limit of 0.1 Jy, the calculations of Dabrowski et al (1995) (their Figure 4) suggest that orientation bias can be ignored. Nevertheless, in what follows I do not discount the possibility that this might not be the case.…”

“…Additionally, relativistic beaming is important in compact sources, so that the apparent radio power increases and the angular size decreases when the beam is close to the line of sight. For this reason the use of compact sources as standard measuring rods has been questioned by Dabrowski, Lasenby and Saunders (1995). Dabrowski et al (1995) use a simple model, comprising two identical but oppositely directed jets, and assume that the measured angular size corresponds to their separation projected onto the plane of the sky.…”

“…For this reason the use of compact sources as standard measuring rods has been questioned by Dabrowski, Lasenby and Saunders (1995). Dabrowski et al (1995) use a simple model, comprising two identical but oppositely directed jets, and assume that the measured angular size corresponds to their separation projected onto the plane of the sky. This model might be very misleading, as the counter-jet would be very much fainter than the forward one, and the source would appear to be very asymmetric.…”

“…A cosmology is needed for this plot, and I have pre-empted the results to be presented here by using Ω m = 0.24 and Ω Λ = 0.76; however, the significant qualitative aspects of the arguments I shall present are not sensitive to this choice. The dashed line is a curve according to the Dabrowski et al (1995) beaming model, for typical source parameters: actual luminosity 10 24 Watts/Hz, actual separation 65 parsecs and Lorentz factor 10. There is clear evidence that at any particular epoch individual luminosities are a function of size, and that for sources with z > ∼ 0.2 this relationship is an inverse correlation; empirically for z > ∼ 0.5 the relationship is well represented by…”