Unified models revisited I: modelling the effect of source geometry on radio galaxy/quasar unification

TL;DR

This paper revisits the radio-loud AGN unification paradigm by replacing the simple stick-model projection with a cylindrical geometry that includes finite width and realistic distributions of intrinsic sizes and axial ratios. Using Mullin et al. (2008) FRII data to calibrate an intrinsic-length and axial-ratio model, the authors perform extensive Monte Carlo simulations across critical angles and sample sizes, incorporating resolution and redshift effects from LoTSS data. They find that the predicted quasar-to-radio-galaxy length-ratio distributions peak near 0.8–0.9, and that robust tests require large, homogeneous samples (approximately 800 sources or more). Resolution and redshift truncation have only modest impacts, suggesting that previous mixed conclusions arose from small or biased samples and overly simplistic projection assumptions. The work underscores the importance of large, uniform surveys (such as LOFAR deep/wide fields) for definitively testing orientation-based unification models.

Abstract

The orientation-based unification model proposes that radio-loud quasars and radio galaxies are the same objects observed at different angles. A key prediction of this model is that the quasars are seen at smaller angles to the line of sight and so should be more affected by projection, and hence apparently physically smaller, than corresponding radio galaxies, but this has not always been found in earlier studies. We argue that the interpretation of observations requires a less simplistic model for the effects of projection, which takes into account radio sources' finite width and their intrinsic axial ratio distribution. Using this cylindrical configuration as a basis for the simulation of radio galaxies and quasars, we simulate the distribution of the linear size ratio of quasars to radio galaxies for different sample sizes and critical angles. Our simulations that predict the ratio of observed lengths in the presence of a distribution of intrinsic physical sizes and axial ratios that we derive from observation. We conclude that to test the unified scheme, samples should be completely optically identified, sizes should be measurable for all targets, and the sample size should be greater than $\sim 800$ sources. Such large samples with uniform optical identification and accurate size measurements have not been available in previous work, but should become available from wide-area sky surveys in the near future.

Figures (6)

• Figure 1: Representation of the cylindrical model for a source tilted at angle $\theta$ from the line of sight.
• Figure 2: Quasar versus radio galaxy diagnostics in different models. The left plot shows ratio of source type numbers ratio (quasar to radio galaxy) and the right plot shows the mean length ratio (quasar to radio galaxy) as a function of the critical angle. Note the strong dependence of measured length ratio on the axial ratio.
• Figure 3: Distribution of projected lengths obtained for a sample of 98 sources using simulation. Overlaid is the distribution of projected lengths reported by Mullinetal2008. The legends also show the statistic and p-value obtained using the two-sample KS test.
• Figure 4: Distribution of projected axial ratios obtained for a sample of 98 sources using simulation. Overlaid is the distribution of projected axial ratios reported by Mullinetal2008. The legends also show the statistic and $p$-value obtained using the two-sample KS test.
• Figure 5: Projected length of the sources versus the observed axial ratio of the source, comparing the results of our simulation with 150 sources and the distribution observed by Mullinetal2008. We see good agreement between the two distributions.