Large-scale clustering amongst Fermi blazars; evidence for axis alignments?

TL;DR

This study finds significant large-scale clustering among Fermi-selected BL Lac objects, suggesting possible large-volume AGN axis alignments, while FSRQs do not show such clustering, supporting the hypothesis of axis alignments influencing polarization observations.

Contribution

It provides evidence for large-scale clustering of BL Lac objects and supports the idea of AGN axis alignments over large volumes of space, using gamma-ray data and machine learning classifications.

Findings

Significant clustering signal for BL Lac objects up to 30 degrees.

No clustering detected among FSRQs.

Supports the hypothesis of large-scale AGN axis alignments.

Abstract

We find evidence for large-scale clustering amongst Fermi-selected BL Lac objects but not amongst Fermi-selected FSRQs. Using two-point correlation functions we have investigated the clustering properties of different classes of objects from the Fermi LAT 4FGL catalogue. We wanted to test the idea based on optical polarization observations that there might be large volumes of space in which AGN axes are aligned. To do this we needed a clean sample of blazars as these are objects with their jet axes pointing towards the observer and Fermi sources provide such a sample. We find that high latitude Fermi sources taken as a whole show a significant clustering signal on scales up to 30 degrees. To investigate if all blazars behave in the same way we used the machine learning classifications of Kovacevic, et al. (2020), which are based only on gamma-ray information, to separate BL Lac-like objects from FSRQ-like objects. A possible explanation for the clustering signal we find amongst the BL Lac-like objects is that there are indeed large volumes of space in which AGN axes are aligned. This signal might be washed out in FSRQs since they occupy a much larger volume of space. Thus our results support the idea that large scale polarization alignments could originate from coherent alignments of AGN axes. We speculate that these axis alignments may be related to the well-known intrinsic alignments of galaxy optical position angles.