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

M J M Marchã,I W A Browne

doi:10.1093/mnras/stab1872

Abstract

ABSTRACT We find evidence for large-scale clustering amongst Fermi-selected BL Lac objects but not amongst Fermi-selected flat spectrum radio quasars (FSRQs). Using two-point correlation functions, we have investigated the clustering properties of different classes of objects from the Fermi–LAT(Large Area Telescope) 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°. To investigate if all blazars behave in the same way we used he machine learning classifications from the literature, 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.

Highlights

In the widely accepted concordance model of cosmology the largest structures expected in the Universe are 1 Gpc (Yadav et al, 2010; Marinello et al, 2016)
Since in our initial analysis we find that the objects associated with BL Lacs and flat-spectrum radio quasars (FSRQs) have di erent clustering properties we suspected that this could arise from a selection e ect, a ecting BL Lac associations
When the objects classified by machine learning algorithms as listed by Kovacevi∆ et al are considered, a significant clustering signal on scales in excess of 6 degrees is present for the objects classified as BL Lac-like but not for those classified as FSRQs

Summary

INTRODUCTION

In the widely accepted concordance model of cosmology the largest structures expected in the Universe are 1 Gpc (Yadav et al, 2010; Marinello et al, 2016). A dipole stronger than expected is found when the distribution of quasars detected in the infrared by WISE is analysed (Secrest et al, 2020) and for AGN in the infrared-selected MIRAGN sample (Singal, 2021) If these claims, those related to the polarization alignment, are substantiated the most likely astrophysical interpretation of the polarization position angle results is that the axes, presumably the angular momentum axes of the black holes powering the AGN, are coherently aligned over large volumes of space. The distribution of structural position angles of radio sources in the FIRST survey (Panwar et al.), FIRST and TGSS (Contigiani et al.) and LoTSS survey (Osinga et al.) Both investigations find some evidence for alignments. We will discuss if the di erence we find between the clustering properties of BL Lac-like and FSRQ-like objects is of astrophysical origin

THE TWO-POINT ANALYSIS

Assessing the statistical significance

DISCUSSION

Findings

Evidence for clustering using machine learning classifications

SUMMARY AND CONCLUSIONS