First polarization study of the M87 jet and active galactic nuclei at submillimeter wavelengths with ALMA

TL;DR

This study presents the first submillimeter polarization observations of M87 and other AGN with ALMA, revealing high Faraday rotation measures and magnetic field structures, crucial for understanding jet physics and calibrating VLBI data.

Contribution

It provides the first submillimeter polarized images of M87 and AGN, measuring high Faraday rotation and revealing magnetic field structures, aiding future VLBI imaging at 345 GHz.

Findings

High Faraday rotation measures exceeding 10^5 rad/m^2

Detection of RM gradients and sign reversals in M87 jet

First submillimeter polarized images of M87 and AGN

Abstract

We present full-polarization observations at $\lambda = 0.87$ mm (345 GHz) conducted with the Atacama Large Millimeter/submillimeter Array (ALMA) toward Messier 87 (M87) and seven other radio-loud active galactic nuclei (AGN). We determine polarization and Faraday properties of the targets, with linear polarization (LP) fractions spanning 1\% to 17\%, and Faraday rotation measures (RMs) exceeding $10^{5}$ rad m$^{-2}$. These RM values, 1-2 orders of magnitude higher than typically observed at $\lambda > 3$ mm, suggest denser Faraday screens or stronger magnetic fields in the (sub)millimeter emission regions. Additionally, we present the first submillimeter polarized images of the M87 jet and the observed AGN. In the M87 jet, we identify RM gradients and sign reversals, suggestive of a helical magnetic field structure extending over kiloparsec scales. These observations were part of the ALMA Band~7 Very Long Baseline Interferometry (VLBI) commissioning during the April 2021 Event Horizon Telescope (EHT) campaign. The results provide essential constraints for calibrating, analyzing, and interpreting VLBI data from the EHT at 345 GHz, representing a critical step toward submillimeter VLBI imaging.