ALMA ACA detection of submillimeter emission associated with the west hot spot of the radio galaxy Pictor A

TL;DR

This study uses ALMA ACA submillimeter observations to analyze the far-infrared excess in the west hot spot of Pictor A, confirming its origin within the hot spot and providing insights into its magnetic turbulence and field strength.

Contribution

First submillimeter detection of the hot spot with ACA, clarifying the nature of the far-infrared excess and its relation to magnetic turbulence within the hot spot.

Findings

The submillimeter source matches the extrapolated synchrotron spectrum.

The far-infrared excess contribution is smaller than previously estimated.

The excess spectrum is harder than diffusive shock acceleration predictions.

Abstract

In order to investigate the far-infrared excess detected from the west hot spot of the radio galaxy Pictor A with the Herschel observatory, a submillimeter photometry is performed with the Atacama Compact Array (ACA) of the Atacama Large Millimeter/submillimeter Array at Band 8 with the reference frequency of 405 GHz. A submillimeter source is discovered at the radio peak of the hot spot. Because the 405 GHz flux density of the source, $80.7\pm3.1$ mJy, agrees with the extrapolation of the synchrotron radio spectrum, the far-infrared excess is suggested to exhibit no major contribution at the ACA band. In contrast, by subtracting the power-law spectrum tightly constrained by the radio and ACA data, the significance of the excess in the Herschel band is well confirmed. No diffuse submillimeter emission is detected within the ACA field of view, and thus, the excess is ascribed to the west hot spot itself. In comparison to the previous estimate based on the Herschel data, the relative contribution of the far-infrared excess is reduced by a factor of $\sim 1.5$. The spectrum of the excess below the far-infrared band is determined to be harder than that of the diffusive shock acceleration. This strengthens the previous interpretation that the excess originates via the magnetic turbulence in the substructures within the hot spot. The ACA data are utilized to evaluate the magnetic field strength of the excess and of diffuse radio structure associated to the hot spot.