The Peculiar Radio Source M17 JVLA 35

TL;DR

This paper investigates the radio source M17 JVLA 35, presenting multi-frequency observations that suggest it is an extragalactic High Frequency Peaker affected by plasma scattering, challenging previous size expectations.

Contribution

It provides the first multi-frequency analysis of M17 JVLA 35, proposing plasma scattering as a key factor in its observed size and spectrum, and discusses its classification as an HFP.

Findings

Spectral turnover at ~13 GHz with a negative spectral index at higher frequencies.

Angular size of ~0.5 arcseconds at 8.46 GHz, larger than typical HFPs.

Proposed plasma scattering broadens the source size, consistent with observed spectrum.

Abstract

M17 JVLA 35 is a radio source detected in projection against the M17 HII region. In recent observations, its spectrum between 4.96 and 8.46 GHz was found to be positive and very steep, with $\alpha \geq 2.9 \pm 0.6$ ($S_\nu \propto \nu^\alpha$). Here we present Very Large Array observations made in the 18.5 to 36.5 GHz region that indicate a spectral turnover at $\sim$13 GHz and a negative spectral index ($\alpha \simeq -2.0$) at higher frequencies. The spectrum is consistent with that of an extragalactic High Frequency Peaker (HFP). However, M17 JVLA 35 has an angular size of $\sim0\rlap.{''}5$ at 8.46 GHz, while HFPs have extremely compact, milliarcsecond dimensions. We discuss other possible models for the spectrum of the source and do not find them feasible. Finally, we propose that M17 JVLA35 is indeed an HFP but that its angular size becomes broadened by plasma scattering as its radiation travels across M17. If our interpretation is correct, accurate measurements of the angular size of M17 JVLA35 across the centimeter range should reveal the expected $\nu^{-2}$ dependence.