Constraints on OH Megamaser Excitation from a Survey of OH Satellite Lines

TL;DR

This study conducted the first comprehensive survey of all four 18 cm OH lines, including satellite lines, in 77 OH megamasers, revealing rare satellite line emission and supporting models where all lines share the same excitation temperature.

Contribution

It provides the first observational constraints on satellite line excitation in OH megamasers, including the discovery of conjugate satellite lines in one source.

Findings

Satellite line emission detected in 5 sources.

First observation of conjugate satellite lines in an OHM.

Results support models with uniform excitation temperature for all OH lines.

Abstract

We report the results of a full-Stokes survey of all four 18 cm OH lines in 77 OH megamasers (OHMs) using the Arecibo Observatory. This is the first survey of OHMs that included observations of the OH satellite lines; only 4 of the 77 OHMs have existing satellite line observations in the literature. In 5 sources, satellite line emission is detected, with 3 of the 5 sources re-detections of previously published sources. The 2 sources with new detections of satellite line emission are IRAS F10173+0829, which was detected at 1720 MHz, and IRAS F15107+0724, for which both the 1612 MHz and 1720 MHz lines were detected. In IRAS F15107+0724, the satellite lines are partially conjugate, as 1720 MHz absorption and 1612 MHz emission have the same structure at some velocities within the source, along with additional broader 1612 MHz emission. This is the first observed example of conjugate satellite lines in an OHM. In the remaining sources, no satellite line emission is observed. The detections and upper limits are generally consistent with models of OHM emission in which all of the 18 cm OH lines have the same excitation temperature. There is no evidence for a significant population of strong satellite line emitters among OHMs.