Sensitive Observations of Radio Recombination Lines in Orion and W51: The Data and Detection of Systematic Recombination Line Blueshifts Proportional to Impact Broadening

TL;DR

This study presents sensitive radio recombination line observations in Orion and W51, revealing systematic blueshifts proportional to impact broadening, with detection of high quantum number lines and unprecedented spectral dynamic range.

Contribution

It reports the first detection of systematic recombination line blueshifts proportional to impact broadening in Orion, using high-sensitivity spectral data with advanced baseline removal techniques.

Findings

Detection of hydrogen recombination lines with Δn up to 25 in Orion.

Identification of systematic line center shifts proportional to linewidth.

Achieved high dynamic range enabling detection of very weak lines.

Abstract

Sensitive spectral observations made in two frequency bands near 6.0 and 17.6 GHz are described for Orion and W51. Using frequency switching we were able to achieve a dynamic range in excess of 10,000 without fitting sinusoidal or polynomial baselines. This enabled us to detect lines as weak as T$_{A} ~1mK in these strong continuum sources. Hydrogen recombination lines with $\Delta n$ as high as 25 have been detected in Orion. In the Orion data, where the lines are stronger, we have also detected a systematic shift in the line center frequencies proportional to linewidth that cannot be explained by normal optical depth effects.