A determination of the cosmic microwave background temperature using Galactic molecules

TL;DR

This study measures the CN excitation temperature in diffuse Galactic clouds with high precision, providing a new independent estimate of the cosmic microwave background temperature that aligns with previous measurements.

Contribution

The paper presents a reliable, high-resolution spectroscopic method to determine CN excitation temperature, offering an independent and precise measurement of the CMB temperature.

Findings

CN excitation temperature is consistent with the CMB temperature.

Achieved a 3% measurement accuracy of the CN excitation temperature.

Data can be fit with a temperature of 2.725 K, matching the CMB temperature.

Abstract

We report a new, reliable determination of the CN excitation temperature of diffuse molecular clouds in the Milky Way, based on ultra high spectral resolution observations. Our determination is based on CN $B^{2}\Sigma^{+}-X^{2}\Sigma^{+}$ (0,0) vibronic absorption spectra seen along the lines of sight to eight bright Galactic stars. Our analysis is conducted blind, and we account for multiple sources of systematic uncertainty. Like previous studies, our excitation temperature measures exhibit an intrinsic scatter that exceeds the quoted uncertainties. Accounting for this scatter, we derive a 3% determination of the typical CN excitation temperature, $T_{01}=2.769^{+0.084}_{-0.072}~{\rm K}$, which is consistent with the direct determination of the cosmic microwave background temperature. We also perform a single joint fit to all sightlines, and find that our data can be simultaneously fit with an excitation temperature $T_{01}=2.725\pm0.015~{\rm K}$ -- a 0.55% measure that is consistent with the CMB temperature. We propose a future observational strategy to reduce systematic uncertainties and firmly test the limitations of using CN as a cosmic microwave background thermometer.