On the Beam Filling Factors of Molecular Clouds

TL;DR

This study investigates how observational parameters like beam size and noise affect measurements of molecular clouds in the Milky Way, providing practical references for interpreting survey data.

Contribution

It introduces a method to quantify the effects of angular resolution and sensitivity on molecular cloud observations using high-quality CO data as a benchmark.

Findings

12CO has the highest beam filling and sensitivity clip factors.

The beam filling factor relates to a characteristic size of 0.762 times the beam size.

Sensitivity clip factor correlates with the mean voxel SNR of clouds.

Abstract

Imaging surveys of CO and other molecular transition lines are fundamental to measuring the large-scale distribution of molecular gas in the Milky Way. Due to finite angular resolution and sensitivity, however, observational effects are inevitable in the surveys, but few studies are available on the extent of uncertainties involved. The purpose of this work is to investigate the dependence of observations on angular resolution (beam sizes), sensitivity (noise levels), distances, and molecular tracers. To this end, we use high-quality CO images of a large-scale region (25.8 <l< 49.7 deg and |b|<5 deg) mapped by the Milky Way Imaging Scroll Painting (MWISP) survey as a benchmark to simulate observations with larger beam sizes and higher noise levels, deriving corresponding beam filling and sensitivity clip factors. The sensitivity clip factor is defined to be the completeness of observed flux. Taking the entire image as a whole object, we found that 12CO has the largest beam filling and sensitivity clip factors and C18O has the lowest. For molecular cloud samples extracted from images, the beam filling factor can be described by a characteristic size, $l_{1/4}$=0.762 (in beam size), at which the beam filling factor is approximately 1/4. The sensitivity clip factor shows a similar relationship but is more correlated with the mean voxel signal-to-noise ratio of molecular clouds. This result may serve as a practical reference on beam filling and sensitivity clip factors in further analyses of the MWISP data and other observations.