Effects of Galactic Disc Inclination and Resolution on Observed GMC Properties and Larson's Scaling Relations

TL;DR

This study investigates how galactic disc inclination and observational resolution affect the measured properties of giant molecular clouds (GMCs) and the applicability of Larson's scaling relations, using hydrodynamical simulations and simulated ALMA observations.

Contribution

It provides a systematic comparison of GMC properties in simulated physical space and observational space under different inclinations and resolutions, highlighting biases in observational measurements.

Findings

PPV_face properties closely match PPP at high resolution

PPV_edge deviations increase at lower resolution

Larson's relations are sensitive to inclination and resolution

Abstract

With ALMA making it possible to resolve giant molecular clouds (GMCs) in other galaxies, it is becoming necessary to quantify the observational bias on measured GMC properties. Using a hydrodynamical simulation of a barred spiral galaxy, we compared the physical properties of GMCs formed in position-position-position space (PPP) to the observational position-position-velocity space (PPV). We assessed the effect of disc inclination: face-on (PPV_face) and edge-on (PPV_edge), and resolution: 1.5 pc versus 24 pc, on GMC properties and the further implications of using Larson's scaling relations for mass-radius and velocity dispersion-radius. The low-resolution PPV data are generated by simulating ALMA Cycle 3 observations using the CASA package. Results show that the median properties do not differ strongly between PPP and PPV_face under both resolutions, but PPV_edge clouds deviate from these two. The differences become magnified when switching to the lower, but more realistic resolution. The discrepancy can lead to opposite results for the virial parameter's measure of gravitational binding, and therefore the dynamical state of the clouds. The power-law indices for the two Larson's scaling relations decrease going from PPP, PPV_face to PPV_edge and decrease from high to low resolutions. We conclude that the relations are not entirely driven by the underlying physical origin and therefore have to be used with caution when considering the environmental dependence, dynamical state, and the extragalactic CO-to-H2 conversion factor of GMCs.