Effects of galactic disc inclination and resolution on observed GMC properties and Larson's scaling relations
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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.
