Bird's eye view of molecular clouds in the Milky Way: I. Column density
  and star formation from sub-pc to kpc scales
  Describing how the properties of the interstellar medium combine across
size-scales is crucial for understanding star formation scaling laws and
connecting Galactic and extragalactic data of molecular clouds. We describe how
the statistical structure of clouds, and its connection to star formation,
changes from sub-parsec to kiloparsec scales in a complete region within the
Milky Way disk. We build a census of molecular clouds within 2 kpc from the Sun
using literature. We examine the dust-based column density probability
distributions (N-PDFs) of the clouds and their relation to star formation
traced by young stellar objects (YSOs). We then examine our survey region from
the outside, within apertures of varying sizes, and describe how the N-PDFs and
their relation to star formation changes with the size-scale. The N-PDFs of the
clouds are not well described by any single simple model; use of any single
model may bias the interpretation of the N-PDFs. The top-heaviness of the
N-PDFs correlates with star formation activity, and the correlation changes
with Galactic environment (spiral-/inter-arm regions). We find that the density
contrast of clouds may be more intimately linked to star formation than the
dense gas mass fraction. The aperture-averaged N-PDFs vary with the size-scale
and are more top-heavy for larger apertures. The top-heaviness of the aperture
N-PDFs correlates with star formation activity up to roughly 0.5 kpc
size-scale, depending on the environment. Our results suggest that the
relations between cloud structure and star formation are environment specific
and best captured by relative quantities (e.g., the density contrast). Finally,
we show how the density structures of individual clouds give rise to a
kpc-scale Kennicutt-Schmidt relationship as a combination of sampling effects
and blending of different galactic environments.
