Giant molecular filaments in the Milky Way II: The fourth Galactic
  quadrant
  Filamentary structures are common morphological features of the cold,
molecular interstellar medium (ISM). Recent studies have discovered massive,
hundred-parsec-scale filaments that may be connected to the large-scale,
Galactic spiral arm structure. Addressing the nature of these Giant Molecular
Filaments (GMFs) requires a census of their occurrence and properties. We
perform a systematic search of GMFs in the fourth Galactic quadrant and
determine their basic physical properties. We identify GMFs based on their dust
extinction signatures in near- and mid-infrared and velocity structure probed
by ^{13}CO line emission. We use the ^{13}CO line emission and ATLASGAL dust
emission data to estimate the total and dense gas masses of the GMFs. We
combine our sample with an earlier sample from literature and study the
Galactic environment of the GMFs. We identify nine GMFs in the fourth Galactic
quadrant; six are located in the Centaurus spiral arm and three in inter-arm
regions. Combining this sample with an earlier study using the same
identification criteria in the first Galactic quadrant results in 16 GMFs, nine
of which are located within spiral arms. The GMFs have sizes of 80-160 pc and
^{13}CO-derived masses between 5-90 x 10^{4} Msun. Their dense gas mass
fractions are between 1.5-37%, being higher in the GMFs connected to spiral
arms. We also compare the different GMF-identification methods and find that
emission and extinction based techniques overlap only partially, highlighting
the need to use both to achieve a complete census.
