Filamentary Accretion Flows in the Infrared Dark Cloud G14.225-0.506
  Revealed by ALMA
  Filaments are ubiquitous structures in molecular clouds and play an important
role in the mass assembly of stars. We present results of dynamical stability
analyses for filaments in the infrared dark cloud G14.225$-$0.506, where a
delayed onset of massive star formation was reported in the two hubs at the
convergence of multiple filaments of parsec length. Full-synthesis imaging is
performed with the Atacama Large Millimeter/submillimeter Array (ALMA) to map
the $\mathrm{N_2H^+} \; (1-0)$ emission in two hub-filament systems with a
spatial resolution of $\sim 0.034 \; \mathrm{pc}$. Kinematics are derived from
sophisticated spectral fitting algorithm that accounts for line blending, large
optical depth, and multiple velocity components. We identify five velocity
coherent filaments and derive their velocity gradients with principal component
analysis. The mass accretion rates along the filaments are up to $10^{-4} \;
\mathrm{M_\odot \, \mathrm{yr^{-1}}}$ and are significant enough to affect the
hub dynamics within one free-fall time ($\sim 10^5 \; \mathrm{yr}$). The
$\mathrm{N_2H^+}$ filaments are in equilibrium with virial parameter
$\alpha_\mathrm{vir} \sim 1.2$. We compare $\alpha_\mathrm{vir}$ measured in
the $\mathrm{N_2H^+}$ filaments, $\mathrm{NH_3}$ filaments, $870 \;
\mu\mathrm{m}$ dense clumps, and $3 \; \mathrm{mm}$ dense cores. The decreasing
trend in $\alpha_\mathrm{vir}$ with decreasing spatial scales persists,
suggesting an increasingly important role of gravity at small scales.
Meanwhile, $\alpha_\mathrm{vir}$ also decreases with decreasing non-thermal
motions. In combination with the absence of high-mass protostars and massive
cores, our results are consistent with the global hierarchical collapse
scenario.
