Mapping the Multiphase Structure of HI in the Low-latitude Intermediate-velocity Arch 1

We have analyzed the thermal and turbulent properties of the low-latitude intermediate-velocity Arch 1 (LLIV1). This was accomplished using archival H I emission and absorption data from two 21 cm line surveys: GHIGLS at 9'.4 resolution and DHIGLS at 1' resolution. The spectral decomposition code ROHSA was used to model the column density of different thermal phases and also to analyze an absorption measurement against the radio source 4C +66.09. From the latter, we found spin temperature T-s similar to 75 K, cold gas mass fraction f similar to 0.5, and turbulent sonic Mach number M-t similar to 3.4. Similar to the absorption line modeling against 4C +66.09, our best emission line decomposition model has no unstable gas across the whole field of view, suggesting that the thermal condensation and phase transition are not ongoing but rather have reached an equilibrium state. The cold phase of LLIV1 appears as a collection of elongated filaments that forms a closed structure within the field decomposed. These substructures follow the orientation of the overall large-scale cloud, along the diagonal of the GHIGLS field from northwest to southeast (in Galactic coordinates). The angular power spectrum of the cold phase is slightly shallower than that of the warm phase, quantifying that the cold phases have relatively more structure on small scales. Our spatially resolved map of the cold gas mass fraction in LLIV1 from DHIGLS reveals significant variations spanning the possible range of f, with a mean and standard deviation of 0.33 and 0.19, respectively.