Figuring Out Gas & Galaxies In Enzo (FOGGIE). XIII. On the Observability of Extended HI Disks and Warps

TL;DR

This study uses cosmological simulations to analyze the properties and observability of extended HI disks around Milky Way-like galaxies, highlighting the impact of observational limitations on detecting diffuse gas.

Contribution

It provides synthetic HI emission maps and quantifies how interferometric observations can underestimate extended HI structures, emphasizing the need for combined observational approaches.

Findings

Interferometric maps can miss up to 15% of HI outside the central disk.

Most HI emission is retained at 20 Mpc with proper filtering, but diffuse gas detection is affected.

Observability of extended HI depends on system orientation and distribution, complicating corrections.

Abstract

Atomic Hydrogen (HI) is a useful tracer of gas in and around galaxies, and can be found in extended disk-like structures well beyond a system's optical extent. Here we investigate the properties of extended HI disks that emerge in six Milky Way-mass galaxies using cosmological zoom-in simulations from the Figuring Out Gas & Galaxies in Enzo (FOGGIE) suite. This paper focuses on the observability of the extended HI in these systems. We find overall agreement with observational constraints on the HI size-mass relation. To facilitate direct comparisons with observations, we present synthetic HI 21-cm emission cubes. By spatially filtering our synthetic cubes to characterize the absence of short baselines in interferometric maps, we find that such observations at 20 Mpc retain ~96%-99% of total HI emission on average, but can miss up to ~15% of HI signal outside the central disk due to missing short spacings. This effect is small for more isolated systems, but more significant for more strongly interacting systems, as there is more diffuse signal. This preferentially removes low column density, low velocity dispersion gas in the circumgalactic medium (CGM). The amount of observable material depends strongly on its distribution, distance, and the system's observed orientation, preventing the formulation of a simple correction factor. Therefore, to fully characterize extended disks, their CGMs, and the interfaces between them, including data from large single-dish radio telescopes is likely necessary.