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

TL;DR

FOGGIE-based simulations show extended HI disks in Milky Way–mass halos and quantify their observability with interferometric surveys via synthetic 21-cm cubes. The study forwards models observational pipelines, including short-baseline filtering and CLEANing, across multiple survey configurations, revealing that 10–40% of CGM HI can be missed depending on CGM morphology and viewing angle. While the HI size–mass relation is generally consistent with observations, the CGM component is highly sensitive to instrument design, necessitating dual convolution with single-dish data to recover diffuse emission. Robust forward modeling and awareness of warp-induced kinematic signatures are essential for accurate interpretation and comparison between simulations and HI surveys. The results highlight the significant role of CGM-disk coupling in galaxy evolution studies and point toward integrated interferometric–single-dish approaches for a complete census of extended HI.

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 mimic the absence of short baselines in interferometric maps, we find that such observations can miss ~10-40% of diffuse emission, which preferentially removes low column density, low velocity dispersion gas outside the central disk. The amount of observable material depends strongly on its distribution and the system's observed orientation, preventing the formulation of a simple correction factor. Therefore, to fully characterize extended disks, their circumgalactic mediums, and the interfaces between them, dual convolutions including data from interferometers and large single-dish radio telescopes are required.

Figures (9)

• Figure 1: Scaling relations for the six H1$\,$ disks presented in this study. The points correspond to the mean values for a given system between $z=0$ and $z=0.5$, with bars showing the standard deviation. Top: The H1$\,$ Size-Mass relation broeils97-HISizeMass for these systems. $M_{HI}$ is calculated as the total disk H1$\,$ mass. $D_{HI}$ is twice the radius at which the mean column density drops below $1.25\times10^{20}~\rm cm^{-2}$. The solid (dashed) line corresponds to the observational correlation (and 3$\sigma$ scatter) found in wang16-HISizeMass. Bottom: The Baryonic Tully-Fisher relation. $M_{*}$ is calculated as the stellar mass within the disk. We plot the median values of the density-weighted rotational velocity curve, as it best approximates an asymptotic value. All six systems fall below the observed relation mcGaugh00-TullyFisher, although they show similar trends with increasing mass. The additional points (stars) denote the time-averaged peak of the rotational velocity curves, which align more closely.
• Figure 2: Effects of various steps of the synthetic H1$\,$ imaging pipeline for the three Less Populated galaxies at 10 Mpc at an inclination of 40°. Imaging parameters are selected to be analogous to lower resolution, high sensitivity MeerKAT observations (i.e., sensitivity $= 10^{18}$ cm$^{-2}$, beam size = 65, minimum baseline length = 35 m). The scale bar on the top right shows the maximum observable angle at this minimum baseline. The first column (Ideal) shows the ideal column density projections. The second column (Noisy) adds Gaussian noise with a standard deviation equal to 0.2 times the sensitivity limit. For this column and the final two, only sources identified as significant are plotted. The third column (Smoothed) shows the effects of a Gaussian smoothing kernel being applied to the data, which removes signals with small spatial scales. The final column (Filtered) shows the effect of spatially filtering the data. The three galaxies presented respond to this step in distinct ways, related to how much diffuse gas is present around the disk. This has little effect on Tempest and Maelstrom, but has a larger effect on Blizzard due to the presence of more diffuse signal.
• Figure 3: Same as Fig. \ref{['fig:interferometric_projections']}, but for the three More Populated systems. These systems have a much larger amount of both small-scale and diffuse material in the CGM that is lost in the smoothing and filtering steps, respectively.
• Figure 4: H1$\,$ covering fraction ($\mathcal{L}_{HI}$) as a function of impact parameter for the "smoothed" (solid) and "filtered" (dashed) moment-0 maps shown in Fig. \ref{['fig:interferometric_projections']} and Fig. \ref{['fig:interferometric_projections_2']}. As expected, the curve for Maelstrom does not change significantly, as there is little diffuse material that is filtered out. For both Blizzard and Hurricane, the filtered curve is shifted to the left by a few kpc due to diffuse material being filtered out. Covering fractions are calculated in a 2 kpc thick annuli centered on the given impact parameter.
• Figure 5: Log histogram of H1$\,$ column density ($N_{HI}$) versus velocity dispersion ($\sigma$) summed for the three Less Populated systems (Tempest, Maelstrom, and Blizzard). $N_{HI}$ was corrected for inclination (i) by a factor of $\cos{i}$. The color scale shows the normalized probability distribution of pixels. The contours show the 25th, 50th, 75th, and 95th percentile curves. The synthetic survey parameters are analogous to the MHONGOOSE survey's low-resolution case (beam FWHM $\sim 65$", sensitivity $\sim10^{18}~\rm{cm}^{-2}$, minimum baseline $= 29$ m). The plot on the left shows the distribution for the smoothed datacube (without filtering). There are two peaks in the distribution: high column density and high velocity dispersion gas in the top right, and low column density and lower velocity dispersion gas in the bottom left. The plot on the right shows the same distribution for the filtered datacubes. The high column density peak is largely unaffected; however, the low column density peak is greatly reduced by the filtering step. This implies that this gas is primarily diffuse and may not be visible to interferometric observations.