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arxiv: 2606.28150 · v1 · pith:ORPCWDVPnew · submitted 2026-06-26 · 🌌 astro-ph.GA

Deep HI observations of cold gas inflow and outflow

W.J.G. de Blok , D.B. Fisher , K. Tachihara , F.M. Maccagni , J. Wang , R. Enokiya , L. Chemin , T. Hayakawa
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D. Kleiner D.J. Pisano S.H. Oh
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keywords HI observationsgas inflowgas outflowgalaxy evolutionSKA-Midcold gasdisk galaxiesdwarf galaxies
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The pith

Deep HI observations with high resolution and low column density sensitivity are key missing ingredients for mapping gas flows in galaxy evolution.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes that galaxies need ongoing gas supply to maintain star formation, yet current HI data lack the angular resolution and depth to map inflows and outflows at very low column densities. It reviews recent progress on these gas processes and identifies SKA-Mid AA4 observations of nearby disk and dwarf galaxies as the practical route to fill the gap. A sympathetic reader would care because these flows determine how galaxies grow and evolve, and the same mechanisms must be understood to interpret distant-universe observations and to calibrate simulation sub-grid recipes.

Core claim

The central claim is that high-angular-resolution, high-sensitivity HI observations capable of reaching very low column densities remain essential but unavailable, and that targeted deep observations of individual nearby galaxies with SKA-Mid will supply the missing constraints on inflow and outflow processes.

What carries the argument

Deep HI mapping of inflow and outflow in individual nearby disk and dwarf galaxies, enabled by next-generation radio telescope sensitivity and resolution.

Load-bearing premise

That the processes controlling gas supply and removal in galaxies cannot be correctly interpreted without spatially resolved maps that reach much lower column densities than current observations provide.

What would settle it

A set of SKA-Mid AA4 maps of nearby galaxies that show no additional inflow or outflow structures below current column-density limits would indicate that the claimed observational gap is smaller than stated.

Figures

Figures reproduced from arXiv: 2606.28150 by D.B. Fisher, D.J. Pisano, D. Kleiner, F.M. Maccagni, J. Wang, K. Tachihara, L. Chemin, R. Enokiya, S.H. Oh, T. Hayakawa, W.J.G. de Blok.

Figure 1
Figure 1. Figure 1: MHONGOOSE H i distributions (cyan), combined with Galex UV (pink) and Legacy Survey optical images (background) of four sample galaxies each differing about an order of magnitude in H i mass. A 10 kpc scale bar is shown in the top-left of each panel, the H i mass in the top-right. The HIPASS identification is shown in the bottom-left. These correspond to (top-left to bottom-right) KK98-195, ESO 302-G014, N… view at source ↗
Figure 2
Figure 2. Figure 2: Sensitivity versus resolution in H i surveys. Colored symbols show the 3𝜎 column density sensitivity over 16 km s−1 for various interferometric and single-dish surveys, as indicated by the labels and legend and listed in the text. The thick blue line shows the observed MHONGOOSE sensitivities. MHONGOOSE reaches single-dish sensitivities but at a 10–50 times better angular resolution. To give an indication … view at source ↗
Figure 3
Figure 3. Figure 3: H i moment 0 maps from VLA observations of the nearest, accessible outflow systems, M 82 (left, Martini et al., 2018) and NGC 1569 (right, Johnson et al., 2012). In both cases streams of H i extend away from the disk of the galaxy. These galaxies are understood from optical and X-ray observations to be driving large scale winds (review in Veilleux et al., 2005, 2020), and the H i observed perpendicular to … view at source ↗
Figure 4
Figure 4. Figure 4: Mazzilli Ciraulo et al. (2025) compares ionised gas to H i mass outflow rates in all 3 galaxies that have resolved observations of each. All current data suggests that H i dominates that mass-outflow rate by a factor of ∼ 5 − 10× over the warm ionised gas. The trend in the relationship of 𝑀¤ 𝑜𝑢𝑡/𝑆𝐹𝑅 with 𝑀𝑠𝑡 𝑎𝑟 is a direct test of galaxy evolution models (Pandya et al., 2021; Wright et al., 2024). galaxy e… view at source ↗
Figure 5
Figure 5. Figure 5: H i moment 0 map of the outflow in NGC 4666 is compared to H𝛼 map of the same target. The biconical outflow is evident at higher spatial resolution, and H i contours show a response to the outflow. Using current facilities this can be achieved only on the nearest targets (≲15 Mpc), and requires prohibitively long exposures with MeerKAT or VLA. of the galaxy (Ostriker and Shetty, 2011). The feedback acts in… view at source ↗
Figure 6
Figure 6. Figure 6: Left: Integrated H i intensity map of the IVC “PP Arch” identified from HI4PI data. A characteristic head–tail morphology is evident. Right: Longitude–velocity diagram around the head region of the PP Arch. The IVC shows a blue-shift of about 40 km s−1 relative to the nearby local cloud at ∼ 0 km s−1 , and a diffuse bridge structure connects the two. A cavity is also seen in the local cloud, likely formed … view at source ↗
Figure 7
Figure 7. Figure 7: (a) Integrated H i intensity map of the head region of the PP Arch for velocities between −60 and −30 km s−1 . Two IVCs, IVC +86–36 and IVC +97–30, are seen. (b) The same region integrated over −30 to +30 km s−1 , showing the nearby local cloud. Contours indicate the IVCs from panel (a). (c) Distribution of dust optical depth 𝜏353 in the same region. Contours again trace the IVCs from panel (a). The high-l… view at source ↗
Figure 8
Figure 8. Figure 8: Correlation plots between the integrated H i intensity 𝑊(HI) and the dust optical depth 𝜏353 for the region shown in [PITH_FULL_IMAGE:figures/full_fig_p014_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Distribution of IVCs in the northern Galactic hemisphere (𝑏 > 0 ◦ ), covering Galactic longitudes 90◦–270◦ . Colors indicate the relative metallicity of the interstellar medium, with blue representing metal￾poor and red representing metal-rich gas. White circles mark the positions of background sources toward which absorption-line measurements have been obtained. Modified from Hayakawa and Fukui (2024). on… view at source ↗
Figure 10
Figure 10. Figure 10: Histograms of the relative dust-to-gas ratio with respect to the average value of local clouds near the Sun, for (a) local clouds, (b) IVCs, (c) HVCs, and (d) the Magellanic Stream. Modified from Hayakawa and Fukui (2024). 1998; Brüns et al., 2005). Its total mass is estimated to be about 1.6 × 108 , 𝑀⊙ (Brüns et al., 2005). In comparison, the total mass of all HVCs is estimated to be ∼ 2.5 × 108 , 𝑀⊙ (Wa… view at source ↗
read the original abstract

A major question in galaxy evolution is how galaxies acquire sufficient gas to sustain their star formation rates. HI observations with high angular resolution and sensitivity to very low column densities are some of the important observational ingredients that are currently still missing. Answers to these questions are necessary for a correct interpretation of observations of galaxy evolution in the high-redshift universe and will provide crucial input for the sub-grid physics in hydrodynamical simulations of galaxy evolutions. In this chapter we discuss the progress that has been made over the past years, describe the various processes that lead to inflow and outflow of gas, and discuss how SKA-Mid AA4 observations can contribute to further understanding these important aspects of galaxy evolution using deep observations of nearby individual disk and dwarf galaxies.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

0 major / 0 minor

Summary. The manuscript is a forward-looking discussion chapter on the observational requirements for studying cold gas inflow and outflow via HI in nearby galaxies. It states that high angular resolution and sensitivity to very low column densities remain missing ingredients, reviews progress and physical processes, and argues that SKA-Mid AA4 deep observations of individual disk and dwarf galaxies will be essential for interpreting high-redshift galaxy evolution and constraining sub-grid physics in simulations.

Significance. If the field-status assessment is accurate, the chapter usefully synthesizes current gaps and the prospective role of SKA-Mid AA4. Its contribution is primarily as a review of observational needs rather than new data, derivations, or predictions; no machine-checked proofs, reproducible code, or falsifiable quantitative forecasts are presented.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for their positive assessment of the manuscript as a forward-looking discussion chapter on HI observations of cold gas inflow and outflow, and for the recommendation to accept. No major comments were raised in the report.

Circularity Check

0 steps flagged

No derivation chain; forward-looking review with no quantitative claims

full rationale

The manuscript is a discussion chapter reviewing observational gaps in HI studies of inflows/outflows and the prospective role of SKA-Mid AA4. It advances no new empirical result, derivation, model, or quantitative prediction. The statement that high-resolution low-column-density observations are 'still missing' is a field-status assessment rather than a proposition resting on a specific, falsifiable assumption or internal logic that reduces to inputs. No equations, fits, or self-citation chains are present that could exhibit circularity. The text is self-contained as a review.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

No free parameters, axioms, or invented entities are introduced; the text is a review chapter.

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