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The H IX galaxy survey IV: What does the molecular gas reveal in HI eXtreme galaxies

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V. A. Capa, H. Dénes, K. A. Lutz, T. H. Brown

Astrophysics of Galaxies · Submitted 20 Oct 2026 · 82 references

Pith's verdict: Useful first ALMA CO(1-0) data on five HIX galaxies with a plausible HI-to-H2 bottleneck story, but the paper's own Table 7 contradicts its three-stable-discs claim, so the stability part needs a major correction.

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CitationH. D\'enes, K. A. Lutz, T. H. Brown, V. A. Capa. "The H IX galaxy survey IV: What does the molecular gas reveal in HI eXtreme galaxies." arXiv:2610.28970 (2026). https://pith.science/paper/I4XGDTFK
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The paper

The paper claims

The star-formation bottleneck in HI-eXtreme galaxies is the atomic-to-molecular hydrogen conversion step, because their giant atomic discs are turbulent, angular-momentum-rich, and globally stable against gravitational collapse.

The reading

Galaxies with extreme atomic-hydrogen reservoirs, the HI-eXtreme galaxies of the HIX survey, form stars at only normal rates, and this paper sets out to find where the star-formation chain is blocked. New ALMA CO(1-0) observations of five such systems show that their molecular gas discs are compact and ordinary in mass, while their atomic gas discs are enormous, turbulent, and carry most of the galaxy's angular momentum. The paper computes a global stability parameter $q$ and finds that in three of the five galaxies the atomic disc lies above the theoretical limit for gravitational collapse, so the atomic gas cannot easily fragment into the molecular clouds that feed star formation. Atomic-hydrogen depletion times are accordingly longer than the age of the universe in the most extreme cases, while molecular-gas depletion times are normal or short, pinning the bottleneck on the HI-to-H2 conversion step rather than on star formation itself, and pointing future work at the physics that lets atomic gas become molecular.

What carries the argument

The central object is the dimensionless global stability parameter $q = j_{\rm bar}\sigma_{\rm HI}/(G M_{\rm bar})$ introduced by Obreschkow et al. (2016), a disc-averaged analogue of the Toomre parameter that combines baryonic specific angular momentum, HI velocity dispersion, and baryonic mass; a disc with $q$ above $1/\sqrt{2e}$ is globally stable against gravitational collapse. The parameter is fed by dynamical modelling of the ALMA CO and ATCA HI data cubes with the 3DBarolo code, which reconstructs each galaxy as concentric rotating rings and yields the rotation curves, surface density profiles, and velocity dispersions; the specific angular momentum of each component is the mass-weighted sum $j_{\rm disc} = \sum_i M_i v_{{\rm rot},i} r_i / \sum_i M_i$ over the fitted rings, with the stellar component assumed to follow the HI rotation curve. The velocity dispersion entering $q$ is the mode of a log-normal fit to the second-moment maps, a choice meant to avoid the long beam-smearing tail of the distribution.

Extended reading notes

Core claim

The paper's central claim is that HI-eXtreme galaxies fail to make stars because they fail to make molecular gas, not because the molecular gas they already have is used inefficiently. In all five galaxies the CO(1-0) disc is compact, roughly coincident with the inner stellar disc, and has a normal $\mathrm{H_2}$ mass for its stellar mass, whereas the extended HI disc dominates the baryonic angular momentum, carrying roughly an order of magnitude more specific angular momentum than the stellar or molecular discs. Dynamical modelling with 3DBarolo shows that the HI and CO discs are kinematically consistent and that both show velocity dispersions above the thermal value of about 8 km/s, indicating genuine turbulence. From the baryonic specific angular momentum $j_{\rm bar}$, the HI velocity dispersion mode $\sigma_{\rm HI}$, and the baryonic mass $M_{\rm bar}$, the paper forms the dimensionless stability parameter $q = j_{\rm bar}\sigma_{\rm HI}/(G M_{\rm bar})$; for ESO208-G026, ESO378-G003, and ESO417-G018, $q$ exceeds $1/\sqrt{2e}$, the threshold above which an atomic disc is globally stable against gravitational collapse. The resulting HI depletion times are extremely long, in the most extreme cases longer than the age of the universe, while $\mathrm{H_2}$ depletion times are normal or short, and the paper attributes the angular-momentum excess to past gas accretion or recent tidal interactions, with positive tidal indices for ESO378-G003 and ESO381-G005.

Load-bearing premise

The stability argument assumes that the HI velocity-dispersion mode of 11-22 km/s used in $q$ measures true disc turbulence; if part of that dispersion is beam smearing or non-circular motion rather than turbulence, $q$ is biased upward and the claimed global stability of the HI discs weakens.

Editorial extensions

If this is right

  • HI-eXtreme galaxies can be read as a transient phase of galaxy growth: their excess atomic gas has not yet condensed into molecular form, so the sample traces how externally supplied gas is processed after accretion.
  • Galaxy-scale surveys that rely on atomic gas alone to forecast star formation will systematically overpredict star formation in high-angular-momentum discs, so molecular gas must be measured directly.
  • The stability parameter $q$ can serve as a selection tool for finding galaxies where the HI-to-H2 step is suppressed, offering a clean testbed for the physics of molecular cloud formation.
  • Tidal interactions and cold gas accretion, rather than internal secular evolution, emerge as the delivery mechanisms of the high-angular-momentum gas, making HI-eXtreme galaxies useful probes of how galaxies acquire and process accreted gas.
  • The same bottleneck behaviour appears in the comparison galaxies UGC 6168 and UGC 7899 in the baryonic angular-momentum plane, suggesting the mechanism operates beyond the five galaxies observed here.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If the stability reading is right, a testable corollary follows: high-resolution HI observations should find few self-gravitating atomic clouds in the outer discs, since global stability suppresses fragmentation, and counting such clouds at sub-kpc resolution would test this prediction directly.
  • The same logic implies a radial gradient within each galaxy: locally computed $q$ should rise with radius, so HI-to-H2 conversion efficiency should drop outward, and deep CO mapping of the outer discs could verify that no molecular gas forms where $q$ is highest.
  • The mechanism may generalise beyond the extreme sample, with ordinary high-spin spirals showing the same suppression in milder form, placing HI-eXtreme galaxies at the end of a continuous angular-momentum sequence rather than in a separate class.
  • Comparing HI-eXtreme galaxies with gas-rich dwarfs that occupy the same high-$q$, high-atomic-fraction parameter space would show whether one stability mechanism explains low star formation across both populations, which the paper itself does not test.
Original abstract

Galaxies with extreme amounts of neutral atomic hydrogen (HI), known as HI-eXtreme (HIX) galaxies, paradoxically exhibit normal or low star formation rates. To investigate the bottleneck in their star formation, we present ALMA CO(1-0) observations for five HIX survey galaxies: ESO208-G026, ESO378-G003, ESO381-G005, ESO417-G018, and ESO055-G013. While these systems possess massive, extended HI discs, their molecular gas discs are compact with normal H$_{2}$ masses relative to their stellar mass. Dynamical modelling with 3DBarolo reveals that the HI and molecular gas are kinematically consistent. Both gas phases show high velocity dispersions, indicating enhanced turbulence compared to regular late-type galaxies. We calculate the specific angular momentum for the stellar, atomic, and molecular components alongside the global stability parameter ($q$). Most of the system angular momentum is stored in the HI discs, rendering them globally stable against collapse. This stability suppresses the atomic-to-molecular gas conversion, as evidenced by extremely long HI depletion times contrasted with typical H$_{2}$ depletion times. The high angular momentum and HI excess likely originate from past gas accretion or recent tidal interactions; this is especially apparent in ESO378-G003 and ESO381-G005, which exhibit extended HI structures and positive tidal indices.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13.0 via TypeSafe API, question bank abstract-p1-20260923, 2026-10-22: 1 request, 7,055 input tokens.

What the abstract itself states, as probabilities: claims first 0.03 · states limitation 0.11 · human participants 0.01 · controlled comparison 0.47 · randomized assignment 0.01 · states quantitative result 0.34 · compares with prior results 0.54 · mentions code or data availability 0.03.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; .

Galaxies with extreme amounts of neutral atomic hydrogen (HI), known as HI-eXtreme (HIX) galaxies, paradoxically exhibit normal or low star formation rates. To investigate the bottleneck in their star formation, we present ALMA CO(1-0) observations for five HIX survey galaxies: ESO208-G026, ESO378-G003, ESO381-G005, ESO417-G018, and ESO055-G013. While these systems possess massive, extended HI discs, their molecular gas discs are compact with normal H$_{2}$ masses relative to their stellar mass. Dynamical modelling with 3DBarolo reveals that the HI and molecular gas are kinematically consistent. Both gas phases show high velocity dispersions, indicating enhanced turbulence compared to regular late-type galaxies. We calculate the specific angular momentum for the stellar, atomic, and molecular components alongside the global stability parameter ($q$). Most of the system angular momentum is stored in the HI discs, rendering them globally stable against collapse. This stability suppresses the atomic-to-molecular gas conversion, as evidenced by extremely long HI depletion times contrasted with typical H$_{2}$ depletion times. The high angular momentum and HI excess likely originate from past gas accretion or recent tidal interactions; this is especially apparent in ESO378-G003 and ESO381-G005, which exhibit extended HI structures and positive tidal indices.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#sentencerolethe authors' own workstates the main conclusionstrength
s0 Galaxies with extreme amounts of neutral atomic hydrogen (HI), known as HI-eXtreme (HIX) galaxies, paradoxically exhibit normal or low star formation rates. background 1.00 0.11 0.08 asserted 1.00
s1 To investigate the bottleneck in their star formation, we present ALMA CO(1-0) observations for five HIX survey galaxies: ESO208-G026, ESO378-G003, ESO381-G005, ESO417-G018, and ESO055-G013. contribution 0.65 0.96 0.02 asserted 0.95
s2 While these systems possess massive, extended HI discs, their molecular gas discs are compact with normal H$_{2}$ masses relative to their stellar mass. result 1.00 0.90 0.11 asserted 1.00
s3 Dynamical modelling with 3DBarolo reveals that the HI and molecular gas are kinematically consistent. result 0.84 0.94 0.05 asserted 0.77
s4 Both gas phases show high velocity dispersions, indicating enhanced turbulence compared to regular late-type galaxies. result 0.97 0.90 0.05 asserted 0.56
s5 We calculate the specific angular momentum for the stellar, atomic, and molecular components alongside the global stability parameter ($q$). method 0.85 0.96 0.03 asserted 0.98
s6 Most of the system angular momentum is stored in the HI discs, rendering them globally stable against collapse. result 0.92 0.91 0.26 asserted 1.00
s7 This stability suppresses the atomic-to-molecular gas conversion, as evidenced by extremely long HI depletion times contrasted with typical H$_{2}$ depletion times. interpretation 0.91 0.90 0.75 asserted 0.99
s8 The high angular momentum and HI excess likely originate from past gas accretion or recent tidal interactions; this is especially apparent in ESO378-G003 and ESO381-G005, which exhibit extended HI structures and positive tidal indices. interpretation 1.00 0.81 0.10 hedged possibility 0.99

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from observation or experiment 1.00.

What was examined, and what was not

sources read
  • arXiv listing abstract v1
not examined
  • full text
  • figures and tables
  • references and citing sentences
  • relations to other papers
levels run
  • P abstract
not run
  • S sections
  • T body sentences
  • C citations
  • R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures
Figure 1. Legacy Sky Survey (DR10) 𝑔-band image overlaid with ALMA CO contour levels (binned, dilated data set). The synthesised beam of the CO data is shown in the lower left corner of each image. Contour levels are colour coded according to the integrated CO intensity in units of the rms. All galaxies show a slight asymmetry or distortion in their optical image. sets to compare the properties of the H ix galaxies to global H i and H2 scaling relations. For typical nearby galaxies the xGASS and xCOLD GASS (Saintonge et al. 2017; Catinella et al. 2018) surveys provide insights into the…
Figure 2. Integrated spectrum of each galaxy using the 3D SoFiA mask. Dashed line spectra are from the pipeline data cubes, and solid lines are from the binned cubes. We use pbcorr (7m array only) data for ESO055-G013 and ESO208-G026, and combined (7m array and total power) data of ESO378-G003, ESO381-G005 and ESO417-G018. frequency of the emission (𝜈rest) corrected for redshift, to obtain the intrinsic source brightness temperature (Solomon et al. 1997), 𝐿 ′ CO =  𝑐 2 2𝑘  𝑆COΔ𝑣𝐷2 L 𝜈rest(1 + 𝑧) . (1) The molecular gas is directly proportional to this CO(1-0) line luminosity by a co
Figure 3. Intensity-weighted velocity maps of the CO and H i gas discs of each galaxy in the sample, as labelled. To the right of each graph is its respective colour scale. The synthesised beams of the H i (in green) and CO (in orange) data are shown respectively in the lower left and right corners of each plot. stant 𝛼CO = 4.35 M⊙ (K km s−1 pc2 ) −1 , corresponding to 𝑋CO = 2.0 × 1020 cm−2 (K km s−1 ) −1 that is the recommended value for the Milky Way (see Bolatto et al. 2013, Saintonge et al. 2017, Accurso et al. 2017). We use this 𝛼CO value to be consistent with the compar￾ison sample from…
Figure 4. Comparison of (a) H i gas mass, (b) H2 gas mass, and (c) H2-H i ratio of the five galaxies with xCOLD GASS survey data. The black stars represent UGC 6168, UGC 7899, NGC 5230, and the black star with error bars represents UGC 2885. The conversion factor between CO(1-0) radiation and H2 is 𝛼CO = 4.35 M⊙ (K km s−1 pc2 ) −1 . Metallicity-dependent 𝛼CO Fixed 𝛼CO = 4.35 M⊙ (K km s−1 pc2 ) −1 (1) (2) 𝑦 𝑥 𝑥0 𝑚 𝑏 𝜎 𝑚 𝑏 𝜎 𝑅mol log [𝑀★/M⊙ ] 10.70 0.200 ± 0.043 −0.457 ± 0.034 0.425 0.382 ± 0.045 −0.326 ± 0.036 0.447 log 𝑀H2 /𝑀★
Figure 5. Peak-normalised radial surface density profiles of H i and CO gas for the sample of galaxies from 3DBarolo. Left: H i surface density. Right: CO surface density. The galactocentric radius 𝑟 is normalised by the optical radius 𝑅25 of each galaxy. ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆  0 10 20 30 40 0 50 100 150 200 r / kpc vrot / (km s -1 ) ESO055-G013 ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆        0 10 20 30 40 0 20 40 60 80 100 120 140 r / kpc vrot / (km s -1 ) ESO208-G026 ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆ ◆         0 10 20 30 40 50 60 0 20 40 60 80 100 120 140 r / kpc vrot / (km s -1 )…
Figure 6. Rotation velocity calculated by rings with 3DBarolo (green for H i and blue for CO, each point, one ring) and rotation curves fitted with function (8) in red, for each galaxy as labelled. MNRAS 000, 1–19 (2024)
Figure 7. Left: the 2nd moment map for each galaxy (pbcor data or combined if available). Right: Velocity dispersion histograms of the CO (red) and H i (cyan) data. Bin sizes are the same as the velocity resolution of the data. The black solid line shows the fitted probability density function to the data. The blue dashed line indicates the velocity resolution cut-off. MNRAS 000, 1–19 (2024)
Figure 8. Stellar mass versus stellar specific angular momentum. In light grey, the projection of the plane 𝑀★-𝑗★- 𝑓★,atm from the xCOLDGASS data. The dark line indicates the 2D best-fitting linear relation while the dashed lines indicate the 1𝜎 scatter of the relation. of gas ( 𝑓atm = 1), even though their actual atomic gas fractions are between 0.6 and 0.7. Mancera Piña et al. (2021) and Hardwick et al. (2022) found that the distance from the main baryonic relation is correlated with the gas fraction and our results agree well with their findings for galaxies with high atomic gas fractions.…
Figure 10. Atomic gas fraction versus global stability factor in logarithmic scale. The solid black line is the relation found by Obreschkow et al. (2016), and the shaded area represents 40% of scatter. The dashed grey line is the limit for global stability, 𝑞 ≈ 1/(√ 2 𝑒), where a higher 𝑞 parameter implies 𝑓atm = 1. The black stars represent UGC 6168 and UGC 7899. The purple shaded histograms on the left and on the bottom show the distribution of the xCOLD GASS data points on the respective axes. fraction and the stability parameter 𝑞 are approximately related by 𝑓atm = min  1, 2.5𝑞 1.12 .…
Figure 11. Star formation properties of our sample compared with xCOLD GASS. The black stars represent UGC 6168, UGC 7899, NGC 5230, and the black star with error bars represents UGC 2885. Top left: Specific star formation rate against H i mass. Top right: Specific star formation rate against H2 mass. Bottom left: H i depletion time against H i mass. Bottom right: H2 depletion time against H2 mass. The solid blue lines represent the linear regression (best fit) for xCOLD GASS data. The shaded region indicates 1𝜎 interval. MNRAS 000, 1–19 (2024)

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