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Weak Evolution of Cosmic Atomic Hydrogen over the Past 4.5 Billion Years

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Chuan-Peng Zhang, Hong Guo, Yizhou Gu, Amélie Saintonge, Xiaohu Yang, Dirk Scholte, Ming Zhu, Peng Jiang
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Hu Zou, Manasvee Saraf, Wenlin Ma, Yirong Wang, Y. P. Jing, Zheng Zheng, Zhejie Ding, J. Aguilar, S. Ahlen, D. Bianchi, D. Brooks, T. Claybaugh, A. de la Macorra, P. Doel, E. Gaztañaga, G. Gutierrez, M. Ishak, R. Joyce, S. Juneau, R. Kehoe, D. Kirkby, A. Kremin, O. Lahav, C. Lamman, M. Landriau, L. Le Guillou, M. Manera, A. Meisner, R. Miquel, J. Moustakas, S. Nadathur, W. J. Percival, F. Prada, I. Pérez-Ràfols, G. Rossi, E. Sanchez, D. Schlegel, M. Schubnell, H. Seo, J. Silber, D. Sprayberry, G. Tarlé, B. A. Weaver, FASHI Collaboration

Astrophysics of Galaxies · Submitted 6 Jul 2026 · Published version · 93 references

Pith's verdict: The central qualitative result — that Ω_HI evolves much more weakly than the CSFRD — is robust and well-supported. The quantitative headline (factor 1.35, or 1.12 after systematics) is sensitive to an extrapolation below 10^9 M⊙ that the paper's own forward model cannot validate.

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arXiv ID2607.05326 (version 1)
Pith IDU2ZVSWQE, Pith's permanent record of this paper and its review
CitationA. de la Macorra, A. Kremin, A. Meisner, Am\'elie Saintonge, B. A. Weaver, Chuan-Peng Zhang, C. Lamman, D. Bianchi, D. Brooks, Dirk Scholte, D. Kirkby, D. Schlegel, D. Sprayberry, E. Gazta\~naga, E. Sanchez, FASHI Collaboration, F. Prada, G. Gutierrez, G. Rossi, G. Tarl\'e, Hong Guo, H. Seo, Hu Zou, I. P\'erez-R\`afols, J. Aguilar, J. Moustakas, J. Silber, L. Le Guillou, Manasvee Saraf, Ming Zhu, M. Ishak, M. Landriau, M. Manera, M. Schubnell, O. Lahav, P. Doel, Peng Jiang, R. Joyce, R. Kehoe, R. Miquel, S. Ahlen, S. Juneau, S. Nadathur, T. Claybaugh, Wenlin Ma, W. J. Percival, Xiaohu Yang, Yirong Wang, Yizhou Gu, Y. P. Jing, Zhejie Ding, Zheng Zheng. "Weak Evolution of Cosmic Atomic Hydrogen over the Past 4.5 Billion Years." arXiv:2607.05326 (2026). https://pith.science/paper/U2ZVSWQE
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The paper

The paper claims

Cosmic atomic hydrogen barely declined while star formation plummeted

The reading

The paper combines FAST 21-cm radio observations with DESI optical spectroscopy for roughly 2.5 million galaxies across 12,000 square degrees, measuring how the cosmic density of atomic hydrogen (Omega_HI) has changed over the past 4.5 billion years. The authors find that Omega_HI decreased by only a factor of 1.35±0.10 from z=0.41 to the present, and by just 1.12±0.10 after conservative systematic corrections. This is far weaker than the factor-of-2.46 decline in the cosmic star formation rate density over the same period. Crucially, the average HI gas fraction at fixed stellar mass evolves by less than 0.2 dex across the full galaxy population, showing that the weak evolution is not a cancellation between different galaxy types but a universal trend. The central mechanism the authors identify is that the late-time decline in cosmic gas supply does not deplete the atomic hydrogen reservoir directly; instead, its impact is mediated through the baryon cycle, affecting the conversion of atomic gas into molecular gas and stars rather than draining the atomic reservoir itself.

What carries the argument

The paper introduces a luminosity-bias correction method that models the HI gas fraction as a function of both r-band luminosity and stellar mass, f_HI(L_r|M_*), using an 8-parameter double-power-law fit. This correction accounts for the fact that flux-limited optical samples are biased toward more luminous (and gas-richer) galaxies at fixed mass. By measuring how f_HI depends on luminosity within each stellar-mass bin, the method reconstructs the intrinsic gas fraction that would be observed in a complete sample, without requiring mass-completeness cuts that would sacrifice the low-mass galaxy population.

Extended reading notes

Core claim

The cosmic atomic hydrogen density has remained nearly constant over the past 4.5 billion years, declining by at most a factor of 1.35 (or 1.12 after systematic corrections), while the cosmic star formation rate density fell by a factor of 2.46 over the same interval. This mismatch, present at fixed stellar mass across the galaxy population, rules out rapid HI depletion as the cause of declining star formation and points to the HI-to-H2 phase conversion or gas accretion regulation as the operative bottleneck.

Load-bearing premise

The luminosity bias correction assumes that the intrinsic distribution of r-band luminosity at fixed stellar mass does not evolve over 0 < z < 0.41, using the local (z < 0.05) sample as the reference for all redshifts. If this distribution actually evolves with redshift, the corrected HI gas fractions at z > 0.25 could be systematically offset.

Editorial extensions

If this is right

  • Galaxy formation models must reproduce a baryon cycle where the atomic hydrogen reservoir stays nearly invariant while molecular gas and star formation decline substantially — a tighter constraint than matching Omega_HI alone.
  • The bottleneck driving late-time cosmic star formation decline lies downstream of the atomic gas reservoir, in the atomic-to-molecular phase conversion or in the regulation of gas accretion onto galactic disks, not in the depletion of HI itself.
  • Future 21-cm intensity mapping surveys can use the measured Omega_HI(z) relation, Omega_HI proportional to (1+z)^0.9, as a benchmark over 0 < z < 0.41.
  • The f_HI(M_*) relation's near-invariance with redshift provides a stable calibration target for semi-analytic and hydrodynamical models aiming to predict gas partitioning across cosmic time.

Reading between the lines

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

  • If the HI reservoir is stable while H2 declines, the star formation efficiency per unit molecular gas may itself be evolving, or the molecular gas depletion timescale may be lengthening — a question the paper does not directly address but that follows from the decoupling it establishes.
  • The near-invariance of f_HI(M_*) suggests that gas accretion and gas loss are finely balanced across all stellar masses, which may point to a self-regulating equilibrium in the atomic gas phase that is insensitive to the processes driving the molecular phase decline.
  • Extending the same f_HI(L_r|M_*) correction framework to higher redshifts (z > 0.4) with deeper surveys would test whether the weak evolution breaks down at earlier epochs when cosmic gas accretion rates were higher.
Original abstract

The cosmic star formation rate density (CSFRD) has declined sharply toward the present day, but the roles of the atomic and molecular gas reservoirs remain uncertain. We measure the cosmic HI density, $\Omega_{\mathrm{HI}}$, over $0<z<0.41$ by combining HI spectra from the Five-hundred-meter Aperture Spherical Telescope with optical spectroscopy from the Dark Energy Spectroscopic Instrument for $\sim2.5$ million galaxies across $\sim12,000\,{\rm deg}^2$. We measure a raw decrease in $\Omega_{\mathrm{HI}}$ by a factor of $1.35\pm0.10$ over the past 4.5 Gyr. Even after applying the conservative systematic corrections from our forward model, the inferred decline is only $1.12\pm0.10$ -- still far weaker than the CSFRD decline (a factor of 2.46). The molecular gas density, in contrast, is known to evolve more closely with star formation. At fixed stellar mass, the average HI gas fraction evolves by less than 0.2 dex, showing that the weak evolution is present across the galaxy population. These quantitative differences rule out rapid depletion of galaxy HI as the primary driver of the late-time CSFRD decline, and provide a stringent benchmark for models of gas accretion, phase conversion and star-formation regulation.

Figures
Figure 1. Atomic gas fraction as a function of stellar mass.
Figure 2. Comparisons with literature. Left: we compare our measurements of fH i at 0 < z < 0.05 with those of Refs.[26, 30] (blue symbols) using ALFALFA in the same redshift range. The model predictions from the hydrodynamical simulations of TNG and SIMBA are also shown as lines of different colours. Right: our measurements of fH i at 0.25 < z < 0.41 are compared with previous measurements at similar redshifts of Refs.[17, 32–36]. We note that the measurements of Refs.[32–34] are made using only the star-forming galaxies, while that of Ref.[36] is for quiescent galaxies. studies. In
Figure 3. Cosmic HI gas density (ΩHI) as a function of redshift. Left: we compare our measurements of ΩH i (black filled circles) with previous results of 21-cm observations[6, 9–21] (brown open circles) and DLA absorption measurements[2] (gray stars; without contribution from helium). All measurements are corrected to the same cosmological parameters. Our measurement errors of ΩH i show 1σ statistical uncertainties propagated from the stacked fH i measurements and the stellar-mass function. Forward-modelled systematic offsets are discussed in Methods. Our best-fitting relation is displayed…

Figures reproduced from arXiv:2607.05326 by the authors.

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