Weak Evolution of Cosmic Atomic Hydrogen over the Past 4.5 Billion Years
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, ΩHI, 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 ∼2.5 million galaxies across ∼12,000 deg2. We measure a raw decrease in ΩHI by a factor of 1.35±0.10 over the past 4.5 Gyr. Even after applying the conservative systematic corrections from our forward model, the inferred decline is only 1.12±0.10 -- 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.
Versions
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BibTeX
@misc{zhang2026weak,
title = {{Weak Evolution of Cosmic Atomic Hydrogen over the Past 4.5 Billion Years}},
author = {Chuan-Peng Zhang and Hong Guo and Yizhou Gu and Amélie Saintonge and Xiaohu Yang and Dirk Scholte and Ming Zhu and Peng Jiang and Hu Zou and Manasvee Saraf and Wenlin Ma and Yirong Wang and Y. P. Jing and Zheng Zheng and Zhejie Ding and J. Aguilar and S. Ahlen and D. Bianchi and D. Brooks and T. Claybaugh and A. de la Macorra and P. Doel and E. Gaztañaga and G. Gutierrez and M. Ishak and R. Joyce and S. Juneau and R. Kehoe and D. Kirkby and A. Kremin and O. Lahav and C. Lamman and M. Landriau and L. Le Guillou and M. Manera and A. Meisner and R. Miquel and J. Moustakas and S. Nadathur and W. J. Percival and F. Prada and I. Pérez-Ràfols and G. Rossi and E. Sanchez and D. Schlegel and M. Schubnell and H. Seo and J. Silber and D. Sprayberry and G. Tarlé and B. A. Weaver and FASHI Collaboration},
howpublished = {arXiv},
year = {2026},
month = jul,
doi = {10.48550/arxiv.2607.05326},
eprint = {2607.05326},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2607.05326},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Weak Evolution of Cosmic Atomic Hydrogen over the Past 4.5 Billion Years
AU - Chuan-Peng Zhang
AU - Hong Guo
AU - Yizhou Gu
AU - Amélie Saintonge
AU - Xiaohu Yang
AU - Dirk Scholte
AU - Ming Zhu
AU - Peng Jiang
AU - Hu Zou
AU - Manasvee Saraf
AU - Wenlin Ma
AU - Yirong Wang
AU - Y. P. Jing
AU - Zheng Zheng
AU - Zhejie Ding
AU - J. Aguilar
AU - S. Ahlen
AU - D. Bianchi
AU - D. Brooks
AU - T. Claybaugh
AU - A. de la Macorra
AU - P. Doel
AU - E. Gaztañaga
AU - G. Gutierrez
AU - M. Ishak
AU - R. Joyce
AU - S. Juneau
AU - R. Kehoe
AU - D. Kirkby
AU - A. Kremin
AU - O. Lahav
AU - C. Lamman
AU - M. Landriau
AU - L. Le Guillou
AU - M. Manera
AU - A. Meisner
AU - R. Miquel
AU - J. Moustakas
AU - S. Nadathur
AU - W. J. Percival
AU - F. Prada
AU - I. Pérez-Ràfols
AU - G. Rossi
AU - E. Sanchez
AU - D. Schlegel
AU - M. Schubnell
AU - H. Seo
AU - J. Silber
AU - D. Sprayberry
AU - G. Tarlé
AU - B. A. Weaver
AU - FASHI Collaboration
PY - 2026
DA - 2026/07/06
PB - arXiv
DO - 10.48550/arxiv.2607.05326
UR - https://arxiv.org/abs/2607.05326
AB - 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, Ω_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 ∼2.5 million galaxies across ∼12,000 deg^2. We measure a raw decrease in Ω_HI by a factor of 1.35±0.10 over the past 4.5 Gyr. Even after applying the conservative systematic corrections from our forward model, the inferred decline is only 1.12±0.10 -- 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.
ER -