Nature | Vol 586 | 15 October 2020 | 371have ⟨tdep,Hi⟩ = 1.00 ± 0.25 Gyr. This is similar to the H 2 depletion time,
tdep,H 2 ≈ 0.7 Gyr, obtained for main-sequence galaxies at these redshifts
lying at the upper end of the stellar mass and SFR distributions^19. In
the local Universe, star-forming galaxies with a similar stellar mass
distribution have tdep,Hi ≈ 7.8 Gyr, substantially longer than the H 2 deple-
tion timescale, about 1 Gyr (ref.^20 ). Thus, in the local Universe,
main-sequence galaxies can continue to form stars quiescently at the
current SFR for approximately 7.8 Gyr without the need for fresh gas
accretion, as long as there is efficient conversion of H i to H 2 (on time-
scales shorter than tdep,H 2 ). Conversely, main-sequence galaxies at z ≈ 1
can sustain their current SFR for only approximately 1−2 Gyr, unless
their atomic gas reservoir is replenished via gas accretion. This H i
depletion time is similar to the timescale on which the cosmic SFR
density is observed to decline steeply. This indicates that the quench-
ing of star-formation activity at z < 1 is likely to arise owing to insuf-
ficient gas infall (from the circumgalactic medium or via minor
mergers), resulting in a paucity of neutral gas to fuel further star
formation.
Measurement of the redshift evolution of the co-moving cosmologi-
cal H i mass density in galaxies (ΩHi) is important for understanding
the global flow of gas into galaxies. In the local Universe, ΩHi can be
measured from unbiased H i 21-cm emission surveys^21 , whereas at high
redshifts, z ≳ 2, ΩHi has been measured from damped Lyman-α absorb-
ers (DLAs) detected in quasar absorption spectra^22. These studies have
shown that ΩHi declines by a factor of about 2 from z ≈ 2.2 to z = 0 (ref.^23 ).
However, the nature of the evolution of ΩHi between z ≈ 0.4 and z ≈ 2.2
remains unclear, owing to the difficulty of carrying out both H i 21-cm
emission studies^9 ,^24 and direct DLA surveys^25 at these redshifts.
Our measurement of the average H i mass in a sample of blue,
star-forming galaxies allows us to measure ΩHi at z ≈ 1 (see Methods).
For this purpose, we use a subsample of galaxies with MB ≤ −20, for
which the DEEP2 survey is expected to be spectroscopically complete
at z ≈ 1 (ref.^5 ). To estimate ΩHi, we used the known B-band luminos-
ity function for blue galaxies at z ≈ 1 (ref.^26 ) and the relation between
MHi and MB from the local Universe (which we find to be consistent
with our measurements for galaxies with MB ≤ −20; see Methods).
We find that blue, star-forming galaxies with MB ≤ −20 contribute
ΩHi,Bright = (2.31 ± 0.58) × 10−4 to the total co-moving H i mass density at
⟨z⟩ = 1.06. We emphasize that this estimate is a lower limit to the total
ΩHi in galaxies at z ≈ 1, as contributions from H i in faint blue galaxies
(and red galaxies) could only increase the total ΩHi. Extrapolating this
estimate to all blue galaxies, again using the B-band luminosity func-
tion at z ≈ 1 and the MHi–MB relation of the local Universe, we obtain
ΩHi = (4.5 ± 1.1) × 10−4 in blue galaxies at ⟨z⟩ = 1.06 (see Methods).
Figure 3 shows a compilation of ΩHi measurements at different red-
shifts. Our measurement of ΩHi at ⟨z⟩ = 1.06 is consistent within the
uncertainties with all measurements of ΩHi at z ≤ 1, but is lower (at about
3 σ significance) than the DLA measurement of ΩHi at z ≈ 2.15 (ref.^23 ).
Our results thus indicate that the cosmic H i mass density in galaxies
declines substantially by z ≈ 1, and then remains unchanged at later
times. This also indicates that H i in star-forming galaxies is not suf-
ficiently replenished to fuel star-formation at the same level after the
peak of star-formation activity.Online content
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0 1 2 3 4 5 6
Redshift0.51.01.5ΩH(10I3 )0 46 8 10 12Lookback time (Gyr)Fig. 3 | Redshift and temporal evolution of the cosmic H i mass density in
galaxies, ΩHi. Vertical error bars indicate 1σ measurement errors, and
horizontal error bars indicate redshift bins. In cases where there are multiple
statistically significant (≥3σ) measurements of ΩHi from a single technique at
similar redshifts, only the most sensitive result has been included. The green
symbols are measurements from DLAs (triangles)^23 ,^27 or Mg ii absorbers
(inverted triangles)^25. The blue square is the local Universe measurement of ΩHi
(ref.^21 ). The purple circles are from low-z H i 21-cm emission stacking
experiments^8 ,^10. The filled red circle indicates our estimate of ΩHi from all blue
galaxies, after correcting for the contribution of faint blue galaxies. Our
measurement of ΩHi is consistent within the errors with all measurements at
z ≤ 1, but is lower (at about 3σ significance) than the measurement of ΩHi at
z ≈ 2.15 from DLA surveys^23.