Insufficient Gas Accretion Caused the Decline in Cosmic Star-formation Activity Eight Billion Years Ago

Measurements of the atomic hydrogen (Hi) properties of high-redshift galaxies are critical to understanding the decline in the star formation rate (SFR) density of the universe after its peak ≈8–11 Gyr ago. Here, we use ≈510 hr of observations with the upgraded Giant Metrewave Radio Telescope to measure the dependence of the average Hi mass of star-forming galaxies at z = 0.74–1.45 on their average stellar mass and redshift by stacking their Hi 21 cm emission signals. We divide our sample of 11,419 main-sequence galaxies at z = 0.74–1.45 into two stellar-mass (M*) subsamples, with M* > 1010 M⊙ and M* < 1010 M⊙, and obtain clear detections, at >4.6σ significance, of the stacked Hi 21 cm emission in both subsamples. We find that galaxies with M* > 1010 M⊙, which dominate the decline in the cosmic SFR density at z ≲ 1, have Hi reservoirs that can sustain their SFRs for only a short period, 0.86 ± 0.20 Gyr, unless their Hi is replenished via accretion. We also stack the Hi 21 cm emission from galaxies in two redshift subsamples, at z = 0.74–1.25 and z = 1.25–1.45, again obtaining clear detections of the stacked Hi 21 cm emission signals, at >5.2σ significance in both subsamples. We find that the average Hi mass of galaxies with 〈M*〉 ≈ 1010 M⊙ declines steeply over a period of ≈1 billion years, from (33.6 ± 6.4) × 109M⊙ at 〈z〉 ≈ 1.3 to (10.6 ± 1.9) × 109M⊙ at 〈z〉 ≈ 1.0, i.e., by a factor ≳3. We thus find direct evidence that accretion of Hi onto star-forming galaxies at z ≈ 1 is insufficient to replenish their Hi reservoirs and sustain their SFRs, thus resulting in the decline in the cosmic SFR density 8 billion years ago.