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) × 10⁹ M _⊙ at <z> ≈ 1.3 to (10.6 ± 1.9) × 10⁹ M _⊙ 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.