Insufficient Gas Accretion Caused the Decline in Cosmic Star-Formation Activity 8 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 $\approx 8-11$ Gyr ago. Here, we use $\approx 510$ hours 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 ($M_{\ast }$) subsamples, with $M_{\ast }>{10}^{10}{M}_{\odot }$ and $M_{\ast }<{10}^{10}{M}_{\odot }$, and obtain clear detections, at $>4.6\sigma$ significance, of the stacked HI 21 cm emission in both subsamples. We find that galaxies with $M_{\ast }>{10}^{10}{M}_{\odot }$, which dominate the decline in the cosmic SFR density at $z\lesssim 1$, have HI reservoirs that can sustain their SFRs for only a short period, $0.86\pm 0.20$ 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 $>5.2\sigma$ significance in both subsamples. We find that the average HI mass of galaxies with $\langle M_* \rangle\approx10^{10} M_\odot$declinessteeplyoveraperiodof$\approx1$billionyears,from$(33.6\pm6.4) \times 10^9 M_\odot$at$\langle z\rangle\approx1.3$to$(10.6\pm1.9)\times10^9 M_\odot$at$\langle z\rangle\approx1.0$, i.e. by a factor $\gtrsim 3$. We thus find direct evidence that accretion of HI onto star-forming galaxies at $z\approx 1$ 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.