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Measurements of the atomic hydrogen (HI) properties of high-redshift galaxiesare critical to understanding the decline in the star-formation rate (SFR)density of the Universe after its peak $\approx8-11$ Gyr ago. Here, we use$\approx510$ hours of observations with the upgraded Giant Metrewave RadioTelescope to measure the dependence of the average HI mass of star-forminggalaxies at $z=0.74-1.45$ on their average stellar mass and redshift, bystacking their HI 21 cm emission signals. We divide our sample of 11,419main-sequence galaxies at $z=0.74-1.45$ into two stellar-mass ($M_*$)subsamples, with $M_*>10^{10} M_\odot$ and $M_*<10^{10} M_\odot$, and obtainclear detections, at $>4.6\sigma$ significance, of the stacked HI 21 cmemission in both subsamples. We find that galaxies with $M_*>10^{10} M_\odot$,which dominate the decline in the cosmic SFR density at $z\lesssim1$, have HIreservoirs that can sustain their SFRs for only a short period, $0.86\pm0.20$Gyr, unless their HI is replenished via accretion. We also stack the HI 21 cmemission 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 cmemission signals, at $>5.2\sigma$ significance in both subsamples. We find thatthe average HI mass of galaxies with $\langle M_* \rangle\approx10^{10}M_\odot$ declines steeply over a period of $\approx1$ billion years, 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 afactor $\gtrsim3$. We thus find direct evidence that accretion of HI ontostar-forming galaxies at $z\approx1$ is insufficient to replenish their HIreservoirs and sustain their SFRs, thus resulting in the decline in the cosmicSFR density 8 billion years ago.

Insufficient Gas Accretion Caused the Decline in Cosmic Star-Formation Activity 8 Billion Years Ago