A golden nearby SN sample from the Zwicky Transient Facility-II Survey (ZTF-II) – ZTF

Type Ia supernovae (SNIa) are the only cosmological probe able to map accurately the cosmic expansion and structure formation in the late Universe -- the other techniques (BAO, RSD) being cosmic variance limited at low redshifts.

The Zwicky Transient Facility (ZTF) is the only instrument able to cover extragalactic sky twice a night, making it the most fantastic transient discovery machine ever built. By 2023, the ZTF SNIa sample will reach O(4500) SNIa with an exquisite photometric sampling. We request support from ANR to carry out the calibration effort that will turn this sample into a (legacy) cosmology-grade dataset. We will then build on this effort to produce (1) the best constraints on the nature of dark energy (2) a measurement of the growth rate of structures from SN peculiar velocities (3) a new measurement of H0, along with expansion isotropy tests. These measurements are key to investigate the tensions that have appeared between probes of the late and early Universe.

The project is structured into two interconnected parts: first, a technical effort to produce from the raw ZTF pixels the calibrated light curves of all the transients discovered by the project. Second, an analysis effort to produce standardized luminosity distances and the three key cosmological constraints of our scientific programme.

At the core of our technical effort is the control of the survey calibration, with a precision about 5 times better than the current state-of-the-art (~0.1% for the flux calibration and survey uniformity, 0.1-nm, for the characterization of the instrument passbands). These aggressive requirements are motivated by the measurement of the Dark Energy equation of state, dominated by calibration systematic uncertainties. The two other probes are less demanding and would benefit from a fast increase of low-z statistics.

The project is organized around incremental data releases (DR). Each DR will bring a sizable increase of the sample size, and improvements in data reduction. DR1 ("state of the art") will occur 14 months after the beginning of the project, and will allow us to initiate growth rate measurements. DR2 ("High statistics"), one year later, with a preliminary calibration, will unlock the two other probes. DR3 ("final data release") will occur one year before the end of the project, in time for the final round of cosmology papers.

The risk mitigation analysis of this project shows that it is of the Low Risk/High Yield category: the survey instrument is in operation and one third of the expected sample is on disk. Prototypes of the instrument characterization hardware exist already. Computing resources matching our requirements have been allocated by the IN2P3 computing center. The ZTF robotic telescopes are currently unaffected by the COVID19 pandemic. In summary: the overall risk level of the project is extremely low, for a very high scientific return on investment, and our most serious risk is lack of the manpower required to complete our programme.

Joining ZTF-II as a Major Partner costs 600k$. The technical work we propose, along with an in-kind hardware contribution has been valued 400k$. If we manage to find 100k$, IN2P3 has pledged to match with 100k$ in cash which will finalize our cash contribution. With this proposal, we ask for (1) 100 k$ as a contribution to the ZTF running costs (2) three 3-year junior postdoc positions, each devoted to one of our three key cosmological probes. Each ANR postdoc will work ~12 months on a technical contribution critical for her/his science topic, ~2 months to the preparation of the data releases, and the remaining of her time to one of the three key cosmological analyses. ANR support in cash and manpower is critical to enable this project, and secure its return on investment. Our program will have a strong impact on the pre-LSST cosmological constraints. The ZTF dataset will dominate the low-z statistics for many years after the beginning of LSST.