ZTF's high-cadence data stream will enable new investigations in a wide variety of fields. The ZTF survey will average more than 300 epoch each year over the entire Northern sky, giving nearly four times the number of exposures of SDSS Stripe 82 over 100 times the sky area. Public access to the ZTF data will provide a wide variety of community science, much unanticipated. We describe a few selected highlights below.
AGN & TDEs
The ZTF Northern Sky Survey will be a transformative survey for the study of tidal disruption events (TDEs): outbursts from massive black holes found in the centers of galaxies caught in the act of feasting on an unlucky star that wanders close enough to the black hole to be ripped apart by tidal forces. ZTF is expected to increase the current census of TDEs by an order of magnitude, yielding 30 TDEs per year, some fraction of which will be discovered very early before peak, enabling prompt multiwavelength observations that can probe the geometry of the accreting stellar debris, constrain the mass of the central black hole powering the events, and look for evidence for the launching of outflows and jets.
Stellar Astrophyiscs
Because the stellar density is at least a factor of 1000 higher in the Galactic Plane (known as the Milky Way which is visible as the milky strip across the night sky), previous surveys have avoided this area. But to study all the different kinds of stars, the Galactic Plane is the place to look, as that is where the majority of stars live out their lives. As an analogy: Observing stars outside the Galactic Plane is like trying to find a pretty Christmas tree in the desert. If you go to Oregon, there will be lots of them and some will be very unique. The same is true for stars inside the Galactic Plane vs. outside the Galactic Plane. Or another analogy: The galactic plane is largely unexplored, like the ocean, and that instead of only looking at other galaxies, it might be interesting to understand our own a bit more. ZTF is the first optical survey which will observe the visible part of the Galactic Plane every single night in two colors This survey will provide hundreds of observations in the Galactic Plane, an unprecedented dataset that will be analyzed by astronomers for decades. We expect to find millions of new variable stars with periods as short as a few minutes (white dwarfs) or as long as years (pulsating giant stars). We also expect rare objects where a normal star orbits a black hole or a neutron star.
Besides fantastic lightcurves we will get night-to-night variability information for every observed object and will see when objects change their brightness significantly within a single day. For example stars with masses about the half the mass of the sun show strong flares which are sudden flashes of increased brightness of the star similar to a solar flare but 100-1000 times stronger compared to the sun. Another possibility are so-called cataclysmic variables which are binary stars which transfer matter from a low mass companion star that is first accreted onto an accretion disc before it falls onto the more massive primary star. These accretion discs can become unstable and increase their brightness by a factor of 100-1000 within a few hours and come back to quiescence brightness within a few days. ZTF will find all of these. If we are lucky we could possibly witness the rare event when two stars come so close that they merge to form one massive, very dense star. Such a rare stellar merger event is seen due to a rapid brightness increase that takes place during the event.
Cosmology with SNe Ia and Gravitational Lensing SWG
Thermonuclear explosions of White Dwarf stars, Type Ia supernovae, can be used for accurate distance measurements throughout the Universe and were used to provide the first evidence for dark energy and the accelerated expansion of space-time. The Zwicky Transient Facility will find thousands of such supernovae at a key range where precision distances can be derived and explosion characteristics determined through spectroscopic follow-up. These supernova distances will be used to study how dark matter is distributed in the local universe, how the Milky Way is moving through the Universe and provide the basis for how to use supernovae to search for dark energy variability in the future. The enormous ZTF search volume will also allow the detection of rare supernovae that illuminates details of the detonation or are aligned such that we observe multiple copies of the same object through strong gravitational lensing.
EM/GW and Neutrino Counterparts
ZTF should promptly and regularly detect bright, blue counterparts now shown to be associated with gravitational waves from neutron star mergers. ZTF is designed to pinpoint exactly which galaxy is the home of the merger among the hundreds of galaxies in the coarse localization by the gravitational wave interferometers. The ZTF localization will enable a quantification of how prolific are these cosmic mines of r-process nucleosynthesis responsible for half the elements in the periodic table heavier than iron. ZTF would quantify how much of the heavy elements (such as gold, platinum, neodymium) are synthesized in a neutron star merging with another neutron stars or a neutron star merging with a black hole.
Solar System Bodies
ZTF will be a valuable tool to conduct a range of science for small bodies, such as detection of fast moving asteroids and/or low elongation objects, search of monolithic asteroids that rotate at a very high rate, and monitoring of outbursting comets.Asteroids that are very close to the Earth usually have high motion rates and leave streaks on typical survey exposures, presenting a challenge for any detection algorithm. ZTF will make use of a streak detection pipeline originally developed at IPAC/Caltech and tested for PTF to search for these fast moving asteroids. The twilight sky is difficult to observe but is known to contain a number of interesting phenomena. The ZTF twilight survey will make use of the twilight hours to repeatedly scan the small elongation region for incoming asteroids and comets from that direction.Most asteroids are gravitationally bounded “rubble-piles”. Rubble-pile asteroids cannot have rotation periods less than a critical limit. It has been found that a small number of asteroids have rotation periods shorter than this limit, implying that they may be monolithic. PTF had discovered 3 of 6 super-fast rotators (SFRs) known to date. With its large sky coverage and high cadence, ZTF can improve our knowledge of the SFR population. Comet outbursts can be spectacular, turning a modestly active comet into a naked eye objects. From Rosetta and Deep Impact we know that smaller outburst may occur daily. Larger outburst happens occasionally, but we do not know the frequency and intensity distribution. Many are caught by amateur astronomers. This will change with ZTF, which will pick up between 30 to 50 comets every time it scans the whole sky. Comets are found all over the sky, so we’re interested in seeing as many of them as we can, in as much detail as possible.
Physics of Supernovae and Relativistic Explosions
Why and how stars explode as SNe is poorly understood. Previous SN progenitor studies were limited to identifying progenitor stars in pre-explosion Hubble images, if available, or serendipitous spectroscopic observations of massive stars, like in the case of SN1987A?. Flash spectroscopy (Gal-Yam et al., 2014, Nature, 509) offers a new path to systematically study SN progenitors. The carbon copy of this novel technique is iPTF13dyq (SN2013fs), detected by the intermediate Palomar Transient Factory (iPTF) on 13 October 2013 (Yaron et al. 2017, Nature Physics, 13, 510). The short-cadence experiment of the iPTF survey detected this supernova a mere three hours after the massive star exploded. After the second epoch, obtained 50 minutes later, confirmed that the brightness of the transient is rapidly rising, follow-up observations were initiated around the world, with an orchestra of telescopes. About two hours later, the spectrum obtained with the spectrograph LRIS at the 10-m Keck telescope showed highly ionized oxygen and nitrogen recombination lines that vanished over the next 24 hours. Those lines are produced in the circumstellar material that was ionized by the SN shock break-out, before the SN ejecta swept up the circumstellar material. The modelling of the lines provided an unprecedented view on the distribution of material in the immediate environment of the exploding star. This offered a unique opportunity to examine the mass-loss history shortly before the star exploded and therefore information about the progenitor star. Today, only a handful of events have such precious data sets. The high-cadence experiment of the Zwicky Transient Facility will routinely detect such young supernovae and empower us to finally understand not only why but also how massive stars explode as supernovae.
M31
The galaxy Andromeda is our nearest cosmic neighbor; only a few hundred kiloparsecs away. Due to its proximity and the fact that it hosts different stellar populations, including both young ones (in its spiral arms) and old ones (in its bulge), it is an ideal location for the hunt of varied kinds of variable stars and transient events.
As an example, given the sensitivity of ZTF (take R~21mag), at the distance of M31, you can easily track the pulsation of stars brighter than about -4mag---including those of Cepheids, and other giant stars. In fact, during this 3-day-all sky survey, you can look at one of the Cepheids (at Right Ascension of 11.29108 degrees and Declination of 41.50875 degrees) breathe. These stars are also representative of young stellar population, as they evolve from massive stars, and thus by following a (pre-identified) group of them pulsate, you can trace the relatively young parts of the galaxy. The wealth of variable phenomena that can be directly observed in this galaxy makes it a crucial testbed for many astrophysical stellar theories. Furthermore, "guest events" (otherwise invisible to ZTF) tend to easily pop up here, for example, it is likely you will be able to spot a thermonuclear flash from a white dwarf in a binary during the 3-day period.
