The design and optimization of the LSST system leverages its unique capability to scan a large sky area to a faint flux limit in a short amount of time. The main product of the LSST system will be a multi-color ugrizy image of about half the sky to unprecedented depth (r ~ 27.5), with superior image quality (0.7 arsec median delivered seeing in the r band), and exquisite photometric (1% or better) and astrometric (10 mas per epoch) accuracy. The catalogs based on these imaging data will include about 10 billion galaxies and a similar number of stars. For a comparison, the best analogous contemporary dataset is that of SDSS, which provides ugriz image of about a quarter of the sky to r~22.5, with 1.5 arcsec seeing, about a factor of two larger photometric errors and three times larger astrometric errors, and about two orders of magnitude fewer detected sources.
Another major advantage of LSST is the fact that the deep sky map is produced by taking hundreds of shorter exposures. Each sky position within the survey area will be observed over 800 times over time scales spanning seven orders of magnitude (from 30 sec to 10 years). Hence, LSST will open the time domain for massive and accurate studies of photometrically and astrometrically varying sources with unprecedented coverage in flux, wavelength, and timescale.
It would be impossible to list all the possible projects that LSST data will enable. However, here we list a few to give a flavor of these studies, and organize them by the four science themes that drive the LSST design (although some span more than one theme).
Taking an Inventory of the Solar System
LSST, with its unprecedented power for discovering moving objects, will make a giant leap forward in solar system studies. The baseline LSST cadence will result in orbital parameters for several million moving objects; these will be dominated by main-belt asteroids, with light curves and colorimetry for a substantial fraction of detected objects. This will give 10 to 100 times more objects than are currently available with orbits, colors, and variability information. LSST is capable of reaching the Congressional target completeness of 9% for PHAs larger than 140 m, and will detect over 30,000 TNOs brighter than r~24.5 using its baseline cadence. Because each object will be observed several hundred times, accurate orbital elements, colors, and variability information will be available for most of these objects.
- Studies of the distribution of orbital elements for about 100,000 NEOs as a function of color and size. Correlations with the analogous distributions for main-belt objects, studies of object shapes and structure using light curves.
- Studies of the distribution of orbital elements for several million main-belt asteroids as a function of color-based taxonomy and size: size distributions of asteroid families, correlations with age, dynamical effects, studies of object shapes and structure using light curves.
- Studies of the distribution of orbital elements for about 100,000 Jovian Trojan asteroids as a function of color and size; the search for dynamical families, studies of shapes and structure using light curves.
- TNOs, Centaurs, non-Jovian Trojans: studies of the distribution of orbital elements for over 30,000 TNOs as a function of color and size; the search for dynamical families, studies of shapes and structure using light curves.
- Unbiased and complete census of both Jupiter-family and Oort-cloud comets; 6-band sub-arcsecond spatial profiles to a faint surface brightness limit, temporally resolved activity.
- Extremely distant solar system: the search for objects with perihelia a several hundred AU (e.g. Sedna will be observable to 130 AU).
- Multi-color studies of interplanetary (zodiacal) dust with high spatial resolution.
Exploring the Transient Optical Sky
Time domain science will greatly benefit from unique LSST capability to simultaneously provide large area coverage, dense temporal coverage, accurate color information, good image quality , and rapid data reduction and classification. Since LSST extends time-volume space a thousand times over current surveys, the most interesting science may well be the discovery of new classes of objects. There are many known applications for LSST data products:
- Studies of dwarf novae, including their use as probes of stellar populations and structure in the Local Group.
- Gamma Ray Burst afterglows and transients to high redshift.
- Gravitational micro-lensing in the local group and beyond. Studies of SNe populations and parametrization of light curves.
- A deep search for dolichonovae (slow) and macronovae populations (newly discovered subclasses).
- Search for stellar tidal disruptions by nuclear supermassive black holes.
- Accretion of nuclear gas clouds or large-scale accretion-disk instabilities.
- Optical bursters (varying faster than 1 mag/hr) to r~25 mag: new phenomena.
- Optical identifications for transients detected at other wavelengths, from gamma-rays to radio.
- A study of quasar variability using 2% accurate, multicolor light curves for 2 million low-redshift (z<2) quasars: constraints on the accretion physics.
- The superb continuum light curves will enable economical "piggyback" reverberation-mapping efforts using emission lines. These results will greatly broaden the luminosity-redshift plane of reverberation-mapped AGNs, upon which the whole industry of AGN black-hole mass estimates relies. For LSST data alone, the inter-band continuum lags will give useful structural information.
- LSST should provide a dramatic increase in the number of known AGNs at the end of the dark ages, z~6.5-7.5 (about 1000).
- By pushing substantially further down the luminosity function over a very large solid angle, will lead to a much clearer understanding of black-hole growth during the first Gyr.
- LSST data will allow good constraints on AGN lifetimes.
Constraining Dark Energy and Dark Matter
A unique aspect of LSST as a probe of dark energy and matter is the use of several billion galaxies to reach unprecedented precision through multiple cross-checking probes. Of particular interest is the dynamical behavior of dark energy, i.e. how it behaves with cosmic time or with redshift. We note that some recent models of dark energy predict complicated dynamics and will require a survey which delivers simultaneous independent probes. The main LSST deliverables will be:
- Multiple cosmic shear probes. Using 3-D shear tomography (correlation of shear of a galaxy at one redshift bin with another galaxy in a different redshift bin, averaged of all pairs of billions of galaxies). There will be over 50 of these independent probes.
- Baryon Acoustic Oscillations. The standard ruler of the sound horizon at decoupling which is imprinted on the mass distribution at all redshifts provides a direct way of measuring distance vs redshift. When combined with CMB and WL cosmic shear, this combination yields the best constraints on the dynamical behavior of dark energy.
- Supernovae. The two LSST samples are complementary: the main survey will get 6-band colors and redshifts of a million Type-I supernovae (permitting a search for a "third parameter" which if not understood might introduce systematic error in luminosity evolution), and the "movie mode" rapid sampling survey of selected areas (yielding well sampled lightcurves of tens of thousands of supernovae, leading to an independent test of dark energy dynamics.)
- The distribution of giant WL shear peaks with redshift. This is a simultaneous probe of the universal mass function and the growth of structure -- an important constraint on dark energy. LSST will find over 200,000 such giant peaks.
- Several hundred special alignment cluster lenses are expected in the LSST sample. This will be used to explore the existence of any mass singularity.
- A million galaxy-galaxy lenses will provide the needed statistics to constrain the dark matter mass function on small scales.
- Multi-image lensed SN time delays form a separate test of cosmology.
- Time delays for QSOs multiply lensed by clusters as a function of redshift are an independent test of dark energy. The natural timescale (many months to years) is well matched to the LSST survey.
- Detailed studies of galaxy formation and evolution using their distribution in luminosity-color-morphology space as a function of redshift (LSST will extend GALEX studies to a redshift of 2.) This projects onto many axes:
- the evolution of galaxy luminosity function with redshift, as a function of morphology and color
- the evolution of galaxy color distribution over a wide range of rest-frame wavelengths, and as a function of luminosity and morphology
- the bulge-disk decomposition, as a function of luminosity and color, over a large redshift range
- detailed distribution of satellite galaxies in luminosity-color-morphology space as a function of luminosity, color, and morphology of the primary galaxy
- correlations of luminosity, color and morphology with local environment
- the properties of galaxy groups as a function of cosmic time
- Galaxy correlation functions as a function of redshift.
- The first half-sky survey of ultra low surface brightness objects, with redshift information.
- Studies of the solar wind propagation speed using induced activity in a large sample of comets at different heliocentric distances.
- Multi-wavelength studies of faint optical sources using X-ray, IR and radio data (e.g. SDSS detected only 1/3 of 20cm FIRST sources because it was too shallow by ~4 magnitudes.)
Synergy with other projects
LSST will not operate in isolation and will benefit from other pre-cursor and coeval data and multiple wavelengths, depth, and timescales. For example, most of the Celestial Sphere will be covered to a limit several magnitudes fainter than LSST saturation (r~16), first by the combination of SDSS and SkyMapper, and then by the Gaia survey. The Pan-STARRS surveys will provide multi-epoch data deeper than SDSS in the northern sky, and the Dark Energy Survey in the southern sky. LSST and Gaia will be complementary datasets for studying the Milky Way in the multi-dimensional space of three-dimensional positions, proper motions and metallicity. The Gaia survey will provide calibration checks at the bright end for proper motions and trigonometric parallax measurements by LSST, and LSST will extend the Gaia survey with excellent astrometry four magnitudes deeper. It is likely that LSST data stream will invigorate follow-up by numerous other systems that will provide additional temporal, spectral and spatial resolution coverage. We are already collaborating on a world-wide deep spectroscopy program to provide exquisite calibration of photometric redshifts, and are planning to cross-correlate LSST data with a large number of other surveys using VO technology. Plans are now being developed by the world astronomical community for follow-up of optical transients (multi-band concurrent imaging, and IFU spectroscopy.)