11 Supernovae W. Michael Wood-Vasey, David Arnett, S. J. Asztalos, Stephen Bailey, Joseph P. Bernstein, Rahul Biswas, David Cinabro, Kem H. Cook, Jeff Cooke, Willem H. de Vries, Benjamin Dilday, Brian D. Fields, Josh Frieman, Peter Garnavich, Mario Hamuy, Saurabh W. Jha, Richard Kessler, Stephen Kuhlman, Amy Lien, Sergei Nikolaev, Masamune Oguri, Scot S. Olivier, Philip A. Pinto, Jeonghee Rho, Evan Scannapieco, Benjamin D. Wandelt, Yun Wang, Patrick Young, Hu Zhan 11.1 Introduction Josh Frieman, W. Michael Wood-Vasey In 1998, measurements of the Hubble diagram of Type Ia supernovae (SNe Ia) provided the first direct evidence for cosmic acceleration (Riess et al. 1998; Perlmutter et al. 1999). This discovery rested on observations of several tens of supernovae at low and high redshift. In the intervening decade, several dedicated SN Ia surveys have together measured light curves for over a thousand SNe Ia, confirming and sharpening the evidence for accelerated expansion. Despite these advances (or perhaps because of them), a number of concerns have arisen about the robustness of current SN Ia cosmology constraints. The SN Ia Hubble diagram is constructed from combining low- and high-redshift SN Ia samples that have been observed with a variety of telescopes, instruments, and photometric passbands. Photometric offsets between these samples are degenerate with changes in cosmological parameters. In addition, the low-redshift SN Ia measurements that are used both to anchor the Hubble diagram and to train SN Ia distance estimators were themselves compiled from combinations of several surveys using different telescopes and selection criteria. When these effects are combined with uncertainties in intrinsic SN Ia color variations and in the effects of dust extinction, the result is that the current constraints are largely dominated by systematic as opposed to statistical errors. Roughly 103 supernovae have been discovered in the history of astronomy. In comparison, LSST will discover over ten million supernovae during its ten-year survey, spanning a very broad range in redshift and with precise, uniform photometric calibration. This will enable a dramatic step forward in supernova studies, using the power of unprecedented large statistics to control systematic errors and thereby lead to major advances in the precision of supernova cosmology. This overwhelming compendium of stellar explosions will also allow for novel techniques and insights to be brought to bear in the study of large-scale structure, the explosion physics of supernovae of all types, and star formation and evolution. The LSST sample will include hundreds of thousands of well-measured Type Ia supernovae (SNe Ia), replicating the current generation of SN Ia cosmology experiments several hundred times over in different directions and regions across the sky, providing a stringent test of homogeneity and isotropy. Subsamples as a function of redshift, galaxy type, environment, 379 Chapter 11: Supernovae and supernova properties will allow for detailed investigations of supernova evolution and of the relationship between supernovae and the processing of baryons in galaxies, one of the keys to understanding galaxy formation. Such large samples of supernovae, supplemented by follow-up observations, will reveal details of supernova explosions both from the large statistics of typical supernovae and the extra leverage and perspective of the outliers of the supernova population. The skewness of the brightness distribution of SNe Ia as a function of redshift will encode the lensing structure of the massive systems traversed by the SN Ia light on the way to us on Earth. There will even be enough SNe Ia to construct a SN Ia-only baryon acoustic oscillation measurement that will provide independent checks on the more precise galaxy-based method as well as allowing for a SN Ia-only constraint on Ωm. This will allow SNe Ia alone to constrain σ8 and Ωm, as well as the properties of dark energy over the past 10 billion years of cosmic history. Finally, millions of supernovae will allow for population investigations using supernovae that were once reserved for large galaxy surveys (Wood-Vasey et al. 2009). Supernovae are dynamic events that occur on time scales of hours to months, but they allow us to probe billions of years into the past. From studying nearby progenitors of these violent deaths of stars, to studying early massive stellar explosions and connecting with the star formation in between, to probing properties of host galaxies, clusters and determining the arrangement of the galaxies themselves, supernovae probe the scales of the Universe from an AU to a Hubble radius. The standard LSST cadence of revisits every several days carried out over years, in addition to specialized “deep-drilling” fields to monitor for variations on the time scale of hours (§ 2.1), offers the perfect laboratory to study supernovae. In this chapter we consider the science possible with two complementary cadences for observations with the LSST, one based on the standard cadence of 3–4 days (the “main” survey in what follows) and another survey over a smaller area of sky using a more rapid cadence, going substantially deeper in a single epoch (the “deep” survey). As described in § 2.1, the detailed plans for the deep-drilling fields are still under discussion, but they have two potential benefits: allowing us to get improved light curve coverage for supernovae at intermediate redshifts (z ∼ 0.5), and pushing photometry deep enough to allow SNe Ia to z ∼ 1 to be observed. With this in mind, the ideal cadence would be repeat observations of a single field on a given night, totalling 10-20 minutes in any given band. We would return to the field each night with a different filter, thus cycling through the filters every five or six nights. Because the SNe Ia discovered in this mode will be in a small number of such deep fields, it is plausible to imagine carrying out a follow-up survey of their host galaxies with a wide-field, multi-object spectrograph to obtain spectroscopic redshifts. The outline of this chapter is as follows. § 11.2 describes detailed simulations of SN Ia light curves for both the main and deep LSST fields and the resulting expected numbers of measured SNe Ia as a function of redshift for different selection criteria, while § 11.3 quantifies the contamination of the SN Ia sample by core-collapse supernovae. In § 11.4 we discuss photometric redshift estimation with SN Ia light curves, the expected precision of such redshift estimates, and their suitability for use in cosmological studies. § 11.5 describes the constraints on dark energy that will result from such a large SN Ia sample. The large sky coverage of the LSST SN Ia sample will enable a novel probe of large-scale homogeneity and isotropy, as described in § 11.6. § 11.7, presents the issue of SN Ia evolution, a potential systematic for SN Ia cosmology studies. In § 11.8 we discuss SN Ia rate models and the use of LSST to probe them. § 11.9 describes how SN Ia measurements can be used to measure the baryon acoustic oscillation feature in a manner complementary to that using 380 11.2 Simulations of SN Ia Light Curves and Event Rates the galaxy distribution. The effects of weak lensing on the distribution of SN Ia brightness and its possible use as a cosmological probe are described in § 11.10. We then turn to core-collapse SN, discussing their rates in § 11.12 and their use for distance measurements and cosmology in § 11.12. § 11.13 discusses the prospects for measuring SN light echoes with LSST. Pair-production SNe, the hypothetical endpoints of the evolution of supermassive stars, are the subject of § 11.14. Finally, we conclude with a discussion of the opportunities for education and outreach with LSST supernovae in § 11.15. 11.2 Simulations of SN Ia Light Curves and Event Rates Stephen Bailey, Joseph P. Bernstein, David Cinabro, Richard Kessler, Steve Kuhlman We begin with estimates for the anticipated SN Ia sample that LSST will observe. We put emphasis on those objects with good enough photometry that detailed light curves can be fit to them, as these are the objects that can be used in cosmological studies as described below. SNe Ia are simulated assuming a volumetric rate of −5 1.5 3 −3 −1 rV (z) = 2.6 × 10 (1 + z) SNe h70 Mpc yr (11.1) based on the rate analysis in Dilday et al. (2008). See § 11.8 for how LSST can improve the measurement of SNe Ia rates. At redshifts beyond about z ' 1, the above rate is likely an overestimate since it does not account for a correlated decrease with star formation rates at higher redshifts. We simulated one year of LSST data; thus the total numbers of SNe we present below should be multiplied by ten for the full survey. For each observation the simulation determines the (source) flux and sky noise in photoelectrons measured by the telescope CCDs, and then estimates the total noise, assuming PSF fitting, that would be determined from an image subtraction algorithm that subtracts the host galaxy and sky background. For this initial study we have ignored additional sky noise from the deep co-added template used for the image subtraction, as well as noise from the host galaxy; we expect these effects to be small. We use the outputs of the Operations Simulator (§ 3.1) to generate SN Ia light curves with a realistic cadence, including the effects of weather. The light curves were made by SNANA (Kessler et al. 2009b), a publicly available1 simulation and light curve fitter. We carry out simulations separately for the universal cadence, and for seven deep drilling fields, which are visited in each filter on a five-day cadence (see the discussion in § 2.1).