The cosmic supernova rate at high redshift

I have chosen to discuss this science case in some detail (rather than the perhaps more exciting ones of the terrestrial exo-planets or the direct detection of the cosmic deceleration that I also described in my talk) because it gives a clear example of the kind of detectors we need to achieve the science objectives OWL will make possible. Some details on the other cases may be found in the paper by Sandro D'Odorico in these proceedings.

The detection and the study of high redshift (z) supernovae (SNe) is important not only because their use as calibrated standard candles allows measurements of the acceleration of the Universe and to probe the different cosmological models, but also because the evolution of the cosmic SN rate provides a direct measurement of the cosmic star formation rate.

Figure 4. Simulation of OWL observations of SNe. The points between z = 0 and z = 1 are actual observations, the black points to z = 10 are core-collapse supernovae (Type II and Ib/c), the dark gray points between z = 1 and z = 5 are Type Ia SNe (binary systems). The SNe above z = 10 are hypothetical SNe from the very first generation of stars in the Universe. The curves represent models of the Universe. The simulation is based to today's "favorite" model, but the OWL observations will determine if a different one is should be considered.

Figure 4. Simulation of OWL observations of SNe. The points between z = 0 and z = 1 are actual observations, the black points to z = 10 are core-collapse supernovae (Type II and Ib/c), the dark gray points between z = 1 and z = 5 are Type Ia SNe (binary systems). The SNe above z = 10 are hypothetical SNe from the very first generation of stars in the Universe. The curves represent models of the Universe. The simulation is based to today's "favorite" model, but the OWL observations will determine if a different one is should be considered.

SNe are enormously energetic explosions that can be seen at large distances in the Universe. They come from different physical mechanisms: core-collapse explosion of massive stars at the end of their 'life' (due to the star having consumed all its usable nuclear fuel), or thermonuclear explosion of a white dwarf in a binary star system (due to its accreting matter from the companion to above a critical threshold). These different types of SNe have specific signatures that allow astronomers to distinguish among them. Determining their rate at various redshifts (i.e. at various look back times) will tell us about the distribution of their parent stars, from which we can deduce the history of star formation, one of the critical elements to understand the evolution of the Universe from the initial fireball to today.

Figure 4 is a simulation that Massimo della Valle and I did a couple of years ago, showing the expected results of OWL observations based on some assumptions on the SN rates valid in the far Universe (extrapolated from our knowledge up to redshift z = 1) and on the current best knowledge of the "model" describing the Universe. The anticipated quality of the data is such that if it turns out that a different model would describe better the Universe, it would immediately be shown by the data.

To obtain this data set one needs a substantial amount of observing time, and a reasonable field of view. The data shown in Fig. 4 require that one find the SNe with a suitable search strategy, characterize them (to know what their "intrinsic" luminosity is, and at which redshift they are), and follow them during their light curve (to be sure that they are detected at their maximum light). All this translates into about 120 nights observing in the J, H and K infrared bands with a field of view of 2 x 2 arcminutes sampled at the half the diffraction limit (1.6 mas at J). In other words, one would need a 75Kx75K detector (more than 5 Gpixel, about 1 m2 if the pixels are 15 ^m in size). Ways to resize the science case may possibly be found, whereby the observations are made sampling the focal plane in a smart way, or taking a longer time with a smaller field of view, or accepting a less optimal sample statistics. But one thing is certain: to avoid that the detectors cost more than the telescope (!) we need to find ways to decrease the costs from the current 10 0/pixel to below 1 0/pixel. A breakthrough may come from the fact that by next decade the request for pixels will have increased more than 100-fold with respect to today, and this will probably require some paradigm shifts (e.g. the development of a new mass production, cost saving approach).

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