## Supernova Rates

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The evolution of the rate of Supernovae with redshift contains unique information on the galaxy star formation history and initial mass function. In particular, the rate of core collapse (CC) SNe (Type II + Ib/c), originating from the collapse of the core of massive stars $(M > 8M_{\odot})$, is a direct tracer of the ongoing star formation rate and of the chemical enrichment of elements like O and Ca. By measuring the evolution of the CC SN rate with redshift it is possible to reconstruct the history of star formation and to put constraints on different models of galaxy evolution. On the other hand, the thermonuclear explosions of accreting white dwarf stars in close binary systems, which originate Type Ia SNe, give a major contribution to the iron-peak elements and echo the long term star formation history. The actual configuration of the precursor binary system is still an open issue that can be investigated by studying the evolution of the Type Ia SN rate with redshift.
Despite the obvious interest, so far an accurate estimate of the rate of SNe has been obtained only for the local Universe. For more distant galaxies the current estimates are mainly by-products of surveys optimised to tackle other scientific purposes. The vast majority of SN rate estimates at intermediate and high redshifts have been obtained in the framework of programs aimed at exploiting Type Ia SNe as distance indicators to probe the geometry of the Unverse. To achieve their goals, these programs need a large sample of well observed, un-reddened Type Ia SNe spanning a large redshift range. The limiting factor is mainly due to the requirements imposed by obtaining a well sampled SN light curve, which is the key ingredient in the procedure used to make Type Ia SNe, actually, standard candles.
For this reason the sample of objects collected by these programs are affected by a severe, intrinsic bias. This is even more the case for CC SNe which are normally considered as “contamination” by such programs. The candidate screening process is indeed tuned to avoid false positives getting through to the expensive spectroscopic follow-up stage.
Addressing these issues was among the main goal of the southern intermediate redshifts ESO supernova search (STRESS) program. The project run between 2000 and 2005 discovering several SNe between redshift 0.1 and 0.7. The novelties of the study was represented by the completely unbiased selection of both CC and Type Ia SNe, by the adoption of the photometric redshift technique to characterise the galaxy sample and by the first quantification of the actual AGN/QSO contamination in a SN search at intermediate redshifts. Preliminary results based on a subset of the data where presented in Cappellaro, Riello, Altavilla et al. 2005 providing the first direct measurement of the rate of CC SNe at z=0.26 and evidence for an increase of a factor of three (+/- 50%) with respect to local values.

Currently I am contributing to the preparation of the final paper (Botticella, Riello, Cappellaro, et al. 2007 – hopefully). This new work is not a mere copy of the previous work with just a larger sample but it actually includes some novelties. First, we used dust extinction models developed by Riello & Patat 2006 to correct the observed rates for the effect of host galaxy extinction. Typically a SN search of this kind is mainly limited by the limited telescope time that can be allocated for the spectroscopic follow-up that provides the most reliable evidence to actually prove a candidate to be a SN rather than an AGN or a QSO. In the attempt to overcome such a limitation we performed a short, post-mortem program aimed at obtaining spectra of the host galaxies of those SN candidates that could not make it to the spectroscopic follow up because of the lack of telescope time. In this way we have been able to put a reliable constraint on the actual AGN/QSO contamination and to use the unconfirmed candidates (in a statistical fashion).
Watch out for more results soon !

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