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Survey for Supernovae at High Redshift

Prepared by Danziger J., Benetti, S., Cappellaro, E., Della Valle, M., Mazzali, P., Patat, F., Turatto, M.

A search for SNe at high redshift will give a unique opportunity to address two crucial cosmological issues:

a) - type Ia SNe are probably the best distance indicators to test cosmological models. The reason is that they are very bright ( $M \sim
-19$), can be easily recognised at large distances and their absolute magnitude can be calibrated based on the observed luminosity decline rate.

Two major programs are running today aimed at discovering SNe at intermediate redshift ( z = 0.3-0.8) (Perlmutter et al. 1998; Garnavich et al. 1998). In a few years, after sufficient statistics are accumulated, this data will be able to settle definitively the debate on the value of the Hubble constant and to better define the geometry of the Universe.

On the other hand to really constrain the value of the density parameter and to disentangle the relative contribution of the density of matter from the cosmological constant we need to accumulate statistics on SNe at higher redshift.

The WF camera at the LBT will allow the discovery of SNIa up to z=1.5 and would therefore give a fundamental contribution in this respect.

b) - the evolution of the rate of Supernovae (SNe) with redshift contains unique information on the star formation history, the initial mass function of stars and the progenitor scenario (e.g. Madau et al., 1998).

In particular estimates of the rates of SN Ia, Ib/c+II up to redshift z=1 can be used as an independent test for the star formation and heavy element enrichment history of the Universe and significantly improve our understanding of the intrinsic nature and the age of the populations involved in the SN explosions. Also for Ia's, the observed rates depend on the timescale of the explosion delay after the collapse of the primary star to a White Dwarf.

This means that measurements of the SN rates at $z \geq 0.5$ will give important insights in the physics of SN Ia explosions and in the galaxy evolution scenarios.

Because of the presence of different selection effects in SN searches relative SN rates have smaller uncertainties than absolute SN rates. The estimate of the relative SN rates as a function of z will be limited by the detection threshold for the faintest SN types. With a WF camera at LBT we will be able to detect normal SNII up to z=0.6-0.8 and therefore derive relative SN rates up to these distances.

We stress that at present estimates of the relative SN rates are available only for the local universe (Cappellaro et al. 1997) and there have been only very preliminary attempts to derive the rate of SN Ia up to $z \le 0.3-0.4$ (Pain et al. 1998).

Conservatively, both for the use of SN Ia as distance indicators and for the determination of the rates of SNe as a function of z one would like to obtain spectroscopic confirmation of the candidates. This is the strategy adopted by ongoing SN search programs.

However, even for today's intermediate z searches this is a very expensive strategy since for the faintest candidates it requires long exposures at 8m class telescope. Actually the spectroscopic classification of the faintest SNe discovered with the WF camera of LBT may require a spectrograph feed by the telescope adaptive optics.

For this reason we are exploring also the alternative strategy of a statistical approach based only on broad band photometry for the confirmation and classification of the SN candidates. The basic idea is that the different types of SNe have in general very characteristic light and colour evolution. This means that by properly sampling the light and colour curves one is able to confirm that the object is indeed a SN, and assign it a type with a given confidence level.

One of the problems for deriving the SN rates at intermediate redshift is that usually one does not know the redshift and morphological type of the observed galaxy. Again a conservative approach would be to concentrate our effort on a few selected fields where this information is or will become available. Another possibility is the use the same multicolor images used for the SN search and confirmation to derive photometric redshift and colour for all galaxy in the field.

The fact that for each single event there will be a non-negligible chance of misclassification will be compensated by the possibility to build much better statistics, which is mandatory for the purposes discussed above.

Cappellaro, E. et al., 1997, A&A, 322, 431
Madau, P., Della Valle, M. & Panagia, N., 1998, M.N.R.A.S., in press
Pain, R. et al. 1997, ApJ 473, 356
Perlmutter, S. et al., 1998 Nature 391, 51
Garnavich, P.M. et al., 1998 ApJ 493, L53

next up previous contents
Next: Observations of Intracluster Planetary Up: The Scientific Cases for Previous: Search and Luminosity Function
Guido Buscema