1 Introduction

It is of paramount importance to determine distances to external galaxies: such knowledge impacts both stellar and extragalactic astrophysics as well as cosmology and the structure of the Universe including the derivation of the Hubble constant (H₀). Unfortunately the standard "luminosity-calibrated'' methods of distance determination have not yet allowed an unambiguous determination of the Hubble constant (see the Proceedings of "The Extragalactic Distance Scale'' STScI May 1996 Symposium for an overview), and all luminosity based indicators are extinction and metallicity dependent in some way. Thus (new) physical and geometric methods of distance determination are highly desirable and should be pursued wherever possible.

Sparks (1994) proposed that distances to external galaxies (well in excess of the distance to the Virgo cluster) could be determined by means of supernova (SN) light echoes. The method is appealing as it is purely geometrical, does not need any secondary distance indicators or calibration and might be used to relate to luminosity distance indicators when applied to galaxies hosting Cepheids and/or Type Ia supernovae (Sparks 1994, 1996).

The technique requires high resolution imaging polarization observations of SN light echoes. A light echo is expected to be produced by scattering of the SN light by dust in the interstellar medium, and would be visible to the observer at some time after the supernova explosion (due to light travel time effects). The evolution of such a feature is well understood (e.g. Chevalier 1986) and mathematically straightforward. To summarize, the light echo is a paraboloid at the focus of which the historical supernova lies. The observer looks down the axis, "into'' the paraboloid. While the intensity distribution of the echo is likely to be complex, depending primarily on dust density, by contrast, the polarization distribution should be simple. Angular distance from the supernova is related to the scattering angle, and polarization depends simply on scattering angle, maximizing at $90^{\circ}$ scattering. Hence at the intersection between the plane of the sky containing the supernova and the parabola, light is scattered at an angle of 90 degrees and forms a ring in an image of the degree of polarization (this light having the maximum degree of polarization). The linear diameter of the ring is 2ct, where c is the speed of light and t is the time since the explosion, therefore, if ${\rm {\phi}}$, the angular diameter of the ring is measured, the distance D is derived geometrically by ${D}={2ct}/{\rm {\phi}}$.

Light echoes have been observed around SN 1987A (Crotts 1988; Sparks et al. 1989) and SN 1991T (Schmidt et al. 1994). From the ground we would not expect to resolve light echoes around known historical supernovae in most cases. Yet from the ground candidate light echoes can be sought and identified by using imaging and, when appropriate, polarization criteria: a good light echo candidate shows optical emission at/near the site of the historical supernova, is blue in color, because of both the intrinsic "blueness'' of the SN and the additional blueing introduced by the scattering process, and is polarized. The discovery of light echo candidates and subsequent confirmation either through ground-based spectroscopic observations, or direct space imaging polarimetry observations is an essential step in the process of enabling general geometric galaxy distance determinations by this technique. Here we describe a program to search for candidate echoes via CCD imaging. Sparks et al. (1999) show how space imaging polarimetry can be used in the well known case of the SN 1991T light echo. The region of maximum linear polarization emission cannot be resolved yet due to the small time since the explosion and the distance to the galaxy. However, we give a distance estimate to SN 1991T (an upper limit of 15 Mpc is found) via simple modeling of the scattering process.

Although the main objective of such a search program is to find light echoes and use them to determine distances, the photometric and spectroscopic data collected, together with the polarimetry, should also provide important information on the supernova environments in the host galaxy. More generally, what we learn about environments of different SN Types in turn tells us something about the SN progenitor and the stellar populations of galaxies. In particular we think of SNe of Type Ia whose star progenitor systems have not unambiguously been identified yet. A careful investigation of both circumstellar and interstellar environments of these SNe may provide clues on the nature of their progenitors and in some instances allow to test some of the proposed pre-SN scenarios (Boffi & Branch 1995; Branch et al. 1995). The identification of Type Ia SN star system progenitors is important (among other reasons) because the nature of the progenitors is connected to the use of these SNe as distance indicators and to derive cosmological parameters (Perlmutter et al. 1998; Riess et al. 1998). In fact to determine the mass and energy density of the Universe from SNe of Type Ia in the Hubble diagram, the evolution with cosmic epoch of both the Type Ia SN rate and their luminosity function are relevant; these functions depend on the nature of the progenitor systems.

In Sect. 2 the sample of galaxies is presented and we explain the selection criteria used. The observations are described and the analysis process is presented in Sect. 3. Our results are summarized in Sect. 4 where we discuss (a) the results from our search for candidate light echoes; (b) some by-products of the present investigation in terms of SN environments and of SN star progenitors. Conclusions and plans for future work are discussed in the last section.