VLT optical BVR observations of two bright supernova Ia hosts in the Virgo cluster

Surface brightness fluctuation analysis

¹ INAF – Osservatorio Astronomico di Teramo, via M. Maggini snc, 64100 Teramo, Italy
e-mail: cantiello@oa-teramo.inaf.it
² Dipartimento di Fisica – Universitá di Roma Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy

Received: 7 February 2011
Accepted: 6 June 2011

Abstract

Aims. We study the characteristics of field stars in the two bright ellipticals NGC 4621 and NGC 4374 in the Virgo cluster to derive accurate distances and stellar population properties. Moreover, since the target galaxies have hosted three type Ia supernova events, we investigate the possible correlations between the SNe Ia properties and their host stellar systems.

Methods. Using deep imaging BVR data, obtained with the FORS2 camera mounted at the VLT, we analysed the surface brightness fluctuations (SBF) properties of the targets. We adopted our measurements and existing empirical or theoretical calibrations to estimate the distance of the NGC 4621 and NGC 4374. For stellar population analysis, we measured SBF amplitudes in different galaxy regions, allowing us to study the changes in field star properties. The three-band coverage of present data, coupled with existing SBF measures available from the literature, provides us with the broadest wavelength coverage of SBF magnitudes for single objects. We present a detailed comparison between SBF data and models to constrain the physical characteristics of the dominant stellar components at i) various galactic radii; and ii) in the regions where SNe Ia events were recorded.

Results. Our V and R SBF measures, coupled with either empirical or theoretical calibrations, provide distance moduli in agreement with literature estimates. The median of our and literature SBF-based distances agrees with the median obtained from non-SBF methods, indicating the absence of any systematic effect in the SBF technique. The same result holds for SBF and SN Ia distances. Comparing either the SBF versus integrated colour diagrams, or the SBF versus SBF colour diagrams, with SPoT models, we find that stellar population properties do not change significantly along the galactic radius, with a dominant population having old age and nearly solar chemical composition. The galaxies appear similar in all properties analysed, except for B-band SBF. Since the SBF magnitudes in this band are sensitive to the properties of the hot stellar component, we speculate that such behaviour can be a consequence of different diffuse hot stellar components in the two objects. By using specific models, we find that the presence of a percentage of hot HB stars in old and metal-rich stellar population could be at the origin of the observed differences. Finally, we find a good uniformity in the V and R SBF and integrated colours in the regions where the three SNe Ia, presenting different absolute luminosities, exploded. On the other hand, the SBF signal measured in the B-band shows intriguing differences.