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Register a new userVLT Optical BVR observations of two bright Supernova Ia hosts in the Virgo cluster
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Michele Cantiello
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We study the two galaxies NGC4621 and NGC4374 in the Virgo cluster to derive their distances and stellar population properties. The targets have hosted three type Ia Supernova events allowing to investigate the correlations between the SNeIa and their host stellar systems. Using deep BVR data, obtained with FORS2 at the VLT, we analyse the Surface Brightness Fluctuations (SBF) properties of the targets. We adopt our measurements and existing calibrations to estimate the distance of NGC4621 and NGC4374. For stellar population analysis, we measured SBF amplitudes in different galaxy regions. We present a detailed comparison between data and models to constrain the characteristics of the dominant stellar components at i) various galactic radii, and ii) in the regions where SNeIa were recorded. Our V and R SBF measures provide distances in agreement with literature estimates. The median of our and literature SBF-based distances agrees with the one from non-SBF methods. Comparing data with models we find that stellar populations properties do not change significantly along galactic radius, with a dominant population having old age and 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 a hot stellar component, we speculate that such behaviour is a consequence of different diffuse hot components in the galaxies. We find that the presence of a percentage of hot-HB stars in old and metal rich stellar populations could be at the origin of the observed differences. We find a good uniformity in the V and R SBF and integrated colours in the regions where the three SNeIa exploded. On the other hand, the B-band SBF signal shows intriguing differences.
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We aim to present 70 spectra of 68 new high-redshift type Ia supernovae (SNeIa) measured at ESOs VLT during the final two years of operation (2006-2008) of the Supernova Legacy Survey (SNLS). We use the full five year SNLS VLT spectral set to investigate a possible spectral evolution of SNeIa populations with redshift and study spectral properties as a function of lightcurve fit parameters and the mass of the host-galaxy. Reduction and extraction are based on both IRAF standard tasks and our own reduction pipeline. Redshifts are estimated from host-galaxy lines whenever possible or alternatively from supernova features. We used the spectrophotometric SNIa model SALT2 combined with a set of galaxy templates that model the host-galaxy contamination to assess the type Ia nature of the candidates. We identify 68 new SNeIa with redshift ranging from z=0.207 to z=0.98 (<z>=0.62). Each spectrum is presented individually along with its best-fit SALT2 model. The five year dataset contains 209 spectra corresponding to 192 SNeIa identified at the VLT. We also publish the redshifts of other candidates (host galaxies or other transients) whose spectra were obtained at the same time as the spectra of live SNe Ia. Using the full VLT SNeIa sample, we build composite spectra around maximum light with cuts in color, lightcurve shape parameter (stretch), host-galaxy mass and redshift. We find that high-z SNeIa are bluer, brighter and have weaker intermediate mass element absorption lines than their low-z counterparts at a level consistent with what is expected from selection effects. We also find a flux excess in the range [3000-3400] A for SNeIa in low mass host-galaxies or with locally blue U-V colors, and suggest that the UV flux (or local color) may be used in future cosmological studies as a third standardization parameter in addition to stretch and color.
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