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December 9th, 2022

The first paper has been already published in A&A! (Gallego-Cano et al. 2022)
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In the early 20th century, Edwin Hubble observed that the Universe is expanding at a rate given by the Hubble constant (Hubble 1929). Today, the best model to explain its late expansion acceleration and the formation of the structure of the Universe is the standard cosmological model ΛCMD (Peebles 2012), thanks to the presence of still unexplained “dark” components (dark matter and dark energy).

The Hubble constant (H0) is a key parameter for the standard cosmological model since it establishes the size and age scale of the Universe. The value of H0 can be estimated both at early times, by the sound horizon from the cosmic microwave background (CMB, Planck Collaboration et al. 2020), and in the local present-time Universe using luminosity distance indicators (e.g. Freedman et al. 2001; Riess et al. 2016). However, these estimates show a discrepancy, which represents one of the main problems in cosmology. This “tension” may indicate new physics beyond the standard cosmological model. Therefore, it is mandatory to explore the possible systematics in the local values of H0 and obtain more precise measurements.

Type Ia supernovae (SNe Ia) play an essential role as distance indicators for cosmological studies and their observations show that the expansion of the Universe is accelerating (Riess et al. al. 1998; Perlmutter et al. 1998). SNe Ia can be used as “standard candles” for calculating distances in the Universe, after applying various corrections to their absolute luminosity, absolute magnitude-decay, and intrinsic color-reddening relationships (Phillips 1993; Hamuy et al. 1993; Riess et al. 1996; Tripp 1997). Recent studies suggest that the calibration of SNe Ia depends on the environment of SN progenitors, which can affect their physical properties, and thus the luminosity emitted during the SN event (see Figure 1).

The SN2 project aims at investigating SN Ia progenitors' dependence on their environments by searching for correlations between SN photometric observables with intrinsic properties of their host galaxies (metallicity, star-formation rate, stellar ages, and masses). All this information can be obtained from host galaxy spectra using line indicators, such as lick indices, gas emissions lines, and/or analysis of the stellar population component(s).

Figure 1. The distribution of the stretch-colour (sBV) parameter in the Combined Pantheon dataset according to the morphological type of the SN host galaxies (early-type vs late-type). The distribution reflects the trend observed for the calibration samples, with low values found more in early-type galaxies, for which we have the majority of SBF distances measured, and high values mainly observed in SNe within late-type galaxies, where we easily observe Cepheid.

Main objectives

  • Building a statistically-significant photometric sample of ∼ 70 SNe Ia.
  • Including events from different SN host environments, in a redshift range spanning from the local Universe (z = 0.001) up to z ∼ 0.07.
  • Anchoring SN luminosity relations using Cepheids, the tip of the red giant branch (TRGB), surface brightness fluctuations (SBF), and gravitational wave standard sirens detected by LIGO-Virgo from the coalescence of binary neutron stars (Abbott et al. 2017).
  • Studying galaxies that hosted two or more SNe Ia (“sibling”), than can mitigate systematics in their distance estimates.
  • Providing accurate estimates of their luminosity measurements, and reducing various systematic errors involved in estimating the Hubble constant, contributing to the understanding of the “Hubble constant problem”.