Feeling Blue: Constructing a Robust SALT3 UV Template and Constraining its Redshift Dependency

TL;DR

This work addresses the challenge of calibrating Type Ia supernovae in the rest-frame UV, where data are currently sparse and UV diversity is large. It introduces SALT3-UV, a robust UV-extended SED template trained with high-quality HST/STIS UV spectra via the SALTshaker framework, improving UVtemplate precision and enabling exploration of redshift evolution. The authors demonstrate substantial UV-area improvements over the previous SALT3-K21 model, quantify potential UV-driven evolutionary effects on rest-frame observables, and assess implications for the dark energy parameter $w$ in future cosmological surveys like LSST and Roman. The findings suggest that, if UV evolution is real, neglecting it could bias $w$ by a few percent, underscoring the need for UV-calibrated SN Ia templates in high-$z$ cosmology and motivating re-training as ground-based UV calibrations improve.

Abstract

Upcoming cosmological surveys will obtain numerous rest-frame ultraviolet (UV) observations of Type Ia supernovae (SNe Ia), yet there is concern about how standardizable SNe Ia are in the UV. In this work, we train a robust optical--UV SED model for SNe Ia (SALT3-UV) with the open-source model-training software $\texttt{SALTshaker}$. We incorporate a spectroscopic UV data sample from HST, including 67 UV spectra from 18 nearby SNe Ia. Unlike previous training spectra, the HST spectra have sufficiently precise calibration that they do not require additional warping to match coincident photometric data. Additionally, while including this new SN Ia sample necessitates incorporating auxiliary photometric data from ZTF and ATLAS that have insufficient calibration for cosmological analyses, the improvements in the calibration of these data is anticipated in the near future. Compared to the previous SALT3-K21 model, the SALT3-UV model shows a significant improvement in the UV down to $2000\mathring{\text{A}}$, with over a threefold improvement in model uncertainty and a more physically accurate continuum and line features. We further evaluate potential redshift evolution in the UV template by separating the UV training sample into low- and high-$z$ subsamples. Our results reveal a non-negligible $\gtrsim 0.05$ mag difference between low- and high-$z$ SALT3-UV models in the $g-$band at $z\gtrsim0.5$ and the $u-$band at $z\gtrsim0.2$. We demonstrate that, if confirmed, such evolution could lead to a few-percent bias in the measurement of $w$ if high-$z$ rest-frame UV data are included in future cosmological surveys such as LSST and $\textit{Roman}$.

Figures (6)

• Figure 1: The number of UV spectra in the training sample as a function of redshift. The blue solid line represents the new HST/STIS G230L spectra in the SALT3-UV training sample, all of which are at low $z$ and with minimum wavelengths around $1660$Å. Dashed lines in different colors represent the spectra in SALT3-K21 with different minimum wavelengths in the rest frame, $\lambda_{min}^{rest}$, and the black solid line shows the summed training sample for the SALT3-UV model. Most low-$z$ UV spectra in the previous SALT3-K21 training sample only marginally probe the UV, and at $\lambda<2500$Å the SALT3-K21 sample is dominated by low S/N spectra at high $z$.
• Figure 2: The density of photometric (top) and spectroscopic (bottom) data in our SALT3-UV training sample, as a function of rest-frame wavelength and days relative to peak brightness (phase). The sample can be divided into low-$z$ ($z<0.1$, left) and high-$z$ ($z\geq0.1$, middle) subsamples with distinct UV data. The low-$z$ subsample and high-$z$ subsample (supplemented with low-$z$ optical data) are used for separate model trainings in Section \ref{['sec:ztraining']} to evaluate potential redshift dependence in the UV.
• Figure 3: The comparison between the $M_0$ and $M_1$ component of the SALT3-UV and SALT3-K21 model between $2200-5000$Å . Note that the relatively high regularization factors of the SALT3-K21 model over-smooth the UV templates and create a non-vanishing continuum at the short wavelength end.
• Figure 4: The SALT3-UV model flux integrated over optical bands in comparison with the SALT3.K21 model, as a function of phase and $x_1$ parameter.
• Figure 5: Left: Comparison between the color scatter of the SALT3-K21 and SALT3-UV models. Right: Difference between the color law of the SALT3-UV model and the SALT3-K21 model. $\sigma_c = 0.1$ is the standard deviation of the color parameter $c$ in a typical SN Ia sample.