Carnegie Supernova Project Research Articles

Optical and near-infrared photometry of 94 type II supernovae from the Carnegie Supernova Project

Type II supernovae (SNe II) mark the endpoint in the lives of hydrogen-rich massive stars. Their large explosion energies and luminosities allow us to measure distances, metallicities, and star formation rates into the distant Universe. To fully exploit their use in answering different astrophysical problems, high-quality low-redshift data sets are required. Such samples are vital to understand the physics of SNe II, but also to serve as calibrators for distinct -- and often lower-quality -- samples. We present $uBgVri$ optical and $YJH$ near-infrared (NIR) photometry for 94 low-redshift SNe II observed by the Carnegie Supernova Project (CSP). A total of 9817 optical and 1872 NIR photometric data points are released, leading to a sample of high-quality SN II light curves during the first sim 150 days post explosion on a well-calibrated photometric system. The sample is presented and its properties are analysed and discussed through comparison to literature events. We also focus on individual SNe II as examples of classically defined subtypes and outlier objects. Making a cut in the plateau decline rate of our sample ($s_2$), a new subsample of fast-declining SNe II is presented. The sample has a median redshift of 0.015, with the nearest event at 0.001 and the most distant at 0.07. At optical wavelengths ($V$), the sample has a median cadence of 4.7 days over the course of a median coverage of 80 days. In the NIR ($J$), the median cadence is 7.2 days over the course of 59 days. The fast-declining subsample is more luminous than the full sample and shows shorter plateau phases. Of the non-standard SNe II highlighted, SN 2009A particularly stands out with a steeply declining then rising light curve, together with what appears to be two superimposed P-Cygni profiles of in its spectra. We outline the significant utility of these data, and finally provide an outlook of future SN II science.

Carnegie Supernova Project I and II: Measurements of H 0 Using Cepheid, Tip of the Red Giant Branch, and Surface Brightness Fluctuation Distance Calibration to Type Ia Supernovae* *This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

We present an analysis of Type Ia supernovae (SNe Ia) from the Carnegie Supernova Project I and II and extend the Hubble diagram from optical to near-infrared wavelengths (uBgVriYJH). We calculate the Hubble constant, H 0, using various distance calibrators: Cepheids, the tip of the red giant branch (TRGB), and surface brightness fluctuations (SBFs). Combining all methods of calibration, we derive H 0 = 71.76 ± 0.58 (stat) ± 1.19 (sys) km s−1 Mpc−1 from the B band and H 0 = 73.22 ± 0.68 (stat) ± 1.28 (sys) km s−1 Mpc−1 from the H band. By assigning equal weight to the Cepheid, TRGB, and SBF calibrators, we derive the systematic errors required for consistency in the first rung of the distance ladder, resulting in a systematic error of 1.2 ∼ 1.3 km s−1 Mpc−1 in H 0. As a result, relative to the statistics-only uncertainty, the tension between the late-time H 0 we derive by combining the various distance calibrators and the early-time H 0 from the cosmic microwave background is reduced. The highest precision in SN Ia luminosity is found in the Y band (0.12 ± 0.01 mag), as defined by the intrinsic scatter (σ int). We revisit SN Ia Hubble residual-host mass correlations and recover previous results that these correlations do not change significantly between the optical and near-infrared wavelengths. Finally, SNe Ia that explode beyond 10 kpc from their host centers exhibit smaller dispersion in their luminosity, confirming our earlier findings. A reduced effect of dust in the outskirts of hosts may be responsible for this effect.

Type Ia Supernova Progenitor Properties and their Host Galaxies

We present an eigenfunction method to analyze 161 visual light curves (LCs) of Type Ia supernovae (SNe Ia) obtained by the Carnegie Supernova Project to characterize their diversity and host-galaxy correlations. The eigenfunctions are based on the delayed-detonation (DD) scenario using three parameters: the LC stretch s determined by the amount of deflagration burning governing the 56Ni production, the main-sequence mass M MS of the progenitor white dwarf controlling the explosion energy, and its central density ρ c shifting the 56Ni distribution. Our analysis tool (Supernova Parameter Analysis Tool) extracts the parameters from observations and projects them into physical space using their allowed ranges (M MS ≤ 8 M ⊙, ρ c ≤ 7–8 × 109 g cm−3). The residuals between fits and individual LC points are ≈1%–3% for ≈92% of objects. We find two distinct M MS groups corresponding to a fast (≈4–65 Myr) and a slow(≈200–500 Myr) stellar evolution. Most underluminous SNe Ia have hosts with low star formation but high M MS, suggesting slow evolution times of the progenitor system. 91T-like SNe show very similar LCs and high M MS and are correlated to star formation regions, making them potentially important tracers of star formation in the early Universe out to z ≈ 4–11. Some ∼6% outliers with nonphysical parameters using DD scenarios can be attributed to superluminous SNe Ia and subluminous SNe Ia with hosts of active star formation. For deciphering the SNe Ia diversity and high-precision SNe Ia cosmology, the importance is shown for LCs covering out to ≈60 days past maximum. Finally, our method and results are discussed within the framework of multiple explosion scenarios, and in light of upcoming surveys.

Optical Spectroscopy of Type Ia Supernovae by the Carnegie Supernova Projects I and II

We present the second and final release of optical spectroscopy of Type Ia supernovae (SNe Ia) obtained during the first and second phases of the Carnegie Supernova Project (CSP-I and CSP-II). The newly released data consist of 148 spectra of 30 SNe Ia observed in the course of CSP-I and 234 spectra of 127 SNe Ia obtained during CSP-II. We also present 216 optical spectra of 46 historical SNe Ia, including 53 spectra of 30 SNe Ia observed by the Calán/Tololo Supernova Survey. We combine these observations with previously published CSP data and publicly available spectra to compile a large sample of measurements of spectroscopic parameters at maximum light, consisting of pseudo-equivalent widths and expansion velocities of selected features for 232 CSP and historical SNe Ia (including more than 1000 spectra). Finally, we review some of the strongest correlations between spectroscopic and photometric properties of SNe Ia. Specifically, we define two samples: one consisting of SNe Ia discovered by targeted searches (most of them CSP-I objects) and the other composed of SNe Ia discovered by untargeted searches, which includes most of the CSP-II objects. The analyzed correlations are similar for both samples. We find a larger incidence of SNe Ia belonging to the cool and broad-line Branch subtypes among the events discovered by targeted searches, shallow-silicon SNe Ia are present with similar frequencies in both samples, while core normal SNe Ia are more frequent in untargeted searches.

Extrapolation of Type Ia Supernova Spectra into the Near-infrared Using Principal Component Analysis

We present a method of extrapolating the spectroscopic behavior of Type Ia supernovae (SNe Ia) in the near-infrared (NIR) wavelength regime up to 2.30 μm using optical spectroscopy. Such a process is useful for accurately estimating K-corrections and other photometric quantities of SNe Ia in the NIR. A principal component analysis is performed on data consisting of Carnegie Supernova Project I & II optical and NIR FIRE spectra to produce models capable of making these extrapolations. This method differs from previous spectral template methods by not parameterizing models strictly by photometric light-curve properties of SNe Ia, allowing for more flexibility of the resulting extrapolated NIR flux. A difference of around −3.1% to −2.7% in the total integrated NIR flux between these extrapolations and the observations is seen here for most test cases including Branch core-normal and shallow-silicon subtypes. However, larger deviations from the observation are found for other tests, likely due to the limited high-velocity and broad-line SNe Ia in the training sample. Maximum-light principal components are shown to allow for spectroscopic predictions of the color-stretch light-curve parameter, s BV, within approximately ±0.1 units of the value measured with photometry. We also show these results compare well with NIR templates, although in most cases the templates are marginally more fitting to observations, illustrating a need for more concurrent optical+NIR spectroscopic observations to truly understand the diversity of SNe Ia in the NIR.

Using Rest-Frame Optical and NIR Data from the RAISIN Survey to Explore the Redshift Evolution of Dust Laws in SN Ia Host Galaxies

Abstract We use rest-frame optical and near-infrared (NIR) observations of 42 Type Ia supernovae (SNe Ia) from the Carnegie Supernova Project at low-z and 37 from the RAISIN Survey at high-z to investigate correlations between SN Ia host galaxy dust, host mass, and redshift. This is the first time the SN Ia host galaxy dust extinction law at high-z has been estimated using combined optical and rest-frame NIR data (YJ-band). We use the BayeSN hierarchical model to leverage the data’s wide rest-frame wavelength range (extending to ∼1.0–1.2 μm for the RAISIN sample at 0.2 ≲ z ≲ 0.6). By contrasting the RAISIN and CSP data, we constrain the population distributions of the host dust RV parameter for both redshift ranges. We place a limit on the difference in population mean RV between RAISIN and CSP of −1.16 < Δμ(RV) < 1.38 with 95% posterior probability. For RAISIN we estimate μ(RV) = 2.58 ± 0.57, and constrain the population standard deviation to σ(RV) < 0.90 [2.42] at the 68 [95]% level. Given that we are only able to constrain the size of the low- to high-z shift in μ(RV) to ≲ 1.4 – which could still propagate to a substantial bias in the equation of state parameter w – these and other recent results motivate continued effort to obtain rest-frame NIR data at low and high redshifts (e.g. using the Roman Space Telescope).

Underluminous 1991bg-like Type Ia supernovae are standardizable candles

ABSTRACT It is widely accepted that the width–luminosity relation used to standardize normal Type Ia supernovae (SNe Ia) breaks down in underluminous, 1991bg-like SNe Ia. This breakdown may be due to the choice of parameter used as a stand-in for the width of the SN Ia light curve. Using the colour stretch parameter sBV instead of older parameters resolves this issue. Here, I assemble a sample of 14 nearby 1991bg-like SNe Ia from the literature, all of which have independent host-galaxy distance moduli and little to no reddening. I use Gaussian process regression to fit the light curves of these SNe in U/u, B, V, g, R/r, I/i, and H, and measure their peak absolute magnitudes. I find statistically significant (>5σ confidence level in the optical and >4σ in the near-infrared) correlations between the peak absolute magnitudes of the 1991bg-like SNe Ia and their sBV values in the range 0.2 < sBV < 0.6. These correlations are broadly consistent with fits to sBV < 0.7 SNe Ia with preliminary B- and V-band peak absolute magnitudes from the Carnegie Supernova Project and significantly inconsistent with similar fits to normal and transitional SNe Ia (with 0.7 < sBV < 1.1). The underluminous width–luminosity relation shown here needs to be properly calibrated with a homogeneous sample of 1991bg-like SNe Ia, after which it could be used as a rung on a new cosmological distance ladder. With surface-brightness fluctuations (or another non-Cepheid method) used to calibrate distances to nearby 1991bg-like SNe Ia, such a ladder could produce an independent measurement of the Hubble–Lemaître constant, H0.

SIDE-real: Supernova Ia Dust Extinction with truncated marginal neural ratio estimation applied to real data

Abstract We present the first fully simulation-based hierarchical analysis of the light curves of a population of low-redshift type Ia supernov aelig; (SNaelig; Ia). Our hardware-accelerated forward model, released in the Python package slicsim, includes stochastic variations of each SN’s spectral flux distribution (based on the pre-trained BayeSN model), extinction from dust in the host and in the Milky Way, redshift, and realistic instrumental noise. By utilising truncated marginal neural ratio estimation (TMNRE), a neural network-enabled simulation-based inference technique, we implicitly marginalise over 4000 latent variables (for a set of ≈ 100 SNaelig; Ia to efficiently infer SN Ia absolute magnitudes and host-galaxy dust properties at the population level while also constraining the parameters of individual objects. Amortisation of the inference procedure allows us to obtain coverage guarantees for our results through Bayesian validation and frequentist calibration. Furthermore, we show a detailed comparison to full likelihood-based inference, implemented through Hamiltonian Monte Carlo, on simulated data and then apply TMNRE to the light curves of 86 SNaelig; Ia from the Carnegie Supernova Project,, deriving marginal posteriors in excellent agreement with previous work. Given its ability to accommodate arbitrarily complex extensions to the forward model—e.g. different populations based on host properties, redshift evolution, complicated photometric redshift estimates, selection effects, and non-la contamination—without significant modifications to the inference procedure, TMNRE has the potential to become the tool of choice for cosmological parameter inference from future, large SN Ia samples.

The Carnegie Supernova Project I

An analysis leveraging 170 optical spectra of 35 stripped-envelope (SE) core-collapse supernovae (SNe) observed by the Carnegie Supernova Project I and published in a companion paper is presented. Mean template spectra were constructed for the SNe IIb, Ib, and Ic subtypes, and parent ions associated with designated spectral features are identified with the aid of the spectral synthesis code SYNAPPS. Our modeled mean spectra suggest the ∼6150 Å feature in SNe IIb may have an underlying contribution due to silicon, while the same feature in some SNe Ib may have an underlying contribution due to hydrogen. Standard spectral line diagnostics consisting of pseudo-equivalent widths (pEWs) and blue-shifted Doppler velocity were measured for each of the spectral features. Correlation matrices and rolling mean values of both spectral diagnostics were constructed. A principle component analysis (PCA) was applied to various wavelength ranges of the entire dataset and suggests clear separation among the different SE SN subtypes, which follows from trends previously identified in the literature. In addition, our findings reveal the presence of two SNe IIb subtypes, a select number of SNe Ib displaying signatures of weak, high-velocity hydrogen, and a single SN Ic with evidence of weak helium features. Our PCA results can be leveraged to obtain robust subtyping of SE SNe based on a single spectrum taken during the so-called photospheric phase, separating SNe IIb from SNe Ib with ∼80% completion.

Carnegie Supernova Project. II. Near-infrared Spectral Diversity and Template of Type Ia Supernovae

We present the largest and most homogeneous collection of near-infrared (NIR) spectra of Type Ia supernovae (SNe Ia): 339 spectra of 98 individual SNe obtained as part of the Carnegie Supernova Project-II. These spectra, obtained with the FIRE spectrograph on the 6.5 m Magellan Baade telescope, have a spectral range of 0.8–2.5 μm. Using this sample, we explore the NIR spectral diversity of SNe Ia and construct a template of spectral time series as a function of the light-curve-shape parameter, color stretch s BV . Principal component analysis is applied to characterize the diversity of the spectral features and reduce data dimensionality to a smaller subspace. Gaussian process regression is then used to model the subspace dependence on phase and light-curve shape and the associated uncertainty. Our template is able to predict spectral variations that are correlated with s BV , such as the hallmark NIR features: Mg ii at early times and the H-band break after peak. Using this template reduces the systematic uncertainties in K-corrections by ∼90% compared to those from the Hsiao template. These uncertainties, defined as the mean K-correction differences computed with the color-matched template and observed spectra, are on the level of 4 × 10−4 mag on average. This template can serve as the baseline spectral energy distribution for light-curve fitters and can identify peculiar spectral features that might point to compelling physics. The results presented here will substantially improve future SN Ia cosmological experiments, for both nearby and distant samples.

Neural network reconstruction of H'(z) and its application in teleparallel gravity

In this work, we explore the possibility of using artificial neural networks to impose constraints on teleparallel gravity and its f(T) extensions. We use the available Hubble parameter observations from cosmic chronometers and baryon acoustic oscillations from different galaxy surveys. We discuss the procedure for training a network model to reconstruct the Hubble diagram. Further, we describe the procedure to obtain H'(z), the first order derivative of H(z), using artificial neural networks which is a novel approach to this method of reconstruction. These analyses are complemented with further studies on the impact of two priors which we put on H 0 to assess their impact on the analysis, which are the local measurements by the SH0ES team (H 0 R20 = 73.2 ± 1.3 km Mpc-1 s-1) and the updated TRGB calibration from the Carnegie Supernova Project (H 0 TRGB = 69.8 ± 1.9 km Mpc-1 s-1), respectively. Additionally, we investigate the validity of the concordance model, through some cosmological null tests with these reconstructed data sets. Finally, we reconstruct the allowed f(T) functions for different combinations of the observational Hubble data sets. Results show that the ΛCDM model lies comfortably included at the 1σ confidence level for all the examined cases.

The Absolute Magnitudes of 1991T-like Supernovae * * This paper includes data gathered with the 6.5 meter Magellan telescopes at Las Campanas Observatory, Chile.

1991T-like supernovae are the luminous, slow-declining extreme of the Branch shallow-silicon (SS) subclass of Type Ia supernovae. They are distinguished by extremely weak Ca ii H & K and Si ii λ6355 and strong Fe iii absorption features in their optical spectra at pre-maximum phases, and have long been suspected to be over-luminous compared to normal Type Ia supernovae. In this paper, the pseudo-equivalent width of the Si ii λ6355 absorption obtained at light curve phases from ≤ +10 days is combined with the morphology of the i-band light curve to identify a sample of 1991T-like supernovae in the Carnegie Supernova Project II. Hubble diagram residuals show that, at optical as well as near-infrared wavelengths, these events are over-luminous by ∼0.1–0.5 mag with respect to the less extreme Branch SS (1999aa-like) and Branch core-normal supernovae with similar B-band light-curve decline rates.

Constraining the SN Ia host galaxy dust law distribution and mass step: hierarchical BayeSN analysis of optical and near-infrared light curves

ABSTRACT We use the BayeSN hierarchical probabilistic SED model to analyse the optical–NIR (BVriYJH) light curves of 86 Type Ia supernovae (SNe Ia) from the Carnegie Supernova Project to investigate the SN Ia host galaxy dust law distribution and correlations between SN Ia Hubble residuals and host mass. Our Bayesian analysis simultaneously constrains the mass step and dust RV population distribution by leveraging optical–NIR colour information. We demonstrate how a simplistic analysis where individual RV values are first estimated for each SN separately, and then the sample variance of these point estimates is computed, overestimates the RV population variance $\sigma _R^2$. This bias is exacerbated when neglecting residual intrinsic colour variation beyond that due to light curve shape. Instead, Bayesian shrinkage estimates of σR are more accurate, with fully hierarchical analysis of the light curves being ideal. For the 75 SNe with low-to-moderate reddening (peak apparent B − V ≤ 0.3), we estimate an RV distribution with population mean μR = 2.59 ± 0.14, and standard deviation σR = 0.62 ± 0.16. Splitting this subsample at the median host galaxy mass (1010.57 M⊙) yields consistent estimated RV distributions between low- and high-mass galaxies, with μR = 2.79 ± 0.18, σR = 0.42 ± 0.24, and μR = 2.35 ± 0.27, σR = 0.74 ± 0.36, respectively. When estimating distances from the full optical–NIR light curves while marginalizing over various forms of the dust RV distribution, a mass step of ≳0.06 mag persists in the Hubble residuals at the median host mass.

Cosmological Results from the RAISIN Survey: Using Type Ia Supernovae in the Near Infrared as a Novel Path to Measure the Dark Energy Equation of State

Type Ia supernovae (SNe Ia) are more precise standardizable candles when measured in the near-infrared (NIR) than in the optical. With this motivation, from 2012 to 2017 we embarked on the RAISIN program with the Hubble Space Telescope (HST) to obtain rest-frame NIR light curves for a cosmologically distant sample of 37 SNe Ia (0.2 ≲ z ≲ 0.6) discovered by Pan-STARRS and the Dark Energy Survey. By comparing higher-z HST data with 42 SNe Ia at z < 0.1 observed in the NIR by the Carnegie Supernova Project, we construct a Hubble diagram from NIR observations (with only time of maximum light and some selection cuts from optical photometry) to pursue a unique avenue to constrain the dark energy equation-of-state parameter, w. We analyze the dependence of the full set of Hubble residuals on the SN Ia host galaxy mass and find Hubble residual steps of size ∼0.06-0.1 mag with 1.5σ−2.5σ significance depending on the method and step location used. Combining our NIR sample with cosmic microwave background constraints, we find 1 + w = −0.17 ± 0.12 (statistical + systematic errors). The largest systematic errors are the redshift-dependent SN selection biases and the properties of the NIR mass step. We also use these data to measure H 0 = 75.9 ± 2.2 km s−1 Mpc−1 from stars with geometric distance calibration in the hosts of eight SNe Ia observed in the NIR versus H 0 = 71.2 ± 3.8 km s−1 Mpc−1 using an inverse distance ladder approach tied to Planck. Using optical data, we find 1 + w = −0.10 ± 0.09, and with optical and NIR data combined, we find 1 + w = −0.06 ± 0.07; these shifts of up to ∼0.11 in w could point to inconsistency in the optical versus NIR SN models. There will be many opportunities to improve this NIR measurement and better understand systematic uncertainties through larger low-z samples, new light-curve models, calibration improvements, and eventually by building high-z samples from the Roman Space Telescope.

Comparing Tip of the Red Giant Branch Distance Scales: An Independent Reduction of the Carnegie-Chicago Hubble Program and the Value of the Hubble Constant

The tip of the red giant branch has been used to measure distances to 500 nearby galaxies with the Hubble Space Telescope (HST) which are available in the Color–Magnitude Diagrams and Tip of the Red Giant Branch (CMDs/TRGB) catalog on the Extragalactic Distance Database (EDD). Our established methods are employed to perform an independent reduction of the targets presented by the Carnegie-Chicago Hubble Program (CCHP) in the series of papers culminating in Freedman (2021). Our distinct methodology involves modeling the observed luminosity function of red giant branch and asymptotic giant branch stars, which differs from the edge-detection algorithms employed by the CCHP. We find excellent agreement between distances for 11 hosts with new imaging, all at D < 20 Mpc. However, we are unable to measure the TRGB for four hosts that use archival data designed to measure distances with Cepheids, all at D > 23 Mpc. With two new HST observations taken in the halo of the megamaser host NGC 4258, the first with the same ACS F606W and F814W filters and state of the electronics used for SN Ia hosts, we then calibrate our TRGB distance scale to the geometric megamaser distance. Using our TRGB distances, we find a value of the Hubble Constant of H 0 = 71.5 ± 1.8 km s−1 Mpc−1 when using either the Pantheon or Carnegie Supernova Project (CSP) samples of supernovae. In the future, the James Webb Space Telescope will extend measurements of the TRGB to additional hosts of SN Ia and surface-brightness fluctuation measurements for separate paths to H 0.

SN 2012ij: A Low-luminosity Type Ia Supernova and Evidence for a Continuous Distribution from a 91bg-like Explosion to Normal Ones*

Abstract In this paper, we present photometric and spectroscopic observations of the subluminous Type Ia supernova (SN Ia) 2012ij, which has an absolute B-band peak magnitude M B , max = − 17.95 ± 0.15 mag. The B-band light curve exhibits a fast postpeak decline with Δm 15(B) = 1.86 ± 0.05 mag. All the R- and I/i-band light curves show a weak secondary peak/shoulder feature at about 3 weeks after the peak, like some transitional subclass of SNe Ia, which could result from an incomplete merger of near-infrared (NIR) double peaks. The spectra are characterized by Ti ii and strong Si ii λ5972 absorption features that are usually seen in low-luminosity objects like SN 1999by. The NIR spectrum before maximum light reveals weak carbon absorption features, implying the existence of unburned materials. We compare the observed properties of SN 2012ij with those predicted by the sub-Chandrasekhar-mass and the Chandrasekhar-mass delayed-detonation models and find that both optical and NIR spectral properties can be explained to some extent by these two models. By comparing the secondary maximum features in the I and i bands, we suggest that SN 2012ij is a transitional object linking normal SNe Ia to typical 91bg-like ones. From the published sample of SNe Ia from the Carnegie Supernova Project II, we estimate that the fraction of SN 2012ij–like SNe Ia is not lower than ∼2%.

Carnegie Supernova Project: kinky i-band light curves of Type Ia supernovae

ABSTRACT We present detailed investigation of a specific i-band light-curve feature in Type Ia supernovae (SNe Ia) using the rapid cadence and high signal-to-noise ratio light curves obtained by the Carnegie Supernova Project. The feature is present in most SNe Ia and emerges a few days after the i-band maximum. It is an abrupt change in curvature in the light curve over a few days and appears as a flattening in mild cases and a strong downward concave shape, or a ‘kink’, in the most extreme cases. We computed the second derivatives of Gaussian Process interpolations to study 54 rapid-cadence light curves. From the second derivatives we measure: (1) the timing of the feature in days relative to i-band maximum; tdm2(i) and (2) the strength and direction of the concavity in mag d−2; dm2(i). 76 per cent of the SNe Ia show a negative dm2(i), representing a downward concavity – either a mild flattening or a strong ‘kink’. The tdm2(i) parameter is shown to correlate with the colour-stretch parameter sBV, a SN Ia primary parameter. The dm2(i) parameter shows no correlation with sBV and therefore provides independent information. It is also largely independent of the spectroscopic and environmental properties. Dividing the sample based on the strength of the light-curve feature as measured by dm2(i), SNe Ia with strong features have a Hubble diagram dispersion of 0.107 mag, 0.075 mag smaller than the group with weak features. Although larger samples should be obtained to test this result, it potentially offers a new method for improving SN Ia distance determinations without shifting to more costly near-infrared or spectroscopic observations.

The Foundation Supernova Survey: Photospheric Velocity Correlations in Type Ia Supernovae

Abstract The ejecta velocities of Type Ia supernovae (SNe Ia), as measured by the Si ii λ6355 line, have been shown to correlate with other supernova properties, including color and standardized luminosity. We investigate these results using the Foundation Supernova Survey, with a spectroscopic data release presented here, and photometry analyzed with the SALT2 light-curve fitter. We find that the Foundation data do not show significant evidence for an offset in color between SNe Ia with high and normal photospheric velocities, with Δc = 0.004 ± 0.015. Our SALT2 analysis does show evidence for redder high-velocity SNe Ia in other samples, including objects from the Carnegie Supernova Project, with a combined sample yielding Δc = 0.018 ± 0.008. When split on velocity, the Foundation SNe Ia also do not show a significant difference in Hubble diagram residual, ΔHR = 0.015 ± 0.049 mag. Intriguingly, we find that SN Ia ejecta velocity information may be gleaned from photometry, particularly in redder optical bands. For high-redshift SNe Ia, these rest-frame red wavelengths will be observed by the Nancy Grace Roman Space Telescope. Our results are in line with previous work that suggests SN Ia host-galaxy stellar mass is correlated with ejecta velocity: high-velocity SNe Ia are found nearly exclusively in high-stellar-mass hosts. However, host-galaxy properties alone do not explain velocity-dependent differences in supernova colors and luminosities across samples. Measuring and understanding the connection between intrinsic explosion properties and supernova environments, across cosmic time, will be important for precision cosmology with SNe Ia.

Carnegie Supernova Project: The First Homogeneous Sample of Super-Chandrasekhar-mass/2003fg-like Type Ia Supernovae

Abstract We present a multiwavelength photometric and spectroscopic analysis of 13 super-Chandrasekhar-mass/2003fg-like Type Ia supernovae (SNe Ia). Nine of these objects were observed by the Carnegie Supernova Project. The 2003fg-like SNe have slowly declining light curves (Δm 15(B) &lt; 1.3 mag), and peak absolute B-band magnitudes of −19 &lt; M B &lt; −21 mag. Many of the 2003fg-like SNe are located in the same part of the luminosity–width relation as normal SNe Ia. In the optical B and V bands, the 2003fg-like SNe look like normal SNe Ia, but at redder wavelengths they diverge. Unlike other luminous SNe Ia, the 2003fg-like SNe generally have only one i-band maximum, which peaks after the epoch of the B-band maximum, while their near-IR (NIR) light-curve rise times can be ≳40 days longer than those of normal SNe Ia. They are also at least 1 mag brighter in the NIR bands than normal SNe Ia, peaking above M H = −19 mag, and generally have negative Hubble residuals, which may be the cause of some systematics in dark-energy experiments. Spectroscopically, the 2003fg-like SNe exhibit peculiarities such as unburnt carbon well past maximum light, a large spread (8000–12,000 km s−1) in Si ii λ6355 velocities at maximum light with no rapid early velocity decline, and no clear H-band break at +10 days. We find that SNe with a larger pseudo-equivalent width of C ii at maximum light have lower Si ii λ6355 velocities and more slowly declining light curves. There are also multiple factors that contribute to the peak luminosity of 2003fg-like SNe. The explosion of a C–O degenerate core inside a carbon-rich envelope is consistent with these observations. Such a configuration may come from the core-degenerate scenario.

The ZTF-BTS Type Ia supernovae luminosity function is consistent with a single progenitor channel for the explosions

ABSTRACT We construct the Type Ia supernovae (SNe Ia) luminosity function (LF) using the Zwicky Transient Facility Bright Transient Survey (BTS) catalogue. While this magnitude-limited survey has an unprecedented number of objects, it suffers from large distance uncertainties and lacks an estimation of host extinction. We bypass these issues by calculating the intrinsic luminosities from the shape parameters of the light curve’s g and r bands, with the luminosities calibrated from the well observed SNe Ia sample of the Carnegie Supernova Project, allowing us to construct, for the first time, the intrinsic LF of SNe Ia. We then use a novel tight relation between the colour stretch and the synthesized 56Ni mass, MNi56, to determine the MNi56 distribution of SNe Ia. We find that the LFs are unimodal, with their peaks in line with previous results, but have a much lower rate of dim events and luminous events. We show that the features on top of the unimodal LF-derived distributions are all compatible with statistical noise, consistent with a single progenitor channel for the explosions. We further derive, for the first time, the SNe Ia distribution of host galaxy extinction, and find a mean selective extinction of E(B − V) ≈ 0.1 and a non-negligible fraction with large, $\gt 1\, \text{mag}$, extinction in the optical bands. The high extinction is typical for luminous SNe, supporting their young population origin.