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Item Open Access A Reference Survey for Supernova Cosmology with the Nancy Grace Roman Space TelescopeRose, BM; Baltay, C; Hounsell, R; Macias, P; Rubin, D; Scolnic, D; Aldering, G; Bohlin, R; Dai, M; Deustua, SE; Foley, RJ; Fruchter, A; Galbany, L; Jha, SW; Jones, DO; Joshi, BA; Kelly, PL; Kessler, R; Kirshner, RP; Mandel, KS; Perlmutter, S; Pierel, J; Qu, H; Rabinowitz, D; Rest, A; Riess, AG; Rodney, S; Sako, M; Siebert, MR; Strolger, L; Suzuki, N; Thorp, S; Dyk, SD Van; Wang, K; Ward, SM; Wood-Vasey, WMThis note presents an initial survey design for the Nancy Grace Roman High-latitude Time Domain Survey. This is not meant to be a final or exhaustive list of all the survey strategy choices, but instead presents a viable path towards achieving the desired precision and accuracy of dark energy measurements using Type Ia supernovae (SNe Ia). We describe a survey strategy that use six filters (RZYJH and F) and the prism on the Roman Wide Field Instrument. This survey has two tiers, one "wide" which targets SNe Ia at redshifts up to 1 and one "deep" targeting redshifts up to 1.7; for each, four filters are used (with Y and J used in both tiers). We propose one field each in the north and south continuous viewing zones, and expect to obtain high-quality distances of $\sim$12,000 SNe Ia with $\sim$5,000 at z > 1. We propose a wide-tier area of $\sim$19 deg$^2$ and a deep tier of $\sim$5 deg$^2$. Exposure times range from 100 s to 900 s for imaging and 900 s to 3600 s for the prism. These exposure times would reach $\sim$25.5 mag and $\sim$26.5 mag for the wide and deep tiers respectively, with deep co-add stacks reaching $\sim$28 mag and $\sim$29 mag. The total survey spans two years, with a total allocation time of six months, and a cadence of $\sim$5 days.Item Open Access First Cosmology Results using Type Ia Supernovae from the Dark Energy Survey: The Effect of Host Galaxy Properties on Supernova LuminositySmith, M; Sullivan, M; Wiseman, P; Kessler, R; Scolnic, D; Brout, D; D'Andrea, CB; Davis, TM; Foley, RJ; Frohmaier, C; Galbany, L; Gupta, RR; Gutiérrez, CP; Hinton, SR; Kelsey, L; Lidman, C; Macaulay, E; Möller, A; Nichol, RC; Nugent, P; Palmese, A; Pursiainen, M; Sako, M; Swann, E; Thomas, RC; Tucker, BE; Vincenzi, M; Carollo, D; Lewis, GF; Sommer, NE; Abbott, TMC; Aguena, M; Allam, S; Avila, S; Bertin, E; Bhargava, S; Brooks, D; Buckley-Geer, E; Burke, DL; Rosell, AC; Kind, MC; Costanzi, M; da Costa, LN; de Vicente, J; Desai, S; Diehl, HT; Doel, P; Eifler, TF; Everett, S; Flaugher, B; Fosalba, P; Frieman, J; García-Bellido, J; Gaztanaga, E; Glazebrook, K; Gruen, D; Gruendl, RA; Gschwend, J; Gutierrez, G; Hartley, WG; Hollowood, DL; Honscheid, K; James, DJ; Krause, E; Kuehn, K; Kuropatkin, N; Lima, M; MacCrann, N; Maia, MAG; Marshall, JL; Martini, P; Melchior, P; Menanteau, F; Miquel, R; Paz-Chinchón, F; Plazas, AA; Romer, AK; Roodman, A; Rykoff, ES; Sanchez, E; Scarpine, V; Schubnell, M; Serrano, S; Sevilla-Noarbe, I; Suchyta, E; Swanson, MEC; Tarle, G; Thomas, D; Tucker, DL; Varga, TN; Walker, ARWe present improved photometric measurements for the host galaxies of 206 spectroscopically confirmed type Ia supernovae discovered by the Dark Energy Survey Supernova Program (DES-SN) and used in the first DES-SN cosmological analysis. Fitting spectral energy distributions to the $griz$ photometric measurements of the DES-SN host galaxies, we derive stellar masses and star-formation rates. For the DES-SN sample, when considering a 5D ($z$, $x_1$, $c$, $\alpha$, $\beta$) bias correction, we find evidence of a Hubble residual `mass step', where SNe Ia in high mass galaxies ($>10^{10} \textrm{M}_{\odot}$) are intrinsically more luminous (after correction) than their low mass counterparts by $\gamma=0.040\pm0.019$mag. This value is larger by $0.031$mag than the value found in the first DES-SN cosmological analysis. This difference is due to a combination of updated photometric measurements and improved star formation histories and is not from host-galaxy misidentification. When using a 1D (redshift-only) bias correction the inferred mass step is larger, with $\gamma=0.066\pm0.020$mag. The 1D-5D $\gamma$ difference for DES-SN is $0.026\pm0.009$mag. We show that this difference is due to a strong correlation between host galaxy stellar mass and the $x_1$ component of the 5D distance-bias correction. To better understand this effect, we include an intrinsic correlation between light-curve width and stellar mass in simulated SN Ia samples. We show that a 5D fit recovers $\gamma$ with $-9$mmag bias compared to a $+2$mmag bias for a 1D fit. This difference can explain part of the discrepancy seen in the data. Improvements in modeling correlations between galaxy properties and SN is necessary to determine the implications for $\gamma$ and ensure unbiased precision estimates of the dark energy equation-of-state as we enter the era of LSST.Item Open Access It's Dust: Solving the Mysteries of the Intrinsic Scatter and Host-Galaxy Dependence of Standardized Type Ia Supernova BrightnessesBrout, Dillon; Scolnic, DanielThe use of Type Ia Supernovae (SNe Ia) as cosmological tools has motivated significant effort to: understand what drives the intrinsic scatter of SN Ia distance modulus residuals after standardization, characterize the distribution of SN Ia colors, and explain why properties of the host galaxies of the SNe correlate with SN Ia distance modulus residuals. We use a compiled sample of $\sim1450$ spectroscopically confirmed, photometric light-curves of SN Ia and propose a solution to these three problems simultaneously that also explains an empirical 11$\sigma$ detection of the dependence of Hubble residual scatter on SN Ia color. We introduce a physical model of color where intrinsic SN Ia colors with a relatively weak correlation with luminosity are combined with extrinsic dust-like colors ($E(B-V)$) with a wide range of extinction parameter values ($R_V$). This model captures the observed trends of Hubble residual scatter and indicates that the dominant component of SN Ia intrinsic scatter is from variation in $R_V$. We also find that the recovered $E(B-V)$ and $R_V$ distributions differ based on global host-galaxy stellar mass and this explains the observed correlation ($\gamma$) between mass and Hubble residuals seen in past analyses as well as an observed 4.5$\sigma$ dependence of $\gamma$ on SN Ia color. This finding removes any need to prescribe different intrinsic luminosities to different progenitor systems. Finally we measure biases in the equation-of-state of dark energy ($w$) up to $|\Delta w|=0.04$ by replacing previous models of SN color with our dust-based model; this bias is larger than any systematic uncertainty in previous SN Ia cosmological analyses.Item Open Access No Evidence for Type Ia Supernova Luminosity Evolution: Evidence for Dark Energy is RobustRose, BM; Rubin, D; Cikota, A; Deustua, SE; Dixon, S; Fruchter, A; Jones, DO; Riess, AG; Scolnic, DMType Ia Supernovae (SNe Ia) are powerful standardized candles for constraining the cosmological model and provided the first evidence of accelerated expansion. Their precision derives from empirical correlations now measured from $>1000$ SNe Ia between their luminosities, light curve shapes, colors and most recently a modest relationship with the mass of their host galaxy. As mass correlates with other host properties, these have been investigated to improve SN Ia standardization though none have been shown to significantly alter the determination of cosmological parameters. We re-examine a recent claim, based on 34 SN Ia in nearby passive host galaxies, of a 0.05 mag/Gyr dependence of standardized SN Ia luminosity on host age which if extrapolate to higher redshifts, might accrue to 0.25 mag challenging the inference of dark energy. We reanalyze this sample of hosts using both the original method and a Bayesian Hierarchical Model and find after a fuller accounting of the errors the significance for a dependence on age to be $\leq2\sigma$ and $\sim1\sigma$ after removal of a single poorly-measured SN. To test the claim that a trend seen in old stellar populations can be applied to younger ages, we extend our analysis to a larger sample which includes young hosts. We find the residual dependence of host age (after all standardization typically employed for cosmological measurements) to be $0.0011\pm0.0018$ mag/Gyr ($0.6\sigma$) for 254 SNe Ia from the Pantheon sample, consistent with no trend and strongly ruling out the large but low significance trend claimed from the passive hosts.Item Open Access Supernova Host Galaxies in the Dark Energy Survey: I. Deep Coadds, Photometry, and Stellar MassesWiseman, P; Smith, M; Childress, M; Kelsey, L; Möller, A; Gupta, RR; Swann, E; Angus, CR; Brout, D; Davis, TM; Foley, RJ; Frohmaier, C; Galbany, L; Gutiérrez, CP; Inserra, C; Kessler, R; Lewis, GF; Lidman, C; Macaulay, E; Nichol, RC; Pursiainen, M; Sako, M; Scolnic, D; Sommer, NE; Sullivan, M; Tucker, BE; Abbott, TMC; Aguena, M; Allam, S; Avila, S; Bertin, E; Brooks, D; Buckley-Geer, E; Burke, DL; Rosell, AC; Carollo, D; Kind, MC; da Costa, LN; de Vicente, J; Desai, S; Diehl, HT; Doel, P; Eifler, TF; Everett, S; Fosalba, P; Frieman, J; García-Bellido, J; Gaztanaga, E; Gerdes, DW; Gill, MSS; Glazebrook, K; Gruendl, RA; Gschwend, J; Hartley, WG; Hinton, SR; Hollowood, DL; Honscheid, K; James, DJ; Kuehn, K; Kuropatkin, N; Lima, M; Maia, MAG; March, M; Martini, P; Melchior, P; Menanteau, F; Miquel, R; Ogando, RLC; Paz-Chinchón, F; Plazas, AA; Romer, AK; Roodman, A; Sanchez, E; Scarpine, V; Serrano, S; Suchyta, E; Swanson, MEC; Tarle, G; Thomas, D; Tucker, DL; Varga, TN; Walker, AR; Wilkinson, RDThe five-year Dark Energy Survey supernova programme (DES-SN) is one of the largest and deepest transient surveys to date in terms of volume and number of supernovae. Identifying and characterising the host galaxies of transients plays a key role in their classification, the study of their formation mechanisms, and the cosmological analyses. To derive accurate host galaxy properties, we create depth-optimised coadds using single-epoch DES-SN images that are selected based on sky and atmospheric conditions. For each of the five DES-SN seasons, a separate coadd is made from the other 4 seasons such that each SN has a corresponding deep coadd with no contaminating SN emission. The coadds reach limiting magnitudes of order $\sim 27$ in $g$-band, and have a much smaller magnitude uncertainty than the previous DES-SN host templates, particularly for faint objects. We present the resulting multi-band photometry of host galaxies for samples of spectroscopically confirmed type Ia (SNe Ia), core-collapse (CCSNe), and superluminous (SLSNe) as well as rapidly evolving transients (RETs) discovered by DES-SN. We derive host galaxy stellar masses and probabilistically compare stellar-mass distributions to samples from other surveys. We find that the DES spectroscopically confirmed sample of SNe Ia selects preferentially fewer high mass hosts at high redshift compared to other surveys, while at low redshift the distributions are consistent. DES CCSNe and SLSNe hosts are similar to other samples, while RET hosts are unlike the hosts of any other transients, although these differences have not been disentangled from selection effects.Item Open Access Supernova Siblings: Assessing the Consistency of Properties of Type Ia Supernovae that Share the Same Parent GalaxiesScolnic, D; Smith, M; Massiah, A; Wiseman, P; Brout, D; Kessler, R; Davis, TM; Foley, RJ; Galbany, L; Hinton, SR; Hounsell, R; Kelsey, L; Lidman, C; Macaulay, E; Morgan, R; Nichol, RC; Möller, A; Popovic, B; Sako, M; Sullivan, M; Thomas, BP; Tucker, BE; Abbott, TMC; Aguena, M; Allam, S; Annis, J; Avila, S; Bechtol, K; Bertin, E; Brooks, D; Burke, DL; Rosell, AC; Carollo, D; Kind, MC; Carretero, J; Costanzi, M; Da Costa, LN; De Vicente, J; Desai, S; Diehl, HT; Doel, P; Drlica-Wagner, A; Eckert, K; Eifler, TF; Everett, S; Flaugher, B; Fosalba, P; Frieman, J; Garciá-Bellido, J; Gaztanaga, E; Gerdes, DW; Glazebrook, K; Gruen, D; Gruendl, RA; Gschwend, J; Gutierrez, G; Hartley, WG; Hollowood, DL; Honscheid, K; James, DJ; Kuehn, K; Kuropatkin, N; Lewis, GF; Li, TS; Lima, M; Maia, MAG; Marshall, JL; Menanteau, F; Miquel, R; Palmese, A; Paz-Chinchón, F; Plazas, AA; Pursiainen, M; Sanchez, E; Scarpine, V; Schubnell, M; Serrano, S; Sevilla-Noarbe, I; Sommer, NE; Suchyta, E; Swanson, MEC; Tarle, G; Varga, TN; Walker, AR; Wilkinson, RWhile many studies have shown a correlation between properties of the light curves of Type Ia SN (SNe Ia) and properties of their host galaxies, it remains unclear what is driving these correlations. We introduce a new direct method to study these correlations by analyzing `parent' galaxies that host multiple SNe Ia 'siblings'. Here, we search the Dark Energy Survey SN sample, one of the largest samples of discovered SNe, and find 8 galaxies that hosted two likely Type Ia SNe. Comparing the light-curve properties of these SNe and recovered distances from the light curves, we find no better agreement between properties of SNe in the same galaxy as any random pair of galaxies, with the exception of the SN light-curve stretch. We show at $2.8\sigma$ significance that at least 1/2 of the intrinsic scatter of SNe Ia distance modulus residuals is not from common host properties. We also discuss the robustness with which we could make this evaluation with LSST, which will find $100\times$ more pairs of galaxies, and pave a new line of study on the consistency of Type Ia supernovae in the same parent galaxies. Finally, we argue that it is unlikely some of these SNe are actually single, lensed SN with multiple images.Item Open Access The Foundation Supernova Survey: Photospheric Velocity Correlations in Type Ia SupernovaeDettman, Kyle G; Jha, Saurabh W; Dai, Mi; Foley, Ryan J; Rest, Armin; Scolnic, Daniel M; Siebert, Matthew R; Chambers, KC; Coulter, DA; Huber, ME; Johnson, E; Jones, DO; Kilpatrick, CD; Kirshner, RP; Pan, Y-C; Riess, AG; Schultz, ASBThe ejecta velocities of type-Ia supernovae (SNe Ia), as measured by the Si II $\lambda 6355$ 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 $\Delta c = 0.005 \pm 0.014$. Our SALT2 analysis does show evidence for redder high-velocity SN Ia in other samples, including objects from the Carnegie Supernova Project, with a combined sample yielding $\Delta c = 0.017 \pm 0.007$. When split on velocity, the Foundation SN Ia also do not show a significant difference in Hubble diagram residual, $\Delta HR = 0.015 \pm 0.049$ mag. Intriguingly, we find that SN Ia ejecta velocity information may be gleaned from photometry, particularly in redder optical bands. For high-redshift SN Ia, these rest-frame red wavelengths will be observed by the Nancy Grace Roman Space Telescope. Our results also confirm previous work that SN Ia host-galaxy stellar mass is strongly correlated with ejecta velocity: high-velocity SN 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.Item Open Access The Pantheon+ Analysis: Forward-Modeling the Dust and Intrinsic Colour Distributions of Type Ia Supernovae, and Quantifying their Impact on Cosmological InferencesPopovic, Brodie; Brout, Dillon; Kessler, Richard; Scolnic, DanielRecent studies have shown that the observed colour distributions of Type Ia SNe (SNIa) are well-described by a combination of distributions from dust and intrinsic colour. Here we present a new forward-modeling fitting method (Dust2Dust) to measure the parent dust and colour distributions, including their dependence on host-galaxy mass. At each fit step, the SNIa selection efficiency is determined from a large simulated sample that is re-weighted to reflect the proposed distributions. We use five separate metrics to constrain the Dust2Dust parameters: distribution of fitted light-curve colour $c$, cosmological residual trends with $c$, cosmological residual scatter with $c$, fitted colour-luminosity relationship $\beta_{\rm SALT2}$, and intrinsic scatter $\sigma_{\rm int}$. Using the Pantheon+ data sample, we present results for a Dust2Dust fit that includes 4 parameters describing intrinsic colour variations and 8 parameters describing dust. Furthermore, we propagate the Dust2Dust parameter uncertainties and covariance to the dark energy equation-of-state $w$ and Hubble constant H$_0$: we find $\sigma_w = 0.005$ and $\sigma_{\textrm{H}_0} = 0.145~$km/s/Mpc. The Dust2Dust code is publically available.