Type Ia Supernova Host Galaxies Research Articles

WiFeS observations of nearby southern Type Ia supernova host galaxies

Abstract We present high-resolution observations of nearby ( $z\lesssim0.1$ ) galaxies that have hosted Type Ia supernovae to measure systemic spectroscopic redshifts using the wide field spectrograph (WiFeS) instrument on the Australian National University 2.3 m telescope at Siding Spring Observatory. While most of the galaxies targeted have previous spectroscopic redshifts, we provide demonstrably more accurate and precise redshifts with competitive uncertainties, motivated by potential systematic errors that could bias estimates of the Hubble constant ( $H_0$ ). The WiFeS instrument is remarkably stable; after calibration, the wavelength solution varies by $\lesssim$ 0.5 Å in red and blue with no evidence of a trend over the course of several years. By virtue of the $25\times 38$ arcsec field of view, we are always able to measure the redshift of the galactic core, or the entire galaxy in the cases where its angular extent is smaller than the field of view, reducing any errors due to galaxy rotation. We observed 185 southern SN Ia host galaxies and measured the redshift of each via at least one spatial region of (a) the core and (b) the average over the full-field/entire galaxy. Overall, we find stochastic differences between historical redshifts and our measured redshifts on the order of $\lesssim10^{-3}$ with a mean offset of 4.3 ${\times 10^{-5}}$ and normalised median absolute deviation of 1.2 ${\times 10^{-4}}$ . We show that a systematic redshift offset at this level is not enough to bias cosmology, as $H_0$ shifts by $+0.1$ km s $^{-1}$ Mpc $^{-1}$ when we replace Pantheon+ redshifts with our own, but the occasional large differences are interesting to note.

Iron-rich Metal-poor Stars and the Astrophysics of Thermonuclear Events Observationally Classified as Type Ia Supernovae. I. Establishing the Connection

The progenitor systems and explosion mechanisms responsible for the thermonuclear events observationally classified as Type Ia supernovae are uncertain and difficult to uniquely constrain using traditional observations of Type Ia supernova host galaxies, progenitors, light curves, and remnants. For the subset of thermonuclear events that are prolific producers of iron, we use published theoretical nucleosynthetic yields to identify a set of elemental abundance ratios infrequently observed in metal-poor stars but shared across a range of progenitor systems and explosion mechanisms: [Na, Mg, Co/Fe] < 0. We label stars with this abundance signature “iron-rich metal-poor,” or IRMP stars. We suggest that IRMP stars formed in environments dominated by thermonuclear nucleosynthesis and consequently that their elemental abundances can be used to constrain both the progenitor systems and explosion mechanisms responsible for thermonuclear explosions. We identify three IRMP stars in the literature and homogeneously infer their elemental abundances. We find that the elemental abundances of BD +80 245, HE 0533–5340, and SMSS J034249.53–284216.0 are best explained by the (double) detonations of sub-Chandrasekhar-mass CO white dwarfs. If our interpretation of IRMP stars is accurate, then they should be very rare in globular clusters and more common in the Magellanic Clouds and dwarf spheroidal galaxies than in the Milky Way’s halo. We propose that future studies of IRMP stars will quantify the relative occurrences of different thermonuclear event progenitor systems and explosion mechanisms.

A Spectroscopic Model of the Type Ia Supernova–Host-galaxy Mass Correlation from SALT3

The unknown cause of the correlation between Type Ia supernova (SN Ia) Hubble residuals and their host-galaxy masses (the “mass step”) may bias cosmological parameter measurements. To better understand the mass step, we develop a SALT3 light-curve model for SN cosmology that uses the host-galaxy masses of 296 low-redshift SNe Ia to derive a spectral energy distribution–host-galaxy mass relationship. The resulting model has larger Ca ii H and K, Ca ii near-infrared triplet, and Si ii equivalent widths for SNe in low-mass host galaxies at 2.2–2.7σ significance; this indicates higher explosion energies per unit mass in low-mass-hosted SNe. The model has phase-dependent changes in SN Ia colors as a function of host mass, indicating intrinsic differences in mean broadband light curves. Although the model provides a better fit to the SN data overall, it does not substantially reduce data–model residuals for a typical light curve in our sample nor does it significantly reduce Hubble residual dispersion. This is because we find that previous SALT models parameterized most host-galaxy dependencies with their first principal component, although they failed to model some significant spectral variations. Our new model is luminosity and cosmology independent, and applying it to data reduces the mass step by 0.021 ± 0.002 mag (uncertainty accounts for correlated data sets); these results indicate that ∼35% of the mass step can be attributed to luminosity-independent effects. This SALT model version could be trained using alternative host-galaxy properties and at different redshifts, and therefore will be a tool for understanding redshift-dependent correlations between SNe Ia and their host properties as well as their impact on cosmological parameter measurements.

Synthetic Population of Binary Cepheids. II. The Effect of Companion Light on the Extragalactic Distance Scale

Because of their period–luminosity relation (PLR), classical Cepheids play a key role in the calibration of the extragalactic distance scale and the determination of the Hubble–Lemaître constant H 0. Recent findings show that the majority of classical Cepheids should be in binary or multiple systems, which might undermine their accuracy, as the extra—and unaccounted for—light from the companions of Cepheids causes a shift in the PLR. We quantify this shift using synthetic populations of binary Cepheids that we developed for this purpose, as described in Paper I of this series. We find that while all PLRs are shifted toward brighter values due to the excess light from the companions, the bias in the relative distance modulus between two galaxies hosting binary Cepheids can be either positive or negative, depending on the percentage of binary Cepheids in them. If the binarity percentage in the two galaxies is similar, the effect of binarity is canceled. Otherwise, it introduces a shift in the distance modulus of the order of millimagnitudes in the near-infrared passbands and Wesenheit indices, and tens of millimagnitudes in the visual domain; its exact value depends on the variant of the synthetic population (a unique combination of metallicity, star formation history, shape and location of the instability strip, and initial parameter distributions). Such shifts in the distance moduli to Type Ia supernova host galaxies introduce an additional statistical error on H 0, which however does not prevent measuring H 0 with a precision of 1%.

Stellar Populations in type Ia supernova host galaxies at intermediate-high redshift: Star formation and metallicity enrichment histories

ABSTRACT We present a summary of our project that studies galaxies hosting type Ia supernova (SN Ia) at different redshifts. We present Gran Telescopio de Canarias (GTC) optical spectroscopy of six SN Ia host galaxies at redshift z ∼ 0.4–0.5. They are joined to a set of SN Ia host galaxies at intermediate-high redshift, which include galaxies from surveys SDSS and COSMOS. The final sample, after a selection of galaxy spectra in terms of signal-to-noise and other characteristics, consists of 680 galaxies with redshift in the range 0.04 &amp;lt; z &amp;lt; 1. We perform an inverse stellar population synthesis with the code fado to estimate the star formation and enrichment histories of this set of galaxies, simultaneously obtaining their mean stellar age and metallicity and stellar mass. After analysing the correlations among these characteristics, we look for possible dependencies of the Hubble diagram residuals and supernova features (luminosity, colour and strength parameter) on these stellar parameters. We find that the Hubble residuals show a clear dependence on the stellar metallicity weighted by mass with a slope of −0.061 mag dex−1, when represented in logarithmic scale, log 〈ZM/Z⊙〉. This result supports our previous findings obtained from gas oxygen abundances for local and SDSS-survey galaxies. Comparing with other works from the literature that also use the stellar metallicity, we find a similar value, but with more precision and a better significance (2.08 versus ∼ 1.1), due to the higher number of objects and wider range of redshift of our sample.

The Hubble Tension Revisited: Additional Local Distance Ladder Uncertainties

In a recent paper, we investigated possible systematic uncertainties related to the Cepheid color–luminosity calibration method and their influence on the tension between the Hubble constant as inferred from distances to Type Ia supernovae and the cosmic microwave background as measured with the Planck satellite. Here, we study the impact of other sources of uncertainty in the supernova distance ladder, including Cepheid temperature and metallicity variations, supernova magnitudes, and GAIA parallax distances. Using Cepheid data in 19 Type Ia supernova host galaxies from Riess et al., anchor data from Riess et al., and a set of recalibrated Milky Way Cepheid distances, we obtain H 0 = 71.9 ± 2.2 km s−1 Mpc−1, 2.0σ from the Planck value. Excluding Cepheids with estimated color excesses mag to mitigate the impact of the Cepheid color–luminosity calibration, the inferred Hubble constant is H 0 = 68.1 ± 2.6 km s−1 Mpc−1, removing the tension with the Planck value.

Sensitivity of the Hubble Constant Determination to Cepheid Calibration

Motivated by the large observed diversity in the properties of extragalactic extinction by dust, we reanalyze the Cepheid calibration used to infer the Hubble constant, H 0, from Type Ia supernovae, using Cepheid data in 19 Type Ia supernova host galaxies from Riess et al. and anchor data from Riess et al. Unlike the SH0ES team, we do not enforce a fixed universal color–luminosity relation to correct the Cepheid magnitudes. Instead, we focus on a data-driven method, where the optical colors and near-infrared magnitudes of the Cepheids are used to derive individual color–luminosity relations for each Type Ia supernova host and anchor galaxy. We present two different analyses, one based on Wesenheit magnitudes, resulting in H 0 = 73.2 ± 1.3 km s−1 Mpc−1, a 4.2σ tension with the value inferred from the cosmic microwave background. In the second approach, we calibrate an individual extinction law for each galaxy, with noninformative priors using color excesses, yielding H 0 = 73.9 ± 1.8 km s−1 Mpc−1, in 3.4σ tension with the Planck value. Although the two methods yield similar results, in the latter approach, the Hubble constants inferred from the individual Cepheid absolute distance calibrator galaxies range from H 0 = 68.1 ± 3.5 km s−1 Mpc−1 to H 0 = 76.7 ± 2.0 km s−1 Mpc−1. Taking the correlated nature of H 0 inferred from individual anchors into account, and allowing for individual extinction laws, the Milky Way anchor is in 2.1–3.1 σ tension with the NGC 4258 and Large Magellanic Cloud anchors, depending on prior assumptions regarding the color–luminosity relations and the method used for quantifying the tension.

Infrared Surface Brightness Fluctuation Distances for MASSIVE and Type Ia Supernova Host Galaxies*

Abstract We measured high-quality surface brightness fluctuation (SBF) distances for a sample of 63 massive early-type galaxies using the WFC3/IR camera on the Hubble Space Telescope. The median uncertainty on the SBF distance measurements is 0.085 mag, or 3.9% in distance. Achieving this precision at distances of 50–100 Mpc required significant improvements to the SBF calibration and data analysis procedures for WFC3/IR data. Forty-two of the galaxies are from the MASSIVE Galaxy Survey, a complete sample of massive galaxies within ∼100 Mpc; the SBF distances for these will be used to improve the estimates of the stellar and central supermassive black hole masses in these galaxies. Twenty-four of the galaxies are Type Ia supernova hosts, useful for calibrating SN Ia distances for early-type galaxies and exploring possible systematic trends in the peak luminosities. Our results demonstrate that the SBF method is a powerful and versatile technique for measuring distances to galaxies with evolved stellar populations out to 100 Mpc and constraining the local value of the Hubble constant.

Inspecting the Cepheid Distance Ladder: the Hubble Space Telescope Distance to the SN Ia Host Galaxy NGC 5584

Abstract The current tension between the direct and the early-universe measurements of the Hubble constant, H 0, requires detailed scrutiny of all the data and methods used in the studies on both sides of the debate. The Cepheids in the Type Ia supernova (SN Ia) host galaxy NGC 5584 played a key role in the local measurement of H 0. The SH0ES project used the observations of this galaxy to derive a relation between the Cepheids’ periods and ratios of their amplitudes in different optical bands of the Hubble Space Telescope and used these relations to analyze the light curves of the Cepheids in around half of the current sample of local SN Ia host galaxies. In this work, we present an independent detailed analysis of the Cepheids in NGC 5584. We employ different tools for our photometric analysis and a completely different method for our light-curve analysis, and we do not find a systematic difference between our period and mean magnitude measurements compared to those reported by SH0ES. By adopting a period–luminosity relation calibrated by the Cepheids in the Milky Way, we measure a distance modulus μ = 31.810 ± 0.047 (mag), which is in agreement with μ = 31.786 ± 0.046 (mag) measured by SH0ES. In addition, the relations we find between the periods and amplitude ratios of the Cepheids in NGC 5584 are significantly tighter than those of SH0ES, and their potential impact on the direct H 0 measurement will be investigated in future studies.

Specific Effect of Peculiar Velocities on Dark-energy Constraints from Type Ia Supernovae

Abstract Peculiar velocities of Type Ia supernova (SN Ia) host galaxies affect the dark-energy parameter constraints in a small but very specific way: the parameters are biased in a single direction in parameter space that is a priori knowable for a given SN Ia data set. We demonstrate the latter fact with a combination of inference from a cosmological N-body simulation with overwhelming statistics applied to the Pantheon SN Ia data set, then confirm it by simple quantitative arguments. We quantify small modifications to the current analyses that would ensure that the effect of cosmological parameters is essentially guaranteed to be negligible.

The Carnegie Chicago Hubble Program. VI. Tip of the Red Giant Branch Distances to M66 and M96 of the Leo I Group

We determine the distances to the Type Ia supernova host galaxies M66 (NGC 3627) and M96 (NGC 3368) of the Leo I Group using the Tip of the Red Giant Branch (TRGB) method. We target the stellar halos of these galaxies using the Hubble Space Telescope ACS/WFC in the F606W and F814W bandpasses. By pointing to the stellar halos we sample RGB stars predominantly of Population II, minimize host-galaxy reddening, and significantly reduce the effects of source crowding. Our absolute calibration of the I-band TRGB is based on a recent detached eclipsing binary distance to the Large Magellanic Cloud. With this geometric zero-point in hand, we find for M66 and M96, respectively, true distance moduli μ0 = 30.23 ± 0.04 (stat) ± 0.06 (sys) mag and μ0 = 30.29 ± 0.02 (stat) ± 0.06 (sys) mag.

Milky Way Cepheid Standards for Measuring Cosmic Distances and Application to Gaia DR2: Implications for the Hubble Constant

Abstract We present Hubble Space Telescope (HST) photometry of a selected sample of 50 long-period, low-extinction Milky Way Cepheids measured on the same WFC3 F555W-, F814W-, and F160W-band photometric system as extragalactic Cepheids in Type Ia supernova host galaxies. These bright Cepheids were observed with the WFC3 spatial scanning mode in the optical and near-infrared to mitigate saturation and reduce pixel-to-pixel calibration errors to reach a mean photometric error of 5 mmag per observation. We use the new Gaia DR2 parallaxes and HST photometry to simultaneously constrain the cosmic distance scale and to measure the DR2 parallax zeropoint offset appropriate for Cepheids. We find the latter to be −46 ± 13 μas or ±6 μas for a fixed distance scale, higher than found from quasars, as expected for these brighter and redder sources. The precision of the distance scale from DR2 has been reduced by a factor of 2.5 because of the need to independently determine the parallax offset. The best-fit distance scale is 1.006 ± 0.033, relative to the scale from Riess et al. with H 0 = 73.24 km s−1 Mpc−1 used to predict the parallaxes photometrically, and is inconsistent with the scale needed to match the Planck 2016 cosmic microwave background data combined with ΛCDM at the 2.9σ confidence level (99.6%). At 96.5% confidence we find that the formal DR2 errors may be underestimated as indicated. We identify additional errors associated with the use of augmented Cepheid samples utilizing ground-based photometry and discuss their likely origins. Including the DR2 parallaxes with all prior distance-ladder data raises the current tension between the late and early universe route to the Hubble constant to 3.8σ (99.99%). With the final expected precision from Gaia, the sample of 50 Cepheids with HST photometry will limit to 0.5% the contribution of the first rung of the distance ladder to the uncertainty in H 0.

RECONSIDERING THE EFFECTS OF LOCAL STAR FORMATION ON TYPE Ia SUPERNOVA COSMOLOGY

Recent studies found a correlation with $\sim$3 sigma significance between the local star formation measured by GALEX in Type Ia supernova (SN Ia) host galaxies and the distances or dispersions derived from these SNe. We search for these effects by using data from recent cosmological analyses to greatly increase the SN Ia sample; we include 179 GALEX-imaged SN Ia hosts with distances from the JLA and Pan-STARRS SN Ia cosmology samples and 157 GALEX-imaged SN Ia hosts with distances from the Riess et al. (2011) H$_0$ measurement. We find little evidence that SNe Ia in locally star-forming environments are fainter after light curve correction than SNe Ia in locally passive environments. We find a difference of only 0.000$\pm$0.018 (stat+sys) mag for SNe fit with SALT2 and 0.029$\pm$0.027 (stat+sys) mag for SNe fit with MLCS2k2 (R$_V$ = 2.5), which suggests that proposed changes to recent measurements of H$_0$ and w are not significant and numerically smaller than the parameter measurement uncertainties. We find the greatly reduced significance of these distance modulus differences compared to Rigault et al. (2013) and Rigault et al. (2015) result from two improvements with fairly equal effects, our larger sample size and the use of JLA and Riess et al. (2011) sample selection criteria. Without these improvements, we recover the results of Rigault et al. (2015). We find that both populations have more similar dispersion in distance than found by Rigault et al. (2013), Rigault et al. (2015), and Kelly et al. (2015), with slightly smaller dispersion for locally passive SNe Ia fit with MLCS, the opposite of the effect seen by Rigault et al. (2015) and Kelly et al. (2015). We caution that measuring local SNe Ia environments in the future may require a higher-resolution instrument than GALEX and that SN sample selection has a significant effect on local star formation biases.

THE TIP OF THE RED GIANT BRANCH DISTANCES TO TYPE IA SUPERNOVA HOST GALAXIES. III. NGC 4038/39 AND NGC 5584

We present the tip of the red giant branch (TRGB) distances to Type Ia supernova (SNe Ia) host galaxies NGC 4038/39 and NGC 5584. Based on the deep images constructed using archival Hubble Space Telescope data, we detect red giant branch stars in each galaxy. VI photometry of the resolved stars and corresponding I-band luminosity functions show the TRGB to be at I_{TRGB} = 27.67 \pm 0.05 for NGC 4038/39 and I_{TRGB} = 27.77 \pm 0.04 for NGC 5584. From these estimates, we determine the distance modulus to NGC 4038/39 to be (m-M)_0 = 31.67 \pm 0.05 (random) \pm 0.12 (systematic) (corresponding to a linear distance of 21.58 \pm 0.50 \pm 1.19 Mpc) and the distance modulus to NGC 5584 to be (m-M)_0 = 31.76 \pm 0.04 (random) \pm 0.12 (systematic) (corresponding to a linear distance of 22.49 \pm 0.41 \pm 1.24 Mpc). We derive a mean absolute maximum magnitude of SNe Ia of M_V = -19.29 \pm 0.08 from the distance estimates of five SNe Ia (including two SNe in this study and three SNe Ia from our previous studies), and we derive a value of M_V = -19.19 \pm 0.10 using three low-reddened SNe Ia among the five SNe Ia. With these estimates, we derive a value of the Hubble constant, H_0 = 69.8 \pm 2.6 (random) \pm 3.9 (systematic) km/s/Mpc and 72.2 \pm 3.3 (random) \pm 4.0 (systematic) km/s/Mpc, respectively. The value from the five SNe is similar to those from the cosmic microwave background analysis, and not much different within errors, from those of recent Cepheid calibrations of SNe Ia. The value from the three SNe is between the values from the two methods.

PARALLAX BEYOND A KILOPARSEC FROM SPATIALLY SCANNING THE WIDE FIELD CAMERA 3 ON THEHUBBLE SPACE TELESCOPE

We use a newly developed observing mode on the Hubble Space Telescope (HST) and Wide Field Camera 3 (WFC3), spatial scanning, to increase source sampling a thousand-fold and measure changes in source positions to a precision of 20--40 microarcseconds, more than an order of magnitude better than attainable in pointed observations. This observing mode can usefully measure the parallaxes of bright stars at distances of up to 5 kpc, a factor of ten farther than achieved thus far with HST. Long-period classical Cepheid variable stars in the Milky Way, nearly all of which reside beyond 1 kpc, are especially compelling targets for parallax measurements from scanning, as they may be used to anchor a determination of the Hubble constant to ~1%. We illustrate the method by measuring to high precision the parallax of a classical Cepheid, SY Aurigae, at a distance of more than 2 kpc, using 5 epochs of spatial-scan data obtained at intervals of 6 months. Rapid spatial scans also enable photometric measurements of bright Milky Way Cepheids---which would otherwise saturate even in the shortest possible pointed observations---on the same flux scale as extragalactic Cepheids, which is a necessity for reducing a leading source of systematic error in the Hubble constant. We demonstrate this capability with photometric measurements of SY Aur on the same system used for Cepheids in Type Ia supernova host galaxies. While the technique and results presented here are preliminary, an ongoing program with HST is collecting such parallax measurements for another 18 Cepheids to produce a better anchor for the distance scale.

THE DWARF STARBURST HOST GALAXY OF A TYPE Ia SUPERNOVA ATz= 1.55 FROM CANDELS

We present VLT/X-shooter observations of a high redshift, type Ia supernova host galaxy, discovered with HST/WFC3 as part of the CANDELS Supernova project. The galaxy exhibits strong emission lines of Ly{\alpha}, [O II], H{\beta}, [O III], and H{\alpha} at z = 1.54992(+0.00008-0.00004). From the emission-line fluxes and SED fitting of broad-band photometry we rule out AGN activity and characterize the host galaxy as a young, low mass, metal poor, starburst galaxy with low intrinsic extinction and high Ly{\alpha} escape fraction. The host galaxy stands out in terms of the star formation, stellar mass, and metallicity compared to its lower redshift counterparts, mainly because of its high specific star-formation rate. If valid for a larger sample of high-redshift SN Ia host galaxies, such changes in the host galaxy properties with redshift are of interest because of the potential impact on the use of SN Ia as standard candles in cosmology.

Type Ia supernova host galaxies as seen with IFU spectroscopy

(abridged) We used the wide-field IFU spectrograph PMAS/PPAK at the 3.5m telescope of Calar Alto Observatory to observe six nearby spiral galaxies that hosted SNe Ia. Spatially resolved 2D maps of the properties of the ionized gas and the stellar populations were derived. Five of the observed galaxies have an ongoing star formation rate of 1-5 M_sun/yr and mean stellar population ages ~5 Gyr. The sixth galaxy shows no star formation and has an about 12 Gyr old stellar population. All galaxies have stellar masses larger than 2E+10 M_sun and metallicities above solar. Four galaxies show negative radial metallicity gradients of the ionized gas up to -0.058 dex/kpc and one has nearly uniform metallicity with a possible shallow positive slope. The stellar components show shallower negative metallicity gradients up to -0.03 dex/kpc. We find no clear correlation between the properties of the galaxy and those of the supernovae, which may be because of the small ranges spanned by the galaxy parameters. However, we note that the Hubble residuals are on average positive while negative Hubble residuals are expected for SNe Ia in massive hosts such as the galaxies in our sample. In conclusion, IFU spectroscopy on 4-m telescopes is a viable technique for studying host galaxies of nearby SNe Ia. It allows one to correlate the supernova properties with the properties of their host galaxies at the projected positions of the supernovae. Our current sample of six galaxies is too small to draw conclusions about the SN Ia progenitors or correlations with the galaxy properties, but the ongoing CALIFA IFU survey will provide a solid basis to exploit this technique more and improve our understanding of SNe Ia as cosmological standard candles.

Improving Type Ia supernova characterization through multiwavelength studies of Type Ia supernova host galaxies

AbstractPast investigations have shown a connection between the properties of Type Ia supernovae (SNe Ia) and their host galaxies. We refine these studies using ultraviolet through mid-infrared observations of both nearby and distant SN Ia hosts. We present new results showing that the properties of SNe Ia, both intrinsic and with respect to their use as distance indicators, appear to depend on a combination of metallicity, stellar age, and star-formation rate of the host. We suggest that the stellar population age and location of the supernova progenitor all can play a roll in using SNe Ia as precision distance indicators, and advocate that a multiwavelength approach is one way to disentangle the different influences, resulting in an improvement of 8% in distance measurements.

IMPROVED CONSTRAINTS ON TYPE Ia SUPERNOVA HOST GALAXY PROPERTIES USING MULTI-WAVELENGTH PHOTOMETRY AND THEIR CORRELATIONS WITH SUPERNOVA PROPERTIES

We improve estimates of stellar mass and mass-weighted average age of Type Ia supernova (SN Ia) host galaxies by combining UV and near-IR photometry with optical photometry in our analysis. Using 206 SNe Ia drawn from the full three-year SDSS-II Supernova Survey (median redshift of z {\approx} 0.2) and multi-wavelength host-galaxy photometry from SDSS, GALEX, and UKIDSS, we present evidence of a correlation (1.9{\sigma} confidence level) between the residuals of SNe Ia about the best-fit Hubble relation and the mass-weighted average age of their host galaxies. The trend is such that older galaxies host SNe Ia that are brighter than average after standard light-curve corrections are made. We also confirm, at the 3.0{\sigma} level, the trend seen by previous studies that more massive galaxies often host brighter SNe Ia after light-curve correction.

KECK OBSERVATIONS OF THE YOUNG METAL-POOR HOST GALAXY OF THE SUPER-CHANDRASEKHAR-MASS TYPE Ia SUPERNOVA SN 2007if

We present Keck LRIS spectroscopy and $g$-band photometry of the metal-poor, low-luminosity host galaxy of the super-Chandrasekhar mass Type Ia supernova SN 2007if. Deep imaging of the host reveals its apparent magnitude to be $m_g=23.15\pm0.06$, which at the spectroscopically-measured redshift of $z_{helio}=0.07450\pm0.00015$ corresponds to an absolute magnitude of $M_g=-14.45\pm0.06$. Galaxy $g-r$ color constrains the mass-to-light ratio, giving a host stellar mass estimate of $\log(M_*/M_\odot)=7.32\pm0.17$. Balmer absorption in the stellar continuum, along with the strength of the 4000\AA\ break, constrain the age of the dominant starburst in the galaxy to be $t_\mathrm{burst}=123^{+165}_{-77}$ Myr, corresponding to a main-sequence turn-off mass of $M/M_\odot=4.6^{+2.6}_{-1.4}$. Using the R$_{23}$ method of calculating metallicity from the fluxes of strong emission lines, we determine the host oxygen abundance to be $12+\log(O/H)_\mathrm{KK04}=8.01\pm0.09$, significantly lower than any previously reported spectroscopically-measured Type Ia supernova host galaxy metallicity. Our data show that SN 2007if is very likely to have originated from a young, metal-poor progenitor.