SN Ia Luminosity Research Articles

Effect of a varying gravitational constant on the SN Ia Hubble diagram, AGN luminosity evolution, and X-ray source counts

The impact of a cosmic time evolution of the gravitational constant on SN Ia luminosity and AGN/QSO luminosity functions is studied. The gravitational constant scales linearly with the Hubble parameter, its present-day variation being $\dot{G}_{0}/G_{0}\approx 1.9\times 10^{-4}$ Gyr−1, compatible with current bounds from lunar laser ranging. Distance moduli of Type Ia supernovae are fitted with a cosmic expansion factor derived from temperature variations of planetary paleoclimates, and a luminosity dependence on look-back time proportional to the varying gravitational constant is inferred from the Hubble diagram. A fit is performed to the comoving space density of X-ray-selected active galactic nuclei (AGNs) and optically selected quasars (QSOs) extending to redshifts z≈6. The initial steep increase of the AGN space density is reproduced by a redshift evolution depending solely on the Hubble parameter as scaling variable. The AGN luminosity scales with the Hubble parameter, and the scaling exponents of the luminosity function, composed of two competing power laws with exponential cutoff, are obtained. Based on the AGN luminosity function, flux-limited X-ray source counts are investigated. The counting functions are derived and put to test by fitting cumulative number counts of soft X-ray point sources compiled from ROSAT, XMM-Newton, and Chandra surveys.

IMPROVED DARK ENERGY CONSTRAINTS FROM ∼100 NEW CfA SUPERNOVA TYPE Ia LIGHT CURVES

We combine the CfA3 supernovae Type Ia (SN Ia) sample with samples from the literature to calculate improved constraints on the dark energy equation of state parameter, w. The CfA3 sample is added to the Union set of Kowalski et al. to form the Constitution set and, combined with a BAO prior, produces 1 + w = 0.013+0.066 –0.068 (0.11 syst), consistent with the cosmological constant. The CfA3 addition makes the cosmologically useful sample of nearby SN Ia between 2.6 and 2.9 times larger than before, reducing the statistical uncertainty to the point where systematics play the largest role. We use four light-curve fitters to test for systematic differences: SALT, SALT2, MLCS2k2 (RV = 3.1), and MLCS2k2 (RV = 1.7). SALT produces high-redshift Hubble residuals with systematic trends versus color and larger scatter than MLCS2k2. MLCS2k2 overestimates the intrinsic luminosity of SN Ia with 0.7 < Δ < 1.2. MLCS2k2 with RV = 3.1 overestimates host-galaxy extinction while RV 1.7 does not. Our investigation is consistent with no Hubble bubble. We also find that, after light-curve correction, SN Ia in Scd/Sd/Irr hosts are intrinsically fainter than those in E/S0 hosts by 2σ, suggesting that they may come from different populations. We also find that SN Ia in Scd/Sd/Irr hosts have low scatter (0.1 mag) and reddening. Current systematic errors can be reduced by improving SN Ia photometric accuracy, by including the CfA3 sample to retrain light-curve fitters, by combining optical SN Ia photometry with near-infrared photometry to understand host-galaxy extinction, and by determining if different environments give rise to different intrinsic SN Ia luminosity after correction for light-curve shape and color.

A REDETERMINATION OF THE HUBBLE CONSTANT WITH THEHUBBLE SPACE TELESCOPEFROM A DIFFERENTIAL DISTANCE LADDER

We report observations of 240 Cepheid variables obtained with the Near Infrared Camera (NICMOS) through the F160W filter on the Hubble Space Telescope (HST). The Cepheids are distributed across six recent hosts of Type Ia supernovae (SNe Ia) and the "maser galaxy" NGC 4258, allowing us to directly calibrate the peak luminosities of the SNe Ia from the precise, geometric distance measurements provided by the masers. New features of our measurement include the use of the same instrument for all Cepheid measurements across the distance ladder and homogeneity of the Cepheid periods and metallicities thus necessitating only a differential measurement of Cepheid fluxes and reducing the largest systematic uncertainties in the determination of the fiducial SN Ia luminosity. The NICMOS measurements reduce differential extinction in the host galaxies by a factor of 5 over past optical data. Combined with an expanded of 240 SNe Ia at z<0.1 which define their magnitude-redshift relation, we find H_0=74.2 +/-3.6, a 4.8% uncertainty including both statistical and systematic errors. We show that the factor of 2.2 improvement in the precision of H_0 is a significant aid to the determination of the equation-of-state of dark energy, w = P/(rho c^2). Combined with the WMAP 5-year measurement of Omega_M h^2, we find w= -1.12 +/- 0.12 independent of high-redshift SNe Ia or baryon acoustic oscillations (BAO). This result is also consistent with analyses based on the combination of high-z SNe Ia and BAO. The constraints on w(z) now with high-z SNe Ia and BAO are consistent with a cosmological constant and improved by a factor of 3 from the refinement in H_0 alone. We show future improvements in H_0 are likely and will further contribute to multi-technique studies of dark energy.

Using spectral flux ratios to standardize SN Ia luminosities

We present a new method to standardize Type Ia supernova (SN Ia) luminosities to ~<0.13 magnitudes using flux ratios from a single flux-calibrated spectrum per SN. Using Nearby Supernova Factory spectrophotomery of 58 SNe Ia, we performed an unbiased search for flux ratios which correlate with SN Ia luminosity. After developing the method and selecting the best ratios from a training sample, we verified the results on a separate validation sample and with data from the literature. We identified multiple flux ratios whose correlations with luminosity are stronger than those of light curve shape and color, previously identified spectral feature ratios, or equivalent width measurements. In particular, the flux ratio R(642/443) = F(642 nm) / F(443 nm) has a correlation of 0.95 with SN Ia absolute magnitudes. Using this single ratio as a correction factor produces a Hubble diagram with a residual scatter standard deviation of 0.125 +- 0.011 mag, compared with 0.161 +- 0.015 mag when fit with the SALT2 light curve shape and color parameters x1 and c. The ratio R(642/443) is an effective correction factor for both extrinsic dust reddening and instrinsic variations such as those of SN 1991T-like and SN 1999aa-like SNe. When combined with broad-band color measurements, spectral flux ratios can standardize SN Ia magnitudes to ~0.12 mag. These are the first spectral metrics that improve over the standard normalization methods based upon light curve shape and color and they provide among the lowest scatter Hubble diagrams ever published.

THE EFFECT OF PROGENITOR AGE AND METALLICITY ON LUMINOSITY AND56Ni YIELD IN TYPE Ia SUPERNOVAE

Timmes, Brown & Truran found that metallicity variations could theoretically account for a 25% variation in the mass of 56Ni synthesized in Type Ia supernovae (SNe Ia), and thus account for a large fraction of the scatter in observed SN Ia luminosities. Higher-metallicity progenitors are more neutron-rich, producing more stable burning products relative to radioactive 56Ni. We develop a new method for estimating bolometric luminosity and 56Ni yield in SNe Ia and use it to test the theory with data from the Supernova Legacy Survey. We find that the average 56Ni yield does drop in SNe Ia from high metallicity environments, but the theory can only account for 7%--10% of the dispersion in SN Ia 56Ni mass, and thus luminosity. This is because the effect is dominant at metallicities significantly above solar, whereas we find that SN hosts have predominantly subsolar or only moderately above-solar metallicities. We also show that allowing for changes in O/Fe with the metallicity [Fe/H] does not have a major effect on the theoretical prediction of Timmes, Brown & Truran, so long as one is using the O/H as the independent variable. Age may have a greater effect than metallicity -- we find that the luminosity weighted age of the host galaxy is correlated with 56Ni yield, and thus more massive progenitors give rise to more luminous explosions. This is hard to understand if most SNe Ia explode when the primaries reach the Chandrasekhar mass. Finally, we test the findings of Gallagher et al., that the residuals of SNe Ia from the Hubble diagram are correlated with host galaxy metallicity, and we find no such correlation.

Luminosity Indicators in the Ultraviolet Spectra of Type Ia Supernovae

We present a complete sample of International Ultraviolet Explorer and Hubble Space Telescope ultraviolet (UV) spectra of Type Ia supernovae (SNe Ia) through 2004. We measure the equivalent width (EW) and blueshifted velocity of the minimum of the one strong UV feature, Fe II λ3250. We also quantify the slope of the near-UV spectra using a new parameter, the "UV ratio." We find that the velocity of the Fe II line does not correlate with light-curve shape, while the EW shows distinct behavior for the slow- and fast-declining objects. Using precise Cepheid and surface brightness fluctuation distance measurements of six objects with UV spectra observed near maximum light (a total of 12 spectra), we determine that the UV ratio at maximum light is highly correlated with SN Ia luminosity. A larger sample of UV spectra is necessary to determine the validity of these luminosity indicators and whether they can be combined with light-curve shape to improve measured SN Ia distances.

Distribution of 56 Ni Yields of Type Ia Supernovae and its Implication for Progenitors

The amount of $^{56}$Ni produced in type Ia supernova (SN Ia) explosion is probably the most important physical parameter underlying the observed correlation of SN Ia luminosities with their light curves. Based on an empirical relation between the $^{56}$Ni mass and the light curve parameter $\triangle m_{15}$, we obtained rough estimates of the $^{56}$Ni mass for a large sample of nearby SNe Ia with the aim of exploring the diversity in SN Ia. We found that the derived $^{56}$Ni masses for different SNe Ia could vary by a factor of ten (e.g., $M_{\rm Ni}=0.1 - 1.3$ $M_{\odot}$), which cannot be explained in terms of the standard Chandrasekhar-mass model (with a $^{56}$Ni mass production of 0.4 -- 0.8 $M_{\odot}$). Different explosion and/or progenitor models are clearly required for various SNe Ia, in particular, for those extremely nickel-poor and nickel-rich producers. The nickel-rich (with $M_{\rm Ni}$ $>$ 0.8 $M_{\odot}$) SNe Ia are very luminous and may have massive progenitors exceeding the Chandrasekhar-mass limit since extra progenitor fuel is required to produce more $^{56}$Ni to power the light curve. This is also consistent with the finding that the intrinsically bright SNe Ia prefer to occur in stellar environments of young and massive stars. For example, 75% SNe Ia in spirals have $\Delta m_{15} < 1.2$ while this ratio is only 18% in E/S0 galaxies. On the other hand, the nickel-poor SNe Ia (with $M_{\rm Ni}$ $<$ 0.2 $M_{\odot}$) may invoke the sub-Chandrasekhar model, as most of them were found in early-type E/S0 galaxies dominated by the older and low-mass stellar populations. This indicates that SNe Ia in spiral and E/S0 galaxies have progenitors of different properties.

Comparison of Distances from RR Lyrae Stars, the Tip of the Red Giant Branch, and Classical Cepheids

The extragalactic distance scale relies heavily on Cepheids. However, it has become clear from observations and pulsation models that the slope and zero point of their P-L relations differ from galaxy to galaxy. This makes the determination of Cepheid distances complex and calls for an independent test of their differences. The test is provided by RR Lyrae star distances of 24 galaxies that calibrate the tip of the red giant branch (TRGB; -->MITRGB = − 4.05), which in turn confirms the adopted Cepheid distances on our 2006 distance scale in 18 cases to within 0.1 mag on average. Relative SN Ia and velocity distances deny a remaining significant metallicity effect of the adopted distances. The new support for these Cepheid distances increases the weight of our previous calibration of the SN Ia luminosity and of the 21 cm line width-luminosity (TF) relation. The value of -->H0 = 62.3 (± 5) is confirmed on all scales.

Calibrating Type Ia SNe Using the Planetary Nebula Luminosity Function

Using the [O III] 5007 Planetary Nebula Luminosity Function (PNLF) distance indicator, we can double the number of known distances to nearby Type Ia SNe, and hence improve their zero point calibration while searching for systematic offsets in SN Ia luminosities between young populations (spiral galaxy) and old populations (ellipticals). We report three new PNLF distances and two new lower limits for five galaxies (NGC 524, 1316, 1380, 1448, &amp; 4526) that have hosted well-observed Type Ia SNe. From the PNLF, we find a Hubble constant of 75 km/s/Mpc, whereas we derive H=73 from SBF.

SALT: a spectral adaptive light curve template for type Ia supernovae

We present a new method to parameterize Type Ia Supernovae (SN Ia) multi-color light curves. The method was developed in order to analyze the large number of SN Ia multi-color light curves measured in current high-redshift projects. The technique is based on empirically modeling SN Ia luminosity variations as a function of phase, wavelength, a shape parameter, and a color parameter. The model is trained with a sample of well measured nearby SN Ia and then tested with an independent set of supernovae by building an optimal luminosity distance estimator combining the supernova rest-frame luminosity, shape parameter and color reconstructed with the model. The distances we measure using B- and V-band data show a dispersion around the Hubble line comparable or lower than obtained with other methods. With this model, we are able to measure distances using U- and B-band data with a dispersion around the Hubble line of 0.16 +- 0.05.

Properties of the Dark Energy

A non-zero cosmological constant is only one of many possible explanations for the observed accelerating expansion of the Universe. Any smoothly distributed, “dark” energy with a significant negative pressure can drive the acceleration. One possible culprit is a dynamical scalar field, but there are many less popular models such as tangled cosmic strings or domain walls. Soon theorists are likely to think up a number of new energies that can accelerate the expansion, meaning that only better observations can solve this question. Dark energy can be parameterized by its equation of state, w = p/ρ, which in the most general form can vary over time. Unlike the CMB, supernova observations cover a range of redshift so they can, in principle, probe the variation in the equation of state of the unknown component. The current SN observations loosely constrain the equation of state to w &lt; −0.6, ruling out non-intercommuting strings and textures (w = −1/3), but consistent with a cosmological constant (w = −1). The constraints achievable from future large SN surveys are limited by our ability to understand systematic effects in SN Ia luminosities. But a large sample of supernovae reaching out to z ˜ 2 should at least discriminate between a cosmological constant and a dynamical scalar field as the source of the observed acceleration.

Direct Analysis of Spectra of the Peculiar Type Ia Supernova 2000cx

The Type Ia SN 2000cx exhibited multiple peculiarities, including a lopsided B-band light-curve peak that does not conform to current methods for using shapes of light curves to standardize SN Ia luminosities. We use the parameterized supernova synthetic-spectrum code SYNOW to study line identifications in the photospheric-phase spectra of SN 2000cx. Previous work established the presence of Ca II infrared-triplet features forming above velocity about 20,000 km/s, much higher than the photospheric velocity of about 10,000 km/s. We find Ti II features forming at the same high velocity. High-velocity line formation is partly responsible for the photometric peculiarities of SN 2000cx: for example, B-band flux blocking by Ti II absorption features that decreases with time causes the B light curve to rise more rapidly and decline more slowly than it otherwise would. SN 2000cx contains an absorption feature near 4530 A that may be H-beta, forming at the same high velocity. The lack of conspicuous H-alpha and P-alpha signatures does not necessarily invalidate the H-beta identification if the high-velocity line formation is confined to a clump that partly covers the photosphere and the H-alpha and P-alpha source functions are elevated relative to that of resonance scattering. The H-beta identification is tentative. If it is correct, the high-velocity matter must have come from a nondegenerate companion star.

Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant

We present spectral and photometric observations of 10 Type Ia supernovae (SNe Ia) in the redshift range 0.16 ≤ z ≤ 0.62. The luminosity distances of these objects are determined by methods that employ relations between SN Ia luminosity and light curve shape. Combined with previous data from our High-z Supernova Search Team and recent results by Riess et al., this expanded set of 16 high-redshift supernovae and a set of 34 nearby supernovae are used to place constraints on the following cosmological parameters: the Hubble constant (H0), the mass density (ΩM), the cosmological constant (i.e., the vacuum energy density, ΩΛ), the deceleration parameter (q0), and the dynamical age of the universe (t0). The distances of the high-redshift SNe Ia are, on average, 10%–15% farther than expected in a low mass density (ΩM = 0.2) universe without a cosmological constant. Different light curve fitting methods, SN Ia subsamples, and prior constraints unanimously favor eternally expanding models with positive cosmological constant (i.e., ΩΛ > 0) and a current acceleration of the expansion (i.e., q0 < 0). With no prior constraint on mass density other than ΩM ≥ 0, the spectroscopically confirmed SNe Ia are statistically consistent with q0 < 0 at the 2.8 σ and 3.9 σ confidence levels, and with ΩΛ > 0 at the 3.0 σ and 4.0 σ confidence levels, for two different fitting methods, respectively. Fixing a "minimal" mass density, ΩM = 0.2, results in the weakest detection, ΩΛ > 0 at the 3.0 σ confidence level from one of the two methods. For a flat universe prior (ΩM + ΩΛ = 1), the spectroscopically confirmed SNe Ia require ΩΛ > 0 at 7 σ and 9 σ formal statistical significance for the two different fitting methods. A universe closed by ordinary matter (i.e., ΩM = 1) is formally ruled out at the 7 σ to 8 σ confidence level for the two different fitting methods. We estimate the dynamical age of the universe to be 14.2 ± 1.7 Gyr including systematic uncertainties in the current Cepheid distance scale. We estimate the likely effect of several sources of systematic error, including progenitor and metallicity evolution, extinction, sample selection bias, local perturbations in the expansion rate, gravitational lensing, and sample contamination. Presently, none of these effects appear to reconcile the data with ΩΛ = 0 and q0 ≥ 0.

Gravitational microlensing of high-redshift supernovae by compact objects

An analysis of the effect of microlensing by a cosmologically dominant density of compact objects is performed, using high-redshift Type Ia supernovae (SN Ia's) as probes. The compact objects are modeled as a three-dimensional distribution of point masses, and Monte Carlo simulations are done to calculate the resulting amplification probability distributions for several column densities and cosmologies. By combining these distributions with the intrinsic SN Ia luminosity function and comparing with the results for a perfectly smooth universe, estimates are made of the number of supernovae that would need to be observed to confirm or rule out this lensing scenario. It is found that about 1000 SN Ia's with redshifts of z = 1 would be needed to perform this test, which is beyond what current searches can hope to accomplish. Observations of many fewer high-redshift supernovae, used merely as standard candles, appears a promising way of distinguishing between different cosmological models.