Sloan Digital Sky Survey-II Research Articles

Aperture-corrected spectroscopic type Ia supernova host galaxy properties

We use type Ia supernova (SN Ia) data obtained by the Sloan Digital Sky Survey-II Supernova Survey (SDSS-II SNS) in combination with the publicly available SDSS DR16 fiber spectroscopy of supernova (SN) host galaxies to correlate SN Ia light-curve parameters and Hubble residuals with several host galaxy properties. Fixed-aperture fiber spectroscopy suffers from aperture effects: the fraction of the galaxy covered by the fiber varies depending on its projected size on the sky, and thus measured properties are not representative of the whole galaxy. The advent of integral field spectroscopy has provided a way to correct the missing light, by studying how these galaxy parameters change with the aperture size. Here we study how the standard SN host galaxy relations change once global host galaxy parameters are corrected for aperture effects. We recover previous trends on SN Hubble residuals with host galaxy properties, but we find that discarding objects with poor fiber coverage instead of correcting for aperture loss introduces biases into the sample that affect SN host galaxy relations. The net effect of applying the commonly used g-band fraction criterion is that intrinsically faint SNe Ia in high-mass galaxies are discarded, thus artificially increasing the height of the mass step by 0.02 mag and its significance. Current and next-generation fixed-aperture fiber-spectroscopy surveys, such as OzDES, DESI, or TiDES with 4MOST, that aim to study SN and galaxy correlations must consider, and correct for, these effects.

A Template-based Approach to the Photometric Classification of SN 1991bg-like Supernovae in the SDSS-II Supernova Survey

Abstract The use of SNe Ia to measure cosmological parameters has grown significantly over the past two decades. However, there exists a significant diversity in the SN Ia population that is not well understood. Overluminous SN 1991T-like and subluminous SN 1991bg-like objects are two characteristic examples of peculiar SNe. The identification and classification of such objects is an important step in studying what makes them unique from the remaining SN population. With the upcoming Vera C. Rubin Observatory promising on the order of a million new SNe over a 10 year survey, spectroscopic classifications will be possible for only a small subset of observed targets. As such, photometric classification has become an increasingly important concern in preparing for the next generation of astronomical surveys. Using observations from the Sloan Digital Sky Survey II (SDSS-II) SN Survey, we apply here an empirically based classification technique targeted at the identification of SN 1991bg-like SNe in photometric data sets. By performing dedicated fits to photometric data in the rest-frame redder and bluer bandpasses, we classify 16 previously unidentified 91bg-like SNe. Using SDSS-II host galaxy measurements, we find that these SNe are preferentially found in host galaxies with an older average stellar age than the hosts of normal SNe Ia. We also find that these SNe are found at a further physical distance from the center of their host galaxies. We find no statistically significant bias in host galaxy mass or specific star formation rate for these targets.

The Data Release of the Sloan Digital Sky Survey-II Supernova Survey

This paper describes the data release of the Sloan Digital Sky Survey-II (SDSS-II) Supernova Survey conducted between 2005 and 2007. Light curves, spectra, classifications, and ancillary data are presented for 10,258 variable and transient sources discovered through repeat ugriz imaging of SDSS Stripe 82, a 300 deg2 area along the celestial equator. This data release is comprised of all transient sources brighter than r ≃ 22.5 mag with no history of variability prior to 2004. Dedicated spectroscopic observations were performed on a subset of 889 transients, as well as spectra for thousands of transient host galaxies using the SDSS-III BOSS spectrographs. Photometric classifications are provided for the candidates with good multi-color light curves that were not observed spectroscopically, using host galaxy redshift information when available. From these observations, 4607 transients are either spectroscopically confirmed, or likely to be, supernovae, making this the largest sample of supernova candidates ever compiled. We present a new method for SN host-galaxy identification and derive host-galaxy properties including stellar masses, star formation rates, and the average stellar population ages from our SDSS multi-band photometry. We derive SALT2 distance moduli for a total of 1364 SN Ia with spectroscopic redshifts as well as photometric redshifts for a further 624 purely photometric SN Ia candidates. Using the spectroscopically confirmed subset of the three-year SDSS-II SN Ia sample and assuming a flat ΛCDM cosmology, we determine ΩM = 0.315 ± 0.093 (statistical error only) and detect a non-zero cosmological constant at 5.7σ.

The Influence of Host Galaxies in Type Ia Supernova Cosmology

Abstract We use a sample of 1338 spectroscopically confirmed and photometrically classified Type Ia supernovae (SNe Ia) sourced from Carnegie Supernova Project, Center for Astrophysics Supernova Survey, Sloan Digital Sky Survey-II, and SuperNova Legacy Survey SN samples to examine the relationships between SNe Ia and the galaxies that host them. Our results provide confirmation with improved statistical significance that SNe Ia, after standardization, are on average more luminous in massive hosts (significance >5σ), and decline more rapidly in massive hosts (significance >9σ) and in hosts with low specific star formation rates (significance >8σ). We study the variation of these relationships with redshift and detect no evolution. We split SNe Ia into pairs of subsets that are based on the properties of the hosts and fit cosmological models to each subset. Including both systematic and statistical uncertainties, we do not find any significant shift in the best-fit cosmological parameters between the subsets. Among different SN Ia subsets, we find that SNe Ia in hosts with high specific star formation rates have the least intrinsic scatter (σ int = 0.08 ± 0.01) in luminosity after standardization.

Photometric properties of intermediate-redshift Type Ia supernovae observed by the Sloan Digital Sky Survey-II Supernova Survey

We have analyzed multi-band light curves of 328 intermediate redshift (0.05 <= z < 0.24) type Ia supernovae (SNe Ia) observed by the Sloan Digital Sky Survey-II Supernova Survey (SDSS-II SN Survey). The multi-band light curves were parameterized by using the Multi-band Stretch Method, which can simply parameterize light curve shapes and peak brightness without dust extinction models. We found that most of the SNe Ia which appeared in red host galaxies (u - r > 2.5) don't have a broad light curve width and the SNe Ia which appeared in blue host galaxies (u - r < 2.0) have a variety of light curve widths. The Kolmogorov-Smirnov test shows that the colour distribution of SNe Ia appeared in red / blue host galaxies is different (significance level of 99.9%). We also investigate the extinction law of host galaxy dust. As a result, we find the value of Rv derived from SNe Ia with medium light curve width is consistent with the standard Galactic value. On the other hand, the value of Rv derived from SNe Ia that appeared in red host galaxies becomes significantly smaller. These results indicate that there may be two types of SNe Ia with different intrinsic colours, and they are obscured by host galaxy dust with two different properties.

Determining H 0 using a model-independent method

By using type Ia supernovae (SNIa) to provide the luminosity distance (LD) directly, which depends on the value of the Hubble constant $H_0= 100 h\; {\rm km\; s^{-1}\; Mpc^{-1}}$, and the angular diameter distance from galaxy clusters or baryon acoustic oscillations (BAOs) to give the derived LD according to the distance duality relation, we propose a model-independent method to determine $h$ from the fact that different observations should give the same LD at a given redshift. Combining the Sloan Digital Sky Survey II (SDSS-II) SNIa from the MLCS2k2 light curve fit and galaxy cluster data, we find that at the $1\sigma$ confidence level (CL), $h=0.5867\pm0.0303$ for the sample of the elliptical $\beta$ model for galaxy clusters, and $h=0.6199\pm0.0293$ for that of the spherical $\beta$ model. The former is smaller than the values from other observations, whereas the latter is consistent with the Planck result at the $2\sigma$ CL and agrees very well with the value reconstructed directly from the $H(z)$ data. With the SDSS-II SNIa and BAO measurements, a tighter constraint, $h=0.6683\pm0.0221$, is obtained. For comparison, we also consider the Union 2.1 SNIa from the SALT2 light curve fitting. The results from the Union 2.1 SNIa are slightly larger than those from the SDSS-II SNIa, and the Union 2.1 SNIa + BAOs give the tightest value. We find that the values from SNIa + BAOs are quite consistent with those from the Planck and the BAOs, as well as the local measurement from Cepheids and very-low-redshift SNIa.

How SN Ia host-galaxy properties affect cosmological parameters

We present a systematic study of the relationship between Type Ia Supernova (SN Ia) properties, and the characteristics of their host galaxies, using a sample of 581 SNe Ia from the full Sloan Digital Sky Survey II (SDSS-II) SN Survey. We also investigate the effects of this on the cosmological constraints derived from SNe~Ia. Compared to previous studies, our sample is larger by a factor of $>4$, and covers a substantially larger redshift range (up to z~0.5), which is directly applicable to the volume of cosmological interest. We measure a significant correlation (>5\sigma) between the host-galaxy stellar-mass and the SN~Ia Hubble Residuals (HR). We find a weak correlation (1.4\sigma) between the host-galaxy metallicity as measured from emission lines in the spectra, and the SN~Ia HR. We also find evidence that the slope of the correlation between host-galaxy mass and HR is -0.11 $\mathrm{mag}/\mathrm{log}(\mathrm{M}_{\mathrm{host}}/\mathrm{M}_{\odot})$ steeper in lower metallicity galaxies. We test the effects on a cosmological analysis using both the derived best-fitting correlations between host parameters and HR, and by allowing an additional free parameter in the fit to account for host properties which we then marginalize over when determining cosmological parameters. We see a shift towards more negative values of the equation of state parameter $w$, along with a shift to lower values of $\Omega_\mathrm{m}$ after applying mass or metallicity corrections. The shift in cosmological parameters with host-galaxy stellar-mass correction is consistent with previous studies. We find a best-fitting cosmology of $\Omega_{\mathrm{m}} =0.266_{-0.016}^{+0.016}$, $\Omega_{\Lambda}=0.740_{-0.018}^{+0.018}$ and $w=-1.151_{-0.121}^{+0.123}$ (statistical errors only).

HOST GALAXY SPECTRA AND CONSEQUENCES FOR SUPERNOVA TYPING FROM THE SDSS SN SURVEY

We present the spectroscopy from 5254 galaxies that hosted supernovae (SNe) or other transient events in the Sloan Digital Sky Survey II (SDSS-II). Obtained during SDSS-I, SDSS-II, and the Baryon Oscillation Spectroscopic Survey (BOSS), this sample represents the largest systematic, unbiased, magnitude limited spectroscopic survey of supernova (SN) host galaxies. Using the host galaxy redshifts, we test the impact of photometric SN classification based on SDSS imaging data with and without using spectroscopic redshifts of the host galaxies. Following our suggested scheme, there are a total of 1166 photometrically classified SNe Ia when using a flat redshift prior and 1126 SNe Ia when the host spectroscopic redshift is assumed. For 1024 (87.8%) candidates classified as likely SNe Ia without redshift information, we find that the classification is unchanged when adding the host galaxy redshift. Using photometry from SDSS imaging data and the host galaxy spectra, we also report host galaxy properties for use in future nalysis of SN astrophysics. Finally, we investigate the differences in the interpretation of the light curve properties with and without knowledge of the redshift. When using the SALT2 light curve fitter, we find a 21% increase in the number of fits that converge when using the spectroscopic redshift. Without host galaxy redshifts, we find that SALT2 light curve fits are systematically biased towards lower photometric redshift estimates and redder colors in the limit of low signal-to-noise data. The general improvements in performance of the light curve fitter and the increased diversity of the host galaxy sample highlights the importance of host galaxy spectroscopy for current photometric SN surveys such as the Dark Energy Survey and future surveys such as the Large Synoptic Survey Telescope.

Simultaneous constraints on the growth of structure and cosmic expansion from the multipole power spectra of the SDSS DR7 LRG sample

The anisotropic galaxy clustering on large scales provides us with a unique opportunity to probe into the gravity theory through the redshift-space distortions (RSDs) and the Alcock-Paczynski effect. Using the multipole power spectra up to hexadecapole (ell=4), of the Luminous Red Galaxy (LRG) sample in the data release 7 (DR7) of the Sloan Digital Sky Survey II (SDSS-II), we obtain simultaneous constraints on the linear growth rate f, angular diameter distance D_A, and Hubble parameter H at redshift z = 0.3. For this purpose, we first extensively examine the validity of a theoretical model for the non-linear RSDs using mock subhalo catalogues from N-body simulations, which are constructed to match with the observed multipole power spectra. We show that the input cosmological parameters of the simulations can be recovered well within the error bars by comparing the multipole power spectra of our theoretical model and those of the mock subhalo catalogues. We also carefully examine systematic uncertainties in our analysis by testing the dependence on prior assumption of the theoretical model and the range of wavenumbers to be used in the fitting. These investigations validate that the theoretical model can be safely applied to the real data. Thus, our results from the SDSS DR7 LRG sample are robust including systematics of theoretical modeling; f(z = 0.3) sigma_8(z = 0.3) =0.49+-0.08(stat.)+-0.04(sys.), D_A (z = 0.3) =968+-42(stat.)+-17(sys.)[Mpc], H (z = 0.3) =81.7+-5.0(stat.)+-3.7(sys.)[km/s/Mpc]. We believe that our method to constrain the cosmological parameters using subhaloes catalogues will be useful for more refined samples like CMASS and LOWZ catalogues in the Baryon Oscillation Spectroscopic Survey in SDSS-III.

Properties of Type Ia supernovae inside rich galaxy clusters

We used the Gaussian Mixture Brightest Cluster Galaxy catalogue and Sloan Digital Sky Survey-II supernovae data with redshifts measured by the Baryon Oscillation Spectroscopic Survey to identify 48 Type Ia supernovae (SNe Ia) residing in rich galaxy clusters and compare their properties with 1015 SNe Ia in the field. Their light curves were parametrized by the SALT2 model and the significance of the observed differences was assessed by a resampling technique. To test our samples and methods, we first looked for known differences between SNe Ia residing in active and passive galaxies. We confirm that passive galaxies host SNe Ia with smaller stretch, weaker colour–luminosity relation [β of 2.54(22) against 3.35(14)], and that are ∼0.1 mag more luminous after stretch and colour corrections. We show that only 0.02 per cent of random samples drawn from our set of SNe Ia in active galaxies can reach these values. Reported differences in the Hubble residuals scatter could not be detected, possibly due to the exclusion of outliers. We then show that, while most field and cluster SNe Ia properties are compatible at the current level, their stretch distributions are different (∼3σ): besides having a higher concentration of passive galaxies than the field, the cluster's passive galaxies host SNe Ia with an average stretch even smaller than those in field passive galaxies (at 95 per cent confidence). We argue that the older age of passive galaxies in clusters is responsible for this effect since, as we show, old passive galaxies host SNe Ia with smaller stretch than young passive galaxies (∼4σ).

TESTING MODELS OF INTRINSIC BRIGHTNESS VARIATIONS IN TYPE Ia SUPERNOVAE AND THEIR IMPACT ON MEASURING COSMOLOGICAL PARAMETERS

For spectroscopically confirmed Type Ia supernovae we evaluate models of intrinsic brightness variations with detailed data/Monte Carlo comparisons of the dispersion in the following quantities: Hubble-diagram scatter, color difference (B-V-c) between the true B-V color and the fitted color (c) from the SALT-II light curve model, and photometric redshift residual. The data sample includes 251 ugriz light curves from the 3-season Sloan Digital Sky Survey-II, and 191 griz light curves from the Supernova Legacy Survey 3-year data release. We find that the simplest model of a wavelength-independent (coherent) scatter is not adequate, and that to describe the data the intrinsic scatter model must have wavelength-dependent variations. We use Monte Carlo simulations to examine the standard approach of adding a coherent scatter term in quadrature to the distance-modulus uncertainty in order to bring the reduced chi2 to unity when fitting a Hubble diagram. If the light curve fits include model uncertainties with the correct wavelength dependence of the scatter, we find that the bias on the dark energy equation of state parameter $w$ is negligible. However, incorrect model uncertainties can lead to a significant bias on the distance moduli, with up to ~0.05 mag redshift-dependent variation. For the recent SNLS3 cosmology results we estimate that this effect introduces an additional systematic uncertainty on $w$ of ~0.02, well below the total uncertainty. However, this uncertainty depends on the samples used, and thus this small $w$-uncertainty is not guaranteed in future cosmology results.

COSMOLOGY WITH PHOTOMETRICALLY CLASSIFIED TYPE Ia SUPERNOVAE FROM THE SDSS-II SUPERNOVA SURVEY

We present the cosmological analysis of 752 photometrically-classified Type Ia Supernovae (SNe Ia) obtained from the full Sloan Digital Sky Survey II (SDSS-II) Supernova (SN) Survey, supplemented with host-galaxy spectroscopy from the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS). Our photometric-classification method is based on the SN typing technique of Sako et al. (2011), aided by host galaxy redshifts (0.05<z<0.55). SNANA simulations of our methodology estimate that we have a SN Ia typing efficiency of 70.8%, with only 3.9% contamination from core-collapse (non-Ia) SNe. We demonstrate that this level of contamination has no effect on our cosmological constraints. We quantify and correct for our selection effects (e.g., Malmquist bias) using simulations. When fitting to a flat LambdaCDM cosmological model, we find that our photometric sample alone gives omega_m=0.24+0.07-0.05 (statistical errors only). If we relax the constraint on flatness, then our sample provides competitive joint statistical constraints on omega_m and omega_lambda, comparable to those derived from the spectroscopically-confirmed three-year Supernova Legacy Survey (SNLS3). Using only our data, the statistics-only result favors an accelerating universe at 99.96% confidence. Assuming a constant wCDM cosmological model, and combining with H0, CMB and LRG data, we obtain w=-0.96+0.10-0.10, omega_m=0.29+0.02-0.02 and omega_k=0.00+0.03-0.02 (statistical errors only), which is competitive with similar spectroscopically confirmed SNe Ia analyses. Overall this comparison is re-assuring, considering the lower redshift leverage of the SDSS-II SN sample (z<0.55) and the lack of spectroscopic confirmation used herein. These results demonstrate the potential of photometrically-classified SNe Ia samples in improving cosmological constraints.

A MISMATCH IN THE ULTRAVIOLET SPECTRA BETWEEN LOW-REDSHIFT AND INTERMEDIATE-REDSHIFT TYPE Ia SUPERNOVAE AS A POSSIBLE SYSTEMATIC UNCERTAINTY FOR SUPERNOVA COSMOLOGY

We present Keck high-quality rest-frame ultraviolet (UV) through optical spectra of 21 Type Ia supernovae (SNe Ia) in the redshift range 0.11 < z < 0.37 and a mean redshift of 0.22 that were discovered during the Sloan Digital Sky Survey-II (SDSS-II) SN Survey. Using the broad-band photometry of the SDSS survey, we are able to reconstruct the SN host-galaxy spectral energy distributions (SEDs), allowing for a correction for the host-galaxy contamination in the SN Ia spectra. Comparison of composite spectra constructed from a subsample of 17 high-quality spectra to those created from a low-redshift sample with otherwise similar properties shows that the Keck/SDSS SNe Ia have, on average, extremely similar rest-frame optical spectra but show a UV flux excess. This observation is confirmed by comparing synthesized broad-band colors of the individual spectra, showing a difference in mean colors at the 2.4 - 4.4 sigma level for various UV colors. We further see a slight difference in the UV spectral shape between SNe with low-mass and high-mass host galaxies. Additionally, we detect a relationship between the flux ratio at 2770 and 2900 A and peak luminosity that differs from that observed at low redshift. We find that changing the UV SED of an SN Ia within the observed dispersion can change the inferred distance moduli by ~0.1 mag. This effect only occurs when the data probe the rest-frame UV. We suggest that this discrepancy could be due to differences in the host-galaxy population of the two SN samples or to small-sample statistics.

THE RELATION BETWEEN EJECTA VELOCITY, INTRINSIC COLOR, AND HOST-GALAXY MASS FOR HIGH-REDSHIFT TYPE Ia SUPERNOVAE

Recently, using a large low-redshift sample of Type Ia supernovae (SNe Ia), we discovered a relation between SN Ia ejecta velocity and intrinsic color that improves the distance precision of SNe Ia and reduces potential systematic biases related to dust reddening. No SN Ia cosmological results have yet made a correction for the "velocity–color" relation. To test the existence of such a relation and constrain its properties at high redshift, we examine a sample of 75 SNe Ia discovered and observed by the Sloan Digital Sky Survey-II Supernova Survey and Supernova Legacy Survey. From each spectrum, we measure ejecta velocities at maximum brightness for the Ca H&K and Si ii λ6355 features, v0Ca H&K and v0Si ii, respectively. Using SN light curve parameters, we determine the intrinsic Bmax − Vmax for each SN. Similar to what was found at low redshift, we find that SNe Ia with higher ejecta velocity tend to be intrinsically redder than SNe Ia with lower ejecta velocity. The distributions of ejecta velocities for SNe Ia at low and high redshift are similar, indicating that current cosmological results should have little bias related to the velocity–color relation. Additionally, we find a slight (2.4σ significant) trend between SN Ia ejecta velocity and host-galaxy mass such that SNe Ia in high-mass host galaxies tend to have lower ejecta velocities as probed by v0Ca H&K. These results emphasize the importance of spectroscopy for SN Ia cosmology.

Interpreting large-scale redshift-space distortion measurements

The simplest theory describing large-scale redshift-space distortions (RSD), based on linear theory and distant galaxies, depends on the growth of cosmological structure, suggesting that strong tests of General Relativity can be constructed from galaxy surveys. As data sets become larger and the expected constraints more precise, the extent to which the RSD follow the simple theory needs to be assessed in order that we do not introduce systematic errors into the tests by introducing inaccurate simplifying assumptions. We study the impact of the sample geometry, non-linear processes, and biases induced by our lack of understanding of the radial galaxy distribution on RSD measurements. Using LasDamas simulations of the Sloan Digital Sky Survey II (SDSS-II) Luminous Red Galaxy (LRG) data, these effects are shown to be important at the level of 20 per cent. Including them, we can accurately model the recovered clustering in these mock catalogues on scales 30 -- 200 Mpc/h. Applying this analysis to robustly measure parameters describing the growth history of the Universe from the SDSS-II data, gives $f(z=0.25)\sigma_8(z=0.25)=0.3512\pm0.0583$ and $f(z=0.37)\sigma_8(z=0.37)=0.4602\pm0.0378$ when no prior is imposed on the growth-rate, and the background geometry is assumed to follow a $\Lambda$CDM model with the WMAP + SNIa priors. The standard WMAP constrained $\Lambda$CDM model with General Relativity predicts $f(z=0.25)\sigma_8(z=0.25)=0.4260\pm0.0141$ and $f(z=0.37)\sigma_8(z=0.37)=0.4367\pm0.0136$, which is fully consistent with these measurements.

THE EFFECT OF PECULIAR VELOCITIES ON SUPERNOVA COSMOLOGY

We analyze the effect that peculiar velocities have on the cosmological inferences we make using luminosity distance indicators, such as Type Ia supernovae. In particular we study the corrections required to account for (1) our own motion, (2) correlations in galaxy motions, and (3) a possible local under- or overdensity. For all of these effects we present a case study showing the impact on the cosmology derived by the Sloan Digital Sky Survey-II Supernova Survey (SDSS-II SN Survey). Correcting supernova (SN) redshifts for the cosmic microwave background (CMB) dipole slightly overcorrects nearby SNe that share some of our local motion. We show that while neglecting the CMB dipole would cause a shift in the derived equation of state of Δw ∼ 0.04 (at fixed Ωm), the additional local-motion correction is currently negligible (Δw ≲ 0.01). We then demonstrate a covariance-matrix approach to statistically account for correlated peculiar velocities. This down-weights nearby SNe and effectively acts as a graduated version of the usual sharp low-redshift cut. Neglecting coherent velocities in the current sample causes a systematic shift of Δw ∼ 0.02. This will therefore have to be considered carefully when future surveys aim for percent-level accuracy and we recommend our statistical approach to down-weighting peculiar velocities as a more robust option than a sharp low-redshift cut.

Clustering of photometric luminous red galaxies - I. Growth of structure and baryon acoustic feature

The possibility of measuring redshift-space distortions (RSDs) using photometric data has been recently highlighted. This effect complements and significantly alters the detectability of baryon acoustic oscillations (BAO) in photometric surveys. In this paper we present measurements of the angular correlation function of luminous red galaxies (LRGs) in the photometric catalogue of the final data release [data release 7 (DR7)] of the Sloan Digital Sky Survey II (SDSS-II). The sample compromises ∼1.5 × 106 LRGs distributed in 0.45 < z < 0.65, with a characteristic photometric error of ∼0.05. Our measured correlation centred at z= 0.55 is in very good agreement with predictions from standard Λcold dark matter (ΛCDM) in a broad range of angular scales, 0°.5 < θ < 6°. We find that the growth of structure can indeed be robustly measured, with errors matching expectations. The velocity growth rate is recovered as fσ8= 0.53 ± 0.42 when no prior is imposed on the growth factor and the background geometry follows a ΛCDM model with 7-year Wilkinson Microwave Anisotropy Probe (WMAP7)+SNIa priors. This is compatible with the corresponding General Relativity (GR) prediction fσ8= 0.45 for our fiducial cosmology. If we adopt a parametrization such that f=Ωγm(z), with γ≈ 0.55 in GR, and combine our fσ8 measurement with the corresponding ones from spectroscopic LRGs at lower redshifts, we obtain γ= 0.54 ± 0.17. In addition we find evidence for the presence of the baryon acoustic feature matching the amplitude, location and shape of ΛCDM predictions. The photometric BAO feature is detected with 98 per cent confidence level at z= 0.55.

Results from the Supernova Photometric Classification Challenge

ABSTRACTWe report results from the Supernova Photometric Classification Challenge (SNPhotCC), a publicly released mix of simulated supernovae (SNe), with types (Ia, Ibc, and II) selected in proportion to their expected rates. The simulation was realized in the griz filters of the Dark Energy Survey (DES) with realistic observing conditions (sky noise, point-spread function, and atmospheric transparency) based on years of recorded conditions at the DES site. Simulations of non–Ia-type SNe are based on spectroscopically confirmed light curves that include unpublished non-Ia samples donated from the Carnegie Supernova Project (CSP), the Supernova Legacy Survey (SNLS), and the Sloan Digital Sky Survey-II (SDSS-II). A spectroscopically confirmed subset was provided for training. We challenged scientists to run their classification algorithms and report a type and photo- z for each SN. Participants from 10 groups contributed 13 entries for the sample that included a host-galaxy photo- z for each SN and nine entries for the sample that had no redshift information. Several different classification strategies resulted in similar performance, and for all entries the performance was significantly better for the training subset than for the unconfirmed sample. For the spectroscopically unconfirmed subset, the entry with the highest average figure of merit for classifying SNe Ia has an efficiency of 0.96 and an SN Ia purity of 0.79. As a public resource for the future development of photometric SN classification and photo- z estimators, we have released updated simulations with improvements based on our experience from the SNPhotCC, added samples corresponding to the Large Synoptic Survey Telescope (LSST) and the SDSS-II, and provided the answer keys so that developers can evaluate their own analysis.

THE PROPERTIES OF TYPE Ia SUPERNOVA HOST GALAXIES FROM THE SLOAN DIGITAL SKY SURVEY

We investigate the properties and environments of Type Ia Supernova (SN Ia) host galaxies in the Stripe 82 of the Sloan Digital Sky Survey-II Supernova Survey centered on the celestial equator. Host galaxies are defined as the galaxy nearest to the supernova (SN) in terms of angular distance whose velocity difference from the SN is less than 1000 km s^{-1}. Eighty seven SN Ia host galaxies are selected from the SDSS Main galaxy sample with the apparent r-band magnitude m_r < 17.77, and compared with the SDSS Main galaxies. The SN Ia rates for early and late-type galaxies are 0.81 +- 0.19 SN (100yr)^{-1} and 0.99 +- 0.21 SN (100yr)^{-1}, respectively. We find that the host galaxies have a color distribution consistent with that of the Main galaxies, regardless of their morphology. However, host galaxies are on average brighter than the Main galaxies by ~ 0.3 mag over the range of -18.3 > M_r > -21.3. But the brighter ends of their luminosity distributions are similar. The distribution of the distance to the nearest neighbor galaxy shows that SNe Ia are more likely to occur in isolated galaxies without close neighbors. We also find that the SN Ia host galaxies are preferentially located in a region close to massive galaxy clusters compared to the Main galaxies.

Gamma-ray bursts and dark energy-dark matter interaction

In this work Gamma Ray Burst (GRB) data is used to place constraints on a putative coupling between dark energy and dark matter. Type Ia supernovae (SNe Ia) constraints from the Sloan Digital Sky Survey II (SDSS-II) first-year results, the cosmic microwave background radiation (CMBR) shift parameter from WMAP seven year results and the baryon acoustic oscillation (BAO) peak from the Sloan Digital Sky Survey (SDSS) are also discussed. The prospects for the field are assessed, as more GRB events become available.