Christophe Balland

Supernova Constraints and Systematic Uncertainties from the First Three Years of the Supernova Legacy Survey

We combine high-redshift Type Ia supernovae from the first three years of the Supernova Legacy Survey (SNLS) with other supernova (SN) samples, primarily at lower redshifts, to form a high-quality joint sample of 472 SNe (123 low-z, 93 SDSS, 242 SNLS, and 14 Hubble Space Telescope). SN data alone require cosmic acceleration at >99.999% confidence, including systematic effects. For the dark energy equation of state parameter (assumed constant out to at least z = 1.4) in a flat universe, we find w = –0.91^(+0.16)_(–0.20)(stat)^(+0.07)_(–0.14)(sys) from SNe only, consistent with a cosmological constant. Our fits include a correction for the recently discovered relationship between host-galaxy mass and SN absolute brightness. We pay particular attention to systematic uncertainties, characterizing them using a systematic covariance matrix that incorporates the redshift dependence of these effects, as well as the shape-luminosity and color-luminosity relationships. Unlike previous work, we include the effects of systematic terms on the empirical light-curve models. The total systematic uncertainty is dominated by calibration terms. We describe how the systematic uncertainties can be reduced with soon to be available improved nearby and intermediate-redshift samples, particularly those calibrated onto USNO/SDSS-like systems.

SNLS3: CONSTRAINTS ON DARK ENERGY COMBINING THE SUPERNOVA LEGACY SURVEY THREE-YEAR DATA WITH OTHER PROBES

We present observational constraints on the nature of dark energy using the Supernova Legacy Survey three-year sample (SNLS3) of Guy et al. and Conley et al. We use the 472 Type Ia supernovae (SNe Ia) in this sample, accounting for recently discovered correlations between SN Ia luminosity and host galaxy properties, and include the effects of all identified systematic uncertainties directly in the cosmological fits. Combining the SNLS3 data with the full WMAP7 power spectrum, the Sloan Digital Sky Survey luminous red galaxy power spectrum, and a prior on the Hubble constant H_0 from SHOES, in a flat universe we find Ω_m = 0.269 ± 0.015 and w = –1.061^(+0.069)_(–0.068) (where the uncertainties include all statistical and SN Ia systematic errors)—a 6.5% measure of the dark energy equation-of-state parameter w. The statistical and systematic uncertainties are approximately equal, with the systematic uncertainties dominated by the photometric calibration of the SN Ia fluxes—without these calibration effects, systematics contribute only a ~2% error in w. When relaxing the assumption of flatness, we find Ω_m = 0.271 ± 0.015, Ω_k = –0.002 ± 0.006, and w = –1.069^(+0.091)_(–0.092). Parameterizing the time evolution of w as w(a) = w_0 + w_a (1–a) gives w_0 = –0.905 ± 0.196, w_a = –0.984^(+1.094)_(– 1.097) in a flat universe. All of our results are consistent with a flat, w = –1 universe. The size of the SNLS3 sample allows various tests to be performed with the SNe segregated according to their light curve and host galaxy properties. We find that the cosmological constraints derived from these different subsamples are consistent. There is evidence that the coefficient, β, relating SN Ia luminosity and color, varies with host parameters at >4σ significance (in addition to the known SN luminosity-host relation); however, this has only a small effect on the cosmological results and is currently a subdominant systematic.

The Supernova Legacy Survey 3-year sample: Type Ia supernovae photometric distances and cosmological constraints ,

Aims. We present photometric properties and distance measurements of 252 high redshift Type Ia supernovae (0.15 < z < 1.1) discovered during the first three years of the Supernova Legacy Survey (SNLS). These events were detected and their multi-colour light curves measured using the MegaPrime/MegaCam instrument at the Canada-France-Hawaii Telescope (CFHT), by repeatedly imaging four one-square degree fields in four bands. Follow-up spectroscopy was performed at the VLT, Gemini and Keck telescopes to confirm the nature of the supernovae and to measure their redshifts. Methods. Systematic uncertainties arising from light curve modeling are studied, making use of two techniques to derive the peak magnitude, shape and colour of the supernovae, and taking advantage of a precise calibration of the SNLS fields. Results. A flat ΛCDM cosmological fit to 231 SNLS high redshift type Ia supernovae alone gives Ω_M = 0.211 ± 0.034(stat) ± 0.069(sys). The dominant systematic uncertainty comes from uncertainties in the photometric calibration. Systematic uncertainties from light curve fitters come next with a total contribution of ± 0.026 on Ω_M. No clear evidence is found for a possible evolution of the slope (β) of the colour-luminosity relation with redshift.

A Semiempirical Model of the Infrared Universe

We present a simple model of the infrared universe, based as much as possible on local observations. We model the luminosity and number evolution of disk and starburst galaxies, including the eUects of dust, gas, and spectral evolution. Although simple, our approach is able to reproduce observations of galaxy number counts and the infrared and submillimeter extragalactic backgrounds. It provides a useful probe of galaxy formation and evolution out to high redshift. The model demonstrates the signi—cant role of the starburst population and predicts high star formation rates at z D 3¨4, consistent with recent extinction-corrected observations of Lyman break galaxies. Starbursting galaxies are predicted to domi- nate the current Submillimeter Common-User Bolometer Array surveys. Their star formation is driven predominantly by strong tidal interactions and mergers of galaxies. This leads to the creation of spher- oidal stellar systems, which may act as the seeds for disk formation as gas infalls. We predict that the present-day baryonic mass in bulges and halos is comparable to that in disks. From observations of the extragalactic background, the model predicts that the vast majority of star formation in the universe occurs at z ( 5. Subject headings: cosmology: theorygalaxies: evolutiongalaxies: statistics ¨ galaxies: stellar contentinfrared: galaxies

EVIDENCE FOR A CORRELATION BETWEEN THE Si II λ4000 WIDTH AND TYPE Ia SUPERNOVA COLOR

We study the pseudo equivalent width of the Siii �4000 feature of Type Ia supernovae (SNe Ia) in the redshift range 0.0024 � z � 0.634. We find that this spectral indicator correlates with the light curve color excess (SALT2 c) as well as previously defined spectroscopic subclasses (Branch types) and the evolution of the Siii �6150 velocity, i.e., the so called velocity gradient. Based on our study of 55 objects from different surveys, we find indications that the Siii �4000 spectral indicator could provide important information to improve cosmological distance measurements with SNe Ia. Subject headings: cosmology: observations – distance scale – dust, extinction – supernovae: general

Collision-induced galaxy formation: semi-analytical model and multiwavelength predictions

A semi-analytical model is proposed that couples the Press‐Schechter formalism for the number of galaxies with a prescription for galaxy‐galaxy interactions that enables us to follow the evolution of galaxy morphologies along the Hubble sequence. Within this framework, we calculate the chemo-spectrophotometric evolution of galaxies to obtain spectral energy distributions. We find that such an approach is very successful in reproducing the statistical properties of galaxies as well as their time evolution. We are able to make predictions as a function of galaxy type. For clarity, we restrict ourselves to two categories of galaxies: early and late types that are identified with ellipticals and discs. In our model, irregulars are simply an early stage of galaxy formation. In particular, we obtain good matches for the galaxy counts and redshift distributions of sources from ultraviolet to submillimetre wavelengths. We also reproduce the observed cosmic star formation history and the diffuse background radiation, and make predictions as to the epoch and wavelength at which the dust-shrouded star formation of spheroids begins to dominate over the star formation that occurs more quiescently in discs. A new prediction of our model is a rise in the far-infrared luminosity density with increasing redshift, peaking at about z ∼ 3, and with a ratio to the local luminosity density ρ L,ν(z = zpeak)/ρ L,ν(z = 0) about 10 times higher than that in the blue (B band) which peaks near z ∼ 2.

Collision-induced Galaxy Formation

We present a semianalytical model in which galaxy collisions and strong tidal interactions, both in the field and during the collapse phase of groups and clusters, help determine galaxy morphology. From a semianalytical analysis based on simulation results of tidal collisions (Aguilar & White), we propose simple rules for energy exchanges during collisions that allow one to discriminate between different Hubble types: efficient collisions result in the disruption of disks and substantial star formation, leading to the formation of elliptical galaxies; inefficient collisions allow a large gas reservoir to survive and form disks. Assuming that galaxy formation proceeds in an Ω0 = 1 cold dark matter universe, the model both reproduces a number of observations and makes predictions, among which are the redshifts of formation of the different Hubble types in the field. When the model is normalized to the present-day abundance of X-ray clusters, the amount of energy exchange needed to produce elliptical galaxies in the field implies that they formed at z 2.5 while spiral galaxies formed at z 1.5. The model also offers a natural explanation for biasing between the various morphological types. We find that the present-day morphology-density relation in the field is well reproduced under the collision hypothesis. Finally, predictions of the evolution of the various galaxy populations with redshift are made, in the field as well as in clusters.

The Rise-Time of Normal and Subluminous Type Ia Supernovae

We calculate the average stretch-corrected rise-time of type Ia supernovae (SNe Ia) in the Supernova Legacy Survey. We use the aggregate lightcurves of spectroscopic and photometrically identified SNe Ia to fit the rising part of the lightcurve with a simple quadratic model. We obtain a lightcurve shape corrected, i .e. stretch-corrected, fiducial rise-time of 17.02^{+0.18}_{-0.28} (stat) days. The measured rise-time differs from an earlier finding by the SNLS (Conley et al. 2006) due to the use of different SN Ia templates. We compare it to nearby samples using the same methods and find no evolution in the early part of the lightcurve of SNe Ia up to z=1. We search for variations among different populations, particularly subluminous objects, by dividing the sample in stretch. Bright and slow decliners (s>1.0) have consistent stretch-corrected rise-times compared to fainter and faster decliners (0.8

On the Uncertainty in X-Ray Cluster Mass Estimates from the Equation of Hydrostatic Equilibrium

We study the uncertainty in galaxy cluster mass estimates derived from X-ray data assuming hydrostatic equilibrium (HE) for the intra cluster gas. Using a Monte-Carlo procedure we generate a general class of mass models allowing very massive clusters. We then compute the corresponding temperature profiles via the HE equation and compare them to observational data on some clusters. We find several massive clusters that pass the observational constraints, with integrated masses varying in a quite wide range. The resulting accuracy of the mass estimates is rather poor, larger than what is generally claimed. We argue that the tight constraints on cluster masses previously obtained come from the fact that a too restricted class of mass density profiles has been investigated so far. Applying our procedure to Perseus then Coma, we find that the improvement of the observational constraints results in a quite modest improvement in the accuracy of the mass estimate. For Coma, using the best current available data, we end up with a factor of two of uncertainty in the mass within the Abell radius. This uncertainty rapidly increases at further radius.

The SNLS-VLT Type Ia Spectrum Evolution with Redshift: a Demographic Effect?

We build composite spectra at z 0.5 and z > 0.5 from the 3 year SuperNova Legacy Survey (SNLS) VLT spectral sample and study their differences around maximum light. We use 93 spectra near maximum of confirmed SNe Ia observed at the VLT between 2003 and 2006 as part of the 3 first years of operation of the SNLS. We find differences in the absorption depth of some intermediate mass elements (Ca ii , Si ii). Average stretches of observed distributions are 0.975 ± 0.016 and 0.983 ± 0.014 for the z 0.5 and z > 0.5 sample respectively. We use the A+B model of Scannapieco & Bildsten (2005) to interpret these results in terms of a possible demographic evolution.