dawson by liuqingyan

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									                   Decelerating and Dustfree:
      Dark Energy Studies of Supernovae with the Hubble Space
                                   Telescope




                                  Kyle Dawson
                                 March 16, 2008

                       For the SuperNova Cosmology Project




RDCS 1252.9 @ z=1.23
(ISAAC and ACS)
Supernovae and the Accelerating Universe


   Brief discussion of systematics – probably heard
    it all before from J. Guy
   Our HST program to combat the dust systematic
   Some initial results
                              The Problem with Dust

Host galaxy dust extinction
                                               Before extinction
If host galaxy is similar to Milky Way then:
                                               correction
DMB=RBE(B-V) with RB~ 4

Color error ~ 0.1 => Magnitude error ~0.4

Extinction correction dominates
measurement error!



                                               After extinction
                                               correction
                                What About Intrinsic Color of SNe?

                                                          Extinction: DMB=RBE(B-V)
Residual without c-correction




                                 β=4.1
                                                          Conventional dust extinction predicts
                                                            RB~ 4. However, dispersion in SNe
                                                            color contributes to observed B-V as
                                                            well.

                                                          Introduce color correction b to account
                                                              for intrinsic color + host extinction

                                     Sullivan et al. 08
                                                          mB=mmaxB – M + a(s-1) - bc

                                                          Measure s, c, mmaxB, fit M, a, and b

                                                          SNe are better fit with b~ 2.5
                                                            (depending on who you ask).
                            The Problem with Dust
Host galaxy dust extinction
DMB=RBE(B-V) with RB~ 4     Or is it 2.5???
                                                 Before extinction
Degeneracy between intrinsic color of SNe and    correction
reddening from host lead to poorly constrained
extinction correction
The error increases at higher redshift
AND
Corrections may be biased due to incomplete
understanding of dust properties at high z!


                                                 After extinction
                                                 correction
One Possible Solution: Use SNe Hosted by Dust-Free
                 Elliptical Galaxies
            M. Sullivan et al. (SCP) 2003,
              MNRAS, 340, 1057.
                                              Galaxy type    Dispersion (no Ext Corr)
                                             •Elliptical: E/S0    s=0.16 mag
                                             •Spiral: Sa/Sb/Sc     s=0.20 mag
                                             •Irregular Scd/Irr   s=0.27 mag




                                                 Solution: Use Ellipticals and don’t
                                                 apply the extinction correction.

                                                 But, at z~0.5, only ~1 in 5 SNe Ia is
                                                 hosted by an elliptical galaxy.
         Minimizing host galaxy dust extinction
SN color: With few exceptions, SNe
hosted by early type galaxies exhibit
color consistent with no host extinction




                     B-V color
                     of low-z SNe          Elliptical Galaxies & Clusters
                                           Elliptical galaxies (E/S0) have
                                           little star-formation & dust.

                                           Clusters: concentration of nearly
                                           dust free Elliptical galaxies.
           No dust in elliptical (cluster) galaxies?


Ellipticals in massive clusters
are among the earliest (oldest)
objects in the Universe.




          How do we know these galaxies are dust-free?
              * Small scatter in color for cluster galaxies – uniform population
              * Uniform color within a galaxy (dust is clumpy)
              * No emission at ~200 microns (dust emission)
                         Coherent evolution of
                         cluster environment                             25 clusters
                                                                         with HST
                                                                         resolution
                                                                         will be added
Scatter of color of ellipticals
galaxies in clusters is very small
(consistent with current resolution).


Evolution from date of formation
(z~3) appears consistent for clusters
over wide range of redshifts.
                                          Blakeslee et al. ApJL (2003)
Strong constraints on host environment.
             Decelerating and Dustfree:
Targeting SNe in Very High Redshift Galaxy Clusters
Major cycle 14 HST Program in collaboration with several galaxy cluster groups (219 orbits over 18
months).
 •    searched 25 recently discovered massive galaxy clusters at z ≥ 1 with ACS.

Why clusters?
 •    Dominated by nearly dust-free Elliptical (E/S0) galaxies.
 •    5 times higher density of Elliptical galaxies at high redshift.

Scientific goals:
 •    Significantly improve SN constraints of dark energy - reduce the statistical and systematic
      uncertainties.
 •    Cluster studies: weak lensing, galaxy morphology, and color-magnitude relationship.

     Spitzer                          VLT                        XMM+VLT                    ACS
What Sort of Improvement are we Looking For?




  Left Panel: Simulated 68% confidence region on w' vs. w0 for the current literature SN sample but with underlying
        cosmology(w0 = -1; w’ = 0). The parameters are poorly constrained because color errors are magnified by R B ~ 4.
  Center Panel: Demonstration of bias introduced from assuming a prior. Solid filled curve-RB=4.1, dashed curve, RB=2.5.
  Right Panel: The goal of this project is shown as a confidence region for a simulated new sample of 10 z > 1 SNe Ia
        found in ellipticals, together with 5 in ellipticals from the past and ongoing GOODS searches, as well as 120 SNe Ia
        in ellipticals at the lower redshifts now being produced by the ground-based CFHT SN Legacy Survey, the CTIO
        Essence survey, and (at z < 0.1) the Nearby SN Factory
                      The Clusters
 Weak lensing mass estimates
  for 18 clusters
 Red-sequence modeled from
  extensive ground-based
  spectroscopy + 2 band ACS
  imaging
 Richness estimates for all 25
  clusters – Ngal from red-seq
 Consistent with model from
  SDSS (dotted line)
M proportional to Ngal1.2


All SNe discovered in richest clusters  unshaded region
 Observing Program: 25 massive galaxy
            clusters at z ≥ 1
                                                 HST Rolling Search
                                                 20-26 day cadence
                                                 ACS z’, i’ bands.




Search and follow: Continuous flow of ACS data
Followup: NICMOS J-band scheduled ~10 days
after discovery
Spectroscopy: At least one night/month
    Intensive Ground-based Spectroscopy Follow-up

Goal: Establish redshift, host
  type, and often SN type
 Subaru: 10 half-
  nights/semester
 Keck: 2 nights/semester

 VLT: 10 hours/semester
  (queue mode) + Director’s
  Discretionary time
 E Host  SNIa with high
  confidence




                                 Preliminary spectra from this program
Results: SNe Discoveries in HST Program
                    15 z>1 SNe discovered, 9 hosted by E
                    type galaxies


                    Full multi-color light curves for 8/9
                    hosted by E type galaxies


                    Full light curves for 4/7 z>0.95 SNe
                    hosted by late-type galaxies


                    Twice the SN/orbit as previous SN
                    searches in random fields using HST


                    Three times the E hosted-SN/orbit!
         Factors to Consider for ACS Photometry
    Many issues contributing at the ~5%
    level were solved in 2007:
                                                 K-corrections using various
   Time variation of distortion
                                                 lightcurve templates
    corrections
   Time variation of bias levels
   Relative astrometry corrections
   PSF variation across FOV
   Variation/gradient in flat field
   Backscattering of NIR photons
   K-corrections of different templates
   Gravitational lensing of SNe behind
    clusters

Almost there! Still blinded but expect to put E-hosted SNe on Hubble diagram very soon
   The Elliptical host (E/S0) Hubble Diagram
            No extinction correction
Example of E–only Hubble Diagram               Seven SNe Ia in elliptical galaxies
           Sullivan et al.                     observed with complete lightcurves.
           & Knop et al.
                                               No extinction correction applied
                                This program


                                               other analyses
                                                   • SN Ia rate in cluster
                                                   environment
                                                   •Host Properties
                                                   • Cluster studies (collaborators)
                                                       •Weak lensing
                                                       •Red Sequence Scatter
                             (David Rubin)             •Etc.
                          Summary


   219 orbits with HST to search for SNe in high z clusters
   Efficient observing strategy, finding 3x E hosted SNe/orbit
    over previous GOODS searches
   Each E-hosted SN worth 9 normal SNe
   Results soon, photometry complete, unblinding
   SNe results will include cosmology, properties of SNe by host
    type, rates
   Lots of parallel science: weak lensing mass estimates,
    morphology, evolution, spectral properties

								
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