Cosmology with Type-Ia Supernovae

Cosmology with Type-Ia Supernovae

Ramon Miquel

Lawrence Berkeley National Laboratoryand

ICREA / IFAE, Barcelona

IRGAC, July 11-15 2006, Barcelona

July 13, 2006 Ramon Miquel IRGAC 2006 2

Outline

• Type-Ia SNe as cosmological tools• Cosmological analysis• Systematic uncertainties• Current surveys: SNLS• Future surveys: SNAP• Summary


Type-Ia SNe probe dark energy through the history of the expansion rate

• Friedmann-Lemaître Equations (GR + homogeneity and isotropy):

a : scale factor : energy density p : pressure k : curvature

• After specifying a equation of state p = p() for each component:

H2(z) = H20 [M (1+z)3 + DE (1+z)3(1+w)] , a = (1+z)-1

matter dark energy flat universe, constant w = p/

• Measuring the history of the expansion rate, H(z), we can learn about the universe constituents, M, DE, w.

Universe Constituents and Dynamics

2

2

3

8

33

4

a

kρ

πG

a

a

pρπG

a

a

aaH /.


• Standard candles provide a measurement of the luminosity distance as a function of redshift:

: flux

L : intrinsic luminosity

dL : luminosity distance

r(z) : co-moving distance

• Astronomers measure the apparent magnitude and redshift:

– M is the (assumed unknown) absolute magnitude of a type-Ia SN.– H0 dL does NOT depend on H0

Probing Dark Energy with Type-Ia SNe

z

L

zH

dzzr

r(z)z)(zd

0 )'(

')(

1)(

]Mpcs km100/[log525

)]([log5)/(log5.2)(11

010

010010

HM

zdHzm L

M

M

24/ LdL

(geometric test of dark energy)


Type-Ia Supernovae (I)

• Defined empirically as supernovae without Hydrogen but with Silicon in spectrum.

• Progenitor understood as a white dwarf accreting material from a binary companion.

• As the white dwarf approaches Chandrasekhar mass, a thermonuclear runaway is triggered.

• A naturally triggered and standard bomb.


Type-Ia Supernovae (II)

• General properties:– Homogeneous class of events: luminosity, color, spectrum at

maximum light. Only small (correlated) variations– Rise time: ~ 15 – 20 days– Decay time: ~ 2 months

– Bright: MB ~ –19.5 at peak

• No hydrogen in the spectra:– Early spectra: Si, Ca, Mg, ...(absorption)– Late spectra: Fe, Ni,…(emission)

• SN Ia found in all types of galaxies, including ellipticals– Progenitor systems must have long lifetimes


Discovering Supernovae

8Ramon Miquel IRGAC 2006July 13, 2006

Are Type-Ia SNe Standard Candles?

apparent magnitude → distance → time


Type-Ia SNe as Standardizable Candles

• Nearby (z < 0.1) supernovae used to study SNe light curves• Brightness not quite standard• Intrinsically brighter SNe last longer• Correction needed

Peak-magnitude dispersion of 0.25 – 0.30 mag

~ 0.10 – 0.15 mag dispersion(5 – 7% precision in distance)

• After correction, standardcandles in optical region (at least).


Near-Optical Bands

g r i z


Near-Optical Bands

U B V Rz = 0.5

→ ·(1+z)

I

z = 1.0 U B V R


Type-Ia SN Spectral Features

• Spectra at near maximum light are used to determine type of SN (Si-II feature)

• And to measure the redshift, z, by observing the shift in the spectrum

Si-II


Images

Spectra

Redshift & spectral properties

Light Curves

Data Analysis Science

M and

w and wa

SN Analysis


Hubble Diagram


Discovery of Acceleration


High-z Results

Riess et al. 2004; also Knop et al. 2003

• Expansion went from deceleration to acceleration

• Exclude simple gray dust models

(m

-M)

(mag

)

redshift


Current Surveys

(300)


Systematic Errors

• Statistical error is dominated by intrinsic SN peak magnitude dispersion int = 0.10–0.15

• Many systematic errors will be totally correlated for SNe at similar redshifts

– Current and near-future surveys will have O(100) SNe for z = 0.1 redshift bin.

– Therefore, systematic errors of order int/√NSN = 0.01–0.02 will already become important or even dominant.


Sources of Systematic Errors

Error Source ControlHost-galaxy dust extinction

Wavelength-dependent absorption identified with high S/N multi-band photometry.

Supernova evolution

Supernova sub-classified with high S/N light curves and peak-brightness spectrum.

Flux calibration error

Program to construct a set of 1% error flux standard stars.

Malmquist bias Supernova discovered early with high S/N multi-band photometry.

K-corrections Construction of a library of supernova spectra.

Gravitational lensing

Measure the average flux for a large number of supernovae in each redshift bin.

Non-Type-Ia contamination

Classification of each event with a peak-brightness spectrum.

*

*

*

*

Kim, Linder, Miquel, Mostek, MNRAS 347 (2004) 909


Extinction by Dust (I)

Dust in the path between the SN and the telescope attenuates the amount of light measured

• Milky Way dust is well measured and understood (Schlegel, Finkbeiner & Davis 1998)

• Host galaxy extinction leads to reddening of supernova colors:

AV = RV · E(B-V)

• In another band j, the extinction is (Cardelli, Clayton & Mathis 1989)

AV : increase in magnitude in V bandE(B-V): excess in B-V color over expectedRV ≈ 3.1 in nearby galaxies

)()()()(

)( jjVjV

jjVjj baRVBEm

R

baAmm

known

(≈ 0-0.10)

2600 citations

1400 citations

What is the value of RV in distant galaxies?


Extinction by Dust (II)

Different approaches to RV determination:

• Riess et al. 2004 (HZT) assume RV = 3.1, as it appears to be in the local universe. Include exponential prior on AV. Bias?

• Astier et al. 2006 (SNLS) instead determine one RV value for all their high-z SNe, coming up with a much lower value RV = 0.57 ± 0.15

– Their RV effectively includes any other effect that might correlate SN color and magnitude.

• SNAP will determine RV for each SN independently.

– Needs at least 3 bands for each SN

)()()()(

)( jjVjV

jjVjj baRVBEm

R

baAmm


Dust Biases

Linder & Miquel 2004

Current data quality

No extinction (e.g. only SNe in ellipticals) Extinction corrected With AV bias With AV and RV biases

w(z) = w0 + (1-a) wa

Linder 2003


Gray Dust?

• Gray dust would be dust that does not lead to any measurable reddening (equivalently, RV → ∞)

• Therefore, it’s not correctable with the usual methods.• “Natural” models would lead to a dimming of SNe at all redshifts.

Simple gray dust models excluded

Some contrived models are just

indistinguishable from CDM

Riess et al. 2004; also Knop et al. 2003

(m

-M)

(mag

)


U B V Rz = 0.5

→ ·(1+z)

I

• At high z, one needs to relate measured fluxes in, say, R, I, z filters with fluxes in SN rest frame B, V, R bands.

• Good empirical model for SN spectrum from B to z is needed.

K-corrections

))1('()'('

)()(log5.2

)()(

)()(log5.2 10

0

0

10zTd

Td

Td

TdK

RSN

BSN

R

B

BR

≈ O(0.5 mag)


Calibration• Calibration ≡ determining the “zero-points” 0,j of each filter j

• Overall normalization is irrelevant• Relative filter-to-filter normalization is crucial (K-corrections, dust-

extinction corrections) Standard cal = 0.005 Standard cal = 0.001 Self cal = 0.005

w0

wa

68% CL contours

*

Kim & Miquel 2006

Standard procedure uses well-understood stars to get cal = 0.01 at best

Alternative procedure using also SN data themselves achieves a large degree of self-calibration (Kim & Miquel 2006)

Example for SNF + SNAP (300 + 2000 SNe up to z = 1.7)


Current SNe Surveys

SNLS ESSENCE

SDSS-II / SNe SuperNova Factory


The SuperNova Legacy Survey (SNLS)

• Ongoing (2003-2008) SN survey using CFHT (Mauna Kea):

– 3.6 m aperture

– 1 deg2 field of view

– 328 Megapixel camera (MegaCam)

• Photometry for 40 nights/yr during 5 years.

– 4-night cadence rolling search in four 1-deg2 fields in g, r, i, z bands.

• Expect to discover 500-700 type-Ia SNe up to z = 1.

• Spectroscopic follow-up of most good SN candidates in VLT, Gemini, Keck…


SNLS Dataz = 0.36 z = 0.91

day

z = 0.285


SNLS Analysis

• Light-curve fit performs K-corrections and returns miB at peak (SN

rest frame), stretch si and color excess Ei(B-V).

• Every available filter is used in fit, provided it corresponds to U, B, V, R in SN rest frame.

• At least two filters are required.

• The cosmology fit then proceeds as:

• 44 published nearby (z < 0.1) SNe and 73 new high-z SNe are used in the fit.

• Statistical errors dominate now, but systematic errors will dominate with final sample.

– Main systematic error: calibration.

)()1()],([log5 010 VBEszdHm iiiLB

iB M x : free parameter

cosmological params.


SNLS Hubble Diagram

First-Year SNLS Hubble Diagram 73 high-z SNe

Astier et al. 2006

int = 0.12 mag

mag

nitu

de (

B-b

and)

+ c

onst

ant

redshift


SNLS Results

SNLS + BAO (Einsestein et al. 2005) SNLS + WMAP-3 (Spergel et al. 2006)

021.0029.0

066.0085.0

281.0

984.0

M

w

022.0271.0

105.0023.1

M

wFlat universe assumed

1 XM


Next Generation SNe Surveys from the Ground

DES Pan-STARRS

LSST

(2008-2012)


SNe without spectroscopy?

• Next generation SN surveys from the ground will gather about 2000 type-Ia SNe with redshifts up to z = 1.2.– Practically impossible to get spectroscopy for all those SNe.

• Is it possible to do cosmology with type-Ia SNe without spectroscopy?– Redshift determination

• Photometric redshifts

• Host galaxy redshift?

– Typing

• Typing from goodness of light-curve fit.

– Systematic tests: ???


SNLS Photo-z’s and Photo-z Typing

– Photo-z’s:• <|zphot - zspec|> = 0.03*(1+z)

assuming cosmology known. • But small dependency on the

assumed cosmological values.

– Photo-typing: • 90% purity using a real-time

analysis of pre-maximum light curves.

• Presumably, it can be improved using all light-curve information.

M=0.25, =0.75

M = 1, = 0

Sullivan et al. 2006

◊ Fail 2 cut


Future SNe Surveys from Space

JDEM/SNAP JDEM/Destiny

JDEM/JEDIDUNE

(2013-2016)


Why Space?

• Precision on wa increases by going to z > 1

• Window into deceleration (z > 1) era can help with syst. errors.

• For z > 1-1.2, rest-frame B band redshifts into observer IR region ( > 1.2 m)

• Atmospheric absorption is large in IR region

Need space-based telescope

SNAP simulation

Miquel 2004


The SNAP Satellite

• 2m-class wide-field telescope with state of the art optical and NIR camera and spectrograph

– Collect about 2000 type-Ia SNe with z < 1.7

– Study weak lensing from space

• Could fly in ~2013. Part of JDEM (DOE/NASA) competition.


Fixed filters atop the sensors

SNAP Focal Plane

Guider

Spectrograph port

VisibleNIRFocus starprojectors

Calibration projectors

D=56.6 cm (13.0 mrad)

0.7 square degrees

Field beforeslicing

Pseudo-slit

Slicing mirror (S1)

Spectrogram

Pupil mirrors(S2)

To spectrograph

Field optics (slit mirrors S3)

From telescopeand fore-optics

Integral Field Spectrograph


SNAP (and DES) Optical Detectors

• New LBNL technology: thick back-illuminated CCD detector.

• Better red response (up to = 1 m) than “thinned” CCDs devices in use at most telescopes.

• High-purity silicon has better radiation tolerance for space applications.


(Some) SNAP Systematics

• Dust extinction:

– Measure each SN in 9 → 3 (low to high z) filters

– Can determine AV and RV for each SN independently.

• Evolution:– Properties of SNe that correlate with luminosity can change with z

– Get precise spectrum at maximum light for all SNe

– Classify SNe according to sub-type. This needs a large database of nearby SNe with good photometry and spectra (SNF)

– Perform cosmology fits within sub-types including low- and high-z SNe (“like-to-like” comparison).

– In practice, allow for several Mi in cosmology fit (one for each sub-type).

– Statistical degradation because of extra parameters is only few % (Kim, Linder, Miquel, Mostek 2004)


• For a fiducial CDM model

– w0 measured to 10%

– w’ ( ≈ wa / 2) to 10%

• Better for most other models (more sensitive to late-time dark-energy)

• Big improvement after adding weak lensing

w’ ≈

wa /

2

w0

SNAP Reach

Linder 2005


Summary

• Type-Ia SNe provided the “smoking gun” for acceleration.

• Mature technique still being perfected.

• Control of systematic errors key to future improvements.

• Vigorous current and future program:• Low-z from ground: SNF, SDSS-II/SNe, CfA, Carnegie…

• Medium- to high-z from ground: Essence, SNLS, DES, Pan-STARRS, LSST

• High-z from space: HST, JDEM, DUNE

Expect more insight on the nature of Dark Energy from type-Ia SNe studies

Documents

Cosmology with Type-Ia Supernovae