Dust and Lenses in Future Surveys

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Dust and Lenses in Future Surveys

Árdís Elíasdóttir

Department of Astrophysical Sciences Princeton University

(ardis@astro.princeton.edu)

Strong Gravitational Lensing in the Next Decade

Cogne, Aosta Valley, Italy 25.06.2009

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Department of Astrophysical Sciences www.astro.princeton.edu

Outline

• Extinction Curves

• The Lens Perspective

• Future Surveys

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EXTINCTION CURVES

“The true harvest of my life is intangible - a little star dust caught, a portion of the rainbow I have clutched”

- Henry David Thoreau (1817-1862)

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Dusty Worlds

• Dust between the stars in galaxies causes the dimming of light from background sources

• Extinction curves measure this

dimming as a function of wavelength

• Traditionally measured by comparing two stars of the same spectral type

• Important for

• Galaxy formation studies

• Dark energy surveys

• Well determined for the Milky Way

• Very little known about extragalactic

dust extinction

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The Galactic Extinction Curve

!

R

_V

= A(V )

E(B " V ) = A(V ) A(B) " A(V )

!

A( " ⁾

A(V ) = a( " ^#1 ) + b( " ^#1 ⁾ R _V

(Cardelli et al. 1989)

• Empirically determined

• Mean value is R

_V

= 3.1 (blue)

• Extreme values: R

_V

= 1.8 (green) and R

_V

= 5.6-5.8 (red) ( Cardelli et al. 1989, Fitzpatrick et al. 1999,Udalski 2003 )

• Larger R

_V

-> larger dust grains

• Bump at 2175 Å (4.6 µm

^-1

) – Unknown origin

“Reddening”

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Other nearby galaxies

• LMC: Smaller bump and steeper rise into the UV ( Nandy et al. 1981 )

• SMC: No bump, well fitted by A(λ) ∝ 1/ λ ( Prevót et al. 1984 )

• M31: Average Galactic extinction law ( Bianchi et al.

1996 )

Pei (1992)

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Other not-so-nearby galaxies

• From QSOs

• Galaxies

• SNe IA

– Lower R

_V

values

• GRBs

– Predominantly SMC type

• Gravitational lenses

(York et al. 2006)(Noll et al. 2009)(ÁE et al. 2009) (Noterdaeme et al. 2009)

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The Lens Perspective

“You may look in front of you, and on both

sides, if you like," said the Sheep; "but you can't look all round you - unless you've got eyes at

the back of your head.”

- Lewis Carroll (1832-1898)

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Gravitational Lenses

• Compare two images, where ideally one should suffer no extinction and the other go through the galaxy

• For more than doubly imaged quasars have the

possibility of getting more than one curve for the

lensing galaxy

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Extinction along both lines of sight

!

A

^diff

( " ) = A

^B

( " ) # A

^A

( " )

= ( E

^B

# E

^A

) [ ^R

^V^diff

^a( ^"

^#1

^{) + b(} ^"

^#1

⁾ ]

!

R

_V^diff

R

_V^B

= 1+ E

^A

/ E

^B

1" E

^A

/ E

^B

1" R

_V^A

R

_V^B

#

$ % &

' ( ) 1+ *

!

E " E(B # V ) = A(B) # A(V )

( )

Galactic extinction:

The deviation of the real R _V to the

deduced R _V ^diff is

given by η, where:

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Microlensing

• The lensing by stars or other objects in the lens galaxy

• Affects the continuum part of the emission

• Effects can look like ‘extinction’

• If the data are taken on a timescale smaller than

that of the microlensing signal, then any achromatic microlensing signal should, to first order, only affect the estimate of the intrinsic magnitude ratio

• Time variable data allows to correct for chromatic

microlensing

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Extinction in Lensing Galaxies

• SBS 0909+532

– Double

– z _l = 0.83; z _Q = 1.38

• Extinction:

– E(B-V) = 0.21 ± 0.02 – R _V = 2.1 ± 0.9

⇒ A(V) ≈ 0.44

– Strong detection of the 2175 Å bump

(Motta et al. 2002)

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Extinction in Lensing Galaxies

• MG0414+0534

– Quad

– Early type

– z _l = 0.96; z _Q = 2.64

• Extinction (for both A2-B and A2-C):

– A(V) = 0.9 ± 0.1 – R _V = 2.7 ± 0.2

(ÁE et al. 2006)

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Moving towards surveys

“ There are three kinds of lies: lies, damned lies, and statistics. ”

- Mark Twain (1835-1910)

- Paraphrased from Benjamin Disraeli (1804-1881)

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Future Surveys

• SNAP and LSST are proposed space/ground telescopes

• Both missions will provide an

extensive sample to study extinction and its evolution with redshift

100-1000 50.000

Wide (WL)

~10 5.000

Deep (SNIa)

Quasar Galaxy

(Marshall et al. 2005)

Compared to 10 systems in the VLT survey!

500 Supernova

4000 Quasar

SNAP

LSST

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Goals

• Measure typical reddening (E(B-V))

• Gives a lower limit

• Measure the steepness of the slope (R _V )

• Important for dark energy surveys

⇒ Independent measurement of dust extinction

• Measure the frequency of the 2175 bump

• Can we find tracers for the bump?

• Is the bump created or destroyed?

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Ideal LSST data

z=0 z=1 z=2

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Dealing with Biases - LSST

• Microlensing:

• Data will be obtained simultaneously in all bands

• LSST will provide time varying datasets

• Extinction along both lines of sight

• Will find lower limits on E(B-V)

• The mean R _V is not expected to be strongly biased - although individual systems can

be

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Ambitious Goals

• Can we locate tracers of the bump?

– Metallicity?

– UV radiation field - CI lines?

– Dust to gas ratio?

– PAH emission lines?

• Need extensive

followup for selected systems

(ÁE et al. 2009) (Draine 2003)

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How competitive?

• Supernovae

– Hard to separate the extinction effects from intrinsic effects

• GRBs

– Very few systems, usually dust free

– Hard to determine the absolute intrinsic slope

• Quasars

– The original spectrum not known – Usually requires stacking

– Emission lines can be confused with 2175 bump

• Galaxies

– Depends on the modelling of the light distribution

– Stacking required

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Dust and Lenses in Future Surveys

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