Lindsay King, IoA Cambridge

Regimes of gravitational lensing

Insensitive to lens being luminous or dark

and to dynamical state of matter –

Light deflection

!

"#$

Why use gravitational lensing?

A unique way to investigate the spatial distribution of DM and substructure beyond the Local Group.

Complementary to direct detection experiments in

providing information on DM at large scales.

To understand the interplay between DM and baryons in

the formation and properties of massive objects such as

galaxies and galaxy clusters.

What does/can strong lensing tell us about DM in

massive galaxies and about substructures?

What is gravitational flexion, and why is it a useful tool

in addition to weak and strong lensing?

Bullet clusters and CDM

Galaxy clusters provide powerful tests of CDM and

our structure fomation paradigm….

Galaxy Scale Strong Lensing

Auger et al. 2010

DM fraction inside rEinstein

!! ~200 systems known

!! Reveals DM on kpc scales

!! Massive elliptical galaxies have

isothermal density profiles

!! Light and mass quite well aligned

Gavazzi, Treu / SLACS team

Example: double einstein ring

Signatures of millilensing by

substructures of ~105-1010 M"#

Zackrisson & Riehm 2010 review

(Some) lensing signatures of mass substructure – luminous

dwarfs or CDM substructure?

Vegetti, Czoske & Koopmans 2010

!! Distort extended images

!! Cause “flux ratio anomaly”

McKean et al. 2007

The clone CLASS 2045+265

Vegetti & Koopmans 2009

Constraint on CDM substructure

fraction and mass function

slope from 200 arcs/Einstein

rings (assuming sensitivity to

108 Msol)

Claim detection of dark

substructure in double Einstein

ring system (limits weak since

single system).

Recent lower limit of f~0.3%

from Metcalf & Amara (2010)

using flux ratio anomalies.

Harnessing gravitational flexion to detect DM

From Bacon et al. 2006

shear flexion

!! Sensitive to gradients in mass

!! Acts over larger region than strong lensing

!! Traces smaller scales than weak lensing

Statistic for detecting and quantifying substructure

Use weighted flexion signal in apertures on data field; weight factor can

be chosen to maximise signal:noise for particular types of mass profile.

We call it the FMap statistic.

Leonard, King & Wilkins 2009

Clowe et al. 2006

The original bullet cluster

“A direct empirical proof of the existence of dark matter”

Russell et al. 2010

A new bullet: Abell 2146

Z=0.23

Map of the mass distribution

using weak lensing analysis

(Subaru data).

Confirm cluster galaxy

membership (Beccy Canning).

Deep observations of hot X-ray gas using Chandra

(Helen Russell).

Coming soon!

Most fits to lensing data are broadly consistent with NFW profiles

as seen in simulations performed assuming LCDM.

(e.g. samples from Clowe et al. 2006 (EDisCS), Dahle et al.).

Some claims of galaxy cluster profiles in conflict with CDM paradigm,

e.g. profile deviates from NFW form, or mass is too concentrated.

(e.g. Broadhurst and collaborators, Oguri et al. 2009).

Meneghetti et al. (2007) show that fitting

circularly symmetric mass models doesn’t

give the correct inner profile slope… Need to

add ellipticity to the mass models used for

strong lensing / dynamics

Problem with CDM? Analysis?

Mead, King, Sijacki, Leonard, Puchwein, McCarthy 2010

Stars

Gas

DM

strong weak

Clowe, De Lucia & King 2004 Clusters have 3-D structure

Important to account for

triaxiality in error budget.

Corless & King 2007

Prolate Oblate

(cigar) (pancake)

Account for cluster shape

+ incorporate other priors

in Corless + King (2008)

MCMC fitting method.

Corless, King & Clowe 2009

Abell 1689

Weak lensing peak counts are very sensitive to

the relationship between mass and concentration

King & Mead 2010

Oguri et al.

2009 M-c

Duffy et al.

2008 M-c

Possible with next generation surveys $

Gravitational lensing is a powerful tool for

investigating the distribution of (dark) matter over

a wide range of scales in the universe.

Large samples of strong lenses will provide

interesting constraints on DM substructure.

Gravitational flexion measurements look promising in probing substructure on group and cluster scales.

Clusters are a powerful test of the CDM paradigm -

but be cautious with models and error bars!