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!