Strong and Weak Lensing

Strong and Weak Lensing
TOMMASO TREU (UCSB)

Outline
• Introduction:
– Strong lensing: what is it and what can it do for us?
– Weak lensing: what is it and what can it do for us?
– Are lenses “normal” galaxies?
– Lensing++
• Recent progress:
– Finding gravitational lenses
– The mass structure of early-type galaxies
– The mass structure of spiral galaxies
– Clusters and their brightest galaxies

Introduction

Strong lensing:
What is it and what can it do for us?
P. Marshall

Strong lensing by galaxies
Lensed quasar Lensed galaxy

Strong lensing measures mass
• Projected mass inside
the Einstein Radius to a
few % independent of
kinematic state
• Good azimuthal
information (i.e.
ellipticity)
• Radial information only
for extended sources or
special configurations

Weak lensing:
What is it and what can it do for us?
• Image of background sources are
perturbed by intervening matter
• Sources are slightly distorted
(sheared) and magnified
• Image quality is key
• Main source of noise is intrinsic
shape of objects (shape noise)
• Signal can be detected statistically
• Discussed here only in combination
with strong lensing (see Hoekstra’s
talk for more on weak lensing)

Finding Strong
Gravitational Lenses

Why do we need more strong
lenses and why is it hard?
• Most applications of strong gravitational lensing are
currently limited by sample size.
• Galaxy lenses are rare:
– Density on the sky is of order 10/deg 2 at typical HST
depth/resolution
– One galaxy in 200-1000 is a strong lens
– Lensing gives you extremely precise measurements for a
subset of galaxies
• High quality data are needed
– Subarcsecond imaging, ideally HST-like resolution
– Deep spectroscopy for redshifts

SLACS: the strong lens factory
(www.slacs.org)
• Candidate lenses selected from SDSS as red galaxies with
“spurious” emission lines (Bolton et al. 2004,2005,2006)
• SDSS velocity dispersion can be used to pre-select masses
and estimate success rate
• 98 Strong lenses confirmed to date using HST (Bolton et al.
2008; Auger et al. 2009)

Lensing++

Lensing plays well with others
• Strong Lensing: total mass on small scales
– little spatial information
• Weak lensing: total mass on large scales;
– poor spatial resolution; too weak for individual galaxies
• Stellar kinematics: total mass
– depends on kinematic state of the tracers (e.g. anisotropy)
• Stellar population synthesis models: stellar mass
– degeneracy with initial mass function
• Kinematics of hot gas: total mass/gas mass;
– only for most massive systems; poor spatial resolution;
Combination of different techniques is most powerful

Are lensing galaxies special?

Lenses are “normal” galaxies
Lenses are normal in terms of their position on the
Fundamental Plane, stellar mass, size, environment
(Treu et al. 2006, 2009; Bolton et al. 2008; Auger et al. 2008,2009)

The mass structure of
early-type galaxies

• How much dark
matter is there in
galaxies?
• How is it
distributed?
• What are the
properties of dark
matter halos
– NFW or not?
Smooth…
Gerhard et al. 2001

…and clumpy
• Not covered here: Koopmans’s talk….
Moore et al.

Early-type galaxies live in dark
matter halos: e.g. 0047
Koopmans & Treu 2003

Galaxies have approximately
isothermal profiles
Treu & Koopmans 2004
• Mass density profile slope can be measured to ~10%
over scales 1-10 kpc.
• It’s harder to measure DARK MATTER only slope on
galaxy scales

Homogeneity of lens galaxies
Koopmans et al. 2006 Bolton et al 2008
The logarithmic slope is -2 with little intrinsic scatter.
More in Koopmans’ talk

There are exceptions/caveats
HE0435; Kochanek et al. 2006
Slope
σ/σSIE
-2
1
Treu & Koopmans 2004
R Einst /R e
Also PG1115!!!

Trends with mass:
a more Fundamental Plane
• MFP has no “tilt”, i.e.
• Tilt of the classic FP due to varying dark matter
content?
Bolton et al. 2007, 2008

Enter Stellar Masses
New Bayesian stellar mass code; Auger et al. 2009
Ask me if interested

Total M/L
Stellar masses
and the “tilt” of the FP
Stellar M/L
Velocity dispersion (km/s)
New Bayesian stellar mass estimator: Auger et al. 2009;
see also Cardone et al. 2009; Tortora et al. 2009; Grillo et al. 2009

The IMF of early-type galaxies
• Lensing+dynamics
gives absolute
calibration of the
IMF
• Closer to Salpeter
than Chabrier
(preliminary)
Treu et al. 2009

+Weak Lensing

ΔΣ (h M sun/pc 2 )
How about at larger radii?
Shear profile
Gavazzi, TT et al. 2007

Reaching out into the halo:
Strong and weak lensing analysis
Constant M/L ratio doesn’t work
Need an extended halo
M * /M vir =2±1%
=2e13 Msun
Gavazzi et al. 2007

Is there evolution?
Lagattuta et al. 2009

The mass structure of
spiral galaxies

What about spirals?
How “heavy” is the disk?
• Gravitational lensing (total projected mass)
• Stellar rotation curves (total 3D mass)
• High resolution OIR photometry (total stellar mass)
Need good targets: find more edge-on disk galaxy lenses
See Marshall’s poster and Dutton’s talk

A case study: 2237
Q2237
Trott et al. 2008
see also van de Ven et al. 2008

New samples: Haggles
Marshall et al. 2009

15 targets
observed
SWELLS (HST-16s)

Dust is sometimes a problem…
but we are working on it!
Marshall’s poster

Clusters and their brightest
galaxies

Clusters are not large ellipticals
Abell 611; Newman et al. 2009

The mass profile of clusters
• Total mass density
profile NOT
isothermal
• Dark matter halo
not NFW
Newman et al. 2009

Conclusions
• Strong and weak lensing are extremely powerful techniques
• Combining techniques is essential to disentangle luminous
and dark matter and establish radial trends
• Early-type galaxies live in dark matter halos:
– M*/M vir ~2±1%
– Dark matter fraction within Re increases with mass
– IMF is closer to Salpeter than Chabrier
– Total mass profile close to isothermal with scatter
• More soon on spiral galaxies from lensing
• Clusters of galaxies have different mass profiles:
– Total is not isothermal
– Dark matter halo can be shallower than NFW

UNIVERSITY OF CALIFORNIA SANTA BARBARA
The end
PhD and postdoc positions open Fall 2009; e-mail me if interested