Transcript Rubber sheet analogy to GR Einstein’s view of orbits The closer the light beam passes to the deflector, the greater.

Rubber sheet analogy to GR
Einstein’s view of orbits
The closer the light beam passes to the deflector, the greater is the
deflection; for large impact parameters the image location IS the
source location. In general, the source location is not observable.
Lensing can create multiple images
“SOURCE”
Fritz Zwicky & Coma
QSO 0957+561
lens z = 0.36, source z = 1.41, separation = 6.1’’ (huge!!)
The very first known lens (Walsh, Weyman & Carswell 1979)
Einstein Cross/Huchra’s Lens
MACHO-96-BLG5 (candidate microlensing event in the bulge)
Can’t resolve the two stellar images but
can measure the increase in brightness
accurately using differential photometry
Genuine lensing events should follow the
predicted amplification curve, never
repeat, and be achromatic (except for
limb darkening effects)
Sample MACHO light curves
Complications with interpretations: don’t know the distance to the dark object, so don’t know its tangential velocity, so
don’t know the mass accurately. Most “MACHOs” were seen towards the bulge and most were consistent with star-star
lensing.
Giant Arc in Abell 370
cluster z = 0.374, arc z = 0.725
One of the first giant arcs to be discovered (Lynds & Petrosian 1986)
arc length = 21”, mean thickness = 2”, radius of curvature = 15”
Giant arc in cluster Cl2244-02
One of the first arcs to be discovered (Lynds & Petrosian 1986)
Existence of SINGLE
giant arcs proves the
cluster mass distribution
cannot be spherical
Note: HST FWPC2 FOV=3sq.
arcmin. (= most cluster images
prior to ACS are single pointings
= only the central regions of
most clusters)
Galaxy clusters make great lenses (massive & centrallycondensed)
Cl0024+16, with multiple
images of the same galaxy
Abell 2218 - the “Poster Child” of cluster lenses
Cluster lens discovered in the SDSS
Note: ACS
FOV = 2x
WFPC2
First known 5-image QSO
Galaxy Lens Candidates (HST Medium Deep Survey; Griffths, Ratnatunga et al.)
Galaxy Lenses (most are ellipticals!)
PG115+080
one of the first “Einstein rings” to be discovered
ring is lensed image of the QSO host galaxy
“Einstein rings” made by galaxy lenses (SLACS Team; SDSS galaxies)
“Double” Einstein Ring (galaxy lens)
Cluster lenses magnify very distant galaxies! (Zwicky was right.)
The recordholder in 1997
Lens is
Cl1358+62
(z=0.33)
Source is
galaxy at
z=4.92 (and the
arc is not the
only image of
it!)
XXX
“SCUBA” sources first discovered via cluster lensing using JCMT (Blain et al. 1997)
Top: 850 micron maps
Bottom: 450 micron maps
Left: A370
Right: Cl2244
Sub-mm galaxies at z=2
to z=3 that are undergoing
massive amounts of star
formation and are highly
dust enshrouded; redshifts
mostly come from radio
counterparts
A z=10 galaxy lensed by A1853 (?); Pello et al. (2004)
Pello et al. have a 4-5 sigma detection of a single line in the spectrum; if
that line is Lyman alpha, then the galaxy is at z=10
Star formation rate is ~60 solar
masses per year (uncorrected for
lensing)
Stellar mass is ~8x108 solar masses
Luminosity is ~2x1010 solar
luminosities
Lensing magnification is ~25 to 100
Graham et al. (2005) failed to detect the galaxy with long Keck integrations, Spitzer
observations, and a re-reduction of Pello et al.’s original data (from VLT archive)
Confirmed z=7 galaxy seen through A2218 (Kneib et al. 2006)
In principle, high-z galaxies seen
through lenses should form an
unbiased sample (but they are
rare to find)
zL = 0.63
zS = 1.39
H0 = 69.7+4.9-5.0 km/s/Mpc
w = -0.94+0.17-0.19
Sherry Suyu, Roger
Blandford, et al. (2010)
Abell 2218
The strong lensing is obvious by eye; the weak
lensing has to be detected statistically.
Get weak lensing by an ensemble average over
the shapes of the images of background galaxies
that have no obvious distortion.
Implicit assumption is that, in the absence of a
lens, the background galaxies are RANDOMLY
oriented. Lensing introduces a (weak) preference
for the background galaxies to be tangentiallyoriented with respect to the center of the potential.
Shape parameter in the complex plane
e1 = real part, e2 = imaginary part
Weak lensing shear in A2218 (Kneib et al. 1996)
A2218 lensing-based mass map (Kneib et al. 1996)
Complex Image Shapes of Stars in a Globular Cluster
HST/WFPC2, Hoekstra et al. (1998)
weak lensing “image polarization” ~ 2x shear
Stars should be round! Here
the PSF is not only not round,
it is variable in location on the
chips AND it’s time-variable as
well (due to “breathing” of the
telescope)
Note: things are much better
with ACS, but still have to
worry about systematics
Mean “camera shear” for WFPC2
Hoekstra et al. (1998)
“Tangential” alignment makes it seem
like there is a lens at the center of each
chip, but the artificial shear
INCREASES with distance from the
center.
Must REMOVE this signal for accurate
mass maps.
All detectors on all telescopes have to
have such an analysis for EACH
FIELD (distortion may vary with
pointing due to instrument sag)
Key to accurate weak lensing:
exquisite control of SYSTEMATICS.
HST mosaic image + weak lensing mass map for Cl1358 (Hoekstra et al. 1998)
Light rays from distant galaxies pass by ALL of the mass along the line of sight
(clusters are big deflectors, galaxies are smaller deflectors, but galaxies and clusters
aren’t the only possible deflectors!
Note: all weak deflections add vectorially. In the limit of multiple weak deflections,
each deflection can be treated as being independent of all the rest.
Two Degree Field Galaxy Redshift Survey (very shallow & incomplete beyond z ~ 0.15)
Light from ALL background galaxies must be deflected at some (small) level as the photons
travel towards the observer!
There is a LOT of mass in the universe (not all of which is luminous)
CDM lightcone “necktie” (Evrard et al. 2002; Hubble Volume Project)
The “tie” extends all the way to z=2 and shows the evolution of clustering in the
MASS of the universe over cosmic time, visualized like a traditional galaxy clustering
“pie diagram”
Even the large-scale structure of the universe causes lensing: “cosmic shear”
IF you can detect this, you can place constraints on the fundamental cosmological
parameters (Hubble const., mass density of the univese, dark energy density)
“Cosmic Shear” makes galaxies appear to be (very weakly) aligned in the
plane of the sky; degree of induced alignment depends on angular scale
Assumption: in the absence of lensing, galaxies are randomly-oriented. In reality, this is not
true for galaxies in CDM universes because of formation within filaments (but there are
ways of dealing with this “intrinsic alignment” signal).
Hoekstra et al. (2006) - mean square cosmic shear from the CFHT Legacy Survey
Expect signal to decrease with angular scale because on large scales structures are
no longer correlated with each other on the sky.
Open circles are a control statistic (a.k.a. a “sanity check”) in which the images of
the galaxies have been rotated randomly by 45 degrees.
Cosmological Evolution Survey (COSMOS); 2 square degrees over a wide
range of wavelengths
1000 hours of HST time
used to get the optical
imaging of the galaxies
and do weak lensing
“tomography”
Mass versus Light from COSMOS (Massey et al. 2007), as projected on the sky
Dark matter mass
map comes from
weak lensing (HST
ACS imaging)
The first 3-d mass map of the universe (COSMOS; Massey et al. 2007)
Projected mass and light from COSMOS
These are really only the “first returns”; much more work will be done in future by
many groups. These maps seem to reveal a lot of systematics in the data, but you
have to start somewhere! (This is a HARD problem, but worth doing.)
Problems for “cosmic shear”: intrinsic galaxy alignments?
“Host” and “satellite” galaxies are aligned with each other (Agustsson & Brainerd 2006)
Galaxy-Galaxy Intrinsic Alignments on Large Scales?
CDM predicts that they *ought* to exist (formation of galaxies in
filaments = not entirely random process + tidal torquing of nearby
neighbours)
“Luminous Red Galaxies” in SDSS appear to be intrinsically
aligned (Okumura, Jing & Lee 2009, ApJ, 694, 214)
Signal is small, but present for 1 h-1 Mpc < rp < 100 h-1 Mpc !!
Problems for “cosmic shear” theory (?)
Shear due to galaxies ALONE (no large scale structure) in the HDF-North
Changing the lens mass by only 20% changes the small-scale weak lensing shear
by an amount equal to the difference between two different cosmologies