Download Astronomy with mm-Mpc lenses - SLAC

Astronomy with
cm – Mpc lenses
Phil Marshall
KIPAC – SLAC – Stanford University
February 28th 2004
The Human Eye
has an aperture of 7mm or so when dark-adapted
●
provides an image updated every eighth of a second
●
has a logarithmic response to brightness, which has led
astronomers to measure observed flux in magnitudes:
m = -2.5 log10(flux) + constant
●
gives an angular resolution of about 1arcmin
●
Faintest star visible by eye from a dark site has magnitude 6
In Palo Alto one can sometimes see the Big Dipper – mag 2
Collecting photons
Use CCDs (charge coupled devices) to detect photons
Amount of charge built up in pixel ≈ no. of photons
Images manipulated as arrays of numbers
Astronomy with a digital camera
Exposure time = 16 secs
Aperture diameter = 30mm
⇒ see to magnitude 10.4?
Wide field! 48x36 degrees...
Zoom in (after the exposure!):
The pleiades star cluster:
Resolution
limited by
camera
optics,
~3arcmin
Human eye
does 3 times
better!
Comparison with
Palomar digitized
sky survey (1949)
http://www.astro.caltech.edu/ob
servatories/palomar/
Comparison with
Palomar digitized
sky survey
http://archive.stsci.edu/dss/
Magnitude limits:
Naked eye in Palo
Alto: ~2
Camera image: ~5
(predicted 10.4)
DSS: ~21
Telescopes:
Faintest star visible by eye from a dark site has magnitude 6
Ron got comparable results in Palo Alto by storing photons
An 8.4m lens would collect (8.4m/7mm)2 times more light
than a dark-adapted eye
⇒ 15 magnitudes fainter (bit less for inefficency)
Integrate for an hour:
⇒ another 10 magnitudes (bit less for inefficency)
Resolution is (8.4m/7mm) times higher: 0.05 arcsec?
( = 1.22/D when “diffraction-limited”)
Refracting
Reflecting
Parabolic mirrors
Making an 8.4m parabolic mirror:
Melt glass – rotate furnace – cool carefully – polish.
Do not drop.
cf. Palomar 200inch
http://medusa.as.arizona.edu/mlab/mlab.html
http://wood.phy.ulaval.ca/english/intro/what.htm
Example images – nearby galaxies
Filters used to make
separate red and blue
images
+
=
Then combine to make
colour picture
Spiral
cf. Digicam
Elliptical
http://www.astro.princeton.edu/~frei/catalog.htm
Spectroscopy
Diffraction grating:
d sin() = m 
Best to use reflection
grating:
A stellar spectrum:
No prizes for guessing
which star...
Continuum with absorption
lines – temperature and
composition
Continuum is a
5700K black body
A typical galaxy spectrum:
Absorption
and emission
lines
Positions
known from
atomic
physics
http://www.sdss.org/
Redshift:
Galaxies appear to be
receding from us:
spectral lines are
redshifted
Doppler shift is not quite
right – the wavelengths
are stretched by the
expansion of the
Universe
Redshift z
Universe scale size
R = 1/(1+z)
Ned Wright's cosmology tutorial
http://www.astro.ucla.edu/~wright/
Limits to image quality
Night sky is bright (even on Mountain tops!)
Scattered light from moon, cities
Airglow (chemiluminescence)
Faint objects are lost in noise
Atmosphere is turbulent
Twinkling of stars = blurring of images (“seeing”)
Resolution ≤ 1 arcsec at good site
Solution – get above atmosphere!
http://hubblesite.org
Deflection of light by massive bodies
Hyperbolic orbit r(t)
Does this happen?
Deflection angle:
http://www.theory.caltech.edu/people/patricia/lclens.html
http://www.mathpages.com/rr/s6-03/6-03.htm
Deflection of light by massive bodies
GR – light is deflected by, and travels slower in, a
gravitational field (latter accounts for missing 2)
Refractive index is given by
Index is greater than 1, and gravity is an attractive force:
massive bodies focus light, acting as “gravitational lenses”
Effect is greatest for rays passing close to point mass, or
through regions of high density
Index varies over field of view: a highly aberrated system!
Lens geometry
On axis source S produces ring image when c
Off axis: partial ring, or “arcs”
Magnification: image sizes increase roughly as 1/(1-c)2
Demonstrating gravitational lensing
http://vela.astro.ulg.ac.be/themes/extragal/gravlens/bibdat/engl/DE/didac.html
Numbers
c = 1 g cm-2 (Dd / 700 Mpc)-1 (1 Mpc = 3 x 1022 m)
c = 2x1025 g cm-2 (Dd / 0.5m)-1 (nuclear ~ 1015 g cm-3)
700 Mpc is a cosmological distance (z=0.35)
1 g cm-2 = 1011 Mo / (0.3 kpc)2
Galaxies make good gravitational lenses!
Gravitational lensing by galaxies
Galaxy lens lying in front of small light source
Yellow ring marks “critical curve”, cross is optical axis
Lens demo by Jim Lovell
http://www-ra.phys.utas.edu.au/~jlovell/simlens/
RXJ0911+055
1
2 lens galaxies,
1 source quasar
Lens galaxies are
different colour
4 images of quasar
Many more lens images
at http://cfa-www.harvard.edu/castles/
RXJ0911+055
1
Refractive index is
independent of
wavelength
This is an
X-ray image!
No visible lens galaxy
– we are not seeing
stars...
X-ray Astronomy
Ionising radiation, absorbed
by most things – including
the atmosphere
All X-ray telescopes
are satellites
X-ray Telescopes
Particle behaviour makes focusing tricky: absorption not
reflection
Refractive index is <1
for most materials
esp. metals
Total external
reflection
occurs at
grazing incidence
X-ray telescopes
are long!
http://www.chandra.harvard.edu
http://xmm.vilspa.esa.es/
X-ray Detectors
Band gap in silicon is a few eV
One optical photon excites one electron in the CCD pixel
No energy information
X-ray photons deposit all their energy: charge proportional to
energy. Dependent on frequent readout
X-ray images are colour!
Reflection grating spectrometers can be used too: problem is
always getting enough photons...
Cosmic telescope design
Wide field to catch chance alignments – try a few hundred
times bigger angular size: expect strong lensing in dense
central regions
Stay at cosmological distance:
c = 1 g cm-2 = 1015 Mo / (30 kpc)2
Clusters of galaxies contain typically:
100 galaxies at 1011 Mo each
3 x 1014 Mo hot (transparent) plasma
7 x 1014 Mo cold (transparent) dark matter
Clusters make good gravitational lenses!
A wide field cosmic telescope:
Abell 2218
Abell 2218:
Many muliply-imaged galaxies are visible
Mass distribution of lens can be precisely modelled
Lensing geometry is an important constraint on galaxy
redshift, as well as (faint) spectrum
Galaxy appears to have magnitude 28 – but has been
magnified 25x by the lens...
z=7 would make it the most distant galaxy known to date
(last week). Universe was 1/8 its current scale and a
very
different place...
http://xxx.arxiv.org/abs/astro-ph/0402319
st
21
Century Astronomy
Has grown out of our frustration at being stuck on Earth
combined with the usual thirst for more information
Uses large telescopes with sensitive detectors at dark sites
or in space
Involves collecting EM radiation over the whole spectrum,
measuring its intensity, colour and polarisation; particles
arrive from the sky as well
Makes extensive use of basic physics,
and some cunning and guile!