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 6In 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 secsAperture diameter = 30mm ⇒ see to magnitude 10.4?

Wide field! 48x36 degrees...

Zoom in (after the exposure!):

The pleiades star cluster:

Resolutionlimited by camera optics, ~3arcmin

Human eye does 3 times better!

Comparison with Palomar digitized sky survey (1949)

http://www.astro.caltech.edu/observatories/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

cf. Digicam

http://www.astro.princeton.edu/~frei/catalog.htm

+ =

Filters used to make separate red and blue images

Then combine to make colour picture

Spiral

Elliptical

Spectroscopy

Diffraction grating: d sin() = m

Best to use reflection grating:

A stellar spectrum:

Continuum with absorption lines – temperature and composition

No prizes for guessing which star...

Continuum is a 5700K black body

A typical galaxy spectrum:

http://www.sdss.org/

Absorption and emissionlines

Positions known fromatomic physics

Redshift:

Ned Wright's cosmology tutorial http://www.astro.ucla.edu/~wright/

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 zUniverse scale size R = 1/(1+z)

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

Does this happen?

Hyperbolic orbit r(t)

Deflection angle:

Deflection of light by massive bodies

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 Lovellhttp://www-ra.phys.utas.edu.au/~jlovell/simlens/

RXJ0911+0551

Many more lens images at http://cfa-www.harvard.edu/castles/

2 lens galaxies, 1 source quasar

Lens galaxies are different colour

4 images of quasar

RXJ0911+0551Refractive 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 <1for most materials esp. metals

Total external reflection occurs at grazing incidence

X-ray telescopes are long!

http://www.chandra.harvard.eduhttp://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 designWide 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

21st Century Astronomy

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

Has grown out of our frustration at being stuck on Earth combined with the usual thirst for more information

Makes extensive use of basic physics, and some cunning and guile!