Astronomy with cm – Mpc lenses Phil Marshall KIPAC – SLAC – Stanford University February 28 th 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 log 10 (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) servatories/palomar/

Comparison with Palomar digitized sky survey 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

Example images – nearby galaxies cf. Digicam + = 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: Absorption and emission lines Positions known from atomic physics

Redshift: Ned Wright's cosmology tutorial 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)

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!

Does this happen? Hyperbolic orbit r(t) Deflection angle: Deflection of light by massive bodies

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

Numbers  c = 1 g cm -2 (D d / 700 Mpc) -1 (1 Mpc = 3 x m)  c = 2x10 25 g cm -2 (D d / 0.5m) -1 (nuclear ~ g cm -3 ) 700 Mpc is a cosmological distance (z=0.35) 1 g cm -2 = M o / (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

RXJ Many more lens images at 2 lens galaxies, 1 source quasar Lens galaxies are different colour 4 images of quasar

RXJ 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!

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 = M o / (30 kpc) 2 Clusters of galaxies contain typically: 100 galaxies at M o each 3 x M o hot (transparent) plasma 7 x M o 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...

21 st 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!