Notes…•Handouts available for today and yesterday

•Some bits might not come out too well, if you want to check against the originals I will put them all on my website:

http://www.star.bris.ac.uk/katy

•The versions on there currently are 2007 so please be careful!

Suggested Reading…

•Couple of useful texts (I may add to these):

•CMB: ‘The Cosmological background Radiation’. Lachieze-Rey and Gunzig. Available in the library.

•SZ: ‘The Sunyaev Zel’dovich Effect’. A review by Mark Birkinshaw, Physics Reports ‘99, available online (I’ll link it from my website).

•Many useful papers, websites etc are linked from the ‘4th year seminar’ section on my website.

Lecture 2•Why is SZ useful? Unique property, science we can derive from measurements

•Real examples, comparisons between experiments

•What improvements are required in order to progress with ‘traditional’ aims

•Science prospects for the future•Observational Issues

A Brief History of SZ•Postulated by Sunyaev and Zel’dovich in

1970

•Many observational attempts with little success until Birkinshaw 1978 - trustworthy measurement put the technique on the map

•First SZ image - Jones 1993. •1990s, 2000s - plethora of SZ studies. 50+ detections, unresolved images

•2000s onwards - purpose built instruments, surveys, high resolution images, samples

Revision of concept•Low energy CMB photon scatters

from high energy cluster electron

•CMB spectrum shifted to higher energy

•Observe:

-Decrement at low frequency

-Null at 220GHz

-Increment at high frequency

•Strength depends on density and temperature of the cluster gas

SZ Science Basics•SZ can be exploited alone, or in

combination with data from other wavebands

•Most astronomy relies on multi-frequency observations (i.e. optical, infrared, X-ray, radio)

•Can combine SZ with:•X-rays (discussed at length here)•Strong lensing (total mass)•Weak lensing•Velocity dispersions from optical spectra

X-ray observations•In these lectures, we will focus predominantly

on how we combine SZ with X-ray data•X-ray surface brightness is given by:

•More usual to remember that:

•So the X-ray emission has a different dependence on the cluster temperature and density

Redshift Independance•Unique property of the SZ effect

•SZ is a spectral distortion rather than a process of emission. Recall:

•No redshift dependence•For central measures, completely independent of redshift. Total flux density depends on angular size

•Extremely useful for surveys - currently detect clusters out to redshift ~1 (optical, IR, X-ray)

Strength of decrement independent of redshift

Scientific Applications•We can learn a great deal of science from

SZ measurements:

-Thermal SZ - Cosmology: Angular diameter distance, Hubble’s constant, Hubble diagrams

-Thermal SZ - Cluster properties: Cluster gas fractions, Universal Baryon fraction

-Number counts - Test cosmological models-Kinematic SZ - Cluster peculiar velocities

•We will also look at some major results found in the literature

Distance Estimates•Back to the equations for SZ

and X-ray surface brightness. Approximate the temperature and density distribution as constants:

•Equate and eliminate the density term:

Distance Estimates•Relate the size of the cluster on the

sky to the line-of-sight distance through it

-If spherical, size on sky = l-o-s distance

-Could also assume an elliptical model-In reality: fit a model to the X-ray data

•Simplest case:

Literature....•Mason et al. 2001: ‘A measurement of H

0 from the S-Z effect’. ~7 clusters,

•Reese et al 2002: ‘Determing the cosmic distance scale from interferometric measurements of the S-Z Effect’. ~20 clusters,

•Jones et al. 2005: ‘H0 from an

orientation unbiased sample of S-Z and X-ray clusters’, small sample but more sensible selection,

Hubble Diagram•Angular diameter distances determined from SZ measurements, plotted against redshift

•Lines correspond to different cosmologies

•Clearly need higher redshift data, higher precision measurements

Reese et al 2002

Distance scale - future?•Accuracy of distance estimates

sensitive to calibration uncertainties

•Best SZ calibration accuracy ~2.5%•ROSAT calibration ~10% (XMM and Chandra are better)

•SYSTEMATICS - uncertain about assumptions of isothermality, substructure, point source contaminations

Need higher resolution information - purpose built instruments

Distance scale - future?•Also limited by sample size, and

incomplete sample selection

•‘Complete sample’ - e.g. all clusters above some flux limit, regardless of size, shape, radio source population......

•.....believed to more accurately represent the Universe, i.e. less bias

SZ Surveys will produce more statistically robust samples, mass limited

Gas Properties and b •Have already seen that we can find the

gas density from SZ if we know the temperature - take this from X-ray data.

•Fit cluster-density model to 2-D SZ signal, e.g. King model:

•Empirical relation, established for globular clusters (!) but works well here

..Gas mass, gas fraction..•Integrate density distribution out to some

radius to find the gas mass:

•Can find total mass from SZ by assuming hydrostatic equilibrium, otherwise use X-rays / lensing. Then:

•Compare with findings from e.g. X-rays to test models and assumptions

Literature....•For •Grego et al 2001: ‘Galaxy cluster gas mass fractions from Sunyaev Zel’dovich measurements: Constraints on M.’

•Lancaster et al 2005: ‘Very Small Array observations of the Sunyaev Zel’dovich effect in nearby galaxy clusters.’

Cluster Evolution?

Unable to constrain this well at the moment!

Grego et al 2001

VSA ⇒

•Expect ~90% of cluster baryons to exist as ICM. Remaining ~10% in galaxies.

•Gas fraction is lower limit on Universal baryon fraction

•So, measure baryon fraction from SZ, take baryon fraction from eg BBN or primordial CMB, leads directly to an estimate of the matter density:

Constraining M

ICM properties - future?

•Again, larger SZ samples will enable better determination of parameters for individual clusters

•High resolution observations will allow us to fit sophisticated models to the cluster gas - substructure

•SZ imaging needs to progress in order to keep up with developments in X-rays

Virgo - Rosat

Virgo - Chandra

Peculiar Velocities•Can only be derived from the kinematic SZ

effect

•Observe at the thermal null, or use multi-frequency data

•Spectrally the same as primordial CMB - difficult to measure peculiar velocity for individual objects.

•Samples more promising - uncertainties average out

•Measure velocity fields at high redshift by finding peculiar velocities for many clusters

Literature...

SuZIE

Diabolo + SuZIE

BIMA

Thermal + Kinematic SZ for Abell 2163

Best-fit Thermal Best-fit Kinematic Best-fit Combined

Literature...

SuZIE

Diabolo + SuZIE

BIMA

Thermal + Kinematic SZ for Abell 2163

Why are peculiar velocities useful? •Measure for a number of clusters in

a particular redshift ‘bin’ and minimise errors

•Repeat for a range of redshift bins•Can derive something about the formation of large scale structure - i.e. how quickly things are moving around at different redshifts

•Clusters move under gravity - learn about distribution of matter at different epochs

Surveys: New Science•SZ selected samples will allow us to

improve on ‘traditional’ SZ applications (Hubble const. etc)

•New frontier - cluster number density and its evolution with time

•The potential of this application will be realised with the release of cluster catalogues from SZ surveys

•One aim is simply to record how many clusters are found in e.g. different redshift bins

•Examine cluster evolution (e.g. mass functions) and the geometry of the Universe

Cluster Abundance

Distinguish between cosmological models

Carlstrom et al 2002

SZ-selected samples

•Previous SZ samples are often chosen somewhat arbitrarily - i.e. clusters picked because they are easy to observe

•Some attempts to select representative samples from X-ray catalogues (e.g. Jones et al 2005, Lancaster et al 2005)

•Still subject to ‘selection effects’ (i.e. X-rays point preferentially to dense clusters)

•SZ catalogues will be mass-limited only

SZ-selected samples•X-ray catalogues are limited in

numbers due to rapid fall off of detectable flux with distance

•SZ catalogues do not suffer from this limitation - will yield large numbers of new clusters, enabling studies of large scale structure via methods currently applied to galaxy catalogues e.g. 2DF

•Will also provide the first picture of the high-redshift Universe

SZ Science to date•Distance estimates to

reasonable precision

-Good agreement between different experiments

•ICM properties e.g. gas fractions

-Large errors but consistent between experiements

Future Science Prospects

•Detailed images - physics of clusters as individuals, and Universal population

•Large samples - more statistically robust estimates of cosmological parameters

•Blind surveys - direct view of the growth of large-scale structure over entire redshift range

SZ Practicalities•SZ is a tiny signal - requires

sophisticated observing techniques

•Various sources of contamination and confusion, which observing techniques deal with in different ways

-Radio sources (galaxies, planets)-Atmospheric emission, ground emission-Primordial CMB fluctuations•Today, a few details. We will discuss the various observing techniques and how they cope with these issues tomorrow

Radio Sources•If a radio source is present in the field of a galaxy cluster, it will ‘fill in’ the SZ decrement

•This could be true for sources in front of / behind the cluster, or indeed member galaxies

•Problem greater at low frequency: most sources are ‘steep spectrum’

•Can choose to observe clusters with no sources - introduce bias

•Better to ‘subtract’ effects•No high-freq. radio surveys - further complication

Atmosphere, Ground•Atmosphere is ‘warm’ - radiates.

•Time variable emission•Ground also a source of thermal emission

•Varies with pointing angle or telescope

•Can minimise this using a ‘ground shield’

Various ways exist of dealing with these contaminant signals

Primordial CMB•Primordial anisotropies look remarkably similar to the SZ effect on large angular scales (tens of arcminutes)

•Seem unsurprising that telescopes such as the VSA and CBI (built to observe the primordial CMB) suffer drastically from this type of contamination....

•.....We were still surprised!

Last Lecture.....

•More on the practicalities of observing the SZ effect - telescopes and observing techniques

•Recent results and future prospects