Sunyaev-Zel dovich effect observations with the SVT

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1 Sunyaev-Zel dovich effect observations with the SVT M. E. Jones, A. C. Taylor 1 S. Kay, R. Battye, M. Peel 2 A. Scaife, K. Grainge 3 1. Sub-dept of Astrophysics, University of Oxford 2. Jodrell Bank Observatory, University of Manchester 3. Cavendish Laboratory, University of Cambridge 1 Science background The Sunayaev-Zeldovich (SZ) effect is the inverse Compton scattering of cosmic microwave background (CMB) photons as they pass through the hot electron gas in intracluster media. For a rich cluster the optical depth of scattering is around 1%, and is usually characterized in terms of a Compton y-distortion of the CMB spectrum, which at low frequencies << 217GHz appears as a temperature decrement of around 1µK for a cluster of M M. The effect has been detected in known clusters at a range of redshifts and angular resolutions. At the present time the only uniformly-selected samples that exist have used X-ray observations as their primary source. In this proposal we will discuss the prospects for high resolution SZ measurements using the various stages of the SVT. We will concentrate on performing follow up of SZ-detected samples likely to be available on the relevant timescales, as well as considering how the later stages of the SVT could be used to perform a survey. Although the SZ effect is not currently one of the primary science goals of the full SKA, given the specifications of the SVT and the relevant timescales it seems sensible to consider it as an important science goal for the SKA community. The brightness temperature of the SZ effect is proportional to total integrated gas pressure along the line-ofsight, which is directly related to the cluster mass, and is independent of redshift. For this reason it has long been realised that large, homogeneous, high-redshift cluster SZ surveys could be performed which would have a very flat mass cut-off as a function of redshift, and a number of instruments have been or are being built to do this. The first generation (AMI [Kneissl et al 21] and SZA [Muchovej et al 27]), which are already operational, are 1 2 element interferometers working at low frequencies (15-3GHz), while the second generation, some which have already seen first light, will work at higher frequencies ( 15GHz) with arcminute resolution and use large, dedicated single dishes operating at either high altitude sites (eg ACT, [Kosowsky 26]) or the South Pole (SPT) [Ruhl et al 24]. It is likely that the first-generation instruments will generate samples containing 1 clusters and the second generation 1 1, many of which will be at redshifts in excess of 1. For example, the planned SPT survey will cover 4 sq degrees with a mass limit around M. As well as samples from these purpose-built facilities, a shallow (M > 1 15 M ) all-sky survey, finding up to 1 clusters, will be made using the PLANCK satellite due its substantial frequency coverage. The basic premise of this proposal is thus that there will be large ( 2) samples of clusters available for follow-up observations by the SVT, with probably 1 having redshifts greater than 1. This seems perfectly plausible if the next generation of instrument perform close their specifications. As well as the SZ-selected surveys, on the timescale of the SVT there will also be much improved X-ray samples. One of the cluster samples we will use for follow-up SVT observations is the XMM Cluster Survey (XCS). The XCS is an ongoing serendipitous X-ray survey, designed to find all clusters within the XMM archive. By 21, the expected number of clusters will be more than a thousand, covering an area of around 5 square degrees scattered across the sky, and going to a mass limit significantly lower than the proposed SZ surveys. It is also expected that around 1 per cent of those clusters will lie at z > 1, making it the most extensive X-ray selected cluster survey to date. There is also a substantial redshift follow-up programme, notably the NOAO-XCS, which will determine photometric redshifts for clusters in over 3 XMM pointings to 5 per cent accuracy. Having said that there will be large samples of clusters available, most of these objects will be close to the detection thresholds of the instruments and only barely resolved. Typically PLANCK clusters will have angular sizes 1arcmin and have redshifts <.5 whereas SPT/ACT/APEX-SZ clusters will be 2arcmin and be at 1

$Figure 1: Map of fractional pressure fluctuations in the Perseus cluster, inferred from a 1 Ms Chandra observation [Fabian et al 26].$

2 Figure 1: Map of fractional pressure fluctuations in the Perseus cluster, inferred from a 1 Ms Chandra observation [Fabian et al 26]. These kind of pressure fluctuations, due to interaction between the central AGN and the cluster gas, could potentially be observed in many clusters with a sufficiently sensitive and high-resolution SZ instrument such as the SVT. much higher redshifts. The science goals of the worldwide SZ program are two fold: (i) to set constraints on cosmological parameters such as σ 8, Ω m and the properties dark energy; and (ii) find a better understanding of the intracluster medium and relevant structure formation. It is essential that high-resolution follow-up observations are performed to help understand the detailed features of the gas within the cluster in order to refine the selection criteria for cosmological applications (which depend sensitively on the profile of the gas) and to explore the complicated dynamics of shocks, cold fronts and other exotic phenomena within the gas. Current X-ray observations are beginning to reveal complex phenomena in cluster atmospheres associated with merger events and AGN, which would demand impossible amounts of X-ray observatory time to investigate fully. For example, Figure 1 shows a pressure map inferred from a 1 6 second exposure with Chandra of the Perseus cluster[fabian et al 26], showing complex features interpreted as pressure waves emanating from the central AGN plus bubbles blown in the plasma by the radio source. SZ studies will be highly complementary to observations such as this as they are unaffected by isobaric clumping which dominates the contrast in the X-ray image, imaging the pressure directly. A further advantage of radio imaging of clusters is that, if a large frequency range is available with good surface-brightness sensitivity, it will be possible to simultaneously image the synchrotron plasma of cluster radio sources along with the cluster gas, and to study the interaction between the two. 2 SVT properties Since the SVT technical description is not fully specified, we have pushed the parameters somewhat in the direction of optimizing its performance for SZ observations. Since the cluster signal is strongest on baselines of a few hundred wavelengths (corresponding to a few arcminutes angular scale), we have designed array configurations which maximize the number of short baselines, while still providing the specified full resolution. We have also assumed that the highest frequencies will be available from the outset, and that the available bandwidth will be somewhat more than the.25 GHz quoted, which is more characteristic of previous generation, rather than next generation, instruments. By pushing the specification envelope in this way we hope to encourage the development of actual SKA precursors by showing what science is possible with an instrument which is certainly within the realms of technical possibility. We thus assume observations made at a centre frequency of 17 GHz, with a bandwidth of 4 GHz. As in the SVT specifications, the antennas are assumed to be 6-m dishes, with an aperture efficiency of.6, and system temperatures of 5K in SVT 1 and 3K in SVT2 and SVT3. The centre frequency is chosen assuming that the maximum frequency available is 2 GHz. Higher frequencies may be useful if available. 2

3 Figure 2: Array configurations assumed for the different SVT configurations. Top left: SVT1; top centre: SVT2 core; Top right SVT2 full; bottom left: SVT3 core; bottom right: SVT3 central 5km, where each point in the arms is a station of twenty antennas. 2.1 Array configurations The array configuration requirements for SZ observations are very particular. There must be enough baselines short enough to detect a significant SZ signal, but also enough long baselines to detect the point sources, both those in the cluster and those projected on to it, that would otherwise obscure the SZ signal. One of the great advantages of the SVT over existing SZ instruments is that the very large number of antennas permits a large range of angular scales to be sampled, with very good temperature and flux sensitivity. As well as allowing radio sources to be distinguished from the SZ effect, this means that the sources can be imaged, and in particular the sources that are in the cluster and interacting with the cluster gas can be examined in detail. The configurations proposed have the right general characteristics, but are by no means fully optimized yet. For SVT1 (12 antennas total) we propose a core of 81 antennas arranged in a dense spiral, with roughly equal nearest-neighbour spacings, plus three arms totaling 39 antennas in a logarithmic spiral. The core diameter is around 9 m, with the arms giving a maximum baseline of 4 m (see Figure 2). All baselines are correlated. For SVT2 (5 antennas) the core is increased up to 35 antennas, now 2m in diameter, with 15 antennas in spiral arms extending up to 5km baselines, and again all baselines are correlated. In SVT3 (2 antennas), we have assumed the core is increased to 5 antennas, and the arms are formed in clusters of 2 antennas which are phased up in stations. Each arm station is treated as a single antenna in the correlator. There are 39 arm stations within 5 km, totalling 78 antennas; the remaining 72 antennas are also formed in to arms stretching out to 5 km baselines, but we do not consider them in our simulations, as this degree of resolution is not necessary for SZ observations. 3 Simulated cluster observations with the SVTs In order to demonstrate the capabilities of the SVTs to observe clusters, we have simulated observations of clusters in each configuration. 3

4 3.1 Cluster Simulations We wish to simulate SZ observations of galaxy clusters contaminated by point sources of synchrotron emission (we do not consider the fact that some sources may be extended at this stage). The first stage was to construct maps of the SZ effect that can then be used as test data for the mock SVT observations. Our first set of simulations is that of a single galaxy cluster, originally drawn from a ΛCDM cosmological N-body simulation performed by the Virgo Consortium. We re-simulated the cluster with non-radiative gas physics (the simplest assumption), using the gadget N-body/SPH code. By z = the cluster has grown to have a virial mass of M vir = 1 15 h 1 M, simulated with approximately 2 million particles each of gas and dark matter. The virial radius of the cluster at z = is R vir = 2h 1 Mpc and the force resolution was set to 5h 1 kpc, giving a spatial dynamic range of 4. We consider two simulation outputs, z = and z =.5. In the former case, the cluster is fairly relaxed and there is no significant substructure. In the latter case, some substructure exists as the cluster is dynamically active. Compton-y maps of the two outputs were then constructed with 8 8 pixels, corresponding to a width of 2R vir (thus the length of each pixel was set to the force resolution). All ionised (T > 1 5 K) gas particles were identified within 3R vir from the cluster s centre along the line of sight and smoothed in projection onto the map using the projected version of the gadget SPH kernel. The estimate of y at projected position, R, is thus y(r) =.88k Bσ T m gas 2 m e c 2 m p Σ Ngas i=1 T iw( R i R, h i ), (1) where is the pixel size and m gas is the mass of each gas particle. The sum in the equation runs over all ionised gas particles with position, R i, temperature, T i and SPH smoothing length, h i. The projected kernel, w, was suitably normalised to conserve the quantity (in this case temperature) that was being smoothed over the pixels. We also computed mass surface density maps, where a similar procedure was performed as before, but for all (gas and dark matter) particles Σ(R) = 1 2 ΣN i=1 m i w( R i R, h i ), (2) where m i is the mass of each particle. These maps were used for assigning positions of point sources (see below). The y(r) and Σ(R) maps were then converted to T(θ, φ) and Σ(θ, φ) maps before adding point sources. For this conversion, we placed the z = output at redshifts, z =.2,.5, 1 (thus progressively decreasing the angular size of the cluster). For the z =.5 output, we did not change its redshift. We also constructed a mock sky map in order to simulate a blind survey, in which clusters masses and positions were taken from the Hubble Volume simulation (the LCDM Deep Wedge, Beta-model cluster images were placed at each cluster location, covering a mass range from to M. The next stage was to add the synchrotron point sources. For this, we used the mean field source counts, dn/ds, from the Toffolatti model at 3GHz. The total number of sources in each map is thus Smax N tot = β Ω S min dn ds, (3) ds where Ω is the solid angle subtended by the maps and S min = mjy and S max = 1 5 mjy respectively. We also include an enhancement factor, β, which can be used to account for any enhancement in the number of point sources towards a cluster. A list of sources was then created, with the fluxes of the sources calculated by randomly sampling from a probability distribution defined by a normalized dn/ds between the flux limits. The fluxes are scaled to 17GHz by S ν α, where the spectral index, α, is gaussianly distributed about a mean of α =.3, with σ α =.36. The sources are spatially distributed randomly with an enhancement factor dependant on some power of the surface density, P Σ γ, where we have used γ = 1/3 and an enhancement factor, β = 3. This method biases sources towards the centre of the cluster and in dark matter substructures, where the galaxies are expected to reside. This is in accordance with studies of actual source counts within clusters [Cooray et al 1998]. This recipe is somewhat simplistic in several ways. Firstly, the source counts are continued down to a very low flux level (3 njy), when in fact the counts must turn over at some low flux in order not to exceed either 4

5 5e :1 45:5 45:5 44:55 44:55 44:5 44:5 h1ms hm3s hms 23h59m3s h1ms hm3s hms 23h59m3s (y) e 5 45:8 45:1 45:4 45:5 44:56 44:55 44:52 44:5 hm4s hm2s hms 23h59m4s 23h59m2s h1ms hm3s hms 23h59m3s 23h59ms.1 5e 5 3e 5 2e 5 1e 5 1e 5 Figure 3: Simulated observations of a merging cluster at redshift z =.5. Top left: the input cluster map; top centre: simulated observation with the AMI small array (ten 4-m antennas at 15 GHz), representative of current technology (resolution 2 arcmin); top right: observation with SVT1 (smoothed to 45 arcsec resolution); bottom left: SVT2 (smoothed to 3 arcsec); bottom right: SVT3 (1 arcsec resolution). In each case the observation time is 12h, and point sources have been added below five times the thermal noise limit to simulate unsubtracted confusion noise. In this last image it is difficult to display the full spatial dynamic range adequately; the low spatial frequency information evident in the SVT2 images is of course also still present. the known galaxy counts or the limits on the CMB spectrum. In fact the simulation results are insensitive to a much higher truncation levels, around 1 µjy, with fainter sources contributing an rms level around 1 µjy. Secondly, the enhancement in the direction of clusters is applied at all flux levels, such that the number of enhanced sources can actually exceed the number of galaxies in the cluster. Neverthless, source confusion is modelled in a sufficiently realistic way to determine if it is a limiting factor in the observations. 4 Simulated observations The clusters observations are simulated through a mock interferometer pipeline, producing visibilities which are mapped and cleaned in the usual way. Point sources are added in the map plane, and to simulate point source removal, only sources less than the 5-σ flux sensitivity of the observation are included if the instrument meets its contimuum imaging dynamic range specification, residuals from brighter sources will be negligible. We have simulated both pointed observations of a merging cluster with all three SVT configurations (Figure 3), and a small deep survey with SVT2 and SVT3 (Figure 4). 5 Proposed observing programme Based on the simulation results, we can propose observing programmes with the SVT configurations that would produce unique science on the proposed timescales. 5

6 23h58m 45:15 45:15 44:45 44:45 h2m h1m hm 23h59m h2m h1m hm 23h59m 23h58m 2e 5 1e 5 1e 5 2e 5 4e 5 6e 5 8e 5.1 (y) 5e 6 5e 6 Figure 4: Simulated observations of a small-area deep survey with SVT2 and 3. The survey area is about half a square degree, representing about twelve pointings. The integration time per pointing is 12 hours. Centre: input cluster map. The lowest mass clusters are around M, at a redshift greater than 4. Left: survey with SVT2; right: survey with SVT3. In both cases point sources have been added at fluxes below the five times the thermal noise, down to 5 njy. Note that, although the point source sensitivity and resolution are better in the SVT3 case, in both surveys essentially all the input clusters are recovered, with the exception of the diffuse low-redshift cluster at the lower left, which is resolved out. With SVT1 we would propose to observe a sample of around 1 2 clusters, selected from SZ and X-ray surveys, mostly at lower redshift (z < 1) where there is good X-ray imaging data. These observations would be used to verify the modelling and scalings used in the larger SZ surveys (eg from Planck). With an average integration time around 6 hours, this would require 5 1 nights. With the higher sensitivity and resolution of SVT2, we would be able to turn to the higher redshift clusters and make detailed images of clusters out to redshifts beyond 1 with resolution of 3 arcsec and better. The SVT2 observation in Figure 3 represents a 5-σ detection of a 1 15 M cluster, which (assuming a Y M 5/3 scaling) implies a 5-σ detection limit of M. 1 nights of observation would allow detailed imaging of several hundred bright clusters at 3 arcsec resolution, plus detections of lower mass clusters selected from deep X-ray surveys below the limit of SZ surveys. We could also begin a deep blind survey, although SVT3 is even better suited to this. As seen in Figure 4, an SVT3 deep survey cluster survey at 12h per pointing would reach a cluster mass limit of a few times 1 13 M, with sufficient flux sensitivity and resolution to be thermal noise rather than confusion limited. At this level we expect of order 12 clusters per square degree (heavily cosmology-dependent, of course). A 2-day survey would thus cover some 16 square degrees detecting 2 clusters down to a mass limit ten times lower than the SPT survey, opening up a complementary range of parameter space for detailed follow-up, and allowing for the first time an accurate census of the low-mass cluster population at z >> 1. In addition, relatively short exposures (hours) on bright clusters will reveal imaging detail down to 1 arcsec, allowing detailed comparison of substructures in SZ and X-ray imaging, modelling non-equillibrium structures such as pressure waves, cold fronts, shocks and radio bubbles. References [Fabian et al 26] Fabian, A.C. et al, MNS, 366, 417, 26 [Muchovej et al 27] Muchovej, S. et al, Ap J, 663, 78, 27 [Kneissl et al 21] Kneissl, R. et al, MNS, 328, 783, 21 [Ruhl et al 24] Ruhl, J. E. et al, SPIE, 5498, 11, 24 [Kosowsky 26] Kosowsky, A., New Astron. Rev, 5, 969, 26 [Cooray et al 1998] Cooray A. R., et al, AJ, 115, 1388,

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