MAPPING CLUSTERS OF GALAXIES WITH A STRATOSPHERIC BALLOON EXPERIMENT

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1 MAPPING CLUSTERS OF GALAXIES WITH A STRATOSPHERIC BALLOON EXPERIMENT S. Masi¹, D. Brienza¹, L. Conversi¹, P. de Bernardis¹, M. De Petris¹, P. Fiadino¹, A. Iacoangeli¹, L. Lamagna¹, C. Marini Bettolo¹, L. Moncelsi¹, L. Nati¹, F. Nati¹, F. Piacentini¹, G. Polenta¹, R. Rispoli¹, P.A.R. Ade², P. Hargrave², P. Mauskopf², G. Pisano², G. Savini², C. Tucker², A. Boscaleri³, S. Peterzen 4,1, D. Spoto 4, D. Carpinteri 4, S. Colafrancesco 5, Y. Rephaeli 6, G. Di Stefano 7, G. Romeo 7, A. Delbart 8, L. Doumoulin 8, P. Camus 8, J.B. Juin 8, C. Magneville 8, J.P. Pansart 8, D. Yvon 8, V. Gromov 9, I. Maslov 9 1. Dipartimento di Fisica, Universita La Sapienza, P.le A. Moro 2, Roma, Italy, silvia.masi@roma1.infn.it 2. Department of Physics and Astronomy, University of Cardiff, The Parade, Cardiff, UK 3. IFAC-CNR, Via Panciatichi 64, Firenze, Italy 4. ISTAR and Agenzia Spaziale Italiana, Rome, Italy 5. INAF, Osservatorio di Monte Porzio, Rome, Italy 6. Tel Aviv University, Israel 7. Istituto Nazionale di Geofisica, Rome, Italy 8. DAPNIA/SPP CEA-Saclay, Gif sur Yvette, France 9. IKI, Moscow, Russia ABSTRACT We describe the scientific rationale and the preliminary calibrations of the OLIMPO experiment: a 2.6 m stratospheric balloon borne telescope, to be flown in 2008 in a long duration circumpolar flight. One of the main goals of the experiment is to map with high sensitivity and resolution at mm and submm wavelengths the Sunyaev-Zel dovich effect in many clusters of galaxies, both in the Northern and in the Southern hemisphere. Key words: long duration stratospheric balloons, mm/sub-mm astronomy, cosmology, clusters of galaxies 1. INTRODUCTION Our knowledge of the distribution of visible matter in the universe is nowadays very good, thanks to the 3D galaxy surveys like 2DF and SDSS. Galaxy filaments form a sort of cosmic web with clusters and voids. From X-rays images of the clusters (Chandra etc ) we have evidence that the potential wells of clusters of galaxies are full of hot (around 10keV), ionized and diluted gas, which is bright in the X-rays. The photons of the Cosmic Microwave Background can interact with the hot gas, receiving a small boost in energy from the electrons in the gas (by inverse Compton): this is the so called Sunyaev-Zeldovich (S-Z) effect [see e.g. 1,2]. A first order calculation of the Inverse Compton Effect for CMB photons against charged particles in the hot gas of clusters can be done as follows: The cluster optical depth is τ=nσl where l = a few Mpc = cm ; n < 10-3 cm -3 and σ = 6.65x10-25 cm 2. So τ = nσl is of the order or less than 0.01: there is a 1% likelihood that a CMB photon crossing the cluster is scattered by an electron of the hot gas. Since E e >> E γ, the electron transfers part of his energy to the photon. To first order, the energy gain of the photon is ν kt = ν m c 5keV 500keV e = 2 e 0.01 The resulting CMB temperature anisotropy is T ν 4 τ = 10 T ν This is not a small signal: maps of the CMB with sensitivity of 10-5 of the background per pixel are now routinely obtained by CMB anisotropy experiments. In Figure 1 we plot the spectral behaviour of the effect: since all photons get a positive boost in energy and the number of photons is conserved, there is a shift of the spectrum of the CMB anisotropy in the direction of the cluster, which means a decrement of the brightness at frequencies below 217 GHz, where the CMB anisotropy spectrum is increasing, and an increment at frequencies above 217 GHz. This spectrum is very peculiar and can be measured by comparing the signal from the cluster to the signal from a reference region outside the cluster.

2 I (mjy/sr) 6.0x x x x x10-4 7keV 10keV 15keV 20keV ν (GHz) Figure 1. Spectrum of the S-Z effect for different energy of the ionised intracluster gas.the vertical bands refer to the frequency bands of the OLIMPO experiment. The S-Z effect is one of the three main sources of anisotropy in the microwave sky at high galactic latitudes and millimetre wavelengths. The primary anisotropy of the CMB, and the anisotropy of the Extragalactic Far Infrared Background (FIRB) are the other main contributors. The "cosmological window" where these components are dominant extends roughly from 90 to 600 GHz : at lower frequencies interstellar emission of spinning dust grains, free-free and synchrotron dominate over the cosmological background; at higher frequencies the clumpy foreground from cirrus dust dominates the sky brightness even at high Galactic latitudes. It is clear that the only way to separate these different emissions and to extract cosmological information is to use multi-band experiments. OLIMPO, a 2.6 m stratospheric balloon borne telescope, will carry out its survey in four frequency bands centered at 140, 220, 410 and 540 GHz, in order to be optimally sensitive to the S-Z effect and to efficiently reject competing sources of emission. 2. WHY DO WE WANT TO MEASURE THIS EFFECT? The intensity of the S-Z effect is proportional to the density of the intergalactic electrons (n), while the X- ray brightness of the same cluster is proportional to n 2. So the S-Z measurements are sensitive to the intracluster gas in the peripheral regions of the cluster, while X-ray measurements are not. Moreover, combining measurements of the two quantities it is possible to derive the angular diameter distance of the clusters of galaxies [3,4,5]. Observations of many clusters would allow to build an Hubble diagram, and from this to measure the Hubble constant. These measurements are being carried out [6], but the error in the determination of H o is still quite large. To improve these measurements we need to collect a larger sample of clusters (and the forthcoming South Pole Telescope will do a wonderful job in this respect [7]), and also to improve our knowledge of the details of the S-Z effect (cooling flows, inhomogeneities, relativistic corrections and so on): a survey of nearby clusters with excellent interchannel calibration and wide frequency coverage, will be instrumental in this. The S-Z effect depends on the optical depth, but it does not depend on the distance of the clusters (it is like an opacity effect). So we can see clusters that are too faint to be visible in the optical or in the X-rays bands. The number of clusters seen at different distances is a strong function of the Dark Energy density: clusters can in principle be used as probes of the history of the Dark Energy [8]. Observing selected clusters where dark matter is separated from baryons it is possible to study the SZ effect generated by annihilation products of the Dark Matter, thus testing the nature of Dark Matter [9]. 3. WHY FROM A BALLOON? Even in very highest altitude and lowest temperature sites on the earth, like the South Pole, this measurement can be performed only from windows clean from atmospheric emission and fluctuations. This means wavelengths longer than 1 mm. In fact, a number of ground based telescopes is performing or will perform deep, high resolution surveys at 90, 150, 220 GHz (SPT, ACT, VSA, AMI, MITO ). In the sub-millimeter region, where the S-Z effect is an increment, even in high mountain dry sites it is very hard to perform accurate measurements. At these shorter wavelengths the atmosphere is highly opaque and noisy, and going to space is the only option. This can be done either by stratospheric balloons or by satellites. Planck will perform a shallow, full-sky survey of clusters, limited by angular resolution and integration time. A balloon-borne experiment can use a larger telescope, improving the angular resolution, and concentrate its integration time on a selected set of key clusters (about 100) to measure their positive S-Z in detail. It can also perform a blind, deep survey of a small region (about 100 square degrees) looking for

3 unknown distant clusters, not detected in the optical and X bands. For the determination of non standard S-Z, like the one produced by the annihilation products of dark matter clouds, it is extremely important the detection of the effect at submm wavelengths: being these annihilation products relativistic, the S-Z effect will be negative even at sub-mm wavelengths: a very distinctive signature of supersymmetric dark matter, if detected. detail the OLIMPO measurement of a sky patch around a rich cluster of galaxies. The result is shown in figure 3. Detailed simulations show that for a Y=10-5 cluster, in a dust optical depth of 1 mm, in presence of a 100 µk CMB anisotropy in 2 hours of integration over 1 square degree of sky centered on the cluster Y can be determined to +10-6, T CMB can be measured to +10µK, and T e can be measured to +3keV. 4. OLIMPO OLIMPO is the combination of A large (2.6m diameter) mm/submm telescope with pointing and scanning capabilities Four multi-frequency arrays of bolometers A long duration He3 cryostat A long duration balloon flight. The subsystems of OLIMPO have been described in detail in [10]. The instrument will produce a set of high resolution (few arcminutes) sensitive (few µk per resolution element) maps of the mm/submm sky, with optimal frequency coverage (150, 220, 340, 540 GHz) for SZ detection, for the determination of the parameters of the clusters with optimal control of foreground and background contamination [11]. In fig. 2 we compare the arrays of OLIMPO, projected on the sky, to realistic simulations of the three main components of the mm/submm sky at high galactic latitudes: CMB anisotropy, SZ effect, and emission from unresolved galaxies. Figure 3. Simulated observation of a SZ cluster at 2 mm with the Olimpo array. The large scale signals are CMB anisotropy. The cluster is the dark spot evident in the middle of the figure. Parameters of this observation: scans at 1o/s, amplitude of the scans 3op-p, detector noise 150 µk s 1/2, 1/f knee = 0.1 Hz, total observing time = 4 hours, comptonization parameter for the cluster y=10-4. OLIMPO will also detect the tail of the CMB anisotropy power spectrum with high accuracy. This is very sensitive to the average density of matter in the Universe. CMB anisotropy SZ clusters Galaxies 150 GHz 220 GHz 340 GHz 540 GHz Additional targets will be compact, insulated HII regions for calibration, and cirrus clouds at high Galactic latitudes. 30 mm-wave sky vs OLIMPO arrays Figure 2.Comparing the arrays of OLIMPO to realistic simulations of the mm sky. In fig.1 it is evident that the frequency bands of the 4 arrays are optimally located to sample the negative, the null, and the positive SZ effects. We have simulated in 5. COMPONENTS SEPARATION SIMULATIONS As previously anticipated, the main components that contribute in the four OLIMPO frequency bands are the CMB, the S-Z effect, the galactic dust emission and the FIRB. Intensive simulations were carried out in order to predict the maps in the four bands and to see if OLIMPO is able to separate the signals, reconstruct the input maps and disentangle between different theoretical models. CMB and SZ behaviours are well known, hence

4 producing maps with these signals at different frequencies is quite simple. Galactic dust emission and the far infrared background are not well known yet, in particular at mm and sub-mm wavelengths. Thus, theoretical models for these contributions are still uncertain. We adopted the most popular ones, i.e. model #8 by Schlegel, Finkbeiner and Davis [12] for the galactic dust emission, and that of Lagache, Dole and Puget [13] for the FIRB. Once the maps of each component are created for each frequency channel, they are summed to obtain the four total maps at 143, 217, 353 and 545 GHz, also taking into account the expected noise at the different frequencies (see Fig. 4). It is clear how the signal due to S-Z effect is subdominant in each band; CMB anisotropies dominate the two lower frequency channels, while galactic dust is the main component at 353 and 545 GHz. The far infrared background contribution is comparable to the galactic dust one, but it results more difficult to distinguish, since it lacks of structures at scales larger than beam size. In order to separate the various components, we compute a χ2 and minimize it: this method requires the assumption that all the components, including noise, are uncorrelated. The χ2 is computed varying on each pixel of the four maps the parameters all over the 4- dimensional space of the components, with the proper scaling in frequency of the three non-thermal components, and finally summing over the frequency channels. Then, for each pixel and each component, the signal is extracted computing the normalized product of the χ2 likelihood function and the parameter relative to the selected component. The results of such extraction are shown in Fig. 5. CMB anisotropies are optimally extracted, as well as the S-Z contribution. Instead, galactic dust and the FIRB are not easily separated, their spectra being too similar. A possible solution is to remove the scales larger than ~1, which are related to the dominant dust component. Since the model generating the FIRB emission is the most uncertain among the four considered, we decided to slightly change some of the assumptions of the model used, in order to get different source counts and luminosities and verify if OLIMPO is able distinguish between different models. We found that variations in the FIRB signal are always larger than the confusion limit and OLIMPO intrinsic noise: hence, in principle it is possible to disentangle between different models. The main issue remains the separation of the FIRB and galactic dust components. We finally carried out the analysis on the counts of clusters which produce Sunyaev-Zel dovich effect. We used the SExtractor program to select the clusters present in the original map and in the extracted one. The complete analysis of an image is performed in two steps: first, a model of sky background is built and the variance of the local background σ is computed, then such estimated background is subtracted, the map is filtered and thresholded. Figure 4. Total simulated maps at the four frequency bands of OLIMPO. Signals are expressed in K, the integration time is 100 h on a region of 10 x10.

5 to 2.5σ, the sources detected become 423 and 152, respectively. Figure 5. Resulting maps of the four components after the extraction, at 143GHz. Signals are expressed in K. Detections are then deblended, pruned, photometered, classified and finally written to the output catalog. With a 3σ threshold, 268 clusters are detected in the input map, and 89 in the extracted one; lowering the threshold 6. ANGULAR RESPONSE MEASUREMENTS We recently made intensive calibration measurements to reconstruct the beam profile of the OLIMPO telescope. To achieve this we needed to control its pointing with sub-arcmin accuracy. This accuracy level was achieved in both the scan directions of the telescope by means of two absolute 16-bit rotary encoders. These have 360 / angular resolution. The whole gondola supporting the telescope was mounted on spherical bearings and was rotated by a linear actuator, thus providing azimuthal scans. The elevation angle was controlled also by a linear actuator, allowing to tilt the whole inner frame with respect to the outer part of the gondola. A coherent modulated microwave source was located in the far field of the telescope: for such a 2.6 m diameter telescope, with working wavelengths of 2-3 mm, the far field is located at distances greater than about 2D 2 / λ ~6.7 km. OLIMPO is currently hosted by the CNR research area in Tor Vergata : from this place it's possible to point the telescope at an elevation up to about 20 and see the hills around Rome, where Villa Mondragone is located, about 7 km away from OLIMPO. The source was placed on a balcony of Villa Mondragone, with clear and direct sight of the experiment down in the CNR area (in 1610 the windows of the same villa were observed from the Gianicolo by Galileo Galilei, to estimate the resolution of his telescope!). In order to avoid any disturbance effect from the environment surrounding the telescope down in the CNR area and also to prevent possible reflections from the ground, the whole gondola with the telescope, together with the electronics and all the measuring instruments, were raised about 5 meters from the ground by means of a 6m*6m platform. The sources were two microwave Gunn oscillators, working in the frequency ranges of GHz and GHz. We choose respectively, ν 0 = 90 GHz λ 0= 3.3 mm and ν 0 = 137 GHz λ 0= 2.2 mm. The reason of these choices can be easily understood looking at the atmospheric absorption spectrum at millimetric wavelengths (see e.g. [14]): near 60 GHz we find the blended forest of fine structure transitions from O 2, while a fine structure line of O 2 is centered at GHz. On the other hand water completely dominates atmospheric absorption above 150 GHz. An HDPE lens was used to convert the beam from the source horn in a collimated beam. The reference signal used to modulate the source was transmitted down to the CNR area for synchronous demodulation, by means of a pair of dual band radio TX/RX. In the focal plane of the telescope we put the receiving system, made of a feed horn matching the f/# of the telescope and feeding a broadband diode detector, followed by a low-noise amplifier and by a lock-in demodulator. The scans were mainly performed varying

6 the elevation angle, while azimuth was kept fixed. After one scan was completed, we stepped ahead in azimuth and repeated the scan in elevation. After such mapping was complete, we always provided at least two azimuthal scans at fixed elevation, in order to recover the beam profile as a convolution of all the previous elevation scans. In fig.6 we present two dimensional plots representing the slices of the beam map, both in elevation and azimuth. Note that the 2-D slices we present are only those who lie very close to the absolute maximum of the signal. From Fig.6 we see that the agreement with the expected modified Airy function is very good in the main lobe: the fitted values for the aperture and the occlusion substantially coincide with the nominal ones. The sidelobes present instead a systematic asymmetry, both at 90 GHz and 137 GHz, either in azimuth or elevation: this excludes the eventuality of possible effects due to reflections on the ground, which should not emerge during azimuthal scans. Preliminary simulations of a 2 mm decentering or, alternatively, a 0.15 tilt of the secondary mirror optical axis with respect to the primary, provide an asymmetry in the sidelobes which is comparable to the observed one. where the fitted parameters are the aperture D (primary mirror diameter), and the occlusion diameter εd (secondary mirror diameter); the second curve (blue) is a Gaussian, from which we are able evaluate the FWHM of the main beam. The fit results are shown in Tab. 1: the measured FWHMs at 90 GHz and 137 GHz are well consistent with those of a diffraction-limited 2.6 m telescope. Table 1. Summary of measured beam sizes (in arcmin) at 90 and 137 GHz, both in elevation and azimuth. These results are compared to the diffraction limit for a 2.6 m diameter telescope. Acknowledgements: this work is being supported by Agenzia Spaziale Italiana and MIUR. The experiment is part of the Coordinated Investigation Project NOBILE (Long Duration Balloon Program) within the IPY REFERENCES 1. Birkinshaw M., 1999, Physics Reports, 310, Sunyaev R., Zeldovich Y.B., 1970, Astrophysics and Space Science, 7, 3 3. Gunn J.E., et al., 1978, Observational Cosmology 1 Geneva Obs., Sauverny, p.3 4. Silk J., White S., 1978, Astrophysical Journal Letters, 226, L Cavaliere A., et al., 1979, A&A, 75, Bonamente M., et al., 2006, Ap. J. 647, 25B 7. Ruhl J.E., 2004, Proc. SPIE, 5498, Nunes N.J., et al., 2006, A&A, 450, Colafrancesco et al., 2007, A&A Letters, A Masi S. et al., 2005, ESA SP590, Masi S. et al., 1999, AIP 476, D. P. Finkbeiner, M. Davis, and D. J. Schlegel, 1999, ApJ, 524: G. Lagache, H. Dole, and J.-L. Puget, 2003, MNRAS, 338: Pardo J.R., et al., 2001, IEEE Trans. On Antennas and Propagation, 49/12, 1683 Figure 6. 2-D slices of the OLIMPO telescope beam profile at 90 and 137 GHz, both in elevation. In the 2-D plots we also plot 2 curves: the red one is the appropriate Airy-modified Point Spread Function (PSF),