OBSERVATIONS OF THE CORONA BOREALIS SUPERCLUSTER WITH VSA

OBSERVATIONS OF THE CORONA BOREALIS SUPERCLUSTER WITH VSA: FURTHER CONSTRAINTS ON THE NATURE OF THE NONGAUSSIAN COLD SPOT Astrophysics Group Cavendish Laboratory Cambridge Ricardo Génova-Santos VSA collaboration Instituto de Astrofísica de Canarias (IAC) 43 rd Rencontres de Moriond Cosmology session

Statement of the problem v The baryon problem: • Total baryon budget at z=0 (Fukugita & Peebles 2004) • Lyα forest at z>2 (Rauch et al. 1997) CMB at z~1000 (Hinshaw et al. 2008) BBN (Burles et al. 2001) ΩB = (0. 010± 0. 003)h-2 ΩB > 0. 021 h-2 ΩB = (0. 023± 0. 001)h-2 ΩB = (0. 020± 0. 002)h-2 Half of the baryons in the present-day Universe are yet to be detected! v Hydrodinamical simulations (Cen & Ostriker, 2006): predict the formation, at low redshift (z<1), of the socalled Warm/Hot Intergalactic Medium (WHIM), a very diffuse gas phase with Te ~ 105 – 107 K, δB ~ 10 – 30 and L~10 Mpc, distributed in large-scale sheet-like structures and filaments connecting clusters of galaxies v Means for detecting it: • Sunyaev-Zel’dovich (SZ) effect. Correlations of CMB maps with galaxy templates (Hernández-Monteagudo et al. 2004) 0 M (Cen & Ostriker, 2006) δB ~ 10 / 1000 12 • UV (Nicastro et al. 2005) or X-ray (Barcons et al. 2005) absorption lines pc • Soft (0. 1 -2. 4 ke. V) X-ray emission (Zappacosta et al. 2005)

Selection of the supercluster v The WHIM is likely to be located in large regions with a high matter density, such as supercluster of galaxies v We selected the Corona Borealis supercluster for VSA observations in the wake of: • Its appropiate angular size • Observavility by the VSA • Relatively high LX of its member clusters • Lack of bright radio sources v The Cr. B-SC is: • in the northern hemisphere at Dec. ~27º • located at z≈0. 07 • formed by 8 galaxy clusters. 4 of them have reported X-ray emission, with LX ~ 1 -5 x 1044 h 50 -2 erg s-1 v We have focused on its core, which contains 6 clusters distributed in a region of 3 x 3 deg 2 v Observations of this region has been carried out at microwaves with the VSA (Génova. Santos et al. 2005, 2008) and AMI, millimeter wavelenghts with MITO (Battistelli et al. 2006) and infrared range with the ING (Padilla-Torres et al. 2008)

The Very Small Array v Interferometer with 14 antenna (91 baselines) located at the Teide Observatory at 2400 m asl v Frequency: 33 GHz, bandwidth: 1. 5 GHz v Belongs to the joint collaboration: Cambridge + Manchester + IAC v Aimed for doing primordial CMB observations (Taylor et al. 2003, Grainge et al. 2003, Dickinson et al. 2004). Also succesfully used to do SZ observations on galaxy clusters (Lancaster et al. 2005). Currently performing high-l primordial CMB observations Compact (Oct. 00 -Sep. 01) Extended (Oct. 01 -Nov. 04) v Compact / Extended / Superextended: • FOV: FWHMpb= 4. 6º / 2. 1º / 0. 9º Superextended (Dec. 05 -) • Resolution: FWHMsb= 20’ / 11’ / 7’ • Multipole coverage: l~ 150 -900 / 300 -1500 / 4502400

The Very Small Array v Source subtractor, aimed for simultaneous observations of the radio sources in the observed fields v Sources were identified in the NVSS (1. 4 GHz) and GB 6 (4. 85 GHz) catalogues and their fluxes extrapolated to 33 GHz v Sources with extrapolated flux above 18 m. Jy were selected for SS observations, and finally subtracted from the data • 2 -element interferometer with a N-S baseline of 9. 2 m • Parabolic dishes of 3. 7 m • FWHMpb= 9’ • FWHMsb = 4’

Extended VSA observations 24 deg 2 mosaic in the Cr. B core region, built up from 9 pointings with the extended VSA Very deep decrement at ΔTRJ=-230± 23μK and coordinates 15 22 11. 47 +28 54 06. 2 (J 2000) (Genova-Santos et al. 2005)

Superextended VSA observations 3 deg 2 mosaic in the region of the decrement, built up from 6 pointings with the superextended VSA Pointed observation on the position of the decrement. Tapered map (Genova-Santos et al. 2008) Minimum decrement: ΔTRJ=-308± 26μK ΔTRJ=-190± 9μK

WMAP-5 th-yr on Corona Borealis Q-band V-band v WMAP W-band (freq=33 GHz, FWHMbeam=13. 2’) temperature in the position of the minimum: ΔTRJ=-120± 53μK v Disagrees at 1. 3σ with the minimum in the VSA tapered map (FWHMsb=13. 2’) v Agrees at 0. 1σ with a SZ espectrum W-band

Origin of the decrement v Primary CMB anisotropy? • 15000 simulations of Gaussian CMB in the aperture plane adding: primordial CMB (σCMB=41μK), thermal noise (σn=30μK) and residual radio sources (σsour=15μK) • The decrement is a 4. 8σ deviation • The probability of finding an at least as high deviation is only 0. 19% • Computed the power spectrum of the central pointing • 4. 0σ deviation from pure primordial CMB at l≈500

Origin of the decrement v Distant galaxy cluster? • Number of collapsed halos above the minimum mass the cluster needs to have to produce such SZ effect: • Sheth-Tormen (1999) mass function, predicts 0. 3 clusters in the entire region initially surveyed which could produce an at least as intense decrement • The optical survey carried out with the ING revealed a high concentration of infrared galaxies at z≈0. 07 and ≈0. 11, but without a cluster-like distribution (Padilla-Torres et al. 2008) (Padilla-Torres et al. , 2008)

Origin of the decrement v No X-ray excess emission is found in the position of the decrement in the ROSAT-R 6 (0. 73 -1. 56 ke. V) data v We may consider a large filament aligned along the l. o. s • Central SZ decrement: • Length < 40 Mpc: • Lx < ROSAT-R 6 map: v Within the range of typical WHIM Te a filament like this could contribute up to ≈70% of the total decrement v A filament with <δb>~500 and Te~0. 6 would give f=0. 5, and would have a total mass comparable to the total baryon content in the Cr. B clusters

MITO observations v Observations with the Millimeter and Infrared Testa Grigia Observatory (Italian Alps) at 143, 214 and 272 GHz (Battistelli et al. 2006) Thermodynamic temperature maps derived from the MEM reconstruction of the VSA and the 3 MITO channels (Battistelli et al. 2006)

MITO observations Primary anisotropy + SZE maps derived from a ML analysis VSA+MITO detections spectral dependency. Overplotted SZ spectrum + CMB component (Battistelli et al. 2006) v These observations support the hypothesis of a combination of a primary anisotropy (ΔTCMB=-128± 20μK) with an SZ effect (ΔTSZ=-42± 26μK), being 25% the contribution from the latter one

Conclusions v We have confirmed, with the superextended VSA, the presence of a very deep decrement on the CMB of -308± 26 µK, in the Corona Borealis supercluster in a position with no known galaxy clusters v This feature is present in WMAP-5 yr data. It’s minimum temperature disagrees with the VSA value, but matches the SZ spectral dependency very well v Our analyses revealed that it is a 4. 8σ deviation from the Gaussian CMB, and that the probability of finding a primordial CMB decrement at least as negative is only 0. 19% v The hypothesis of an SZ galaxy cluster is disfavoured by the fact that, due to the absence of X-ray emission, a distant cluster may would be needed, and this is in conflict with the large angular extension of the decrement v A large-scale filamentary structure seems in some ways one of the most attractive options. However, the absence of X-ray emission set constraints to its physical shape and parameters that rule out such deep SZ effect within the typical WHIM temperatures and length scales v Therefore, the most reliable explanation would be a combination between a negative primordial CMB feature and an SZ decrement