The Sunyaev-Zeldovich effect in superclusters of galaxies using gasdynamical simulations: the case of Corona Borealis

[Abridged] We study the thermal and kinetic Sunyaev-Zel'dovich (SZ) effect associated with superclusters of galaxies using the MareNostrum Universe SPH simulation. We consider superclusters similar to the Corona Borealis Supercluster (CrB-SC). This paper is motivated by the detection at 33GHz of a strong temperature decrement in the CMB towards the core of this supercluster. Multifrequency observations with VSA and MITO suggest the existence of a thermal SZ effect component in the spectrum of this cold spot, which would account for roughly 25% of the total observed decrement. We identify nine regions containing superclusters similar to CrB-SC, obtain the associated SZ maps and calculate the probability of finding such SZ signals arising from hot gas within the supercluster. Our results show that WHIM produces a thermal SZ effect much smaller than the observed value. Neither can summing the contribution of small clusters and galaxy groups in the region explain the amplitude of the SZ signal. When we take into account the actual posterior distribution from the observations, the probability that WHIM can cause a thermal SZ signal like the one observed is <1%, rising up to a 3.2% when the contribution of small clusters and galaxy groups is included. If the simulations provide a suitable description of the gas physics, then we conclude that the thermal SZ component of the CrB spot most probably arises from an unknown galaxy cluster along the line of sight. The simulations also show that the kinetic SZ signal associated with the supercluster cannot provide an explanation for the remaining 75% of the observed cold spot in CrB.Comment: Accepted for publication in MNRAS. 14 pages, 9 figure

SDSS J092609.45+334304.1: a nearby unevolved galaxy

We present the results of observations of the very low surface brightness (VLSB) dwarf galaxy SDSS J092609.45+334304.1 with extreme parameters which indicate its unevolved status. Namely, its value of O/H, derived as an average of that in two adjacent HII regions at the NE edge of the disc, corresponds to the parameter 12+log(O/H)=7.12+-0.02, which is amongst two lowest known. The total HI flux measurement obtained with the Nancay Radio Telescope and the photometric results imply that the galaxy ratio M(HI)/L_B ~3.0, is among the top known in the Local Volume. The galaxy is situated in the region of a nearby underdense region known as the Lynx-Cancer void, where other, unevolved galaxies, including DDO 68, HS 0832+3542 and SAO 0822+3545, are known to be present. The total mass of this almost edge-on VLSB galaxy is ~8.3 times larger than its baryonic mass, implying the dynamical dominance of Dark Matter (DM) halo. The (u-g), (g-r) colours of outer parts of this galaxy are consistent with the ages of its main stellar population of 1--3 Gyr. Thanks to the galaxy isolation, the small effect of current or recent star formation (SF), its proximity and rather large HI flux (~2.5 Jy km/s), this VLSB dwarf is a good laboratory for the detailed study of DM halo properties through HI kinematics and the star formation processes in very metal-poor low surface density environment. This finding, along with the discovery of other unusual dwarf galaxies in this void, provides evidence for the relation between galaxy evolution and its very low-density environment for the baryonic mass range of 10^{8} to 10^{9} Mo. This relation seems to be consistent with that expected in the LambdaCDM models of galaxy and structure formation.Comment: 10 pages, 5 figures, 6 tables, accepted to MNRA