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تسجيل مستخدم جديدThe Universe behind the Milky Way
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Due to the foreground extinction of the Milky Way, galaxies appear increasingly fainter the closer they lie to the Galactic Equator, creating a zone of avoidance of about 25% in the distribution of optically visible galaxies. A whole-sky map of galaxies is essential, however, for understanding the dynamics in our local Universe, in particular the peculiar velocity of the Local Group with respect to the Cosmic Microwave Background and velocity flow fields such as in the Great Attractor region. Various dynamically important structures behind the Milky Way have only recently been made ``visible through dedicated deep surveys at various wavelengths. The wide range of observational searches (optical, near infrared, far infrared, radio and X-ray) for galaxies in the Zone of Avoidance are reviewed, including a discussion on the limitations and selection effects of these partly complementary approaches. The uncovered and suspected large-scale structures are summarized. Reconstruction methods of the density field in the Zone of Avoidance are described and the resulting predictions compared with observational evidence. The comparison between reconstructed density fields and the observed galaxy distribution allow derivations of the density and biasing parameters Omega_0 and b.
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A first analysis of a deep blind HI survey covering the southern Zone of Avoidance plus an extension towards the north (196 < l < 52 deg) obtained with the Multibeam receiver at the 64m Parkes telescope reveals slightly over a thousand galaxies withi
n the latitude completeness limit of |b| < 5deg. The characteristics and the uncovered large-scale structures of this survey are described, in particular the prominence of the Norma Supercluster, the possible cluster around PKS 1343-601 (both in the Great Attractor region), as well as the Local Void and the clustering in the Puppis region. In this blind HI survey, HIZOA J0836-43, one of the most massive spiral galaxies known to date was discovered (M(HI) = 7.3 10^10 Msun M(tot) = 1.1 10^12 Msun; Ho = 75 km/s/Mpc). Although of similar mass as Malin 1-like objects, this galaxy does not share their typical low-surface brightness properties but seems an exceptionally massive but normal, high-surface brightness, star-forming galaxy.
Dust and stars in the plane of the Milky Way create a Zone of Avoidance in the extragalactic sky. Galaxies are distributed in gigantic labyrinth formations, filaments and great walls with occasional dense clusters. They can be traced all over the sky
, except where the dust within our own galaxy becomes too thick - leaving about 25% of the extragalactic sky unaccounted for. Our Galaxy is a natural barrier which constrains the studies of large-scale structures in the Universe, the peculiar motion of our Local Group of galaxies and other streaming motions (cosmic flows) which are important for understanding formation processes in the Early Universe and for cosmological models. Only in recent years have astronomers developed the techniques to peer through the disk and uncover the galaxy distribution in the Zone of Avoidance. I present the various observational multi-wavelength procedures (optical, far infrared, near infrared, radio and X-ray) that are currently being pursued to map the galaxy distribution behind our Milky Way. Particular emphasis is given to discoveries in the Great Attractor region -- a from streaming motions predicted huge overdensity centered behind the Galactic Plane. The recently unveiled massive rich cluster A3627 seems to constitute the previously unidentified core of the Great Attractor.
Current studies of the peculiar velocity flow field in the Local Universe are limited by the lack of detection of galaxies behind the Milky Way. The contribution of the largely unknown mass distribution in this Zone of Avoidance (ZoA) to the dynamics
of the Local group remains contraversial. We have undertaken a near infrared (NIR) survey of HI detected galaxies in the ZoA. The photomety derived here will be used in the NIR Tully-Fisher (TF) relation to derive the peculiar velocities of this sample of galaxies in the ZoA.
About 25% of the optical extragalactic sky is obscured by the dust and stars of our Milky Way. Dynamically important structures might still lie hidden in this zone. Various approaches are presently being employed to uncover the galaxy distribution in
this Zone of Avoidance (ZOA). Results as well as the different limitations and selection effects from these multi-wavelengths explorations are being discussed. Galaxies within the innermost part of the Milky Way - typically at a foreground obscuration in the blue of A_B > 5mag and |b| < 5 deg - remain particularly difficult to uncover except for HI-surveys: the Galaxy is fully transparent at the 21cm line and HI-rich galaxies are easy to trace. We will report here on the first results from the systematic blind HI-search (v < 12700 km/s) in the southern Zone of Avoidance which is currently being conducted with the Parkes Multibeam (MB) Receiver.
Our Galaxy blocks a significant portion of the extragalactic sky from view, hampering studies of large-scale structure. This produces an incomplete knowledge of the distribution of galaxies, and, assuming galaxies trace mass, of the gravity field. Fu
rther, just one unrecognized, nearby massive galaxy could have large influence over the Milky Ways motion with respect to the Cosmic Microwave Background. Diligent surveys in the optical and infrared wavebands can find galaxies through moderate Galactic gas and dust, but close to the Galactic Plane, only radio surveys are effective. The entire northern Zone of Avoidance is being searched at 21 cm for galaxies using the Dwingeloo 25-m telescope. A shallow search for nearby, and/or massive galaxies has been completed, yielding five objects. Two of these galaxies were previously unknown, and although they are not likely members of the Local Group, are part of the nearby Universe. A deeper search continues, which will produce a flux-limited catalog of hidden galaxies. This portion of the survey is one-third complete, and has detected about 40 objects to date. Based on present understanding of the HI mass function, the complete survey should uncover 50 - 100 galaxies.
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