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Register a new userThe Mass Distribution of the Great Attractor as Revealed by a Deep NIR Survey
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Renee C. Kraan-Korteweg
Publication date
2011
fields
Physics
and research's language is
English
Authors
R.C. Kraan-Korteweg
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This paper presents the analysis of a deep near-infrared J,H,Ks-imaging survey (37.5 sq deg) aimed at tracing the galaxy distribution of the Great Attractor (GA) in the Zone of Avoidance along the so-called Norma Wall. The resulting galaxy catalog is complete to extinction-corrected magnitudes Ks^o = 14.8 mag for extinctions less than A_K = 1.0 mag and star densities below log N(Ks<14.0) < 4.72. Of the 4360 cataloged galaxies, 99.2% lie in the hereby constrained 89.5% of the survey area. Although the analyzed galaxy distribution reveals no new major galaxy clusters at the GA distance (albeit some more distant ones), the overall number counts and luminosity density indicate a clear and surprisingly smooth overdensity at the GA distance that extends over the whole surveyed region. A mass estimate of the Norma Wall overdensity derived from (a) galaxy number counts and (b) photometric redshift distribution gives a lower value compared to the original prediction by Lynden-Bell et al. 1988 (~14%), but is consistent with more recent independent assessments.
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A blind HI survey using the Parkes telescope at |b|<5 deg, 300 deg < l < 332 deg has so far revealed 305 galaxies, most of which were previously unknown. These galaxies are used to map out the distribution of filaments and voids out to 10000 km/s. A preliminary measurement of the galaxy overdensity suggests only a moderate overdensity is present, and that the excess mass (above the background density) is ~2.10^{15}.Omega.M(sun). This is below the mass predicted in POTENT reconstructions of the local velocity field, and implies that the `Great Attractor (GA) is not as massive as these reconstructions indicate, or does not lie hidden in the region investigated.
The redshift distribution of near-IR selected galaxies is often used to attempt to discriminate between the classical view of galaxy formation, in which present-day luminous galaxies were assembled at early times and evolve due to the passive aging of their stellar populations, and that of hierarchical structure formation, in which galaxies were assembled more recently via the merging of smaller objects. We carry out such a test here, by computing the distribution of photometric redshifts of K_AB<22 galaxies in the Great Observatories Origins Deep Field Survey GOODS Southern Field, and comparing the results with predictions from a semi-analytic model based on hierarchical structure formation, and a classical `passive evolution model. We find that the redshift distributions at z <~ 1.5 of both the hierarchical and passive models are very similar to the observed one. At z>~1.5, the hierarchical model shows a deficit of galaxies, while the passive model predicts an excess. We investigate the nature of the observed galaxies in the redshift range where the models diverge, and find that the majority have highly disturbed morphologies, suggesting that they may be merger-induced starbursts. While the hierarchical model used here does not produce these objects in great enough numbers, the appearance of this population is clearly in better qualitative agreement with the hierarchical picture than with the classical passive evolution scenario. We conclude that the observations support the general framework of hierarchical formation, but suggest the need for new or modified physics in the models.
We compare the measured peculiar velocities of 98 local (<150/h Mpc) type Ia supernovae (SNIa) with predictions derived from the PSCz. There is excellent agreement between the two datasets with a best fit beta_I (=Omega_m^0.6/b_I) of 0.55+-0.06. Subsets of the SNIa dataset are further analysed and the above result is found to be robust with respect to culls by distance, host-galaxy extinction and to the reference frame in which the analysis is carried out. We briefly review the peculiar motions in the direction of the Great Attractor. Most clusters in this part of the sky out to a distance of 14,000 km/s, i.e. those closer than the Shapley Concentration, have sizable positive peculiar velocities, i.e. (~ +400 km/s). There are nine local SNIa in the GA direction that are in the foreground of Shapley. All these SNIa have positive peculiar velocities. Hence both the cluster and local SNIa data strongly support the idea of a sizable flow into Shapley.
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.
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