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Posts: 131433
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The Local Void
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Title: Our Peculiar Motion Away from the Local Void
Author: R. Brent Tully, Edward J. Shaya, Igor D. Karachentsev, Helene M. Courtois, Dale D. Kocevski, Luca Rizzi, Alan Peel
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The peculiar velocity of the Local Group of galaxies manifested in the Cosmic Microwave Background dipole is found to decompose into three dominant components. The three components are clearly separated because they arise on distinct spatial scales and are fortuitously almost orthogonal in their influences. The nearest, which is distinguished by a velocity discontinuity at ~7 Mpc, arises from the evacuation of the Local Void. We lie in the Local Sheet that bounds the void. Random motions within the Local Sheet are small. Our Galaxy participates in the bulk motion of the Local Sheet away from the Local Void. The component of our motion on an intermediate scale is attributed to the Virgo Cluster and its surroundings, 17 Mpc away. The third and largest component is an attraction on scales larger than 3000 km/s and centred near the direction of the Centaurus Cluster. The amplitudes of the three components are 259, 185, and 455 km/s, respectively, adding collectively to 631 km/s in the reference frame of the Local Sheet. Taking the nearby influences into account causes the residual attributed to large scales to align with observed concentrations of distant galaxies and reduces somewhat the amplitude of motion attributed to their pull. On small scales, in addition to the motion of our Local Sheet away from the Local Void, the nearest adjacent filament, the Leo Spur, is seen to be moving in a direction that will lead to convergence with our filament. Finally, a good distance to an isolated galaxy within the Local Void reveals that this dwarf system has a motion of at least 230 km/s away from the void center. Given the velocities expected from gravitational instability theory in the standard cosmological paradigm, the distance to the center of the Local Void must be at least 23 Mpc from our position. The Local Void is large!

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Posts: 131433
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RE: The Local Volume
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Title: Too Big to Fail in the Local Group
Author: Shea Garrison-Kimmel, Michael Boylan-Kolchin, James S. Bullock, Evan N. Kirby

We compare the dynamical masses of dwarf galaxies in the Local Group (LG) to the predicted masses of halos in the ELVIS suite of LambdaCDM simulations, a sample of 48 Galaxy-size hosts, 24 of which are in paired configuration similar to the LG. We enumerate unaccounted-for dense halos (Vmax \gtrsim 25 km s^-1) in these volumes that at some point in their histories were massive enough to have formed stars in the presence of an ionsing background (Vpeak>30 km s^-1). Within 300 kpc of the Milky Way, the number of unaccounted-for massive halos ranges from 2 - 25 over our full sample. Moreover, this "too big to fail" count grows as we extend our comparison to the outer regions of the Local Group: within 1.2 Mpc of either giant we find that there are 12-40 unaccounted-for massive halos. This count excludes volumes within 300 kpc of both the MW and M31, and thus should be largely unaffected by any baryonically-induced environmental processes. According to abundance matching -- specifically abundance matching that reproduces the Local Group stellar mass function -- all of these missing massive systems should have been quite bright, with stella masses >10^6 solar masses. Finally, we use the predicted density structure of outer LG dark matter halos together with observed dwarf galaxy masses to derive an stellar masses -Vmax relation for LG galaxies that are outside the virial regions of either giant. We find that there is no obvious trend in the relation over three orders of magnitude in stellar mass (a "common mass" relation), from stellar masses ~108-10^5 solar masses, in drastic conflict with the tight relation expected for halos that are unaffected by reionisation. Solutions to the too big to fail problem that rely on ram pressure stripping, tidal effects, or statistical flukes appear less likely in the face of these results.

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Posts: 131433
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Local universe
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Title: Evidence for a ~300 Mpc Scale Local Under-density in the Distribution of Galaxies
Authors: Ryan C. Keenan, Amy J. Barger, Lennox L.Cowie

Galaxy counts and recent measurements of the luminosity density in the near-infrared (NIR) have indicated the possibility that the local universe may be under-dense on scales of several hundred megaparsecs. The presence of a large-scale under-density in the local universe could introduce significant biases into the interpretation of cosmological observables, and, in particular, into the inferred effects of dark energy on the expansion rate. Here we measure the K-band luminosity density as a function of redshift to test for such a local under-density. We find that the overall shape of the z = 0 rest-frame K-band luminosity function (M* = -21.6 ± 0.04 and alpha = -0.99 ± 0.03) appears to be relatively constant as a function of environment and redshift out to z ~0.2. We find a local (z < 0.07) luminosity density that is in good agreement with previous studies. At z > 0.07 we detect a rising luminosity density, and at z > 0.1, it is roughly ~1.5 times higher than that measured locally. This suggests that the stellar mass density as a function of redshift follows a similar trend. Assuming that luminous matter traces the underlying dark matter distribution, this implies that the local mass density of the universe may be lower than the global value on a scale and amplitude sufficient to introduce significant biases into the determination of basic cosmological observables, such as the expansion rate. An under-density on this scale and amplitude, for example, would be more than sufficient to resolve the apparent tension between direct measurements of the Hubble constant and those inferred by Planck.

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Posts: 131433
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RE: The Local Volume
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Title: Identifying Local Group Field Galaxies which have interacted with the Milky Way
Authors: Maureen Teyssier, Kathryn V. Johnston, Michael Kuhlen

We distinguish between Local Group field galaxies which may have passed through the virial volume of the Milky Way, and those which have not, via a statistical comparison against populations of dark matter haloes in the Via Lactea II (VLII) simulation with known orbital histories. Analysis of VLII provides expectations for this escaped population: they contribute 13 per cent of the galactic population between 300 and 1500 kpc from the Milky Way, and hence we anticipate that about 7 of the 54 known Local Group galaxies in that distance range are likely to be Milky Way escapees. These objects can be of any mass below that of the Milky Way, and they are expected to have positive radial velocities with respect to the Milky Way. Comparison of the radius-velocity distributions of VLII populations and measurements of Local Group galaxies presents a strong likelihood that Tucana, Cetus, NGC3109, SextansA, SextansB, Antlia, NGC6822, Phoenix, LeoT, and NGC185 have passed through the Milky Way. Most of these dwarfs have a lower HI mass fraction than the majority of dwarfs lying at similar distances to either the Milky Way or M31. Indeed, several of these galaxies - especially those with lower masses - contain signatures in their morphology, star formation history and/or gas content indicative of evolution seen in simulations of satellite/parent galactic interactions. Our results offer strong support for scenarios in which dwarfs of different types form a sequence in morphology and gas content, with evolution along the sequence being driven by interaction history.

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Posts: 131433
Date:
Local Universe
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Title: Cosmic Structure and Dynamics of the Local Universe
Authors: Francisco-Shu Kitaura, Pirin Erdogdu, Sebastian E. Nuza, Arman Khalatyan, Raul E. Angulo, Yehuda Hoffman, Stefan Gottloeber

We present a cosmography analysis of the Local Universe based on the recently released Two-Micron All-Sky Redshift Survey (2MRS). Our method is based on a Bayesian Networks Machine Learning algorithm (the Kigen-code) which self-consistently samples the initial density fluctuations compatible with the observed galaxy distribution and a structure formation model given by second order Lagrangian perturbation theory (2LPT). From the initial conditions we obtain an ensemble of reconstructed density and peculiar velocity fields which characterize the local cosmic structure with high accuracy unveiling nonlinear structures like filaments and voids in detail. Coherent redshift space distortions are consistently corrected within 2LPT. From the ensemble of cross-correlations between the reconstructions and the galaxy field and the variance of the recovered density fields we find that our method is extremely accurate up to k ~ 1 h Mpc^-1 and still yields reliable results up to k ~ 2 h Mpc^-1. The motion of the local group we obtain within ~ 80 h^-1 Mpc (v_LG=522±86 km s^-1, l_LG=291° ± 16°, b_LG=34°±8°) is in good agreement with measurements derived from the CMB and from direct observations of peculiar motions and is consistent with the predictions of LambdaCDM.

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Posts: 131433
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RE: The Local Volume
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Title: 3D Velocity and Density Reconstructions of the Local Universe with Cosmicflows-1
Authors: Helene M. Courtois, Yehuda Hoffman, R. Brent Tully, Stefan Gottlober

This paper presents an analysis of the local peculiar velocity field based on the Wiener Filter reconstruction method. We used our currently available catalogue of distance measurements containing 1,797 galaxies within 3000 km/s: Cosmicflows-1. The Wiener Filter method is used to recover the full 3D peculiar velocity field from the observed map of radial velocities and to recover the underlying linear density field. The velocity field within a data zone of 3000 km/s is decomposed into a local component that is generated within the data zone and a tidal one that is generated by the mass distribution outside that zone. The tidal component is characterized by a coherent flow toward the Norma-Hydra-Centaurus (Great Attractor) region while the local component is dominated by a flow toward the Virgo Cluster and away from the Local Void. A detailed analysis shows that the local flow is predominantly governed by the Local Void and the Virgo Cluster plays a lesser role. The analysis procedure was tested against a mock catalogue. It is demonstrated that the Wiener Filter accurately recovers the input velocity field of the mock catalogue on the scale of the extraction of distances and reasonably recovers the velocity field on significantly larger scales. The Bayesian Wiener Filter reconstruction is carried out within the ?CDM WMAP5 framework. The Wiener Filter reconstruction draws particular attention to the importance of voids in proximity to our neighbourhood. The prominent structure of the Local Supercluster is wrapped in a horseshoe collar of under density with the Local Void as a major component.

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Posts: 131433
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Two Micron All-Sky Survey
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The Universe Nearby

2mass1.gif
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Credit: 2MASS, T. H. Jarrett, J. Carpenter, & R. Hurt

This plot shows nearly 50,000 galaxies in the nearby universe detected by the Two Micron All Sky Survey (2MASS) in infrared light.

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Posts: 131433
Date:
The Local Group
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Title: A Dynamical Model of the Local Group
Authors: P. J. E. Peebles, R. Brent Tully, Edward J. Shaya

This dynamical model for the 28 galaxies with distances less than 1.5 Mpc, and not apparently tight satellites, is constrained by the initial condition that peculiar velocities at high redshift are small and growing in accordance with the standard cosmology. The solution is a satisfactory fit to most of the measured redshifts, distances, and proper motions, with some interesting exceptions that call for further investigation. The model predicts Milky Way rotation speed 256 km/s, consistent with Reid et al. (2009a). Ten Local Group galaxies emanate from low supergalactic latitude and supergalactic longitude ~ 70 degrees, perhaps as remnants from failed assembly of a larger galaxy. NGC 6822 passes close to the Milky Way at redshift ~0.27, in an orbit similar to the Magellanic Clouds. Leo I has heliocentric angular velocity 0.33 mas/yr, perhaps measurable by the mean stellar motion, and 15 galaxies have proper motions greater than 0.05 mas/yr, measurable for any with masers.

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The Local Universe
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Astronomers Unveil Most Complete 3-D Map of Local Universe

Today, astronomers unveiled the most complete 3-D map of the local universe (out to a distance of 380 million light-years) ever created. Taking more than 10 years to complete, the 2MASS Redshift Survey (2MRS) also is notable for extending closer to the Galactic plane than previous surveys - a region that's generally obscured by dust.
Karen Masters (University of Portsmouth, UK) presented the new map today in a press conference at the 218th meeting of the American Astronomical Society.

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Posts: 131433
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The Local Group
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Il y a 6 milliards d'années, crash majeur dans le Groupe local de galaxies

Notre environnement cosmique proche - le Groupe local de galaxies - a-t-il vécu un épisode de collision particulièrement violente il y a 6 milliards d'années ? C'est ce qu'indiquent les simulations numériques effectuées par six chercheurs de l'Observatoire de Paris, du CNRS, et des Observatoires astronomiques nationaux de l'Académie des sciences de Chine NAOC. Cette fusion de deux galaxies serait à l'origine de la formation de la spirale géante d'Andromède et des deux Nuages de Magellan, qui s'approchent de notre Voie lactée. Les résultats seront publiés les 20 novembre et 10 décembre 2010 dans deux articles d'Astrophysical Journal.
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