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Search Results: 1 - 10 of 1666 matches for " Joe Silk "
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The massive black hole-velocity dispersion relation and the halo baryon fraction: a case for positive AGN feedback
Joe Silk,Adi Nusser
Physics , 2010, DOI: 10.1088/0004-637X/725/1/556
Abstract: Force balance considerations put a limit on the rate of AGN radiation momentum output, $L/c$, capable of driving galactic superwinds and reproducing the observed $\mbh -\sigma $ relation between black hole mass and spheroid velocity dispersion. We show that black holes cannot supply enough momentum in radiation to drive the gas out by pressure alone. Energy-driven winds give a $\mbh -\sigma $ scaling favoured by a recent analysis but also fall short energetically once cooling is incorporated. We propose that outflow-triggering of star formation by enhancing the intercloud medium turbulent pressure and squeezing clouds can supply the necessary boost, and suggest possible tests of this hypothesis. Our hypothesis simultaneously can account for the observed halo baryon fraction.
Observational Constraints on Open Inflation Models
Martin White,Joe Silk
Physics , 1996, DOI: 10.1103/PhysRevLett.77.4704
Abstract: We discuss observational constraints on models of open inflation. Current data from large-scale structure and the cosmic microwave background prefer models with blue spectra and/or Omega_0 >= 0.3--0.5. Models with minimal anisotropy at large angles are strongly preferred.
Broaden the search for dark matter
Mario Livio,Joe Silk
Physics , 2014, DOI: 10.1038/507029a
Abstract: Bold strategies are needed to identify most of the elusive particles that should make up most of the Universe's mass
The Current Status of Galaxy Formation
Joe Silk,Gary A. Mamon
Physics , 2012, DOI: 10.1088/1674-4527/12/8/004
Abstract: Understanding galaxy formation is one of the most pressing issues in cosmology. We review the current status of galaxy formation from both an observational and a theoretical perspective, and summarise the prospects for future advances.
Cosmological constraints in the presence of ionizing and resonance radiation at recombination
Bean, Rachel;Melchiorri, Alessandro;Silk, Joe
High Energy Physics - Phenomenology , 2007, DOI: 10.1103/PhysRevD.75.063505
Abstract: With the recent measurement of full sky cosmic microwave background polarization from WMAP, key cosmological degeneracies have been broken, allowing tighter constraints to be placed on cosmological parameters inferred assuming a standard recombination scenario. Here we consider the effect on cosmological constraints if additional ionizing and resonance radiation sources are present at recombination. We find that the new CMB data significantly improve the constraints on the additional radiation sources, with $\log_{10}[\epsilon_{\alpha}] < -0.5$ and $\log_{10}[\epsilon_{i}] <-2.4$ at 95% c.l. for resonance and ionizing sources respectively. Including the generalized recombination scenario, however, we find that the constraints on the scalar spectral index $n_s$ are weakened to $n_s=0.98\pm0.03$, with the $n_s=1$ case now well inside the 95% c.l.. The relaxation of constraints on tensor modes, scale invariance, dark energy and neutrino masses are also discussed.
Cosmic microwave background anomalies viewed via Gumbel Statistics
Gatis Mikelsons,Joseph Silk,Joe Zuntz
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.15503.x
Abstract: We describe and discuss the application of Gumbel statistics, which model extreme events, to WMAP 5-year measurements of the cosmic microwave background. We find that temperature extrema of the CMB are well modelled by the Gumbel formalism and describe tests for Gaussianity that the approach can provide. Comparison to simulations reveals Gumbel statistics to have only weak discriminatory power for the conventional statistic: $f_{NL}<1000$, though it may probe other regimes of non-Gaussianity. Tests based on hemispheric cuts reveal interesting alignment with other reported CMB anomalies. The approach has the advantage of model independence and may find further utility with smaller scale data.
A Semi-Empirical Model of the Infra-Red Universe
Jonathan C. Tan,Joe Silk,Christophe Balland
Physics , 1999, DOI: 10.1086/307685
Abstract: We present a simple model of the infra-red universe, based as much as possible on local observations. We model the luminosity and number evolution of disk and starburst galaxies, including the effects of dust, gas and spectral evolution. Although simple, our approach is able to reproduce observations of galaxy number counts and the infra-red and sub-millimeter extra-galactic backgrounds. It provides a useful probe of galaxy formation and evolution out to high redshift. The model demonstrates the significant role of the starburst population and predicts high star formation rates at redshifts 3 to 4, consistent with recent extinction-corrected observations of Lyman break galaxies. Starbursting galaxies are predicted to dominate the current SCUBA surveys. Their star formation is driven predominantly by strong tidal interactions and mergers of galaxies. This leads to the creation of spheroidal stellar systems, which may act as the seeds for disk formation as gas infalls. We predict the present-day baryonic mass in bulges and halos is comparable to that in disks. From observations of the extra-galactic background, the model predicts that the vast majority of star formation in the Universe occurs at z<5.
Cosmological constraints in the presence of ionizing and resonance radiation at recombination
Rachel Bean,Alessandro Melchiorri,Joe Silk
Physics , 2007, DOI: 10.1103/PhysRevD.75.063505
Abstract: With the recent measurement of full sky cosmic microwave background polarization from WMAP, key cosmological degeneracies have been broken, allowing tighter constraints to be placed on cosmological parameters inferred assuming a standard recombination scenario. Here we consider the effect on cosmological constraints if additional ionizing and resonance radiation sources are present at recombination. We find that the new CMB data significantly improve the constraints on the additional radiation sources, with $\log_{10}[\epsilon_{\alpha}] < -0.5$ and $\log_{10}[\epsilon_{i}] <-2.4$ at 95% c.l. for resonance and ionizing sources respectively. Including the generalized recombination scenario, however, we find that the constraints on the scalar spectral index $n_s$ are weakened to $n_s=0.98\pm0.03$, with the $n_s=1$ case now well inside the 95% c.l.. The relaxation of constraints on tensor modes, scale invariance, dark energy and neutrino masses are also discussed.
From Microwave Anisotropies to Cosmology
Douglas Scott,Joe Silk,Martin White
Physics , 1995, DOI: 10.1126/science.268.5212.829
Abstract: Fluctuations in the temperature of the cosmic microwave background have now been detected over a wide range of angular scales, and a consistent picture seems to be emerging. This article describes some of the implications for cosmology. Analysis of all the published detections suggests the existence of a peak on degree scales of height 2.4 to 10 (90%CL) relative to the amplitude of the power spectrum at large angular scales. This result confirms an early prediction, implies that the universe did in fact recombine, and limits theories of structure formation. Illustrative examples are provided of how the comparison of microwave background and large-scale structure data will be a potentially powerful means of answering fundamental questions about the universe.
Enhancements to velocity-dependent dark matter interactions from tidal streams and shells in the Andromeda galaxy
Robyn E. Sanderson,Roya Mohayaee,Joe Silk
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.20215.x
Abstract: Dark matter substructure around nearby galaxies provides an interesting opportunity for confusion-free indirect detection of dark matter. We calculate the boost over a smooth background distribution of dark matter for gamma-ray emission from dark matter self-annihilations in tidal structure in M31, assuming a cross-section inversely proportional to the relative velocities of the dark matter particles as proposed by the Sommerfeld effect. The low velocity of the material in the structure results in a significant increase in gamma-ray emission compared to both the background halo and the predicted emission for a velocity-independent cross section. We also calculate the expected signal for Fermi, for reasonable choices of the dark matter parameters. We find that for a cross section proportional to the inverse-square of the relative velocity, the enhancement to the annihilation rate is sufficient to test the velocity dependence of the cross section by spatial correlation with the stellar component of the stream, given sufficient detector sensitivity.
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