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Shaw J.R.,Kavli Institute for Cosmology | Shaw J.R.,Canadian Institute for Theoretical Astrophysics | Lewis A.,University of Sussex
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

Primordial magnetic fields could provide an explanation for the galactic magnetic fields observed today; in which case, they may leave interesting signals in the CMB and the small-scale matter power spectrum. We discuss how to approximately calculate the important nonlinear magnetic effects within the guise of linear perturbation theory and calculate the matter and CMB power spectra including the Sunyaev-Zel'dovich contribution. We then use various cosmological data sets to constrain the form of the magnetic field power spectrum. Using solely large-scale CMB data (WMAP7, QUaD, and ACBAR) we find a 95% C.L. on the variance of the magnetic field at 1 Mpc of B λ<6.4nG. When we include South Pole Telescope data to constrain the Sunyaev-Zel'dovich effect, we find a revised limit of B λ<4.1nG. The addition of Sloan Digital Sky Survey Lyman-α data lowers this limit even further, roughly constraining the magnetic field to B λ<1.3nG. © 2012 American Physical Society.

Pettini M.,Institute of Astronomy | Pettini M.,Kavli Institute for Cosmology | Cooke R.,Institute of Astronomy | Cooke R.,Kavli Institute for Cosmology
Monthly Notices of the Royal Astronomical Society | Year: 2012

The metal-poor (Z {minus tilde} 1/100 Z⊙) damped Lyman a system (DLA) at redshift zabs = 3.049 84 in the zem {minus tilde} 3.030 QSO SDSS J1419+0829 has near-ideal properties for an accurate determination of the primordial abundance of deuterium (D/H)p. We have analysed a high-quality spectrum of this object with software specifically designed to deduce the best-fitting value of D/H and to assess comprehensively the random and systematic errors affecting this determination. We find (D/H)DLA = (2.535 ± 0.05) × 10-5, which in turn implies ωb,0h2 = 0.0223 ± 0.0009, in very good agreement with σb,0h2(CMB) = 0.0222 ± 0.0004 deduced from the angular power spectrum of the cosmic microwave background (CMB). If the value in this DLA is indeed the true (D/H)p produced by big bang nucleosynthesis (BBN), there may be no need to invoke non-standard physics nor early astration of D to bring together ωb,0 h2(BBN) and ωb,0 h2(CMB). The scatter between most of the reported values of (D/H)p in the literature may be due largely to unaccounted systematic errors and biases. Further progress in this area will require a homogeneous set of data comparable to those reported here and analysed in a self-consistent manner. Such an endeavour, while observationally demanding, has the potential of improving our understanding of BBN physics, including the relevant nuclear reactions, and the subsequent processing of light nuclides through stars. © 2012 The Authors Monthly Notices of the Royal Astronomical Society.

Shaw J.R.,Kavli Institute for Cosmology | Lewis A.,Institute of Astronomy
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2010

Primordial magnetic fields and massive neutrinos can leave an interesting signal in the CMB temperature and polarization. We perform a systematic analysis of general perturbations in the radiation-dominated universe, accounting for any primordial magnetic field and including leading-order effects of the neutrino mass. We show that massive neutrinos qualitatively change the large-scale perturbations sourced by magnetic fields, but that the effect is much smaller than previously claimed. We calculate the CMB power spectra sourced by inhomogeneous primordial magnetic fields, from before and after neutrino decoupling, including scalar, vector and tensor modes, and consistently modeling the correlation between the density and anisotropic stress sources. In an appendix we present general series solutions for the possible regular primordial perturbations. © 2010 The American Physical Society.

Cantalupo S.,Kavli Institute for Cosmology | Porciani C.,Argelander Institute For Astronomie
Monthly Notices of the Royal Astronomical Society | Year: 2011

We present a new three-dimensional radiative transfer (RT) code, radamesh (Radiative-transfer on ADAptive MESH), based on a ray-tracing, photon-conserving and adaptive (in space and time) scheme. radamesh uses a novel Monte Carlo approach to sample the radiation field within the computational domain on a 'cell-by-cell' basis. Thanks to this algorithm, the computational efforts are now focused where actually needed, i.e. within the Ionization-fronts (I-fronts). This results in an increased accuracy level and, at the same time, a huge gain in computational speed with respect to a 'classical' Monte Carlo RT, especially when combined with an Adaptive Mesh Refinement (AMR) scheme. Among several new features, radamesh is able to adaptively refine the computational mesh in correspondence of the I-fronts, allowing to fully resolve them within large, cosmological boxes. We follow the propagation of ionizing radiation from an arbitrary number of sources and from the recombination radiation produced by H and He. The chemical state of six species (Hi, Hii, Hei, Heii, Heiii, e) and gas temperatures are computed with a time-dependent, non-equilibrium chemistry solver. We present several validating tests of the code, including the standard tests from the RT code comparison project and a new set of tests aimed at substantiating the new characteristics of radamesh. Using our AMR scheme, we show that properly resolving the I-front of a bright quasar during reionization produces a large increase of the predicted gas temperature within the whole Hii region. Also, we discuss how H and He recombination radiation is able to substantially change the ionization state of both species (for the classical Strömgren sphere test) with respect to the widely used 'on-the-spot' approximation. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.

News Article | January 19, 2016

KISSIMMEE, Fla. — Many galaxies are LIERS, says Francesco Belfiore, a graduate student at the Kavli Institute for Cosmology at the University of Cambridge. He's not throwing shade at these objects, but rather trying to explain why new stars are no longer born inside them. Earth lies in a galaxy that is flush with new star birth. The Milky Way's spiral shape and blue color are both signs that baby stars are being made inside it. But in elliptical-shaped galaxies with more reddish hues, star birth has stopped, and scientists don't understand why. In trying to study the chemistry of these "dead" galaxies, researchers have found a different chemical fingerprint than the one that dominates star-forming galaxies. To describe what they were seeing, researchers came up with the acronym LINER, which stands for "low-ionization nuclear emission-line region." Belfiore's explanation of the name is more direct: "The reason why we use this acronym is because we don't know what they are," he said. [Gallery: 65 All-Time Great Galaxy Hits] More specifically, scientists don't know what's creating the "LINER" chemical signature in the dead galaxies, and whether it might help explain why they stopped forming stars. Now, new observations by Belfiore and colleagues have added another twist to this LINER mystery: rather than coming from the black hole at the center of the dead galaxy, as researchers previously thought, the signature can be found throughout the galaxies, all the way out to their fringes. According to Belfiore, this new finding that means the "N" in LINER (which stood for "nuclear," referring to the center of the galaxy) should be removed, and these galaxies should be called "LIERs." Belfiore spoke about the new findings on Jan. 8 during a press briefing here at the 227th meeting of the American Astronomical Society. By itself, the new information doesn't answer the big question of why star formation has stopped in those galaxies, but it may resolve a seemingly contradictory observation found in many previous studies: that select patches of living galaxies also exhibit this LINER/LIER chemical fingerprint. In other words, it may help scientists understand what turns living galaxies into LIERs. What does a galactic LIER look like? These "dead" galaxies are different from star-forming galaxies in many ways. They tend to be redder in color, because blue stars have shorter lives than red stars, so for the most part, the blue stars have all died out in the LIER galaxies. The stellar nurseries in star-forming galaxies like the Milky Way emit large amounts of light, and appear as bright, glowing beacons. These are also missing from images of LIER galaxies, which have a more diffuse glow. Dead galaxies are also shaped differently. They're more often elliptical, like an American football, instead of a flat, circular spiral. Without star birth, these galaxies can't develop the massive arms that wrap around the centers of star-forming galaxies like the Milky Way. Once again, scientists don't fully understand why the stop of star birth also means a change in shape for the galaxy, but there is a theory that many elliptical galaxies are created when two or more galaxies collide and merge together. Perhaps that process somehow shuts off star birth, scientists have suggested. These red, dead, football-shaped galaxies contain a cocktail of chemicals that's different from that of their living counterparts. Previous observations of elliptical galaxies have been limited in their resolution, and suggested that the LINER gas signature was coming from the center of these galaxies. This is important because, at the center of most (if not all) large galaxies is a supermassive black hole. According to Belfiore, the leading theory for why dead galaxies have this LIER signature is because of activity near the black hole at the center of the galaxy. This idea is bolstered by a third category of galaxies called active galactic nuclei, or AGNs. The black holes at the center of AGNs are extremely active, meaning they have lots of material falling into them, producing jets of material that spew out into space, and radiating an incredible amount of light. AGNs also have a chemical signature that's different from that of LINER galaxies and star-forming galaxies, Belfiore noted in his talk. Some scientists suspect that LINER galaxies are simply weaker examples of AGNs, where a lower amount of activity around the black hole produces this unique chemical fingerprint, he said. But the new observations presented by Belfiore have allowed scientists to look at the source of the LINER emission at a higher resolution, and revealed that they are not limited to the galaxy's center. Using the Sloan Digital Sky Survey (SDSS), a 2.5-meter (8.2 feet) telescope in New Mexico, as part of a project called MaNGA (Mapping Nearby Galaxies at Apache Point Observatory), Belfiore and colleagues found that, in some cases, LIER signatures can be found emanating from throughout a LINER galaxy, or from separate locations near the outskirts. The new results indicate that the process creating the LINER signature is, most likely, something that can occur throughout the galaxy, Belfiore said. [Gallery: Black Holes of the Universe] Living galaxies can be LIERS, too Belfiore and his collaborators theorize that the LIER chemical fingerprint might be coming from older stars as they reach the twilight hours of their lives. In that stage of life, many stars shed their outer layers, and it may be the chemical signature of this dispelled stellar material that is being detected, Belfiore explained. This would explain why the LIER chemical signature is seen in some galaxies where star formation is still happening, Belfiore told in an email. In many spiral galaxies, star formation does not shut off suddenly, but gradually. Some spiral galaxies develop regions (often near their centers) where star formation stops, and in those cases, it is possible for scientists to see a LIER emission from the "dead" region of the galaxy. Of course, dying stars are present even where new ones are forming, but the LIER emission is "always fainter than the emission due to star formation," Belfiore told in an email. Hence, in those living galaxies, the weak LIER emission would be swamped by photons from the star-forming regions and would go undetected by telescopes.   In other words, Belfiore said, it's possible that most galaxies are LIERs.  The idea that LINER galaxies may actually be LIERs — that this chemical signature is not coming from the galactic center but from another source, such as dying stars throughout the galaxy — has been building for some years, Belfiore told "Although most astronomers not directly working on this topic would assume that LINERs are weak active galactic nuclei, a paradigm shift has been happening slowly for several years now," he said. He mentioned that some astronomers remain highly skeptical of the idea, but the new SDSS observations may change that. "Compared to previous studies, the new MaNGA data allows, for the first time, a direct test of the stellar hypothesis for LIER emission in a well-defined large sample of galaxies, covering (crucially) both spirals and ellipticals," Belfiore said. "It is the weight of this very direct evidence and its statistical significance that I feel is most compelling about the SDSS MaNGA result." Belfiore said he and his colleagues are preparing to submit their results for publication. Copyright 2016, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

Becker G.D.,Kavli Institute for Cosmology | Hewett P.C.,Kavli Institute for Cosmology | Worseck G.,University of California at Santa Cruz | Xavier Prochaska J.,University of California at Santa Cruz
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present new measurements of the mean transmitted flux in the Lya forest over 2 < z < 5 made using 6065 quasar spectra from the Sloan Digital Sky Survey data release 7 (SDSS DR7). We exploit the general lack of evolution in the mean quasar continuum to avoid the bias introduced by continuum fitting over the Lyα forest at high redshifts, which has been the primary systematic uncertainty in previous measurements of the mean Lyα transmission. The individual spectra are first combined into 26 composites with mean redshifts spanning 2.25 ≤ zcomp ≤ 5.08. The flux ratios of separate composites at the same rest wavelength are then used, without continuum fitting, to infer the mean transmitted flux, F(z), as a fraction of its value at z ~ 2. Absolute values for F(z) are found by scaling our relative values to measurements made from high-resolution data by Faucher-Giguére et al. at z < 2.5, where continuum uncertainties are minimal. We find that F(z) evolves smoothly with redshift, with no evidence of a previously reported feature at z ≲ 3.2. This trend is consistent with a gradual evolution of the ionization and thermal state of the intergalactic medium over 2 < z < 5. Our results generally agree with the most careful measurements to date made from high-resolution data, but offer much greater precision and extend to higher redshifts. This work also improves upon previous efforts using SDSS spectra by significantly reducing the level of systematic errors. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Amin M.A.,Kavli Institute for Cosmology
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

Oscillons are long-lived, localized, oscillatory scalar field configurations. In this work we derive a condition for the existence of small-amplitude oscillons (and provide solutions) in scalar field theories with noncanonical kinetic terms. While oscillons have been studied extensively in the canonical case, this is the first example of oscillons in scalar field theories with noncanonical kinetic terms. In particular, we demonstrate the existence of oscillons supported solely by the noncanonical kinetic terms, without any need for nonlinear terms in the potential. In the small-amplitude limit, we provide an explicit condition for their stability in d+1 dimensions against long-wavelength perturbations. We show that for d≥3, there exists a long-wavelength instability which can lead to radial collapse of small-amplitude oscillons. © 2013 American Physical Society.

Efstathiou G.,Kavli Institute for Cosmology
Monthly Notices of the Royal Astronomical Society | Year: 2014

I reanalyse the Riess et al. (hereafter R11) Cepheid data using the revised geometric maser distance to NGC 4258 of Humphreys et al. (hereafter H13). I explore different outlier rejection criteria designed to give a reduced X2 of unity and compare the results with the R11 rejection algorithm, which produces a reduced X2 that is substantially less than unity and, in some cases, leads to underestimates of the errors on parameters. I show that there are sub-luminous low-metallicity Cepheids in the R11 sample that skew the global fits of the period-luminosity relation. This has a small but non-negligible impact on the global fits using NGC 4258 as a distance scale anchor, but adds a poorly constrained source of systematic error when using the Large Magellanic Cloud as an anchor. I also show that the small Milky Way Cepheid sample with accurate parallax measurements leads to a distance to NGC 4258 that is in tension with the maser distance. I conclude that H0 based on the NGC 4258 maser distance is H0 = 70.6 ± 3.3 kms-1 Mpc-1, compatible within 1s with the recent determination from Planck for the base six-parameter cold dark matter cosmology. If the H-band period-luminosity relation is assumed to be independent of metallicity and the three distance anchors are combined, I find H0 = 72.5 ± 2.5 kms-1 Mpc-1, which differs by 1.9σ from the Planck value. The differences between the Planck results and these estimates of H0 are not large enough to provide compelling evidence for new physics at this stage. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Bonvin C.,Kavli Institute for Cosmology | Bonvin C.,University of Cambridge
Classical and Quantum Gravity | Year: 2014

We present a fully relativistic calculation of the observed galaxy number counts in the linear regime. We show that besides the density fluctuations and redshift-space distortions, various relativistic effects contribute to observations at large scales. These effects all have the same physical origin: they result from the fact that our coordinate system, namely the galaxy redshift and the incoming photons' direction, is distorted by inhomogeneities in our Universe. We then discuss the impact of the relativistic effects on the angular power spectrum and on the two-point correlation function in configuration space. We show that the latter is very well adapted to isolate the relativistic effects since it naturally makes use of the symmetries of the different contributions. In particular, we discuss how the Doppler effect and the gravitational redshift distortions can be isolated by looking for a dipole in the cross-correlation function between a bright and a faint population of galaxies. © 2014 IOP Publishing Ltd.

Cantalupo S.,Kavli Institute for Cosmology
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2010

Current models of galaxy formation lack an efficient and physically constrained mechanism to regulate star formation (SF) in low and intermediate mass galaxies. We argue that the missing ingredient could be the effect of photoionization by local sources on the gas cooling. We show that the soft X-ray and EUV flux generated by SF is able to efficiently remove the main coolants (e.g. He+, O4+ and Fe8+) from the halo gas via direct photoionization. As a consequence, the cooling and accretion time of the gas surrounding star-forming galaxies may increase by one or two orders of magnitude. For a given halo mass and redshift, the effect is directly related to the value of the star formation rate (SFR). Our results suggest that the existence of a critical SFR above which 'cold' mode accretion is stopped, even for haloes with Mvir well below the critical shock-heating mass suggested by previous studies. The evolution of the critical SFR with redshift, for a given halo mass, resembles the respective steep evolution of the observed SFR for z < 1. This suggests that photoionization by local sources would be able to regulate gas accretion and SF, without the need for additional, strong feedback processes. © 2010 The Author. Journal compilation © 2010 RAS.

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