Browsing by Subject "Astrophysics"

Now showing 1 - 17 of 17
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    Are braneworlds born isotropic?
    (2004) Dunsby, Peter K S; Goheer, Naureen; Bruni, Marco; Coley, Alan
    It has recently been suggested that an isotropic singularity may be a generic feature of brane cosmologies, even in the inhomogeneous case. Using the covariant and gauge-invariant approach we present a detailed analysis of linear perturbations of the isotropic model ${cal F}_b$ which is a past attractor in the phase space of homogeneous Bianchi models on the brane. We find that for matter with an equation of state parameter $gamma > 1$, the dimensionless variables representing generic anisotropic and inhomogeneous perturbations decay as $tto 0$, showing that the model ${cal F}_b$ is asymptotically stable in the past. We conclude that brane universes are born with isotropy naturally built-in, contrary to standard cosmology. The observed large-scale homogeneity and isotropy of the universe can therefore be explained as a consequence of the initial conditions if the brane-world paradigm represents a description of the very early universe. Comment: Changed to match published version
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    Atomic processes in nova shells
    (1990) Smits, Derck Peter; Warner, Brian
    The unusual spectra of the spatially-resolved nova shells of DQ Her, T Aur and CP Pup are reviewed. Because calculations for these conditions have not been made, recombination models of HI, HeI and CNO from a neutral to a doubly ionized state were constructed. The models are described and the results presented for densities at various temperatures. A photoionization model is also described. The effects of non-uniform density distributions in nova shells have been investigated and the observational consequences discussed. A model of the CP Pup shell is described and the evolution of this plasma during the nova's constant luminosity phase traced. The properties of neon novae are summarized and compared with the features seen in the optical spectra of old classical novae. Enhanced neon abundances are used in the model of the CP Pup Shell to investigate its effect on the evolution of the shell. Finally, the results of some near-infrared observations made on the shells of CP Pup, RR Pic and T Pyx are reported.
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    The DA + dM eclipsing binary : EC13471-1258
    (2008) Brownstone, Michael R; Donoghue, Darragh O'; Kilkenny, Dave
    Includes abstract. Includes bibliographical references (leaves 76-77).
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    First MeerKAT HI survey results mapping large scale structures hidden behind the Milky Way out to z = 0.08
    (2023) Steyn, Nadia; Kraan-Korteweg, Renee Christine; Frank Bradley
    SKA pathfinders (e.g., ASKAP and MeerKAT) are performing large HI surveys, surpassing previous-generation radio surveys in angular resolution, volume, and depth — going nearly two orders of magnitude deeper. The vast amount of data being produced by these telescopes creates a need for new tools and techniques, such as automated source-extraction. One particular science goal is to trace the large scale structure of galaxies in the local Universe behind the Zone of Avoidance (ZoA). This is difficult to do in the optical and infrared bands because of the thick dust and stellar crowding along the plane of the Milky Way. HI surveys are ideal because the 21 cm spectral line emission of neutral hydrogen (HI) atoms is unaffected by these features. This dissertation presents two HI surveys in the ZoA: the MeerKAT16 Early Science project, conducted before the completion of the full array, with 16-dish subarrays and the ROACH-32k correlator; and later, the Galactic Plane Legacy Survey (GPS), a larger MeerKAT survey project utilising 60-dish subarrays and the SKARAB-4k correlator. The motivation of the MeerKAT16 survey was two-fold: serving as a pilot project for the envisioned large MeerKAT64 Vela Supercluster (VSCL) survey; and a test-bed for optimising galaxy identification processes using automated pipelines. The survey aimed to map the rich galaxy cluster VC04, embedded in one of the prospective VSCL walls. We searched two mosaic cubes for HI sources by means of a deep visual search, and then using the Source Finding Application (SoFiA; version 1.3.2). The purpose of using both methods was to understand and optimise the SoFiA pipeline on real data, which is important for the development of source finding strategies for future large surveys. We catalogued 119 reliable galaxy detections (and an additional 37 candidates with lower certainty) within the early science data. SoFiA found galaxies all the way at the VSCL distance (V(hel) ≈ 18 000 km/s), where we detected hints of two walls, and HI deficiency in the centre of VC04. GPS surveyed a long narrow strip (|b| ≤ 2°) along the Galactic plane, with the aim of penetrating the most obscured part of the ZoA. A segment of the data spanning 302° ≤ l ≤ 332° was reduced, imaged, and analysed as part of this project. This region is interesting because it encompasses the Great Attractor (GA) — a massive overdensity highly influential to the local flow of galaxies. Aided by the newer version of SoFiA (v2.3.1), we found 477 galaxy candidates, most of which are new discoveries, and performed an in-depth comparison against the largest systematic HI survey covering the southern ZoA up to this point — the Parkes HIZOA survey. Additionally, we compared our results to simulations that follow the method used for the SKA HI science case. The GPS redshift distribution reveals a striking overdensity at the GA distance (V(hel) ≈ 4000 km/s), inconsistent with a uniform galaxy distribution, whereas the high-redshift end is more underdense than predicted by the simulation. These deep interferometric HI surveys provided a new glimpse of highly interesting structures crossing the Galactic plane, demonstrating that large scale structure can be mapped even in the deepest part of the ZoA. The success of MeerKAT16 and GPS is a preview of the HI science potential of the upcoming SKA, which will surpass MeerKAT in depth, sensitivity and resolution.
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    Galaxy groups in the updated 2MRS using a graph-theory based friends-of-friends algorithm
    (2020) Lambert, Trystan; Kraan-Korteweg, Renee Christine; Jarrett, Thomas H.
    A galaxy group catalogue for the recently-completed 2MASS Redshift Survey (2MRS, Macri et al., 2019) is presented which consists of 44572 redshifts, including 1041 new measurements for galaxies mostly located within the Zone of Avoidance. The galaxy group catalogue is generated using a novel, graph-theory based, modified version of the Friends-of-Friends algorithm. Several graph-theory examples are presented throughout this paper, and include a new method to identify substructures within groups. The results and graph-theory methods have been thoroughly interrogated against previous 2MRS group catalogues and a Theoretical Astrophysical Observatory (TAO) mock by making use of cutting-edge visualization techniques including immersive facilities, a digital planetarium, and virtual reality. This has resulted in a stable and robust catalogue with on-sky positions and line-of-sight distances within 0.5 Mpc and 2 Mpc of the group respectively. It has recovered all major groups and clusters. The final catalogue consists of 3022 groups, resulting in the most complete “whole-sky” galaxy group catalogue to date. The 3D-positions of the groups are presented, as well as their luminosity and comoving distances, observed and corrected number of members, richness metric, velocity dispersion, and estimates of R200 and M200. Three additional catalogues are provided: 2MRS galaxies found in groups, a catalogue of subgroups, and the catalogue of 687 new group candidates which had no counterparts in previous 2MRS-based analyses.
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    Heavy neutrino matter at finite temperature
    (1998) Joubert, Charles Erasmus
    We study, for a system of massive fermions which interact only gravitationally, the phase transition that is associated with gravitational collapse. It is shown that by cooling a non-degenerate gas of massive neutrinos below some critical temperature, a condensed phase emerges, consisting of quasi-degenerate super-massive neutrino stars. These compact dark objects could play an important role in structure formation in this universe, as they might in fact provide the seeds for galactic nuclei and quasi-stellar objects.
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    Measurement of exclusive γ γ → W + W − production and search for exclusive Higgs boson production in p p collisions at s = 8     TeV using the ATLAS detector
    (2016) Aaboud, M; Aad, G; Abbott, B; Abdallah, J; Abdinov, O; Abeloos, B; Aben, R; AbouZeid, O S; Abraham, N L; Abramowicz, H; Abreu, H; Abreu, R; Abulaiti, Y; Acharya, B S; Adamczyk, L; Adams, D L; Adelman, J; Adomeit, S; Adye, T; Affolder, A A; Agatonovic-Jovin, T; Agricola, J; Aguilar-Saavedra, J A; Ahlen, S P; Ahmadov, F; Aielli, G; Akerstedt, H; Åkesson, T P A; Akimov, A V; Alberghi, G L; Albert, J; Albrand, S
    Searches for exclusively produced W boson pairs in the process p p ( γ γ ) → p W + W - p and an exclusively produced Higgs boson in the process p p ( g g ) → p H p have been performed using e ± μ ∓ final states. These measurements use 20.2 fb - 1 of p p collisions collected by the ATLAS experiment at a center-of-mass energy √ s = 8 TeV at the LHC. Exclusive production of W + W - consistent with the Standard Model prediction is found with 3.0 σ significance. The exclusive W + W - production cross section is determined to be σ ( γ γ → W + W - → e ± μ ∓ X ) = 6.9 ± 2.2 ( stat ) ± 1.4 ( sys ) fb , in agreement with the Standard Model prediction. Limits on anomalous quartic gauge couplings are set at 95% confidence level as - 1.7 × 10 - 6 < a W 0 / Λ 2 < 1.7 × 10 - 6 GeV - 2 and - 6.4 × 10 - 6 < a W C / Λ 2 < 6.3more » × 10 - 6 GeV - 2 . A 95% confidence-level upper limit on the total production cross section for an exclusive Higgs boson is set to 1.2 pb.« less
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    Measurement of the W ± Z boson pair-production cross section in pp collisions at s = 13  TeV with the ATLAS detector
    (2016) Aaboud, M; Aad, G; Abbott, B; Abdallah, J; Abdinov, O; Abeloos, B; Aben, R; AbouZeid, O S; Abraham, N L; Abramowicz, H; Abreu, H; Abreu, R; Abulaiti, Y; Acharya, B S; Adamczyk, L; Adams, D L; Adelman, J; Adomeit, S; Adye, T; Affolder, A A; Agatonovic-Jovin, T; Agricola, J; Aguilar-Saavedra, J A; Ahlen, S P; Ahmadov, F; Aielli, G; Akerstedt, H; Åkesson, T P A; Akimov, A V; Alberghi, G L; Albert, J; Albrand, S
    Here, the production of W ±Z events in proton–proton collisions at a centre-of-mass energy of 13 TeV13 TeV is measured with the ATLAS detector at the LHC. The collected data correspond to an integrated luminosity of 3.2 fb –1. The W ±Z candidates are reconstructed using leptonic decays of the gauge bosons into electrons or muons.
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    Measurement of the correlation between the polar angles of leptons from top quark decays in the helicity basis at √s = 7 TeV using the ATLAS detector
    (2016) Aad, G; Abbott, B; Abdallah, J; Abdinov, O; Aben, R; Abolins, M; AbouZeid, O S; Abramowicz, H; Abreu, H; Abreu, R; Abulaiti, Y; Acharya, B S; Adamczyk, L; Adams, D L; Adelman, J; Adomeit, S; Adye, T; Affolder, A A; Agatonovic-Jovin, T; Agricola, J; Aguilar-Saavedra, J A; Ahlen, S P; Ahmadov, F; Aielli, G; Akerstedt, H; Åkesson, T P A; Akimov, A V; Alberghi, G L; Albert, J; Albrand, S; Alconada Verzini, M J; Aleksa, M
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    Measurement of the cross section for inclusive isolated-photon production in pp collisions at s√=13 TeV using the ATLAS detector
    (2017) Aaboud, M; Aad, G; Abbott, B; Abdallah, J; Abdinov, O; Abeloos, B; Abidi, S H; AbouZeid, O S; Abraham, N L; Abramowicz, H; Abreu, H; Abreu, R; Abulaiti, Y; Acharya, B S; Adachi, S; Adamczyk, L; Adams, D L; Adelman, J; Adersberger, M; Adye, T; Affolder, A A; Agatonovic-Jovin, T; Agheorghiesei, C; Aguilar-Saavedra, J A; Ahlen, S P; Ahmadov, F; Aielli, G; Akatsuka, S; Akerstedt, H; Åkesson, T P A; Akimov, A V; Alberghi, G L
    Here, inclusive isolated-photon production in pp collisions at a centre-of-mass energy of 13TeV is studied with the ATLAS detector at the LHC using a data set with an integrated luminosity of 3.2fb -1. The cross section is measured as a function of the photon transverse energy above 125GeV in different regions of photon pseudorapidity. Next-to-leading-order perturbative QCD and Monte Carlo event-generator predictions are compared to the cross-section measurements and provide an adequate description of the data.
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    Measurement of the total cross section from elastic scattering in pp collisions at s = 8   TeV with the ATLAS detector
    (2016) Aaboud, M; Aad, G; Abbott, B; Abdallah, J; Abdinov, O; Abeloos, B; Aben, R; AbouZeid, O S; Abraham, N L; Abramowicz, H; Abreu, H; Abreu, R; Abulaiti, Y; Acharya, B S; Adachi, S; Adamczyk, L; Adams, D L; Adelman, J; Adomeit, S; Adye, T; Affolder, A A; Agatonovic-Jovin, T; Agricola, J; Aguilar-Saavedra, J A; Ahlen, S P; Ahmadov, F; Aielli, G; Akerstedt, H; Åkesson, T P A; Akimov, A V; Alberghi, G L; Albert, J
    A measurement of the total $pp$ cross section at the LHC at $sqrt{s}=8$ TeV is presented. An integrated luminosity of $500$ $mu$b${-1}$ was accumulated in a special run with high-$beta{star}$ beam optics to measure the differential elastic cross section as a function of the Mandelstam momentum transfer variable $t$. The measurement is performed with the ALFA sub-detector of ATLAS. Using a fit to the differential elastic cross section in the $-t$ range from $0.014$ GeV$2$ to $0.1$ GeV$2$ to extrapolate $trightarrow 0$, the total cross section, $sigma_{mathrm{tot}}(pprightarrow X)$, is measured via the optical theorem to be: $sigma_{mathrm{tot}}(pprightarrow X) = {96.07} ; pm 0.18 ; ({{stat.}}) pm 0.85 ; ({{exp.}}) pm 0.31 ; ({extr.}) ; {mb} ;,$ where the first error is statistical, the second accounts for all experimental systematic uncertainties and the last is related to uncertainties in the extrapolation $trightarrow 0$. In addition, the slope of the exponential function describing the elastic cross section at small $t$ is determined to be $B = 19.74 pm 0.05 ; ({{stat.}}) pm 0.23 ; ({{syst.}}) ; {GeV}{-2}$.
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    Multicolour observations, inhomogeneity and evolution
    (2001) Hellaby, Charles
    We propose a method of testing source evolution theories that is independent of the effects of inhomogeneity, and thus complementary to other studies of evolution. It is suitable for large scale sky surveys, and the new generation of large telescopes. In an earlier paper it was shown that basic cosmological observations -luminosity versus redshift, area distance versus redshift and number counts versus redshift -cannot separate the effects of cosmic inhomogeneity, cosmic evolution and source evolution. We here investigate multicolour observations, and show that by comparing luminosity versus redshift in two or more colours, contraints can be placed on source evolution even if unknown source evolution is present, providing an important test of evolution theories that is complementary to present methods. However, number counts in different colours versus redshift are not useful in separating the effects of source evolution and inhomogeneity.
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    Overdensities of SMGs around WISE-selected, ultraluminous, high-redshift AGNs
    (2017) Jones, Suzy F; Blain, Andrew W; Assef, Roberto J; Eisenhardt, Peter; Lonsdale, Carol; Condon, James; Farrah, Duncan; Tsai, Chao-Wei; Bridge, Carrie; Wu, Jingwen; Wright, Edward L; Jarrett, Tom
    We investigate extremely luminous dusty galaxies in the environments around Wide-field Infrared Survey Explorer (WISE)-selected hot dust-obscured galaxies (Hot DOGs) and WISE/radio-selected active galactic nuclei (AGNs) at average redshifts of z = 2.7 and 1.7, respectively. Previous observations have detected overdensities of companion submillimetre-selected sources around 10 Hot DOGs and 30 WISE/radio AGNs, with overdensities of similar to 2-3 and similar to 5-6, respectively. We find that the space densities in both samples to be overdense compared to normal star-forming galaxies and submillimetre galaxies (SMGs) in the Submillimetre Common-User Bolometer Array 2 (SCUBA-2) Cosmology Legacy Survey (S2CLS). Both samples of companion sources have consistent mid-infrared (mid-IR) colours and mid-IR to submm ratios as SMGs. The brighter population around WISE/radio AGNs could be responsible for the higher overdensity reported. We also find that the star formation rate densities are higher than the field, but consistent with clusters of dusty galaxies. WISE-selected AGNs appear to be good signposts for protoclusters at high redshift on arcmin scales. The results reported here provide an upper limit to the strength of angular clustering using the two-point correlation function. Monte Carlo simulations show no angular correlation, which could indicate protoclusters on scales larger than the SCUBA-2 1.5-arcmin scale maps.
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    Search for heavy resonances decaying to a Z boson and a photon in pp collisions at √s = 13 TeV with the ATLAS detector
    (2017) Aaboud, M; Aad, G; Abbott, B; Abdallah, J; Abdinov, O; Abeloos, B; Aben, R; AbouZeid, O S; Abraham, N L; Abramowicz, H; Abreu, H; Abreu, R; Abulaiti, Y; Acharya, B S; Adachi, S; Adamczyk, L; Adams, D L; Adelman, J; Adomeit, S; Adye, T; Affolder, A A; Agatonovic-Jovin, T; Agricola, J; Aguilar-Saavedra, J A; Ahlen, S P; Ahmadov, F; Aielli, G; Akerstedt, H; Åkesson, T P A; Akimov, A V; Alberghi, G L; Albert, J
    This Letter presents a search for new resonances with mass larger than 250 GeV, decaying to a Z boson and a photon. The dataset consists of an integrated luminosity of 3.2 fb−1 of pp collisions collected at √s = 13 TeV with the ATLAS detector at the Large Hadron Collider. The Z bosons are identified through their decays either to charged, light, lepton pairs (e+e−, μ+μ−) or to hadrons. The data are found to be consistent with the expected background in the whole mass range investigated and upper limits are set on the production cross section times decay branching ratio to Zγ of a narrow scalar boson with mass between 250 GeV and 2.75 TeV.
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    Search for metastable heavy charged particles with large ionization energy loss in p p collisions at s = 13     TeV using the ATLAS experiment
    (2016) Aaboud, M; Aad, G; Abbott, B; Abdallah, J; Abdinov, O; Abeloos, B; Aben, R; AbouZeid, O S; Abraham, N L; Abramowicz, H; Abreu, H; Abreu, R; Abulaiti, Y; Acharya, B S; Adamczyk, L; Adams, D L; Adelman, J; Adomeit, S; Adye, T; Affolder, A A; Agatonovic-Jovin, T; Agricola, J; Aguilar-Saavedra, J A; Ahlen, S P; Ahmadov, F; Aielli, G; Akerstedt, H; Åkesson, T P A; Akimov, A V; Alberghi, G L; Albert, J; Albrand, S
    This paper presents a search for massive charged long-lived particles produced in pp collisions at [sqrt]s=13 TeV at the LHC using the ATLAS experiment. The data set used corresponds to an integrated luminosity of 3.2 fb-2. Many extensions of the Standard Model predict the existence of massive charged long-lived particles, such as R-hadrons. These massive particles are expected to be produced with a velocity significantly below the speed of light, and therefore to have a specific ionization higher than any Standard Model particle of unit charge at high momenta. The Pixel subsystem of the ATLAS detector is used to measure the ionization energy loss of reconstructed charged particles and to search for such highly ionizing particles. The search presented here has much greater sensitivity than a similar search performed using the ATLAS detector in the [sqrt]s=8 TeV data set, thanks to the increase in expected signal cross section due to the higher center-of-mass energy of collisions, to an upgraded detector with a new silicon layer close to the interaction point, and to analysis improvements. No significant deviation from Standard Model background expectations is observed, and lifetime-dependent upper limits on R-hadron production cross sections and masses are set. Gluino R-hadrons with lifetimes above 0.4 ns and decaying to qq plus a 100 GeV neutralino are excluded at the 95% confidence level, with lower mass limit ranging between 740 and 1590 GeV. In the case of stable R-hadrons the lower mass limit at the 95% confidence level is 1570 GeV.
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    Search for the electroweak production of supersymmetric particles in s√=8 TeV pp collisions with the ATLAS detector
    (2016) Aad, G; Abbott, B; Abdallah, J; Abdinov, O; Aben, R; Abolins, M; AbouZeid, O S; Abramowicz, H; Abreu, H; Abreu, R; Abulaiti, Y; Acharya, B S; Adamczyk, L; Adams, D L; Adelman, J; Adomeit, S; Adye, T; Affolder, A A; Agatonovic-Jovin, T; Agricola, J; Aguilar-Saavedra, J A; Ahlen, S P; Ahmadov, F; Aielli, G; Akerstedt, H; Åkesson, T P A; Akimov, A V; Alberghi, G L; Albert, J; Albrand, S; Alconada Verzini, M J; Aleksa, M
    The ATLAS experiment has performed extensive searches for the electroweak production of charginos, neutralinos and staus. This article summarizes and extends the search for electroweak supersymmetry with new analyses targeting scenarios not covered by previously published searches. New searches use vector-boson fusion production, initial-state radiation jets, and low-momentum lepton final states, as well as multivariate analysis techniques to improve the sensitivity to scenarios with small mass splittings and low-production cross-sections. Results are based on 20 fb⁻¹ of proton-proton collision data at s√=8 TeV recorded with the ATLAS experiment at the Large Hadron Collider. No significant excess beyond Standard Model expectations is observed. The new and existing searches are combined and interpreted in terms of 95% confidence-level exclusion limits in simplified models, where a single production process and decay mode is assumed, as well as within phenomenological supersymmetric models.
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    The WiFeS Ionized Gas Kinematics of the Edge-on Galaxy J1447-17
    (2021) Mputle, Omphemetse Kelebogile; Mogotsi, Moses; Bershady, Matthew
    The presence of extraplanar diffuse ionized gas (eDIG) and galactic winds and outflows in the late Universe has been observed to be ubiquitous in nearby star-forming spiral galaxies. Optical integral field spectroscopic data from the Wide Field Spectrograph (WiFeS) on the 2.3m Australian National University Telescope is used to determine if the nearby edge-on galaxy J1447-17 from the SINGG survey hosts eDIG, winds or outflows. Tests used to detect their existence include the presence of multiple components in the Hα signal, an increased velocity dispersion in the off-plane region relative to the disk, residuals in the velocity asymmetry map and an enhanced line-ratio ([N II]λ6583/Hα) corresponding to a decrease in the Hα intensity in the case of eDIG. The absence of profiles meeting the line-ratio condition and absence of multiple Hα components compounded with strong residuals in the velocity asymmetry map are not indicative of the galaxy possessing diffuse ionized gas. Ionization cone signatures in the velocity dispersion map together with the velocity asymmetry map and greater dispersion in the off-plane regions than in the disk indicate that the galaxy hosts a galactic wind.