Galaxies

13 pages, 395 KiB

Open AccessEditor’s ChoiceArticle

Applying MOG to Lensing: Einstein Rings, Abell 520 and the Bullet Cluster

by John W. Moffat, Sohrab Rahvar and Viktor T. Toth

Galaxies 2018, 6(2), 43; https://doi.org/10.3390/galaxies6020043 - 2 Apr 2018

Cited by 20 | Viewed by 5121

We investigate gravitational lensing in the context of the MOG modified theory of gravity. Using a formulation of the theory with no adjustable or fitted parameters, we present the MOG equations of motion for slow, nonrelativistic test particles and for ultrarelativistic test particles, [...] Read more.

We investigate gravitational lensing in the context of the MOG modified theory of gravity. Using a formulation of the theory with no adjustable or fitted parameters, we present the MOG equations of motion for slow, nonrelativistic test particles and for ultrarelativistic test particles, such as rays of light. We demonstrate how the MOG prediction for the bending of light can be applied to astronomical observations. Our investigation first focuses on a small set of strong lensing observations where the properties of the lensing objects are found to be consistent with the predictions of the theory. We also present an analysis of the colliding clusters 1E0657-558 (known also as the Bullet Cluster) and Abell 520; in both cases, the predictions of the MOG theory are in good agreement with observation. Full article

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19 pages, 1151 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

The Train Wreck Cluster Abell 520 and the Bullet Cluster 1E0657-558 in a Generalized Theory of Gravitation

by Norman Israel and John Moffat

Galaxies 2018, 6(2), 41; https://doi.org/10.3390/galaxies6020041 - 26 Mar 2018

Cited by 24 | Viewed by 6954

Abstract

A major hurdle for modified gravity theories is to explain the dynamics of galaxy clusters. A case is made for a generalized gravitational theory called Scalar-Tensor-Vector-Gravity (STVG) or MOG (Modified Gravity) to explain merging cluster dynamics. The paper presents the results of a [...] Read more.

A major hurdle for modified gravity theories is to explain the dynamics of galaxy clusters. A case is made for a generalized gravitational theory called Scalar-Tensor-Vector-Gravity (STVG) or MOG (Modified Gravity) to explain merging cluster dynamics. The paper presents the results of a re-analysis of the Bullet Cluster, as well as an analysis of the Train Wreck Cluster in the weak gravitational field limit without dark matter. The King-

β

model is used to fit the X-ray data of both clusters, and the

κ

-maps are computed using the parameters of this fit. The amount of galaxies in the clusters is estimated by subtracting the predicted

κ

-map from the

κ

-map data. The estimate for the Bullet Cluster is that

14.1 %

of the cluster is composed of galaxies. For the Train Wreck Cluster, if the Jee et al. data are used,

25.7 %

of the cluster is composed of galaxies. The baryon matter in the galaxies and the enhanced strength of gravitation in MOG shift the lensing peaks, making them offset from the gas. The work demonstrates that this generalized gravitational theory can explain merging cluster dynamics without dark matter. Full article

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20 pages, 5956 KiB

Open AccessFeature PaperEditor’s ChoiceReview

X-Ray and Gamma-Ray Observations of the Fermi Bubbles and NPS/Loop I Structures

by Jun Kataoka, Yoshiaki Sofue, Yoshiyuki Inoue, Masahiro Akita, Shinya Nakashima and Tomonori Totani

Galaxies 2018, 6(1), 27; https://doi.org/10.3390/galaxies6010027 - 26 Feb 2018

Cited by 49 | Viewed by 8283

Abstract

The Fermi bubbles were possibly created by large injections of energy into the Galactic Center (GC), either by an active galactic nucleus (AGN) or by nuclear starburst more than ~10 Myr ago. However, the origin of the diffuse gamma-ray emission associated with Loop [...] Read more.

The Fermi bubbles were possibly created by large injections of energy into the Galactic Center (GC), either by an active galactic nucleus (AGN) or by nuclear starburst more than ~10 Myr ago. However, the origin of the diffuse gamma-ray emission associated with Loop I, a radio continuum loop spanning across 100° on the sky, is still being debated. The northern-most part of Loop I, known as the North Polar Spur (NPS), is the brightest arm and is even clearly visible in the ROSAT X-ray sky map. In this paper, we present a comprehensive review on the X-ray observations of the Fermi bubbles and their possible association with the NPS and Loop I structures. Using uniform analysis of archival Suzaku and Swift data, we show that X-ray plasma with kT~0.3 keV and low metal abundance (Z~0.2 Z_◉) is ubiquitous in both the bubbles and Loop I and is naturally interpreted as weakly shock-heated Galactic halo gas. However, the observed asymmetry of the X-ray-emitting gas above and below the GC has still not been resolved; it cannot be fully explained by the inclination of the axis of the Fermi bubbles to the Galactic disk normal. We argue that the NPS and Loop I may be asymmetric remnants of a large explosion that occurred before the event that created the Fermi bubbles, and that the soft gamma-ray emission from Loop I may be due to either π⁰ decay of accelerated protons or electron bremsstrahlung. Full article

(This article belongs to the Special Issue Searching for Connections among the Fermi Bubbles, the Galactic Center GeV Excess, and Loop I)

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7 pages, 2281 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Unitary Issues in Some Higher Derivative Field Theories

by Manuel Asorey, Leslaw Rachwal and Ilya Shapiro

Galaxies 2018, 6(1), 23; https://doi.org/10.3390/galaxies6010023 - 14 Feb 2018

Cited by 26 | Viewed by 3682

Abstract

We analyze the unitarity properties of higher derivative quantum field theories which are free of ghosts and ultraviolet singularities. We point out that in spite of the absence of ghosts most of these theories are not unitary. This result confirms the difficulties of [...] Read more.

We analyze the unitarity properties of higher derivative quantum field theories which are free of ghosts and ultraviolet singularities. We point out that in spite of the absence of ghosts most of these theories are not unitary. This result confirms the difficulties of finding a consistent quantum field theory of quantum gravity. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

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8 pages, 333 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Ultra Light Axionic Dark Matter: Galactic Halos and Implications for Observations with Pulsar Timing Arrays

by Ivan De Martino, Tom Broadhurst, S.-H. Henry Tye, Tzihong Chiueh, Hsi-Yu Schive and Ruth Lazkoz

Galaxies 2018, 6(1), 10; https://doi.org/10.3390/galaxies6010010 - 16 Jan 2018

Cited by 20 | Viewed by 4154

Abstract

The cold dark matter (CDM) paradigm successfully explains the cosmic structure over an enormous span of redshifts. However, it fails when probing the innermost regions of dark matter halos and the properties of the Milky Way’s dwarf galaxy satellites. Moreover, the lack of [...] Read more.

The cold dark matter (CDM) paradigm successfully explains the cosmic structure over an enormous span of redshifts. However, it fails when probing the innermost regions of dark matter halos and the properties of the Milky Way’s dwarf galaxy satellites. Moreover, the lack of experimental detection of Weakly Interacting Massive Particle (WIMP) favors alternative candidates such as light axionic dark matter that naturally arise in string theory. Cosmological N-body simulations have shown that axionic dark matter forms a solitonic core of size of ≃150 pc in the innermost region of the galactic halos. The oscillating scalar field associated to the axionic dark matter halo produces an oscillating gravitational potential that induces a time dilation of the pulse arrival time of ≃400 ns/(m

_{B}

/10

^{- 22}

eV) for pulsar within such a solitonic core. Over the whole galaxy, the averaged predicted signal may be detectable with current and forthcoming pulsar timing array telescopes. Full article

(This article belongs to the Special Issue Cosmology and the Quantum Vacuum)

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8 pages, 1976 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

The Variable Rotation Measure Distribution in 3C 273 on Parsec Scales

by John Wardle

Galaxies 2018, 6(1), 5; https://doi.org/10.3390/galaxies6010005 - 5 Jan 2018

Cited by 22 | Viewed by 4853

Abstract

We briefly review how opacity affects the observed polarization in synchrotron emitting jets. We show some new multi-frequency observations of 3C 273 made with the VLBA in 1999–2000, which add significantly to the available rotation measure (RM) observations of this source. Our findings [...] Read more.

We briefly review how opacity affects the observed polarization in synchrotron emitting jets. We show some new multi-frequency observations of 3C 273 made with the VLBA in 1999–2000, which add significantly to the available rotation measure (RM) observations of this source. Our findings can be summarized as follows: (1) The transverse gradient in RM is amply confirmed. This implies a toroidal component to the magnetic field, which in turn requires a current of 10¹⁷–10¹⁸ A flowing down the jet. (2) The net magnetic field in the jet is longitudinal; however, whether or not the longitudinal component is vector-ordered is an open question. (3) The RM distribution is variable on timescales of months to years. We attribute this to the motion of superluminal components behind a turbulent Faraday screen that surrounds the jet. (4) Finally, we suggest that Faraday rotation measurements at higher resolution and higher frequencies, with the Event Horizon Telescope, may enable useful constraints to be placed on the accretion rate onto the central black hole. Full article

(This article belongs to the Special Issue Polarised Emission from Astrophysical Jets)

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14 pages, 1281 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Microvariability in BL Lacertae: “Zooming” into the Innermost Blazar Regions

by Gopal Bhatta and James R. Webb

Galaxies 2018, 6(1), 2; https://doi.org/10.3390/galaxies6010002 - 3 Jan 2018

Cited by 28 | Viewed by 4529

Abstract

In this work, we present the results of our multi-band microvariability study of the famous blazar BL Lac. We performed microvariablity observations of the source in the optical VRI bands for four nights in 2016. We studied the intranight flux and spectral variability [...] Read more.

In this work, we present the results of our multi-band microvariability study of the famous blazar BL Lac. We performed microvariablity observations of the source in the optical VRI bands for four nights in 2016. We studied the intranight flux and spectral variability of the source in detail with an objective to characterize microvariability in the blazars, a frequently observed phenomenon in blazars. The results show that the source often displays a fast flux variability with an amplitude as large as ~0.2 magnitude within a few hours, and that the color variability in the similar time scales can be characterized as “bluer-when-brighter” trend. We also observed markedly curved optical spectrum during one of the nights. Furthermore, the correlation between multi-band emission shows that in general the emission in all the bands are highly correlated; and in one of the nights V band emission was found to lead the I band emission by ~13 min. The search for characteristic timescale using z-transformed auto-correlation function and the structure function analyses reveals characteristic timescale of ~50 min in one of the R band observations. We try to explain the observed results in the context of the passage of shock waves through the relativistic outflows in blazars. Full article

(This article belongs to the Special Issue Microvariability of Blazars)

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