Galaxies

Research

8 pages, 2086 KiB

Open AccessFeature PaperArticle

Constraints on the Distribution of Gas and Young Stars in the Galactic Centre in the Context of Interpreting Gamma Ray Emission Features

by Steven N. Longmore and J. M. Diederik Kruijssen

Galaxies 2018, 6(2), 55; https://doi.org/10.3390/galaxies6020055 - 15 May 2018

Cited by 6 | Viewed by 3818

Abstract

Gamma ray observations have found evidence of an extremely energetic outflow emanating from the Galactic Centre, and an ‘excess’ of emission at GeV energies towards the Galactic Centre over that expected from current models. Determining whether the outflow is AGN- or star formation-driven, [...] Read more.

Gamma ray observations have found evidence of an extremely energetic outflow emanating from the Galactic Centre, and an ‘excess’ of emission at GeV energies towards the Galactic Centre over that expected from current models. Determining whether the outflow is AGN- or star formation-driven, and whether the ‘excess’ is astrophysical in origin or requires new physics (e.g., self-annihilation of dark matter), requires the accurate modelling of the expected energy injection from astrophysical sources and the subsequent interaction with the surrounding environment. We briefly summarise current constraints on the distribution of gas and young stars in the inner few hundred parsecs of the Galaxy that can be included in future 2D and 3D modelling of the astrophysical gamma ray emission. The key points to highlight with respect to predominantly axisymmetric models currently in use are: (i) the distribution of dense gas, young stars and interstellar radiation field is highly asymmetric around the Galactic Centre; (ii) star formation is almost exclusively constrained to a Galactocentric radius of ∼100 pc; and (iii) the star formation rate in this region has been constant at ≲0.1 M

_{⊙}

yr

^{- 1}

to within a factor of 2 over the last ∼5 Myr. 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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9 pages, 604 KiB

Open AccessFeature PaperArticle

Hadronic Models of the Fermi Bubbles: Future Perspectives

by Soebur Razzaque and Lili Yang

Galaxies 2018, 6(2), 47; https://doi.org/10.3390/galaxies6020047 - 12 Apr 2018

Cited by 12 | Viewed by 3741

Abstract

The origin of sub-TeV gamma rays detected by Fermi-LAT from the Fermi bubbles at the Galactic center is still unknown. In a hadronic model, acceleration of protons and/or nuclei and their subsequent interactions with gas in the bubble volume can produce observed [...] Read more.

The origin of sub-TeV gamma rays detected by Fermi-LAT from the Fermi bubbles at the Galactic center is still unknown. In a hadronic model, acceleration of protons and/or nuclei and their subsequent interactions with gas in the bubble volume can produce observed gamma rays. Such interactions naturally produce high-energy neutrinos, and a detection of those can discriminate between a hadronic and a leptonic origin of gamma rays. Additional constraints on the Fermi bubbles gamma-ray flux in the TeV range from recent HAWC observations restrict hadronic model parameters, which in turn disfavor Fermi bubbles as the origin of a large fraction of neutrino events detected by IceCube along the bubble directions. We revisit our hadronic model and discuss future constraints on parameters from observations in very high-energy gamma rays and neutrinos. 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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9 pages, 713 KiB

Open AccessFeature PaperArticle

Unveiling the Origin of the Fermi Bubbles

by H.-Y. Karen Yang, Mateusz Ruszkowski and Ellen G. Zweibel

Galaxies 2018, 6(1), 29; https://doi.org/10.3390/galaxies6010029 - 28 Feb 2018

Cited by 33 | Viewed by 5977

Abstract

The Fermi bubbles, two giant structures above and below the Galactic center (GC), are among the most important discoveries of the Fermi Gamma-ray Space Telescope. Studying their physical origin has been providing valuable insights into cosmic-ray transport, the Galactic magnetic field, and [...] Read more.

The Fermi bubbles, two giant structures above and below the Galactic center (GC), are among the most important discoveries of the Fermi Gamma-ray Space Telescope. Studying their physical origin has been providing valuable insights into cosmic-ray transport, the Galactic magnetic field, and past activity at the GC in the Milky Way galaxy. Despite their importance, the formation mechanism of the bubbles is still elusive. Over the past few years, there have been numerous efforts, both observational and theoretical, to uncover the nature of the bubbles. In this article, we present an overview of the current status of our understanding of the bubbles’ origin, and discuss possible future directions that will help to distinguish different scenarios of bubble formation. Full article

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

Review

Jump to: Research, Other

18 pages, 2125 KiB

Open AccessFeature PaperEditor’s ChoiceReview

Multi-Wavelength Observations and Modeling of Loop I

by Yuri Shchekinov

Galaxies 2018, 6(2), 62; https://doi.org/10.3390/galaxies6020062 - 7 Jun 2018

Cited by 15 | Viewed by 3568

Abstract

The article aims to overview the origin and current dynamical state of a giant structure on the northern galactic sky—the radio Loop I extending from ≈

- 45^{\circ}

to ≈

+ 45^{\circ}

in longitudes and up to ≈

80^{\circ}

in [...] Read more.

The article aims to overview the origin and current dynamical state of a giant structure on the northern galactic sky—the radio Loop I extending from ≈

- 45^{\circ}

to ≈

+ 45^{\circ}

in longitudes and up to ≈

80^{\circ}

in latitudes over the Galactic center (GC). The main issue addressed here is a description of possible sources of mass and energy able to build up the Loop I and associated structures seen in X-ray, 21 cm, far infrared (FIR), and maintain them on long timescales. This region of the sky is highly crowded, such that contaminations from many projected structures can be tangled, and not always current direct observations look sufficient to disentangle them. At such conditions indirect arguments based on analysis of underlying star formation (SF) rate, morphological features in radio, X-ray and FIR may be important for understanding the origin of Loop I. Simple estimates show that the observed rather weak SF rate is able to create and maintain Loop I, and under certain circumstances can provide the observed east-west asymmetry. However, an explanation of an apparent coexistence of morphologically similar HI and FIR filaments close to Loop I is challenging, indicating that most likely they may belong to the foreground. Recently discovered absorptions in diffuse interstellar bands seem to confirm this picture. 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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16 pages, 9347 KiB

Open AccessFeature PaperReview

Large-Scale Features of the Radio Sky and a Model for Loop I

by Clive Dickinson

Galaxies 2018, 6(2), 56; https://doi.org/10.3390/galaxies6020056 - 22 May 2018

Cited by 22 | Viewed by 6150

Abstract

The large-scale radio/microwave sky has been mapped over a range of frequencies from tens of MHz to tens of GHz, in intensity and polarization. The emission is primarily synchrotron radiation from cosmic ray electrons spiralling in the Galactic magnetic field, in addition to [...] Read more.

The large-scale radio/microwave sky has been mapped over a range of frequencies from tens of MHz to tens of GHz, in intensity and polarization. The emission is primarily synchrotron radiation from cosmic ray electrons spiralling in the Galactic magnetic field, in addition to free–free radiation from warm ionized gas. Away from the Galactic plane, the radio sky is dominated by very large (tens of degrees) loops, arcs, spurs and filaments, including the well-known North Polar Spur (NPS), which forms part of Loop I with a diameter of ∼

120^{\circ}

. In polarization data, such features are often more discernible due to their high polarization fractions suggesting ordered magnetic fields, while the polarization angles suggest fields that are parallel to the filament. The exact nature of these features are poorly understood. We give a brief review of these features, focussing on the NPS/Loop I, whose polarization directions can be explained using a simple expanding shell model, placing the centre of the shell at a distance of ∼100–200 pc. However, there is significant evidence for a larger distance in the range ∼500–1000 pc, while larger distances including the Galactic Centre are unlikely. We also briefly discuss other large-scale curiosities in the radio sky such as the microwave haze and anti-correlation of H

α

filaments and synchrotron polarized intensity. 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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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 46 | Viewed by 7811

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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Other

Jump to: Research, Review

8 pages, 221 KiB

Open AccessFeature PaperCommentary

Positron Transport and Annihilation in the Galactic Bulge

by Fiona Helen Panther

Galaxies 2018, 6(2), 39; https://doi.org/10.3390/galaxies6020039 - 21 Mar 2018

Cited by 15 | Viewed by 4075

Abstract

The annihilation of positrons in the Milky Way Galaxy has been observed for ∼50 years; however, the production sites of these positrons remains hard to identify. The observed morphology of positron annihilation gamma-rays provides information on the annihilation sites of these Galactic positrons. [...] Read more.

The annihilation of positrons in the Milky Way Galaxy has been observed for ∼50 years; however, the production sites of these positrons remains hard to identify. The observed morphology of positron annihilation gamma-rays provides information on the annihilation sites of these Galactic positrons. It is understood that the positrons responsible for the annihilation signal originate at MeV energies. The majority of sources of MeV positrons occupy the star-forming thin disk of the Milky Way. If positrons propagate far from their sources, we must develop accurate models of positron propagation through all interstellar medium (ISM) phases in order to reveal the currently uncertain origin of these Galactic positrons. On the other hand, if positrons annihilate close to their sources, an alternative source of MeV positrons with a distribution that matches the annihilation morphology must be identified. In this work, I discuss the various models that have been developed to understand the origin of the

511 keV

line from the direction of the Galactic bulge, and the propagation of positrons in the ISM. Full article

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

10 pages, 2097 KiB

Open AccessFeature PaperConference Report

A Way Out of the Bubble Trouble?—Upon Reconstructing the Origin of the Local Bubble and Loop I via Radioisotopic Signatures on Earth

by Michael Mathias Schulreich, Dieter Breitschwerdt, Jenny Feige and Christian Dettbarn

Galaxies 2018, 6(1), 26; https://doi.org/10.3390/galaxies6010026 - 25 Feb 2018

Cited by 15 | Viewed by 4401

Abstract

Deep-sea archives all over the world show an enhanced concentration of the radionuclide ⁶⁰Fe, isolated in layers dating from about 2.2 Myr ago. Since this comparatively long-lived isotope is not naturally produced on Earth, such an enhancement can only be attributed to [...] Read more.

Deep-sea archives all over the world show an enhanced concentration of the radionuclide ⁶⁰Fe, isolated in layers dating from about 2.2 Myr ago. Since this comparatively long-lived isotope is not naturally produced on Earth, such an enhancement can only be attributed to extraterrestrial sources, particularly one or several nearby supernovae in the recent past. It has been speculated that these supernovae might have been involved in the formation of the Local Superbubble, our Galactic habitat. Here, we summarize our efforts in giving a quantitative evidence for this scenario. Besides analytical calculations, we present results from high-resolution hydrodynamical simulations of the Local Superbubble and its presumptive neighbor Loop I in different environments, including a self-consistently evolved supernova-driven interstellar medium. For the superbubble modeling, the time sequence and locations of the generating core-collapse supernova explosions are taken into account, which are derived from the mass spectrum of the perished members of certain, carefully preselected stellar moving groups. The release and turbulent mixing of ⁶⁰Fe is followed via passive scalars, where the yields of the decaying radioisotope were adjusted according to recent stellar evolution calculations. The models are able to reproduce both the timing and the intensity of the ⁶⁰Fe excess observed with rather high precision. We close with a discussion of recent developments and give future perspectives. 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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