Fermi Bubbles Research Articles

Sources and Radiations of the Fermi Bubbles

Two enigmatic gamma-ray features in the galactic central region, known as Fermi Bubbles (FBs), were found from Fermi-LAT data. An energy release, (e.g., by tidal disruption events in the Galactic Center, GC), generates a cavity with a shock that expands into the local ambient medium of the galactic halo. A decade or so ago, a phenomenological model of the FBs was suggested as a result of routine star disruptions by the supermassive black hole in the GC which might provide enough energy for large-scale structures, like the FBs. In 2020, analytical and numerical models of the FBs as a process of routine tidal disruption of stars near the GC were developed; these disruption events can provide enough cumulative energy to form and maintain large-scale structures like the FBs. The disruption events are expected to be 10−4∼10−5yr−1, providing an average power of energy release from the GC into the halo of E˙∼3×1041 erg s−1, which is needed to support the FBs. Analysis of the evolution of superbubbles in exponentially stratified disks concluded that the FB envelope would be destroyed by the Rayleigh–Taylor (RT) instabilities at late stages. The shell is composed of swept-up gas of the bubble, whose thickness is much thinner in comparison to the size of the envelope. We assume that hydrodynamic turbulence is excited in the FB envelope by the RT instability. In this case, the universal energy spectrum of turbulence may be developed in the inertial range of wavenumbers of fluctuations (the Kolmogorov–Obukhov spectrum). From our model we suppose the power of the FBs is transformed partly into the energy of hydrodynamic turbulence in the envelope. If so, hydrodynamic turbulence may generate MHD fluctuations, which accelerate cosmic rays there and generate gamma-ray and radio emission from the FBs. We hope that this model may interpret the observed nonthermal emission from the bubbles.

Fermi and eROSITA Bubbles as Persistent Structures of the Milky Way

The Fermi and eROSITA bubbles (FBs and eRBs), large diffuse structures in our Galaxy, can be the by-products of steady star formation activity. To simultaneously explain the star formation history of the Milky Way (MW) and the metallicity of ∼Z ⊙ at the Galactic disk, a steady Galactic wind driven by cosmic rays (CRs) is required. For tenuous gases with a density of ≲10−3 cm−3, CR heating dominates over radiative cooling, and the gas can maintain the virial temperature of ∼0.3 keV, ideal for escape from the Galactic system as the wind. A part of the wind falls back onto the disk like a Galactic fountain flow. We model the wind dynamics according to the Galactic evolution scenario and find that the scale height and surface brightness of the X-ray and the hadronic gamma-ray emissions from such fountain flow region can be consistent with the observed properties of the FBs and eRBs. This implies that the bubbles are persistent structures of the MW existing over (at least) the last ∼1 Gyr rather than evanescent structures formed by nontrivial, ∼10 Myr past Galactic center transient activities.

Past activity of Sgr A⋆ is unlikely to affect the local cosmic-ray spectrum up to the TeV regime

Context The presence of kiloparsec-sized bubble structures on both sides of the Galactic plane suggests active phases of Sgr A⋆, the central supermassive black hole of the Milky Way in the last 1–6 Myr. We investigated the contribution of such events to the cosmic-ray (CR) flux measured in the solar neighborhood with numerical simulations. Aims. We evaluate whether the population of high-energy charged particles emitted by the Galactic center could be sufficient to significantly impact the CR flux measured in the solar neighborhood. Methods. We present a set of 3D magnetohydrodynamical simulations following the anisotropic propagation of CRs in a Milky Way-like Galaxy. We followed independent populations of CRs through time. We followed CRs originating from two different source types, namely supernovae and the Galactic center. To assess the evolution of the CR flux spectrum properties, we split these populations into two independent energy groups of 100 GeV and 10 TeV. Results. We find that the anisotropic nature of CR diffusion dramatically affects the amount of CR energy received in the solar neighborhood. The typical timescale required to observe measurable changes in the CR spectrum slope is of the order 10 Myr, largely surpassing estimated ages of the Fermi bubbles in the active galactic nuclei (AGN) jet-driven scenario. Conclusions. We conclude that a CR outburst from the Galactic center in the last few million years is unlikely have produced any observable feature in the local CR spectrum in the TeV regime within times consistent with current estimates of the age of the Fermi bubbles.

Can the Symmetric Fermi and eROSITA Bubbles Be Produced by Tilted Jets?

The Fermi Gamma-Ray Space Telescope reveals two large bubbles in the Galaxy, extending nearly symmetrically ∼50° above and below the Galactic center (GC). Previous simulations of bubble formation invoking active galactic nucleus (AGN) jets have assumed that the jets are vertical to the Galactic disk; however, in general, the jet orientation does not necessarily correlate with the rotational axis of the Galactic disk. Using three-dimensional special relativistic hydrodynamic simulations including cosmic rays (CRs) and thermal gas, we show that the dense clumpy gas within the Galactic disk disrupts jet collimation (“failed jets” hereafter), which causes the failed jets to form hot bubbles. Subsequent buoyancy in the stratified atmosphere renders them vertical to form the symmetric Fermi and eROSITA bubbles (collectively, Galactic bubbles). We find that (1) despite the relativistic jets emanating from the GC at various angles ≤45° with respect to the rotational axis of the Galaxy, the Galactic bubbles nonetheless appear aligned with the axis; (2) the edge of the eROSITA bubbles corresponds to a forward shock driven by the hot bubbles; (3) followed by the forward shock is a tangling contact discontinuity corresponding to the edge of the Fermi bubbles; (4) assuming a leptonic model we find that the observed gamma-ray bubbles and microwave haze can be reproduced with a best-fit CR power-law spectral index of 2.4; The agreements between the simulated and the observed multiwavelength features suggest that forming the Galactic bubbles by oblique AGN failed jets is a plausible scenario.

Radio Recombination Line Observations toward the Fermi Bubble

We use Green Bank Telescope radio recombination line (RRL) observations to constrain the morphology of warm ionized gas in the Fermi Bubble. Our limits on hydrogen RRL emission allow us to constrain the emission measure of the ionized gas and to estimate the beam filling factor of previous Wisconsin H-Alpha Mapper observations. Our results indicate that the warm ionized gas on the Fermi Bubble boundary spans at least 1000 pc2 with a depth of less than a pc, suggesting either a sheet-like structure or a structure consisting of many small clumps. Our data will enable us to properly calibrate future Hα observations and aid in calculating the Lyman continuum flux needed to maintain the ionized gas.

Reconstruction of Fermi and eROSITA Bubbles from Magnetized Jet Eruption with Simulations

The Fermi bubbles and the eROSITA bubbles around the Milky Way Galaxy are speculated to be the aftermaths of past jet eruptions from a supermassive black hole in the galactic center. In this work, a 2.5D axisymmetric relativistic magnetohydrodynamic (RMHD) model is applied to simulate a jet eruption from our galactic center and to reconstruct the observed Fermi bubbles and eROSITA bubbles. High-energy non-thermal electrons are excited around forward shock and discontinuity transition regions in the simulated plasma distributions. The γ-ray and X-ray emissions from these electrons manifest patterns on the skymap that match the observed Fermi bubbles and eROSITA bubbles, respectively, in shape, size and radiation intensity. The influence of the background magnetic field, initial mass distribution in the Galaxy, and the jet parameters on the plasma distributions and hence these bubbles is analyzed. Subtle effects on the evolution of plasma distributions attributed to the adoption of a galactic disk model versus a spiral-arm model are also studied.

Fermi-bubble Bulk and Edge Analysis Reveals Dust, Cooling Breaks, and Cosmic-Ray Diffusion, Facilitating a Self-consistent Model

The full, radio to γ-ray spectrum of the Fermi bubbles is shown to be consistent with standard strong-shock electron acceleration at the bubble edge, without the unnatural energy cutoffs and unrealistic electron cooling of previous studies, if the ambient interstellar radiation is strong; the γ-ray cooling break should then have a microwave counterpart, undetected until now. Indeed, a broadband bubble-edge analysis uncovers a pronounced downstream dust component, which masked the anticipated ∼35 GHz spectral break, and the same overall radio softening consistent with Kraichnan diffusion previously reported in γ-rays. A self-consistent bulk and edge model implies a few Myr old bubbles, with fairly uniform radiation fields and enhanced magnetization near the edge.

The Signature of the Northern Galactic Center Region in Low-velocity UV Absorption

The Galactic Center (GC) is surrounded by plasma lobes that extend up to ∼14 kpc above and below the plane. Until now, UV absorption studies of these lobes have only focused on high-velocity components (∣v LSR∣ > 100 km s−1) because low- and intermediate-velocity (LIV) components (∣v LSR∣ < 100 km s−1) are blended with foreground interstellar medium. To overcome this difficulty, we present a differential experiment to compare the LIV absorption between different structures within the GC region, including the Fermi Bubbles (FBs; seen in gamma rays), the eROSITA Bubbles (eBs; seen in X-rays), and the Loop I North Polar Spur (LNPS) association, an X-ray and radio feature within the northern eB. We use far-UV spectra from Hubble Space Telescope to measure LIV Si iv absorption in 61 active galactic nuclei sight lines, of which 21 pass through the FBs, 53 pass through the eBs, and 18 pass through the LNPS. We also compare our measurements to those in the literature from sight lines covering the disk–halo interface and circumgalactic medium (CGM). We find that the FBs and eBs have enhancements in measured columns of 0.22–0.29 dex in log. We also remove the contribution of a modeled disk and CGM component from the measured Si iv columns and find that the northern eB still retains a Si iv enhancement of 0.62 dex in log. A similar enhancement is not seen in the southern eB. Since a notable difference between the northern and southern eBs is the presence of the LNPS association in the nothern bubble, the northern eB enhancement may be caused by the LNPS.

Discovery of spectacular quasar-driven superbubbles in red quasars.

Quasar-driven outflows on galactic scales are a routinely invoked ingredient for galaxy formation models. We report the discovery of ionized gas nebulae surrounding three luminous red quasars at z ~ 0.4 from Gemini integral field unit observations. All these nebulae feature unprecedented pairs of "superbubbles" extending ~20 kpc in diameter, and the line-of-sight velocity difference between the red- and blueshifted bubbles reaches up to ~1200 km/s. Their spectacular dual-bubble morphology (in analogy to the galactic "Fermi bubbles") and their kinematics provide unambiguous evidence for galaxy-wide quasar-driven outflows, in parallel with the quasi-spherical outflows similar in size from luminous type 1 and type 2 quasars at concordant redshift. These bubble pairs manifest themselves as a signpost of the short-lived superbubble "break-out" phase, when the quasar wind drives the bubbles to escape the confinement from the dense environment and plunge into the galactic halo with a high-velocity expansion.

Misaligned Jets from Sgr A* and the Origin of Fermi/eROSITA Bubbles

One of the leading explanations for the origin of Fermi Bubbles is past jet activity in the Galactic center supermassive black hole Sgr A*. The claimed jets are often assumed to be perpendicular to the Galactic plane. Motivated by the orientation of pc-scale nuclear stellar disk and gas streams, as well as a low inclination of the accretion disk around Sgr A* inferred by the Event Horizon Telescope, we perform hydrodynamical simulations of nuclear jets significantly tilted relative to the Galactic rotation axis. The observed axisymmetry and hemisymmetry (north–south symmetry) of Fermi/eROSITA bubbles (FEBs) due to quasi-steady jets in Sgr A* could be produced if the jet had a super-Eddington power (≳5 × 1044 erg s−1) for a short time (jet active period ≲6 kyr) for a reasonable jet opening angle (≲10°). Such powerful explosions are, however, incompatible with the observed O viii/O vii line ratio toward the bubbles, even after considering electron–proton temperature nonequilibrium. We argue that the only remaining options for producing FEBs are (i) a low-luminosity (≈1040.5−41 erg s−1) magnetically dominated jet or accretion wind from the Sgr A*, or (ii) a supernovae or tidal disruption event driven wind of a similar luminosity from the Galactic center.

North Polar Spur/Loop I: gigantic outskirt of the Northern Fermi bubble or nearby hot gas cavity blown by supernovae?

North Polar Spur/Loop I: gigantic outskirt of the Northern Fermi bubble or nearby hot gas cavity blown by supernovae?

Possible Counterpart Signal of the Fermi Bubbles at the Cosmic-Ray Positrons

The inner Galaxy has hosted cosmic-ray burst events, including those responsible for the gamma-ray Fermi bubbles and the eROSITA bubbles in X-rays. In this work, we study the Alpha Magnetic Spectrometer positron fraction and find three features around 12, 21, and 48 GeV, of which the lowest energy has a 1.4–4.9σ significance, depending on astrophysical background assumptions. Using background simulations that explain the cosmic-ray positron fraction, positron flux, and electron plus positron flux by primary and secondary cosmic rays and cosmic rays from local pulsars, we test these spectral features as originating from electron/positron burst events from the inner Galaxy. We find the 12 GeV feature to be explained by an event of age τ ≃ 3–10 Myr, in agreement with the proposed age of the Fermi bubbles. Furthermore, the energy in cosmic-ray electrons and positrons propagating along the Galactic disk and not within the Fermi bubbles volume is estimated to be 1051.5–1057.5 erg, or O(10−4) − O(1) the cosmic-ray energy causing the Fermi bubbles. We advocate that these positron fraction features are the counterpart signals of the Fermi bubbles, or of substructures within them, or of the eROSITA bubbles.

QUIJOTE scientific results – VI. The Haze as seen by QUIJOTE

ABSTRACT The Haze is an excess of microwave intensity emission surrounding the Galactic Centre. It is spatially correlated with the γ-ray Fermi bubbles, and with the S-PASS radio polarization plumes, suggesting a possible common provenance. The models proposed to explain the origin of the Haze, including energetic events at the Galactic Centre and dark matter decay in the Galactic halo, do not yet provide a clear physical interpretation. In this paper, we present a reanalysis of the Haze including new observations from the Multi-Frequency Instrument (MFI) of the Q-U-I Joint TEnerife (QUIJOTE) experiment, at 11 and 13 GHz. We analyse the Haze in intensity and polarization, characterizing its spectrum. We detect an excess of diffuse intensity signal ascribed to the Haze. The spectrum at frequencies 11 GHz $\, \le \nu \le \,$ 70 GHz is a power law with spectral index βH = −2.79 ± 0.08, which is flatter than the Galactic synchrotron in the same region (βS = −2.98 ± 0.04), but steeper than that obtained from previous works (βH ∼ −2.5 at 23 GHz $\, \le \, \nu \le \,$ 70 GHz). We also observe an excess of polarized signal in the QUIJOTE-MFI maps in the Haze area. This is a first hint detection of polarized Haze, or a consequence of curvature of the synchrotron spectrum in that area. Finally, we show that the spectrum of polarized structures associated with Galactic Centre activity is steep at low frequencies (β ∼ −3.2 at 2.3 GHz ≤ ν ≤ 23 GHz), and becomes flatter above 11 GHz.

A galactic breeze origin for the Fermi bubbles emission

ABSTRACT The origin of the Fermi bubbles, which constitute two gamma-ray emitting lobes above and below the Galactic plane, remains unclear. The possibility that this Fermi bubbles gamma-ray emission originates from hadronic cosmic rays advected by a subsonic Galactic outflow, or breeze, is here explored. The simulation of a breeze solution and subsequent cosmic ray transport is carried out using the hydrodynamical code, PLUTO, in combination with a cosmic ray transport code. The Galactic outflow model obtained is found to be compatible with both inferences of the decelerating outflow velocity profile of the gas in the Fermi bubbles region, and evidence for the presence of a large amount of hot ionized gas out in the Galactic halo region. Although simple, this model is found to be able to reproduce the observed Fermi-LAT energy flux at high Galactic latitudes. Following these results a prediction concerning the gamma-ray emission for 1–3 TeV photons is made for future comparison with CTA/SWGO measurements.

Evidence for powerful winds and the associated reverse shock as the origin of the Fermi bubbles

ABSTRACT The Fermi bubbles are large gamma-ray-emitting structures. They are symmetric about the Galactic Centre (GC), and their creation is therefore attributed to intensive energy injection at the GC. In this study, we focus on the non-equilibrium X-ray gas structures associated with the bubbles. We show that a combination of the density, temperature, and shock age profiles of the X-ray gas can be used to distinguish the energy-injection mechanisms. By comparing the results of numerical simulations with observations, we indicate that the bubbles were created by a fast wind from the GC because it generates a strong reverse shock and reproduces the observed temperature peak there. On the other hand, instantaneous energy injection at the GC cannot reproduce the temperature profile. The wind had a speed of ${\sim} 1000\rm \: km\: s^{-1}$, and blew for ∼107 yr. Because the mass flux of the wind is large, the entrainment of interstellar gas by wide-angle outflows from the black hole is required. Thus, the wind may be the same as active galactic nuclei outflows often observed in other galaxies and thought to regulate the growth of galaxies and their central black holes.

The quantum character of the Scalar Field Dark Matter

ABSTRACT The scalar field dark matter (SFDM) model, also called Fuzzy, Wave, Bose–Einstein, and Ultra-light Dark Matter, has received a lot of attention because it has been able to provide simpler and more natural explanations for various features of galaxies, such as the number of satellite galaxies and the cusp-core problem. We recently showed that this model is able to explain the vast polar orbits of satellite galaxies around their host, the so-called VPO, and to explain the X-ray and gamma-ray emissions in the vacuum regions of our galaxy, that is, the Fermi Bubbles. In all these phenomena, the quantum character of SFDM has been crucial. In this work, we study the quantum effects of SFDM at the cosmological level, to see these effects not only at the galactic scale, but also at the cosmological scale. Using a convenient ansatz, we were able to integrate the perturbed equations to show that the shape of the SFDM haloes resembling atoms is a generic result. The main conclusion of this work is that quantum mechanics, the successful microworld theory, could also explain the dark side of the Cosmos.

Galactic halo bubble magnetic fields and UHECR deflections

ABSTRACT We consider the synchrotron emission from electrons out in the Galactic halo bubble region where the Fermi bubble structures reside. Utilizing a simple analytical expression for the non-thermal electron distribution and a toy magnetic field model, we simulate polarized synchrotron emission maps at a frequency of 30 GHz. Comparing these maps with the observational data, we obtain constraints on the parameters of our toy Galactic halo bubble magnetic field model. Utilizing this parameter value range for the toy magnetic field model, we determine the corresponding range of arrival directions and suppression factors of ultra high energy cosmic rays (UHECRs) from potential local source locations. We find that high levels of flux suppression (down to 2 per cent) and large deflection angles (≥80°) are possible for source locations whose line of sight pass through the Galactic halo bubble region. We conclude that the magnetic field out in the Galactic halo bubble region can strongly dominate the level of deflection UHECRs experience whilst propagating from local sources to Earth.

Baryon breakdown in black hole

According to relativity theory, a black hole is a distinct region in spacetime; according to astronomical observations, it is a celestial body transforming matter into high-energy jets. We propose that a black hole is, indeed, a star, where particles transform into photons through a specific nuclear reaction, besides radiative accretion disk processes. Our reasoning draws from statistical physics of open quantized systems. The many-body theory describes elementary particles comprising quanta of actions and their reactions as conversions of matter-bound quanta into vacuum quanta. The proposed transformation details the annihilation of neutrons into gamma rays. This reaction, characteristic of a black hole, begins when the strength of gravitation exceeds the strength of the strong force. Then gluons detach from quarks and attach to surrounding high-energy quanta of the gravitational field. Without gluons, the tightly packed neutrons cannot hold up their SU(3) symmetry. The tetrahedral structures flatten out so that quarks of opposite charges end up pairwise on top of each other and annihilate into rays of light quanta as electrons and positrons do. Finally, the quanta jet out along the black hole spinning axis, where the gravitation due to the collapsing core gives in most. Over the eons, these episodic effluxes from a precessing supermassive black hole amass into Fermi bubbles.

Gamma-ray emission from the Sagittarius dwarf spheroidal galaxy due to millisecond pulsars

The Fermi bubbles are giant, γ-ray-emitting lobes emanating from the nucleus of the Milky Way discovered in ~1–100 GeV data collected by the Large Area Telescope on board the Fermi Gamma-Ray Space Telescope. Previous work has revealed substructure within the Fermi bubbles that has been interpreted as a signature of collimated outflows from the Galaxy’s supermassive black hole. Here we show via a spatial template analysis that much of the γ-ray emission associated with the brightest region of substructure—the so-called cocoon—is probably due to the Sagittarius dwarf spheroidal galaxy (dSph). This large Milky Way satellite is viewed through the Fermi bubbles from the position of the Solar System. As a tidally and ram-pressure stripped remnant, the Sagittarius dSph has no ongoing star formation, but we nevertheless demonstrate that the dwarf’s millisecond pulsar population can plausibly supply the γ-ray signal that our analysis associates with its stellar template. The measured spectrum is naturally explained by inverse Compton scattering of cosmic microwave background photons by high-energy electron–positron pairs injected by millisecond pulsars belonging to the Sagittarius dSph, combined with these objects’ magnetospheric emission. This finding plausibly suggests that millisecond pulsars produce significant γ-ray emission among old stellar populations, potentially confounding indirect dark-matter searches in regions such as the Galactic Centre, the Andromeda galaxy and other massive Milky Way dSphs. A bright patch in the Fermi bubbles, previously attributed to a jet launched by the Galaxy’s central black hole, is actually due to gamma-ray emission by millisecond pulsars in a background, satellite galaxy of the Milky Way.

Emission from hadronic and leptonic processes in galactic jet-driven bubbles

ABSTRACT We investigate the multiwavelength emission from hadronic and leptonic cosmic rays (CRs) in bubbles around galaxies, analogous to the Fermi bubbles of the Milky Way. The bubbles are modelled using 3D magnetohydrodynamical simulations, and are driven by a 0.3 Myr intense explosive outburst from the nucleus of Milky Way-like galaxies. We compute their non-thermal emission properties at different stages throughout their evolution, up to 7 Myr, by post-processing the simulations. We compare the spectral and spatial signatures of bubbles with hadronic, leptonic, and hybrid hadro-leptonic CR compositions. These each show broadly similar emission spectra, comprised of radio synchrotron, inverse Compton, and non-thermal bremsstrahlung components. However, hadronic and hybrid bubbles were found to be brighter than leptonic bubbles in X-rays, and marginally less bright at radio frequencies, and in γ-rays between ∼0.1 and a few 10s of GeV, with a large part of their emission being driven by secondary electrons formed in hadronic interactions. Hadronic systems were also found to be slightly brighter in high-energy γ-rays than their leptonic counterparts, owing to the π0 decay emission that dominates their emission between energies of 100s of GeV and a few TeV.