Extragalactic Radio Background Research Articles

Constraining the Clustering and 21 cm Signature of Radio Galaxies at Cosmic Dawn

The efficiency of radio emission is an important unknown parameter of early galaxies at cosmic dawn, as models with high efficiency have been shown to modify the cosmological 21 cm signal substantially, deepening the absorption trough and boosting the 21 cm power spectrum. Such models have been previously directly constrained by the overall extragalactic radio background, as observed by Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission 2 and Long Wavelength Array. In this work, we constrain the clustering of high-redshift radio sources by utilizing the observed upper limits on arcminute-scale anisotropy from the Very Large Array at 4.9 GHz and Australia Telescope Compact Array at 8.7 GHz. Using a seminumerical simulation of a plausible astrophysical model for illustration, we show that the clustering constraints on the radio efficiency are much stronger than those from the overall background intensity by a factor that varies between 18 and 55 in the redshift range of 7–22. As a result, the predicted maximum depth of the global 21 cm signal is lowered by a factor of 6 (to 1400 mK), and the maximum 21 cm power spectrum peak at cosmic dawn is lowered by a factor of 45 (to 1.3 × 105 mK2). We conclude that the observed clustering is the strongest current direct constraint on such models, but strong early radio emission from galaxies remains viable for producing a strongly enhanced 21 cm signal from cosmic dawn.

A relook at the GZK neutrino-photon connection: impact of extra-galactic radio background & UHECR properties

Ultra-high energy cosmic rays (UHECRs) beyond the Greisen-Zatsepin-Kuzmin (GZK) cut-off provide us with a unique opportunity to understand the universe at extreme energies. Secondary GZK photons and GZK neutrinos associated with the same interaction are indeed interconnected and render access to multi-messenger analysis of UHECRs. The GZK photon flux is heavily attenuated due to the interaction with Cosmic Microwave Background (CMB) and the Extra-galactic Radio Background (ERB). The present estimate of the ERB comprising of several model uncertainties together with the ARCADE2 radio results in large propagation uncertainties in the GZK photon flux. On the other hand, the weakly interacting GZK neutrino flux is unaffected by these propagation effects. In this work, we make an updated estimate of the GZK photon and GZK neutrino fluxes considering a wide variation of both the production and propagation properties of the UHECR like, the spectral index, the cut-off energy of the primary spectrum, the distribution of sources and the uncertainties in the ERB estimation. We explore the detection prospects of the GZK fluxes with various present and upcoming UHECR and UHE neutrino detectors such as Auger, TA, GRAND, ANITA, ARA, IceCube and IceCube-Gen2. The predicted fluxes are found to be beyond the reach of the current detectors. In future, proposed IceCube-Gen2, Auger upgrade and GRAND experiments will have the sensitivity to the predicted GZK photon and GZK neutrino fluxes. Such detection can put constraints on the UHECR source properties and the propagation effects due to the ERB. We also propose an indirect limit on the GZK photon flux using the neutrino-photon connection for any future detection of GZK neutrinos by the IceCube-Gen2 detector. We find this limit to be consistent with our GZK flux predictions.

Is there a Radio Excess from the Decoupling of Pre-recombination Bremsstrahlung?

Recently it has been suggested that thermal bremsstrahlung emission, when it decouples prior to recombination, creates an excess over the Planck cosmic microwave background spectrum at sub-GHz frequencies. Remarkable by itself, this would also explain a long-standing unexplained deficit in the predictions of the extragalactic radio background. In this brief note, I reiterate that no such non-thermal component can arise by itself when matter and radiation remain kinetically coupled.

Upper Limit on the Diffuse Radio Background from GZK Photon Observation

Here, we point out that an observation of ultrahigh energy cosmic ray (UHECR) photons, “GZK photons”, could provide an upper limit on the level of the extragalactic radio background, depending on the level of UHECR proton primaries (to be determined after a few years of data taking by the Pierre Auger Observatory upgrade AugerPrime). We also update our 2005 prediction of the range of GZK photon fluxes expected from proton primaries.

The contribution of discrete sources to the sky temperature at 144 MHz

In recent years, the level of the extragalactic radio background has become a point of considerable interest, with some lines of argument pointing to an entirely new cosmological synchrotron background. The contribution of the known discrete source population to the sky temperature is key to this discussion. Because of the steep spectral index of the excess over the cosmic microwave background, it is best studied at low frequencies where the signal is strongest. The Low-Frequency Array (LOFAR) wide and deep sky surveys give us the best constraints yet on the contribution of discrete extragalactic sources at 144 MHz, and in particular allow us to include contributions from diffuse, low-surface-brightness emission that could not be fully accounted for in previous work. We show that, even with these new data, known sources can still only account for around a quarter of the estimated extragalactic sky temperature at LOFAR frequencies.

Signature of excess radio background in the 21-cm global signal and power spectrum

The recent tentative detection by the EDGES Low-Band antenna of the hydrogen 21-cm line from cosmic dawn, if confirmed, is the first ever signature observed from the epoch of primordial star formation. However, the magnitude and the shape of this signal are incompatible with standard astrophysical predictions, requiring either colder than expected gas, or an excess radio background above the Cosmic Microwave Background (CMB) radiation. In this paper we explore the latter option, investigating possible 21-cm signals in models with a phenomenological excess radio background together with the standard astrophysical modeling. Assuming uniform radiation with a synchrotron-like spectrum existing throughout cosmic history, we show that such a radio background, in addition to explaining the EDGES Low-Band signal, enhances the power spectrum. The signal during cosmic dawn and reionization strongly depends on both the intensity of the radio background and the astrophysical parameters. We verify the broad agreement of our models with the detected feature, including additional constraints from the EDGES High-Band data, high-redshift quasars, and observational limits on the extragalactic radio background. The data imply a lower limit on the star formation efficiency of 2.8\%, an upper limit on the minimum mass of star-forming halos of M$_{\rm h}\sim 10^9$ M$_\odot$ at $z = 17$, and a lower limit on the excess radio background of 1.9 times the CMB at the 78 MHz cosmic dawn frequency (corresponding to 0.1\% of the CMB at 1.42 GHz). The properties of X-ray sources remain unconstrained. We also show that during the dark ages, such a radio background saturates the 21-cm signal, imprinting a unique signature in both the global signal and the power spectrum.

Anisotropy of the extragalactic radio background from dark matter annihilation

Observations of the extragalactic radio background have uncovered a significant isotropic emission across multiple frequencies spanning from 22 MHz to 10 GHz. The intensity of this non-thermal emission component significantly exceeds the expected contribution from known astrophysical sources. Interestingly, models have indicated that the annihilation of dark matter particles may reproduce both the flux and spectrum of the excess. However, the lack of a measurable anisotropy in the residual emission remains challenging for both dark matter and standard astrophysical interpretations of the ARCADE-2 data. We calculate the expected synchrotron anisotropy from dark matter annihilation and show that these models can produce very small anisotropies, though this requires galaxy clusters to have large substructure contributions and strong magnetic fields. We show that this constraint can be significantly relaxed, however, in scenarios where electrons produced via dark matter annihilation can be efficiently reaccelerated by Alfv\'en waves in the intra-Cluster medium. Our analysis indicates that any source capable of explaining the intensity and isotropy of the extragalactic radio excess must have a spatial extension far larger than typical for baryons in galaxies, suggesting a novel physics interpretation.

IS THERE AN UNACCOUNTED FOR EXCESS IN THE EXTRAGALACTIC COSMIC RADIO BACKGROUND?

Analyses of measurements of the distribution of absolute brightness temperature over the radio sky have led recently to suggestions that there exists a substantial unexplained extragalactic radio background. Consequently, there have been numerous attempts to place constraints on plausible origins for the `excess'. We suggest here this expectation of a large extragalactic background, over and above that contributed by the sources observed in the surveys, is based on an extremely simple geometry adopted to model the Galactic emission and the procedure adopted in the estimation of the extragalactic contribution. In this paper, we derive the extragalactic radio background from wide-field radio images by using a more realistic modeling of the Galactic emission and decompose the sky maps at 150, 408, and 1420 MHz into anisotropic Galactic and isotropic extragalactic components. The anisotropic Galactic component is assumed to arise from a highly flattened spheroid representing the thick disc, embedded in a spherical halo, both centered at the Galactic center, along with Galactic sources, filamentary structures, Galactic loops and spurs. All components are constrained to be positive and the optimization scheme minimizes the sky area occupied by the complex filaments. We show that in contrast to the simple modeling of the Galactic emission as a plane parallel slab, the more realistic modeling yields estimates for the uniform extragalactic brightness that is consistent with expectations from known extragalactic radio source populations.

Sources of the radio background considered

We investigate different scenarios for the origin of the extragalactic radio background. The surface brightness of the background, as reported by the ARCADE 2 collaboration, is several times higher than that which would result from currently observed radio sources. We consider contributions to the background from diffuse synchrotron emission from clusters and the intergalactic medium, previously unrecognized flux from low surface brightness regions of radio sources, and faint point sources below the flux limit of existing surveys. By examining radio source counts available in the literature, we conclude that most of the radio background is produced by radio point sources that dominate at sub microJy fluxes. We show that a truly diffuse background produced by electrons far from galaxes is ruled out because such energetic electrons would overproduce the obserevd X-ray/gamma-ray background through inverse Compton scattering of the other photon fields. Unrecognized flux from low surface brightness regions of extended radio sources, or moderate flux sources missed entirely by radio source count surveys, cannot explain the bulk of the observed background, but may contribute as much as 10 per cent. We consider both radio supernovae and radio quiet quasars as candidate sources for the background, and show that both fail to produce it at the observed level because of insufficient number of objects and total flux, although radio quiet quasars contribute at the level of at least a few percent. We conclude that if the radio background is at the level reported, a majority of the total surface brightness would have to be produced by ordinary starforming galaxies above redshift 1 characterized by an evolving radio far-infrared correlation, which changes toward the radio loud with redshift.

Imprint of Intergalactic Shocks on the Radio Sky

Strong intergalactic shocks are a natural consequence of structure formation in the universe. These shocks are expected to deposit large fractions of their thermal energy in relativistic electrons (ξe 0.05 according to supernova remnant observations) and magnetic fields (ξB 0.01 according to cluster halo observations). We calculate the synchrotron emission from such shocks using an analytical model, calibrated and verified based on a hydrodynamic ΛCDM simulation. The resulting signal composes a large fraction (up to a few 10%) of the extragalactic radio background below 500 MHz. The associated angular fluctuations, e.g., δTl 260(ξeξB/5 × 10-4)(ν/100 MHz)-3 K for multipoles 400 l 2000, dominate the radio sky for frequencies ν 10 GHz and angular scales 1' θ < 1° (after a modest removal of discrete sources), provided that ξeξB 3 × 10-4. The fluctuating signal is most pronounced for ν 500 MHz, dominating the sky there even for ξeξB = 5 × 10-5. The signal will be easily observable by next-generation telescopes such as the LOFAR and the SKA and is marginally observable with present-day radio telescopes. The signal could also be identified through a cross-correlation with tracers of large-scale structure (such as γ-ray emission from intergalactic shocks), possibly even in existing 10 GHz CMB anisotropy maps and high-resolution ~1 GHz radio surveys. Detection of the signal will provide the first identification of intergalactic shocks and of the warm-hot intergalactic medium (believed to contain most of the baryons in the low-redshift universe), and gauge the unknown strength of the intergalactic magnetic field. We analyze existing observations of the diffuse radio background below 500 MHz and show that they are well fitted by a simple, double-disk Galactic model, precluding a direct identification of the diffuse extragalactic radio background. Modeling the frequency-dependent anisotropy pattern observed at very low (1-10 MHz) frequencies can be used to disentangle the distributions of Galactic cosmic rays, ionized gas, and magnetic fields. Space missions such as the Astronomical Low Frequency Array will thus provide an important insight into the structure and composition of our Galaxy.

Searching for intergalactic shocks with the Square Kilometer Array

Strong intergalactic shocks are a natural consequence of structure formation in the universe. These shocks are expected to deposit large fractions of their thermal energy in relativistic electrons ( ξ e ≃ 0.05 according to supernova remnant observations) and magnetic fields ( ξ B ≃ 0.01 according to cluster halo observations). We discuss the expected synchrotron emission from such shocks, and the observational consequences for next generation radio telescopes such as the Square Kilometer Array. We present an analytical model, calibrated and verified based on a hydrodynamical ΛCDM simulation. The resulting signal composes a large fraction (up to a few 10%) of the extragalactic radio background below 500 MHz. The associated angular fluctuations, e.g. δT l ≳ 260 ( ξ e ξ B /5 × 10 −4)( ν/100 MHz) −3 K for multipoles 400 ≲ l ≲ 2000, dominate the radio sky for frequencies ν ≲ 10 GHz and angular scales 1′ ≲ θ < 1° (after a modest removal of point sources), provided that ξ e ξ B ≳ 3 × 10 −4. The fluctuating signal is most pronounced for ν ≲ 500 MHz, dominating the sky there even for ξ e ξ B = 5 × 10 −5. We find the signal easily observable by next generation telescopes such as the SKA, and marginally observable with present telescopes. The signal may be identified using cross-correlations with tracers of large-scale structure (e.g. γ-ray emission from intergalactic shocks). Detection of the signal will provide the first identification of intergalactic shocks and of the warm-hot intergalactic medium (believed to contain most of the baryons in the low redshift universe), and gauge the unknown intergalactic magnetic field.

Radio Wavelength Constraints on the Sources of the Far-Infrared Background

The cosmic far-infrared background detected recently by the COBE-DIRBE team is presumably due, in large part, to the far-infrared (FIR) emission from all galaxies. We take the well-established correlation between FIR and radio luminosity for individual galaxies and apply it to the FIR background. We find that these sources make up about half of the extragalactic radio background, the other half being due to active galactic nuclei (AGNs). This is in agreement with other radio observations, which leads us to conclude that the FIR-radio correlation holds well for the very faint sources making up the FIR background and that the FIR background is indeed due to star formation activity (not AGNs or other possible sources). If these star-forming galaxies have a radio spectral index between 0.4 and 0.8 and make up 40%-60% of the extragalactic radio background, we find that they have redshifts between roughly 1 and 2, which is in agreement with recent estimates by Madau et al. of the redshift of peak star formation activity. We compare the observed extragalactic radio background to the integral over the logN-logS curve for star-forming radio sources and find that the slope of the curve must change significantly below about 1 μJy. At 1 μJy, the faint radio source counts predict about 25 sources per square arcminute, and these will cause the Space Infrared Telescope Facility to be confusion limited at 160 μm.

A new estimate of the extragalactic radio background and implications for ultra-high-energy γ-ray propagation

We make a new estimate of the extragalactic radio background down to kHz frequencies based on the observed luminosity functions and radio spectra of normal galaxies and radio galaxies. We have constructed models for the spectra of these two classes of objects down to low frequencies based on observations of our Galaxy, other normal galaxies and radio galaxies. We check that the models and evolution of the luminosity functions give source counts consistent with data and calculate the radio background expected from kHz to GHz frequencies. The motivation for this calculation is that the propagation of ultra-high energy gamma-rays in the universe is limited by photon-photon pair production on the radio background. Electromagnetic cascades involving photon-photon pair production and subsequent synchrotron radiation in the intergalactic magnetic field may develop. Such gamma-rays may be produced in acceleration sites of ultra-high energy cosmic rays, as a result of interactions with the microwave background, or emitted as a result of decay or annihilation of topological defects. We find that photon-photon pair production on the radio background remains the dominant attenuation process for gamma-rays from $3 \times 10^{10}$ GeV up to GUT scale energies.

The origin of the extragalactic background radiation between 0.5 and 400 MHz

The calculations described in this paper show that the superposition of extragalactic radio sources forms a large fraction of the extragalactic radio background spectrum between 0.5 and 400 MHz derived from observations. A significant contribution at low frequencies is also expected from the low-luminosity sources which are inadequately represented by the complete sample used. Synchrotron self-absorption in the components of the individual sources which make up the background results in a low-frequency turn-over. This result is independent of the form of cosmological evolution used, and the spectra for both evolving and non-evolving models peak in the range 1–2 MHz. No other absorption effects are important in this frequency range in the sources which give rise to most of the extragalactic background radiation. The form of the spectrum is due to the fact that for non-evolving models the dominant sources at all frequencies are the low-intensity sources which also have synchrotron turn-overs at low frequencies. For evolving models the contributions from the most luminous sources are sufficiently redshifted for the peak to occur at approximately the same frequency. Most of the flux originates at moderate redshifts for all models, so the possibility of absorption by galaxies or intergalactic gas resulting in a further cut-off is very small.

On Cosmological Models with an Antipole

The properties of Lemaître models are summarized and the luminosity–volume test is used to determine which of these models are consistent with the distributions in space of quasars and radio-galaxies respectively. Ranges of models can be found consistent with either, but not both, of these classes of radio-source, including models which could account for the anomalous absorption redshift of 1.95 in quasars. The integrated extragalactic radio background provides an important additional constraint to the problem. No model is found to agree with the observed radio source-counts, without the introduction of evolutionary effects.