Rm GHz Research Articles

An ALMA CO(1-0) survey of the 2Jy sample: large and massive molecular discs in radio AGN host galaxies

ABSTRACT The jets of radio AGN provide one of the most important forms of active galactic nuclei (AGN) feedback, yet considerable uncertainties remain about how they are triggered. Since the molecular gas reservoirs of the host galaxies can supply key information about the dominant triggering mechanism(s), here we present Atacama Large Millimeter/sub-millimeter Array CO(1-0) observations of a complete sample of 29 powerful radio AGN ($P_{1.4\,{\rm GHz}} \gt 10^{25}$ W Hz$^{-1}$ and $0.05 \lt z \lt 0.3$) with an angular resolution of about 2–3 arcsec (corresponding to 2–8 kpc). We detect molecular gas with masses in the range $10^{8.9} \lt M_{{\rm H}_2} \lt 10^{10.2}$ M$_\odot$ in the early-type host galaxies of ten targets, while for the other 19 sources, we derive upper limits. The detection rate of objects with such large molecular masses – $34\pm 9$ per cent – is higher than in the general population of non-active early-type galaxies (ETGs: $\lt $10 per cent). The kinematics of the molecular gas are dominated in most cases by rotating disc-like structures, with diameters up to 25 kpc. Compared with the results for samples of quiescent ETG in the literature, we find a larger fraction of more massive, more extended and less settled molecular gas structures. In most of the CO-detected sources, the results are consistent with triggering of the AGN as the gas settles following a merger or close encounter with a gas-rich companion. However, in a minority of objects at the centres of rich clusters of galaxies, the accretion of gas cooling from the hot X-ray haloes is a plausible alternative to galaxy interactions as a triggering mechanism.

X-ray cool core remnants heated by strong radio AGN feedback

ABSTRACT Strong AGN heating provides an alternative means for the disruption of cluster cool cores (CCs) to cluster mergers. In this work, we present a systematic Chandra study of a sample of 108 nearby (z < 0.1) galaxy clusters, to investigate the effect of AGN heating on CCs. About 40 per cent of clusters with small offsets between the BCG and the X-ray centre (≤50 kpc) have small CCs. For comparison, 14 of 17 clusters with large offsets have small CCs, which suggests that mergers or sloshing can be efficient in reducing the CC size. Relaxed, small CC clusters generally have weak radio AGNs ($P_{1.4\rm GHz}\lt 10^{23}$ W Hz−1), and they show a lack of systems hosting a radio AGN with intermediate radio power ($2\times 10^{23}\lt P_{1.4\rm GHz}\lt 2\times 10^{24}$ W Hz−1). We found that the strongest circumnuclear (<1 kpc) X-ray emission only exists in clusters with strong radio AGN. The duty cycle of relaxed, small CC clusters is less than half of that for large CC clusters. It suggests that the radio activity of BCGs is affected by the properties of the surrounding gas beyond the central ∼10 kpc, and strong radio AGNs in small X-ray CCs fade more rapidly than those embedded in large X-ray CCs. A scenario is also presented for the transition of large CCs and coronae due to radio AGN feedback. We also present a detailed analysis of galaxy cluster 3C 129.1 as an example of a CC remnant possibly disrupted by radio AGN.

Modelling the galaxy radio continuum from star formation and active galactic nuclei in the Shark semi-analytic model

ABSTRACT We present a model of radio continuum emission associated with star formation (SF) and active galactic nuclei (AGNs) implemented in the Shark semi-analytic model of galaxy formation. SF emission includes free-free and synchrotron emission, which depend on the free-electron density and the rate of core-collapse supernovae with a minor contribution from supernova remnants, respectively. AGN emission is modelled based on the jet production rate, which depends on the black hole mass, accretion rate, and spin, and includes synchrotron self-absorption. Shark reproduces radio luminosity functions (RLFs) at $1.4\, \rm GHz$ and $150\, \rm MHz$ for 0 ≤ z ≤ 4, and scaling relations between radio luminosity, star formation rate, and infrared luminosity of galaxies in the local and distant universe in good agreement with observations. The model also reproduces observed number counts of radio sources from 150 MHz to 8.4 GHz to within a factor of 2 on average, though larger discrepancies are seen at the very bright fluxes at higher frequencies. We use this model to understand how the radio continuum emission from radio-quiet AGNs can affect the measured RLFs of galaxies. We find current methods to exclude AGNs from observational samples result in large fractions of radio-quiet AGNs contaminating the ‘star-forming galaxies’ selection and a brighter end to the resulting RLFs.We investigate how this affects the infrared-radio correlation (IRRC) and show that AGN contamination can lead to evolution of the IRRC with redshift. Without this contamination, our model predicts a redshift- and stellar mass-independent IRRC, except at the dwarf-galaxy regime.

Non-LTE radio recombination line analysis towards H ii region G34.25+0.14

ABSTRACT The electron temperature $T^{*}_\mathrm{ e}$ of H ii regions in local thermodynamic equilibrium (LTE) is usually calculated based on the ratio of radio combination lines (RRLs) and continuum emission. In this work, the LTE $T^{*}_\mathrm{ e}$ in the H ii region G34.25+0.14, which is especially luminous and regarded as a prime target to investigate the physical properties of an H ii region, is obtained by analysing both RRLs and continuum in L band, and employing the correction factor $b_n^{*}$ to correct for non-LTE conditions. The LTE $T^{*}_\mathrm{ e}$ values in L band are calculated to be $2580\, {\rm K}$ for H i/OH/Recombination line survey (THOR) observations and $3690\, {\rm K}$ for Survey of Ionized Gas in our Galaxy, using the Arecibo telescope (SIGGMA), respectively. By employing a non-LTE correction, we derive Te of 8900 $\, {\rm K}$ and the corresponding $b^{*}_n$ values are calculated to be 1 and close to 1 in X band and C band, respectively. However, $b_n^{*}$ at L band (${\sim} 1.4\, {\rm GHz}$) is calculated as 4.2 and 2.4 for THOR and SIGGMA, suggesting that non-LTE corrections are necessary and significantly affect the results. This work demonstrates that the assumption of LTE to calculate $T^{*}_\mathrm{ e}$ is not valid for the observed transitions in L band, and hence, non-LTE corrections are needed. Furthermore, Ne for G34.25+0.14 is calculated from multiple bands independently, consistent with previous work. This study provides a method which could be used for other H ii regions to investigate their degree of departure from LTE.

A closer look at dark photon explanations of the excess radio background

ABSTRACT The observed excess radio background has remained a puzzle for over a decade. A recent new physics solution involves dark matter that decays into dark photons in the presence of a thermal dark photon background. The produced non-thermal dark photon spectrum then converts into standard photons around the reionization era, yielding an approximate power-law radio excess with brightness temperature T(ν) ≃ ν−2.5 over a wide range of frequencies, ν. This simple power-law model comes intriguingly close to the current data, even if several ingredients are required to make it work. In this paper, we investigate some of the details of this model, showcasing the importance of individual effects. In particular, significant deviation from a power law is present at $\nu \lesssim 100\, {\rm MHz}$ and $\nu \gtrsim 1\, {\rm GHz}$. These effects result in improving the fit to data compared to a power-law spectrum, and may become testable in future observations. We also highlight independent signatures that can be tested with future cosmic microwave background spectral distortion experiments such as PIXIE. However, there are challenges for the model from the observed radio background anisotropies, as discussed here. We furthermore highlight a possible runaway process due to the finite width of the dark matter decay profile, which suggests that additional work might be required to obtain a viable model.

Bound states in the continuum in a fluxonium qutrit

The heavy fluxonium at zero external flux has a long-lived state when coupled capacitively to any other system. We analyze it by projecting all the fluxonium relevant operators into the qutrit subspace, as this long-lived configuration corresponds to the second excited fluxonium level. This state becomes a bound-state in the continuum (BIC) when the coupling occurs to an extended state supporting a continuum of modes. In the case without noise, we find BIC decay times that can be much larger than seconds $T_1\gg {\rm s}$ when the fluxonium is coupled to superconducting waveguide, while typical device frequencies are in the order of ${\rm GHz}$. We have also analyzed the noise in a realistic situation, arguing that the most dangerous noise source is the well-known 1/f flux noise. Even in its presence, we show that decay times could reach the range of ${T_1\sim \rm 10 ms}.$

Can accreting primordial black holes explain the excess radio background?

ABSTRACT The excess radio background seen at ${\simeq}0.1\rm{-}10\, {\rm GHz}$ has stimulated much scientific debate in the past years. Recently, it was pointed out that the soft photon emission from accreting primordial black holes may be able to explain this signal. We show that the expected ultraviolet photon emission from these accreting black holes would ionize the universe completely at z > 6 and thus wash out the 21-cm absorption signature at z ≃ 20 as well as be in tension with existing cosmic microwave background anisotropy and average spectral distortion limits. We discuss possible augmentations of the model; however, it seems that an explanation of radio excess by accreting primordial black holes is not well-justified.

The dichotomy between low-frequency-peaked BL Lacs and flat-spectrum radio quasars

ABSTRACT By comparing the Fourth Fermi Large Area Telescope active galactic nuclei (AGNs) Catalog Data Release 2 (4LAC-DR2) and the Radio Fundamental Catalog (RFC-2021b), we obtain an AGN sample containing 2816 AGNs. We calculate the γ-ray loudness (Gγ) of the sample sources, and establish the relevant relationship between several gamma/radio properties. The results show that: 1) the synchrotron peak frequency ($\nu _\mathrm{peak}^S$) strongly correlates with the γ-ray photon spectral index (Γγ), γ-ray luminosity (Lγ), and radio luminosity (LR); 2) the low-synchrotron-peaked BL Lac objects (LBLs) and the flat-spectrum radio quasars (FSRQs) partially overlap in log $\nu _\mathrm{peak}^S\!-\!\Gamma _\gamma$, log Lγ–Γγ, log LR–Γγ, log Gγ–Γγ, log $\nu _\mathrm{peak}^S\!-\!$log Lγ, log $\nu _\mathrm{peak}^S\!-\!$log LR, and log $\nu _\mathrm{peak}^S\!-\!$log Gγ space. We plotted the distribution of redshift (z), $\nu _\mathrm{peak}^S$, Γγ, Lγ, VLBI 8.4 GHz flux density ($S_{\rm {8.4\, GHz}}$), and Gγ to distinguish FSRQs and LBLs. We note that $\nu _\mathrm{peak}^S$, Γγ, and $S_{8.4\, {\rm GHz}}$ show similar bimodal distributions. To further determine whether FSRQs are misjudged as LBLs, we examined the Compton dominance (CD), and found that CD < 1 for only two out of 39 sources. This measurement provides further evidence that FSRQs may be mistaken for LBLs.

Filamentation of fast radio bursts in magnetar winds

ABSTRACT Magnetars are the most promising progenitors of fast radio bursts (FRBs). Strong radio waves propagating through the magnetar wind are subject to non-linear effects, including modulation/filamentation instabilities. We derive the dispersion relation for modulations of strong waves propagating in magnetically dominated pair plasmas focusing on dimensionless strength parameters a0 ≲ 1, and discuss implications for FRBs. As an effect of the instability, the FRB-radiation intensity develops sheets perpendicular to the direction of the wind magnetic field. When the FRB front expands outside the radius where the instability ends, the radiation sheets are scattered due to diffraction. The FRB-scattering time-scale depends on the properties of the magnetar wind. In a cold wind, the typical scattering time-scale is τsc ∼ $\mu$s–ms at the frequency $\nu \sim 1\, {\rm GHz}$. The scattering time-scale increases at low frequencies, with the scaling τsc ∝ ν−2. The frequency-dependent broadening of the brightest pulse of FRB 181112 is consistent with this scaling. From the scattering time-scale of the pulse, one can estimate that the wind Lorentz factor is larger than a few tens. In a warm wind, the scattering time-scale can approach $\tau _{\rm sc}\sim \, {\rm ns}$. Then scattering produces a frequency modulation of the observed intensity with a large bandwidth, $\Delta \nu \sim 1/\tau _{\rm sc}\gtrsim 100\, {\rm MHz}$. Broad-band frequency modulations observed in FRBs could be due to scattering in a warm magnetar wind.

The far-infrared/radio correlation for a sample of strongly lensed dusty star-forming galaxies detected by Herschel

Abstract We investigate the radio-far infrared (FIR) correlation for a sample of 28 bright high-redshift (1 ≲ z ≲ 4) star-forming galaxies selected in the FIR from the Herschel-ATLAS fields as candidates to be strongly gravitationally lensed. The radio information comes either from high sensitivity dedicated ATCA observations at 2.1 GHz or from cross-matches with the FIRST survey at 1.4 GHz. By taking advantage of source brightness possibly enhanced by lensing magnification, we identify a weak evolution with redshift out to z ≲ 4 of the FIR-to-radio luminosity ratio qFIR. We also find that the qFIR parameter as a function of the radio power $L_{1.4\, \rm GHz}$ displays a clear decreasing trend, similarly to what is observed for optically/radio selected lensed quasars found in literature, yet covering a complementary region in the $q_{\rm FIR}-L_{1.4\, \rm GHz}$ diagram. We interpret such a behavior in the framework of an in-situ galaxy formation scenario, as a result of the transition from an early dust-obscured star-forming phase (mainly pinpointed by our FIR selection) to a late radio-loud quasar phase (preferentially sampled by the optical/radio selection).

Background of radio photons from primordial black holes

ABSTRACT We compute the isotropic radiation background due to Hawking emission from primordial black holes (PBHs), and examine if this background is a viable option in explaining the excess radiowave background observed by the Absolute Radiometer for Cosmology, Astrophysics and Diffuse Emission (ARCADE2) and Long Wavelength Array (LWA1) experiments at $\lesssim 1\, {\rm GHz}$. We find that even under the extreme assumption that all of the dark matter is in the form of PBHs, the radio brightness temperature induced by Hawking evaporation of PBHs is $\mathcal {O}(10^{-46})\,$K, highly subdominant compared to the cosmic microwave background. The main reason for this is that for PBHs in the mass range ∼1012–${10^{14}}\, {\rm kg}$, which can be constrained by Hawking emission, the spectrum peaks at 107 to ${10^{5}}\, {\rm eV}$. As the Hawking spectrum is power law suppressed towards lower energies, negligible flux of $\mu \mathrm{eV}$ photons is obtained. The peak of the Hawking spectrum shifts to lower energies for higher masses, but the number density is low and so is the specific intensity. Because Hawking emission from PBHs is thus unable to explain the observed excess radio background, we also consider the alternative possibility of radio emission from gas accretion on to supermassive PBHs. These PBHs can readily produce strong radio emission that could easily explain the ARCADE2/LWA1 excess.

Distant probes of rotation measure structure: where is the Faraday rotation towards the Magellanic Leading Arm?

ABSTRACT Faraday rotation measures (RMs) should be interpreted with caution because there could be multiple magneto-ionized medium components that contribute to the net Faraday rotation along sightlines. We introduce a simple test using Galactic diffuse polarized emission that evaluates whether structures evident in RM observations are associated with distant circumgalactic medium or foreground interstellar medium. We focus on the Magellanic Leading Arm region where a clear excess of RM was previously reported. There are two gaseous objects standing out in this direction: the distant Magellanic Leading Arm and the nearby Antlia supernova remnant (SNR). We recognized narrow depolarized filaments in the $2.3\, \rm GHz$ S-band Polarization All Sky Survey image that overlaps with the reported RM excess. We suggest that there is a steep gradient in Faraday rotation in a foreground screen arising from the Antlia SNR. The estimated strength of the line-of-sight component of the magnetic field is $B_{\parallel }\sim 5\, \rm \mu G$, assuming that the excess of RM is entirely an outcome of the magnetized supernova shell. Our analysis indicates that the overlap between the RM excess and the Magellanic Leading Arm is only a remarkable coincidence. We suggest for future RM grid studies that checking Galactic diffuse polarization maps is a convenient way to identify local Faraday screens.

Multi-epoch searches for relativistic binary pulsars and fast transients in the Galactic Centre

ABSTRACT The high stellar density in the central parsecs around the Galactic Centre makes it a seemingly favourable environment for finding relativistic binary pulsars. These include pulsars orbiting other neutron stars, stellar-mass black holes, or the central supermassive black hole, Sagittarius A*. Here, we present multi-epoch pulsar searches of the Galactic Centre at four observing frequencies, $4.85,\, 8.35,\, 14.6,\, 18.95\, {\rm GHz}$, using the Effelsberg 100-m radio telescope. Observations were conducted 1 yr prior to the discovery of, and during monitoring observations of, the Galactic Centre magnetar PSR J1745−2900. Our data analysis features acceleration searches on progressively shorter time series to maintain sensitivity to relativistic binary pulsars. The multi-epoch observations increase the likelihood of discovering transient or nulling pulsars, or ensure orbital phases are observed at which acceleration search methods work optimally. In ${\sim}147\, {\rm h}$ of separate observations, no previously undiscovered pulsars have been detected. Through calibration observations, we conclude this might be due to insufficient instantaneous sensitivity, caused by the intense continuum emission from the Galactic Centre, its large distance, and, at higher frequencies, the aggregate effect of steep pulsar spectral indices and atmospheric contributions to the system temperature. Additionally, we find that for millisecond pulsars in wide circular orbits $({\lesssim}800\, {\rm d})$ around Sagittarius A*, linear acceleration effects cannot be fully corrected in deep observations $(9\, {\rm h})$ with existing software tools. Pulsar searches of the Galactic Centre with the next generation of radio telescopes – such as MeerKat, ngVLA, and SKA1-mid – will have improved chances of uncovering this elusive population.

Unveiling the nature of 11 dusty star-forming galaxies at the peak of cosmic star formation history

ABSTRACT We present a panchromatic study of 11 (sub-)millimetre selected DSFGs with spectroscopically confirmed redshift (1.5 < zspec < 3) in the GOODS-S field, with the aim of constraining their astrophysical properties (e.g. age, stellar mass, dust, and gas content) and characterizing their role in the context of galaxy evolution. The multiwavelength coverage of GOODS-S, from X-rays to radio band, allow us to model galaxy SED by using cigale z with a novel approach, based on a physical motivated modelling of stellar light attenuation by dust. Median stellar mass (≃ 6.5 × 1010 M⊙) and SFR (≃ 241 M⊙ yr−1) are consistent with galaxy main sequence at z ∼ 2. The galaxies are experiencing an intense and dusty burst of star formation (medianLIR ≃ 2 × 1012L⊙), with a median age of 750 Myr. The high median content of interstellar dust (Mdust ≃ 5 × 108 M⊙) suggests a rapid enrichment of the ISM (on time-scales ∼108 yr). We derived galaxy total and molecular gas content from CO spectroscopy and/or Rayleigh–Jeans dust continuum (1010 ≲Mgas/M⊙ ≲ 1011), depleted over a typical time-scale τdepl ∼ 200 Myr. X-ray and radio luminosities (LX = 1042–1044 erg s−1,L$_{1.5\, {\rm GHz}}=10^{30}\!-\!10^{31}$ erg s−1,L$_{6\, {\rm GHz}}=10^{29}\!-\!10^{30}$ erg s−1) suggest that most of the galaxies hosts an accreting radio-silent/quiet SMBH. This evidence, along with their compact multiwavelength sizes (median rALMA ∼ rVLA = 1.8 kpc, rHST = 2.3 kpc) measured from high-resolution imaging (θres ≲ 1 arcsec), indicates these objects as the high-z star-forming counterparts of massive quiescent galaxies, as predicted e.g. by the in situ scenario. Four objects show some signatures of a forthcoming/ongoing AGN feedback, which is thought to trigger the morphological transition from star-forming discs to ETGs.

New constraints on the 1.4 GHz source number counts and luminosity functions in the Lockman Hole field

ABSTRACT We present a study of the 1173 sources brighter than $S_{1.4\, \rm GHz}= 120\, \mu$Jy detected over an area of $\simeq 1.4\, \hbox{deg}^{2}$ in the Lockman Hole field. Exploiting the multiband information available in this field for ∼79 per cent of the sample, sources have been classified into radio loud (RL) active galactic nuclei (AGNs), star-forming galaxies (SFGs), and radio quiet (RQ) AGNs, using a variety of diagnostics available in the literature. Exploiting the observed tight anticorrelations between IRAC band 1 or band 2 and the source redshift we could assign a redshift to 177 sources missing a spectroscopic measurement or a reliable photometric estimate. A Monte Carlo approach was used to take into account the spread around the mean relation. The derived differential number counts and luminosity functions at several redshifts of each population show a good consistency with models and with earlier estimates made using data from different surveys and applying different approaches. Our results confirm that below $\sim 300\, \mu$Jy SFGs+RQ AGNs overtake RL AGNs that dominate at brighter flux densities. We also confirm earlier indications of a similar evolution of RQ AGNs and SFGs. Finally, we discuss the angular correlation function of our sources and highlight its sensitivity to the criteria used for the classification.

The complex large-scale magnetic fields in the first Galactic quadrant as revealed by the Faraday depth profile disparity

ABSTRACT The Milky Way is one of the very few spiral galaxies known to host large-scale magnetic field reversals. The existence of the field reversal in the first Galactic quadrant near the Sagittarius spiral arm has been well established, yet poorly characterized due to the insufficient number of reliable Faraday depths (FDs) from extragalactic radio sources (EGSs) through this reversal region. We have therefore performed broad-band (1–$2\, {\rm GHz}$) spectropolarimetric observations with the Karl G. Jansky Very Large Array (VLA) to determine the FD values of 194 EGSs in the Galactic longitude range of 20°–52° within ±5° from the Galactic mid-plane, covering the Sagittarius arm tangent. This factor of five increase in the EGS FD density has led to the discovery of a disparity in FD values across the Galactic mid-plane in the Galactic longitude range of 40°–52°. Combined with existing pulsar FD measurements, we suggest that the Sagittarius arm can host an odd-parity disc field. We further compared our newly derived EGS FDs with the predictions of three major Galactic magnetic field models, and concluded that none of them can adequately reproduce our observational results. This has led to our development of new, improved models of the Milky Way disc magnetic field that will serve as an important step towards major future improvements in Galactic magnetic field models.

Mapping the relativistic electron gas temperature across the sky

ABSTRACT With increasing sensitivity, angular resolution, and frequency coverage, future cosmic microwave background (CMB) experiments like PICO will allow us to access new information about galaxy clusters through the relativistic thermal Sunyaev–Zeldovich (SZ) effect. We will be able to map the temperature of relativistic electrons across the entire sky, going well beyond a simple detection of the relativistic SZ effect by cluster stacking methods that currently define the state-of-the-art. Here, we propose a new map-based approach utilizing SZ-temperature moment expansion and constrained-ILC methods to extract electron gas temperature maps from foreground-obscured CMB data. This delivers a new independent map-based observable, the electron temperature power spectrum $T_{\rm e}^{yy}(\ell)$, which can be used to constrain cosmology in addition to the Compton-y power spectrum $C_\ell ^{yy}(\ell)$ . We find that PICO has the required sensitivity, resolution, and frequency coverage to accurately map the electron gas temperature of galaxy clusters across the full sky, covering a broad range of angular scales. Frequency coverage at $\nu \gtrsim 300\, {\rm GHz}$ plays an important role for extracting the relativistic SZ effect in the presence of foregrounds. For Coma, PICO will allow us to directly reconstruct the electron temperature profile using the relativistic SZ effect. Coma’s average electron temperature will be measured to 10σ significance after foreground removal using PICO. Low angular resolution CMB experiment like LiteBIRD could achieve 2σ to 3σ measurement of the electron temperature of this largest cluster. Our analysis highlights a new spectroscopic window into the thermodynamic properties of galaxy clusters and the diffuse electron gas at large angular scales.

Photonic instantaneous microwave frequency measurement with adjustable measurement range based on an electro-optical polarization modulator.

This paper presents a simple photonic-assisted instantaneous microwave frequency measurement approach with an adjustable measurement range. In our scheme, different polarization processing is performed on the upper and the lower branches, and then the powers of the two branches are compared to obtain the amplitude comparison function (ACF), which provides the frequency-amplitude mapping. The measurement system is significantly simplified since only one polarization modulator (PolM) and one single laser source are required. In addition, our scheme achieves a larger ACF slope, which significantly increases the measurement resolution. In the simulation, a frequency measurement over the range of 3-42.8 GHz with measurement errors within $\pm 0.1\;{\rm GHz}$±0.1GHz is achieved by optimizing the dc bias voltage applied to the PolM and the polarization angle. This scheme provides a reference for measuring the unknown instantaneous frequency of the received signal from a radar system.

OUP accepted manuscript

Using magnetohydrodynamic simulations of fluctuation dynamos, we perform broad-bandwidth synthetic observations to investigate the properties of polarized synchrotron emission and the role that Faraday rotation plays in inferring the polarized structures in the intracluster medium (ICM) of galaxy clusters. In the saturated state of the dynamo, we find a Faraday depth (FD) dispersion $\sigma_{\rm FD} \approx 100$ rad m$^{-2}$, in agreement with observed values in the ICM. Remarkably, the FD power spectrum is qualitatively similar to $M(k)/k$, where $M(k)$ is the magnetic spectrum and $k$ the wavenumber. However, this similarity is broken at high $k$ when FD is obtained by applying RM synthesis to polarized emission from the ICM due to poor resolution and complexities of spectrum in FD space. Unlike the Gaussian probability distribution function (PDF) obtained for FD, the PDF of the synchrotron intensity is lognormal. A relatively large $\sigma_{\rm FD}$ in the ICM gives rise to strong frequency-dependent variations of the pixel-wise mean and peak polarized intensities at low frequencies ($\lesssim 1.5\,{\rm GHz}$). The mean fractional polarization $\langle p \rangle$ obtained at the resolution of the simulations increases from $<0.1$ at 0.5 GHz to its intrinsic value of $\sim0.3$ at 6 GHz. Beam smoothing significantly affects the polarization properties below $\lesssim 1.5\,{\rm GHz}$, reducing $\langle p \rangle$ to $\lesssim 0.01$ at 0.5 GHz. At frequencies $\gtrsim 5\,{\rm GHz}$, polarization remains largely unaffected, even when recovered using RM synthesis. Thus, our results underline the need for high-frequency ($\gtrsim 5\,{\rm GHz}$) observations with future radio telescopes to effectively probe the properties of polarized emission in the ICM.

Probing primordial 3He from hyperfine line afterglows around supercritical black holes

Abstract We consider the possibility of the detection of 3HeII hyperfine line (rest frequency, $8.67 \, \rm GHz$) emission from ionized zones around accreting black holes (BHs) formed at high redshifts, z = 15–30. We show that the brightness temperature in 8.67GHz line increases and reaches a peak value after the accretion onto the BH exhausts and HeIII recombines into HeII. This period of brightening last up to 40 million years. We find that during this period the maximum brightness temperature reaches ≃ 0.2–0.5μK, depending on the epoch when such a black hole starts growing. The maximum angular size of the region emitting in the hyperfine line is around 0.5′. The flux from such a region ($\simeq 0.3 \, \rm nJy$) is too small to be detected by SKA1-MID. The RMS of the collective flux from many emitting regions from a volume bounded by the synthesized beam and the band-width of SKA1-MID might reach 100 nJy, which is potentially detectable by SKA1-MID.