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Register a new userThe contribution of discrete sources to the sky temperature at 144 MHz
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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.
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We present a morphological and spectral study of a sample of 99 BL Lacs using the LOFAR Two-Metre Sky Survey Second Data Release (LDR2). Extended emission has been identified at gigahertz frequencies around BL Lacs, but with LDR2 it is now possible to systematically study their morphologies at 144 MHz, where more diffuse emission is expected. LDR2 reveals the presence of extended radio structures around 66/99 of the BL Lac nuclei, with angular extents ranging up to 115 arcseconds, corresponding to spatial extents of 410 kpc. The extended emission is likely to be both unbeamed diffuse emission and beamed emission associated with relativistic bulk motion in jets. The spatial extents and luminosities of the extended emission are consistent with the AGN unification scheme where BL Lacs correspond to low-excitation radio galaxies with the jet axis aligned along the line-of-sight. While extended emission is detected around the majority of BL Lacs, the median 144-1400 MHz spectral index and core dominance at 144 MHz indicate that the core component contributes ~42% on average to the total low-frequency flux density. A stronger correlation was found between the 144 MHz core flux density and the gamma-ray photon flux (r = 0.69) compared to the 144 MHz extended flux density and the gamma-ray photon flux (r = 0.42). This suggests that the radio-to-gamma-ray connection weakens at low radio frequencies because the population of particles that give rise to the gamma-ray flux are distinct from the electrons producing the diffuse synchrotron emission associated with spatially-extended features.
The contribution of the Unresolved Extragalactic Radio Sources to the diffuse brightness of the sky was evaluated using the source number - flux measurements available in literature. We first optimized the fitting function of the data based on number counts distribution. We then computed the brightness temperature at various frequencies from 151 MHz to 8440 MHz and derived its spectral dependence. As expected the frequency dependence can be described by a power law with a spectral index $gamma simeq -2.7$, in agreement with the flux emitted by the {it steep spectrum} sources. The contribution of {it flat spectrum} sources becomes relevant at frequencies above several GHz. Using the data available in literature we improved our knowledge of the brightness of the unresolved extragalactic radio sources. The results obtained have general validity and they can be used to disentangle the various contributions of the sky brightness and to evaluate the CMB temperature.
Polarization measurements at low radio frequencies allow detection of small Faraday rotation measures caused by regular magnetic fields in galaxies and in the foreground of the Milky Way. The galaxy M31 was observed in two overlapping pointings with the Westerbork Synthesis Radio Telescope (WSRT) resulting in ~4 resolution in total intensity and linearly polarized emission. The frequency range 310-376 MHz was covered by 1024 channels which allowed the application of RM synthesis. We derived a data cube in Faraday depth and compared two symmetric ranges of negative and positive Faraday depths. This new method avoids the range of high instrumental polarization and allows the detection of very low degrees of polarization. For the first time, diffuse polarized emission from a nearby galaxy is detected below 1 GHz. The degree of polarization is only 0.21 +/- 0.05 %, consistent with extrapolation of internal depolarization from data at higher radio frequency. A catalogue of 33 polarized sources and their Faraday rotation in the M31 field is presented. Their average depolarization is DP(90,20) = 0.14 +/- 0.02, 7 times stronger depolarized than at 1.4 GHz. We argue that this strong depolarization originates within the sources, e.g. in their radio lobes, or in intervening galaxies on the line of sight. On the other hand, the Faraday rotation of the sources is mostly produced in the foreground of the Milky Way and varies significantly across the ~9 square degree M31 field. As expected, polarized emission from M31 and extragalactic background sources is much weaker at low frequencies compared to the GHz range. Future observations with LOFAR, with high sensitivity and high angular resolution to reduce depolarization, may reveal diffuse polarization from the outer disks and halos of galaxies.
The prominent radio source Hercules A features complex structures in its radio lobes. Although it is one of the most comprehensively studied sources in the radio sky, the origin of the ring structures in the Hercules A radio lobes remains an open question. We present the first sub-arcsecond angular resolution images at low frequencies (<300 MHz) of Hercules A, made with the International LOFAR Telescope. With the addition of data from the Karl G. Jansky Very Large Array, we mapped the structure of the lobes from 144 MHz to 7 GHz. We explore the origin of the rings within the lobes of Hercules A, and test whether their properties are best described by a shock model, where shock waves are produced by the jet propagating in the radio lobe, or by an inner-lobe model, where the rings are formed by decelerated jetted plasma. From spectral index mapping our large frequency coverage reveals that the curvature of the different ring spectra increases with distance away from the central active galactic nucleus. We demonstrate that the spectral shape of the rings is consistent with synchrotron aging, which speaks in favor of an inner-lobe model where the rings are formed from the deposition of material from past periods of intermittent core activity.
Recent results have suggested that active galactic nuclei (AGN) could provide enough photons to reionise the Universe. We assess the viability of this scenario using a semi-numerical framework for modeling reionisation, to which we add a quasar contribution by constructing a Quasar Halo Occupation Distribution (QHOD) based on Giallongo et al. observations. Assuming a constant QHOD, we find that an AGN-only model cannot simultaneously match observations of the optical depth $tau_e$, neutral fraction, and ionising emissivity. Such a model predicts $tau_e$ too low by $sim 2sigma$ relative to Planck constraints, and reionises the Universe at $zlesssim 5$. Arbitrarily increasing the AGN emissivity to match these results yields a strong mismatch with the observed ionising emissivity at $zsim 5$. If we instead assume a redshift-independent AGN luminosity function yielding an emissivity evolution like that assumed in Madau & Haardt model, then we can match $tau_e$ albeit with late reionisation, however such evolution is inconsistent with observations at $zsim 4-6$ and poorly motivated physically. These results arise because AGN are more biased towards massive halos than typical reionising galaxies, resulting in stronger clustering and later formation times. AGN-dominated models produce larger ionising bubbles that are reflected in $simtimes 2$ more 21cm power on all scales. A model with equal parts galaxies and AGN contribution is still (barely) consistent with observations, but could be distinguished using next-generation 21cm experiments HERA and SKA-low. We conclude that, even with recent claims of more faint AGN than previously thought, AGN are highly unlikely to dominate the ionising photon budget for reionisation.
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