Optically luminous quasars at
z
> 5 are important probes of super-massive black hole (SMBH) formation. With new and future radio facilities, the discovery of the brightest low-frequency radio ...sources in this epoch would be an important new probe of cosmic reionization through 21-cm absorption experiments. In this work, we systematically study the low-frequency radio properties of a sample of 115 known spectroscopically confirmed
z
> 5 quasars using the second data release of the Low Frequency Array (LOFAR) Two Metre Sky survey (LoTSS-DR2), reaching noise levels of ∼80 μJy beam
−1
(at 144 MHz) over an area of ∼5720 deg
2
. We find that 41 sources (36%) are detected in LoTSS-DR2 at > 2
σ
significance and we explore the evolution of their radio properties (power, spectral index, and radio loudness) as a function of redshift and rest-frame ultra-violet properties. We obtain a median spectral index of −0.29
−0.09
+0.10
by stacking 93 quasars using LoTSS-DR2 and Faint Images of the Radio Sky at Twenty Centimetres (FIRST) data at 1.4 GHz, in line with observations of quasars at
z
< 3. We compare the radio loudness of the high-
z
quasar sample to a lower-
z
quasar sample at
z
∼ 2 and find that the two radio loudness distributions are consistent with no evolution, although the low number of high-
z
quasars means that we cannot rule out weak evolution. Furthermore, we make a first order empirical estimate of the
z
= 6 quasar radio luminosity function, which is used to derive the expected number of high-
z
sources that will be detected in the completed LoTSS survey. This work highlights the fact that new deep radio observations can be a valuable tool in selecting high-
z
quasar candidates for follow-up spectroscopic observations by decreasing contamination of stellar dwarfs and reducing possible selection biases introduced by strict colour cuts.
High-redshift quasars (
z
> 5) that also shine brightly at radio wavelengths are unique signposts of supermassive black hole activity in the early universe. However, bright radio sources at
z
≥ 5 ...are extremely rare and therefore we have started a campaign to search for new high-
z
quasars by combining an optical dropout selection driven by the
g
,
r
, and
z
bands from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys with low-frequency radio observations from the LOFAR Two-metre Sky Survey (LoTSS). Currently, LoTSS covers a large fraction of the northern sky (∼5720 deg
2
) to such a depth (median noise level ∼83 μJy beam
−1
) that about 30% of the general quasar population is detected − which is a factor of 5–10 more than previous large sky radio surveys such as NVSS and FIRST, respectively. In this paper, we present the discovery of 20 new quasars (and the independent confirmation of four) between 4.9 ≤
z
≤ 6.6. Out of the 24 quasars, 21 satisfy the traditional radio-loudness criterion of
R
=
f
5 GHz
/
f
4400 Å
> 10, with the full sample spanning
R
∼ 6–1000, thereby more than doubling the sample of known radio-loud quasars at
z
≥ 5. Our radio detection requirement strongly decreases the contamination of stellar sources and allows one to select these quasars in a broad redshift range. Despite selecting our quasar candidates using fewer and less conservative colour restrictions, both the optical and near-infrared colours, Ly
α
emission line properties, and dust reddening,
E
(
B
−
V
), measurements of our quasar sample do not deviate from the known radio-quiet quasar population, suggesting similar optical quasar properties of the radio-loud and radio-quiet quasar population at high-
z
. Our campaign demonstrates the potential for discovering new high-
z
quasar populations through next generation radio continuum surveys.
The Apollo 15 and 16 missions were the first to explore the Lunar surface chemistry by investigating about 10% of the Lunar surface using a remote sensing X-ray fluorescence spectrometer experiment. ...The data obtained have been extensively used to study Lunar formation history and geological evolution. In this work, a re-evaluation of the Apollo 15 and 16 X-ray fluorescence experiment is conducted with the aim of obtaining up-to-date empirical values for aluminum (Al) and magnesium (Mg) concentrations relative to silicon (Si) of the upper Lunar surface. An updated instrument response, a newly reconstructed Lunar trajectory orbit, and improved intensity ratio calculations were used to obtain new intensity ratio maps. The resulting Lunar Al/Si and Mg/Al X-ray maps show a clear distinction in Lunar mare and highland regions. The mean Al/Si and Mg/Al intensity ratios for the mare regions obtained from the newly obtained maps are 0.54 ± 0.07 and 0.54 ± 0.17, respectively; for the highland regions, the values are 0.76 ± 0.07 and 1.07 ± 0.13, respectively. For the Mg/Si intensity ratio, no clear distinction between Lunar features is obtained and we derived a mean value of 0.47 ± 0.13. Our determined intensity ratios are lower than previously published. These values can be used to infer concentration ratios when accounting for Solar activity, inter-orbit variability, and measurements from different instruments. We employed a correction to infer concentration ratios by comparing our intensity ratios directly to Lunar rock concentrations obtained from various Lunar missions.
Lyman alpha emitters (LAEs) in the Epoch of Reionization (EoR) offer valuable probes of both early galaxy evolution and the process of reionization itself; however, the exact evolution of their ...abundance and the nature of their emission remain open questions. We combine samples of 229 and 349 LAE candidates at
z
= 5.7 and
z
= 6.6, respectively, from the SILVERRUSH narrowband survey with deep Low Frequency Array (LOFAR) radio continuum observations in the European Large Area Infrared Space Observatory Survey-North 1 (ELAIS-N1) field to search for radio galaxies in the EoR and study the low-frequency radio properties of
z
≳ 5.7 LAE emitters. Our LOFAR observations reach an unprecedented noise level of ~20
μ
Jy beam
−1
at 150 MHz, and we detect five candidate LAEs at >5
σ
significance. Based on detailed spectral energy distribution modelling of independent multi-wavelength observations in the field, we conclude that these sources are likely O
II
emitters at
z
= 1.47, yielding no reliable
z
≳ 5.7 radio galaxy candidates. We examine the 111
z
= 5.7 and
z
= 6.6 LAE candidates from our panchromatic photometry catalogue that are undetected by LOFAR, finding contamination rates of 81–92% for the
z
= 5.7 and
z
= 6.6 subset of the LAE candidate samples. This subset of the full sample is biased towards brighter magnitudes and redder near-infrared colours. The contamination rates of the full sample will therefore likely be lower than the reported values. Contamination of these optically bright LAE samples by likely O
II
emitters is lowered significantly through constraints on the near-infrared colours, highlighting the need for infrared observations to robustly identify bright LAEs in narrowband surveys. Finally, the stacking of radio continuum observations for the robust LAE samples yields 2
σ
upper limits on radio luminosity of 8.2 × 10
23
and 8.7 × 10
23
W Hz
−1
at
z
= 5.7 and 6.6, respectively, corresponding to limits on their median star-formation rates of <53 and <56
M
⊙
yr
−1
.
We present the detection of 68 sources from the most sensitive radio survey in circular polarisation conducted to date. We used the second data release of the 144 MHz LOFAR Two-metre Sky Survey to ...produce circularly polarised maps with a median noise of 140 µJy beam
−1
and resolution of 20″ for ≈27% of the northern sky (5634 deg
2
). The leakage of total intensity into circular polarisation is measured to be ≈0.06%, and our survey is complete at flux densities ≥1 mJy. A detection is considered reliable when the circularly polarised fraction exceeds 1%. We find the population of circularly polarised sources is composed of four distinct classes: stellar systems, pulsars, active galactic nuclei, and sources unidentified in the literature. The stellar systems can be further separated into chromospherically active stars, M dwarfs, and brown dwarfs. Based on the circularly polarised fraction and lack of an optical counterpart, we show it is possible to infer whether the unidentified sources are likely unknown pulsars or brown dwarfs. By the completion of this survey of the northern sky, we expect to detect 300±100 circularly polarised sources.
The SCIAMACHY satellite instrument shows enhanced carbon monoxide (CO) columns in the Southern Hemisphere during the local Spring. Chemistry‐transport model simulations using the new GFEDv2 ...biomass‐burning emission database show a similar temporal and spatial CO distribution, indicating that the observed enhancements are mainly due to biomass burning (BB). Large differences between the year 2003 and 2004 are observed in both the measurements and the model for South America and Australia. This study analyzes the origin of these observed enhancements in the Southern Hemisphere. The fact that SCIAMACHY is sensitive to surface CO allows for the observation of enhanced CO columns in both emission areas and in areas that are affected by long‐range transport of CO. Model results show a large contribution of South American BB CO over Australian BB regions during the 2004 BB season of up to ∼30–35% and up to 55% further south, with smaller contributions for 2003. BB CO transported from southern Africa contributes up to ∼40% in 2003 and ∼30% in 2004. The results indicate that differences between SCIAMACHY CO and the model simulations over Australian BB areas are probably not only caused by uncertainties in local emissions but also in overseas emissions.
This paper presents a detailed global comparison of Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) and Measurement of Pollution in the Troposphere (MOPITT) carbon ...monoxide (CO) column measurements for the years 2004 and 2005. Qualitatively, spatiotemporal variations of SCIAMACHY and MOPITT are similar. Quantitative comparisons have been performed taking the effects of instrument noise errors, vertical sensitivities via the averaging kernel and a priori, different spatiotemporal sampling and clouds into account using simulated CO profiles from the TM4 model. SCIAMACHY and MOPITT CO columns are similar over tropical, subtropical, and Northern Hemisphere oceans as well as over boreal regions where SCIAMACHY and MOPITT agree to within 10% or 2 × 1017 molecules/cm2. The short‐wave infrared SCIAMACHY observations also provide information about lower tropospheric CO in Arctic and subarctic regions north of 60°N, where the MOPITT sensitivity is strongly reduced. South of 45°S, SCIAMACHY CO columns are 3–5 × 1017 molecules/cm2 smaller than MOPITT CO columns. Approximately 1.5 × 1017 molecules/cm2 (∼10%) of this difference is attributed to a bias in the SCIAMACHY CO columns, which is currently under investigation. The remaining difference is possibly related to MOPITT biases in this region. In the transition from oceans to dry desert regions, MOPITT CO total columns show a rapid increase of approximately 3 × 1017 molecules/cm2 (∼15%). While MOPITT and SCIAMACHY agree over oceans, MOPITT is approximately 5 × 1017 molecules/cm2 (∼25%) larger than SCIAMACHY results over dry land regions. The origin of this bias needs further investigation.
This paper presents a first quantitative and systematic analysis of one year of SCIAMACHY Carbon Monoxide (CO) total column retrievals from the IMLM algorithm (v6.3) using a chemistry‐transport model ...simulation. The global distribution of modeled and measured CO show similar spatial patterns: a north‐south gradient, low CO over mountains, and high CO over emission regions. CO column errors due to instrument noise are closely related to surface albedo and are less than 6% for monthly means at high surface albedo locations, improving to ∼1% for ideal circumstances: cloud‐free pixels, high surface albedo, and spatial averaging (3° × 2°). Quantitative comparison shows that measured and modeled seasonality agree very well at several locations with different types of seasonal cycles. Differences between SCIAMACHY CO and model results are less than 13% except for regions with large instrument‐noise errors. Differences larger than the 2σ instrument‐noise error (95% confidence interval) occur in some regions with small noise errors, for example southern Africa. In this case the SCIAMACHY CO variations are different from the model biomass‐burning emission seasonal cycle and more in agreement with observed fire count seasonality. The comparison with model results indicates that despite unforeseen time‐dependent instrument‐calibration complications, SCIAMACHY CO total column retrievals are of sufficient quality to provide useful new information on the global distribution and variation of CO.
We present the low-frequency radio spectra of nine high-redshift quasars at 5.6 ≤
z
≤ 6.6 using the Giant Metre Radio Telescope band-3, -4, and -5 observations (∼300−1200 MHz), archival Low ...Frequency Array (LOFAR; 144 MHz), and Very Large Array (VLA; 1.4 and 3 GHz) data. Five of the quasars in our sample have been discovered recently, representing some of the highest redshift radio bright quasars known at low frequencies. We model their radio spectra to study their radio emission mechanism and age of the radio jets by constraining the spectral turnover caused by synchrotron self-absorption (SSA) or free-free absorption (FFA). Besides J0309+2717, a blazar at
z
= 6.1, our quasars show no sign of a spectral flattening between 144 MHz and a few gigahertz, indicating there is no strong SSA or FFA absorption in the observed frequency range. However, we find a wide range of spectral indices between −1.6 and 0.05, including the discovery of 3 potential ultra-steep spectrum quasars. Using further archival Very Long Baseline Array data, we confirm that the radio spectral energy distribution of the blazar J0309+2717 likely turns over at a rest-frame frequency of 0.6−2.3 GHz (90−330 MHz observed frame), with a high-frequency break indicative of radiative ageing of the electron population in the radio lobes. Ultra-low frequency data below 50 MHz are necessary to constrain the absorption mechanism for J0309+2717 and the turnover frequencies for the other high-
z
quasars in our sample. A relation between linear radio jet size and turnover frequency has been established at low redshifts. If this relation were to hold at high redshifts, the limits on the turnover frequency of our sample suggest the radio jet sizes must be more extended than the typical sizes observed in other radio-bright quasars at similar redshift. To confirm this, deep radio follow-up observations with high spatial resolution are required.
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