ABSTRACT
The Large-aperture Experiment to detect the Dark Age (LEDA) was designed to measure the 21-cm signal from neutral hydrogen at Cosmic Dawn, z ≈ 15–30. Using observations made with the ≈ 200 m ...diameter core of the Owens Valley Radio Observatory Long Wavelength Array (OVRO–LWA), we present a 2D cylindrical spatial power spectrum for data at 43.1–53.5 MHz (zmedian ≈ 28) incoherently integrated for 4 h, and an analysis of the array sensitivity. Power from foregrounds is localized to a ‘wedge’ within k⊥, $k_\parallel$ space. After calibration of visibilities using five bright compact sources including Vir A, we measure Δ2(k) ≈ 2 × 1012 mK2 outside the foreground wedge, where an uncontaminated cosmological signal would lie, in principle. The measured Δ2(k) is an upper limit that reflects a combination of thermal instrumental and sky noise, and unmodelled systematics that scatter power from the wedge, as will be discussed. By differencing calibrated visibilities for close pairs of frequency channels, we suppress foreground sky structure and systematics, extract thermal noise, and use a mix of coherent and incoherent integration to simulate a noise-dominated power spectrum for a 3000 h observation and z = 16−37. For suitable calibration quality, the resulting noise level, Δ2(k) ≈ 100 mK2 (k = 0.3 Mpc−1), would be sufficient to detect peaks in the 21-cm spatial power spectrum due to early Ly-α and X-ray sources, as predicted for a range of theoretical model parameters.
ABSTRACT
Emulation of the Global (sky-averaged) 21-cm signal with neural networks has been shown to be an essential tool for physical signal modelling. In this paper, we present globalemu, a Global ...21-cm signal emulator that uses redshift as a character-defining variable alongside a set of astrophysical parameters to estimate the signal brightness temperature. Combined with physically motivated data pre-processing, this makes for a reliable and fast emulator that is relatively insensitive to the network design. globalemu can emulate a high-resolution signal in 1.3 ms in comparison to 133 ms, a factor of 102 improvement, when using the existing public state-of-the-art 21cmGEM. We illustrate, with the standard astrophysical models used to train 21cmGEM, that globalemu is almost twice as accurate and for a test set of ≈1700 signals we achieve a mean root mean squared error of 2.52 mK across the band z = 7–28 ≈10 per cent the expected noise of the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH). The models are parametrized by the star formation efficiency, f*, minimum virial circular velocity, Vc, X-ray efficiency, fX, cosmic microwave background optical depth, τ, the slope and low energy cut-off of the X-ray spectral energy density, α and νmin, respectively, and the mean free path of ionizing photons, Rmfp. globalemu provides a flexible framework for easily emulating updated simulations of the Global signal and in addition the neutral fraction history. The emulator is pip installable and available at https://github.com/htjb/globalemu. globalemu will be used extensively by the REACH collaboration.
ABSTRACT
The ARCADE2 and LWA1 experiments have claimed an excess over the cosmic microwave background (CMB) at low radio frequencies. If the cosmological high-redshift contribution to this radio ...background is between 0.1 per cent and 22 per cent of the CMB at 1.42 GHz, it could explain the tentative EDGES low-band detection of the anomalously deep absorption in the 21-cm signal of neutral hydrogen. We use the upper limit on the 21-cm signal from the Epoch of Reionization (z = 9.1) based on 141 h of observations with LOFAR to evaluate the contribution of the high-redshift Universe to the detected radio background. Marginalizing over astrophysical properties of star-forming haloes, we find (at 95 per cent CL) that the cosmological radio background can be at most 9.6 per cent of the CMB at 1.42 GHz. This limit rules out strong contribution of the high-redshift Universe to the ARCADE2 and LWA1 measurements. Even though LOFAR places limit on the extra radio background, excess of 0.1–9.6 per cent over the CMB (at 1.42 GHz) is still allowed and could explain the EDGES low-band detection. We also constrain the thermal and ionization state of the gas at z = 9.1, and put limits on the properties of the first star-forming objects. We find that, in agreement with the limits from EDGES high-band data, LOFAR data constrain scenarios with inefficient X-ray sources, and cases where the Universe was ionized by stars in massive haloes only.
ABSTRACT
Cosmic rays generated by supernovae carry away a significant portion of the lifetime energy emission of their parent star, making them a plausible mechanism for heating the early universe ...intergalactic medium (IGM). Following a review of the existing literature on cosmic ray heating, we develop a flexible model of this heating mechanism for use in 3D seminumerical 21-cm signal simulations and conduct the first investigations of the signatures it imprints on the 21-cm power spectrum and tomographic maps. We find that cosmic ray heating of the IGM is short-ranged, leading to heating clustered around star-forming sites, and a sharp contrast between heated regions of 21-cm emission and unheated regions of absorption. This contrast results in greater small-scale power for cosmic ray heated scenarios compared to what is found for X-ray heating, thus suggesting a way to test the nature of IGM heating with future 21-cm observations. Finally, we find an unexpectedly rich thermal history in models where cosmic rays can only escape efficiently from low-mass haloes, such as in scenarios where these energetic particles originate from population III star supernovae remnants. The interplay of heating and the Lyman–Werner feedback in these models can produce a local peak in the IGM kinetic temperature and, for a limited parameter range, a flattened absorption trough in the global 21-cm signal.
ABSTRACT
Maximally Smooth Functions (MSFs) are a form of constrained functions in which there are no inflection points or zero crossings in high-order derivatives. Consequently, they have ...applications to signal recovery in experiments where signals of interest are expected to be non-smooth features masked by larger smooth signals or foregrounds. They can also act as a powerful tool for diagnosing the presence of systematics. The constrained nature of MSFs makes fitting these functions a non-trivial task. We introduce maxsmooth, an open-source package that uses quadratic programming to rapidly fit MSFs. We demonstrate the efficiency and reliability of maxsmooth by comparison to commonly used fitting routines and show that we can reduce the fitting time by approximately two orders of magnitude. We introduce and implement with maxsmooth Partially Smooth Functions, which are useful for describing elements of non-smooth structure in foregrounds. This work has been motivated by the problem of foreground modelling in 21-cm cosmology. We discuss applications of maxsmooth to 21-cm cosmology and highlight this with examples using data from the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) and the Large-aperture Experiment to Detect the Dark Ages (LEDA) experiments. We demonstrate the presence of a sinusoidal systematic in the EDGES data with a log-evidence difference of 86.19 ± 0.12 when compared to a pure foreground fit. MSFs are applied to data from LEDA for the first time in this paper and we identify the presence of sinusoidal systematics. maxsmooth is pip installable and available for download at https://github.com/htjb/maxsmooth.
ABSTRACT
Cosmic dawn, the onset of star formation in the early universe, can in principle be studied via the 21-cm transition of neutral hydrogen, for which a sky-averaged absorption signal, ...redshifted to MHz frequencies, is predicted to be O(10–100) mK. Detection requires separation of the 21-cm signal from bright chromatic foreground emission due to Galactic structure, and the characterization of how it couples to instrumental response. In this work, we present characterization of antenna gain patterns for the Large-aperture Experiment to detect the Dark Ages (LEDA) via simulations, assessing the effects of the antenna ground-plane geometries used, and measured soil properties. We then investigate the impact of beam pattern uncertainties on the reconstruction of a Gaussian absorption feature. Assuming the pattern is known and correcting for the chromaticity of the instrument, the foregrounds can be modelled with a log-polynomial, and the 21-cm signal identified with high accuracy. However, uncertainties on the soil properties lead to percentage changes in the chromaticity that can bias the signal recovery. The bias can be up to a factor of two in amplitude and up to few per cent in the frequency location. These effects do not appear to be mitigated by larger ground planes, conversely gain patterns with larger ground planes exhibit more complex frequency structure, significantly compromising the parameter reconstruction. Our results, consistent with findings from other antenna design studies, emphasize the importance of chromatic response and suggest caution in assuming log-polynomial foreground models in global signal experiments.
ABSTRACT
Properties of the first generation of stars referred to as the Population III (Pop III) stars, such as their initial mass function (IMF), are poorly constrained by observations and have yet ...to converge between simulations. The cosmological 21-cm signal of neutral hydrogen is predicted to be sensitive to Lyman-band photons produced by these stars, thus providing a unique way to probe the first stellar population. In this paper, we investigate the impacts of the Pop III IMF on the cosmic-dawn 21-cm signal via the Wouthuysen–Field effect, Lyman–Werner feedback, Ly α heating, and cosmic microwave background heating. We calculate the emission spectra of star-forming haloes for different IMFs by integrating over individual metal-free stellar spectra, computed from a set of stellar evolution histories and stellar atmospheres, and taking into account variability of the spectra with stellar age. Through this study, we therefore relax two common assumptions: that the zero-age main-sequence emission rate of a Pop III star is representative of its lifetime mean emission rate, and that Pop III emission can be treated as instantaneous. Exploring bottom-heavy, top-heavy, and intermediate IMFs, we show that variations in the 21-cm signal are driven by stars lighter than 20 M⊙. For the explored models, we find maximum relative differences of 59 per cent in the cosmic-dawn global 21-cm signal, and 131 per cent between power spectra. Although this impact is modest, precise modelling of the first stars and their evolution is necessary for accurate prediction and interpretation of the 21-cm signal.
The Herschel Multi-tiered Extragalactic Survey (HerMES) has identified large numbers of dusty star-forming galaxies (DSFGs) over a wide range in redshift. A detailed understanding of these DSFGs is ...hampered by the limited spatial resolution of Herschel. We present 870 mu m 0".45 resolution imaging obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) of a sample of 29 HerMES DSFGs that have far-infrared (FIR) flux densities that lie between the brightest of sources found by Herschel and fainter DSFGs found via ground-based surveys in the submillimeter region. The ALMA imaging reveals that these DSFGs comprise a total of 62 sources (down to the 5sigma point-source sensitivity limit in our ALMA sample; sigma approximate 0.2 mJy). Optical or near-infrared imaging indicates that 36 of the ALMA sources experience a significant flux boost from gravitational lensing ( mu > 1.1), but only six are strongly lensed and show multiple images. We introduce and make use of UVMCMCFIT, a general-purpose and publicly available Markov chain Monte Carlo visibility-plane analysis tool to analyze the source properties. Combined with our previous work on brighter Herschel sources, the lens models presented here tentatively favor intrinsic number counts for DSFGs with a break near 8 mJy at 880 mu m and a steep fall-off at higher flux densities. Nearly 70% of the Herschel sources break down into multiple ALMA counterparts, consistent with previous research indicating that the multiplicity rate is high in bright sources discovered in single-dish submillimeter or FIR surveys. The ALMA counterparts to our Herschel targets are located significantly closer to each other than ALMA counterparts to sources found in the LABOCA ECDFS Submillimeter Survey. Theoretical models underpredict the excess number of sources with small separations seen in our ALMA sample. The high multiplicity rate and small projected separations between sources seen in our sample argue in favor of interactions and mergers plausibly driving both the prodigious emission from the brightest DSFGs as well as the sharp downturn above S sub(880) = 8 mJy.
PDF
