Foreground Emission Research Articles

Sensitivity of Bayesian 21 cm power spectrum estimation to foreground model errors

Abstract Power spectrum estimators are an important tool in efforts to detect the 21 cm brightness temperature fluctuations from neutral hydrogen at early times. An initial detection will likely be statistical in nature, meaning that it will not be possible to make a coherent map of the brightness temperature fluctuations; instead, only their variance will be measured against a background of noise and residual systematic effects. Optimal Quadratic Estimator (OQE)-based methods often apply an inverse covariance weighting to the data. However, inaccurate covariance modelling can lead to reduced sensitivity and, in some cases, severe signal loss. We recently proposed a Bayesian method to jointly estimate the 21 cm fluctuations, their power spectrum, and foreground emission. Instead of requiring a fixed a priori estimate of the covariance, we estimate the covariance as part of the inference. Choices of parametrization, particularly of the foregrounds, are subject to model errors and could lead to biases and other ill effects if not properly controlled. In this paper, we investigate the effects of inaccurate foreground models on 21 cm power spectrum recovery. Using simulated visibilities, we find that, even in the most extreme scenarios tested, our approach is capable of recovering 21 cm delay power spectrum estimates consistent with a known input signal for delays ≳ 300 ns (∼88% of the available Fourier modes). This is true even when using foreground models derived from modified foreground catalogs containing spatial and spectral perturbations at the quoted level of uncertainty on our foreground catalogs.

Implications of scattering for CMB foreground emission modelling

The extreme precision and accuracy of forthcoming observations of the cosmic microwave background (CMB) temperature and polarisation anisotropies, aiming to detect the tiny signatures of primordial gravitational waves or of light relic particles beyond the standard three light neutrinos, requires commensurate precision in the modelling of foreground Galactic emission that contaminates CMB observations. We evaluate the impact of second-order effects in Galactic foreground emission due to Thomson scattering off interstellar free electrons and to Rayleigh scattering off interstellar dust particles. We used existing sky survey data and models of the distribution of free electrons and dust within the Milky Way to estimate the amplitude and power spectra of the emission originating from radiation scattered either by free electrons or by dust grains at CMB frequencies. Both processes generate corrections to the total emission that are small compared to direct emission and are small enough not to pose problems for current-generation observations. However, B modes generated by Thomson scattering of incoming radiation off interstellar free electrons at CMB frequencies are within an order of magnitude compared to the sensitivity of the most advanced forthcoming CMB telescopes, and might require more precise evaluation in the future.

Mitigating calibration errors from mutual coupling with time-domain filtering of 21 cm cosmological radio observations

Abstract The 21 cm transition from neutral Hydrogen promises to be the best observational probe of the Epoch of Reionisation (EoR). This has led to the construction of low-frequency radio interferometric arrays, such as the Hydrogen Epoch of Reionization Array (HERA), aimed at systematically mapping this emission for the first time. Precision calibration, however, is a requirement in 21 cm radio observations. Due to the spatial compactness of HERA, the array is prone to the effects of mutual coupling, which inevitably lead to non-smooth calibration errors that contaminate the data. When unsmooth gains are used in calibration, intrinsically spectrally-smooth foreground emission begins to contaminate the data in a way that can prohibit a clean detection of the cosmological EoR signal. In this paper, we show that the effects of mutual coupling on calibration quality can be reduced by applying custom time-domain filters to the data prior to calibration. We find that more robust calibration solutions are derived when filtering in this way, which reduces the observed foreground power leakage. Specifically, we find a reduction of foreground power leakage by 2 orders of magnitude at k∥ ≈ 0.5 h Mpc−1.

Significant impact of Galactic dark matter particles on annihilation signals from Sagittarius analogues

We examine the gamma-ray signal from dark matter (DM) annihilation from analogues of the Sagittarius (Sgr) dwarf spheroidal galaxy in the Auriga cosmological simulations. For velocity-dependent annihilation cross sections, we compute emissions from simulated Sgr subhalos and from the Milky Way (MW) foreground. In addition to the annihilation signals from DM particles bound to Sgr, we consider for the first time the annihilation of DM particles bound to the MW that overlap spatially with Sgr. For p-wave models this contribution can enhance the signal by over an order of magnitude, while for d-wave models the enhancement can be over three orders of magnitude. For Sommerfeld and s-wave models, the corresponding emission does not significantly change. For the Sommerfeld model, the Sgr source can be visible above the MW foreground emission, while for s, p and d-wave models, the signal towards Sgr is most likely dominated by foreground MW emission. We interpret our results within the context of the observed gamma-ray emission from Sgr. We find that, given the background emission estimated from this region, the templates from simulations likely have spatial morphology that is too extended to explain the point-like emission that is observed.

Dissecting the planetary nebula NGC 4361 with MUSE

Optical integral field spectroscopy of planetary nebulae (PNe) offers a unique tool to explore the spatial relationships between the complex mixture of the many components (neutral, low- and high-ionisation gas, dust, and the central star) and their underlying physical conditions. The optical line and continuum emission in the very-high-ionisation Galactic PN, NGC 4361, were mapped to study the distribution of ionisation, extinction, electron temperature, and density. Based on commissioning data, MUSE Wide Field (60times 60$''$) normal-mode (4750-9300\ observations of NGC 4361 were reduced. The PN is larger than a single MUSE field and only the central 1 arcmin2 of the PN was observed in good conditions. Emission images in recombination and collisionally excited lines were extracted and the line ratios provided the dust extinction, electron density and temperature, and ionic abundances using standard techniques. A family of compact low-ionisation knots (dubbed 'freckles') was discovered and techniques developed to measure their spectra, independently of the extended high-ionisation medium. The nebula is confirmed as optically thin in the H-ionising continuum, based on its very low He i emission, even to the edges of the field. The electron temperature, $T_ e $, is shown to have a large-scale spatially coherent structure, as indicated by a previous long-slit spectrum. Prior to this study, no low-ionisation emission had been positively detected, although MUSE revealed both weak extended N ii and O ii and $>$100 spatially unresolved knots. There are several linear associations of these knots, but none of them point convincingly back to the central star. They have low-to-moderate ionisation with $T_ e $ sim 11000\,K e $ sim 1500 cm$^ $ and generally exhibit a higher extinction than the extended high-ionisation nebula. Within the MUSE field, a low-redshift emission-line galaxy was serendipitously found to be hiding behind NGC 4361. The spectrum of this dwarf galaxy was carefully extracted from the bright foreground nebular emission and the galaxy's line and continuum properties were then determined. NGC 4361 is not completely optically thin, as indicated by several extended regions and many compact features of lower ionisation emission. The low-ionisation 'freckles' identified here do not clearly appear to differ in (He, N, O, S) abundance with respect to the extended high-ionisation gas. The spatial distribution and radial velocities of these features suggest that they belong to a thick disk oriented perpendicular to the large-scale nebular gas, which may perhaps be remnants of an earlier structure. The low-luminosity disk galaxy at sim 87\,Mpc has bright H ii regions with metallicity 12+log(O/H) cong 8.4 and is suggested to be a Magellanic irregular or low-mass spiral.

Gamma-ray line emission from the Local Bubble

Deep-sea archives that include intermediate-lived radioactive 60Fe particles suggest the occurrence of several recent supernovae inside the present-day volume of the Local Bubble during the last ~10 Myr. The isotope 60Fe is mainly produced in massive stars and ejected in supernova explosions, which should always result in a sizeable yield of 26Al from the same objects. 60Fe and 26Al decay with lifetimes of 3.82 and 1.05 Myr, and emit γ rays at 1332 and 1809 keV, respectively. These γ rays have been measured as diffuse glow of the Milky Way, and would also be expected from inside the Local Bubble as foreground emission. Based on two scenarios, one employing a geometrical model and the other state-of-the-art hydrodynamics simulations, we estimated the expected fluxes of the 1332 and 1809 keV γ-ray lines, as well as the resulting 511 keV line from positron annihilation due to the 26Al β+ decay. We find fluxes in the range of 10−6–10−5 ph cm−2 s−1 for all three lines with isotropic contributions of 10–50%. We show that these fluxes are within reach for the upcoming COSI-SMEX γ-ray telescope over its nominal satellite mission duration of 2 yr. Given the Local Bubble models considered, we conclude that in the case of 10–20 Myr-old superbubbles, the distributions of 60Fe and26 Al are not co-spatial - an assumption usually made in γ-ray data analyses. In fact, this should be taken into account however when analysing individual nearby targets for their 60Fe to26 Al flux ratio as this gauges the stellar evolution models and the age of the superbubbles. A flux ratio measured for the Local Bubble could further constrain models of 60Fe deposition on Earth and its moon.

Planck Dust Polarization Power Spectra Are Consistent with Strongly Supersonic Turbulence

The polarization of the cosmic microwave background is rich in information but obscured by foreground emission from the Milky Way’s interstellar medium (ISM). To uncover relationships between the underlying turbulent ISM and the foreground power spectra, we simulated a suite of driven, magnetized, turbulent models of the ISM, varying the fluid properties via the sonic Mach number, MS , and magnetic (Alfvén) Mach number, MA . We measure the power spectra of density (ρ), velocity (v), magnetic field (H), total projected intensity (T), parity-even polarization (E), and parity-odd polarization (B). We find that the slopes of all six quantities increase with MS . Most increase with MA , while the magnetic field spectrum steepens with MA . By comparing spectral slopes of E and B to those measured by Planck, we infer typical values of MS and MA for the ISM. As the fluid velocity increases, MS>4 , the ratio of BB power to EE power increases to approach a constant value near the Planck-observed value of ∼0.5, regardless of the magnetic field strength. We also examine correlation coefficients between projected quantities, and find that r TE ≈ 0.3, in agreement with Planck, for appropriate combinations of MS and MA . Finally, we consider parity-violating correlations r TB and r EB.

Time-variable diffuse γ-ray foreground

ABSTRACT While the data analysis of γ-ray telescopes has now become more robust, some signals may be misinterpretations of a time-variable foreground emission from the Solar system, induced by low-energy cosmic-ray interactions with asteroids. Our goal is to provide emission templates for this time-variable diffuse γ-ray foreground by considering the populations of Main Belt Asteroids, Jovian and Neptunian Trojans, Kuiper Belt Objects, and the Oort Cloud. We model the spatial distribution of all known asteroids by performing 3D-fits to determine their density profiles and calculate their appearances by line-of-sight integrations. Because Earth and the asteroids are moving with respect to each other, we obtain diffuse emission templates varying on time-scales of days to decades. We find that the temporal variability can lead to flux enhancements that may mimic emission features unless properly taken into account. This variation is further enhanced by the Solar cycle as the cosmic-ray spectrum is attenuated by the Solar modulation potential, leading to a relative flux increase of the outer asteroids. The cumulative effect of the time-dependent emission is illustrated for the case of the ‘511 keV OSSE fountain’, and for emission features near the Galactic Centre, both being possible misinterpretations of the Solar system albedo. We recommend that γ-ray data analyses should always take into account the possibility of a time-variable foreground. Due to the ecliptic overlap with the Galactic plane, the Galactic emission is expected to be weaker by 0.1–20 per cent, depending on time (relative planetary motion), energy, and Solar cycle, which has consequences for the interpretation of dark matter annihilation cross sections, cosmic-ray spectra and amplitudes, as well as nucleosynthesis yields and related parameters.

Recovering 21cm Monopole Signals Without Smoothness

We expect the monopole signal at the lowest frequencies below 100MHz to be composed to two components: the deep Rayleigh-Jeans tail of the cosmic microwave background and two distinct features: the dark ages trough at ∼17MHz and the cosmic dawn trough at ∼75Mhz. These are hidden under orders of magnitude brighter foregrounds whose emission is approximately a power-law with a spectral index ≈−2.5. It is usually assumed that monopole signals of interest are separable from foregrounds on the basis of spectral smoothness. We argue that this is a difficult approach and likely impossible for the Dark Ages trough. Instead, we suggest that the fluctuations in the foreground emission around the sky should be used to build a model distribution of possible shapes of foregrounds, which can be used to constrain presence of a monopole signal. We implement this idea using normalizing flows and show that this technique allows for efficient unsupervised detection of the amplitude, width and center of the Dark Ages trough as well as Rayleigh-Jeans tail of the cosmic microwave background for a sufficiently sensitive experiment. We show that achromatic and smooth response significantly helps with foreground separation. We discuss the limitations of the inherent assumptions in this method and the impact on the design of future low-frequency experiments.

Mapping the Imprints of Stellar and Active Galactic Nucleus Feedback in the Circumgalactic Medium with X-Ray Microcalorimeters

The Astro2020 Decadal Survey has identified the mapping of the circumgalactic medium (CGM; the gaseous plasma around galaxies) as a key objective. We explore the prospects for characterizing the CGM in and around nearby galaxy halos with a future large-grasp X-ray microcalorimeter. We create realistic mock observations from hydrodynamical simulations (EAGLE, IllustrisTNG, and Simba) that demonstrate a wide range of potential measurements, which will address the open questions in galaxy formation and evolution. By including all background and foreground components in our mock observations, we show why it is impossible to perform these measurements with current instruments, such as X-ray CCDs, and why only microcalorimeters will allow us to distinguish the faint CGM emission from the bright Milky Way (MW) foreground emission lines. We find that individual halos of MW mass can, on average and depending on star formation rate, be traced out to large radii, around R 500, and for larger galaxies even out to R 200, using prominent emission lines, such as O vii, or O viii. Furthermore, we show that emission-line ratios for individual halos can reveal the radial temperature structure. Substructure measurements show that it will be possible to relate azimuthal variations to the feedback mode of the galaxy. We demonstrate the ability to construct temperature, velocity, and abundance ratio maps from spectral fitting for individual galaxy halos, which reveal rotation features, active galactic nucleus outbursts, and enrichment.

Identifying frequency de-correlated dust residuals in B-mode maps by exploiting the spectral capability of bolometric interferometry

Context. Astrophysical polarized foregrounds represent the most critical challenge in cosmic microwave background (CMB) B-mode experiments, requiring multifrequency observations to constrain astrophysical foregrounds and isolate the CMB signal. However, recent observations indicate that foreground emission may be more complex than anticipated. Not properly accounting for these complexities during component separation can lead to a bias in the recovered tensor-to-scalar ratio. Aims. In this paper we investigate how the increased spectral resolution provided by band-splitting in bolometric interferometry (BI) through a technique called spectral imaging can help control the foreground contamination in the case of an unaccounted-for Galactic dust frequency de-correlation along the line of sight (LOS). Methods. We focused on the next-generation ground-based CMB experiment CMB-S4 and compared its anticipated sensitivity, frequency, and sky coverage with a hypothetical version of the same experiment based on BI (CMB-S4/BI). We performed a Monte Carlo analysis based on parametric component separation methods (FGBuster and Commander) and computed the likelihood of the recovered tensor-to-scalar ratio, r. Results. The main result is that spectral imaging allows us to detect systematic uncertainties on r from frequency de-correlation when this effect is not accounted for in the component separation. Conversely, an imager such as CMB-S4 would detect a biased value of r and would be unable to spot the presence of a systematic effect. We find a similar result in the reconstruction of the dust spectral index, and we show that with BI we can more precisely measure the dust spectral index when frequency de-correlation is present and not accounted for in the component separation. Conclusions. The in-band frequency resolution provided by BI allows us to identify dust LOS frequency de-correlation residuals where an imager with a similar level of performance would fail. This creates the possibility of exploiting this potential in the context of future CMB polarization experiments that will be challenged by complex foregrounds in their quest for B-mode detection.

LSPE-Strip on-sky calibration strategy using bright celestial sources

In this paper we describe the global on-sky calibration strategy of the LSPE-Strip instrument. Strip is a microwave telescope operating in the Q- and W-bands (central frequencies of 43 and 95 GHz respectively) from the Observatorio del Teide in Tenerife, with the goal to observe and characterise the polarised Galactic foreground emission, and complement the observations of the polarisation of the cosmic microwave background to be performed by the LSPE-SWIPE instrument and other similar experiments operating at higher frequencies to target the detection of the B-mode signal from the inflationary epoch of the Universe. Starting from basic assumptions on some of the instrument parameters (NET, 1/f noise knee frequency, beam properties, observing efficiency) we perform realistic simulations to study the level of accuracy that can be achieved through observations of bright celestial calibrators in the Strip footprint (sky fraction of 30%) on the determination and characterisation of the main instrument parameters: global and relative gain factors (in intensity and in polarisation), polarisation direction, polarisation efficiency, leakage from intensity to polarisation, beams, window functions and pointing model.

Prototype Faraday Rotation Measure Catalogs from the Polarisation Sky Survey of the Universe’s Magnetism (POSSUM) Pilot Observations

The Polarisation Sky Survey of the Universe’s Magnetism (POSSUM) will conduct a sensitive ∼1 GHz radio polarization survey covering 20,000 deg2 of the southern sky with the Australian Square Kilometre Array Pathfinder. In anticipation of the full survey, we analyze pilot observations of low-band (800–1087 MHz), mid-band (1316–1439 MHz), and combined-band observations for an extragalactic field and a Galactic plane field (low-band only). Using the POSSUM processing pipeline, we produce prototype rotation measure (RM) catalogs that are filtered to construct prototype RM grids. We assess typical RM grid densities and RM uncertainties and their dependence on frequency, bandwidth, and Galactic latitude. We present a median filter method for separating foreground diffuse emission from background components and find that after application of the filter, 99.5% of the measured RMs of simulated sources are within 3σ of their true RM, with a typical loss of polarized intensity of 5% ± 5%. We find RM grid densities of 35.1, 30.6, 37.2, and 13.5 RMs per square degree and median uncertainties on RM measurements of 1.55, 12.82, 1.06, and 1.89 rad m−2 for the median-filtered low-band, mid-band, combined-band, and Galactic observations, respectively. We estimate that the full POSSUM survey will produce an RM catalog of ∼775,000 RMs with median-filtered low-band observations and ∼877,000 RMs with median-filtered combined-band observations. We construct a structure function from the Galactic RM catalog, which shows a break at 0.°7, corresponding to a physical scale of 12–24 pc for the nearest spiral arm.

Deep learning approach for identification of H ii regions during reionization in 21-cm observations – II. Foreground contamination

ABSTRACT The upcoming Square Kilometre Array Observatory will produce images of neutral hydrogen distribution during the epoch of reionization by observing the corresponding 21-cm signal. However, the 21-cm signal will be subject to instrumental limitations such as noise and galactic foreground contamination that pose a challenge for accurate detection. In this study, we present the SegU-Net v2 framework, an enhanced version of our convolutional neural network, built to identify neutral and ionized regions in the 21-cm signal contaminated with foreground emission. We trained our neural network on 21-cm image data processed by a foreground removal method based on Principal Component Analysis achieving an average classification accuracy of 71 per cent between redshift z = 7 and 11. We tested SegU-Net v2 against various foreground removal methods, including Gaussian Process Regression, Polynomial Fitting, and Foreground-Wedge Removal. Results show comparable performance, highlighting SegU-Net v2’s independence on these pre-processing methods. Statistical analysis shows that a perfect classification score with ${\rm AUC}=95~{{\ \rm per\ cent}}$ is possible for 8 < z < 10. While the network prediction lacks the ability to correctly identify ionized regions at higher redshift and differentiate well the few remaining neutral regions at lower redshift due to low contrast between 21-cm signal, noise, and foreground residual in images. Moreover, as the photon sources driving reionization are expected to be located inside ionized regions, we show that SegU-Net v2 can be used to correctly identify and measure the volume of isolated bubbles with $V_{\rm ion}\gt (10\, {\rm cMpc})^3$ at z > 9, for follow-up studies with infrared/optical telescopes to detect these sources.

Statistical properties of galactic synchrotron temperature and polarization maps — a multi-frequency comparison

Understanding the statistical properties of synchrotron emission from our Galaxy is valuable from the perspective of observations targeting signals of cosmological origin, as well as for understanding physical processes in our Galaxy. In this work, we extend the analysis of Rahman et al. [1] to — (a) all-sky observed maps of total foreground emissions at different frequencies provided by WMAP, Planck and Stockert-Villa,(b) component separated synchrotron temperature maps provided by WMAP, Planck and BeyondPlanck, and (c) component separated polarization maps provided by WMAP and Planck. The tools we use are Minkowski functionals and tensors.Our main goals are twofold. First, we determine the variation of morphological properties of the total foreground maps with observing frequency and compare with simulations. This study elucidates how the morphology varies with frequency due to the relative dominance of different foreground components at different frequencies. Secondly, we determine the nature of non-Gaussianity and statistical isotropy of synchrotron fluctuations towards smaller scales using various component separated synchrotron temperature and polarization maps. We find that all maps exhibit kurtosis-type non-Gaussianity, in agreement with the Haslam map. This result can be an important input for the modelling of small-scale synchrotron fluctuations for component separation pipelines. This also suggests that residual synchrotron contamination in CMB will manifest as kurtosis and will not be captured by three-point statistics.From a comparison of the different component separated maps, we find that BeyondPlanck and WMAP MCMC-e agree well with Haslam at all scales. The other maps show differences of varying statistical significance.Our analysis suggests a combination of residual AME and/or free-free emissions and point sources as contributing to these differences, and underscores the need for further improvement of the pipelines.

The advantage of Bolometric Interferometry for controlling Galactic foreground contamination in CMB primordial B-modes measurements

In the quest for the faint primordial B-mode polarization of the Cosmic Microwave Background, three are the key requirements for any present or future experiment: an utmost sensitivity, excellent control over instrumental systematic effects and over Galactic foreground contamination. Bolometric Interferometry (BI) is a novel technique that matches them all by combining the sensitivity of bolometric detectors, the control of instrumental systematics from interferometry and a software-based, tunable, in-band spectral resolution due to its ability to perform band-splitting during data analysis (spectral imaging). In this paper, we investigate how the spectral imaging capability of BI can help in detecting residual contamination in case an over-simplified model of foreground emission is assumed in the analysis. To mimic this situation, we focus on the next generation of ground-based CMB experiment, CMB-S4, and compare its anticipated sensitivities, frequency and sky coverage with a hypothetical version of the same experiment based on BI, CMB-S4/BI, assuming that lineof-sight (LOS) frequency decorrelation is present in dust emission but is not accounted for during component separation. We show results from a Monte-Carlo analysis based on a parametric component separation method (FGBuster), highlighting how BI has the potential to diagnose the presence of foreground residuals in estimates of the tensor-to-scalar ratio r in the case of unaccounted Galactic dust LOS frequency decorrelation.

Mitigating the effect of 1/f noise on the detection of the H i intensity mapping power spectrum from single-dish measurements

ABSTRACT We present and compare several methods to mitigate time-correlated (1/f) noise within the H i intensity mapping component of the MeerKAT Large Area Synoptic Survey (MeerKLASS). By simulating scan strategies, the H i signal, foreground emissions, white and correlated noise, we assess the ability of various data-processing pipelines to recover the power spectrum of H i brightness temperature fluctuations. We use MeerKAT pilot data to assess the level of 1/f noise expected for the MeerKLASS survey and use these measurements to create realistic levels of time-correlated noise for our simulations. We find the time-correlated noise component within the pilot data to be between 10 and 20 times higher than the white noise level at the scale of $k = 0.04 \, {\rm {Mpc}}^{-1}$. Having determined that the MeerKAT 1/f noise is partially correlated across all the frequency channels, we employ Singular Value Decomposition (SVD) as a technique to remove both the 1/f noise and Galactic foregrounds but find that over-cleaning results in the removal of H i power at large (angular and radial) scales; a power loss of 40 per cent is seen for a 3-mode SVD clean at the scale of $k = 0.04 \, {\rm {Mpc}}^{-1}$. We compare the impact of map-making using weighting by the full noise covariance (i.e. including a 1/f component), as opposed to just a simple unweighted binning, finding that including the time-correlated noise information reduces the excess power added by 1/f noise by up to 30 per cent.

A general Bayesian framework to account for foreground map errors in global 21-cm experiments

ABSTRACT Measurement of the global 21-cm signal during Cosmic Dawn and the Epoch of Reionization is made difficult by bright foreground emission which is 2–5 orders of magnitude larger than the expected signal. Fitting for a physics-motivated parametric forward model of the data within a Bayesian framework provides a robust means to separate the signal from the foregrounds, given sufficient information about the instrument and sky. It has previously been demonstrated that, within such a modelling framework, a foreground model of sufficient fidelity can be generated by dividing the sky into N regions and scaling a base map assuming a distinct uniform spectral index in each region. Using the Radio Experiment for the Analysis of Cosmic Hydrogen as our fiducial instrument, we show that, if unaccounted-for, amplitude errors in low-frequency radio maps used for our base map model will prevent recovery of the 21-cm signal within this framework, and that the level of bias in the recovered 21-cm signal is proportional to the amplitude and the correlation length of the base-map errors in the region. We introduce an updated foreground model that is capable of accounting for these measurement errors by fitting for a monopole offset and a set of spatially dependent scale factors describing the ratio of the true and model sky temperatures, with the size of the set determined by Bayesian evidence-based model comparison. We show that our model is flexible enough to account for multiple foreground error scenarios allowing the 21-cm sky-averaged signal to be detected without bias from simulated observations with a smooth conical log spiral antenna.

Robust Foreground Removal for Hydrogen Intensity Mapping (HIM) Experiments

This paper describes work done to develop robust methods for foreground removal in hydrogen intensity mapping (HIM) experiments. In HIM, one measures the 21cm line from neutral hydrogen (HI), which in turn provides information about the distribution of matter in the universe. However, foreground emission from the milky way and extra-galactic sources must be removed in order to obtain useful results. Methods developed for removing these foregrounds either have not been thoroughly tested on realistic data, or perform poorly when the instrument is not precisely calibrated. The goal of the work here is to develop a method to remove foregrounds that works on realistic, imperfectly calibrated data.

Constraints on Early Dark Energy from Isotropic Cosmic Birefringence.

Polarization of the cosmic microwave background (CMB) is sensitive to new physics violating parity symmetry, such as the presence of a pseudoscalar "axionlike" field. Such a field may be responsible for early dark energy (EDE), which is active prior to recombination and provides a solution to the so-called Hubble tension. The EDE field coupled to photons in a parity-violating manner would rotate the plane of linear polarization of the CMB and produce a cross-correlation power spectrum of E- and B-mode polarization fields with opposite parities. In this Letter, we fit the EB power spectrum predicted by the photon-axion coupling of the EDE model with a potential V(ϕ)∝[1-cos(ϕ/f)]^{3} to polarization data from Planck. We find that the unique shape of the predicted EB power spectrum is not favored by the data and obtain a first constraint on the photon-axion coupling constant, g=(0.04±0.16)M_{Pl}^{-1} (68% C.L.), for the EDE model that best fits the CMB and galaxy clustering data. This constraint is independent of the miscalibration of polarization angles of the instrument or the polarized Galactic foreground emission. Our limit on g may have important implications for embedding EDE in fundamental physics, such as string theory.