分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The absorption feature in the global spectrum is likely the first 21 cm observable from the cosmic dawn, which provides valuable insights into the earliest history of structure formation. We run a set of high-resolution hydrodynamic simulations of early structure formation to assess the effect of non-linear structure formation on the maximum absorption level (i.e. assuming the spin temperature coupling is saturated) of the global 21 cm spectrum in the standard cosmological framework. We ignore the star formation and feedbacks, which also tends to reduce the absorption signal, but take into account the inevitable non-linear density fluctuations in the intergalactic medium (IGM), shock heating, and Compton heating, which can reduce the absorption level. We found that the combination of these reduced the maximum absorption signal by $\sim 15\%$ at redshift 17, as compared with the homogeneous or linearly fluctuating IGM. These effects have to be carefully accounted for when interpreting the observational results, especially when considering the necessity of introducing new physics.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The free-free absorption of low frequency radio waves by thermal electrons in the warm ionized medium of our Galaxy becomes very significant at $\lesssim 10$ MHz (ultralong-wavelength), and the absorption strength depends on the radio frequency. Upcoming space experiments such as the Discovering Sky at the Longest wavelength (DSL) and Farside Array for Radio Science Investigations of the Dark ages and Exoplanets (FARSIDE) will produce high-resolution multi-frequency sky maps at the ultralong-wavelength, providing a new window to observe the Universe. In this paper we propose that from these ultralong-wavelength multi-frequency maps, the three-dimensional distribution of the Galactic electrons can be reconstructed. This novel and robust reconstruction of the Galactic electron distribution will be a key science case of those space missions. Ultralong-wavelength observations will be a powerful tool for studying the astrophysics relevant to the Galactic electron distribution, for example, the impacts of supernova explosions on electron distribution, and the interaction between interstellar atoms and ionizing photons escaped from the HII regions around massive stars. An animation shows the reconstructed results using {\tt NE2001} model as input test. On ArXiv, it is given in the directory: Ancillary files. In the paper the animation is linked to Fig. 5.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The free-free absorption of low frequency radio waves by thermal electrons in the warm ionized medium of our Galaxy becomes very significant at $\lesssim 10$ MHz (ultralong-wavelength), and the absorption strength depends on the radio frequency. Upcoming space experiments such as the Discovering Sky at the Longest wavelength (DSL) and Farside Array for Radio Science Investigations of the Dark ages and Exoplanets (FARSIDE) will produce high-resolution multi-frequency sky maps at the ultralong-wavelength, providing a new window to observe the Universe. In this paper we propose that from these ultralong-wavelength multi-frequency maps, the three-dimensional distribution of the Galactic electrons can be reconstructed. This novel and robust reconstruction of the Galactic electron distribution will be a key science case of those space missions. Ultralong-wavelength observations will be a powerful tool for studying the astrophysics relevant to the Galactic electron distribution, for example, the impacts of supernova explosions on electron distribution, and the interaction between interstellar atoms and ionizing photons escaped from the HII regions around massive stars. An animation shows the reconstructed results using {\tt NE2001} model as input test. On ArXiv, it is given in the directory: Ancillary files. In the paper the animation is linked to Fig. 5.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The reionization process is expected to be prolonged by the small-scale absorbers (SSAs) of ionizing photons, which have been seen as Lyman-limit systems in quasar absorption line observations. We use a set of semi-numerical simulations to investigate the effects of absorption systems on the reionization process, especially their impacts on the neutral islands during the late Epoch of Reionization (EoR). Three models are studied, i.e. the extreme case of no-SSA model with a high level of ionizing background, the moderate-SSA model with a relatively high level of ionizing background, and the dense-SSA model with a low level of ionizing background. We find that while the characteristic scale of neutral regions decreases during the early and middle stages of reionization, it stays nearly unchanged at about 10 comoving Mpc during the late stage for the no-SSA and moderate-SSA models. However, in the case of weak ionizing background in the dense-SSA model, the characteristic island scale shows obvious evolution, as large islands break into many small ones that are slowly ionized. The evolutionary behavior of neutral islands during the late EoR thus provides a novel way to constrain the abundance of SSAs. We discuss the 21-cm observation with the upcoming Square Kilometre Array. The different models can be distinguished by either the 21-cm imaging or the 21-cm power spectrum measurements.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： A redshifted 21 cm line absorption signature is commonly expected from the cosmic dawn era, when the first stars and galaxies formed. The detailed traits of this signal can provide important insight on the cosmic history. However, high precision measurement of this signal is hampered by the ionosphere refraction and absorption, as well as radio frequency interference (RFI). A space observation can solve the problem of the ionosphere, and the Moon can shield the RFI from the Earth. In this paper, we present simulations of the global spectrum measurement in the 30 -- 120 MHz frequency band on the lunar orbit, from the proposed Discovering the Sky at the Longest wavelength (DSL) project. In particular, we consider how the measured signal varies as the satellite moves along the orbit, take into account the blockage of different parts of the sky by the Moon and the antenna response. We estimate the sensitivity for such a 21 cm global spectrum experiment. An RMS noise level of $\le 0.05$ K is expected at 75 MHz after 10 orbits ($\sim$ 1 day) observation, for a frequency channel width of 0.4 MHz. We also study the influence of a frequency-dependent beam, which may generate complex spectral structures in the spectrum. Estimates of the uncertainties in the foreground and 21 cm model parameters are obtained.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： The radio sky at frequencies below $\sim10$ MHz is still largely unknown, this remains the last unexplored part of the electromagnetic spectrum in astronomy. The upcoming space experiments aiming at such low frequencies (ultra-long wavelength or ultra-low frequency) would benefit from reasonable expectations of the sky brightness distribution at relevant frequencies. In this work, we develop a radio sky model that is valid down to $\sim1$ MHz. In addition to the discrete HII objects, we take into account the free-free absorption by thermal electrons in the Milky Way's warm ionized medium (WIM). This absorption effect becomes obvious at $\lesssim10$ MHz, and could make the global radio spectrum turn over at $\sim3$ MHz. Our sky map shows unique features at the ultra-long wavelengths, including a darker Galactic plane in contrast to the sky at higher frequencies, and the huge shadows of the spiral arms on the sky map. It would be a useful guidance for designing the future ultra-long wavelength observations. Our Ultralong-wavelength Sky Model with Absorption (ULSA) model could be downloaded at https://doi.org/10.5281/zenodo.4454153.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Ground-based observation at frequencies below 30 MHz is hindered by the ionosphere of the Earth and radio frequency interference. To map the sky at these low frequencies, we have proposed the Discovering the Sky at the Longest wavelength mission (DSL, also known as the "Hongmeng" mission, which means "Primordial Universe" in Chinese) concept, which employs a linear array of micro-satellites orbiting the Moon. Such an array can be deployed to the lunar orbit by a single rocket launch, and it can make interferometric observations achieving good angular resolutions despite the small size of the antennas. However, it differs from the conventional ground-based interferometer array or even the previous orbital interferometers in many aspects, new data-processing methods need to be developed. In this work, we make a series of simulations to assess the imaging quality and sensitivity of such an array. We start with an input sky model and a simple orbit model, generate mock interferometric visibilities, and then reconstruct the sky map. We consider various observational effects and practical issues, such as the system noise, antenna response, and Moon blockage. Based on the quality of the recovered image, we quantify the imaging capability of the array for different satellite numbers and array configurations. For the first time, we make practical estimates of the point source sensitivity for such a lunar orbit array, and predict the expected number of detectable sources for the mission. Depending on the radio source number distribution which is still very uncertain at these frequencies, the proposed mission can detect $10^2 \sim 10^4$ sources during its operation.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： We forecast constraints on cosmological parameters in the interacting dark energy models using the mock data generated for neutral hydrogen intensity mapping (IM) experiments. In this work, we only consider the interacting dark energy models with energy transfer rate $Q=\beta H\rho_{\rm c}$, and take BINGO, FAST, SKA1-MID, and Tianlai as typical examples of the 21 cm IM experiments. We find that the Tianlai cylinder array will play an important role in constraining the interacting dark energy model. Assuming perfect foreground removal and calibration, and using the Tianlai-alone data, we obtain $\sigma(H_0)=0.19$ km s$^{-1}$ Mpc$^{-1}$, $\sigma(\Omega_{\rm m})=0.0033$ and $\sigma(\sigma_8)=0.0033$ in the I$\Lambda$CDM model, which are much better than the results of Planck+optical BAO (i.e. optical galaxy surveys). However, the Tianlai-alone data cannot provide a very tight constraint on the coupling parameter $\beta$ compared with Planck+optical BAO, while the Planck+Tianlai data can give a rather tight constraint of $\sigma(\beta)=0.00023$ due to the parameter degeneracies being well broken by the data combination. In the I$w$CDM model, we obtain $\sigma(\beta)=0.00079$ and $\sigma(w)=0.013$ from Planck+Tianlai. In addition, we also make a detailed comparison among BINGO, FAST, SKA1-MID, and Tianlai in constraining the interacting dark energy models. We show that future 21 cm IM experiments will provide a useful tool for exploring the nature of dark energy and play a significant role in measuring the coupling between dark energy and dark matter.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Fengquan Wu
Jixia Li
Shifan Zuo
Xuelei Chen
Santanu Das
John P. Marriner
Trevor M. Oxholm
Anh Phan
Albert Stebbins
Peter T. Timbie
Reza Ansari
Jean-Eric Campagne
Zhiping Chen
Yanping Cong
Qizhi Huang
Yichao Li
Tao Liu
Yingfeng Liu
Chenhui Niu
Calvin Osinga
摘要： The Tianlai Dish Pathfinder Array is a radio interferometer designed to test techniques for 21~cm intensity mapping in the post-reionization universe as a means for measuring large-scale cosmic structure. It performs drift scans of the sky at constant declination. We describe the design, calibration, noise level, and stability of this instrument based on the analysis of about $\sim 5 \%$ of 6,200 hours of on-sky observations through October, 2019. Beam pattern determinations using drones and the transit of bright sources are in good agreement, and compatible with electromagnetic simulations. Combining all the baselines, we make maps around bright sources and show that the array behaves as expected. A few hundred hours of observations at different declinations have been used to study the array geometry and pointing imperfections, as well as the instrument noise behaviour. We show that the system temperature is below 80~K for most feed antennas, and that noise fluctuations decrease as expected with integration time, at least up to a few hundred seconds. Analysis of long integrations, from 10 nights of observations of the North Celestial Pole, yielded visibilities with amplitudes of 20-30~mK, consistent with the expected signal from the NCP radio sky with $<10\,$mK precision for $1 ~\mathrm{MHz} \times 1~ \mathrm{min}$ binning. Hi-pass filtering the spectra to remove smooth spectrum signal yields a residual consistent with zero signal at the $0.5\,$mK level.
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