Introduction


Scientific Background

The Big Bang theory is the cornerstone of modern cosmology. Its three major predictions have all been confirmed by observations: (1) the expansion of the universe was verified by Hubble's observations; (2) the primordial nucleosynthesis was confirmed through measurements of the primordial chemical abundances of isotopes such as hydrogen, helium, and lithium; (3) the existence of the first light in the universe was verified by the measurement of the cosmic microwave background (CMB). Among these, measurements of primordial light elements have determined the total amount of baryonic (visible) matter in the universe, while precise measurements of the (CMB) demonstrated that all baryonic matter was detectable about 380,000 years after the Big Bang. Thanks to the advancement in large optical telescope technology, surveys of baryonic matter in the visible light spectrum have now almost covered the period from when galaxies began to form. Results indicate that as the universe evolves, the amount of detectable baryonic matter decreases: in the present-day universe, roughly only half is detectable. Where is the other half of the baryonic matter? This is the long-standing "missing baryon" problem, which not only involves the formation and evolution of large-scale structures but has also become a bottleneck for understanding galaxy formation and evolution. Galaxy formation and evolution is one of the hot frontiers in astrophysics. In the National Natural Science Foundation of China's 2021-2035 Strategic Plan for the Development of Astronomy, "galaxies and cosmology" has been listed as a priority development direction.

Cosmological hydrodynamic simulations provide theoretical guidance for solving the "missing baryons" problem. Although different numerical simulations yield results with significant quantitative differences, qualitatively they all indicate that during the formation and evolution of large-scale structures and galaxies, a large amount of baryonic matter is heated to millions of degrees, existing in the form of low-density gas within the cosmic web structure (intergalactic medium, IGM) or surrounding galaxies (circumgalactic medium, CGM). Their radiation signals are extremely weak and lie in the soft X-ray band, which currently lacks effective observational means for direct detection, thus creating the illusion of "missing baryons". This theoretical prediction has received some observational support in recent years. For example, the Planck satellite seems to have detected the disturbance of hot gas on the cosmic microwave background radiation (the so-called Sunyaev-Zeldovich or SZ effect), thus indirectly confirming the existence of hot gas in the IGM. Additionally, long-exposure observations of selected background active galactic nuclei (which contain supermassive black holes) using X-ray gratings on the Chandra and XMM-Newton satellites have detected absorption lines of foreground hot gas in several sightlines, also indirectly revealing the presence of hot gas in the IGM/CGM. However, both the SZ effect and most X-ray absorption line observations have low signal-to-noise ratios, so the results are highly controversial. Furthermore, indirect observations cannot establish the spatial distribution of hot gas or measure its physical and chemical properties, which are crucial to solving the "missing baryons" problem, rather than merely locating the missing baryons.

In the past decade or so, our understanding of galaxies has undergone a revolutionary change, recognizing that the cycling of gas within and around the CGM is an extremely important physical process in their evolutionary history: recaptured cold gas provides material for star formation within galaxies, while supernova explosions or central black hole jets can heat the gas and drive outflows. Clearly, the accretion process from the IGM and the feedback process within galaxies constitute the galactic ecosystem, with the CGM serving as an ideal laboratory for studying these physical processes. The newly released 10-year plan for astronomy (astro2020) came out from the National Academy of Science focuses on three major topics, one of which is the Cosmic Ecosystem. It highlights the importance of studying multiphase CGM / IGM, which contains more than 85% of the cosmic baryonic matter. So far, observations have been limited to the relatively cold CGM / IGM gas, while direct detection of hot gas still remains a blank space.



Methodology

Solving the "missing baryon" problem requires the direct detection of hot gases in the Circumgalactic Medium (CGM) and the Intergalactic Medium (IGM). Theoretical calculations indicate that the X-ray emission spectrum of low-density hot gases is dominated entirely by emission lines. Therefore, high-resolution spectroscopic observations might provide an effective means for detection. However, due to the very low density of the expected hot gases in the CGM/IGM, their emission lines are extremely weak, and currently, no equipment has sufficient sensitivity to directly detect them. Additionally, the signal is likely to be interfered with by radiation from hot gases within or around the Milky Way Galaxy. High-resolution spectroscopic observation can help us to distinguish the foreground, as spectral lines originating outside the Milky Way will be redshifted due to cosmic expansion. . Fig.1 shows the concept of this approach. It is evident that the photon energy resolution of the detector needs to be at least 4 eV to effectively detect signals from outside the Milky Way.


Fig.1 (right)：A simulated emission spectrum of hot gas in outskirts of a galaxy group. The panels show the results of convolving the spectrum with different detector spectral resolutions (in orange). Also shown (in blue) are the foreground and background contaminations. See main text for a detailed description of the simulation (Color figure online). From Cui, W. et al. 2020, "HUBS: Hot Universe Baryon Explorer", J. Low Temp. Phys. 199, 502




mock_spectra

Technology

X-ray gratings can achieve the required spectral resolution, but such dispersive spectrometers are not only inefficient but also incapable of obtaining spectra or images of diffuse sources. Commonly used CCDs are excellent X-ray imagers, but their spectral resolution is nearly two orders of magnitude short of the requirements. To make significant progress on the "missing baryon" problem, developing high-resolution non-dispersive X-ray spectrometers is essential. The X-ray microcalorimeters are internationally recognized as the revolutionary technique for high-resolution spectra, with integrated micrcalorimeter arrays also have the ability of imaging. In the Xiangshan science conference, interdisciplinary experts attending the meeting around "key scientific and technical problems on the exploration of missing baryons in the universe" conducted in-depth discussion, and agreed that the exploration of the "missing baryon" problem provides a good opportunity in leading the international cutting-edge researches. It is also realized that the HUBS project will encounter a lot of technical challenges, for the research and development of X-ray microcalorimeters has just started its pace recently in China.

The X-ray microcalorimeter mainly consists of three parts: absorber, thermometer and thermal connection. When the energy of an X-ray photon is absorbed, the temperature of the absorber rises. By rapidly and accurately measuring the temperature change, the thermometer can accurately obtain the energy of the incident photon. As the heat escapes to the heat sink, the detector's temperature recovers, ready to detect the next photon. The measurement accuracy of deposited energy from single photon (spectral resolution) is determined by system noise. To achieve a resolution of several eV, the microcalorimeter needs to operate at extremely low temperature below 100 mK to suppress thermal noise.


HUBS Scientific Payload

HUBS intends to conduct an all-sky survey of hot baryonic matter for the first time in the world with the support of self-developed cutting-edge technology and the participation of relevant domestic and overseas organizations. After several rounds of demonstration by experts, high degree of optimizations have been carried out around the core scientific objectives, and a preliminary satellite design of HUBS is formed. The main indicators/parameters are shown as follows:

Taking emission lines for detecting highly ionized (hydrogen-like or helium-like) oxygen elements as an example, the following table shows the detection performance of HUBS compared with existing and planned international scientific satellites. Thanks to its large field of view, HUBS is at least an order of magnitude more capable of detecting diffuse emission from hot gas than small-FoV X-ray missions. However, due to the large effective area, Athena is much more sensitive to point sources or point-like sources (such as active galactic nuclei, high redshift galaxies, etc.) than HUBS, so they are highly complementary in scientific objectives, which is very conducive to international cooperation. For example, as one of the main international partners of HUBS, the Netherlands Institute for Space Research is the CO-PI institution of Athena / X-IFU payload and has extensive experience in the development of microcalorimeters and related superconducting electronics. In the area of scientific research, HUBS scientific groups include researchers from universities and research institutes in Europe, the United States, Japan and elsewhere.

missions


HUBS Technology Development

The development of HUBS scientific payload relies on the breakthroughs in four key technologies: superconducting X-ray microcalorimeters, superconducting electronics for multiplexing readout, extremely low temperature cryogenics and wide-field X-ray focusing optics. Detectors on the focal plane are based on TES microcalorimeters and adopt a mixed array design of large and small pixels: the central 12×12 small pixel (sub-) array is surrounded by 60×60 large pixel (main) array. The area of the large pixel array is about 1mm2, which is 16 times that of the small pixel array. Therefore, the central sub-array is equivalent to a 3×3 large pixel array, which is mainly designed to enhance the ability of the absorption line observations. The energy resolution of large pixels is 2eV, while that of small pixels is 0.6eV. The total number of pixels in the detector array is mainly limited by the multiplexing readout technology.

The X-ray optics is designed with a nested Wolter-I structure consisting of multiple layers of thermo-formed thin glass lenses. The design has the advantage of low mass, but it requires a high temperature stability of the lens, so it is suitable for the observation facilities with not very high angular resolution requirements. HUBS optical system is designed to have a collection area of more than 1,000 cm2 (@1keV), a field of view of approximately 1 square degree, and an angular resolution of better than 1 arcmin.

In order to achieve the proposed energy resolution, TES array needs to work in the extremely low temperature environment below 100mK, so there is a high requirement for cryogenics. HUBS intends to use mechanical cooling technology for cooling from room temperature to 4 K, and adiabatic demagnetization refrigerator to cool from 4 K to 50 mK.

In addition, HUBS will feature an integrated platform-payload design, incorporating technologies such as near-zero deformation mechanical support structures, high precision temperature control, and high efficiency heat dissipation. The main indicators / parameters of the satellite are as follows:

The in-depth research on the key technologies for HUBS will also support potential applications in many non-astronomical fields and lead to increased technological capabilities through industrialization.


Timeline

Concept studies

Concept development

Path forward