Research Highlights

DoA faculty reviews the statistical distribution of exoplanets

How do planetary systems form and evolve? It is a scientific question that astronomers have long been wondering and also a key step toward understanding the origin of life and human beings on Earth. Since the discovery of the first planet orbiting around a Sun-like star outside of the Solar System in 1995 (which was awarded the Nobel Physics Prize in 2019), the number of known exoplanets has grown to over 4000. This large sample has enabled many detailed studies into the statistical distribution of planets and has sharpened our understanding of the planet formation process.

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Forming planets by pebble accretion: a metallicity gradient emerges

Following our previous research efforts, we have conducted numerical calculations about the thermal evolution of the envelope of pebble-accreting protoplanets. These protoplanets emerge early, when the protoplanetary gas disk is still present. Due to the accretion of solids, their envelope becomes hot, sublimating the infalling pebbles and transforming mm-sized solid particles (“pebbles”) into a metal vapor (e.g, SiO2). This transformation greatly affects the thermodynamical evolution of the protoplanet. For example, planets end up with a small "core" but may still undergo runaway gas accretion due to the high molecular weight and small pressure scale height of the atmosphere.

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History of the Solar Nebula from Paleomagnetism

The solar system formed about 4.6 billion years ago. During that time, left-over materials of gas and dust orbited the proto-Sun and formed a disk known as the “solar nebula”. Through a sequence of growth by coagulation and accretion, dust in the solar nebula eventually grow to form our planetary system. The solar nebula is the counterpart of “protoplanetary disks”, which are commonly found around other young stars in the Milky Way. Magnetic fields are known to play a crucial role in the formation, evolution, and dynamics of protoplanetary disks, and hence the processes of planet formation. Currently, attempts to directly measure or infer magnetic field from protoplanetary disks from astronomical observations have been unsuccessful. On the other hand, ancient meteorites have the potential to record the magnetic field in the solar nebular when they formed, which can be deciphered from techniques in the field known as paleomagnetism.

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PolarLight in the Orbit for Two Years

On 29 October 2020, the PolarLight X-ray polarimeter has been working in orbit for 2 years. On 29 October 2018, the CubeSat Tongchuan-1 with the PolarLight detector onboard was launched into a Low-Earth orbit from the Jiuquan satellite launching center. On December 18 of that year, the detector was powered on and detected the "first light" event. In March of 2019, after the commissioning phase of the satellite, PolarLight started scientific operations and has since been observing the Crab nebula, Sco X-1, as well as the in-orbit background. After two years, the detector is still working properly, without any performance degradation.

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Lyα forest as an emerging window into the epoch of reionization

During cosmic reionization (redshift z~8), the intergalactic medium (IGM) is heated by ultraviolet photons originating from stars and galaxies. This extreme heating leaves an imprint in the IGM by injecting the gas with additional thermal energy. The IGM will eventually relax and dissipate the surplus energy, leading to a simple relation between the temperature and the density of the IGM (see, e.g., McQuinn, 2016).

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Rotation inferred from proper motions of RR Lyraes in the Galactic bulge

RR Lyrae stars are pulsating, low-metallicity, core-helium-burning horizontal branch giants with ages older than 10 Gyr. Their period-luminosity relation allows their distances to be determined accurately, which enables us to study their spatial distribution and kinematics.

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A new estimator of resolved molecular gas in nearby galaxies

Stars form out of molecular hydrogen in cold, dense regions of the interstellar medium (ISM). Empirically this picture is supported by correlations between tracers of cold gas and the radiation output from young stars such as the Kennicutt-Schmidt (KS) law. One manifestation of the KS law is the correlation between 12 micron luminosity, measured with the Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010), and CO luminosity measured by ground-based radio telescopes. The 12 micron band spans mid-infrared wavelengths of 8 to 16 microns. In nearby galaxies, 12 micron emission traces star-formation rate (SFR), vibrational emission lines from polycyclic aromatic hydrocarbons (PAHs), and warm dust emission. Galaxy-integrated 12 micron luminosity is strongly correlated with CO(1-0) and CO(2-1) luminosity in nearby galaxies (Jiang et al., 2015; Gao et al., 2019). This correlation is useful for predicting molecular gas masses in galaxies since 12 micron images already exist thanks to the WISE survey (which covers the full sky), whereas CO luminosities require dedicated observations.

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Constraining the density and temperature of circumgalactic media with the SZ effect and X-ray data

Galaxies are believed to form and evolve in dark matter halos through the cooling and condensation of hot/warm gas around galaxies, usually termed as Circum-Galactic Medium (CGM). As the repository of baryons, the CGM is important for star formation, gas accretion and the gas-star-gas cycling in/around galaxies. The CGM can be probed by X-ray emission produced by the hot gas in galaxy clusters, which mainly depends on the temperature and density of the gas. The Sunyaev-Zel'dovich effect (SZE) of galaxy clusters on the spectrum of the cosmic microwave background (CMB) provides a complementary way to probe the CGM. The thermal SZE (tSZE) is proportional to the line-of-sight integral of the electron pressure (or thermal energy density), while the kinetic SZE (kSZE) is proportional to the integral of the momentum density along a given line of sight. Measurements of the X-ray emission, tSZE and kSZE have been made for a number of galaxy clusters in the local Universe, and these data should in principle be able to constrain the properties of the IGM. In particular, it is unclear whether the halo gas of clusters is always at the virial temperature and whether the gas can be described simply by a single-phase component or whether it consists of multiple phases.

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Identifying galaxy groups and clusters in the high-z universe

Galaxies are not distributed at random. Rather, they are found to be clustered, with half or more of them residing in groups (with intermediate-to-low abundance of member galaxies) or clusters (with high abundance). Galaxy groups and clusters are commonly used to link galaxies with their host dark matter halos, as current galaxy formation models predict that galaxies form and evolve in dark matter halos. Therefore, identification of galaxy groups from observational samples is a crucial step toward a complete picture of the galaxy-halo connection. In the past decades, much effort has been dedicated to identifying galaxy groups in various galaxy surveys, both photometric and spectroscopic. Most of these studies have been limited to low-redshift surveys such as the Sloan Digital Sky Survey (SDSS). Next-generation redshift surveys will extend these to high redshifts, such as the Subaru Prime Focus Spectrograph (PFS) project which will observe about 0.3 million galaxies at 0.7 < z < 1.7 over ~15 square degrees in the sky -- an ambitious survey that is a factor of 10 larger than existing surveys at similar redshifts such as zCOSMOS.

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Bars enhance the central star formation and gas inflow in nearby galaxies

Two thirds of spiral galaxies in the local Universe are observed to have a bar-shaped structure in their central region. Theoretically, galactic bars are expected to transfer angular moment from the center outwards through driving cold gas inward. As a result, the gas density at the galactic center will increase and thus trigger star formation, leading to the formation and growth of the central bulge. Therefore, galactic bars play important roles in the secular evolution of galaxies, through driving the inflow of cold gas, enhancing the central star formation, the consumption of cold gas, the growth of the (pseudo-)bulge, and probably that of the central super massive black hole as well. These processes have been seen in numerical simulations of disc galaxy evolution. Observationally, however, it is not easy to directly see the whole process due to the lack of suitable data. In recent years surveys of integral field spectroscopy such as CALIFA and MaNGA have obtained spatially resolved spectroscopy for large samples of galaxies at low redshifts, allowing the star formation history and stellar populations to be mapped across the whole galaxy. Meanwhile, mapping of cold gas content has also become available for a considerable number of galaxies at the same redshifts. These new data have enabled astronomers to study the role of bars in star formation and cold gas inflow in great detail.

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PolarLight results featured on the cover of Nature Astronomy

On May 11, Professor Hua Feng and collaborators reported in Nature Astronomy a re-detection of soft X-ray polarization from the Crab nebula with the space program PolarLight, indicating that this long-awaited window in astronomy has been reopened after more than 40 years since the OSO-8 experiment in the 1970s. Interestingly, PolarLight discovered a time variation of polarization that coincides in time with a glitch of the Crab pulsar. The variation is associated with the pulsar emission but not the nebular emission, suggesting that the pulsar magnetosphere may have altered after the glitch.

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Estimating stellar dust attenuation from galactic spectra

The observed spectrum of a galaxy is a combination of several components: a continuum, absorption and emission lines. The continuum and absorption lines are both dominated by starlight, thus usually referred to as the stellar component of the spectrum. The emission-line component is produced in ionized Hydrogen (HII) regions around hot stars, or emission-line regions of active nuclei, or both. All these components, however, are modified by the attenuation of dust grains distributed in the inter-stellar space. Dust attenuation can affect galaxy spectra over a wide range of wavelengths, from ultraviolet (UV), optical to infrared, by absorbing short-wavelength photons in UV/optical and re-emitting photons in the infrared, and the absorption is stronger in shorter wavelength. Consequently, dust attenuation can cause changes in the overall shape of a galaxy spectrum. Such attenuation has to be taken into account before one can measure the different components of an observed spectrum reliably. Traditionally, dust attenuation is treated as a free parameter when fitting the spectrum with a stellar population synthesis model, and so it is hard to measure the dust attenuation given the well-known dust–age–metallicity degeneracy.

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