Research Highlights

Searching for Wolf-Rayet Regions of Massive Stars in Galaxies

Wolf-Rayet (WR) galaxies are a rare population of galaxies that host living high-mass stars during their WR phase (called WR stars). These galaxies can be used to study a variety of important astrophysical questions including constraints on the stellar initial mass function, stellar evolution models, the relation between supernovae and gamma-ray bursts, etc. The first WR galaxy was identified in 1976 and a total of about 130 WR galaxies had been reported by the end of last century. Thanks to the SDSS I & II survey of nearby galaxies (targeting galaxy centers), many more WR galaxies were found, but the number was still limited to a few hundred [1]. Integral field spectroscopy (IFS) surveys have become available recently and provides a more efficient way of identifying WR galaxies, as WR stars are expected to be more preferentially found in discs than central regions that earlier phases of the SDSS probed.

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LHS 1815b: The First Thick-Disk Planet Detected By TESS

To date, more than 4000 exoplanets have been detected, but few of them have been claimed to be in the thick disk of the Milky Way. A common way to separate different components of the Milky Way (for example, thin and thick disks) relies on the three-dimensinal (3D) spatial motions of stars. This has become possible with GAIA, a space telescope which gives distance, proper motions for relatively bright stars. Furthermore, the recent launched Transiting Exoplanet Survey Satellite (TESS) aims to discover a large sample (10,000) of planets around bright stars in the solar neighborhood across the whole sky. Combined with GAIA, TESS offers an exciting opportunity to study the difference in the planet formation efficiency between stars in the thin and thick disks, which have different age and metallicity distributions.

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Relating structure of dark halos to their assembly and environment

Dark matter halos are the building blocks of the cosmic large-scale structures and the bridges between the dark and luminous sector of the Universe. They are diverse in internal structure, mass assembly history and interactions with environment. The figure at the top illustrates various quantities that are commonly adopted in literature to describe the structural properties of a typical dark matter halo, as well as its formation history and environment. A crucial step toward a full picture of dark matter halo formation is to understand the intrinsic relationship between the structure and the assembly history and environment of dark halos.

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Machine learning recovers H II morphology during reionization

The epoch of reionization (EoR) is a unique period of time in cosmic evolution, during which ultraviolet (UV) and X-ray photons emitted from the first luminous objects (e.g. first stars and galaxies) ionize hydrogen atoms first in the surrounding intergalactic medium (IGM) and form bubbles of H II regions, and eventually these H II bubbles fill the whole Universe. The bubble size distribution of ionized hydrogen regions probes the information about the morphology of H II bubbles during the reionization, and it can be derived from the tomographic imaging data of the redshifted 21cm signal.

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Gas Evolution in the Early Universe Revealed with Keck

Over the past few decades, astronomers have studied the process of gas accretion that drives the formation of stars and galaxies within dark matter halos. People have established a theoretical paradigm, which envisions galaxies fed by cool ‘streams’ of gas, linked to the surrounding circumgalactic medium (CGM) and intergalactic medium (IGM) by a web of cosmic filaments. Despite the general agreement among theorists, fundamental questions are yet to be solved and tested empirically. Due to their extremely low densities, the emission of CGM and IGM are very faint with very low surface brightness. Fortunately, recent development of more advanced instrumentations allows astronomers to see the low-surface brightness IGM emission for the first time.

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Constraining nature of ultra-light dark matter particles with 21cm forest

The ultra-light scalar fields can arise ubiquitously, for instance, as a result of the spontaneous breaking of an approximate symmetry such as the axion and more generally the ultra-light particles (ULPs). In addition to the particle physics motivations, these particles can also play a major role in cosmology by contributing to dark matter abundance and affecting the structure formation at sub-Mpc scales.

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New insight into Jupiter’s diluted core

Jupiter was smacked head-on by a massive planetary embryo about 4.5 billion years ago in the early solar system, according to a new study published in the journal Nature on August 15, 2019. An international collaboration team including two astronomers from Tsinghua University, Dr. Xiaochen Zheng from Department of Astronomy and Department of Physics and Prof. Doug Lin from Institute for Advanced Study, demonstrate their giant impact scenario can explain Jupiter’s large diluted core inferred from Jupiter’s gravity field measurement by the Juno mission. The study was led by Dr. Shangfei Liu from Sun Yat-Sen University. The paper can be found at https://www.nature.com/articles/s41586-019-1470-2

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Environmental dependence of alpha abundance for nearby galaxies

The alpha-to-iron ratio ([alpha/Fe]) is an important indicator for star formation histories because alpha-elements (such as O, Ne, Mg, Si, Ca, Ti) are mostly produced in core-collapse supernovae, whose progenitors are high-mass stars, while irons (Fe) are mostly produced by Type Ia supernovae, whose progenitors are low-mass compact stars. High-mass stars generally have a very short lifetime (~ 1 Million year), whilst low-mass compact stars almost all have ages larger than 1 Giga year. So a galaxy formed in a single burst or experienced fast quenching will be enhanced in [alpha/Fe] in comparison with a galaxy formed with an extended star formation history.

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PolarLight detects first light events

On December 18, PolarLight, an X-ray polarimeter onboard a CubeSat, was powered on for in-orbit test, and detected the first events triggered by cosmic X-rays and charged particles. This is the first time that the new technique for X-ray polarimetry is demonstrated in space, implying that a new window in X-ray astronomy can be opened in the future.

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X-rays Reveal Supercritically Accreting Compact Objects

Accretion onto compact objects has a critical rate when the radiation balances the gravity. The physics for supercritical accretion is still an unsolved problem. It is suggested that, because of the presence of strong radiation pressure, supercritical accretion will power a massive wind that is optically thick and Eddington-limited. Based on Chandra observations of nearby galaxies, Zhou et al. found a list of very soft X-ray sources, which are argued to be good candidates for compact objects under supercritical accretion. The results will be published in the Astrophysical Journal.

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Stellar initial mass function varies across galaxies

Stars are the building blocks of galaxies. The stellar initial mass function (IMF), which describes the mass distribution of stars at birth, has been the subject of numerous investigations in the past decades. The first estimate of the IMF was obtained by Salpeter more than half a contrary ago, described simply by a power law function with a slope of 1.3, i.e. ϕ∝m^(-1.3)) across the entire mass range of stars. Subsequent studies of resolved stellar populations in the Milky Way have revealed a more bottom-light IMF, with a shallower slope at the low-mass end (<0.5M_⊙). In most galaxies, however, directly counting the number of stars in resolved stellar populations is impossible due to the limited spatial resolution of our observational facilities. A long-standing debate on IMF is whether the IMF measured from the few very local galaxies is universal to the general population of galaxies, or it varies from galaxy to galaxy or even from region to region within a single galaxy.

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Molecular gas concentration driven by bars and interactions

Theories of galaxy dynamics have long predicted that the bar-like structure in a galaxy may effectively transport gas from the outer disk to the central region through angular momentum exchange with the disk. Tidal forces from galaxy-galaxy interactions are predicted to produce a similar effect by driving gas inwards, where it forms stars in the central region. Both physical processes are believed to play important roles driving the formation and growth of the central bulge. A recent study at the THCA, led by Ryan Chown (visiting from McMaster U.), Prof. Cheng Li (Tsinghua), and Niu Li (Tsinghua) investigated the role of bars and interactions on both the molecular gas component of the interstellar medium and the star formation history of a sample of 64 nearby galaxies, and found clear evidence in support of this theoretical picture. The work makes use of spatially-resolved maps of 12CO 1-0 emission (2.2 mm wavelength) in these galaxies observed using the Combined Array for Research in Millimeter-wave Astronomy (CARMA) interferometer from the CARMA-EDGE survey (Bolatto et al. 2017), as well as optical integral field unit (IFU) data from the Calar Alto Legacy Integral Field Area (CALIFA) survey (Sanchez et al. 2016). Figure 1 shows maps of spectral features that are sensitive to recent star formation history, and those of molecular gas, for some example galaxies.

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