Research Highlights

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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Quantifying the cosmic variance in low-z galaxy distribution

Our Universe is not so homogeneous even at scales larger than hundreds of millions of light years . The properties of cosmic objects, which form and co-evolve with their environments, are also expected to be fluctuated in space. Limited by the sizes of telescopes, the astronomical observations are always constrained in restricted volume, and are not guaranteed to give an un-biased representation of the whole Universe. This inspires studies of such an effect known as cosmic variance (CV). Previous works have found that CV could have un-negligible effect for small surveys (e.g. pencil beam surveys [3], high-resolution shallow surveys [4], etc). Corresponding corrections have been made either in a statistical manner, or by a lowest-order assumption that the spatial distribution of bright and faint galaxies are much similar. However, these are far from enough to give a sample-to-sample quantifying of CV, and give a precise estimates of the statistics of galaxy distribution.

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Low-Luminosity AGNs: Fed More, Spit Up More

In the center of almost every major galaxy there exists a supermassive black hole. When matters fall onto the black hole, the gravitational potential energy can be released and radiated away via some physical processes that are still not well understood. This makes them bright sources called the active galactic nuclei (AGNs). However, it remains unclear how such processes work when the mass feeding rate (the accretion rate) is extremely low. THCA student Rui She led a project to study these AGNs in nearby galaxies and found that, in the low-luminosity regime, the more the AGNs are fed, the more they reject the in-fall materials in the form of outflow. The observed results are in good agreement with numerical simulations. The paper was accepted for publication in the Astrophysical Journal.

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Understanding neutral hydrogen clustering during re-ionization

Upcoming observations of cosmic radio signals will mark the beginning of a new era in cosmology as they will provide a first glance on the neutral hydrogen distribution in the early universe. These observations will give us insights into a key process in the evolution of the universe – the reionization of the neutral hydrogen by the first stars and galaxies. The neutral hydrogen distribution can be observed via photons, which are emitted by neutral hydrogen atoms at the wavelength of 21cm and are redshifted to radio signals during their journey through the expanding universe before being observed on earth.

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A MaNGA view of how galaxies distribute in the mass-size plane

The universe houses hundreds of billions of galaxies with different color, sizes and morphologies. As seen in the Hubble Ultra-Deep Field (Figure 1): some galaxies appear to be red, nearly featureless and elliptical, while some have beautiful and blue spiral arms (spiral galaxies). Surprisingly these diverse galaxies follow simple global scaling relations. Recent work led by an PhD student, Hongyu Li (NAOC), Profs. Shude Mao (Tsinghua) and Michele Cappellari (Oxford) extended such scaling relations using spatially-resolved integral field unit data from MaNGA (Mapping Nearby Galaxies at Apache Point Observatory), one of the world-leading SDSS surveys. This study provides further constraints on how galaxies form and evolve.


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MaNGA reveals how galaxies stop forming stars

Our understanding of the galaxy populations at both low-z and high-z has advanced dramatically, thanks to the large photometric/spectroscopic surveys of galaxies accomplished in the past one and a half decades. It's well established that the galaxies can be divided into two major populations in the space of stellar mass (or luminosity) and color. In addition, the fraction of the red population has steadily increased by a factor of two since redshift of unity, indicating that the star formation cessation in galaxies has been an important process driving the galaxy evolution in the past ~80 Gyr. However, how the star formation gets shutdown and what processes drive the star formation cessation are not fully understood. Processes internal to individual galaxies (e.g. secular evolution driven by bars or minor mergers, AGN feedback, etc.) and external environmental effects (e.g. tidal stripping and ram-pressure stripping) are both believed to play important roles. Therefore, in order to have a complete picture of the star formation cessation, one would need to have deep imaging and spatially resolved spectroscopy for a large sample of galaxies covering wide ranges of galaxy properties and environment. Such samples have become available only recently from the integral field spectroscopy (IFS) surveys.


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THCA AST3-2 telescope observes GW170817's optical counterpart

On August 17, 2017, the now famous LIGO gravitational wave detector and VIRGO, the italo-french detector simultaneously observed a gravitational wave signal (GW170817). The joint detection allowed a more precise positioning of the event, located at about 130 million light years in a 31 square degrees area. This precision, which was not possible with LIGO-only observations allowed more than 70 observatories and telescopes in the world to point to the position in a most exciting and beautifully orchestrated series of multi-band follow-up observations of the electromagnetic signals associated to a gravitational event. This is the first ever observed neutron star collision, and it is in both gravitational waves and its electromagnetic counterparts (from high energy gamma-rays, X-rays, optical, and radio wavelengths) and will be certainly remembered as the start of gravitational waves astronomy.

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THCA members participate in Insight-HXMT obs of GW170817

On Oct. 16th 2017 Beijing Time, the National Science Foundation of the United States (NSF) announced at a news conference the discovery of a binary neutron star coalescence event (designated GW170817) through gravitational waves detected by the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) together with the Advanced Virgo interferometer. This historic event triggered a rare, global joint observational campaign with dozens of astronomical equipment at all wavelengths in search of the elusive electromagnetic counterparts. Insight-HXMT, the first X-ray Satellite of China launched on Jun. 15th 2017, which is still under test, observed GW170817.


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THCA Professor publishes a review paper in ARA&A

On August 18, 2017, a review paper entitled Ultraluminous X-ray Sources was published in this year’s Annual Review of Astronomy and Astrophysics, written by THCA Professor Hua Feng (second author), in collaboration with Philip Kaaret (first author), and Tim Roberts. The paper reviews the observational facts of a special kind of X-ray sources in the sky, so-called ultra-luminous X-ray sources (ULXs), and how theoretical models and numerical simulations can explain the data. The link between the studies of ULXs, the early universe, and the detection of gravitational waves is also discussed. Prof. Feng’s major interests are 1) to understand the physics around compact objects, including ULXs and 2) to develop novel instrumentation for future X-ray astronomy.

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