Research Highlights and News

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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.


Talking about the Sun and solar activities

The 24th “New Worlds, New Horizons” Public Astronomy Lecture took place at 6C300, the No. 6 Teaching Building (the Yuyuan Building) in Tsinghua University on Dec. 1st, 2018. It was delivered by Professor WANG Jingxiu, from the University of Chinese Academy of Sciences. It attracted a large audience on site and over 1000 online viewers via the Tencent News live broadcast.


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.


Dark Matter and the First Stars in the Universe

The 23rd "New Worlds, New Horizons" Public Astronomy Lecture took place at Ming Li Building, Tsinghua University on Sept 28th, 2018. It was delivered by Prof Rennan Barkana from Tel Aviv University on dark matter and how it's implicated in observations of the first stars in the Universe. The lecture attracted over 200 listeners from various backgrounds, apart from about 6000 online viewers via Tencent News live broadcast.


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.


The Dark Ages and the Dawn of the Universe

On the night of May 11th, 2018, the 22nd session of the "New Worlds, New Horizons" Public Astronomy Lecture series-The Dark Ages and the Dawn of the Universe was held in the Back Hall Auditorium in the Main Building of Tsinghua University. The lecturer, Dr. Xuelei Chen is a leading researcher from the National Astronomical Observatory of China. The lecture attracted an audience of over 500, including students, teachers and the general public.