Research Highlights and News

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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.


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.


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.


DOA approved as a first level discipline PhD granting institute

On March 30, 2020, the State Council Academic Degrees Committee formally approved the Department of Astronomy at Tsinghua University as a first level discipline PhD granting institute.


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.


Recovering star formation histories of low-mass galaxies

Characterizing the star formation history (SFH) of galaxies is an important step toward a full picture of galaxy formation and evolution. Despite being the most abundant galaxy population in the universe, dwarf (low-mass) galaxies remain elusive as far as their formation and evolution path is concerned. The left panel of Figure 1 shows the optical image of a typical low-mass galaxy observed by the SDSS-IV MaNGA survey. Impressed by their blue color, people have long believed dwarf galaxies are exclusively composed of massive,young stars that are bright, blue, and formed very recently. Recent investigations about low-mass galaxies challenge this stereotype. From ‘archaeological’ age reconstruction of local group dwarf galaxies in which individual stars can be resolved, most stars in these galaxies are found to be older than 5 Gyr, with an age similar to or even older than the Sun. However, the number of dwarf galaxies for which stars can be resolved is very limited, and so it is difficult to draw reliable statistical conclusions.