SHAO Astrophysics Colloquium
Title: Mapping the stars, gas and dust in nearby galaxies
Speaker: Cheng Li (Tsinghua University)
Time: 3:00 pm, April 18 (Thursday)
Location: Lecture Hall, 3rd floor
Abstract: The cosmic star formation rate density has been declining since z~2-3, suggesting that the cessation of star formation has been the dominating process in galaxy evolution since then. However, it is unclear how the star formation gets quenched inside the galaxies and what physical mechanisms have been driving this process. Current integral field unit (IFU) surveys such as CALIFA and MaNGA are providing resolved spectroscopy for large samples of galaxies in the local Universe, allowing the star formation histories across the whole galaxy area to be studied with high accuracy. In addition, maps of molecular gas are also being obtained by arrays of radio antenna, thus providing additional data for linking the star formation and gas accretion processes in the IFU era. I'll talk about our recent work based on the CALIFA, MaNGA and CARMA-EDGE surveys, particularly focusing on the roles of environmental effect, dark matter halo and internal structure in driving galaxy evolution.
Special Astrophysics Colloquium
Title: Turbulence and non-thermal pressure in galaxy clusters
Speaker: Xun Shi (石洵, Yunnan University)
Time: 10:00 am, April 19 (Friday)
Location: Lecture Hall, 3rd floor
Abstract: Galaxy clusters are promising probes for cosmology, and yet, their astrophysical complexity leads to significant systematical bias as shown by the Planck CMB-cluster discrepancy. Turbulence in the intracluster medium (ICM) and the corresponding non-thermal pressure are a central piece of the astrophysical complexity that has not been well-explored. I will present recent developments in this area, and in particular, discuss a largely neglected effect - the influence of ICM density stratification on turbulence dissipation, as well as its role in shaping the typical radial distribution of ICM turbulence.
Seminar talk
Title: Blue stragglers and other stellar population mysteries beyond the Milky Way
Time：10:30 a.m., April 16(Tuesday)
Location：Middle Conference Room, 3rd floor
Speaker: Richard de Grijs (Macquarie University, Sydney, Australia)
Abstract: We have analyzed populations of blue straggler stars (BSSs) in 24 Magellanic Cloud star clusters using multi-passband Hubble Space Telescope images. We uncovered a correlation between the number of BSSs in the cluster cores and the clusters' core masses, characterized by a power-law index of 0.51±0.07. For low stellar collision rates, the mass-normalized number of BSSs depends only weakly (or perhaps not at all) on the collision rate, implying that the binary-driven BSS formation channel dominates. Comparison with simulations suggests that stellar collisions contribute less than 20% to the total number of BSSs formed. Further tests, including analysis of the BSS specific frequencies and their population numbers at larger cluster radii, suggest that binary interactions may be their main formation channel, hinting at an anti-correlation between a cluster's binary fraction and its core mass. This study forms part of our group's overall approach to understanding the complexities of star cluster stellar populations. We have been making significant progress in recent years towards a better understanding, of which I will present a number of recent and hot-off-the-press highlights.
Title: Disc reflection in low-mass X-ray binaries
Speaker: Dr. Yanan Wang (王亚楠; University of Groningen)
Location：Middle Conference Room, 3rd floor
Time: 2:00 pm, April 18 (Thursday)
Abstract: X-ray binary systems, with either a black hole or a neutron star, provide a unique window into the physics of strong gravity and ultra-dense matter in a realm that cannot be attained in man-made laboratories. In such binaries, matter from the companion star gets accreted onto the compact object forming an accretion disc and the gravitational energy released in the process is believed to be the main source of their power. However, the observed X-ray energy has been found to be much harder than that expected from the disc emission. This Comptonised emission has been proposed to arise from ‘corona’.
Detailed spectral and timing analysis of disc reflection – a product of the interaction between the disc and the corona – can yield insights into the accretion morphology, in particular, the inner radial extent of the accretion disc as well as of the extent and location of the primary Comptonised source. During the course of my PhD, our detailed studies of X-ray binaries yielded several interesting results. For instance, we found that direct and the reflected emission were related differently on different timescales in the black-hole candidate IGR J17091-3624; we were able to offer a solution to the debated high-inclination issue in the neutron star 4U 1636-53 with the model assuming a lamppost geometry of the corona; in the neutron star 4U 1728-34, the disc iron abundance required by the data is as high as 10 times solar. In this talk, I will present these findings from our study of the three X-ray binaries and discuss the pros and cons of the current disc reflection models.
