SHAO Astrophysics Colloquia　　 

The formation and evolution of galaxy clusters and proto-clusters: Recent results from the new LOFAR radio telescope　　

Speaker: Huub J.A. Röttgering (Leiden University)

Time: Tuesday, 10:00am, June 30th

Location: Lecture Hall, 3rd floor

Abstract: At very low frequencies, the new pan-European radio telescope, LOFAR, is opening the last unexplored window of the electromagnetic spectrum for astrophysical studies. Operating at frequencies from 15 to 240 MHz, its superb sensitivity, high angular resolution, large field of view and flexible spectroscopic capabilities represent a dramatic improvement over previous facilities at these wavelengths. LOFAR will carry out a broad range of fundamental astrophysical studies in a number of key science topics including the formation and evolution of clusters, galaxies and black holes. In this contribution we will describe some of the capabilities of LOFAR and how they enable the planned LOFAR low-frequency radio sky surveys. We will outline some of the challenges we have overcome in carrying out sensitive imaging at low radio frequencies and  present some recent results from the ongoing imaging commissioning efforts. In the second part of the talk we will talk about the formation and evolution of galaxy clusters and proto-clusters. Galaxy clusters are unique laboratories to study some of the most fundamental questions in astrophysics, related to the formation and evolution of galaxies, the physics of particle acceleration, the generation of magnetic fields, the growth of large-scale structure, and cosmology　　 

Seminar talk　　 

Title: Stellar metallicity gradients of the Galactic disk from the LAMOST Spectroscopic Surveys　　 

Speaker: Xiang Maosheng (KIAA）　　 

Time: Thursday,10:00am,July 2nd　　 

Location: the middle conference room　　 

Abstract: Accurate measurement of stellar metallicities and their distribution in the Milky Way is of fundamental importance for an archeological study of the formation and evolution of the Galaxy. In the talk, I will first briefly introduce our efforts on the data reduction and stellar parameter determination utilizing the huge spectra database of the LAMOST Spectroscopic Survey of the Galactic Anti-center (LSS-GAC). Then I will present our results on the study of stellar metallicity gradients of the Galactic disk with both a sample of ~300 000 main sequence turn-off stars and a sample of ~70 000 red clump stars from the LAMOST Spectroscopic Surveys.  We find that the stellar metallicity gradients exhibit significant both spatial and temporal variations. The metallicity gradients, as well as their spatial trends of variations, of the oldest stars are significantly different from those of the younger stars,  indicating that the Milky Way disk may have experienced two distinct formation phases, which are probably related to the formation of the Galactic thick and thin disks, respectively.　　 

Galaxy seminar　　 

Title: SPECTRAL PROPERTIES OF GALAXIES IN VOID REGIONS

Speaker: Chenxu Liu　　 

Time: Wednesday,3:00pm,July 1st　　 

Location: Room 1714　　 

Abstract: We present a study of spectral properties of galaxies in underdense large-scale structures, voids. Our void galaxy sample (75,939 galaxies) is selected from the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) with z < 0.107. We find that there are no significant differences in the luminosities, stellar masses, stellar populations, and specific star formation rates between void galaxies of specific spectral types and their wall counterparts. However, the fraction of star-forming galaxies in voids is significantly higher (¸ 9%) than that in walls. Void galaxies, when considering all spectral types, are slightly fainter, less massive, have younger stellar populations and of higher specific star formation rates than wall galaxies. These minor differences are totally caused by the higher fraction of star-forming galaxies in voids. We confirm that AGNs exist in voids, already found by Constantin et al. (2008), with similar abundance as in walls. Type I AGNs contribute »1-2% of void galaxies, similar to their fraction in walls. The intrinsic [O III] luminosities , spanning over 106 L⊙ » 109 L⊙, and Eddington ratios are similar comparing our void AGNs versus wall AGNs. Small scale statistics show that all spectral types of void galaxies are less clustered than their counterparts in walls.Major merger may not be the dominant trigger of black hole accretion in the luminosity range we probe. Our study implies that the growth of black holes relies weakly on large scale structures.