Winter 2017


Jan 4 Sebastiano Cantalupo

What matters around galaxies? Shining a bright light on the cold phase of the circumgalactic medium

Recent observations of circumgalactic medium (CGM) in emission and absorption are challenging our current theoretical understanding of structure formation based on cosmological simulations, suggesting that a large amount of "cold" and dense gaseous "clumps" should be present around high-redshift galaxies. At the same time, current galaxy formation models lack an efficient mechanism to prevent too much cooling of the CGM onto galaxies at later epochs and rely on very strong "ejective" feedback. In this talk, I will discuss our first attempts to theoretically understand the origin and nature of the dense gas in the CGM using high-resolution hydrodynamical simulations and analytical models. In the second part of the talk, I will show how the interaction between high-energy radiation from star-forming galaxies and the CGM - still ignored by almost all galaxy formation models - provides a natural way to prevent excessive CGM cooling onto galaxies ("preventive" feedback) at later epochs. Finally, I will discuss recent COS observations that provide support for the importance of this "preventive" feedback mechanism and, at the same time, can give vital constraints on the SED of star-forming galaxies in the FUV range.

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Feb 15 Xuening Bai

Magnetohydrodynamic-Particle-in-Cell Method for Cosmic-ray-driven Streaming Instabilities

Cosmic-rays (CRs) are an important constituent of the Galaxy. The mutual interaction between the CRs and background thermal plasmas not only controls the origin, propagation, and escape of the CRs, but also provides feedback to the Galaxy, yet none of these processes are very well understood. Particularly important pieces of physics underlying the interaction are the resonant and non-resonant (Bell) streaming instabilities when CRs drift faster than the Alfven speed of the background gas. We have developed a magnetohydrodynamic (MHD)-particle-in-cell (PIC) method, which retains the full kinetic nature of the CRs while bypassing the microscopic scales that conventional PIC methods must resolve, and hence is dramatically more computationally efficient. Using this method, I will discuss our initial studies of particle acceleration in non-relativistic shocks, where the Bell instability mediates the upstream turbulence to provides the source of scattering in the Fermi process. I will then discuss preliminary studies of the resonant streaming instability, which forms the basis for the picture of cosmic-ray self-confinement and cosmic-ray-driven wind.

Feb 22 Blakesley Burkhart

Galaxy Evolution in Medias Res: Is ISM Turbulence Produced by Gravity or Feedback?

In the midst of galaxies' often violent evolution, a smooth, quiescent gas disc supported by thermal pressure alone is rarely found. Rather, interstellar media (ISM) in galaxies are wracked by supersonic turbulence, and turbulent pressure plays a key role in their support. The ultimate source of this turbulence remains unknown: is it star formation feedback, gravitational instability, or some combination? In this talk, I will explore these two dominant paradigms for turbulent driving, looking at them from the perspective of globally averaged parameters such as the star formation rate, gas accretion, and gas fraction within the galaxy. I will show that H-alpha observations of galaxies from redshifts z=0-2 and cosmological simulations suggest that gravity is the ultimate source of ISM turbulence at least in rapidly star-forming, high-velocity dispersion galaxies.

Mar 1
Feb 28
Chang-Goo Kim

Supernova driven galactic winds and synthetic observations using TIGRESS*

*Three-phase ISM in Galaxies Resolving Evolution with Star formation and Supernova feedback

Supernova (SN) explosions inject a prodigious amount of energy into the interstellar medium (ISM). This powerful feedback implies that SNe are a major driver of turbulence and the dominant regulator of star formation (SF) in star-forming galaxies. Also, SNe may be a major driver of galactic winds. Detailed understanding of the interaction of SN(e) with the ISM is necessary, but a complete and self-consistent gas-dynamical model of the ISM including SN(e) is still numerically challenging. For many years, the effect of SN feedback has been both underestimated (based on poorly resolved numerical simulations) and overestimated (based on classical analytic theories with unconfirmed assumptions). In this talk, I first revisit the evolution of radiative SN remnants in the warm and cold ISM driven by single and multiple SN(e) and provide the condition for SNR evolution to be numerically resolved. This shows that (1) the inability of SNe to limit SF in many galaxy formation simulations has been due to lack of resolution, and (2) classical analytic models do not properly account for cooling during post-Sedov SNR evolution. I then show results from TIGRESS simulations that follow the space-time correlation of SNe with dense and diffuse gas realistically, resolve all three thermal phases of the ISM, and fully capture the circulation of the galactic fountain. A fast, hot galactic wind is launched with a mass loading factor of 0.1-1, while the SFR is self-regulated consistent with expectations within the warm and cold ISM. Finally, I present synthetic HI 21cm lines constructed from local galactic disk simulations. I demonstrate how useful synthetic observations are, and discuss future applications using TIGRESS to study HI 21cm lines, polarized dust emission, molecular lines, ionized lines, and escaping fraction of ionizing radiation.

Mar 8 Yan-Fei Jiang

How do the massive stars maintain their super-Eddington Envelope?

Massive stars play important roles in many astrophysical systems by providing the radiative and mechanical energy output. They can also produce black holes and neutron stars when they explode. However, the traditional 1D stellar evolution models provide very uncertain predictions for the structure and evolution of massive stars because the radiation acceleration around the envelopes of massive stars at the iron opacity regions can be much larger than the gravitational acceleration. I will show how we can understand the physical processes in this region based on a series of first principle local and global 3D radiation MHD simulations. I will use the local simulations to calibrate the mixing length theory in the radiation pressure dominated regimes and quantify the porosity effects in the envelopes. Then I will show how the super-Eddington luminosity can drive significant outflow from the envelope based on global simulations. These simulations will revolutionize our understanding of massive star envelopes and evolutions. They provide the physical fundations of realistic models for massive stars.

Mar 15 Alexander Tchekhovskoy