Wednesday Colloquium


"A Fundamental and Often-Overlooked Failure of Models of Photoionized Gases"

Jonathan Stern (MPIA, Heidelberg)

Photo-ionization modelling of emission and absorption lines is widely used to derive metal abundances in quasars and star forming regions, constrain quasar and stellar spectra at unobservable ultraviolet wavelengths, and recently also to constrain the importance of radiation pressure feedback in models of star formation. In the commonly used constant- density models of photoionized gas, the gas density and the incident photon flux are independent free parameters. However, in a wide range of astrophysical environments, observations suggest that the distribution of gas densities scales with the photon flux, in contrast with the assumption that the two parameters are independent. Why does the gas density depend on the photon flux? I will present the mechanism of radiation pressure confinement, in which the gas density distribution is set by pressure equilibrium of the gas with the incident radiation. I will compare the predictions of this mechanism with observations, and discuss its implications for models of quasar feedback.


"Formation of the LG dwarf galaxies by Tidal Dwarf Galaxies and by primordial dwarf gas-rich major mergers"

Sylvain Fouquet (NCAC, Warsaw)

The formation and evolution of the dwarf galaxies in the Local Group is still an open question. New data on global position and internal kinematics and morphology show that the Local Group dwarf galaxies are more complicated than expected. In this talk I am going to talk about two possible formation scenarios which can complete each other. First, I will discuss the possibility that a part of the dwarf galaxies would be remnant of Tidal Dwarf Galaxies which are dwarf galaxies created after a major merger. Second, I will more specifically discuss the case of Andromeda II which could be the result of a major merger between two primordial dwarf galaxies.


"What was the last Nobel Prize in Physics given for?"

Piotr Perlin (Institute of High Pressure Physics "Unipress", Warsaw, Poland)