Wednesday Colloquium


"The formation and build-up of the red-sequence in VIPERS"

Alexander Fritz (INAF-IASF Milano)

Early-type galaxies form a tight sequence within in the colour-magnitude space which is known as the red-sequence. This red-sequence has been used to search for galaxy clusters and study the properties of quiescent galaxies up to z~1. However, the formation and build-up of the red-sequence is still uncertain and a matter of debate. I will present new observational constrains based on our analysis of the luminosity function and colour-magnitude relation using a sample of 50,000 galaxies from the ongoing VIMOS Public Extragalactic Redshift Survey (VIPERS) project. Our data are a combination of multi-wavelength photometry with precise spectroscopic redshifts, complemented by spectroscopic properties derived from stacked spectra. I will show how these data allow us to constrain both the evolution of the luminosity function and the galaxy colours, the star formation history of early-type galaxies, and discuss possible physical mechanisms that are important for the origin and the build-up of the red-sequence.


"Redshift distortions of galaxy clustering"

Michał Chodorowski (NCAC, Warsaw)

At large distances, the only direct distance estimator of a remote galaxy is its redshift. Galaxy velocities, however, are not entirely composed of their Hubble, uniform expansion component. In the gravitational instability scenario, large-scale inhomogeneities in the universe create inflows and outflows, inducing the so-called `peculiar' component of galaxy velocities. Peculiar velocities of galaxies distort three-dimensional maps of the large-scale structure of the universe, when inferred from redshift surveys. (That is, when redshift is adopted as a third coordinate, besides the two angular ones.) Since redshift distortions are due to peculiar velocities which are strongly coupled to the density field, their effect is systematic and can be quantified in a statistical way. In particular, the autocorrelation function of galaxies, when measured using the data from a redshift survey, is no longer isotropic. The amplitude of anisotropy depends on fundamental cosmological parameters and, as a function of the look-back time, on the evolution of the growth-rate of density perturbations in the universe. For various competing cosmological models attempting to explain a mystery of accelerating expansion of the universe in a different way, the evolution of the growth rate is distinctly different. In order to constrain these models, not only very precise measurements of the redshift-space galaxy autocorrelation function are needed, but also an accurate theoretical model of the latter. I will devote part of my talk to describe twenty-year-long efforts to construct such a model and my recent personal contribution to this field.


"Tidal disruption event in NGC 4845"

Marek Nikołajuk (University of Białystok)

The INTEGRAL satellite has discovered a new source of hard X-ray in 2011. The decline of the flux indicated the tidal disruption event. The estimated mass of a disrupted object suggests an encounter of a brown dwarf or a free-floating massive planet.


"Is Cygnus X-3 going to be a prototype of an unusual gamma ray burst?"

Agnieszka Janiuk (Center for Theoretical Physics, PAN, Warsaw)

The most energetic explosions connected with collapse of massive stars or merging compact objects, are the gamma ray bursts. The broad-band emission of their jets, as well as a possible gravitational wave signal, can give an insight to the black hole physics at the very moment of its birth. Especially interesting is an extreme event when the two black holes merge within a collapsar. Based on the results of extensive numerical simulations, I will discuss a particular scenario for the longest duration gamma ray bursts, resulting from the collapse in a close binary system with a companion black hole (BH). After the infall and spiral-in towards the primary, the binary BHs merge inside the circumbinary disk. The remaining matter forms a massive, possibly magnetized torus and subsequently falls onto the merger product.