Wednesday Colloquium


"Solving the puzzles of Milky Way"

Paweł Pietrukowicz (Astronomical Observatory, Warsaw University)

Our location among billions of stars of our disky Galaxy makes the studies on its structure and evolution extremely difficult. On-going large-scale optical and infrared galaxy surveys are on a good way to find answers to key questions.


"Distance to accretion discs; theory versus observation"

Jean-Pierre Lasota (Institut d’Astrophysique de Paris and NCAC, Warsaw)

Accretion discs around compact objects such as white dwarfs, neutron stars and black holes are ubiquitous and have been intensely studied in last 40 years. Their basic properties are quite well understood. In particular the instability which drives the outbursts observed in binary systems containing white dwarfs (« dwarf novae »), neutron stars and black holes (« X-ray transients ») has been clearly identified. This (thermal-viscous) instability is supposed to be present in accretion discs below some critical luminosity and for a long time this had been confirmed by observations: bright disc-containing systems never exhibit outbursts. In 1999 Hubble Space Telescope observations of the famous dwarf nova SS Cygni put this system at a distance at which according to theory it was too bright to have outbursts. I will describe the subsequent « fight » between theory and observations and its (happy for the model) outcome. Next, I will present the recent case of the outbursting black hole system known as HLX-1 which observations put in a galaxy at 95 megaparsecs (310 million light years) whereas the outburst models require it to be much closer to us, maybe even in our Galaxy. To conclude I will briefly discuss some psychological, sociological and philosophical aspects of these (apparent) conflicts between theory and observations.


"On Charles Babbage's difference engine and other old computers"

Andrzej Krasiński (NCAC, Warsaw)

The first (mechanical!) computer was designed by Charles Babbage in the first half of the 19th century. The work on the actual construction of a prototype was far advanced already in 1830, but the project was abandoned in consequence of organisational and psychological problems. In connection with a later version of this project, Ada Lovelace (daugter of George Byron) wrote her pioneering work. Today, she is considered to be the precursor of computer programming theory. In 1985, a team of engineers led by Doron Swade from the Science Museum in London began work on turning Babbage's design into a real machine. The calculator was completed and made accessible to the public in 1991, and in 2002 the construction of the printer was completed (also according to Babbage's design). Today, two complete copies of this machine exist and work (in shows for the public) in the Science Museum in London and in the Computer History Museum in Mountain View (California, USA). In the second


"72-hour WEBT campaign on blazar 0716+714 - A study of microvariability in blazar"

Gopal Bhatta (Astronomical Observatory, Jagiellonian University, Cracow)

The international Whole Earth Blazar Telescope (WEBT) consortium planned and carried out three days of intensive micro-variability observations of S5 0716+714 from February 22, 2009 to February 25, 2009. This object was chosen due to its bright apparent magnitude range, its high declination, and its very large duty cycle for micro-variations. We report here on the long continuous optical micro-variability light curve of 0716+714 obtained during the multi-site observing campaign during which the bazar showed almost constant variability over a 0.5 mag range. Thirty-six observatories in sixteen countries participated in this continuous monitoring program and the resulting light curve was analyzed using several techniques including Fourier transform, Wavelet and noise analysis techniques. Those results led us to model the light curve by attributing the variations to a series of synchrotron pulses. We have interpreted the observed microvariations in this extended light curve in terms of a new model consisting of individual stochastic pulses due to cells in a turbulent jet which are energized by a passing shock and cool by means of synchrotron emission. We obtained an excellent fit to the 72- hour light curve with the synchrotron pulse model.


"Tidally induced bars in dwarf galaxies"

Ewa L. Łokas (NCAC, Warsaw)

One of the plausible mechanisms for the formation of dwarf spheroidal galaxies in the Local Group involves tidal stirring of disky progenitors embedded in dark matter halos. After being accreted by a host galaxy, the dwarfs are affected by tidal forces from the bigger galaxy and lose mass. The stellar disks undergo morphological transformation into spheroids and the rotation of their stars changes into random motions. An important intermediate stage of such evolution is the formation of a tidally induced bar, usually at the first pericenter passage. I will discuss the properties of such a bar, the evolution of its strength, length and pattern speed as a function of time and the orbits of stars within the bar. I will also present possible candidates for barred galaxies among the Local Group dwarfs.


"Firewalls at the horizon"

Włodzimierz Kluźniak (CAMK)

Attempts to marry quantum mechanics with general relativity at the event horizon have created a storm of controversy. It is now universally accepted that no information is lost in black holes: their evolution from collapse to evaporation is (supposed to be) described by a unitary matrix. However, no specific scenario of resolving Hawking's information paradox has gained universal acceptance. A recent suggestion that disentanglement of quantum states leads to the formation of a wall of fire near the black hole horizon, in which any infalling observer will burn up, has been extensively discussed. I will present limits on the firewall density and mass derived in my paper with M. Abramowicz and J-P. Lasota (Phys. Rev. Lett. 2014, accepted). Terms from the current literature such as entanglement monogamy, complementarity, smoke and mirrors, will be mentioned, and may even be explained.


"May be low mass WIMPS: mini-review"

Andrzej Drukier (BioTraces, George Mason University, VA)


"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.


"Modeling radiative accretion disks in general relativity"

Aleksander Sądowski (Harvard, CfA)


"Gauss and the Supermassive Black Holes"

Douglas Heggie (School of Mathematics and Maxwell Institute for Mathematical Sciences, University of Edinburgh)

In Gauss's time, theoretical astronomy was largely celestial mechanics, in which Gauss excelled. A hundred years ago this was the one part of astronomy which was left almost untouched by the spectacular growth of astrophysics, and the two areas went their separate way. But with the discovery of extrasolar planets, celestial mechanics is once again at centre stage. "In this talk I trace the modern history of an old idea in celestial mechanics due to Gauss. It has proved to be a fertile idea, which sheds light on much besides extrasolar planets, including the evolution of nuclear star clusters around supermassive black holes, the death of comets, and even the evolution of the Moon.


"Stochastic Modeling of the Fermi/LAT Gamma-Ray Blazar Variability"

Małgorzata Sobolewska (NCAC, Warsaw)

Properties of the blazar gamma-ray variability have a potential to constrain the location and geometry of the blazar gamma-ray emitting region. However, these properties are still largely unknown, and the origin of the blazar variability remains under debate. We study 13 blazars with the most complete light curves collected during the first 4 years of the Fermi Large Area Telescope sky survey. We model them with stochastic processes characterized with power-law power spectral densities (PSD) with one or two bends. We constrain the model parameters such as characteristic gamma-ray variability time scales and PSD slopes, and discuss the implications of our results for theoretical models of blazar variability.


"Symbiotic stars: challenges to the binary evolution theory"

Joanna Mikołajewska (NCAC, Warsaw)


"The gamma-ray emission from the pulsar magnetosphere: theoretical predictions and comparison with the Fermi LAT young pulsar population"

Marco Pierbattista (NCAC, Toruń)

The exceptional results obtained in the last years from the Fermi Large Area Telescope (LAT) in detecting pulsars have produced a considerable increase of the high-energy pulsar population. The possibility to study the γ-ray pulsars with high statistics represents a unique opportunity to understand pulsar physics. Moreover it offers the possibility to constrain many emission models through different techniques, from the comparison of the collective properties of a simulated population to the structure and morphology analysis of each γ-ray pulsar light-curve. In this seminar I will first give a qualitative description of the pulsar magnetosphere physics and of the some γ-ray emission and geometrical models used so fat to explain high-energy emission from pulsars. In the second part of the seminar, the young and middle aged pulsar population observed by the LAT in the last 4 years will be described. In the third and last part of the talk I will show how to compare model predictions and observations by synthesising a pulsar population and by fitting the observed γ-ray and radio light curves with modelled emission profiles.


"Simulations of microwave skies for blind galaxy cluster and point source surveys"

Bartosz Lew (Centre for Astronomy, Nicolaus Copernicus University, Toruń)

Galaxy clusters are known to be the most massive systems bounded gravitationally and have proved to be useful probes of cosmology. The currently ongoing and the near-future radio observations will detect hundreds new galaxy clusters and provide new insights into calibration of matter power spectra, dynamics of the large scale structure formation and thermodynamics of intra-cluster medium. I will discuss the current work towards enabling a blind radio survey to be pursued in the Polish climate and discuss perspectives for discovering new galaxy clusters with aid of the existing and planned instruments.


"Historyk w poszukiwaniu dokumentów. Moje 60 lat w archiwach Europy (in Polish)"

Jerzy Wojciech Borejsza (Instytut Historii PAN, Warszawa)


"Astrophysics in Kazakhstan: past, present and future"

Chingis Omarov (Fesenkov Astrophysical Institute, Almaty, Kazakhstan)

Astronomical observations in Kazakhstan are carried out for over 60 years. The advantage of the geographical location makes it possible to set and  conduct programs of stationary ground-based observations, that from the most observatories of other countries are difficult or impossible. Today astrophysical research in Kazakhstan is being developed in theeoretical and observational aspects. In particular, computational astrophysics and stellar dynamics is gaining more momentum due to international collaboration.  Meanwhile one of the main project in observational research is to build a new 3,6 ground telescope and to participate in the international space project  "World Space Observatory - Ultraviolet".


"Newtonian dark energy versus Einsteinian structure formation"

Boud Roukema (Astronomy Center, Nicolaus Copernicus University, Toruń)

By definition, the standard, homogeneous models of the Universe are likely to fail when the Universe is inhomogeneous. The virialisation fraction measures inhomogeneity. Its redshift evolution roughly matches the evolution of would-be "dark energy", if the "dark energy" is inferred from the observations by assuming a homogeneous model. This suggests that "dark energy" is a Newtonian misinterpretation of the fact that we live in the inhomogeneous epoch of the Universe. A virialisation-induced approximation based on the general-relativistic, scalar-averaging approach to defining cosmological parameters will be presented as a physically motivated, dark-energy-free approach to modelling extragalactic observations.


"Contributions from jets and accretion to broad-band spectra of black-hole binaries"

Andrzej Zdziarski (CAMK, Warsaw)


"Modelling relativistic stellar tidal disruptions with SPH"

Emilio Tejeda (Stockholm University )

In this talk I will present the implementation of a generalized Newtonian potential for capturing general relativistic features of Schwarzschild spacetime within a smoothed particle hydrodynamics (SPH) code (arXiv:1303.4068). Then I will show the use of this code for studying GR effects on the tidal disruption of a solar-type star by a supermassive black hole.


"Jet-dominated quiescent state in black hole X-ray binaries "

Fu-Guo Xie (Shanghai Astronomical Observatory)

Quiescent state (X-ray luminosity <10^34 ergs) is the dominate state most of black hole X-ray transients (BHXTs) stays, and its properties remain unclear, mainly because of low-luminosity and poor data quantity. We demonstrate that, the simultaneous multi-wavelength (including radio, optical, ultraviolet and X-ray bands) spectrum of V404 Cyg in its bright quiescent state can be well described by the radiation from the companion star and more importantly, the compact jet. Neither the outer thin disc nor the inner hot accretion flow is important in the total spectrum. Together with recent findings, i.e. the power-law X-ray spectrum and the non-variable X-ray spectral shape (or constant photon index) in contrast to the dramatic change in the X-ray luminosity, we argue the quiescent state spectrum of BHXTs is actually jet-dominated. Additional observational properties consistent with this jet model are also discussed as supporting evidences.


"Modeling Reflection Signatures in the RXTE Spectra from X-ray Binaries: The Case of GX 339-4"

Garcia-Martinez, Javier (CfA Harvard)

Despite its limited spectral resolution, the RXTE mission has provided a vast amount of observational data for many X-ray sources over its entire lifespan of 16 years. We have started a camping to analyze all the available data for most X-ray binaries, focusing on the detection and modeling of reflection signatures. We present the results of this camping on the analysis of all available data for GX 339-4 in the hard state. Strong reflection features such as the Fe K emission line, the Fe K-edge, and the Compton hump are clearly observed on a wide range of luminosities. By fitting the spectra with the most advanced relativistic reflection models we are to impose constrains on the ionization state of the gas, the inner radius of the accretion disk, and the inclination of the system. A novel technique to improve the quality of PCA spectral data will also be discussed


"What Can Tidal Disruption Events Teach Us About Black Hole Accretion?"

Mitchell C. Begelman (JILA, Boulder, Colorado)

For a year or more after a star is tidally disrupted by a black hole, debris can fall back at a rate that greatly exceeds the Eddington limit. Both observations and theoretical arguments indicate that mass loss is unable to regulate the rate at which matter is actually swallowed by the hole, leading to black hole growth rates and energy outputs that can exceed the Eddington limit by orders of magnitude. I will explain why regulation fails in such a case, and explain how this alternate mode of black hole growth could also be crucial for gamma-ray bursts and the rapid growth of supermassive black holes during the epoch of galaxy formation. I will also suggest that hyperaccreting black holes may be associated with the fastest jets, and that these are propelled by radiation pressure instead of magnetic forces.


"Polarization as a diagnostic of the emission processes in relativistic jets"

Markus Böttcher (North-West University, Potchefstroom, RSA)

Astrophysical sources exhibiting relativistic jets, such as active galactic nuclei (AGN) and gamma-ray bursts (GRBs) are strong emitters of radiation across the electromagnetic spectrum, from radio to gamma-rays. In spite of intensive, co-ordinated observing efforts over the past several decades, the nature of the radiating particles, the location of the gamma-ray emissino region, and the mechanism leading to the emission of high-energy radiation, are still uncertain. As a new strategy to break the degeneracy of different models proposed for the origin of gamma-rays from relativistic jets, many observatories are now including measurements of optical polarization into multi-wavelength studies of relativistic jet sources, and a new generation of X-ray polarimeters is currently being developed. These studies have, in several instances, revealed large rotations of the position angle of the optical polarization, correlated with gamma-ray flaring activity in blazars, a highly beamed class of jet-dominated AGN. In this talk, I will summarize recent theoretical predictions of the high-energy (X-ray and gamma-ray) polarization from different blazar models, discuss the implications of high-energy polarization on gamma-gamma absorption of high-energy gamma-rays within the emission region, and present a new theoretical interpretation of the optical polarization-angle swings associated with gamma-ray flares in blazars.


"Feedback from deeply-embedded protostars probed with Herschel / PACS"

Agata Karska (MPE Garching / Leiden Observatory / UAM Poznań)


"Simulations of flux variability of oscillating accretion fluid tori around Kerr black holes"

Kateřina Goluchová (Slezská univerzita v Opavě)

We present a study of oscillating accretion tori orbiting in the vicinity of relativistic compact objects. The study was performed on the background of the Kerr spacetime geometry. We have demonstrated that a significant variation of the observed flux can be caused by combination of radial and vertical oscillations modes of a slender, polytropic, perfect fluid, non-selfgraviting torus with constant specific angular momentum.


"Radio polarisation – a tool for study of cosmic magnetic fields"

Richard Wielebinski (Max-Planck-Institut für Radioastronomie, Bonn)

The Zeeman Effect was the first method of remote study of cosmic magnetic fields. Using optical polarisation observations the magnetic field in the Sun (Hale, 1908) and later in stars (Babcock, 1947) were detected. The development of radio astronomy methods led to a first report of the polarisation of Solar emission in 1946 already. In a way this detection paved the way to the interpretation of the origin of radio cosmic radio waves. For a time optical polarisation was also used for studies of magnetic fields (Hiltner; Hall, 1949). However there was a long discussion in the optical community about the origin of the polarisation – scattering, aligned dust, etc. Theoretical work on the origin of radio emission came quickly to the conclusion that the intense radio waves were non-thermal in origin, generated in magnetic fields. The bold suggestion of Shklovsky (1953) that the optical (and radio) emission of the Crab Nebula had its origin from energetic particles entering the magnetic field of the Nebula quickly led to the realisation that the synchrotron emission process was responsible. Since the synchrotron emission is polarised the argument could be inverted: by measuring the radio polarisation the studies of cosmic magnetic fields became possible. At first the observations of radio polarisation were limited due to instrumental problems. In particular it was necessary to move to higher radio frequencies where the Faraday Effect was negligible. The solar community studied magnetic fields since 1946. The first reported detection of radio polarisation in a distant cosmic object was in fact of the Crab Nebula in 1957. The next cosmic object that showed polarisation was Jupiter (1960). The year 1962 was the big bonanza year for polarisation: the Milky Way and radio galaxies were shown to be polarised. The Faraday Effect became a further source of information about cosmic magnetic fields: while the linear polarisation gives information about the field normal to the line of sight the Faraday Effect gives information about the fields parallel in the line of sight. From these early times the research field of cosmic magnetic fields has grown enormously. On the one hand the Milky Way has been studied in great detail. There is a clear connection between magnetic fields and star formation. Studies of nearby galaxies have shown surprisingly well oriented magnetic fields. Going further in the Universe magnetic fields exist in jets and lobes of radio galaxies. Practically every distant cosmic object has been detected as a non-thermal radio source and hence must have magnetic fields. Magnetic fields have been detected in the intergalactic space in clusters of galaxies. In my talk I will sketch both the observational and the theoretical developments up to the present day.


"Cosmic-Ray driven strong outflows from disk galaxies"

Michał Hanasz (Center for Astronomy, Nicolaus Copernicus University, Toruń)

Recent theoretical studies indicate that cosmic rays play an important role in regulating the evolution of the interstellar medium and efficiency of star formation. I will present numerical models of galactic winds driven by cosmic rays (CRs) produced in supernova remnants in massive star-forming disk galaxies. We assume that type II supernovae deposit 10% of their energy into the ISM as cosmic rays. With a typical Galactic diffusion coefficient for CRs (3 × 1028 cm2 s−1), we demonstrate that this process alone can trigger the local formation of a strong low-density galactic wind maintaining vertically open magnetic field lines. Driven by the additional pressure gradient of the relativistic fluid, the wind speed can exceed 103 km s−1, much higher than the escape velocity of the galaxy. The global mass loading, i.e., the ratio of the gas mass leaving the galactic disk in a wind to the star formation rate, becomes of order unity once the system has settled into an equilibrium. We conclude that relativistic particles accelerated in supernova remnants alone provide a natural and efficient mechanism to trigger winds similar to observed mass-loaded galactic winds in high-redshift galaxies. These winds also help in explaining the low efficiencies for the conversion of gas into stars in galaxies, as well as the early enrichment of the intergalactic medium with metals. This mechanism may be at least of similar importance to the traditionally considered momentum feedback from massive stars and thermal and kinetic feedback from supernova explosions.


"On the interplay between the heartbeat oscillations and wind outflow in the microquasar IGR J17091-3624"

Mikolaj Grzędzielski (Center for Theoretical Physics, PAN, Warsaw)

In 2011, the black hole candidate IGR J17091-3624 exhibited in some characteristic states, bright outbursts and strong quasi-periodic oscillations of luminosity (on timescales of tens of seconds), so called 'heartbeat state. That process may be modeled by the radiation instability driven by the domination of radiation pressure and enhanced heating of the plasma. Despite that the mean accretion rate in this source is probably below the Eddington limit, such oscillations will still have large amplitudes. As the observations show, that the wind may occures in some states. Wind helps to stabilize the hearbeat outbursts. We use the hydrodynamical GLADIS code to model the disk and X-Ray radiation. We examined the data archive from Chandra and XMM-Newton satellites to find the observed limitations on the wind physical properties, such as its velocity and ionization state and investigated the long term evolution of this source, spanned over about 600 days of observations. Our model requires a substantial wind component, to heartbeat variability explain the proper variability pattern, and even complete suppression of flares in some states. The wind mass loss rate extracted from the data is in good quantitative agreement with our scenario.


"Variability of accreting black holes connected with the shocks in the low angular momentum flows"

Petra Suková (Center for Theoretical Physics, PAN, Warsaw)

We derive the conditions for shock formation in a quasi-spherical, slightly rotating flows. We verify the results of semi-analytical, stationary calculations with the time evolution studied by numerical hydro-simulations, and we study the oscillations of the shock position. We also study the behaviour of flows with varying specific angular momentum, where the ’hysteresis’ type of loop is found when passing through the multiple sonic points region. Our results are in agreement with the timescales and shapes of the luminosity flares observed in Sgr A*. These or similar models may also be applicable for the Galactic stellar mass black holes, like GX 339-4, where periodic oscillations of X-ray luminosity are detected.


"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)