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Kazantzidis S.,Ohio State University | Lokas E.L.,Nicolaus Copernicus Astronomical Center | Mayer L.,University of Zürich
Astrophysical Journal Letters | Year: 2013

According to the tidal stirring model, late type, rotationally supported dwarfs resembling present day dwarf irregular (dIrr) galaxies can transform into dwarf spheroidals (dSphs) via interactions with Milky-Way-sized hosts. We perform collisionless N-body simulations to investigate for the first time how tidal stirring depends on the dark matter (DM) density distribution in the central stellar region of the progenitor disky dwarf. Specifically, we explore various asymptotic inner slopes γ of the dwarf DM density profiles (ρr -γ). For a given orbit inside the primary galaxy, rotationally supported dwarfs embedded in DM halos with core-like distributions (γ = 0.2) and mild density cusps (γ = 0.6) demonstrate a substantially enhanced likelihood and efficiency of transformation into dSphs compared to their counterparts with steeper DM density profiles (γ = 1). Such shallow DM distributions are akin to those of observed dIrrs highlighting tidal stirring as a plausible model for the Local Group (LG) morphology-density relation. When γ < 1, a single pericentric passage can induce dSph formation and disky dwarfs on low-eccentricity or large-pericenter orbits are able to transform; these new results allow tidal stirring to explain virtually all known dSphs across a wide range of distances from their hosts. A subset of disky dwarfs initially embedded in DM halos with shallow density profiles are eventually disrupted by the primary; those that survive as dSphs are generally on orbits with lower eccentricities and/or larger pericenters compared to those of typical cold dark matter satellites. The latter could explain the peculiar orbits of several LG dSphs such as Fornax, Leo I, Tucana, and Cetus. © 2013. The American Astronomical Society. All rights reserved..


Bolejko K.,University of Oxford | Celerier M.-N.,University Paris Diderot | Krasinski A.,Nicolaus Copernicus Astronomical Center
Classical and Quantum Gravity | Year: 2011

Recently, inhomogeneous generalizations of the Friedmann-Lematre-Robertson- Walker (FLRW) cosmological models have gained interest in the astrophysical community and are more often employed to study cosmological phenomena. However, in many papers the inhomogeneous cosmological models are treated as an alternative to the FLRW models. In fact, they are not an alternative, but an exact perturbation of the latter, and are gradually becoming a necessity in modern cosmology. The assumption of homogeneity is just a first approximation introduced to simplify equations. So far this assumption is commonly believed to have worked well, but future and more precise observations will not be properly analysed unless inhomogeneities are taken into account. This paper reviews recent developments in the field and shows the importance of an inhomogeneous framework in the analysis of cosmological observations. © 2011 IOP Publishing Ltd.


Ziolkowski J.,Nicolaus Copernicus Astronomical Center
Memorie della Societa Astronomica Italiana, Supplementi - Journal of the Italian Astronomical Society, Supplement | Year: 2010

Three classes of galactic black holes (BHs): supermassive, intermediate mass and stellar mass ones are briefly reviewed. The recent radio observations of our only supermassive black hole (Sgr A∗) permitted us to see (for the first time) the structures on the scale of the event horizon. The evidence for the presence of the intermediate mass black holes in the centers of globular clusters is now somewhat weaker than one year ago (but their presence is entirely possible). There are no new BH candidates (BHCs) from microlensing events (still only 4 are known). A black hole of mass ≥10.4 M⊙ is probably present in triple system V Pup and BHCs are suspected among invisible components of some W-R binaries. Most information about stellar mass BHCs comes still from X-ray binaries (XRBs), which contain 58 BHCs (among them 23 confirmed BHs with mass estimates). Ten BHs have now spin determinations based on modeling the shape of Kα lines and five BHs have spin estimates from fits to the continuum X-ray spectra. Majority of the spin estimates indicate fast or very fast rotation of the investigated BHs. The last chapter of my review is devoted to the mystery of the apparently missing population of XRBs composed of a Be star and a BH. With the help of the most recent stellar population synthesis calculations, it is shown that, at present, the expected number of such systems (1 ± 1) is consistent with the observed number (zero). © SAIt 2010.


Hellwing W.A.,Nicolaus Copernicus Astronomical Center
Annalen der Physik (Leipzig) | Year: 2010

Based on a set of cosmological N-body simulations we analyze properties of the dark matter (DM) haloes in a galaxy mass range (1011-10 13h-1M⊙) in modified ?CDM cosmology with additional dynamically screened scalar interactions in DM sector. Our simulations show that scalar interactions support picture of the Island Universe. Rapid structure formation processes are shifted into higher redshifts resulting in a much smaller accretion and merging rates for galactic haloes at low redshifts. Finally, we present how this "fifth" force affects halo properties, like density profile, triaxiality, ellipticities and the spin parameter. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.


Wojtak R.,Nicolaus Copernicus Astronomical Center | Lokas E.L.,Nicolaus Copernicus Astronomical Center
Monthly Notices of the Royal Astronomical Society | Year: 2010

We analyse kinematic data of 41 nearby (z < 0.1) relaxed galaxy clusters in terms of the projected phase-space density using a phenomenological, fully anisotropic model of the distribution function. We apply the Markov Chain Monte Carlo approach to place constraints on total mass distribution approximated by the universal Navarro, Frenk and White (NFW) profile and the profile of the anisotropy of galaxy orbits. We find the normalization of the mean mass-concentration relation is c= 6.9+0.6 -0.7 at the virial mass Mv= 5 × 1014 M⊙. By comparison with the calibration from cosmological N-body simulations it is demonstrated that this result is fully consistent with σ8 from 1-yr Wilkinson Microwave Anisotropy Probe (WMAP1) data release and agrees at 1σ level with that from WMAP5. Assuming a one-to-one correspondence between σ8 and the normalization of the mass-concentration relation in the framework of the concordance model we estimate the normalization of the linear power spectrum to be σ8= 0.91+0.07 -0.08. Our constraints on the parameters of the mass profile are compared with estimates from X-ray observations and other methods based on galaxy kinematics. We also study correlations between the virial mass and different mass proxies including the velocity dispersion, the X-ray temperature and the X-ray luminosity. We demonstrate that the mass scaling relations with the velocity dispersion and the X-ray temperature are fully consistent with the predictions of the virial theorem.We show that galaxy orbits are isotropic at the cluster centres (with the mean ratio of the radial-to-tangential velocity dispersions σr/σθ= 0.97 ± 0.04) and radially anisotropic at the virial sphere (with the mean ratio σr/σθ= 1.75+0.23 -0.19). Although the value of the central anisotropy appears to be universal, the anisotropy at the virial radius differs between clusters within the range 1 (σr/σθ) ≲ 2.Utilizing the Bautz-Morgan morphological classification and information on the prominence of a cool core we select two subsamples of galaxy clusters corresponding to less and more advanced evolutionary states. It is demonstrated that less evolved clusters have shallower mass profiles and their galaxy orbits are more radially biased at the virial sphere. This property is consistent with the expected evolution of the mass profiles as well as with the observed orbital segregation of late- and early-type galaxies. © 2010 The Authors. Journal compilation © 2010 RAS.


Vincent F.H.,Nicolaus Copernicus Astronomical Center
Classical and Quantum Gravity | Year: 2014

We investigate the possibility to distinguish the small-coupling, slow-rotation black hole solution of Chern-Simons (CS) gravity from the Kerr solution. We develop simulations of electromagnetic observables in the vicinity of CS and Kerr black holes. We show that the typical relative observable difference between CS and Kerr spacetimes is of the order of 0.1% thus beyond reach of current or near-future instruments. © 2014 IOP Publishing Ltd.


Kluaniak W.,Nicolaus Copernicus Astronomical Center
Astronomy and Astrophysics | Year: 2013

A sudden increase in stellar luminosity may lead to the ejection of a large part of any optically thin gas orbiting the star. Test particles in circular orbits will become unbound and will escape to infinity (if radiation drag is neglected) when the luminosity changes from zero to at least one half the Eddington value, or more generally, from L to (LEdd + L)/2 or more. Conversely, a decrease in luminosity will lead to the tightening of orbits of optically thin fluid. Even a modest fluctuation of luminosity of accreting neutron stars or black holes is expected to lead to substantial coronal heating. Luminosity fluctuations may thus account for the high temperatures of the X-ray corona in accreting black hole and neutron star systems. © 2013 ESO.


Janiak M.,Nicolaus Copernicus Astronomical Center | Sikora M.,Nicolaus Copernicus Astronomical Center | Moderski R.,Nicolaus Copernicus Astronomical Center
Monthly Notices of the Royal Astronomical Society | Year: 2015

The luminosities of many powerful blazars are strongly dominated by γ -rays, which most likely result from Comptonization of radiation produced outside a jet. This observation sets certain constraints on the composition and energetics of the jet, as well as the surrounding quasar environment. We study the dependence of Compton dominance on jet magnetization (the magnetic-to-matter energy flux) and on the location of the 'blazar zone'. Calculations are performed for two geometries of broad emission-line and hot dust regions: spherical and planar. The jet magnetization corresponding to the large observed Compton dominance is found to be ~0.1(θjΓ)2 for spherical geometries and ~0.01(θjΓ)2 for planar geometries, where θj is the jet half-opening angle and Γ is the jet Lorentz factor. This implies that jets in luminous blazars are matter-dominated and that this domination is particularly strong for the flattened geometry of external radiation sources. © 2015 The Authors.


Dyks J.,Nicolaus Copernicus Astronomical Center | Rudak B.,Nicolaus Copernicus Astronomical Center
Monthly Notices of the Royal Astronomical Society | Year: 2012

High-quality integrated radio profiles of some pulsars contain bifurcated, highly symmetric emission components (BECs). They are observed when our line of sight traverses through a split-fan shaped emission beam. It is shown that for oblique cuts through such a beam, the features appear asymmetric at nearly all frequencies, except for a single 'frequency of symmetry'ν sym, at which both peaks in the BEC have the same height. Around ν sym, the ratio of flux in the two peaks of a BEC evolves in a way resembling the multifrequency behaviour of J1012+5307. Because of the inherent asymmetry resulting from the oblique traverse of the sightline, each minimum in double notches can be modelled independently. Such a composed model reproduces the double notches of B1929+10 if the fitted function is the microscopic beam of curvature radiation in the orthogonal polarization mode. These results confirm our view that some of the double components in radio pulsar profiles directly reveal the microscopic nature of the emitted radiation beam as the microbeam of the curvature radiation polarized orthogonally to the trajectory of electrons. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.


Smolec R.,Nicolaus Copernicus Astronomical Center | Moskalik P.,Nicolaus Copernicus Astronomical Center
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present hydrodynamic BL Herculis type models which display a long-term modulation of pulsation amplitudes and phases. The modulation is either strictly periodic or quasi-periodic, with the modulation period and modulation pattern varying from one cycle to another. Such behaviour has not been observed in any BL Herculis variable so far; however, it is a common property of their lower luminosity siblings - RR Lyrae variables showing the Blazhko effect. These models provide support for the recent mechanism proposed by Buchler & Kolláth to explain this still mysterious phenomenon. In their model, a half-integer resonance that causes the period-doubling effect, discovered recently in Blazhko RR Lyrae stars, is responsible for the modulation of pulsation as well. Although our models are more luminous than is appropriate for RR Lyrae stars, they clearly demonstrate, through direct hydrodynamic computation, that the mechanism can indeed be operational. Of great importance are models that show quasi-periodic modulation - a phenomenon observed in Blazhko RR Lyrae stars. Our models coupled with the analysis of the amplitude equations show that such behaviour may be caused by the dynamical evolution occurring in the close proximity of the unstable single periodic saddle point. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

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