Schönau-Berzdorf, Germany
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Sommer M.W.,Argelander Institute For Astronomie | Basu K.,Argelander Institute For Astronomie
Monthly Notices of the Royal Astronomical Society | Year: 2014

We aim at an unbiased census of the radio halo population in galaxy clusters and test whether current low number counts of radio haloes have arisen from selection biases. We construct near-complete samples based on X-ray and Sunyaev-Zel'dovich (SZ) effect cluster catalogues and search for diffuse, extended (Mpc-scale) emission near the cluster centres by analysing data from the National Radio Astronomy Observatory Very Large Array Sky Survey. We remove compact sources using a matched filtering algorithm and model the diffuse emission using two independent methods. The relation between radio halo power at 1.4 GHz and mass observables is modelled using a power law, allowing for a 'dropout' population of clusters hosting no radio halo emission. An extensive suite of simulations is used to check for biases in our methods. Our findings suggest that the fraction of targets hosting radio haloes may have to be revised upwards for clusters selected using the SZ effect: while approximately 60 per cent of the X-ray selected targets are found to contain no extended radio emission, in agreement with previous findings, the corresponding fraction in the SZ selected samples is roughly 20 per cent. We propose a simple explanation for this selection difference based on the distinct time evolution of the SZ and X-ray observables during cluster mergers, and a bias towards relaxed, cool-core clusters in the X-ray selection. © 2013 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Banerjee S.,Argelander Institute For Astronomie | Kroupa P.,Helmholtz Institute For Strahlen Und Kernphysik
Astrophysical Journal | Year: 2014

Understanding how distinct, near-spherical gas-free clusters of very young, massive stars shape out of vast, complex clouds of molecular hydrogen is one of the biggest challenges in astrophysics. A popular thought dictates that a single gas cloud fragments into many newborn stars which, in turn, energize and rapidly expel the residual gas to form a gas-free cluster. This study demonstrates that the above classical paradigm remarkably reproduces the well-observed central, young cluster (HD 97950) of the Galactic NGC 3603 star-forming region, in particular, its shape, internal motion, and mass distribution of stars naturally and consistently follow from a single model calculation. Remarkably, the same parameters (star formation efficiency, gas expulsion timescale, and delay) reproduce HD 97950, as were found to reproduce the Orion Nebula Cluster, Pleiades, and R136. The present results therefore provide intriguing evidence of formation of star clusters through single-starburst events followed by significant residual gas expulsion. © 2014. The American Astronomical Society. All rights reserved..

Braithwaite J.,Argelander Institute For Astronomie
Monthly Notices of the Royal Astronomical Society | Year: 2012

Strong magnetic fields play a crucial role in the removal of angular momentum from collapsing clouds and protostellar discs, and are necessary for the formation of disc winds as well as jets from the inner disc and, indeed, strong large-scale poloidal magnetic fields are observed in protostellar discs at all radii down to ∼10R ⊙. Nevertheless, by the time the star is visible virtually, all of the original magnetic flux has vanished. We explore mechanisms for removing this flux during the formation of the protostar once it is magnetically disconnected from the parent cloud, looking at both radiative and convective protostars. This includes a numerical investigation of buoyant magnetic field removal from convective stars. It is found that if the star goes through a fully convective phase, all remaining flux can easily be removed from the protostar, essentially on an Alfvén time-scale. If, on the other hand, the protostar has no fully convective phase, then some flux can be retained, the quantity depending on the net magnetic helicity, which is probably quite small. Only some fraction of this flux is visible at the stellar surface. We also look at how the same mechanisms could prevent flux from accreting on to the star at all, meaning that mass would only accrete as fast as it is able to slip past the flux. © 2012 The Author Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Oh S.,Argelander Institute For Astronomie | Kroupa P.,Argelander Institute For Astronomie
Monthly Notices of the Royal Astronomical Society | Year: 2012

We perform the largest currently available set of direct N-body calculations of young star cluster models to study the dynamical influence, especially through the ejections of the most massive star in the cluster, on the current relation between the maximum stellar mass and the star cluster mass. We vary several initial parameters such as the initial half-mass radius of the cluster, the initial binary fraction and the degree of initial mass segregation. Two different pairing methods are used to construct massive binaries for more realistic initial conditions of massive binaries. We find that lower mass clusters do not shoot out their heaviest star. In the case of massive clusters, no most massive star escapes the cluster within 3Myr regardless of the initial conditions if clusters have initial half-mass radii, r 0.5, ≥0.8pc. However, a few of the initially smaller sized clusters (r 0.5= 0.3pc), which have a higher density, eject their most massive star within 3Myr. If clusters form with a compact size and their massive stars are born in a binary system with a mass ratio biased towards unity, the probability that the mass of the most massive star in the cluster changes due to the ejection of the initially most massive star can be as large as 20per cent. Stellar collisions increase the maximum stellar mass in a large number of clusters when clusters are relatively dense (and r 0.5= 0.3pc) and binary rich. Overall, we conclude that dynamical effects hardly influence the observational maximum stellar mass-cluster mass relation. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Ludlow A.D.,Argelander Institute For Astronomie | Porciani C.,Argelander Institute For Astronomie
Monthly Notices of the Royal Astronomical Society | Year: 2011

We use two cosmological simulations of structure formation to study the conditions under which dark matter haloes emerge from the linear density field. Our analysis focuses on matching sites of halo collapse to local density maxima, or 'peaks', in the initial conditions of the simulations and provides a crucial test of the central ansatz of the peaks formalism. By identifying peaks on a variety of smoothed, linearly extrapolated density fields, we demonstrate that as many as ~70 per cent of well-resolved dark matter haloes form preferentially near peaks whose characteristic masses are similar to those of the halo, with more massive haloes showing a stronger tendency to reside near peaks initially. We identify a small but significant fraction of haloes that appear to evolve from peaks of substantially lower mass than that of the halo itself. We refer to these as 'peakless haloes' for convenience. By contrasting directly the properties of these objects with the bulk of the protohalo population, we find two clear differences: (1) their initial shapes are significantly flatter and more elongated than the predominantly triaxial majority and (2) they are, on average, more strongly compressed by tidal forces associated with their surrounding large-scale structure. Using the two-point correlation function, we show that peakless haloes tend to emerge from highly clustered regions of the initial density field implying that, at fixed mass, the accretion geometry and mass accretion histories of haloes in highly clustered environments differ significantly from those in the field. This may have important implications for understanding the origin of the halo assembly bias, of galaxy properties in dense environments and how environment affects the morphological transformation of galaxies near groups and rich galaxy clusters. © 2011 The Authors. Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Cantalupo S.,Kavli Institute for Cosmology | Porciani C.,Argelander Institute For Astronomie
Monthly Notices of the Royal Astronomical Society | Year: 2011

We present a new three-dimensional radiative transfer (RT) code, radamesh (Radiative-transfer on ADAptive MESH), based on a ray-tracing, photon-conserving and adaptive (in space and time) scheme. radamesh uses a novel Monte Carlo approach to sample the radiation field within the computational domain on a 'cell-by-cell' basis. Thanks to this algorithm, the computational efforts are now focused where actually needed, i.e. within the Ionization-fronts (I-fronts). This results in an increased accuracy level and, at the same time, a huge gain in computational speed with respect to a 'classical' Monte Carlo RT, especially when combined with an Adaptive Mesh Refinement (AMR) scheme. Among several new features, radamesh is able to adaptively refine the computational mesh in correspondence of the I-fronts, allowing to fully resolve them within large, cosmological boxes. We follow the propagation of ionizing radiation from an arbitrary number of sources and from the recombination radiation produced by H and He. The chemical state of six species (Hi, Hii, Hei, Heii, Heiii, e) and gas temperatures are computed with a time-dependent, non-equilibrium chemistry solver. We present several validating tests of the code, including the standard tests from the RT code comparison project and a new set of tests aimed at substantiating the new characteristics of radamesh. Using our AMR scheme, we show that properly resolving the I-front of a bright quasar during reionization produces a large increase of the predicted gas temperature within the whole Hii region. Also, we discuss how H and He recombination radiation is able to substantially change the ionization state of both species (for the classical Strömgren sphere test) with respect to the widely used 'on-the-spot' approximation. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.

Papadopoulos P.P.,Argelander Institute For Astronomie
Astrophysical Journal | Year: 2010

The high-density star formation typical of the merger/starburst events that power the large IR luminosities of ultraluminous infrared galaxies (ULIRGs) (LIR(8-1000 μm) ≳1012 L⊙) throughout the universe results in extraordinarily high cosmic-ray (CR) energy densities of UCR ∼ few × (103-104) UCR,Gal permeating their interstellar medium, a direct consequence of the large supernova remnant number densities in such systems. Unlike far-UV photons emanating from numerous star-forming (SF) sites, these large CR energy densities in ULIRGs will volumetrically heat and raise the ionization fraction of dense (n > 104 cm -3) UV-shielded gas cores throughout their compact SF volumes. Such conditions can turn most of the large molecular gas masses found in such systems and their high redshift counterparts (∼109-1010 Ṁ) into giant CR-dominated regions (CRDRs) rather than ensembles of photon-dominated regions (PDRs) which dominate in less IR-luminous systems where star formation and molecular gas distributions are much more extended. The molecular gas in CRDRs will have a minimum temperature of Tkin ∼ (80-160) K, and very high ionization fractions of x(e) > 10-6 throughout its UV-shielded dense core, which in turn will fundamentally alter the initial conditions for star formation in such systems. Observational tests of CRDRs can be provided by high-J CO and 13CO lines or multi-J transitions of any heavy rotor molecules (e.g., HCN) and their isotopologs. Chemical signatures of very high ionization fractions in dense UV-shielded gas such as low [DCO +]/[HCO+] and high [HCO+]/[CO] abundance ratios would be good probes of CRDRs in extreme starbursts. These tests, along with direct measurements of the high CO line brightness temperatures expected over the areas of compact dense gas disks found in ULIRGs, will soon be feasible as sub-arcsecond interferometric imaging capabilities and sensitivity at millimeter/submillimeter wavelengths improve in the era of ALMA. © 2010. The American Astronomical Society. All rights reserved.

Papadopoulos P.P.,Argelander Institute For Astronomie | Pelupessy F.I.,Leiden University
Astrophysical Journal | Year: 2010

We utilize detailed time-varying models of the coupled evolution of stars and the H I, H2, and CO-bright H2 gas phases in galaxy-sized numerical simulations to explore the evolution of gas-rich and/or metal-poor systems, which are expected to be numerous in the early universe. The inclusion of the CO-bright H2 gas phase and the realistic rendering of star formation as an H2-regulated process (and the new feedback processes that this entails) allow the most realistic tracking of strongly evolving galaxies and much better comparison with observations. We find that while galaxies eventually settle into states conforming to the Schmidt-Kennicutt (S-K) relations, significant and systematic deviations of their star formation rates (SFRs) from the latter occur, and are especially pronounced and prolonged for metal-poor systems. The largest such deviations occur for gas-rich galaxies during not only the early evolutionary stages but also during brief periods at later stages. Given that gas-rich and/or metal-poor states of present-epoch galaxies are expected in the early universe while a much larger number of mergers frequently reset non-isolated systems to gas-rich states, even brief periods of sustained deviations of their SFRs from those expected from the S-K relations may come to characterize significant periods of their stellar mass built-up. This indicates potentially serious limitations of S-K-type relations as reliable sub-grid elements of star formation physics in simulations of structure formation in the early universe. We anticipate that galaxies with marked deviations from the S-K relations will be found at high redshifts as unbiased inventories of total gas mass become possible with ALMA and the EVLA. © 2010 The American Astronomical Society. All rights reserved.

Dietrich J.P.,ESO | Hartlap J.,Argelander Institute For Astronomie
Monthly Notices of the Royal Astronomical Society | Year: 2010

Weak-lensing searches for galaxy clusters are plagued by low completeness and purity, severely limiting their usefulness for constraining cosmological parameters with the cluster mass function. A significant fraction of 'false positives' are due to projection of large-scale structure and as such carry information about the matter distribution. We demonstrate that by constructing a 'peak function', in analogy to the cluster mass function, cosmological parameters can be constrained. To this end, we carried out a large number of cosmological N-body simulations in the Ωm-σ8 plane to study the variation of this peak function. We demonstrate that the peak statistics is able to provide constraints competitive with those obtained from cosmic-shear tomography from the same data set. By taking the full cross-covariance between the peak statistics and cosmic shear into account, we show that the combination of both methods leads to tighter constraints than either method alone can provide. © 2009 The Authors. Journal compilation © 2009 RAS.

Banerjee S.,Argelander Institute For Astronomie | Kroupa P.,Argelander Institute For Astronomie
Astrophysical Journal | Year: 2013

Based on kinematic data observed for very young, massive clusters that appear to be in dynamical equilibrium, it has recently been argued that such young systems are examples of where the early residual gas expulsion did not happen or had no dynamical effect. The intriguing scenario of a star cluster forming through a single starburst has thereby been challenged. Choosing the case of the R136 cluster of the Large Magellanic Cloud, the most cited one in this context, we perform direct N-body computations that mimic the early evolution of this cluster including the gas-removal phase (on a thermal timescale). Our calculations show that under plausible initial conditions which are consistent with observational data, a large fraction (>60%) of a gas-expelled, expanding R136-like cluster is bound to regain dynamical equilibrium by its current age. Therefore, the recent measurements of velocity dispersion in the inner regions of R136, which indicate that the cluster is in dynamical equilibrium, are consistent with an earlier substantial gas expulsion of R136 followed by a rapid re-virialization (in 1 Myr). Additionally, we find that the less massive Galactic NGC 3603 Young Cluster (NYC), with a substantially longer re-virialization time, is likely to be found to have deviated from dynamical equilibrium at its present age (1 Myr). The recently obtained stellar proper motions in the central part of the NYC indeed suggest this and are consistent with the computed models. This work significantly extends previous models of the Orion Nebula Cluster which already demonstrated that the re-virialization time of young post-gas-expulsion clusters decreases with increasing pre-expulsion density. © 2013. The American Astronomical Society. All rights reserved.

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