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Diaz-Gimenez E.,Institute Astronomia Teorica y Experimental | Diaz-Gimenez E.,CONICET | Mamon G.A.,University Pierre and Marie Curie | Mamon G.A.,University of Oxford
Monthly Notices of the Royal Astronomical Society | Year: 2010

We identify compact groups of galaxies (CGs) within mock galaxy catalogues from the Millennium Simulation at z= 0 with the semi-analytic models (SAMs) of galaxy formation of Bower et al., Croton et al. and De Lucia & Blaizot. CGs are identified using the same 2D criteria as those visually applied by Hickson (1982) to his CGs (HCGs), but with a brightest galaxy magnitude limit, and we also add the important effect of observers blending close projected pairs. Half of the mock CGs identified in projection contain at least four accordant velocities (mvCGs), versus 70 per cent for HCGs. In comparison to mvCGs, the HCGs are only 8 per cent complete at distances <9000 km s-1, missing the CGs with small angular sizes, a strongly dominant galaxy, and (for the second SAM) the mvCGs that are fainter and those with lower surface brightness. 10 per cent of the mock mvCGs are identical to the parent virialized group, meaning that they are isolated, while the remainder are embedded in their parent virialized groups. We explore different ways to determine the fraction of physically dense groups given the data from the simulations. Binding energy criteria turn out to be inapplicable given the segregation between galaxies and dark matter particles. We rely instead on the combination of the three-dimensional length of the CGs (maximum real space galaxy separation) and their elongation along the line of sight (ratio of maximum line-of-sight to maximum projected separations), restricting ourselves in both cases to the smallest quartets within the CGs. We find that between 64 and 80 per cent (depending on the SAM) of the mvCGs have 3D lengths shorter than 200 h-1 kpc, between 71 and 80 per cent have line-of-sight elongations less than 2, while between 59 and 76 per cent have either 3D lengths shorter than 100 h-1 kpc or both lengths shorter than 200 h-1 kpc and elongations smaller than 2. Therefore, chance alignments (CAs) of galaxies concern at most 40 per cent of the mvCGs. These CAs are mostly produced from larger host groups, but a few have galaxies extending a few Mpc beyond the host group. The mvCGs built with the Hickson selection (respectively without the close projected pair blending criterion) have 10 per cent higher (lower) fractions of physically dense systems. © 2010 The Authors. Journal compilation © 2010 RAS. Source

Velazquez P.F.,National Autonomous University of Mexico | Raga A.C.,National Autonomous University of Mexico | Canto J.,National Autonomous University of Mexico | Schneiter E.M.,Institute Astronomia Teorica y Experimental | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We have found that the ballistic trajectory of a precessing, orbiting and time-dependent velocity jet has a semi-analytical solution. Bipolar, multipolar and S-like morphologies, which are observed in young protoplanetary and planetary nebula (PPN and PN, respectively), can be reproduced by setting different values for the ratio between dynamical time and precession periods, the ratio between the precession and orbital periods, and the jet velocity variability period. We have also computed numerical simulations and find a good agreement with the semi-analytical solution for a jet 103 times denser than the surrounding environment. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Yaryura C.Y.,Institute Astronomia Teorica y Experimental | Yaryura C.Y.,CONICET | Baugh C.M.,Durham University | Angulo R.E.,Max Planck Institute for Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2011

We introduce an objective method to assess the probability of finding extreme events in the distribution of cold dark matter such as voids, overdensities or very high mass haloes. Our approach uses an ensemble of N-body simulations of the hierarchical clustering of dark matter to find extreme structures. The frequency of extreme events, in our case the cell or smoothing volume with the highest count of cluster-mass dark matter haloes, is well described by a Gumbel distribution. This distribution can then be used to forecast the probability of finding even more extreme events, which would otherwise require a much larger ensemble of simulations to quantify. We use our technique to assess the chance of finding concentrations of massive clusters or superclusters, like the two found in the two-degree field galaxy redshift survey (2dFGRS), using a counts-in-cells analysis. The Gumbel distribution gives an excellent description of the distribution of extreme cell counts across two large ensembles of simulations covering different cosmologies, and also when measuring the clustering in both real and redshift space. We find examples of structures like those found in the 2dFGRS in the simulations. The chance of finding such structures in a volume equal to that of the 2dFGRS is around 2 per cent. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source

Weidmann W.A.,National University of Cordoba | Gamen R.,National University of La Plata | Van Hoof P.A.M.,Royal Observatory of Belgium | Zijlstra A.,University of Manchester | And 3 more authors.
Astronomy and Astrophysics | Year: 2013

Context. Planetary nebulae (PNe) are powerful tracers of evolved stellar populations. Among the 3000 known PNe in the Galaxy, about 600 are located within the 520 square-degree area covered by the VVV survey. The VVV photometric catalogue provides an important new dataset for the study of PNe, with high-resolution imaging in five near-infrared bands. Aims. There are various colour-colour diagrams that can be obtained from the VVV filters. We investigate the location of PNe in these diagrams and the separation from other types of objects. This includes the new Y-J vs. Z-Y diagram. Methods. Aperture photometry of known PNe in the VVV area was retrieved from source catalogues. Care was taken to minimise any confusion with field stars. The colours of the PNe we are determined for (H-Ks), (J-H), (Z-Y), and (Y-J), and compared to stars and to other types of emission line objects. Cloudy photo-ionisation models were used to predict colours for typical PNe. Results. We present near-infrared photometry for 353 known PNe. The best separation from other objects is obtained in the (H-Ks) vs. (J-H) diagram. We calculated the emission-line contribution to the in-band flux based on a model for NGC 6720: we find that this is highest in the Z and Y bands at over 50%, lower in the J band at 40%, and lowest in the H and Ks bands at 20%. A new view of PNe in the wavelength domain of the Z and Y bands is shown. Photo-ionisation models are used to explore the observed colours in these bands. The Y band is shown to be dominated by He i 1.083 μm and He ii 1.012 μm, and colours involving this band are very sensitive to the temperature of the ionizing star. Conclusions. The VVV survey represents a unique dataset for studying crowded and obscured regions in the Galactic plane. The diagnostic diagrams presented here allow one to study the properties of known PNe and to uncover objects not previously classified. © ESO, 2013. Source

Rodriguez-Kamenetzky A.,Institute Astronomia Teorica y Experimental | Rodriguez-Kamenetzky A.,Institute Radioastronomia y Astrofisica IRyA UNAM | Carrasco-Gonzalez C.,Institute Radioastronomia y Astrofisica IRyA UNAM | Araudo A.,University of Oxford | And 5 more authors.
Astrophysical Journal | Year: 2016

While most protostellar jets present free-free emission at radio wavelengths, synchrotron emission has also been proposed to be present in a handful of these objects. The presence of nonthermal emission has been inferred by negative spectral indices at centimeter wavelengths. In one case (the HH 80-81 jet arising from a massive protostar), its synchrotron nature was confirmed by the detection of linearly polarized radio emission. One of the main consequences of these results is that synchrotron emission implies the presence of relativistic particles among the nonrelativistic material of these jets. Therefore, an acceleration mechanism should be taking place. The most probable scenario is that particles are accelerated when the jets strongly impact against the dense envelope surrounding the protostar. Here we present an analysis of radio observations obtained with the Very Large Array of the triple radio source in the Serpens star-forming region. This object is known to be a radio jet arising from an intermediate-mass protostar. It is also one of the first protostellar jets where the presence of nonthermal emission was proposed. We analyze the dynamics of the jet and the nature of the emission and discuss these issues in the context of the physical parameters of the jet and the particle acceleration phenomenon. © 2016. The American Astronomical Society. All rights reserved. Source

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