Search results for "galaxy"
showing 10 items of 1505 documents
A three-dimensional smoothed particle hydrodynamics simulation of the active phase of SS Cyg-type discs and its implications for the mass transfer bu…
1993
We perform a smoothed particle hydrodynamics (SPH) three-dimensional simulation of the outburst phase of the accretion disc of a typical SS Cyg-like dwarf nova in the framework of the mass transfer burst model (MTBM), where we assume that the active phase is triggered by a sudden increase in the accretion rate due to some instability in the secondary's atmosphere. The evolution of the accretion disc is followed for a single orbital period, starting from the initial quiescent disc configuration obtained by us in a previous paper. This is a suitable integration time for determining the geometrical and physical properties of the disc in the impulsive phase and is comparable with observed outbu…
A fresh look into the interacting dark matter scenario
2018
The elastic scattering between dark matter particles and radiation represents an attractive possibility to solve a number of discrepancies between observations and standard cold dark matter predictions, as the induced collisional damping would imply a suppression of small-scale structures. We consider this scenario and confront it with measurements of the ionization history of the Universe at several redshifts and with recent estimates of the counts of Milky Way satellite galaxies. We derive a conservative upper bound on the dark matter-photon elastic scattering cross section of $\sigma_{\gamma \rm{DM}} < 8 \times 10^{-10} \, \sigma_T \, \left(m_{\rm DM}/{\rm GeV}\right)$ at $95\%$~CL, abou…
Simultaneous uvbyβ Photometry and Hα Spectroscopy of Be Stars in Open Clusters
1994
The usual methods of spectral clasification, equivalent widths of Balmer lines or photometric calibrations are not suitable for the determination of the astrophysical parameters of the underlying star in Be-type objects. The spectrum is distorted by the circumstellar envelope lines, while the contribution of the envelope continuum radiation contaminates the photometric indices.
X-ray emission from early-type stars in the Orion Nebula Cluster
2005
The X-ray properties of twenty ~1 Myr old O, B, and A stars of the Orion Trapezium are examined with data from the Chandra Orion Ultradeep Project (COUP). On the basis of simple theories for X-ray emission, we define two classes separated at spectral type B4: hotter stars have strong winds that may give rise to X-ray emission in small- or large-scale wind shocks, and cooler stars that should be X-ray dark due to their weaker winds and absence of outer convection zones where dynamos can generate magnetic fields. Only two of the massive stars show exclusively the constant soft-spectrum emission expected from the standard model for X-ray emission from hot stars involving many small shocks in a…
SPH simulations of Shakura-Sunyaev instability at intermediate accretion rates
2003
We show that a standard Shakura-Sunyaev accretion disc around a black hole with an accretion rate lower than the critical Eddington limit does show the instability in the radiation pressure dominated zone. We obtain this result performing time-dependent simulations of accretion disks for a set of values of the viscosity parameter and accretion rate. In particular we always find the occurrence of the collapse of the disc: the instability develops always towards a collapsed gas pressure dominated disc and not towards the expansion. This result is valid for all initial configurations we tested. We find significant convective heat flux that increases the instability development time, but is not…
Fractal Aspects of Galaxy Clustering
2008
In the past decade, the mathematical concept of fractal has exerted a great influence in a large variety of scientific disciplines. It is very common to find recent papers on the application of fractals to different fields in Physics, Chemistry, Biology, etc. The success of the fractal geometry in the description of many systems is due to the fact that deep insights into very simple objects show how fractal measures are more natural for their study.
Searching for the scale of homogeneity
1998
We introduce a statistical quantity, known as the $K$ function, related to the integral of the two--point correlation function. It gives us straightforward information about the scale where clustering dominates and the scale at which homogeneity is reached. We evaluate the correlation dimension, $D_2$, as the local slope of the log--log plot of the $K$ function. We apply this statistic to several stochastic point fields, to three numerical simulations describing the distribution of clusters and finally to real galaxy redshift surveys. Four different galaxy catalogues have been analysed using this technique: the Center for Astrophysics I, the Perseus--Pisces redshift surveys (these two lying…
A Study of Gravitational Lens Chromaticity using Ground-based Narrow Band Photometry
2011
We present observations of wavelength-dependent flux ratios for four gravitational lens systems (SDSS~J1650+4251, HE~0435$-$1223, FBQ 0951+2635, and Q~0142$-$100) obtained with the Nordic Optical Telescope. The use of narrowband photometry, as well as the excellent seeing conditions during the observations, allows us to study their chromatic behavior. For SDSS~J1650+4251, we determine the extinction curve of the dust in the $z_L=0.58$ lens galaxy and find that the 2175 \AA \ feature is absent. In the case of HE~0435$-$1223, we clearly detect chromatic microlensing. This allows us to estimate the wavelength-dependent size of the accretion disk. We find an R-band disk size of $r^{R}_s=13\pm5$…
PECULIAR TRANSVERSE VELOCITIES OF GALAXIES FROM QUASAR MICROLENSING. TENTATIVE ESTIMATE OF THE PECULIAR VELOCITY DISPERSION ATZ∼ 0.5
2016
We propose to use the flux variability of lensed quasar images induced by gravitational microlensing to measure the transverse peculiar velocity of lens galaxies over a wide range of redshift. Microlensing variability is caused by the motions of the observer, the lens galaxy (including the motion of the stars within the galaxy), and the source; hence, its frequency is directly related to the galaxy's transverse peculiar velocity. The idea is to count time-event rates (e.g., peak or caustic crossing rates) in the observed microlensing light curves of lensed quasars that can be compared with model predictions for different values of the transverse peculiar velocity. To compensate for the larg…
A Robust Determination of the size of quasar accretion disks using gravitational microlensing
2012
Using microlensing measurements from a sample of 27 image-pairs of 19 lensed quasars we determine a maximum likelihood estimate for the accretion disk size of an {{\em}average} quasar of $r_s=4.0^{+2.4}_{-3.1} $ light days at rest frame $=1736$\AA\ for microlenses with a mean mass of $=0.3M_\odot$. This value, in good agreement with previous results from smaller samples, is roughly a factor of 5 greater than the predictions of the standard thin disk model. The individual size estimates for the 19 quasars in our sample are also in excellent agreement with the results of the joint maximum likelihood analysis.