15,276 research outputs found
Lyman Continuum Escape from Inhomogeneous ISM
We have studied the effects of gas density inhomogeneities on the escape of
ionising Lyman continuum (Lyc) photons from Milky Way-type galaxies via 3D
numerical simulations using the Monte Carlo radiative transfer code CRASH
(Ciardi et al. 2001). To this aim a comparison between a smooth Gaussian
distribution (GDD) and an inhomogeneous, fractal one (FDD) has been made with
realistic assumptions for the ionising stellar sources based on available data
in the solar neighborhood. In both cases the escape fraction f_esc increases
with ionisation rate N_gamma (although for the FDD with a flatter slope) and
they become equal at N_gamma = 2*10^50 s^-1 where f_esc = 0.11. FDD allows
escape fractions of the same order also at lower N_gamma, when Lyc photon
escape is sharply suppressed by GDD. Values of the escape fraction as high as
0.6 can be reached (GDD) for N_gamma ~ 9*10^50 s^-1, corresponding to a star
formation rate (SFR) of roughly 2 M_o yr^-1; at this ionising luminosity the
FDD is less transparent (f_esc ~ 0.28). If high redshift galaxies have gas
column densities similar to local ones, are characterized by such high SFRs and
by a predominantly smooth (i.e.turbulence free) interstellar medium, our
results suggest that they should considerably contribute to - and possibly
dominate - the cosmic UV background.Comment: 12 pages, 9 figures, MNRAS accepte
Evidence for a Very Large-Scale Fractal Structure in the Universe from Cobe Measurements
In this work, we analyse the temperature fluctuations of the cosmic microwave
background radiation observed by COBE and show that the distribution can be
fitted by a fractal distribution with a fractal dimension .
This value is in close agreement with the fractal dimension obtained by Coleman
and Pietronero (1992) and Luo and Schramm (1992) from galaxy-galaxy and
cluster-cluster correlations up to . The fact that the
observed temperature fluctuations correspond to scales much larger than and are signatures of the primordial density fluctuations at the
recombination layer suggests that the structure of the matter at the early
universe was already fractal and thus non-homogeneous on those scales. This
result may have important consequences for the theoretical framework that
describes the universe.Comment: 11 pages, postscript file, 2 figures available upon request. To
appear in ApJ Letter
High directivity fractal boundary microstrip patch antenna
A novel patch antenna with a fractal boundary condition is proposed. Experimental and numerical results corroborate the fact that the fractal characteristic of the perimeter produces localised modes. This property is utilised in the design of a microstrip patch antenna with a measured directivity of 12.7 dB.Peer ReviewedPostprint (published version
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