484 research outputs found
Flare in the Galactic stellar outer disc detected in SDSS-SEGUE data
Aims. We explore the outer Galactic disc up to a Galactocentric distance of
30 kpc to derive its parameters and measure the magnitude of its flare.
Methods. We obtained the 3D density of stars of type F8V-G5V with a colour
selection from extinction-corrected photometric data of the Sloan Digital Sky
Survey - Sloan Extension for Galactic Understanding and Exploration
(SDSS-SEGUE) over 1,400 deg^2 in off-plane low Galactic latitude regions and
fitted it to a model of flared thin+thick disc.
Results. The best-fit parameters are a thin-disc scale length of 2.0 kpc, a
thin-disc scale height at solar Galactocentric distance of 0.24 kpc, a
thick-disc scale length of 2.5 kpc, and a thick-disc scale height at solar
Galactocentric distance of 0.71 kpc. We derive a flaring in both discs that
causes the scale height of the average disc to be multiplied with respect to
the solar neighbourhood value by a factor of 3.3^{+2.2}_{-1.6} at R=15 kpc and
by a factor of 12^{+20}_{-7} at R=25 kpc.
Conclusions. The flare is quite prominent at large R and its presence
explains the apparent depletion of in-plane stars that are often confused with
a cut-off at R>15 kpc. Indeed, our Galactic disc does not present a truncation
or abrupt fall-off there, but the stars are spread in off-plane regions, even
at z of several kpc for R>20 kpc. Moreover, the smoothness of the observed
stellar distribution also suggests that there is a continuous structure and not
a combination of a Galactic disc plus some other substructure or extragalactic
component: the hypothesis to interpret the Monoceros ring in terms of a tidal
stream of a putative accreted dwarf galaxy is not only unnecessary because the
observed flare explains the overdensity in the Monoceros ring observed in SDSS
fields, but it appears to be inappropriate.Comment: Accepted for publication in A&
Peaks in the CMBR power spectrum. I. Mathematical analysis of the associated real space features
The purpose of our study is to understand the mathematical origin in real
space of modulated and damped sinusoidal peaks observed in cosmic microwave
background radiation anisotropies. We use the theory of the Fourier transform
to connect localized features of the two-point correlation function in real
space to oscillations in the power spectrum. We also illustrate analytically
and by means of Monte Carlo simulations the angular correlation function for
distributions of filled disks with fixed or variable radii capable of
generating oscillations in the power spectrum. While the power spectrum shows
repeated information in the form of multiple peaks and oscillations, the
angular correlation function offers a more compact presentation that condenses
all the information of the multiple peaks into a localized real space feature.
We have seen that oscillations in the power spectrum arise when there is a
discontinuity in a given derivative of the angular correlation function at a
given angular distance. These kinds of discontinuities do not need to be abrupt
in an infinitesimal range of angular distances but may also be smooth, and can
be generated by simply distributing excesses of antenna temperature in filled
disks of fixed or variable radii on the sky, provided that there is a non-null
minimum radius and/or the maximum radius is constrained.Comment: accepted to be published in Physica
Complete Zeldovich approximation
We have developed a generalization of the Zeldovich approximation (ZA) that
is exact in a wide variety of situations, including plannar, spherical and
cilyndrical symmetries. We have shown that this generalization, that we call
complete Zeldovich approximation (CZA), is exact to second order at an
arbitrary point within any field. For gaussian fields, the third order error
have been obtained and shown to be very small. For statistical purposes, the
CZA leads to results exact to the third order.Comment: 11 pages+1 figure, accepted in ApJ Letter
Fluctuations of K-band galaxy counts
We measure the variance in the distribution of off-plane (|b|>20 deg.)
galaxies with m_K<13.5 from the 2MASS K-band survey in circles of diameter
between 0.344 deg. and 57.2 deg. The use of a near-infrared survey makes
negligible the contribution of Galactic extinction to these fluctuations. We
calculate these variances within the standard Lambda-CDM model assuming that
the sources are distributed like halos of the corresponding mass, and it
reproduces qualitatively the galaxy counts variance. Therefore, we test that
the counts can be basically explained in terms only of the large scale
structure. A second result of this paper is a new method to determine the two
point correlation function obtained by forcing agreement between model and
data. This method does not need the knowledge of the two-point angular
correlation function, allows an estimation of the errors (which are low with
this method), and can be used even with incomplete surveys.
Using this method we get xi(z=0, r<10 h^{-1}Mpc)=(29.8+/-0.3)
(r/h^{-1}Mpc)^{-1.79+/-0.02}, which is the first measure of the amplitude of xi
in the local Universe for the K-band. It is more or less in agreement with
those obtained through red optical filters selected samples, but it is larger
than the amplitude obtained for blue optical filters selected samples.Comment: 7 pages, 5 figures, accepted to be published in A&
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