3,389 research outputs found
Self-similarity and scaling behavior of scale-free gravitational clustering
We measure the scaling properties of the probability distribution of the
smoothed density field in -body simulations of expanding universes with
scale-free initial power-spectra, with particular attention to the predictions
of the stable clustering hypothesis. We concentrate our analysis on the ratios
, , where is the averaged -body correlation function over a cell of radius
. The behavior of the higher order correlations is studied through that
of the void probability distribution function.
As functions of , the quantities , ,
exhibit two plateaus separated by a smooth transition around . In the weakly nonlinear regime, {\bar \xi}_2 \la 1, the results are in
reasonable agreement with the predictions of perturbation theory. In the
nonlinear regime, , the function is
larger than in the weakly nonlinear regime, and increasingly so with . It
is well-fitted by the expression $S_Q= ({\bar \xi}_2/100)^{0.045(Q-2)}\
{\widetilde S}_Qn. This weak dependence on scale proves {\em a
small, but significant departure from the stable clustering predictions} at
least for n=0n=+1P_0S_Qn=-2n=-1$. In these two cases, our measurements are not accurate enough to be
discriminant.Comment: 31 pages, postscript file, figure 1 missing. Postscript file
including figure 1 available at
ftp://ftp-astro-theory.fnal.gov:/pub/Publications/Pub-95-256-
On the morphology of the electron-positron annihilation emission as seen by SPI/INTEGRAL
The 511 keV positron annihilation emission remains a mysterious component of
the high energy emission of our Galaxy. Its study was one of the key scientific
objective of the SPI spectrometer on-board the INTEGRAL satellite. In fact, a
lot of observing time has been dedicated to the Galactic disk with a particular
emphasis on the central region. A crucial issue in such an analysis concerns
the reduction technique used to treat this huge quantity of data, and more
particularly the background modeling. Our method, after validation through a
variety of tests, is based on detector pattern determination per ~6 month
periods, together with a normalisation variable on a few hour timescale. The
Galactic bulge is detected at a level of ~70 sigma allowing more detailed
investigations. The main result is that the bulge morphology can be modelled
with two axisymmetric Gaussians of 3.2 deg. and 11.8 deg. FWHM and respective
fluxes of 2.5 and 5.4 x 10^-4 photons/(cm^2.s^1). We found a possible shift of
the bulge centre towards negative longitude at l=-0.6 +/- 0.2 degrees. In
addition to the bulge, a more extended structure is detected significantly with
flux ranging from 1.7 to 2.9 x10^-3 photons/(cm^2.s^1) depending on its assumed
geometry (pure disk or disk plus halo). The disk emission is also found to be
symmetric within the limits of the statistical errors.Comment: This paper has 12 pages and 14 figures. Accepted for publication by
the Astrophysical Journa
Extended Perturbation Theory for the Local Density Distribution Function
Perturbation theory makes it possible to calculate the probability
distribution function (PDF) of the large scale density field in the small
variance limit. For top hat smoothing and scale-free Gaussian initial
fluctuations, the result depends only on the linear variance, sigma_linear, and
its logarithmic derivative with respect to the filtering scale
-(n_linear+3)=dlog sigma_linear^2/dlog L (Bernardeau 1994). In this paper, we
measure the PDF and its low-order moments in scale-free simulations evolved
well into the nonlinear regime and compare the results with the above
predictions, assuming that the spectral index and the variance are adjustable
parameters, n_eff and sigma_eff=sigma, where sigma is the true, nonlinear
variance. With these additional degrees of freedom, results from perturbation
theory provide a good fit of the PDFs, even in the highly nonlinear regime. The
value of n_eff is of course equal to n_linear when sigma << 1, and it decreases
with increasing sigma. A nearly flat plateau is reached when sigma >> 1. In
this regime, the difference between n_eff and n_linear increases when n_linear
decreases. For initial power-spectra with n_linear=-2,-1,0,+1, we find n_eff ~
-9,-3,-1,-0.5 when sigma^2 ~ 100.Comment: 13 pages, 6 figures, Latex (MN format), submitted to MNRA
Ground-based follow up of IRAS galaxies
Optical, near infrared, radio continuum and HI observations were undertaken of the galaxies identified with IRAS sources in a few fields roughly of the size of a sky survey plate. Results are presented from two fields at galactic latitude +27 and +43 deg over a total area of 100 sq. deg. These regions contained 115 IRAS point sources, out of which 26 were identified with stars and 81 with faint galaxies, 10 of which were difficult to recognize on the Schmidt plates. Spectroscopy was obtained with the ESO telescopes at a resolution of about 10 A. The vast majority of galaxies have low excitation spectra dominated by low ionization lines. The spectra are typical of HII region type galaxies, however of much lower excitation that other starbursts galaxies. The importance of the reddening as determined from the H alpha/H beta ratio is stressed: the visual absorption A sub v ranges from 2 to 6 magnitudes and as a consequence the corrected L sub IR/L sub B ratios are considerably reduced if those reddenings apply to the whole galaxy
The spectral catalogue of INTEGRAL gamma-ray bursts: results of the joint IBIS/SPI spectral analysis
We present the updated INTEGRAL catalogue of gamma-ray bursts (GRBs) observed
between December 2002 and February 2012. The catalogue contains the spectral
parameters for 59 GRBs localized by the INTEGRAL Burst Alert System (IBAS). We
used the data from the two main instruments on board the INTEGRAL satellite:
the spectrometer SPI (SPectrometer on INTEGRAL) nominally covering the energy
range 18 keV - 8 MeV, and the imager IBIS (the Imager on Board the INTEGRAL
Satellite) operating in the range from 15 keV to 10 MeV. For the spectral
analysis we applied a new data extraction technique, developed in order to
explore the energy regions of highest sensitivity for both instruments, SPI and
IBIS. It allowed us to perform analysis of the GRB spectra over a broad energy
range and to determine the bursts' spectral peak energies. The spectral
analysis was performed on the whole sample of GRBs triggered by IBAS, including
all the events observed in period December 2002 - February 2012. The catalogue
contains the trigger times, burst coordinates, positional errors, durations and
peak fluxes for 28 unpublished GRBs observed between September 2008 and
February 2012. The light curves in 20 - 200 keV energy band of these events
were derived using IBIS data. We compare the prompt emission properties of the
INTEGRAL GRB sample with the BATSE and Fermi samples.Comment: 16 pages, 40 figures, accepted for publication in Astronomy &
Astrophysic
The FIR/submm window on galaxy formation
Our view on the deep universe has been so far biased towards optically bright
galaxies. Now, the measurement of the Cosmic Infrared Background in FIRAS and
DIRBE residuals, and the observations of FIR/submm sources by the ISOPHOT and
SCUBA instruments begin unveiling the ``optically dark side'' of galaxy
formation. Though the origin of dust heating is still unsolved, it appears very
likely that a large fraction of the FIR/submm emission is due to
heavily-extinguished star formation. Consequently, the level of the CIRB
implies that about 2/3 of galaxy/star formation in the universe is hidden by
dust shrouds. In this review, we introduce a new modeling of galaxy formation
and evolution that provides us with specific predictions in FIR/submm
wavebands. These predictions are compared with the current status of the
observations. Finally, the capabilities of current and forthcoming instruments
for all-sky and deep surveys of FIR/submm sources are briefly described.Comment: 10 pages, Latex, 5 postscript figures, to appear in ``The Birth of
Galaxies'', 1999, B. Guiderdoni, F.R. Bouchet, T.X. Thuan & J. Tran Thanh Van
(eds), Editions Frontiere
The effect of point sources on satellite observations of the cosmic microwave background
We study the effect of extragalactic point sources on satellite observations
of the cosmic microwave background (CMB). In order to separate the
contributions due to different foreground components, a maximum-entropy method
is applied to simulated observations by the Planck Surveyor satellite. In
addition to point sources, the simulations include emission from the CMB and
the kinetic and thermal Sunyaev-Zel'dovich (SZ) effects from galaxy clusters,
as well as Galactic dust, free-free and synchrotron emission. We find that the
main input components are faithfully recovered and, in particular, that the
quality of the CMB reconstruction is only slightly reduced by the presence of
point sources. In addition, we find that it is possible to recover accurate
point source catalogues at each of the Planck Surveyor observing frequencies.Comment: 12 pages, 9 figures, submitted to MNRA
Predicted Planck Extragalactic Point Source Catalogue
An estimation of the number and amplitude (in flux) of the extragalactic
point sources that will be observed by the Planck Mission is presented in this
paper. The study is based on the Mexican Hat wavelet formalism introduced by
Cayon et al. 2000. Simulations at Planck observing frequencies are analysed,
taking into account all the possible cosmological, Galactic and Extragalactic
emissions together with noise. With the technique used in this work the Planck
Mission will produce a catalogue of extragalactic point sources above fluxes:
1.03 Jy (857 GHz), 0.53 Jy (545 GHz), 0.28 Jy (353 GHz), 0.24 Jy (217 GHz),
0.32 Jy (143 GHz), 0.41 Jy (100 GHz HFI), 0.34 Jy (100 GHz LFI), 0.57 Jy (70
GHz), 0.54 Jy (44 GHz) and 0.54 Jy (30 GHz), which are only slightly model
dependent (see text). Amplitudes of these sources are estimated with errors
below 15%. Moreover, we also provide a complete catalogue (for the point
sources simulation analysed) with errors in the estimation of the amplitude
below 10%. In addition we discuss the possibility of identifying different
point source populations in the Planck catalogue by estimating their spectral
indices.Comment: 13 pages, 2 figures, submitted to MNRA
The Optical - Infrared Colors of CORALS QSOs: Searching for Dust Reddening Associated With High Redshift Damped Lyman Alpha Systems
The presence of dust in quasar absorbers, such as damped Lyman alpha (DLA)
systems, may cause the background QSO to appear reddened. We investigate the
extent of this potential reddening by comparing the optical-to-infrared (IR)
colors of QSOs with and without intervening absorbers. Our QSO sample is based
on the Complete Optical and Radio Absorption Line System (CORALS) survey of
Ellison et al (2001). We have obtained near-simultaneous B and K band
magnitudes for subset of the CORALS sample and supplemented our observations
with further measurements published in the literature. To account for
redshift-related color changes, the B-K colors are normalized using the Sloan
Digital Sky Survey (SDSS) QSO composite. The mean normalized B-K color of the
DLA sub-sample is +0.12, whereas the mean for the no-DLA sample is -0.10; both
distributions have RMS scatters ~0.5. Neither a student's T-test nor a KS test
indicate that there is any significant difference between the two color
distributions. Based on simulations which redden the colors of QSOs with
intervening DLAs, we determine a reddening limit which corresponds to E(B-V) <
0.04 (SMC-like extinction) at 99% confidence (3 sigma), assuming that E(B-V) is
the same for all DLAs. Finally, we do not find any general correlation between
absorber properties (such as [Fe/Zn] or neutral hydrogen column density) and
B-K color. One of these two QSOs shows evidence for strong associated
absorption from X-ray observations, an alternative explanation for its very red
color. We conclude that the presence of intervening galaxies causes a minimal
reddening of the background QSO.Comment: Accepted for publication in A
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