618 research outputs found
UV Continuum, Physical Conditions and Filling Factor in Active Galactic Nuclei
The narrow line region of active galaxies is formed by gas clouds surrounded
by a diluted gas. Standard one-dimensional photoionization models are usually
used to model this region in order to reproduce the observed emission lines.
Since the narrow line region is not homogeneous, two major types of models are
used: (a) those assuming a homogeneous gas distribution and a filling factor
less than unity to mimic the presence of the emitting clouds; (b) those based
on a composition of single-cloud models combined in order to obtain the
observed spectra. The first method is largely used but may induce to misleading
conclusions as shown in this paper. The second one is more appropriate, but
requires a large number of observed lines in order to limit the number of
single models used. After discussing the case of an extragalactic HII region,
for which the ionizing radiation spectrum is better known, we show that 1-D
models for the narrow line region with a filling factor less than unit do not
properly mimic the clumpiness, but just simulates an overall lower density.
Multi-cloud models lead to more reliable results. Both models are tested in
this paper, using the emission-line spectra of two well-known Seyfert galaxies,
NGC 4151 and NGC 1068. It is shown that ionizing radiation spectra with a blue
bump cannot be excluded by multi-cloud models, although excluded by Alexander
et al. (1999, 2000)using homogeneous models with a filling factor less than
unity.Comment: 23 pages, 7 figures. Accepted for Publication in Ap
Temperature Fluctuations and Abundances in HII Galaxies
There is evidence for temperature fluctuations in Planetary Nebulae and in
Galactic HII regions. If such fluctuations occur in the low-metallicity,
extragalactic HII regions used to probe the primordial helium abundance, the
derived 4He mass fraction, Y_P, could be systematically different from the true
primordial value. For cooler, mainly high-metallicity HII regions the derived
helium abundance may be nearly unchanged but the oxygen abundance could have
been seriously underestimated. For hotter, mainly low-metallicity HII regions
the oxygen abundance is likely accurate but the helium abundance could be
underestimated. The net effect is to tilt the Y vs. Z relation, making it
flatter and resulting in a higher inferred Y_P. Although this effect could be
large, there are no data which allow us to estimate the size of the temperature
fluctuations for the extragalactic HII regions. Therefore, we have explored
this effect via Monte Carlos in which the abundances derived from a fiducial
data set are modified by \Delta-T chosen from a distribution with 0 < \Delta-T
< \Delta-T_max where \Delta-T_max is varied from 500K to 4000K. It is
interesting that although this effect shifts the locations of the HII regions
in Y vs. O/H plane, it does not introduce any significant additional
dispersion.Comment: 11 pages, 9 postscript figures; submitted to the Ap
3-D Photoionization Structure and Distances of Planetary Nebulae II. Menzel 1
We present the results of a spatio-kinematic study of the planetary nebula
Menzel 1 using spectro-photometric mapping and a 3-D photoionization code. We
create several 2-D emission line images from our long-slit spectra, and use
these to derive the line fluxes for 15 lines, the Halpha/Hbeta extinction map,
and the [SII] line ratio density map of the nebula. We use our photoionization
code constrained by these data to derive the three-dimensional nebular
structure and ionizing star parameters of Menzel 1 by simultaneously fitting
the integrated line intensities, the density map, and the observed morphologies
in several lines, as well as the velocity structure. Using theoretical
evolutionary tracks of intermediate and low mass stars, we derive a mass for
the central star of 0.63+-0.05 Msolar. We also derive a distance of 1050+_150
pc to Menzel 1.Comment: To be published in ApJ of 10th February 2005. 12 figure
Nuclear and Extended Spectra of NGC 1068 - I: Hints from Near-Infrared Spectroscopy
We report the first simultaneous zJHK spectroscopy on the archetypical
Seyfert 2 Galaxy NGC 1068 covering the wavelength region 0.9 to 2.4 micron. The
slit, aligned in the NS direction and centred in the optical nucleus, maps a
region 300 pc in radius at sub-arcsec resolution, with a spectral resolving
power of 360 km s^-1. This configuration allow us to study the physical
properties of the nuclear gas including that of the north side of the
ionization cone, map the strong excess of continuum emission in the K-band and
attributed to dust and study the variations, both in flux and profile, in the
emission lines. Our results show that (1) Mid- to low-ionization emission lines
are splitted into two components, whose relative strengths vary with the
position along the slit and seem to be correlated with the jet. (2) The coronal
lines are single-peaked and are detected only in the central few hundred of
parsecs from the nucleus. (3) The absorption lines indicate the presence of
intermediate age stellar population, which might be a significant contributor
to the continuum in the NIR spectra. (4) Through some simple photoionization
models we find photoionization as the main mechanism powering the emitting gas.
(5) Calculations using stellar features point to a mass concentration inside
the 100 - 200 pc of about 10^10 solar masses.Comment: 19 Pages, 14 figures. Accepted for publication in MNRA
Power Spectrum Analysis of BNL Decay-Rate Data
Evidence for an anomalous annual periodicity in certain nuclear decay data
has led to speculation concerning a possible solar influence on nuclear
processes. As a test of this hypothesis, we here search for evidence in decay
data that might be indicative of a process involving solar rotation, focusing
on data for 32Si and 36Cl decay rates acquired at the Brookhaven National
Laboratory. Examination of the power spectrum over a range of frequencies (10 -
15 year^-1) appropriate for solar synodic rotation rates reveals several
periodicities, the most prominent being one at 11.18 year^-1 with power 20.76.
We evaluate the significance of this peak in terms of the false-alarm
probability, by means of the shuffle test, and also by means of a new test (the
"shake" test) that involves small random time displacements. The last two tests
indicate that the peak at 11.18 year^-1 would arise by chance only once out of
about 10^7 trials. Since there are several peaks in the search band, we also
investigate the running mean of the power spectrum, and identify a major peak
at 11.93 year^-1 with peak running-mean power 4.08. Application of the shuffle
test and the shake test indicates that there is less than one chance in 10^11,
and one chance in 10^15, respectively, finding by chance a value as large as
4.08.Comment: 12 pages, 17 figures, to be published in Astroparticle Physic
From (p)reheating to nucleosynthesis
This article gives a brief qualitative description of the possible evolution
of the early Universe between the end of an inflationary epoch and the end of
Big Bang nucleosynthesis. After a general introduction, establishing the
minimum requirements cosmologists impose on this cosmic evolutionary phase,
namely, successful baryogenesis, the production of cosmic dark matter, and
successful light-element nucleosynthesis, a more detailed discussion on some
recent developments follows. This latter includes the physics of preheating,
the putative production of (alternative) dark matter, and the current status of
Big Bang nucleosynthesis.Comment: 18 pages, 6 figures, to be published in "Classical and Quantum
Gravity", article based on a talk presented at ``The Early Universe and
Cosmological Observations: a Critical Review'', Cape Town, July 200
Effective number of neutrinos and baryon asymmetry from BBN and WMAP
We place constraints on the number of relativistic degrees of freedom and on
the baryon asymmetry at the epoch of Big Bang Nucleosynthesis (BBN) and at
recombination, using cosmic background radiation (CBR) data from the Wilkinson
Microwave Anisotropy Probe (WMAP), complemented by the Hubble Space Telescope
(HST) Key Project measurement of the Hubble constant, along with the latest
compilation of deuterium abundances and measurements of the primordial helium
abundance. The agreement between the derived values of these key cosmological
and particle physics parameters at these widely separated (in time or redshift)
epochs is remarkable. From the combination of CBR and BBN data, we find the
2\sigma ranges for the effective number of neutrinos and for the baryon
asymmetry (baryon to photon number ratio \eta) to be 1.7-3.0 and 5.53-6.76
\times 10^{-10}, respectively.Comment: 10 pages, 10 figures, 2 table
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