2,002 research outputs found
Gas Metallicity in the Narrow-Line Regions of High-Redshift Active Galactic Nuclei
We analyze optical (UV rest-frame) spectra of X-ray selected narrow-line QSOs
at redshift 1.5 < z < 3.7 found in the Chandra Deep Field South and of
narrow-line radio galaxies at redshift 1.2 < z < 3.8 to investigate the gas
metallicity of the narrow-line regions and their evolution in this redshift
range. Such spectra are also compared with UV spectra of local Seyfert 2
galaxies. The observational data are inconsistent with the predictions of shock
models, suggesting that the narrow-line regions are mainly photoionized. The
photoionization models with dust grains predict line flux ratios which are also
in disagreement with most of the observed values, suggesting that the
high-ionization part of the narrow-line regions (which is sampled by the
available spectra) is dust-free. The photoionization dust-free models provide
two possible scenarios which are consistent with the observed data: low-density
gas clouds (n < 10^3 cm^-3) with a sub-solar metallicity (0.2 < Z/Z_sun < 1.0),
or high-density gas clouds (n ~ 10^5 cm^-3) with a wide range of gas
metallicity (0.2 < Z/Z_sun < 5.0). Regardless of the specific interpretation,
the observational data do not show any evidence for a significant evolution of
the gas metallicity in the narrow-line regions within the redshift range 1.2 <
z < 3.8. Instead, we find a trend for more luminous active galactic nuclei to
have more metal-rich gas clouds (luminosity-metallicity relation), which is in
agreement with the same finding in the studies of the broad-line regions. The
lack of evolution for the gas metallicity of the narrow-line regions implies
that the major epoch of star formation in the host galaxies of these active
galactic nuclei is at z > 4.Comment: 16 pages, 12 figures, submitted to Astronomy and Astrophysic
Using globular clusters to test gravity in the weak acceleration regime: NGC 7099
A test of Newton's law of gravity in the low acceleration regime using
globular clusters is presented. New results for the core collapsed globular
cluster NGC 7099 are given. The run of the gravitational potential as a
function of distance is probed studying the velocity dispersion profile of the
cluster, as derived from a set of 125 radial velocities with accuracy better
than 1 km/s. The velocity dispersion profile is traced up to ~18 pc from the
cluster center. The dispersion is found to be maximal at the center, then
decrease until 10+-2 pc from the center, well inside the cluster tidal radius
of 42 pc. After that the dispersion remains constant with average value
2.2+-0.3 km/s. Assuming for NGC 7099 a total V mag of M(V)=-7.43 mags and
mass-to-light ratio M/L=1, the acceleration at 10 pc from the center is 1.1e-8
cm/s/s. Thus, the flattening of the velocity dispersion profile occurs for a
value of the internal acceleration of gravity fully consistent with a_0=1.2e-8
cm/s/s observed in galaxies. This new result for NGC 7099 brings to 4 the
clusters with velocity dispersion profile probing acceleration below a_0. All
four have been found to have a flat dispersion profile at large radii where the
acceleration is below a_0, mimicking qualitatively and quantitatively
elliptical galaxies. Whether this indicates a failure of Newtonian dynamics in
the low acceleration limit or some more conventional dynamical effect (e.g.,
tidal heating) is still unclear. However, the similarities emerging between
very different globular clusters, as well as between globular clusters and
elliptical galaxies seem to favor the first of these two possibilities.Comment: Accepted for publication in A&A Letters. Four pages in tota
Using globular clusters to test gravity in the weak acceleration regime
We report on the results from an ongoing program aimed at testing Newton's
law of gravity in the low acceleration regime using globular clusters. It is
shown that all clusters studied so far do behave like galaxies, that is, their
velocity dispersion profile flattens out at large radii where the acceleration
of gravity goes below 1e-8 cm/s/s, instead of following the expected Keplerian
fall off. In galaxies this behavior is ascribed to the existence of a dark
matter halo. Globular clusters, however, do not contain dark matter, hence this
result might indicate that our present understanding of gravity in the weak
regime of accelerations is incomplete and somehow incorrect.Comment: As published on the European Southern Observatory "the Messenger",
Num. 128, June 2007. Seven pages, 4 figures, 2 table
Multidimensional Stationary Probability Distribution for Interacting Active Particles
We derive the stationary probability distribution for a non-equilibrium
system composed by an arbitrary number of degrees of freedom that are subject
to Gaussian colored noise and a conservative potential. This is based on a
multidimensional version of the Unified Colored Noise Approximation. By
comparing theory with numerical simulations we demonstrate that the theoretical
probability density quantitatively describes the accumulation of active
particles around repulsive obstacles. In particular, for two particles with
repulsive interactions, the probability of close contact decreases when one of
the two particle is pinned. Moreover, in the case of isotropic confining
potentials, the radial density profile shows a non trivial scaling with radius.
Finally we show that the theory well approximates the "pressure" generated by
the active particles allowing to derive an equation of state for a system of
non-interacting colored noise-driven particles.Comment: 5 pages, 2 figure
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