42 research outputs found
Technique for detecting warm-hot intergalactic gas in quasar UV spectra
The ionizing spectral energy distribution of quasars exhibits a steepening of
the distribution shortward of ~ 1200 A. The change of the power-law index from
approximately -1 (near-UV) to -2 (far-UV) has so far been interpreted as being
intrinsic to quasars. We consider the possibility that the steepening may
result from a tenuous absorption component that is anticorrelated with large
mass overdensities. UV sensitive satellites, whose detectors can extend down to
1000 A, can set a useful limit to such an absorption component through the
search of a flux increase in the window 1050-1190 A (observer frame) with
respect to an extrapolation of the continuum above 1230 A. Since the recent
FUSE or HST-STIS data do not show any obvious discontinuity in this region,
this effectively rules out the possibility that intergalactic HI absorption is
very important, and it is concluded that most if not all of the steepening is
intrinsic to quasars. A smaller flux discontinuity of order 1% cannot, however,
be ruled out yet and would still be consistent with the warm-hot intergalactic
component if it amounts to 30% of the baryonic mass, as predicted by some
models of large scale structure formation, provided its temperature lies around
10^{5.5} K.Comment: 13 pages, 5 figures and 1 table, accepted for publication in
Astrophysical Journal, typos corrected and correction for the faulty scaling
of variable n_H^0 with redshif
Quasar 3C 298: a test-case for meteoritic nanodiamond 3.5 µm emission
Aims. We calculate the dust emission expected at 3.43 and 3.53 µm if meteoritic (i.e. hydrogenated) nanodiamonds are responsible for most of the far-UV break observed in quasars.
Methods. We integrate the UV flux that hydrogenated nanodiamonds must absorb to reproduce the far-UV break. Based on laboratory spectra of H-terminated diamond surfaces, we analyse the radiative energy budget and derive theoretically the IR emission profiles expected for possible C-H surface stretch modes of the diamonds.
Results. Using as test case a spectrum of 3C 298 provided by the Spitzer Observatory, we do not find evidence of these emission bands.
Conclusions. While diamonds without surface adsorbates remain a viable candidate for explaining the far-UV break observed in quasars, hydrogenated nanodiamonds appear to be ruled out, as they would give rise to IR emission bands, which have not been observed so far
Technique for detecting warm-hot intergalactic gas in quasar UV spectra
The ionizing spectral energy distribution of quasars exhibits a steepening of
the distribution shortward of ~ 1200 A. The change of the power-law index from
approximately -1 (near-UV) to -2 (far-UV) has so far been interpreted as being
intrinsic to quasars. We consider the possibility that the steepening may
result from a tenuous absorption component that is anticorrelated with large
mass overdensities. UV sensitive satellites, whose detectors can extend down to
1000 A, can set a useful limit to such an absorption component through the
search of a flux increase in the window 1050-1190 A (observer frame) with
respect to an extrapolation of the continuum above 1230 A. Since the recent
FUSE or HST-STIS data do not show any obvious discontinuity in this region,
this effectively rules out the possibility that intergalactic HI absorption is
very important, and it is concluded that most if not all of the steepening is
intrinsic to quasars. A smaller flux discontinuity of order 1% cannot, however,
be ruled out yet and would still be consistent with the warm-hot intergalactic
component if it amounts to 30% of the baryonic mass, as predicted by some
models of large scale structure formation, provided its temperature lies around
10^{5.5} K.Comment: 13 pages, 5 figures and 1 table, accepted for publication in
Astrophysical Journal, typos corrected and correction for the faulty scaling
of variable n_H^0 with redshif
Optimized Spectral Energy Distribution for Seyfert Galaxies
The temperature predicted by photoionization models for the Narrow Line
Region of Seyfert 2 galaxies is lower than the value inferred from the observed
[O III] {\lambda}4363A/{\lambda}5007A line ratio. We explore the possibility of
considering a harder ionizing continuum than typically assumed. The spectral
ionizing energy distribution, which can generate the observed
{\lambda}4363A/{\lambda}5007A ratio, is characterized by a secondary continuum
peak at 200 eV.Comment: 17 pages, 4 figures, accepted for publication in Revista Mexicana de
Astronom\'ia y Astrof\'isic
Microbenthic Net Metabolism Along Intertidal Gradients (Cadiz Bay, SW Spain): Spatio-Temporal Patterns and Environmental Factors
Microphytobenthos (MPB), the photosynthetic primary producing component of
microbenthos, shows variable patterns in its biomass distribution along the intertidal
gradient as a result of the interactions of factors such as light, tides, temperature,
and grazing pressure. These patterns have been studied more extensively in northern
European estuaries than southern European coastal systems. Even less information is
available regarding temporal changes in MPB primary production rates in these systems.
For this reason, we followed the seasonal changes in net production in light and dark
respiration rates (determined by oxygen microelectrodes) and MPB biomass (estimated
by sediment chlorophyll a) along the intertidal gradient of the inner Cadiz Bay during
a year. Sediment cores were collected along two transects (five sampling stations per
transect) with distinct sediment granulometry: one muddy [Puerto Real (PR)] and one
muddy-sandy transect [San Fernando (SF)]. Our results show that MPB biomass and
net production increased seawards reaching their maxima in winter. In contrast to what
is observed in northern European systems, the higher solar irradiance and temperatures
occurring in summer in southern Spain likely inhibit MPB production. In Cadiz Bay,
spatial patterns of MPB biomass and net production depended on season and location
due to the environmental heterogeneity observed. Environmental variables, analyzed
by principal component analysis (PCA), were used to explain the variability of MPB
metabolism by multiple regression. Selected principal component (PC) axes explained
60% of the net production in light and 41% of the dark respiration rates variability
in PR, while they only accounted for 25% of the same rates in SF. The differences
observed between transects and the variability in the environmental variables explaining
them highlight the importance of considering the spatial heterogeneity of our system to
estimate the contribution of MPB to the inner Cadiz Bay productivity. In our case, this
contribution is significant accounting for up to 49% of the total benthic production of the
inner Cadiz Bay intertidal sediments, confirming previous global estimates