4 research outputs found
Chemical abundances of Seyfert 2 AGNs â III. Reducing the oxygen abundance discrepancy
We investigate the discrepancy between oxygen abundance estimations for
narrow-line regions (NLRs) of Active Galactic Nuclei (AGNs) type Seyfert 2
derived by using direct estimations of the electron temperature (Te-method) and
those derived by using photoionization models. In view of this, observational
emission-line ratios in the optical range (3000 < \lambda(\AA) < 7000) of
Seyfert 2 nuclei compiled from the literature were reproduced by detailed
photoionization models built with the Cloudy code. We find that the derived
discrepancies are mainly due to the inappropriate use of the relations between
temperatures of the low (t2) and high (t3) ionization gas zones derived for H
II regions in AGN chemical abundance studies. Using a photoionization model
grid, we derived a new expression for t2 as a function of t3 valid for Seyfert
2 nuclei. The use of this new expression in the AGN estimation of the O/H
abundances based on Te-method produces O/H abundances slightly lower (about 0.2
dex) than those derived from detailed photoionization models. We also find that
the new formalism for the Te-method reduces by about 0.4 dex the O/H
discrepancies between the abundances obtained from strong emission-line
calibrations and those derived from direct estimations.ERC
STF
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Metal Enrichment in the Circumgalactic Medium and Lyα Halos around Quasars at z ⌠3
Deep observations have detected extended Ly emission nebulae
surrounding tens of quasars at redshift 2 to 6. However, the metallicity of
such extended haloes is still poorly understood. We perform a detailed analysis
on a large sample of 80 quasars at based on MUSE-VLT data. We find
clear evidence of extended emission of the UV nebular lines CIV and HeII for
about 18 of the sample, while CIII] is only marginally detected in a few
objects. By stacking the cubes we detect emission of CIV, HeII and CIII] out to
a radius of about 45 kpc. CIV and HeII show a radial decline much steeper than
Ly, while CIII] shows a shallower profile similar to Ly in the
inner 45 kpc. We infer that the average metallicity of the circumgalactic gas
within the central 30-50~kpc is 0.5 solar, or even higher. However, we
also find evidence of a component of the Ly haloes, which has much
weaker metal emission lines relative to Ly. We suggest that the high
metallicity of the circumgalactic medium within the central 30-50 kpc is
associated with chemical pre-enrichment by past quasar-driven outflows and that
there is a more extended component of the CGM that has much lower metallicity
and likely associated with near-pristine gas accreted from the intergalactic
medium. We show that our observational results are in good agreement with the
expectations of the FABLE zoom-in cosmological simulations.ERC
STF
Lower-hybrid drift waves and electromagnetic electron space-phase holes associated with dipolarization fronts and field-aligned currents observed by the Magnetospheric Multiscale mission during a substorm
We analyse two ion scale dipolarization fronts associated with field-aligned currents detected by the Magnetospheric Multiscale mission during a large substorm on August 10, 2016. The first event corresponds to a fast dawnward flow with an anti-parallel current and could be generated by the wake of a previous fast earthward flow. It is associated with intense lower-hybrid drift waves detected at the front and propagating dawnward with a perpendicular phase speed close to the electric drift and the ion thermal velocity. The second event corresponds to a flow reversal: from southwward/dawnward to northward/duskward associated with a parallel current consistent with a brief expansion of the plasma sheet before the front crossing, and with a smaller lower-hybrid drift wave activity. Electromagnetic electron phase-space holes are detected near these low-frequency drift waves during both events. The drift waves could accelerate electrons parallel to the magnetic field and produce the parallel electron drift needed to generate the electron holes. Yet, we cannot rule out the possibility that the drift waves are produced by the anti-parallel current associated with the fast flows, leaving the source for the electron holes unexplained
Physical conditions and chemical abundances in photoionized nebulae from optical spectra
This chapter presents a review on the latest advances in the computation of
physical conditions and chemical abundances of elements present in photoionized
gas H II regions and planetary nebulae). The arrival of highly sensitive
spectrographs attached to large telescopes and the development of more
sophisticated and detailed atomic data calculations and ionization correction
factors have helped to raise the number of ionic species studied in
photoionized nebulae in the last years, as well as to reduce the uncertainties
in the computed abundances. Special attention will be given to the detection of
very faint lines such as heavy-element recombination lines of C, N and O in H
II regions and planetary nebulae, and collisionally excited lines of
neutron-capture elements (Z >30) in planetary nebulae.Comment: Book Chapter. 31 pages. 6 Figures. Accepted for publication in the
book "Reviews in Frontiers of Modern Astrophysics: From Space Debris to
Cosmology" (eds Kabath, Jones and Skarka; publisher Springer Nature) funded
by the European Union Erasmus+ Strategic Partnership grant "Per Aspera Ad
Astra Simul" 2017-1-CZ01-KA203-03556