116 research outputs found
Magnetic Field Reconstruction for a Realistic Multi-Point, Multi-Scale Spacecraft Observatory
Future in situ space plasma investigations will likely involve spatially distributed observatories comprised of multiple spacecraft, beyond the four and five spacecraft configurations currently in operation. Inferring the magnetic field structure across the observatory, and not simply at the observation points, is a necessary step towards characterizing fundamental plasma processes using these unique multi-point, multi-scale data sets. We propose improvements upon the classic first-order reconstruction method, as well as a second-order method, utilizing magnetometer measurements from a realistic nine-spacecraft observatory. The improved first-order method, which averages over select ensembles of four spacecraft, reconstructs the magnetic field associated with simple current sheets and numerical simulations of turbulence accurately over larger volumes compared to second-order methods or first-order methods using a single regular tetrahedron. Using this averaging method on data sets with fewer than nine measurement points, the volume of accurate reconstruction compared to a known magnetic vector field improves approximately linearly with the number of measurement points
Interaction effects on galaxy pairs with Gemini/GMOS- II: Oxygen abundance gradients
In this paper we derived oxygen abundance gradients from HII regions located
in eleven galaxies in eight systems of close pairs. Long-slit spectra in the
range 4400-7300A were obtained with the Gemini Multi-Object Spec- trograph at
Gemini South (GMOS). Spatial profiles of oxygen abundance in the gaseous phase
along galaxy disks were obtained using calibrations based on strong
emission-lines (N2 and O3N2). We found oxygen gradients signifi- cantly flatter
for all the studied galaxies than those in typical isolated spiral galaxies.
Four objects in our sample, AM1219A, AM1256B, AM 2030A and AM2030B, show a
clear break in the oxygen abundance at galactocentric radius R/R25 between 0.2
and 0.5. For AM1219A and AM1256B we found negative slopes for the inner
gradients, and for AM2030B we found a positive one. In all these three cases
they show a flatter behaviour to the outskirts of the galaxies. For AM2030A, we
found a positive-slope outer gradient while the inner one is almost compatible
with a flat behaviour. A decrease of star forma- tion efficiency in the zone
that corresponds to the oxygen abundance gradient break for AM1219A and AM2030B
was found. For the former, a minimum in the estimated metallicities was found
very close to the break zone that could be associated with a corotation radius.
On the other hand, AM1256B and AM2030A, present a SFR maximum but not an
extreme oxygen abundance value. All the four interacting systems that show
oxygen gradient breakes the extreme SFR values are located very close to break
zones. Hii regions lo- cated in close pairs of galaxies follow the same
relation between the ionization parameter and the oxygen abundance as those
regions in isolated galaxies.Comment: 30 pages, 14 figures, accepted MNRAS, (Figs. 1 and 2 are in low
resolution
Nuclear spirals as feeding channels to the Supermassive Black Hole: the case of the galaxy NGC 6951
We report the discovery of gas streaming motions along nuclear spiral arms
towards the LINER nucleus of the galaxy NGC 6951. The observations, obtained
using the GMOS integral field spectrograph on the Gemini North telescope,
yielded maps of the flux distributions and gas kinematics in the Halpha,
[NII]6584 and [SII]6717,31 emission lines of the inner 7x5 arcsec^2 of the
galaxy. This region includes a circumnuclear star-forming ring with radius
500pc, a nuclear spiral inside the ring and the LINER nucleus. The kinematics
of the ionized gas is dominated by rotation, but subtraction of a kinematic
model of a rotating exponential disk reveals deviations from circular rotation
within the nuclear ring which can be attributed to (1) streaming motions along
the nuclear spiral arms and (2) a bipolar outflow which seems to be associated
to a nuclear jet. On the basis of the observed streaming velocities and
geometry of the spiral arms we estimate a mass inflow rate of ionized gas of
3x10^(-4) Msun/yr, which is of the order of the accretion rate necessary to
power the LINER nucleus of NGC 6951. Similar streaming motions towards the
nucleus of another galaxy with LINER nucleus -- NGC 1097 -- have been reported
by our group in a previous paper. Taken together, these results support a
scenario in which nuclear spirals are channels through which matter is
transferred from galactic scales to the nuclear region to feed the supermassive
black hole.Comment: 25 pages, 6 eps figures, accepted for publication in Ap
Magnetic Field Reconstruction for a Realistic Multi-Point, Multi-Scale Spacecraft Observatory
Future in situ space plasma investigations will likely involve spatially
distributed observatories comprised of multiple spacecraft, beyond the four and
five spacecraft configurations currently in operation. Inferring the magnetic
field structure across the observatory, and not simply at the observation
points, is a necessary step towards characterizing fundamental plasma processes
using these unique multi-point, multi-scale data sets. We propose improvements
upon the classic first-order reconstruction method, as well as a second-order
method, utilizing magnetometer measurements from a realistic nine-spacecraft
observatory. The improved first-order method, which averages over select
ensembles of four spacecraft, reconstructs the magnetic field associated with
simple current sheets and numerical simulations of turbulence accurately over
larger volumes compared to second-order methods or first-order methods using a
single regular tetrahedron. Using this averaging method on data sets with fewer
than nine measurement points, the volume of accurate reconstruction compared to
a known magnetic vector field improves approximately linearly with the number
of measurement points.Comment: 18 pages, 12 figures, 3 table
Interaction effects on galaxy pairs with Gemini/GMOS- I: electron density
We present an observational study about the impacts of the interactions on the electron density of HII regions located in seven systems of interacting galaxies. The data consist of long-slit spectra in the range 4400-7300 A, obtained with the Gemini Multi-Object Spectrograph at Gemini South (GMOS-S). The electron density was determined using the ratio of emission-lines [S II] lambda 6716/ lambda 6731. Our results indicate that the electron density estimates obtained of HII regions from our sample of interacting galaxies are systematically higher than those derived for isolated galaxies. The mean electron density values of interacting galaxies are in the range of Ne = 24 - 532 cm^{-3}, while those obtained for isolated galaxies are in the range of Ne = 40 - 137 cm^{-3}. Comparing the observed emission lines with predictions of photoionization models, we verified that almost all the HII regions of the galaxies AM1054A, AM2058B, AM2306B have emission lines excited by shocks, such as in AM2322B (one point) and AM 2322A (four points). No correlation is obtained between the presence of shocks and electron densities. Indeed, the highest electron density values found in our sample do not belong to the objects with gas shock excitation. We emphasize the importance of considering theses quantities especially when the metallicity is derived for this type of systems.Fil: Krabbe, A.. Universidade Do Vale Do Paraiba; BrasilFil: Rosa, D. A.. Universidade Do Vale Do Paraiba; BrasilFil: Dors Jr., Oli L.. Universidade Do Vale Do Paraiba; BrasilFil: Pastoriza, Miriani G.. Universidade Federal Do Rio Grande Do Sul; BrasilFil: Winge, C.. Gemini Observatory; ChileFil: Hägele, Guillermo Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Astrofísica de la Plata; Argentina; Argentina. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Cardaci, Monica Viviana. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Astrofísica de La Plata; Argentina; ArgentinaFil: Rodrigues, Irapuan. Universidade Do Vale Do Paraiba; Brasi
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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
The effects of the interaction on the kinematics, stellar population and metallicity of AM2322-821 with Gemini/GMOS
We present an observational study about the impacts of the interactions in
the kinematics, stellar populations, and oxygen abundances of the components of
the galaxy pair AM\,2322-821. A fairly symmetric rotation curve for the
companion (AM\,2322B) galaxy with a deprojected velocity amplitude of 110 km
s was obtained, and a dynamical mass of 1.1 - 1.3 \times 10^{10}
M_{\sun} within a radius of 4 kpc was estimated using this deprojected
velocity. Asymmetries in the radial velocity field were detected for the
companion, very likely due the interaction between the galaxies. The
interaction between the main and companion galaxies was modelled using
numerical N-body/hydrodynamical simulations, with the result indicating that
the current stage of the system would be about 90 Myr after perigalacticum. The
spatial variation in the distribution of the stellar-population components in
both galaxies was analysed using the stellar population synthesis code {\sc
STARLIGHT}. The companion galaxy is dominated by a very young (t yr) population, with the fraction of this population to the
total flux at , increasing outwards in the galaxy disc.
On the other hand, the stellar population of AM\,2322A is heterogeneous along
the slit positions observed. Spatial profiles of oxygen abundance in the
gaseous phase were obtained using the diagnostic diagram R23 vs. [OIII]/[OII],
where we compared the observed values with the ones obtained from
photoionization models.
Such gradients of oxygen abundance are significantly flatter for this pair of
galaxies than in typical isolated spiral galaxies. This metallicity
distribution is interpreted as the gradients having been destroyed by
interaction-induced gas flows from the outer parts to the centre of the galaxyComment: accepted by MNRA
The population of planetary nebulae and HII regions in M81. A study of radial metallicity gradients and chemical evolution
We analyze the chemical abundances of planetary nebulae and HII regions in
the M81 disk for insight on galactic evolution, and compare it with that of
other galaxies, including the Milky Way. We acquired Hectospec/MMT spectra of
39 PNe and 20 HII regions, with 33 spectra viable for temperature and abundance
analysis. Our PN observations represent the first PN spectra in M81 ever
published, while several HII region spectra have been published before,
although without a direct electron temperature determination. We determine
elemental abundances of helium, nitrogen, oxygen, neon, sulfur, and argon in
PNe and HII regions, and determine their averages and radial gradients. The
average O/H ratio of PNe compared to that of the HII regions indicates a
general oxygen enrichment in M81 in the last ~10 Gyr. The PN metallicity
gradient in the disk of M81 is -0.055+-0.02 dex/kpc. Neon and sulfur in PNe
have a radial distribution similar to that of oxygen, with similar gradient
slopes. If we combine our HII sample with the one in the literature we find a
possible mild evolution of the gradient slope, with results consistent with
gradient steepening with time. Additional spectroscopy is needed to confirm
this trend. There are no Type I PNe in our M81 sample, consistently with the
observation of only the brightest bins of the PNLF, the galaxy metallicity, and
the evolution of post-AGB shells. Both the young and the old populations of M81
disclose shallow but detectable negative radial metallicity gradient, which
could be slightly steeper for the young population, thus not excluding a mild
gradients steepening with the time since galaxy formation. During its evolution
M81 has been producing oxygen; its total oxygen enrichment exceeds that of
other nearby galaxies.Comment: A&A, in pres
Atomic and Molecular Gas Components in Spiral Galaxies of the Virgo Cluster
Based on two models, we investigate the molecular-to-atomic gas ratio in
Virgo cluster galaxies in comparison with field galaxies. We show that the
enhanced metallicity for cluster members and the ram pressure stripping of
atomic gas from the disk periphery cannot fully explain the observed gas
component ratios. The additional environmental factors affecting the
interstellar medium and leading to an increase in the molecular gas fraction
should be taken into account for cluster galaxies.Comment: 11 pages, 4 figure
Eliminating Error in the Chemical Abundance Scale for Extragalactic HII Regions
In an attempt to remove the systematic errors which have plagued the
calibration of the HII region abundance sequence, we have theoretically modeled
the extragalactic HII region sequence. We then used the theoretical spectra so
generated in a double blind experiment to recover the chemical abundances using
both the classical electron temperature + ionization correction factor
technique, and the technique which depends on the use of strong emission lines
(SELs) in the nebular spectrum to estimate the abundance of oxygen. We find a
number of systematic trends, and we provide correction formulae which should
remove systematic errors in the electron temperature + ionization correction
factor technique. We also provide a critical evaluation of the various
semi-empirical SEL techniques. Finally, we offer a scheme which should help to
eliminate systematic errors in the SEL-derived chemical abundance scale for
extragalactic HII regions.Comment: 24 pages, 9 Tables, 13 figures, accepted for publication in MNRAS.
Updated considering minor changes during the final edition process and some
few missing reference
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