1,416 research outputs found
Completing HI observations of galaxies II. The Coma Supercluster
High sensitivity 21-cm HI line observations, with an rms noise level of \sim
0.5 mJy, were made of 35 spiral galaxies in the Coma Supercluster, using the
refurbished Arecibo telescope, which resulted in the detection of 25 objects.
These data, combined with the measurements available from the literature,
provide the set of HI data for 94% of all late-type galaxies in the Coma
Supercluster with an apparent photographic magnitude m_p <15.7 mag. We confirm
that the typical scale of HI deficiency around the Coma cluster is 2 Mpc, i.e.
one virial radius. Comparing the HI mass function (HIMF) of cluster with
non-cluster members of the Coma Supercluster we detect a shortage of high HI
mass galaxies among cluster members that can be ascribed to the pattern of HI
deficiency found in rich clusters.Comment: 16 pages, 8 figures, 4 tables. Accepted for publication on A&
Star Formation Histories of Nearby Elliptical Galaxies. II. Merger Remnant Sample
This work presents high spectroscopic observations of a sample of six
suspected merger remnants, selected primarily on the basis of H{\sc i} tidal
debris detections. Single stellar population analysis of these galaxies
indicates that their ages, metallicities, and -enhancement ratios are
consistent with those of a representative sample of nearby elliptical galaxies.
The expected stellar population of a recent merger remnant, young age combined
with low [/Fe], is not seen in any H{\sc i}-selected galaxy. However,
one galaxy (NGC~2534), is found to deviate from the -plane in the sense
expected for a merger remnant. Another galaxy (NGC~7332), selected by other
criteria, best matches the merger remnant expectations.Comment: 12 pages, 10 figures, accepted by A
Signatures of Interchange Reconnection: STEREO, ACE and Hinode Observations Combined
Combining STEREO, ACE and Hinode observations has presented an opportunity to
follow a filament eruption and coronal mass ejection (CME) on the 17th of
October 2007 from an active region (AR) inside a coronal hole (CH) into the
heliosphere. This particular combination of `open' and closed magnetic
topologies provides an ideal scenario for interchange reconnection to take
place. With Hinode and STEREO data we were able to identify the emergence time
and type of structure seen in the in-situ data four days later. On the 21st,
ACE observed in-situ the passage of an ICME with `open' magnetic topology. The
magnetic field configuration of the source, a mature AR located inside an
equatorial CH, has important implications for the solar and interplanetary
signatures of the eruption. We interpret the formation of an `anemone'
structure of the erupting AR and the passage in-situ of the ICME being
disconnected at one leg, as manifested by uni-directional suprathermal electron
flux in the ICME, to be a direct result of interchange reconnection between
closed loops of the CME originating from the AR and `open' field lines of the
surrounding CH.Comment: 13 pages, 13 figures, accepted Annales Geophysica
Flux cancellation and the evolution of the eruptive filament of 2011 June 7
We investigate whether flux cancellation is responsible for the formation of
a very massive filament resulting in the spectacular 2011 June 7 eruption. We
analyse and quantify the amount of flux cancellation that occurs in NOAA AR
11226 and its two neighbouring ARs (11227 & 11233) using line-of-sight
magnetograms from the Heliospheric Magnetic Imager. During a 3.6-day period
building up to the filament eruption, 1.7 x 10^21 Mx, 21% of AR 11226's maximum
magnetic flux, was cancelled along the polarity inversion line (PIL) where the
filament formed. If the flux cancellation continued at the same rate up until
the eruption then up to 2.8 x 10^21 Mx (34% of the AR flux) may have been built
into the magnetic configuration that contains the filament plasma. The large
flux cancellation rate is due to an unusual motion of the positive polarity
sunspot, which splits, with the largest section moving rapidly towards the PIL.
This motion compresses the negative polarity and leads to the formation of an
orphan penumbra where one end of the filament is rooted. Dense plasma threads
above the orphan penumbra build into the filament, extending its length, and
presumably injecting material into it. We conclude that the exceptionally
strong flux cancellation in AR 11226 played a significant role in the formation
of its unusually massive filament. In addition, the presence and coherent
evolution of bald patches in the vector magnetic field along the PIL suggests
that the magnetic field configuration supporting the filament material is that
of a flux rope.Comment: 18 pages, 7 figures. Submitted to ApJ in December 2015, accepted in
June 201
Completing HI observations of galaxies in the Virgo cluster
High sensitivity (rms noise mJy) 21-cm HI line observations were
made of 33 galaxies in the Virgo cluster, using the refurbished Arecibo
telescope, which resulted in the detection of 12 objects. These data, combined
with the measurements available from the literature, provide the first set of
HI data that is complete for all 355 late-type (Sa-Im-BCD) galaxies in the
Virgo cluster with mag. The Virgo cluster HI mass function
(HIMF) that was derived for this optically selected galaxy sample is in
agreement with the HIMF derived for the Virgo cluster from the blind HIJASS HI
survey and is inconsistent with the Field HIMF. This indicates that both in
this rich cluster and in the general field, neutral hydrogen is primarily
associated with late-type galaxies, with marginal contributions from early-type
galaxies and isolated HI clouds. The inconsistency between the cluster and the
field HIMF derives primarily from the difference in the optical luminosity
function of late-type galaxies in the two environments, combined with the HI
deficiency that is known to occur in galaxies in rich clusters.Comment: accepted for publication on A&
On-disc observations of flux rope formation prior to its eruption
Coronal mass ejections (CMEs) are one of the primary manifestations of solar activity and can drive severe space weather effects. Therefore, it is vital to work towards being able to predict their occurrence. However, many aspects of CME formation and eruption remain unclear, including whether magnetic flux ropes are present before the onset of eruption and the key mechanisms that cause CMEs to occur. In this work, the pre-eruptive coronal configuration of an active region that produced an interplanetary CME with a clear magnetic flux rope structure at 1 AU is studied. A forward-S sigmoid appears in extreme-ultraviolet (EUV) data two hours before the onset of the eruption (SOL2012-06-14), which is interpreted as a signature of a right-handed flux rope that formed prior to the eruption. Flare ribbons and EUV dimmings are used to infer the locations of the flux rope footpoints. These locations, together with observations of the global magnetic flux distribution, indicate that an interaction between newly emerged magnetic flux and pre-existing sunspot field in the days prior to the eruption may have enabled the coronal flux rope to form via tether-cutting-like reconnection. Composition analysis suggests that the flux rope had a coronal plasma composition, supporting our interpretation that the flux rope formed via magnetic reconnection in the corona. Once formed, the flux rope remained stable for two hours before erupting as a CME
Criteria for Flux Rope Eruption: Non Equilibrium versus Torus Instability
The coronal magnetic configuration of an active region typically evolves
quietly during few days before becoming suddenly eruptive and launching a
coronal mass ejection (CME). The precise origin of the eruption is still
debated. Among several mechanisms, it has been proposed that a loss of
equilibrium, or an ideal magneto-hydrodynamic (MHD) instability such as the
torus instability, could be responsible for the sudden eruptivity. Distinct
approaches have also been formulated for limit cases having circular or
translation symmetry. We revisit the previous theoretical approaches, setting
them in the same analytical framework. The coronal field results from the
contribution of a non-neutralized current channel added to a background
magnetic field, which in our model is the potential field generated by two
photospheric flux concentrations. The evolution on short Alfvenic time scale is
governed by ideal MHD. We show analytically first that the loss of equilibrium
and the stability analysis are two different views of the same physical
mechanism. Second, we identify that the same physics is involved in the
instability of circular and straight current channels. Indeed, they are just
two particular limiting case of more general current paths. A global
instability of the magnetic configuration is present when the current channel
is located at a coronal height, h, large enough so that the decay index of the
potential field, (d ln |Bp|) / (d ln h) is larger than a critical value. At the
limit of very thin current channels, previous analysis found a critical decay
index of 1.5 and 1 for circular and straight current channels, respectively.
However, with current channels being deformable and as thick as expected in the
corona, we show that this critical index has similar values for circular and
straight current channels, typically in the range [1.1,1.3].Comment: 12 pages, 4 figure
The Cold and Hot Gas Content of Fine-Structure E and S0 Galaxies
We investigate trends of the cold and hot gas content of early-type galaxies
with the presence of optical morphological peculiarities, as measured by the
fine-structure index (Sigma). HI mapping observations from the literature are
used to track the cold-gas content, and archival ROSAT PSPC data are used to
quantify the hot-gas content. We find that E and S0 galaxies with a high
incidence of optical peculiarities are exclusively X-ray underluminous and,
therefore, deficient in hot gas. In contrast, more relaxed galaxies with little
or no signs of optical peculiarities span a wide range of X-ray luminosities.
That is, the X-ray excess anticorrelates with Sigma. There appears to be no
similar trend of cold-gas content with either fine-structure index or X-ray
content. The fact that only apparently relaxed E and S0 galaxies are strong
X-ray emitters is consistent with the hypothesis that after strong disturbances
such as a merger hot-gas halos build up over a time scale of several gigayears.
This is consistent with the expected mass loss from stars.Comment: 12 pages, latex, 5 figures. Accepted for publication in A
Exciton enhancement of spin relaxation in diluted magnetic semiconductor quantum wells
The authors performed pump-probe magneto-optical Kerr spectroscopy to explore the role of excitonic effects on the electron and hole spin relaxation in Cd1-xMnxTe-based quantum well structures. Particularly, important information was obtained from detailed interpretation of the temp. and wavelength dependence of the obsd. transient magneto-optics in specially engineered structures. In Mn-rich wells the electron spin relaxation is strongly enhanced in the exciton due to the increased mass of the bound electron-hole pair, in agreement with earlier predictions. An even stronger enhancement of the relaxation time is found for the hole spin, which is explained by the addnl. mixing of heavy-hole and light-hole states in the bound state. In structures where Mn is confined to the barrier region, a different behavior is obsd., indicative for a competition of various contributions. [on SciFinder (R)
Exciton enhancement of spin relaxation in diluted magnetic semiconductor quantum wells
The authors performed pump-probe magneto-optical Kerr spectroscopy to explore the role of excitonic effects on the electron and hole spin relaxation in Cd1-xMnxTe-based quantum well structures. Particularly, important information was obtained from detailed interpretation of the temp. and wavelength dependence of the obsd. transient magneto-optics in specially engineered structures. In Mn-rich wells the electron spin relaxation is strongly enhanced in the exciton due to the increased mass of the bound electron-hole pair, in agreement with earlier predictions. An even stronger enhancement of the relaxation time is found for the hole spin, which is explained by the addnl. mixing of heavy-hole and light-hole states in the bound state. In structures where Mn is confined to the barrier region, a different behavior is obsd., indicative for a competition of various contributions. [on SciFinder (R)
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