340 research outputs found
How galaxies lose their angular momentum
The processes are investigated by which gas loses its angular momentum during
the protogalactic collapse phase, leading to disk galaxies that are too compact
with respect to the observations. High-resolution N-body/SPH simulations in a
cosmological context are presented including cold gas and dark matter. A halo
with quiet merging activity since z~3.8 and with a high spin parameter is
analysed that should be an ideal candidate for the formation of an extended
galactic disk. We show that the gas and the dark matter have similar specific
angular momenta until a merger event occurs at z~2 with a mass ratio of 5:1.
All the gas involved in the merger loses a substantial fraction of its specific
angular momentum due to tidal torques and falls quickly into the center.
Dynamical friction plays a minor role,in contrast to previous claims. In fact,
after this event a new extended disk begins to form from gas that was not
involved in the 5:1 merger event and that falls in subsequently. We argue that
the angular momentum problem of disk galaxy formation is a merger problem: in
cold dark matter cosmology substantial mergers with mass ratios of 1:1 to 6:1
are expected to occur in almost all galaxies. We suggest that energetic
feedback processes could in principle solve this problem, however only if the
heating occurs at the time or shortly before the last substantial merger event.
Good candidates for such a coordinated feedback would be a merger-triggered
star burst or central black hole heating. If a large fraction of the low
angular momentum gas would be ejected as a result of these processes, late-type
galaxies could form with a dominant extended disk component, resulting from
late infall, a small bulge-to-disk ratio and a low baryon fraction, in
agreement with observations.Comment: 7 pages, 5 figures, submitted to MNRAS. Request for high resolution
figures to the author
Tidally Induced Offset Disks in Magellanic Spiral Galaxies
Magellanic spiral galaxies are a class of one-armed systems that often
exhibit an offset stellar bar, and are rarely found around massive spiral
galaxies. Using a set of N-body and hydrodynamic simulations we consider a
dwarf-dwarf galaxy interaction as the driving mechanism for the formation of
this peculiar class of systems. We investigate here the relation between the
dynamical, stellar and gaseous disk center and the bar. In all our simulations
the bar center always coincides with the dynamical center, while the stellar
disk becomes highly asymmetric during the encounter causing the photometric
center of the Magellanic galaxy disk to become mismatched with both the bar and
the dynamical center. The disk asymmetries persist for almost 2 Gyrs, the time
that it takes for the disk to be re-centered with the bar, and well after the
companion has passed. This explains the nature of the offset bar found in many
Magellanic-type galaxies, including the Large Magellanic Cloud (LMC) and NGC
3906. In particular, these results, once applied to the LMC, suggest that the
dynamical center should reside in the bar center instead of the HI center as
previously assumed, pointing to a variation in the current estimate of the
north component of the LMC proper motion.Comment: 12 pages, 8 figures, corrected for referee comment
Fossil group origins: VIII RXJ075243.6+455653 a transitionary fossil group
It is thought that fossil systems are relics of structure formation in the
primitive Universe. They are galaxy aggregations that have assembled their mass
at high redshift with few or no subsequent accretion. Observationally these
systems are selected by large magnitude gaps between their 1st and 2nd ranked
galaxies. Nevertheless, there is still debate over whether or not this
observational criterium selects dynamically evolved ancient systems. We have
studied the properties of the nearby fossil group RXJ075243.6+455653 in order
to understand the mass assembly of this system. Deep spectroscopic observations
allow us to construct the galaxy luminosity function (LF) of RXJ075243.6+455653
down to M*+ 6. The analysis of the faint-end of the LF in groups and clusters
provides valuable information about the mass assembly of the system. In
addition, we have analyzed the nearby large-scale structure around this group.
We identified 26 group members within r200=0.9 Mpc. The LF of the group shows a
flat faint-end slope ( -1.08 +/- 0.33). This low density of dwarf galaxies is
confirmed by the low value of the dwarf-to-giant ratio (DGR = 0.99 +/- 0.49)
for this system. Both the lack of dwarf galaxies and the low luminosity of the
BGG suggests that RXJ075243.6+455653 still has to accrete mass from its nearby
environment. This mass accretion will be achieved because it is the dominant
structure of a rich environment formed by several groups of galaxies (15)
within 7 Mpc from the group center and with +/- 1000$ km/s. RXJ075243.6+455653
is a group of galaxies that has not yet completed the process of its mass
assembly. This new mass accretion will change the fossil state of the group.
This group is an example of a galaxy aggregation selected by a large magnitude
gap but still in the process of the accretion of its mass (Abridged).Comment: 9 pages, 9 figures, accepted in A&
Spatio-temporal changes of Munida Rutllanti Zariquiey-Alvarez, 1952 (Decapoda: Galatheidae) in the North-Western Ionian Sea (Central Mediterranean)
The spatio-temporal pattern of Munida rutllanti distribution in the north-western Ionian Sea has been studied. Data were collected during 14 experimental trawl surveys conducted from 1997 to 2010 as part of the international MEDITS project. The hauls were carried out during day-light hours between depths of 10 and 800 m in the spring season. A progressive increase in the abundance index (N/km2) of M. rutllanti was observed from 2000 to 2008, then a sharp decrease was shown in the last two years. The greatest and lowest abundance indices were observed in the Apulian and central Calabrian sub-areas, respectively. The species was collected between 107 and 795 m in depth, with a significant increase and decrease over time in the maximum and minimum depth of finding, respectively. A highly significant increase over time in the mean carapace length was also observed in the whole study area. The widespread occurrence and increasing abundance of this species in the Ionian Sea could be related to the increase in temperature and the variation in hydrographic conditions which occurred in the Ionian basin during the EMT-BiOS phenomenon
Early detection of Aspergillus carbonarius and A. niger on table grapes: a tool for quality improvement
Aspergillus carbonarius and A. niger aggregate are the main fungal contaminants of table grapes. Besides their ability to cause black rot, they can produce ochratoxin A (OTA), a mycotoxin that has attracted increasing attention worldwide. The objective of this work was to set up a simple and rapid molecular method for the early detection of both fungi in table grapes before fungal development becomes evident. Polymerase chain reaction (PCR)-based assays were developed by designing species-specific primers based on the polyketide synthases (PKSS) sequences of A. carbonarius and A. niger that have recently been demonstrated to be involved in OTA biosynthesis. Three table grape varieties (Red globe, Crimson seedless, and Italia) were inoculated with A. carbonarius and A. niger aggregate strains producing OTA. The extracted DNA from control (non-inoculated) and inoculated grapes was amplified by PCR using ACPKS2F-ACPKS2R for A. carbonarius and ANPKS5-ANPKS6 for A. niger aggregate. Both primers allowed a clear detection, even in symptomless samples. PCR-based methods are considered to be a good alternative to traditional diagnostic means for the early detection of fungi in complex matrix for their high specificity and sensitivity. The results obtained could be useful for the definition of a 'quality label' for tested grapes to improve the safety measures taken to guarantee the production of fresh table grapes
Antagonism and Antimicrobial Capacity of Epiphytic and Endophytic Bacteria against the Phytopathogen Xylella fastidiosa
Olive quick decline syndrome (OQDS), which is caused by Xylella fastidiosa, poses a severe threat to the agriculture of Mediterranean countries and causes severe damage to the olive trees in Italy. Since no effective control measures are currently available, the objective of this study was the screening of antagonistic bacteria that are potentially deployable as biocontrol agents against X. fastidiosa. Therefore, two approaches were used, i.e., the evaluation of the antagonistic activity of (i) endophytic bacteria isolated from two different cultivars of olive trees (Leccino and Ogliarola salentina) and (ii) epiphytic bacteria isolated from the phyllospheres of different host plant species of X. fastidiosa. In vitro dual culture tests showed that 12 out of 200 isolates inhibited X. fastidiosa growth, with appearances of clear zones between 4.0 and 38.6 mm. 16S rRNA gene sequencing revealed different species of the genera Paenibacillus, Bacillus, Pantoea, Microbacterium, Stenotrophomonas, Delftia, and Pseudomonas. Furthermore, an investigation for antimicrobial activity identified 5 out of the 12 antagonistic bacteria, Paenibacillus rigui, Bacillus subtilis, Bacillus pumilus, Microbacterium oxydans, and Stenotrophomonas rhizophila, that were able to produce culture filtrates with inhibitory activities. Our results are promising for further investigation to develop an eco-sustainable strategy to control X. fastidiosa using biocontrol agents or their secreted metabolites
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