5,980 research outputs found
The relation between magnetic and material arms in models for spiral galaxies
Context. Observations of polarized radio emission show that large-scale
(regular) magnetic fields in spiral galaxies are not axisymmetric, but
generally stronger in interarm regions. In some nearby galaxies such as NGC
6946 they are organized in narrow magnetic arms situated between the material
spiral arms. Aims. The phenomenon of magnetic arms and their relation to the
optical spiral arms (the material arms) call for an explanation in the
framework of galactic dynamo theory. Several possibilities have been suggested
but are not completely satisfactory; here we attempt a consistent
investigation. Methods. We use a 2D mean-field dynamo model in the no-z
approximation and add injections of small-scale magnetic field, taken to result
from supernova explosions, to represent the effects of dynamo action on smaller
scales. This injection of small scale field is situated along the spiral arms,
where star-formation mostly occurs. Results. A straightforward explanation of
magnetic arms as a result of modulation of the dynamo mechanism by material
arms struggles to produce pronounced magnetic arms, at least with realistic
parameters, without introducing new effects such as a time lag between Coriolis
force and {\alpha}-effect. In contrast, by taking into account explicitly the
small-scale magnetic field that is injected into the arms by the action of the
star forming regions that are concentrated there, we can obtain dynamo models
with magnetic structures of various forms that can be compared with magnetic
arms. (abbrev). Conclusions. We conclude that magnetic arms can be considered
as coherent magnetic structures generated by large-scale dynamo action, and
associated with spatially modulated small-scale magnetic fluctuations, caused
by enhanced star formation rates within the material arms.Comment: 13 pages, 8 figures, accepted for publication to A&
Magnetic fields in ring galaxies
Many galaxies contain magnetic fields supported by galactic dynamo action.
However, nothing definitive is known about magnetic fields in ring galaxies.
Here we investigate large-scale magnetic fields in a previously unexplored
context, namely ring galaxies, and concentrate our efforts on the structures
that appear most promising for galactic dynamo action, i.e. outer star-forming
rings in visually unbarred galaxies. We use tested methods for modelling
galactic dynamos, taking into account the available
observational information concerning ionized interstellar matter in ring
galaxies. Our main result is that dynamo drivers in ring galaxies are strong
enough to excite large-scale magnetic fields in the ring galaxies studied. The
variety of dynamo driven magnetic configurations in ring galaxies obtained in
our modelling is much richer than that found in classical spiral galaxies. In
particular, various long-lived transients are possible. An especially
interesting case is that of NGC 4513 where the ring counter-rotates with
respect to the disc. Strong shear in the region between the disc and the ring
is associated with unusually strong dynamo drivers for the counter-rotators.
The effect of the strong drivers is found to be unexpectedly moderate. With
counter-rotation in the disc, a generic model shows that a steady mixed parity
magnetic configuration, unknown for classical spiral galaxies, may be excited,
although we do not specifically model NGC 4513. We deduce that ring galaxies
constitute a morphological class of galaxies in which identification of
large-scale magnetic fields from observations of polarized radio emission, as
well as dynamo modelling, may be possible. Such studies have the potential to
throw additional light on the physical nature of rings, their lifetimes and
evolution.Comment: 25 pages, 12 figure
Magnetic fields near the peripheries of galactic discs
Magnetic fields are observed beyond the peripheries of optically detected
galactic discs, while numerical models of their origin and the typical
magnitudes are still absent. Previously, studies of galactic dynamo have
avoided considering the peripheries of galactic discs because of the very
limited (though gradually growing) knowledge about the local properties of the
interstellar medium. Here we investigate the possibility that magnetic fields
can be generated in the outskirts of discs, taking the Milky Way as an example.
We consider a simple evolving galactic dynamo model in the "no-z" formulation,
applicable to peripheral regions of galaxies, for various assumptions about the
radial and vertical profiles of the ionized gas disc. The magnetic field may
grow as galaxies evolve, even in the more remote parts of the galactic disc,
out to radii of 15 to 30 kpc, becoming substantial after times of about 10 Gyr.
This result depends weakly on the adopted distributions of the half thickness
and surface density of the ionized gas component. The model is robust to
changes in the amplitude of the initial field and the position of its maximum
strength. The magnetic field in the remote parts of the galactic disc could be
generated in situ from a seed field by local dynamo action. Another possibility
is field production in the central regions of a galaxy, followed by transport
to the disc's periphery by the joint action of the dynamo and turbulent
diffusivity. Our results demonstrate the possibilities for the appearance and
strengthening of magnetic fields at the peripheries of disc galaxies and
emphasize the need for observational tests with new and anticipated radio
telescopes (LOFAR, MWA, and SKA).Comment: 8 pages, 5 figure
Blogging in the physics classroom: A research-based approach to shaping students' attitudes towards physics
Even though there has been a tremendous amount of research done in how to
help students learn physics, students are still coming away missing a crucial
piece of the puzzle: why bother with physics? Students learn fundamental laws
and how to calculate, but come out of a general physics course without a deep
understanding of how physics has transformed the world around them. In other
words, they get the "how" but not the "why". Studies have shown that students
leave introductory physics courses almost universally with decreased
expectations and with a more negative attitude. This paper will detail an
experiment to address this problem: a course weblog or "blog" which discusses
real-world applications of physics and engages students in discussion and
thinking outside of class. Specifically, students' attitudes towards the value
of physics and its applicability to the real-world were probed using a
26-question Likert scale survey over the course of four semesters in an
introductory physics course at a comprehensive Jesuit university. We found that
students who did not participate in the blog study generally exhibited a
deterioration in attitude towards physics as seen previously. However, students
who read, commented, and were involved with the blog maintained their initially
positive attitudes towards physics. Student response to the blog was
overwhelmingly positive, with students claiming that the blog made the things
we studied in the classroom come alive for them and seem much more relevant.Comment: 20 pages, 6 figure
Wavelet-based Faraday Rotation Measure Synthesis
Faraday Rotation Measure (RM) Synthesis, as a method for analyzing
multi-channel observations of polarized radio emission to investigate galactic
magnetic fields structures, requires the definition of complex polarized
intensity in the range of the negative lambda square. We introduce a simple
method for continuation of the observed complex polarized intensity into this
domain using symmetry arguments. The method is suggested in context of magnetic
field recognition in galactic disks where the magnetic field is supposed to
have a maximum in the equatorial plane. The method is quite simple when applied
to a single Faraday-rotating structure on the line of sight. Recognition of
several structures on the same line of sight requires a more sophisticated
technique. We also introduce a wavelet-based algorithm which allows us to
consider a set of isolated structures. The method essentially improves the
possibilities for reconstruction of complicated Faraday structures using the
capabilities of modern radio telescopes.Comment: 5 pages, 5 figures, accepted for publication in MNRAS Letter
Magnetic field reversals and galactic dynamos
We argue that global magnetic field reversals similar to those observed in
the Milky Way occur quite frequently in mean-field galactic dynamo models that
have relatively strong, random, seed magnetic fields that are localized in
discrete regions. The number of reversals decreases to zero with reduction of
the seed strength, efficiency of the galactic dynamo and size of the spots of
the seed field. A systematic observational search for magnetic field reversals
in a representative sample of spiral galaxies promises to give valuable
information concerning seed magnetic fields and, in this way, to clarify the
initial stages of galactic magnetic field evolution
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