22 research outputs found
A multi-frequency study of the peculiar interacting system Arp 206
Arp 206 is a nearby, relatively large, and bright interacting system comprising unequal members: NGC 3432 and UGC 5983. A third anonymous galaxy, Arp 206c, is visible in the field. The CCD images show a well-developed bridge between NGC 3432 and UGC 5983. On the other hand, the complex H I tails are not visible in the optical. In the total H I map, the bridge is lost in a general envelope encompassing both galaxies. The bridge also appears to have some radio emission. On the Total H I map the system is rather edge-on, far more than it would appear in optical wavelengths. UGC 5983 falls exactly in line with NGC 3432. The velocity of the centers of mass of NGC 3432 and UGC 5983 are 530 km s(exp -1) and 630 km s(exp -1), respectively. In view of the considerable damage sustained by NGC 3432 and the apparent low mass of UGC 5983, it appears that the passage must have been at near parabolic speed, with a small pericentric distance and a very low inclination with rspect to the disk of NGC 3432. The apparent distribution of H I along the z axis of the galaxy could be accounted for by projection effects. The tidal appendage found at higher velocities, which rises at a P.A. approx. equal to 25 degrees west of the main body of the galaxy is probably the tail, the part of the tidal damage away from the perturbing companion. The bridge may be rising north-east from the galaxy and then continue under to the south of the galaxy. The relative sizes of the appendages would indicate that the pericenter was crossed recently. Any further inferences about the collision parameters will need to await the results of detailed computational modelling of the interaction. The authors also summarize the observational characteristics of NGC 3432, UGC 5983, and Arp 206c
The Cosmic Large-Scale Structure in X-rays (CLASSIX) Cluster Survey I: Probing galaxy cluster magnetic fields with line of sight rotation measures
To search for a signature of an intracluster magnetic field, we compare
measurements of Faraday rotation of polarised extragalactic radio sources in
the line of sight of galaxy clusters with those outside. We correlated a
catalogue of 1383 rotation measures (RM) of extragalactic polarised radio
sources with X-ray luminous galaxy clusters from the CLASSIX survey (combining
REFLEX II and NORAS II). We compared the RM in the line of sight of clusters
within their projected radii of r_500 with those outside and found a
significant excess of the dispersion of the RM in the cluster regions. Since
the observed RM is the result of Faraday rotation in several presumably
uncorrelated magnetised cells of the intracluster medium, the observations
correspond to quantities averaged over several magnetic field directions and
strengths. Therefore the interesting quantity is the standard deviation of the
RM for an ensemble of clusters. We found a standard deviation of the RM inside
r_500 of about 120 +- 21 rad m^-2. This compares to about 56 +- 8 rad m^-2
outside. We show that the most X-ray luminous and thus most massive clusters
contribute most to the observed excess RM. Modelling the electron density
distribution in the intracluster medium with a self-similar model, we found
that the dispersion of the RM increases with the column density, and we deduce
a magnetic field value of about 2 - 6 (l/10kpc)^-1/2 microG assuming a constant
magnetic field strength, where l is the size of the coherently magnetised
intracluster medium cells. This magnetic field energy density amounts to a few
percent of the average thermal energy density in clusters. When we assumed the
magnetic energy density to be a constant fraction of the thermal energy
density, we deduced a slightly lower value for this fraction of 3 - 10
(l/10kpc)^-1/2 per mille.Comment: 7 pages, 6 figures, in press, Astronomy and Astrophysics, 201
Strong magnetic fields and large rotation measures in protogalaxies by supernova seeding
We present a model for the seeding and evolution of magnetic fields in
protogalaxies. Supernova (SN) explosions during the assembly of a protogalaxy
provide magnetic seed fields, which are subsequently amplified by compression,
shear flows and random motions. We implement the model into the MHD version of
the cosmological N-body / SPH simulation code GADGET and we couple the magnetic
seeding directly to the underlying multi-phase description of star formation.
We perform simulations of Milky Way-like galactic halo formation using a
standard LCDM cosmology and analyse the strength and distribution of the
subsequent evolving magnetic field. A dipole-shape divergence-free magnetic
field is injected at a rate of 10^{-9}G / Gyr within starforming regions, given
typical dimensions and magnetic field strengths in canonical SN remnants.
Subsequently, the magnetic field strength increases exponentially on timescales
of a few ten million years. At redshift z=0, the entire galactic halo is
magnetized and the field amplitude is of the order of a few G in the
center of the halo, and 10^{-9} G at the virial radius. Additionally, we
analyse the intrinsic rotation measure (RM) of the forming galactic halo over
redshift. The mean halo intrinsic RM peaks between redshifts z=4 and z=2 and
reaches absolute values around 1000 rad m^{-2}. While the halo virializes
towards redshift z=0, the intrinsic RM values decline to a mean value below 10
rad m^{-2}. At high redshifts, the distribution of individual starforming, and
thus magnetized regions is widespread. In our model for the evolution of
galactic magnetic fields, the seed magnetic field amplitude and distribution is
no longer a free parameter, but determined self-consistently by the star
formation process occuring during the formation of cosmic structures.Comment: 13 pages, 14 figures, accepted to MNRAS after moderate revisio
The Centaurus A Ultrahigh-Energy Cosmic Ray Excess and the Local Extragalactic Magnetic Field
The ultrahigh-energy cosmic-ray anisotropies discovered by the Pierre Auger
Observatory give the potential to finally address both the particles' origins
and properties of the nearby extragalactic magnetic field (EGMF). We examine
the implications of the excess of ~ 10^20 eV events around the nearby radio
galaxy Centaurus A. We find that, if Cen A is the source of these cosmic rays,
the angular distribution of events constrains the EGMF strength within several
Mpc of the Milky Way to > 20 nG for an assumed primary proton composition. Our
conclusions suggest that either the observed excess is a statistical anomaly or
the local EGMF is stronger then conventionally thought. We discuss the
implications of this field, including UHECR scattering from more distant
sources, time delays from transient sources, and the possibility of using
magnetic lensing signatures to attain tighter constraints.Comment: 8 pages, 8 figures; Matches published version in AP
Strong magnetic fields in normal galaxies at high redshifts
The origin and growth of magnetic fields in galaxies is still something of an
enigma. It is generally assumed that seed fields are amplified over time
through the dynamo effect, but there are few constraints on the timescale. It
has recently been demonstrated that field strengths as traced by rotation
measures of distant quasars are comparable to those seen today, but it was
unclear whether the high fields were in the exotic environments of the quasars
themselves or distributed along the line of sight. Here we demonstrate that the
quasars with strong MgII absorption lines are unambiguously associated with
larger rotation measures. Since MgII absorption occurs in the haloes of normal
galaxies along the sightline to the quasars, this association requires that
organized fields of surprisingly high strength are associated with normal
galaxies when the Universe was only about one-third of its present age.Comment: 11 pages, 2 figures, Letter to Nature, accepted. Final version
available at
http://www.nature.com/nature/journal/v454/n7202/abs/nature07105.htm
Detailed Radio Spectra of Selected Compact Sources in the Nucleus of M82
We have determined detailed radio spectra for 26 compact sources in the
starburst nucleus of M82, between 74 and 1.3 cm. Seventeen show low-frequency
turnovers. One other has a thermal emission spectrum, and we identify it as an
HII region. The low frequency turnovers are due to absorption by the
interstellar gas in M82. New information on the AGN candidate 44.01+595, shows
it to have a non-thermal falling powerlaw spectrum at the highest frequencies,
and that it is strongly absorbed below 2 GHz. We derive large magnetic fields
in the supernova remnants, of order 1-2 milliGauss, hence large pressures in
the sources suggest that the brightest ones are either expanding or are
strongly confined by a dense interstellar medium. From the largest source in
our sample, we derive a supernova rate of 0.016 SN/yr.Comment: 19 pages, 7 tables, 29 figures, LaTeX, requires AAS macros v. 4.0. To
appear in ApJ July 20, 199
A Faraday Rotation Search for Magnetic Fields in Large Scale Structure
Faraday rotation of radio source polarization provides a measure of the
integrated magnetic field along the observational lines of sight. We compare a
new, large sample of Faraday rotation measures (RMs) of polarized extragalactic
sources with galaxy counts in Hercules and Perseus-Pisces, two nearby
superclusters. We find that the average of RMs in these two supercluster areas
are larger than in control areas in the same galactic latitude range. This is
the first RM detection of magnetic fields that pervade a supercluster volume,
in which case the fields are at least partially coherent over several
megaparsecs. Even the most conservative interpretation of our observations,
according to which Milky Way RM variations mimic the background supercluster
galaxy overdensities, puts constraints on the IGM magneto-ionic ``strength'' in
these two superclusters. We obtain an approximate typical upper limit on the
field strength of about 0.3 microGauss l/(500 kpc), when we combine our RM data
with fiducial estimates of electron density from the environments of giant
radio galaxies, and of the warm-hot intergalactic medium (WHIM).Comment: 8 pages, 3 figures, 1 table, to appear in the Astrophysical Journa
Filaments in the southern giant lobe of Centaurus A : Constraints on nature and origin from modelling and GMRT observations
Date of acceptance: 22/05/2014We present results from imaging of the radio filaments in the southern giant lobe of CentaurusA using data from Giant Metrewave Radio Telescope observations at 325 and 235 MHz, and outcomes from filament modelling. The observations reveal a rich filamentary structure, largely matching the morphology at 1.4 GHz. We find no clear connection of the filaments to the jet. We seek to constrain the nature and origin of the vertex and vortex filaments associated with the lobe and their role in high-energy particle acceleration. We deduce that these filaments are at most mildly overpressuredwith respect to the global lobe plasma showing no evidence of largescale efficient Fermi I-type particle acceleration, and persist for ~2-3 Myr. We demonstrate that the dwarf galaxy KK 196 (AM 1318-444) cannot account for the features, and that surface plasma instabilities, the internal sausage mode and radiative instabilities are highly unlikely. An internal tearing instability and the kink mode are allowed within the observational and growth time constraints and could develop in parallel on different physical scales. We interpret the origin of the vertex and vortex filaments in terms of weak shocks from transonic magnetohydrodynamical turbulence or from a moderately recent jet activity of the parent AGN, or an interplay of both.Peer reviewe
Signatures in a Giant Radio Galaxy of a Cosmological Shock Wave at Intersecting Filaments of Galaxies
Sensitive images of low-level, Mpc-sized radio cocoons offer new
opportunities to probe large scale intergalactic gas flows outside clusters of
galaxies. New radio images of high surface brightness sensitivity at
strategically chosen wavelengths of the giant radio galaxy NGC 315 (Mack et al.
1997,1998) reveal significant asymmetries and particularities in the
morphology, radio spectrum and polarization of the ejected radio plasma. We
argue that the combination of these signatures provides a sensitive probe of an
environmental shock wave. Analysis of optical redshifts in NGC 315 vicinity
confirms its location to be near, or at a site of large-scale flow collisions
in the 100 Mpc sized Pisces-Perseus Supercluster region. NGC 315 resides at the
intersection of several galaxy filaments, and its radio plasma serves there as
a `weather station' (Burns 1998) probing the flow of the elusive and previously
invisible IGM gas. If our interpretation is correct, this is the first
indication for a shock wave in flows caused by the cosmological large scale
structure formation, which is located in a filament of galaxies. The
possibility that the putative shock wave is a source of gamma-rays and ultra
high energy cosmic rays is briefly discussed.Comment: accepted by Astrophysical Journal Letters, 4 pages, 3 figures (incl.
2 color), uses emulateapj5.sty (included), aastex.sty (included) and
psfig.st