118 research outputs found
The magnetic field of M31 from multi-wavelength radio polarization observations
The configuration of the regular magnetic field in M31 is deduced from radio
polarization observations at the wavelengths 6, 11 and 20 cm. By fitting the
observed azimuthal distribution of polarization angles, we find that the
regular magnetic field, averaged over scales 1--3 kpc, is almost perfectly
axisymmetric in the radial range 8 to 14 kpc, and follows a spiral pattern with
pitch angles of p\simeq -19\degr to p\simeq -8\degr. In the ring between 6
and 8 kpc a perturbation of the dominant axisymmetric mode may be present,
having the azimuthal wave number m=2. A systematic analysis of the observed
depolarization allows us to identify the main mechanism for wavelength
dependent depolarization -- Faraday rotation measure gradients arising in a
magneto-ionic screen above the synchrotron disk. Modelling of the
depolarization leads to constraints on the relative scale heights of the
thermal and synchrotron emitting layers in M31; the thermal layer is found to
be up to three times thicker than the synchrotron disk. The regular magnetic
field must be coherent over a vertical scale at least similar to the scale
height of the thermal layer, estimated to be h\therm\simeq 1 kpc. Faraday
effects offer a powerful method to detect thick magneto-ionic disks or halos
around spiral galaxies.Comment: 17 pages, 16 figures, accepted for publication in A&
Magnetic fields and spiral arms in the galaxy M51
(Abridged) We use new multi-wavelength radio observations, made with the VLA
and Effelsberg telescopes, to study the magnetic field of the nearby galaxy M51
on scales from 200\pc to several \kpc. Interferometric and single dish data
are combined to obtain new maps at \wwav{3}{6} in total and polarized emission,
and earlier \wav{20} data are re-reduced. We compare the spatial distribution
of the radio emission with observations of the neutral gas, derive radio
spectral index and Faraday depolarization maps, and model the large-scale
variation in Faraday rotation in order to deduce the structure of the regular
magnetic field. We find that the \wav{20} emission from the disc is severely
depolarized and that a dominating fraction of the observed polarized emission
at \wav{6} must be due to anisotropic small-scale magnetic fields. Taking this
into account, we derive two components for the regular magnetic field in this
galaxy: the disc is dominated by a combination of azimuthal modes, , but
in the halo only an mode is required to fit the observations. We disuss
how the observed arm-interarm contrast in radio intensities can be reconciled
with evidence for strong gas compression in the spiral shocks. The average
arm--interam contrast, representative of the radii r>2\kpc where the spiral
arms are broader, is not compatible with straightforward compression: lower
arm--interarm contrasts than expected may be due to resolution effects and
\emph{decompression} of the magnetic field as it leaves the arms. We suggest a
simple method to estimate the turbulent scale in the magneto-ionic medium from
the dependence of the standard deviation of the observed Faraday rotation
measure on resolution. We thus obtain an estimate of 50\pc for the size of
the turbulent eddies.Comment: 21 pages, 18 figures (some at lower resolution than submitted
version), accepted for publication in MNRA
The distance to the SNR CTB109 deduced from its environment
We conducted a study of the environment around the supernova remnant CTB109.
We found that the SNR is part of a large complex of HII regions extending over
an area of 400 pc along the Galactic plane at a distance of about 3 kpc at the
closer edge of the Perseus spiral arm. At this distance CTB109 has a diameter
of about 24 pc. We demonstrated that including spiral shocks in the distance
estimation is an ultimate requirement to determine reliable distances to
objects located in the Perseus arm. The most likely explanation for the high
concentration of HII regions and SNRs is that the star formation in this part
of the Perseus arm is triggered by the spiral shock.Comment: 6 pages, 6 figures. accepted for publication in the Astrophysical
Journa
Scaling and correlation analysis of galactic images
Different scaling and autocorrelation characteristics and their application
to astronomical images are discussed: the structure function, the
autocorrelation function, Fourier spectra and wavelet spectra. We recommend as
the optimal mathematical tool the wavelet spectrum with a suitable choice of
the analysing wavelet. We introduce the wavelet cross-correlation function
which enables to study the correlation between images as a function of scale.
The wavelet cross-correlation coefficient strongly depends on the scale. The
classical cross-correlation coefficient can be misleading if a bright, extended
central region or an extended disk exists in the galactic images. An analysis
of the scaling and cross-correlation characteristics of 9 optical and radio
maps of the nearby spiral galaxy NGC 6946 is presented. The wavelet spectra
allow to separate structures on different scales like spiral arms and diffuse
extended emission. Only the images of thermal radio emission and Halpha
emission give indications of 3-dimensional Kolmogorov-type turbulence on the
smallest resolved scales (160-800 pc). The cross-correlations between the
images of NGC 6946 show strong similarities between the images of total radio
emission, red light and mid-infrared dust emission on all scales. The best
correlation is found between total radio emission and dust emission. Thermal
radio continuum and Halpha emission are best correlated on a scale of about 1'
\simeq 1.6 kpc, the typical width of a spiral arm. On a similar scale, the
images of polarised radio and Halpha emission are anticorrelated, which remains
undetected with classical ross-correlation analysis.Comment: 15 pages with 12 figures. Accepted for publication in MNRA
Supernova Remnants in the Fossil Starburst in M82
We report the discovery of ten compact H-alpha-bright sources in the
post-starburst region northeast of the center of M82, ``M82 B.'' These objects
have H alpha luminosities and sizes consistent with Type II supernova remnants
(SNRs). They fall on the same H alpha surface brightness-diameter (Sigma-D)
relation defined by SNRs in other nearby star-forming galaxies, with the M82
candidates lying preferentially at the small diameter end. These are the first
candidates for optically-visible SNRs in M82 outside the heavily obscured
central starburst within ~250 pc from the galactic center. If these sources are
SNRs, they set an upper limit to the end of the starburst in region ``B2,''
about 500 pc from the galaxy's core, of ~50 Myr. Region ``B1,'' about 1000 pc
from the core, lacks good SNR candidates and is evidently somewhat older. This
suggests star formation in the galaxy has propagated inward toward the
present-day intense starburst core.Comment: Re-submitted to AJ, referee's comments taken into account, 15 pages
LaTeX preprint style, 4 postscript figures; full-resolution figures available
from http://www.astro.virginia.edu/~rd7a/snrs/ Changes: minor textual changes
and orientation/axes of Fig.
OVI, NV and CIV in the Galactic Halo: II. Velocity-Resolved Observations with Hubble and FUSE
We present a survey of NV and OVI (and where available CIV) in the Galactic
halo, using data from the Far Ultraviolet Spectroscopic Explorer (FUSE) and the
Hubble Space Telescope (HST) along 34 sightlines. These ions are usually
produced in nonequilibrium processes such as shocks, evaporative interfaces, or
rapidly cooling gas, and thus trace the dynamics of the interstellar medium.
Searching for global trends in integrated and velocity-resolved column density
ratios, we find large variations in most measures, with some evidence for a
systematic trend of higher ionization (lower NV/OVI column density ratio) at
larger positive line-of-sight velocities. The slopes of log[N(NV)/N(OVI)] per
unit velocity range from -0.015 to +0.005, with a mean of
-0.0032+/-0.0022(r)+/-0.0014(sys) dex/(km/s). We compare this dataset with
models of velocity-resolved high-ion signatures of several common physical
structures. The dispersion of the ratios, OVI/NV/CIV, supports the growing
belief that no single model can account for hot halo gas, and in fact some
models predict much stronger trends than are observed. It is important to
understand the signatures of different physical structures to interpret
specific lines of sight and future global surveys.Comment: ApJ in press 43 pages, 22 fig
The Velocity Distribution of the Nearest Interstellar Gas
The bulk flow velocity for the cluster of interstellar cloudlets within about
30 pc of the Sun is determined from optical and ultraviolet absorption line
data, after omitting from the sample stars with circumstellar disks or variable
emission lines and the active variable HR 1099. Ninety-six velocity components
towards the remaining 60 stars yield a streaming velocity through the local
standard of rest of -17.0+/-4.6 km/s, with an upstream direction of l=2.3 deg,
b=-5.2 deg (using Hipparcos values for the solar apex motion). The velocity
dispersion of the interstellar matter (ISM) within 30 pc is consistent with
that of nearby diffuse clouds, but present statistics are inadequate to
distinguish between a Gaussian or exponential distribution about the bulk flow
velocity. The upstream direction of the bulk flow vector suggests an origin
associated with the Loop I supernova remnant. Groupings of component velocities
by region are seen, indicating regional departures from the bulk flow velocity
or possibly separate clouds. The absorption components from the cloudlet
feeding ISM into the solar system form one of the regional features. The
nominal gradient between the velocities of upstream and downstream gas may be
an artifact of the Sun's location near the edge of the local cloud complex. The
Sun may emerge from the surrounding gas-patch within several thousand years.Comment: Typographical errors corrected; Five tables, seven figures;
Astrophysical Journal, in pres
Density probability distribution functions of diffuse gas in the Milky Way
In a search for the signature of turbulence in the diffuse interstellar
medium in gas density distributions, we determined the probability distribution
functions (PDFs) of the average volume densities of the diffuse gas. The
densities were derived from dispersion measures and HI column densities towards
pulsars and stars at known distances. The PDFs of the average densities of the
diffuse ionized gas (DIG) and the diffuse atomic gas are close to lognormal,
especially when lines of sight at |b|=5 degrees are
considered separately. The PDF of at high |b| is twice as wide as that
at low |b|. The width of the PDF of the DIG is about 30 per cent smaller than
that of the warm HI at the same latitudes. The results reported here provide
strong support for the existence of a lognormal density PDF in the diffuse ISM,
consistent with a turbulent origin of density structure in the diffuse gas.Comment: 5 pages, 3 figures, accepted for publication in MNRAS Letter
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