40,257 research outputs found
The far-infrared - radio correlation in dwarf galaxies
The far-infrared - radio correlation connects star formation and magnetic
fields in galaxies, and has been confirmed over a large range of far-infrared
luminosities. Recent investigations indicate that it may even hold in the
regime of local dwarf galaxies, and we explore here the expected behavior in
the regime of star formation surface densities below 0.1 M_sun kpc^{-2}
yr^{-1}. We derive two conditions that can be particularly relevant for
inducing a change in the expected correlation: a critical star formation
surface density to maintain the correlation between star formation rate and the
magnetic field, and a critical star formation surface density below which
cosmic ray diffusion losses dominate over their injection via supernova
explosions. For rotation periods shorter than 1.5x10^7 (H/kpc)^2 yrs, with H
the scale height of the disk, the first correlation will break down before
diffusion losses are relevant, as higher star formation rates are required to
maintain the correlation between star formation rate and magnetic field
strength. For high star formation surface densities Sigma_SFR, we derive a
characteristic scaling of the non-thermal radio to the far-infrared / infrared
emission with Sigma_SFR^{1/3}, corresponding to a scaling of the non-thermal
radio luminosity L_s with the infrared luminosity L_{th} as L_{th}^{4/3}. The
latter is expected to change when the above processes are no longer steadily
maintained. In the regime of long rotation periods, we expect a transition
towards a steeper scaling with Sigma_SFR^{2/3}, implying L_s~L_th^{5/3}, while
the regime of fast rotation is expected to show a considerably enhanced
scatter. These scaling relations explain the increasing thermal fraction of the
radio emission observed within local dwarfs, and can be tested with future
observations by the SKA and its precursor radio telescopes.Comment: 16 pages, 11 figures, accepted at A&
A new interpretation of the far-infrared - radio correlation and the expected breakdown at high redshift
(Abrigded) Observations of galaxies up to z 2 show a tight correlation
between far-infrared and radio continuum emission. We explain the far-infrared
- radio continuum correlation by relating star formation and magnetic field
strength in terms of turbulent magnetic field amplification, where turbulence
is injected by supernova explosions from massive stars. We calculate the
expected amount of turbulence in galaxies based on their star formation rates,
and infer the expected magnetic field strength due to turbulent dynamo
amplification. We estimate the timescales for cosmic ray energy losses via
synchrotron emission, inverse Compton scattering, ionization and bremsstrahlung
emission, probing up to which redshift strong synchrotron emission can be
maintained. We find that the correlation between star formation rate and
magnetic field strength in the local Universe can be understood as a result of
turbulent magnetic field amplification. If the typical gas density in the
interstellar medium increases at high z, we expect an increase of the magnetic
field strength and the radio emission, as indicated by current observations.
Such an increase would imply a modification of the far-infrared - radio
correlation. We expect a breakdown when inverse Compton losses start dominating
over synchrotron emission. For a given star formation surface density, we
calculate the redshift where the breakdown occurs, yielding z (Sigma_SFR/0.0045
M_solar kpc^{-2} yr^{-1})^{1/(6-alpha/2)}. In this relation, the parameter
\alpha describes the evolution of the characteristic ISM density in galaxies as
(1+z)^\alpha. Both the possible raise of the radio emission at high redshift
and the final breakdown of the far-infrared -- radio correlation at a critical
redshift will be probed by the Square Kilometre Array (SKA) and its
pathfinders, while the typical ISM density in galaxies will be probed with
ALMA.Comment: 13 pages, 14 figures, 1 table, accepted at A&A (proof corrections
included
Can spicules be detected at disc centre in broad-band Ca II H filter imaging data ?
We estimate the formation height range contributing to broad-band and
narrow-band filter imaging data in Ca II H to investigate whether spicules can
be detected in such observations at the centre of the solar disc. We apply
spectral filters of FWHMs from 0.03 nm to 1 nm to observed Ca line profiles to
simulate Ca imaging data. We estimate the relative intensity contributions of
off-limb and on-disc structures. We compare the synthetic Ca filter imaging
data with intensity maps of Ca spectra at different wavelengths and temperature
maps at different optical depths. We determine the intensity response function
for the wavelengths covered by the filters of different FWHM.
The intensity emitted off the solar limb is about 5% of the intensity at disc
centre. For a 0.3 nm-wide Ca II H filter, up to about 1/3 of the off-limb
intensity comes from emission in Hepsilon. On the disc, only about 15% of the
intensity transmitted through a broad-band filter comes from the line-core
region. No traces of elongated fibrillar structures are visible in imaging data
at disc centre, opposite to the line-core images of the Ca spectra. The
response function for a 0.3 nm-wide filter peaks at about 200 km. Relative
contributions from atmospheric layers above 800 km are about 10%. The inversion
results suggest that the slightly enhanced emission around the photospheric
magnetic network in broad-band Ca imaging data is caused by a thermal canopy at
a height of about 600 km. Broad-band Ca II H imaging data do not trace upper
chromospheric structures such as spicules in observations at the solar disc
because of the too small relative contribution of the line core to the total
wavelength-integrated filter intensity.Comment: 10 pages, 11 figures, accepted for publication in A&
Transition from a mixed to a pure d-wave symmetry in superconducting optimally doped YBaCuO thin films under applied fields
We have probed the Landau levels of nodal quasi-particles by tunneling along
a nodal direction of (110) oriented YBaCuO thin films with a
magnetic field applied perpendicular to the planes, and parallel to the
film's surface. In optimally doped films and at low temperature, finite energy
nodal states are clearly observed in films thinner than the London penetration
depth. Above a well defined temperature the order parameter reverts to a pure
\emph{d}-wave symmetry.Comment: 4 pages, 4 figure
Linear response theory around a localized impurity in the pseudogap regime of an anisotropic superconductor: precursor pairing vs the d-density-wave scenario
We derive the polarizability of an electron system in (i) the superconducting
phase, with d-wave symmetry, (ii) the pseudogap regime, within the precursor
pairing scenario, and (iii) the d-density-wave (dDW) state, characterized by a
d-wave hidden order parameter, but no pairing. Such a calculation is motivated
by the recent proposals that imaging the effects of an isolated impurity may
distinguish between precursor pairing and dDW order in the pseudogap regime of
the high-Tc superconductors. In all three cases, the wave-vector dependence of
the polarizability is characterized by an azymuthal modulation, consistent with
the d-wave symmetry of the underlying state. However, only the dDW result shows
the fingerprints of nesting, with nesting wave-vector Q=(pi,pi), albeit
imperfect, due to a nonzero value of the hopping ratio t'/t in the band
dispersion relation. As a consequence of nesting, the presence of hole pockets
is also exhibited by the (q,omega) dependence of the retarded polarizability.Comment: accepted in Phys. Rev.
Spectroscopy at the solar limb: II. Are spicules heated to coronal temperatures ?
Spicules of the so-called type II were suggested to be relevant for coronal
heating because of their ubiquity on the solar surface and their eventual
extension into the corona. We investigate whether solar spicules are heated to
transition-region or coronal temperatures and reach coronal heights (>6 Mm)
using multi-wavelength observations of limb spicules in different chromospheric
spectral lines (Ca II H, Hepsilon, Halpha, Ca II IR at 854.2 nm, He I at 1083
nm). We determine the line width of individual spicules and throughout the
field of view and estimate the maximal height that different types of off-limb
features reach. We derive estimates of the kinetic temperature and the
non-thermal velocity from the line width of spectral lines from different
chemical elements. We find that most regular spicules reach a maximal height of
about 6 Mm above the solar limb. The majority of features found at larger
heights are irregularly shaped with a significantly larger lateral extension
than spicules. Both individual and average line profiles in all spectral lines
show a decrease in their line width with height above the limb with very few
exceptions. Both the kinetic temperature and the non-thermal velocity decrease
with height above the limb. We find no indications that the spicules in our
data reach coronal heights or transition-region or coronal temperatures.Comment: Accepted for publication in Solar Physics, 52 pages, 32 figure
Remarkable change of tunneling conductance in YBCO films in fields up to 32.4T
We studied the tunneling density of states in YBCO films under strong
currents flowing along node directions. The currents were induced by fields of
up to 32.4T parallel to the film surface and perpendicular to the
planes. We observed a remarkable change in the tunneling conductance at high
fields where the gap-like feature shifts discontinuously from 15meV to a lower
bias of 11meV, becoming more pronounced as the field increases. The effect
takes place in increasing fields around 9T and the transition back to the
initial state occurs around 5T in decreasing fields. We argue that this
transition is driven by surface currents induced by the applied magnetic field.Comment: 4 pages, 7 figure
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