8,729 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
3D simulations of rising magnetic flux tubes in a compressible rotating interior: The effect of magnetic tension
Context: Long-term variability in solar cycles represents a challenging
constraint for theoretical models. Mean-field Babcock-Leighton dynamos that
consider non-instantaneous rising flux tubes have been shown to exhibit
long-term variability in their magnetic cycle. However a relation that
parameterizes the rise-time of non-axisymmetric magnetic flux tubes in terms of
stellar parameters is still missing. Aims: We aim to find a general
parameterization of the rise-time of magnetic flux tubes for solar-like stars.
Methods: By considering the influence of magnetic tension on the rise of
non-axisymmetric flux tubes, we predict the existence of a control parameter
referred as . This parameter is a measure of the
balance between rotational effects and magnetic effects (buoyancy and tension)
acting on the magnetic flux tube. We carry out two series of numerical
experiments (one for axisymmetric rise and one for non-axisymmetric rise) and
demonstrate that indeed controls the rise-time
of magnetic flux tubes. Results: We find that the rise-time follows a power law
of with an exponent that depends on the
azimuthal wavenumber of the magnetic flux loop. Conclusions: Compressibility
does not impact the rise of magnetic flux tubes, while non-axisymmetry does. In
the case of non-axisymmetric rise, the tension force modifies the force balance
acting on the magnetic flux tube. We identified the three independent
parameters required to predict the rise-time of magnetic flux tubes, that is,
the stellar rotation rate, the magnetic flux density of the flux tube, and its
azimuthal wavenumber. We combined these into one single relation that is valid
for any solar-like star. We suggest using this generalized relation to
constrain the rise-time of magnetic flux tubes in Babcock-Leighton dynamo
models.Comment: 18 pages, 15 figures, 6 tabula
Optical full Hadamard matrix multiplexing and noise effects
Hadamard multiplexing provides a considerable SNR boost over additive random noise but Poisson noise such as photon noise reduces the boost. We develop the theory for full H-matrix Hadamard transform imaging under additive and Poisson noise effects. We show that H-matrix encoding results in no effect on average on the noise level due to Poisson noise sources while preferentially reducing additive noise. We use this result to explain the wavelength-dependent varying SNR boost in a Hadamard hyperspectral imager and argue that such a preferential boost is useful when the main noise source is indeterminant or varying
On the Treatment of Non-Original Sample Members in the German Household Panel Study (SOEP): Tracing, Weighting, and Frequencies
In this paper we discuss the rationale for tracing non-original sample members (Non-OSMs) in household panel studies, and in particular in SOEP, and the implications for weighting. We present results on the incidence, survival rates, and thus the relevance of Non-OSMs in the SOEP
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