696 research outputs found
Resolution of the Compact Radio Continuum Sources in Arp220
We present 2 cm and 3.6 cm wavelength very long baseline interferometry
images of the compact radio continuum sources in the nearby ultra-luminous
infrared galaxy Arp220. Based on their radio spectra and variability
properties, we confirm these sources to be a mixture of supernovae (SNe) and
supernova remnants (SNRs). Of the 17 detected sources we resolve 7 at both
wavelengths. The SNe generally only have upper size limits. In contrast all the
SNRs are resolved with diameters {\geq} 0.27 pc. This size limit is consistent
with them having just entered their Sedov phase while embedded in an
interstellar medium (ISM) of density 10^4 cm^{-3} . These objects lie on the
diameter-luminosity correlation for SNRs (and so also on the diameter-surface
brightness relation) and extend these correlations to very small sources. The
data are consistent with the relation L {\propto} D^{-9/4}. Revised
equipartition arguments adjusted to a magnetic field to relativistic particle
energy density ratio of 1% combined with a reasonable synchrotron-emitting
volume filling factor of 10% give estimated magnetic field strengths in the SNR
shells of ~ 15-50 mG. The SNR shell magnetic fields are unlikely to come from
compression of ambient ISM fields and must instead be internally generated. We
set an upper limit of 7 mG for the ISM magnetic field. The estimated energy in
relativistic particles, 2%-20% of the explosion kinetic energy, is consistent
with estimates from models that fit the IR-radio correlation in compact
starburst galaxies.Comment: 16 pages, 5 figure
Dependence of radio halo properties on star formation activity and galaxy mass
We investigate the relation between the existence and size of radio halos,
which are believed to be created by star formation (SF) related energy input
into the interstellar medium, and other galaxy properties, most importantly
star formation activity and galaxy mass. Based on radio continuum and H-alpha
observations of a sample of seven late-type spiral galaxies we find a direct,
linear correlation of the radial extent of gaseous halos on the size of the
actively star-forming parts of the galaxy disks. Data of a larger sample of 22
galaxies indicate that the threshold energy input rate into the disk ISM per
unit surface area for the creation of a gaseous halo depends on the mass
surface density of the galaxy, in the sense that a higher threshold must be
surpassed for galaxies with a higher surface density. Because of the good
prediction of the existence of a radio halo from these two parameters, we
conclude that they are important, albeit not the only contributors. The
compactness of the SF-related energy input is also found to be a relevant
factor. Galaxies with relatively compact SF distributions are more likely to
have gaseous halos than others with more widespread SF activity. These results
quantify the so-called "break-out" condition for matter to escape from galaxy
disks, as used in all current models of the interstellar medium and first
defined by Norman and Ikeuchi (1989).Comment: accepted for publication in Astronomy & Astrophysic
Manned Mars landing missions using electric propulsion
Manned Mars landing missions using electric propulsion - evaluation of various mission profile
Imaging Fabry-Perot Spectroscopy of NGC 5775: Kinematics of the Diffuse Ionized Gas Halo
We present imaging Fabry-Perot observations of Halpha emission in the nearly
edge-on spiral galaxy NGC 5775. We have derived a rotation curve and a radial
density profile along the major axis by examining position-velocity (PV)
diagrams from the Fabry-Perot data cube as well as a CO 2-1 data cube from the
literature. PV diagrams constructed parallel to the major axis are used to
examine changes in azimuthal velocity as a function of height above the
midplane. The results of this analysis reveal the presence of a vertical
gradient in azimuthal velocity. The magnitude of this gradient is approximately
1 km/s/arcsec, or about 8 km/s/kpc, though a higher value of the gradient may
be appropriate in localized regions of the halo. The evidence for an azimuthal
velocity gradient is much stronger for the approaching half of the galaxy,
although earlier slit spectra are consistent with a gradient on both sides.
There is evidence for an outward radial redistribution of gas in the halo. The
form of the rotation curve may also change with height, but this is not
certain. We compare these results with those of an entirely ballistic model of
a disk-halo flow. The model predicts a vertical gradient in azimuthal velocity
which is shallower than the observed gradient, indicating that an additional
mechanism is required to further slow the rotation speeds in the halo.Comment: 18 pages, 18 figures. Uses emulateapj.cls. Accepted for publication
in Ap
The multi-phase gaseous halos of star forming late-type galaxies - II. Statistical analysis of key parameters
In Paper I we showed that multi-phase gaseous halos of late-type spiral
galaxies, detected in the radio continuum, in Halpha, and in X-rays, are
remarkably well correlated regarding their morphology and spatial extent. In
this work we present new results from a statistical analysis in order to
specify and quantify these phenomenological relations. This is accomplished by
investigating soft X-ray (0.3-2.0keV) luminosities, FIR, radio continuum,
Halpha, B-band, and UV luminosities for a sample of 23 edge-on late-type spiral
galaxies. Typical star formation indicators, such as SFRs, are determined and a
statistical correlation analysis is carried out. We find strong linear
correlations, covering at least two orders of magnitude, between star formation
indicators and integrated (disk+halo) luminosities in all covered wavebands. In
addition to the well established L_FIR/L_1.4GHz-relation, we show new and
highly significant linear dependencies between integrated soft X-ray
luminosities and FIR, radio continuum, Halpha, B-band, and UV luminosities.
Moreover, integrated soft X-ray luminosities correlate well with SFRs and the
energy input into the ISM by SNe. The same holds if these quantities are
plotted against soft halo X-ray luminosities. Only a weak correlation exists
between the dust mass of a galaxy and the corresponding X-ray luminosity. Among
soft X-ray luminosities, baryonic, and HI-gas masses, no significant
correlations are found. There seems to exist a critical input energy by SNe
into the ISM or a SFR threshold for multi-phase halos to show up. It is still
not clear whether this threshold is a physical or an instrument dependent
sensitivity limit. These findings strongly support our previous results, but
conflict with the concept of halos being due to infalling gas from the IGM.Comment: 8 pages, 3 figures, 1 table, accepted for publication in A&
- âŠ