319 research outputs found
HALOGAS: HI Observations and Modeling of the Nearby Edge-on Spiral Galaxy NGC 4565
We present 21-cm observations and models of the neutral hydrogen in NGC 4565,
a nearby, edge-on spiral galaxy, as part of the Westerbork Hydrogen Accretion
in LOcal GAlaxieS (HALOGAS) survey. These models provide insight concerning
both the morphology and kinematics of HI above, as well as within, the disk.
NGC 4565 exhibits a distinctly warped and asymmetric disk with a flaring layer.
Our modeling provides no evidence for a massive, extended HI halo. We see
evidence for a bar and associated radial motions. Additionally, there are
indications of radial motions within the disk, possibly associated with a ring
of higher density. We see a substantial decrease in rotational velocity with
height above the plane of the disk (a lag) of -40 +5/-20 km/s/kpc and -30
+5/-30 km s/kpc in the approaching and receding halves, respectively. This lag
is only seen within the inner ~4.75' (14.9 kpc) on the approaching half and
~4.25' (13.4 kpc) on the receding, making this a radially shallowing lag, which
is now seen in the HI layers of several galaxies. When comparing results for
NGC 4565 and those for other galaxies, there are tentative indications of high
star formation rate per unit area being associated with the presence of a halo.
Finally, HI is found in two companion galaxies, one of which is clearly
interacting with NGC 4565.Comment: 17 pages, 16 figures, accepted for publication in the Astrophysical
Journal, modified affiliatio
The radial variation of HI velocity dispersions in dwarfs and spirals
Gas velocity dispersions provide important diagnostics of the forces
counteracting gravity to prevent collapse of the gas. We use the 21 cm line of
neutral atomic hydrogen (HI) to study HI velocity dispersion and HI phases as a
function of galaxy morphology in 22 galaxies from The HI Nearby Galaxy Survey
(THINGS). We stack individual HI velocity profiles and decompose them into
broad and narrow Gaussian components. We study the HI velocity dispersion and
the HI surface density, as a function of radius. For spirals, the velocity
dispersions of the narrow and broad components decline with radius and their
radial profiles are well described by an exponential function. For dwarfs,
however, the profiles are much flatter. The single Gaussian dispersion profiles
are, in general, flatter than those of the narrow and broad components. In most
cases, the dispersion profiles in the outer disks do not drop as fast as the
star formation profiles, derived in the literature. This indicates the
importance of other energy sources in driving HI velocity dispersion in the
outer disks. The radial surface density profiles of spirals and dwarfs are
similar. The surface density profiles of the narrow component decline more
steeply than those of the broad component, but not as steep as what was found
previously for the molecular component. As a consequence, the surface density
ratio between the narrow and broad components, an estimate of the mass ratio
between cold HI and warm HI, tends to decrease with radius. On average, this
ratio is lower in dwarfs than in spirals. This lack of a narrow, cold HI
component in dwarfs may explain their low star formation activity.Comment: Accepted for publication in The Astronomical Journal, 13 pages, 10
figures, 4 table
A large-scale, regular intergalactic magnetic field associated with Stephan's Quintet?
Regular magnetic fields are frequently found within and in the outskirts of
galaxies, but their presence, properties, and origin has not yet been
established for galaxy groups. On the basis of broadband radio polarimetric
imaging with the Westerbork Synthesis Radio Telescope (WSRT), we made use of
Rotation Measure Synthesis to disentangle contributions from magnetic fields on
various scales for several polarised radio sources inside, behind, or in the
vicinity of the Stephan's Quintet (HCG92, SQ). We recognise the signature of a
large-scale, genuinely regular, magnetised screen, seemingly constrained to the
Quintet itself. Although we cannot exclude a contribution from the Milky Way,
our analysis favours a magnetic structure within the SQ system. If indeed
associated with the galaxy group in question, it would span a volume of at
least and have a strength at least
as high as that previously detected within large spiral galaxies. This field
would then surpass the extent of any other known galactic, regular magnetic
fields, have a considerable strength of a few microgauss, and would be the
first known example of such a structure in a galaxy system other than a galaxy
pair. Several other explanations are also presented and evaluated.Comment: 13 pages, 7 figures, 3 table
CHANG-ES V: Nuclear Radio Outflow in a Virgo Cluster Spiral after a Tidal Disruption Event
We have observed the Virgo Cluster spiral galaxy, NGC~4845, at 1.6 and 6 GHz
using the Karl G. Jansky Very Large Array, as part of the `Continuum Halos in
Nearby Galaxies -- an EVLA Survey' (CHANG-ES). The source consists of a bright
unresolved core with a surrounding weak central disk (1.8 kpc diameter). The
core is variable over the 6 month time scale of the CHANG-ES data and has
increased by a factor of 6 since 1995. The wide bandwidths of
CHANG-ES have allowed us to determine the spectral evolution of this core which
peaks {\it between} 1.6 and 6 GHz (it is a GigaHertz-peaked spectrum source).We
show that the spectral turnover is dominated by synchrotron self-absorption and
that the spectral evolution can be explained by adiabatic expansion (outflow),
likely in the form of a jet or cone. The CHANG-ES observations serendipitously
overlap in time with the hard X-ray light curve obtained by Nikolajuk \& Walter
(2013) which they interpret as due to a tidal disruption event (TDE) of a
super-Jupiter mass object around a black hole. We outline a
standard jet model, provide an explanation for the observed circular
polarization, and quantitatively suggest a link between the peak radio and peak
X-ray emission via inverse Compton upscattering of the photons emitted by the
relativistic electrons. We predict that it should be possible to resolve a
young radio jet via VLBI as a result of this nearby TDE.Comment: 45 pages, 10 figures, accepted July 2, 2015 to the Astrophysical
Journa
Accurate Recovery of H i Velocity Dispersion from Radio Interferometers
Gas velocity dispersion measures the amount of disordered motion of a rotating disk. Accurate estimates of this parameter are of the utmost importance because the parameter is directly linked to disk stability and star formation. A global measure of the gas velocity dispersion can be inferred from the width of the atomic hydrogen (H I) 21 cm line. We explore how several systematic effects involved in the production of H I cubes affect the estimate of H I velocity dispersion. We do so by comparing the H I velocity dispersion derived from different types of data cubes provided by The H I Nearby Galaxy Survey. We find that residual-scaled cubes best recover the H I velocity dispersion, independent of the weighting scheme used and for a large range of signal-to-noise ratio. For H I observations, where the dirty beam is substantially different from a Gaussian, the velocity dispersion values are overestimated unless the cubes are cleaned close to (e.g., ˜1.5 times) the noise level
An outburst of SS 433 observed on milliarcsecond scale
SS 433 is a high-mass X-ray binary system (XRB) and one of the most persistent sources of relativistic jets in the Milky Way. The object has been intensively studied in radio at arcsecond scales, however the high-resolution observations (i.e. VLBI) are relatively scarce. In 2008 November the system was in outburst. Using the e-VLBI capabilities of the European VLBI Network (EVN) we observed SS 433 for three epochs during the active phase. The data offered a detailed view of the system’s behaviour in outburst at milliarcsecond scales. We used the “kinematic model" (which predicts the position along the jet of any knot ejected at some particular time in the past) to investigate the dynamic parameters of SS 433 and we examined the polarization properties of the ejected material. We report here the preliminary results
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