112 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
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
HALOGAS: HI Observations and Modeling of the Nearby Edge-on Spiral Galaxy NGC 4244
We present 21-cm observations and models of the HI kinematics and
distribution of NGC 4244, a nearby edge-on Scd galaxy observed as part of the
Westerbork Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) survey. Our models
give insight into the HI kinematics and distribution with an emphasis on the
potential existence of extra-planar gas as well as a negative gradient in
rotational velocity with height above the plane of the disk (a lag). Our models
yield strong evidence against a significantly extended halo and instead favor a
warp component along the line of sight as an explanation for some of the
observed thickening of the disk. Based on these models, we detect a lag of -9
+3/-2 km s-1 kpc-1 in the approaching half and -9 +/-2 km s-1 kpc-1 in the
receding half. This lag decreases in magnitude to -5+/-2 km s-1 kpc-1 and
-4+/-2 km s-1 kpc-1 near a radius of 10 kpc in the approaching and receding
halves respectively. Additionally, we detect several distinct morphological and
kinematic features including a shell that is probably driven by star formation
within the disk.Comment: 18 pages, 14 figures, Full resolution version may be found at:
http://www.astron.nl/halogas/papers/NGC4244.Zschaechner.arXiv.p
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
Integral Field Unit Observations of NGC 891: Kinematics of the Diffuse Ionized Gas Halo
We present high and moderate spectral resolution spectroscopy of diffuse
ionized gas (DIG) emission in the halo of NGC 891. The data were obtained with
the SparsePak integral field unit at the WIYN Observatory. The wavelength
coverage includes the [NII]6548,6583, Halpha, and [SII]6716,6731 emission
lines. Position-velocity (PV) diagrams, constructed using spectra extracted
from four SparsePak pointings in the halo, are used to examine the kinematics
of the DIG. Using two independent methods, a vertical gradient in azimuthal
velocity is found to be present in the northeast quadrant of the halo, with
magnitude approximately 15-18 km/s/kpc, in agreement with results from HI
observations. The kinematics of the DIG suggest that this gradient begins at
approximately 1 kpc above the midplane. In another part of the halo, the
southeast quadrant, the kinematics are markedly different, and suggest rotation
at about 175 km/s, much slower than the disk but with no vertical gradient. We
utilize an entirely ballistic model of disk-halo flow in an attempt to
reproduce the kinematics observed in the northeast quadrant. Analysis shows
that the velocity gradient predicted by the ballistic model is far too shallow.
Based on intensity cuts made parallel to the major axis in the ballistic model
and an Halpha image of NGC 891 from the literature, we conclude that the DIG
halo is much more centrally concentrated than the model, suggesting that
hydrodynamics dominate over ballistic motion in shaping the density structure
of the halo. Velocity dispersion measurements along the minor axis of NGC 891
seem to indicate a lack of radial motions in the halo, but the uncertainties do
not allow us to set firm limits.Comment: 31 pages, 10 figures. Accepted for publication in the Astrophysical
Journa
The association of polypharmacy with COVID-19 outcomes independent of comorbidities in people with type 2 diabetes: implications for the unforeseen consequences of polypharmacy
Signatures from a merging galaxy cluster and its AGN population : LOFAR observations of Abell 1682
Reproduced with permission from Astronomy & Astrophysics. © 2019 ESOWe present LOFAR data from 110-180 MHz of the merging galaxy cluster Abell 1682, alongside archival optical, radio, and X-ray data. Our images of 6 arcsec in resolution at low frequencies reveal new structures associated with numerous radio galaxies in the cluster. At a resolution of 20 arcsec we see diffuse emission throughout the cluster over hundreds of kiloparsecs, indicating particle acceleration mechanisms are in play as a result of the cluster merger event and powerful active galactic nuclei. We show that a significant part of the cluster emission is from an old radio galaxy with very steep spectrum emission (having a spectral index of α < -2.5). Furthermore, we identify a new region of diffuse steep-spectrum emission (α < -1.1) as a candidate for a radio halo which is co-spatial with the centre of the cluster merger. We suggest its origin as a population of old and mildly relativistic electrons left over from radio galaxies throughout the cluster which have been re-accelerated to higher energies by shocks and turbulence induced by the cluster merger event. We also note the discovery of six new giant radio galaxies in the vicinity of Abell 1682.Peer reviewedFinal Accepted Versio
Calibrating the relation of low-frequency radio continuum to star formation rate at 1 kpc scale with LOFAR
9 figures, 6 tables and 17 pages. This paper is part of the LOFAR surveys data release 1 and has been accepted for publication in a special edition of A&A that will appear in Feb 2019, volume 622. The catalogues and images from the data release will be publicly available on lofar-surveys.org upon publication of the journal. Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO.Radio continuum (RC) emission in galaxies allows us to measure star formation rates (SFRs) unaffected by extinction due to dust, of which the low-frequency part is uncontaminated from thermal (free-free) emission. We calibrate the conversion from the spatially resolved 140 MHz RC emission to the SFR surface density () at 1 kpc scale. We used recent observations of three galaxies (NGC 3184, 4736, and 5055) from the LOFAR Two-metre Sky Survey (LoTSS), and archival LOw-Frequency ARray (LOFAR) data of NGC 5194. Maps were created with the facet calibration technique and converted to radio maps using the Condon relation. We compared these maps with hybrid maps from a combination of GALEX far-ultraviolet and Spitzer 24 data using plots tracing the relation at -kpc resolution. The RC emission is smoothed with respect to the hybrid owing to the transport of cosmic-ray electrons (CREs). This results in a sublinear relation , where (140 MHz) and (1365 MHz). Both relations have a scatter of . If we restrict ourselves to areas of young CREs (; ), the relation becomes almost linear at both frequencies with and a reduced scatter of . We then simulate the effect of CRE transport by convolving the hybrid maps with a Gaussian kernel until the RC-SFR relation is linearised; CRE transport lengths are -5 kpc. Solving the CRE diffusion equation, we find diffusion coefficients of - at 1 GeV. A RC-SFR relation at GHz can be exploited to measure SFRs at redshift using MHz observations.Peer reviewe
Smooth HI Low Column Density Outskirts In Nearby Galaxies
This is an author-created, un-copyedited version of an article published in The Astronomical Journal. The Version of Record is available online at https://doi.org/10.3847/1538-3881/aabbaa.The low column density gas at the outskirts of galaxies as traced by the 21 cm hydrogen line emission (H i) represents the interface between galaxies and the intergalactic medium, i.e., where galaxies are believed to get their supply of gas to fuel future episodes of star formation. Photoionization models predict a break in the radial profiles of H i at a column density of ∼5 × 10 19 cm -2 due to the lack of self-shielding against extragalactic ionizing photons. To investigate the prevalence of such breaks in galactic disks and to characterize what determines the potential edge of the H i disks, we study the azimuthally averaged H i column density profiles of 17 nearby galaxies from the H i Nearby Galaxy Survey and supplemented in two cases with published Hydrogen Accretion in LOcal GAlaxieS data. To detect potential faint H i emission that would otherwise be undetected using conventional moment map analysis, we line up individual profiles to the same reference velocity and average them azimuthally to derive stacked radial profiles. To do so, we use model velocity fields created from a simple extrapolation of the rotation curves to align the profiles in velocity at radii beyond the extent probed with the sensitivity of traditional integrated H i maps. With this method, we improve our sensitivity to outer-disk H i emission by up to an order of magnitude. Except for a few disturbed galaxies, none show evidence of a sudden change in the slope of the H i radial profiles: the alleged signature of ionization by the extragalactic background.Peer reviewedFinal Accepted Versio
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