144 research outputs found
The dark matter halo shape of edge-on disk galaxies - I. HI observations
This is the first paper of a series in which we will attempt to put
constraints on the flattening of dark halos in disk galaxies. We observe for
this purpose the HI in edge-on galaxies, where it is in principle possible to
measure the force field in the halo vertically and radially from gas layer
flaring and rotation curve decomposition respectively. In this paper, we define
a sample of 8 HI-rich late-type galaxies suitable for this purpose and present
the HI observations.Comment: Accepted for publication by Astronomy & Astrophysics. For a higher
resolution version see
http://www.astro.rug.nl/~vdkruit/jea3/homepage/12565.pd
Constraints on the halo density profile using HI flaring in the outer Galaxy
The observed flaring of HI disk in the outer region of galaxies has been used
in the past to determine the shape of the dark matter halo. Previous studies
based on this concept suggest a slightly oblate halo (axis ratio ~ 0.8) for our
Galaxy. We reinvestigate this problem by calculating the HI scaleheight in the
outer Galaxy to a larger radial distance, and by studying its dependence on the
shape and the density profile of the halo. We find that a simple isothermal
infinite halo of any shape- oblate or prolate, is not able to account for the
observed flaring. Instead we show that a spherical halo with density falling
faster than isothermal halo in the outer region provides a better fit to the
observed HI flaring as well as the observed rotation curve of our Galaxy. These
halos have about 95% of their mass within a few hundreds of kpc. For R_solar =
8.5 kpc and \Theta_solar = 220 km/s, the central density and core radius can be
constrained to the range \rho_0 = 0.035 - 0.06 M_sun/pc^3 and R_c = 8 - 10 kpc.
Our claim for such 'finite-sized' spherical halos is supported by recent
literature on numerical simulation studies of halo formation as well as
analyses of SDSS data.Comment: Accepted for publication in A &
The dark matter halo shape of edge-on disk galaxies - II. Modelling the HI observations: methods
This is the second paper of a series in which we attempt to put constraints
on the flattening of dark halos in disk galaxies. For this purpose, we observe
the HI in edge-on galaxies, where it is in principle possible to measure the
force field in the halo vertically and radially from gas layer flaring and
rotation curve decomposition respectively. To calculate the force fields, we
need to analyse the observed XV diagrams to accurately measure all three
functions that describe the planar kinematics and distribution of a galaxy: the
radial HI surface density, the rotation curve and the HI velocity dispersion.
In this paper, we discuss the improvements and limitations of the methods
previously used to measure these HI properties. We extend the constant velocity
dispersion method to include determination of the HI velocity dispersion as a
function of galactocentric radius and perform extensive tests on the quality of
the fits. We will apply this 'radial decomposition XV modelling method' to our
HI observations of 8 HI-rich, late-type, edge-on galaxies in the third paper of
this series.Comment: Accepted for publication by Astronomy & Astrophysics. For a higher
resolution version see
http://www.astro.rug.nl/~vdkruit/jea3/homepage/12566.pd
Case Notes
For decades, optical time-domain searches have been tuned to find ordinary supernovae, which rise and fall in brightness over a period of weeks. Recently, supernova searches have improved their cadences and a handful of fast-evolving luminous transients have been identified(1-5). These have peak luminosities comparable to type Ia supernovae, but rise to maximum in less than ten days and fade from view in less than one month. Here we present the most extreme example of this class of object thus far: KSN 2015K, with a rise time of only 2.2 days and a time above half-maximum of only 6.8 days. We show that, unlike type Ia supernovae, the light curve of KSN 2015K was not powered by the decay of radioactive elements. We further argue that it is unlikely that it was powered by continuing energy deposition from a central remnant (a magnetar or black hole). Using numerical radiation hydrodynamical models, we show that the light curve of KSN 2015K is well fitted by a model where the supernova runs into external material presumably expelled in a pre-supernova mass-loss episode. The rapid rise of KSN 2015K therefore probes the venting of photons when a hypersonic shock wave breaks out of a dense extended medium.NASA
NNH15ZDA001N
NNX17AI64G
Australian Research Council Centre of Excellence for All-sky Astrophysics
CE11000102
The great disk of Milky-Way satellites and cosmological sub-structures
We show that the shape of the observed distribution of Milky Way (MW)
satellites is inconsistent with being drawn from a cosmological sub-structure
population with a confidence of 99.5 per cent. Most of the MW satellites
therefore cannot be related to dark-matter dominated satellites.Comment: accepted by A&A, 5 page
The spiral structure of the Galaxy revealed by CS sources and evidence for the 4:1 resonance
We present a map of the spiral structure of the Galaxy, as traced by
molecular CS emission associated with IRAS sources which are believed to be
compact HII regions. The CS line velocities are used to determine the kinematic
distances of the sources, in order to investigate their distribution in the
galactic plane. This allows us to use 870 objects to trace the arms, a number
larger than that of previous studies based on classical HII regions. The
distance ambiguity of the kinematic distances, when it exists, is solved by
different procedures, including the latitude distribution and an analysis of
the longitude-velocity diagram. The well defined spiral arms are seen to be
confined inside the co-rotation radius, as is often the case in spiral
galaxies. We identify a square-shaped sub-structure in the CS map with that
predicted by stellar orbits at the 4:1 resonance (4 epicycle oscillations in
one turn around the galactic center). The sub-structure is found at the
expected radius, based on the known pattern rotation speed and epicycle
frequency curve. An inner arm presents an end with strong inward curvature and
intense star formation that we tentatively associate with the region where this
arm surrounds the extremity of the bar, as seen in many barred galaxies.
Finally, a new arm with concave curvature is found in the Sagitta to Cepheus
region of the sky
Self-consistent nonspherical isothermal halos embedding zero-thickness disks
Disk-halo decompositions of galaxy rotation curves are generally performed in
a parametric way. We construct self-consistent models of nonspherical
isothermal halos embedding a zero-thickness disk, by assuming that the halo
distribution function is a Maxwellian. The method developed here can be used to
study other physically-based choices for the halo distribution function and the
case of a disk accompanied by a bulge. In a preliminary investigation we note
the existence of a fine tuning between the scalelengths R_{\Omega} and h,
respectively characterizing the rise of the rotation curve and the luminosity
profile of the disk, which surprisingly applies to both high surface brightness
and low surface brightness galaxies. This empirical correlation identifies a
much stronger conspiracy than the one required by the smoothness and flatness
of the rotation curve (disk-halo conspiracy). The self-consistent models are
characterized by smooth and flat rotation curves for very different
disk-to-halo mass ratios, hence suggesting that conspiracy is not as dramatic
as often imagined. For a typical rotation curve, with asymptotically flat
rotation curve at V_{\infty} (the precise value of which can also be treated as
a free parameter), and a typical density profile of the disk, self-consistent
models are characterized by two dimensionless parameters, which correspond to
the dimensional scales (the disk mass-to-light ratio M/L and the halo central
density) of standard disk-halo decompositions. We show that if the rotation
curve is decomposed by means of our self-consistent models, the disk-halo
degeneracy is removed and typical rotation curves are fitted by models that are
below the maximum-disk prescription. Similar results are obtained from a study
of NGC 3198. Finally, we quantify the flattening of the spheroidal halo, which
is significant, especially on the scale of the visible disk.Comment: accepted for publication in A&
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