123 research outputs found
Dynamics of stellar and HI streams in the Milky Way halo
Stellar streams are key players in many aspects of Milky Way studies and, in
particular, studying their orbital dynamics is crucial for furthering our
understanding of the Milky Way's gravitational potential. Although this is not
a trivial task when faced with incomplete dynamical phase-space information,
transverse motions of streams can nevertheless be comprehended by harnessing
the information contained within their radial velocity gradients. Such methods
are not only applicable to stellar streams, but also to HI streams residing in
the Milky Way halo. Here, I present the results of two studies that use radial
velocity gradients to determine the system's orbit: for Hercules, one of the
'ultra-faint' dwarf galaxies exhibiting a large ellipticity and located at a
distance of 140kpc, showing that it may in fact be a stellar stream, and for a
string of high-velocity HI clouds belonging to the GCN complex, indicating its
likelihood for being a gaseous stream at a distance of approximately 20kpc.Comment: 4 pages, 2 figures, to appear in the proceedings of "Assembling the
Puzzle of the Milky Way", Le Grand-Bornand (Apr 17-22, 2011
Are Complex A and the Orphan Stream related?
We consider the possibility that the Galactic neutral hydrogen stream Complex
A and the stellar Orphan stream are related, and use this hypothesis to
determine possible distances to Complex A and the Orphan stream, and
line-of-sight velocities for the latter. The method presented uses our current
knowledge of the projected positions of the streams, as well as line-of-sight
velocities for Complex A, and we show that a solution exists in which the two
streams share the same orbit. If Complex A and the Orphan stream are on this
orbit, our calculations suggest the Orphan stream to be at an average distance
of 20 kpc, with heliocentric radial velocities of approximately -110 km/s.
Complex A would be ahead of the Orphan stream in the same wrap of the orbit,
with an average distance of 10 kpc, which is consistent with the distance
constraints determined through interstellar absorption line techniques.Comment: 4 pages, 2 figures; typos corrected, fig 2 and numerical predictions
updated; accepted for publication in MNRAS Letter
What can Gaia proper motions tell us about Milky Way dwarf galaxies?
We present a proper-motion study on models of the dwarf spheroidal galaxy
Sculptor, based on the predicted proper-motion accuracy of Gaia measurements.
Gaia will measure proper motions of several hundreds of stars for a
Sculptor-like system. Even with an uncertainty on the proper motion of order
1.5 times the size of an individual proper-motion value of ~10 mas/century, we
find that it is possible to recover Sculptor's systemic proper motion at its
distance of 79 kpc.Comment: 5 pages, 1 figure; to appear in the proceedings of the GREAT-ITN
conference on "The Milky Way Unravelled by Gaia", Barcelona (Dec 1-5 2014
Geometrodynamical Distances to the Galaxy's Hydrogen Streams
We present a geometrodynamical method for determining distances to orbital
streams of HI gas in the Galaxy. The method makes use of our offset from the
Galactic centre and assumes that the gas comprising the stream nearly follows a
planar orbit about the Galactic centre. We apply this technique to the
Magellanic Stream and determine the distances to all points along it; a
consistency check shows that the angular momentum is approximately constant.
Applying this technique to the Large Magellanic Cloud itself gives an
independent distance which agrees within its accuracy of around 10%. Relaxing
the demand for exact conservation of energy and angular momentum at all points
along the stream allows for an increase in orbital period between the lagging
end and the front end led by the Magellanic Clouds. Similar methods are
applicable to other long streams of high-velocity clouds, provided they also
nearly follow planar orbits; these would allow otherwise unknown distances to
be determined.Comment: 9 pages, 7 figures; typos corrected after being accepted by MNRA
GCN: a gaseous Galactic halo stream?
We show that a string of HI clouds that form part of the high-velocity cloud
complex known as GCN is a probable gaseous stream extending over more than 50
deg in the Galactic halo. The radial velocity gradient along the stream is used
to deduce transverse velocities as a function of distance, enabling a family of
orbits to be computed. We find that a direction of motion towards the Galactic
disk coupled with a mid-stream distance of ~20 kpc provides a good match to the
observed sky positions and radial velocities of the HI clouds comprising the
stream. With an estimated mass of 10^5 Msun, its progenitor is likely to be a
dwarf galaxy. However, no stellar counterpart has been found amongst the
currently known Galactic dwarf spheroidal galaxies or stellar streams and the
exact origin of the stream is therefore currently unknown.Comment: 6 pages, 4 figures, accepted for publication in MNRA
Variable stars in the field of the Hydra II ultra-faint dwarf galaxy
We report the discovery of one RR Lyrae star in the ultra--faint satellite
galaxy Hydra II based on time series photometry in the g, r and i bands
obtained with the Dark Energy Camera at Cerro Tololo Interamerican Observatory,
Chile. The RR Lyrae star has a mean magnitude of which
translates to a heliocentric distance of kpc for Hydra II; this
value is larger than the estimate from the discovery paper based on
the average magnitude of several blue horizontal branch star candidates. The
new distance implies a slightly larger half-light radius of pc
and a brighter absolute magnitude of , which keeps this
object within the realm of the dwarf galaxies. The pulsational properties of
the RR Lyrae star ( d, mag) suggest Hydra II may be
a member of the intermediate Oosterhoff or Oosterhoff II group. A comparison
with other RR Lyrae stars in ultra--faint systems indicates similar pulsational
properties among them, which are different to those found among halo field
stars and those in the largest of the Milky Way satellites. We also report the
discovery of 31 additional short period variables in the field of view (RR
Lyrae, SX Phe, eclipsing binaries, and a likely anomalous cepheid). However,
given their magnitudes and large angular separation from Hydra II, they must be
field stars not related to Hydra II.Comment: Revised version after comments from the referee. Accepted for
publication in A
The enigmatic pair of dwarf galaxies Leo IV and Leo V: coincidence or common origin?
We have obtained deep photometry in two 1x1 degree fields covering the close
pair of dwarf spheroidal galaxies (dSph) Leo IV and Leo V and part of the area
in between. We find that both systems are significantly larger than indicated
by previous measurements based on shallower data and also significantly
elongated. With half-light radii of r_h=4'.6 +- 0'.8 (206 +- 36 pc) and
r_h=2'.6 +- 0'.6 (133 +- 31 pc), respectively, they are now well within the
physical size bracket of typical Milky Way dSph satellites. Their ellipticities
of epsilon ~0.5 are shared by many faint (M_V>-8) Milky Way dSphs. The large
spatial extent of our survey allows us to search for extra-tidal features with
unprecedented sensitivity. The spatial distribution of candidate red giant
branch and horizontal branch stars is found to be non-uniform at the ~3 sigma
level. This substructure is aligned along the direction connecting the two
systems, indicative of a possible `bridge' of extra-tidal material. Fitting the
stellar distribution with a linear Gaussian model yields a significance of 4
sigma for this overdensity, a most likely FWHM of ~16 arcmin and a central
surface brightness of ~32 mag arcsec^{-2}. We investigate different scenarios
to explain the close proximity of Leo IV and Leo V and the possible tidal
bridge between them. Orbit calculations demonstrate that they are unlikely to
be remnants of a single disrupted progenitor, while a comparison with
cosmological simulations shows that a chance collision between unrelated
subhalos is negligibly small. Leo IV and Leo V could, however, be a bound
`tumbling pair' if their combined mass exceeds 8 +- 4 x 10^9 M_sun. The
scenario of an internally interacting pair appears to be the most viable
explanation for this close celestial companionship. (abridged)Comment: 9 pages, 8 figures, small number of minor textual changes, accepted
for publication in Astrophysical Journa
Pilot KaVA monitoring on the M87 jet: confirming the inner jet structure and superluminal motions at sub-pc scales
We report the initial results of our high-cadence monitoring program on the
radio jet in the active galaxy M87, obtained by the KVN and VERA Array (KaVA)
at 22 GHz. This is a pilot study that preceded a larger KaVA-M87 monitoring
program, which is currently ongoing. The pilot monitoring was mostly performed
every two to three weeks from December 2013 to June 2014, at a recording rate
of 1 Gbps, obtaining the data for a total of 10 epochs. We successfully
obtained a sequence of good quality radio maps that revealed the rich structure
of this jet from <~1 mas to 20 mas, corresponding to physical scales
(projected) of ~0.1-2 pc (or ~140-2800 Schwarzschild radii). We detected
superluminal motions at these scales, together with a trend of gradual
acceleration. The first evidence for such fast motions and acceleration near
the jet base were obtained from recent VLBA studies at 43 GHz, and the fact
that very similar kinematics are seen at a different frequency and time with a
different instrument suggests these properties are fundamental characteristics
of this jet. This pilot program demonstrates that KaVA is a powerful VLBI array
for studying the detailed structural evolution of the M87 jet and also other
relativistic jets.Comment: 10 pages, 9 figures, accepted for publication in PAS
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