32 research outputs found
The four leading arms of the Magellanic Cloud system
The Magellanic Cloud System (MCS) interacts via tidal and drag forces with
the Milky Way galaxy. Using the Parkes Galactic All-Sky Survey (GASS) of atomic
hydrogen we explore the role of drag on the evolution of the so-called Leading
Arm (LA). We present a new image recognition algorithm that allows us to
differentiate features within a 3-D data cube (longitude, latitude, radial
velocity) and to parameterize individual coherent structures. We compiled an HI
object catalog of LA objects within an area of 70 degr x 85 degr (1.6 sr) of
the LA region. This catalog comprises information of location, column density,
line width, shape and asymmetries of the individual LA objects above the
4-sigma threshold of Delta T_b simeq 200 mK. We present evidence of a fourth
arm segment (LA4). For all LA objects we find an inverse correlation of
velocities v_GSR in Galactic Standard of Rest frame with Magellanic longitude.
High-mass objects tend to have higher radial velocities than low-mass ones.
About 1/4 of all LA objects can be characterized as head-tail (HT) structures.
Using image recognition with objective criteria, it is feasible to isolate most
of LA emission from the diffuse Milky Way HI gas. Some blended gas components
(we estimate 5%) escape detection, but we find a total gas content of the LA
that is about 50% higher than previously assumed. These methods allow the
deceleration of the LA clouds to be traced towards the Milky Way disk by drag
forces. The derived velocity gradient strongly supports the assumption that the
whole LA originates entirely in the Large Magellanic Cloud (LMC). LA4 is
observed opposite to LA1, and we propose that both arms are related, spanning
about 52kpc in space. HT structures trace drag forces even at tens of kpc
altitudes above the Milky Way disk.Comment: 12 pages, 7 figures, 2 tables, accepted for publication Astronomy &
Astrophysics 201
Do High-Velocity Clouds trace the Dark Matter subhalo population?
Within the cosmological concordance model, Cold Dark Matter (CDM) subhalos
form the building blocks which merge hierarchically to more massive galaxies.
Since intergalactic gas is accreted by massive galaxies, observable e.g. as
high- velocity clouds (HVCs) around the Milky Way, with extremely low
metallicities, these can be suggested to represent the baryonic content of
primordial Dark Matter (DM) subhalos. Another possibility of their origin is
that they stem from disrupted satellite galaxies, but in this case, these gas
clouds move unaccompanied by a bound DM structure. Since HVCs are observed with
long gas tails and with irregular substructures, numerical models are performed
aiming at exploring their structure and compare them with observations. If HVCs
are engulfed by DM subhalos, their gas must leave the DM gravitational
potential and reflect this in their dynamics. On the other hand, the evolution
and survival of pure gas models must be tested to distinguish between
DM-dominated and DM-free clouds and to allow conclusions on their origin. The
models demonstrate that purely baryonic HVCs with low masses are disrupted by
ram-pressure stripping and Kelvin-Helmholtz instabilities, while more massive
ones survive, losing their initially spherical shape and develop significant
substructures including cometary elongations in the column density distribution
("head-tail structure"). On the contrary, HVCs with DM subhalos survive with
more than 90% of their gas mass still bound and spherically shaped, approaching
the Galactic disk like bullets. In addition, we find that velocity gradients
along the cometary head-tail structures does not necessarily offer a
possibility to distinguish between DM-dominated and purely gaseous HVCs.
Comparison of models with observations let us conclude that HVCs are not
embedded in a DM substructure and do not trace the cosmological subhalo
population.Comment: Accepted for publication in A&
Physical Properties of Complex C Halo Clouds
Observations from the Galactic Arecibo L-Band Feed Array HI (GALFA-HI) Survey
of the tail of Complex C are presented and the halo clouds associated with this
complex cataloged. The properties of the Complex C clouds are compared to
clouds cataloged at the tail of the Magellanic Stream to provide insight into
the origin and destruction mechanism of Complex C. Magellanic Stream and
Complex C clouds show similarities in their mass distributions (slope = -0.7
and -0.6, respectively) and have a common linewidth of 20 - 30 km/s (indicative
of a warm component), which may indicate a common origin and/or physical
process breaking down the clouds. The clouds cataloged at the tail of Complex C
extend over a mass range of 10^1.1 to 10^4.8 solar masses, sizes of 10^1.2 to
10^2.6 pc, and have a median volume density of 0.065 cm^(-3) and median
pressure of (P/k) = 580 K cm^{-3}. We do not see a prominent two-phase
structure in Complex C, possibly due to its low metallicity and inefficient
cooling compared to other halo clouds. From assuming the Complex C clouds are
in pressure equilibrium with a hot halo medium, we find a median halo density
of 5.8 x 10^(-4) cm^(-3), which given a constant distance of 10 kpc, is at a
z-height of ~3 kpc. Using the same argument for the Stream results in a median
halo density of 8.4 x 10^(-5) x (60kpc/d) cm^(-3). These densities are
consistent with previous observational constraints and cosmological
simulations. We also assess the derived cloud and halo properties with three
dimensional grid simulations of halo HI clouds and find the temperature is
generally consistent within a factor of 1.5 and the volume densities, pressures
and halo densities are consistent within a factor of 3.Comment: Accepted for publication in AJ. 54 pages, including 6 tables and 16
figure
ROSAT X-ray sources in the field of the LMC I.Total LMC gas from the background AGN spectral fits
We analyzed a sample of 26 background X-ray sources in a ~60 square degree
field of the Large Magellanic Cloud observed with the ROSAT PSPC. The sample
has been selected from previously classified and optically identified X-ray
sources. In addition pointlike and spectrally hard sources with at least 100 to
200 observed counts have been used for the analysis. We performed X-ray
spectral fitting and derived total hydrogen absorbing column densities due to
LMC gas in the range 10^20 - 2. 10^21 cm^-2. We compared these columns with the
HI columns derived from a 21-cm Parkes survey of the LMC. For 7 optically
identified sources we find, within the uncertainties derived from the X-ray
spectral fit, agreement for both columns. For further 19 sources we constrain
the LMC columns from the X-ray spectral fit assuming that the powerlaw photon
index is that of AGN type spectra. We derive for 20 sources gas columns which
are within the uncertainties in agreement with the HI columns. We derive for
two background sources (RX J0536.9-6913 and RX J0547.0-7040) hydrogen absorbing
column densities due to LMC gas, which are in excess to the HI columns. These
sources - located in regions of large (~3. 10^21 cm^-2) LMC HI column densities
- could be seen through additional gas which may be warm and diffuse, cold or
molecular. For 10 sources we derive upper limits for the gas columns additional
to HI and constrain the molecular mass fraction to <(30-140)%.Comment: Accepted by A&
The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way
It has been known for a long time that the satellite galaxies of the Milky
Way (MW) show a significant amount of phase-space correlation, they are
distributed in a highly inclined Disc of Satellites (DoS). We have extended the
previous studies on the DoS by analysing for the first time the orientations of
streams of stars and gas, and the distributions of globular clusters within the
halo of the MW. It is shown that the spatial distribution of MW globular
clusters classified as young halo clusters (YH GC) is very similar to the DoS,
while 7 of the 14 analysed streams align with the DoS. The probability to find
the observed clustering of streams is only 0.3 per cent when assuming isotropy.
The MW thus is surrounded by a vast polar structure (VPOS) of subsystems
(satellite galaxies, globular clusters and streams), spreading from
Galactocentric distances as small as 10 kpc out to 250 kpc. These findings
demonstrate that a near-isotropic infall of cosmological sub-structure
components onto the MW is essentially ruled out because a large number of
infalling objects would have had to be highly correlated, to a degree not
natural for dark matter sub-structures. The majority of satellites, streams and
YH GCs had to be formed as a correlated population. This is possible in tidal
tails consisting of material expelled from interacting galaxies. We discuss the
tidal scenario for the formation of the VPOS, including successes and possible
challenges. The potential consequences of the MW satellites being tidal dwarf
galaxies are severe. If all the satellite galaxies and YH GCs have been formed
in an encounter between the young MW and another gas-rich galaxy about 10-11
Gyr ago, then the MW does not have any luminous dark-matter substructures and
the missing satellites problem becomes a catastrophic failure of the standard
cosmological model.Comment: 21 pages, 8 figures, 2 tables. Accepted for publication in MNRAS. An
animation of Figure 5 can be found at http://youtu.be/nUwxv-WGfH
Optical BVI Imaging and HI Synthesis Observations of the Dwarf Irregular Galaxy ESO 364-G 029
As part of an effort to enlarge the number of well-studied Magellanic-type
galaxies, we obtained broadband optical imaging and neutral hydrogen radio
synthesis observations of the dwarf irregular galaxy ESO 364-G 029. The optical
morphology characteristically shows a bar-like main body with a one-sided
spiral arm, an approximately exponential light distribution, and offset
photometric and kinematic centers. The HI distribution is mildly asymmetric
and, although slightly offset from the photometric center, roughly follows the
optical brightness distribution, extending to over 1.2 Holmberg radii (where
mu_B = 26.5 mag/arcsec^2). In particular, the highest HI column densities
closely follow the bar, one-arm spiral, and a third optical extension. The
rotation is solid-body in the inner parts but flattens outside of the optical
extent. The total HI flux F_HI = 23.1 pm 1.2 Jy km/s, yielding a total HI mass
M_HI= (6.4 pm 1.7) x 10^8 Msun (for a distance D = 10.8 pm 1.4 Mpc) and a total
HI mass-to-blue-luminosity ratio M_HI/L_B = (0.96 pm 0.14) Msun / Lsun,B
(distance independent). The HI data suggest a very complex small-scale HI
structure, with evidence of large shells and/or holes, but deeper observations
are required for a detailed study. Follow-up observations are also desirable
for a proper comparison with the Large Magellanic Cloud, where despite an
optical morphology very similar to ESO 364-G 029 the HI bears little
resemblance to the optical.Comment: 10 pages, 11 figures, accepted by A&
Dissipative phenomena in extended-bodies interactions I: Methods Dwarf galaxies of the Local Group and their synthetic CMDs
Dissipative phenomena occurring during the orbital evolution of a dwarf
satellite galaxy around a host galaxy may leave signatures in the star
formation activity and signatures in the colour magnitude diagram of the galaxy
stellar content. Our goal is to reach a simple and qualitative description of
the these complicated phenomena. We develop an analytical and numerical
technique able to study ram pressure, Kelvin-Helmholtz instability,
Rayleigh-Taylor and tidal forces acting on the star formation processes in
molecular clouds. We consider it together with synthetic colour magnitude
diagrams techniques. We developed a method to investigate the connections
existing between gas consumption processes and star formation processes in the
context of the two extended-body interaction with special attention to the
dwarf galaxies dynamical regime.Comment: Accepted on A&
GASKAP -- The Galactic ASKAP Survey
A survey of the Milky Way disk and the Magellanic System at the wavelengths
of the 21-cm atomic hydrogen (HI) line and three 18-cm lines of the OH molecule
will be carried out with the Australian Square Kilometre Array Pathfinder
telescope. The survey will study the distribution of HI emission and absorption
with unprecedented angular and velocity resolution, as well as molecular line
thermal emission, absorption, and maser lines. The area to be covered includes
the Galactic plane (|b|< 10deg) at all declinations south of delta = +40deg,
spanning longitudes 167deg through 360deg to 79deg at b=0deg, plus the entire
area of the Magellanic Stream and Clouds, a total of 13,020 square degrees. The
brightness temperature sensitivity will be very good, typically sigma_T ~ 1 K
at resolution 30arcsec and 1 km/s. The survey has a wide spectrum of scientific
goals, from studies of galaxy evolution to star formation, with particular
contributions to understanding stellar wind kinematics, the thermal phases of
the interstellar medium, the interaction between gas in the disk and halo, and
the dynamical and thermal states of gas at various positions along the
Magellanic Stream.Comment: 45 pages, 8 figures, Pub. Astron. Soc. Australia (in press
Observational Constraints on the Modified Gravity Model (MOG) Proposed by Moffat: Using the Magellanic System
A simple model for the dynamics of the Magellanic Stream (MS), in the
framework of modified gravity models is investigated. We assume that the galaxy
is made up of baryonic matter out of context of dark matter scenario. The model
we used here is named Modified Gravity (MOG) proposed by Moffat (2005). In
order to examine the compatibility of the overall properties of the MS under
the MOG theory, the observational radial velocity profile of the MS is compared
with the numerical results using the fit method. In order to obtain
the best model parameters, a maximum likelihood analysis is performed. We also
compare the results of this model with the Cold Dark Matter (CDM) halo model
and the other alternative gravity model that proposed by Bekenstein (2004), so
called TeVeS. We show that by selecting the appropriate values for the free
parameters, the MOG theory seems to be plausible to explain the dynamics of the
MS as well as the CDM and the TeVeS models.Comment: 14 pages, 3 Figures, accepted in Int. J. Theor. Phy