195 research outputs found
Relics of structure formation: extra-planar gas and high-velocity clouds around the Andromeda Galaxy
Using the 100-m radio telescope at Effelsberg, we mapped a large area around
the Andromeda Galaxy in the 21-cm line emission of neutral hydrogen to search
for high-velocity clouds (HVCs) out to large projected distances in excess of
100 kpc. Our 3-sigma HI mass sensitivity for the warm neutral medium is 8x10^4
solar masses. We can confirm the existence of a population of HVCs with typical
HI masses of a few times 10^5 solar masses near the disc of M31. However, we
did not detect any HVCs beyond a projected distance of about 50 kpc from M31,
suggesting that HVCs are generally found in proximity of large spiral galaxies
at typical distances of a few 10 kpc. Comparison with CDM-based models and
simulations suggests that only a few of the detected HVCs could be associated
with primordial dark-matter satellites, whereas others are most likely the
result of tidal stripping. The lack of clouds beyond a projected distance of 50
kpc from M31 is also in conflict with the predictions of recent CDM structure
formation simulations. A possible solution to this problem could be ionisation
of the HVCs as a result of decreasing pressure of the ambient coronal gas at
larger distances from M31. A consequence of this scenario would be the presence
of hundreds of mainly ionised or pure dark-matter satellites around large
spiral galaxies like the Milky Way and M31.Comment: 21 pages, 15 figures, accepted for publication in MNRA
A Unified Formation Scenario for the Zoo of Extended Star Clusters and Ultra-Compact Dwarf Galaxies
In the last decade, very extended old stellar clusters, which cover a large mass range, have been found in various types of galaxies in different environments. Objects with masses comparable to normal globular clusters (GCs) are called extended clusters (ECs), while objects with masses in the dwarf galaxy regime are called ultra-compact dwarf galaxies (UCDs). Moreover, observations indicate that star clusters tend to form in larger conglomerations called star cluster complexes (CCs) in heavily interacting galaxies. The CCs have typical masses between MCC = 105 and a few 107 M⊙, radii of tens to a few hundred parsecs, and they typically consist of few to several dozens of young massive star clusters. I have compiled a catalog of 835 ECs and UCDs with effective radii larger than 10 pc from the literature. At each magnitude objects are found with effective radii between 10 pc and an upper size limit, which increases for brighter luminosities. For objects associated with early and late-type galaxies, the turnover of the luminosity function of the extended objects is about one magnitude fainter than that of the GC luminosity function. The extended objects and GCs form a coherent structure in the reff vs. MV parameter space, while there is a clear gap between extended objects and early-type dwarf galaxies except for the high-mass end, where the most extended objects are close to the parameters of some compact elliptical galaxies. The rapidly increasing number of observed objects allows for the first time an in-depth investigation of their nature. In this thesis I investigate the question whether CCs are the progenitors of ECs associated with galactic disks (also known as faint fuzzies, FFs), of ECs located in galactic halos, and of UCDs by performing extensive numerical simulations (in total 154). It is the first systematic research on the evolution of CCs. In this formation scenario these extended objects evolve from CCs by merging of their constituent star clusters. The basic and most important parameters of a CC are itsmass and size. These two parameters were varied in all parametric studies to investigate how the structural parameters of the final merger objects correlate with the underlying CC parameter space. The third important factor is the external tidal field which has a large impact on the evolution of a CC. The influence of the external tidal field was studied by varying the orbital parameters of the CCs. These three parameters are varied systematically and the resulting merger objects are compared with specific extended objects like the faint fuzzies in the galaxy NGC1023 and the Milky Way cluster NGC2419 and with the overall properties of the extended objects in the catalog. A comparison of the observed sample of extended objects of the catalog with the numerical models demonstrates that the merging star cluster scenario reproduces the structural parameters, the distribution, and the overall trends of the observed extended objects very well. Even specific features of some extended objects are well reproduced in the simulations. All extended objects can be very well explained by a star cluster origin, where they are the results of merged star clusters of cluster complexes. The distinction made between FFs, ECs, and UCDs is no longer existent in this formation scenario. They all stem from the same formation process and are therefore united under the name “extended stellar dynamical object” (EO)
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
Effelsberg HI observations of compact high-velocity clouds
We have mapped 11 compact high-velocity clouds (CHVCs) in the 21-cm line
emission of neutral, atomic hydrogen, using the Effelsberg 100-m radio
telescope. The aim of our observations was to study the overall distribution of
the warm neutral medium of CHVCs with high sensitivity. The achieved baseline
rms of about 50 mK at the original 2.6 km/s velocity resolution allows us to
search for evidence of ram-pressure interaction with the ambient medium. In
addition, we have obtained spectra along an appropriate axis across each CHVC
with longer integration times and denser angular sampling. These deep slices
with a spectral baseline rms of about 25...35 mK allow us to determine the
column density profile in greater detail as well as the velocity and line width
gradient across each cloud. The most outstanding result of our observations is
the complexity of the HI column density distribution and the line profiles of
the investigated CHVCs. We have found only one cloud with a
spherically-symmetric appearance. Among the remaining clouds we observe
head-tail structures, bow-shock shapes, and objects with irregular shapes.
These complex morphologies in combination with the obtained physical parameters
suggest that ram-pressure interactions with an ambient medium may play a
significant role in shaping some of the CHVCs from our sample. These results
are consistent with a circumgalactic distribution of CHVCs with typical
distances of the order of 100 kpc. The pressure of the ambient medium might
also stabilise CHVCs in addition to their own gravitational potential.Comment: 18 pages, 11 figures, accepted for publication by Astronomy &
Astrophysic
Origin of structural and kinematical properties of the Small Magellanic Cloud
We investigate structural, kinematical, and chemical properties of stars and
gas in the Small Magellanic Cloud (SMC) interacting with the Large Magellanic
Cloud (LMC) and the Galaxy based on a series of self-consistent chemodynamical
simulations. We adopt a new "dwarf spheroidal model" in which the SMC initially
has both old stars with a spherical spatial distribution and an extended HI gas
disk. We mainly investigate SMC's evolution for the last 3 Gyr within which the
Magellanic stream (MS) and the Magellanic bridge (MB) can be formed as a result
of the LMC-SMC-Galaxy interaction. Our principal results, which can be tested
against observations, are as follows. The final spatial distribution of the old
stars projected onto the sky is spherical even after the strong LMC-SMC-Galaxy
interaction, whereas that of the new ones is significantly flattened and
appears to form a bar structure. Old stars have the line-of-sight velocity
dispersion (sigma) of ~ 30 km/s and slow rotation with the maximum rotational
velocity (V) of less than slow rotation with the maximum rotational velocity
(V) of less than 20 km/s and show asymmetry in the radial profiles. New stars
have a smaller sigma than old ones and a significant amount of rotation
(V/sigma >1). HI gas shows velocity dispersions of sigma = 10-40 km/s a high
maximum rotational velocity (V ~ 50 km/s), and the spatial distribution similar
to that of new stars. The new stars with ages younger than 3 Gyr show a
negative metallicity gradient in the sense that more metal-rich stars are
located in the inner regions of the SMC.Comment: 21 pages, 21 figures (5 color), accepted by PAS
Magnetic field transport from disk to halo via the galactic chimney process in NGC 6946
The interstellar medium (ISM) in galaxies is directly affected by the mass
and energy outflows originating in regions of star formation. Magnetic fields
are an essential ingredient of the ISM, but their connection to the gaseous
medium and its evolution remains poorly understood. Here we present the
detection of a gradient in Faraday rotation measure (RM), co-located with a
hole in the neutral hydrogen (HI) distribution in the disk of the nearby spiral
galaxy NGC 6946. The gas kinematics in the same location show evidence for
infall of cold gas. The combined characteristics of this feature point to a
substantial vertical displacement of the initially plane-parallel ordered
magnetic field, driven by a localized star formation event. This reveals how
the large-scale magnetic field pattern in galaxy disks is directly influenced
by internal energetic phenomena. Conversely, magnetic fields are observed to be
an important ingredient in disk-halo interactions, as predicted in MHD
simulations. Turbulent magnetic fields at smaller spatial scales than the
observed RM gradient will also be carried from the disk and provide a mechanism
for the dynamo process to amplify the ordered magnetic field without quenching.
We discuss the observational biases, and suggest that this is a common feature
of star forming galaxies with active disk-halo flows.Comment: 11 pages, 3 figures. Accepted for publication in ApJ Letters.
Slightly updated to adjust labeling in Fig.
The Effelsberg-Bonn HI Survey (EBHIS)
The Effelsberg-Bonn HI survey (EBHIS) comprises an all-sky survey north of
Dec = -5 degrees of the Milky Way and the local volume out to a red-shift of z
~ 0.07. Using state of the art Field Programmable Gate Array (FPGA)
spectrometers it is feasible to cover the 100 MHz bandwidth with 16.384
spectral channels. High speed storage of HI spectra allows us to minimize the
degradation by Radio Frequency Interference (RFI) signals. Regular EBHIS survey
observations started during the winter season 2008/2009 after extensive system
evaluation and verification tests. Until today, we surveyed about 8000 square
degrees, focusing during the first all-sky coverage of the Sloan-Digital Sky
Survey (SDSS) area and the northern extension of the Magellanic stream. The
first whole sky coverage will be finished in 2011. Already this first coverage
will reach the same sensitivity level as the Parkes Milky Way (GASS) and
extragalactic surveys (HIPASS). EBHIS data will be calibrated, stray-radiation
corrected and freely accessible for the scientific community via a
web-interface. In this paper we demonstrate the scientific data quality and
explore the expected harvest of this new all-sky survey.Comment: 19 pages, 11 figures, accepted for publication by Astronomical Note
Westerbork HI observations of high-velocity clouds near M31 and M33
We have undertaken high-resolution follow-up of a sample of high velocity HI
clouds apparently associated with M31. Our sample was chosen from the
population of high-velocity clouds (HVCs) detected out to 50 kpc projected
radius of the Andromeda Galaxy by Thilker et al. (2004) with the Green Bank
Telescope. Nine pointings were observed with the Westerbork Synthesis Radio
Telescope to determine the physical parameters of these objects and to find
clues to their origin. One additional pointing was directed at a similar object
near M33. At 2' resolution we detect 16 individual HVCs around M31 and 1 HVC
near M33 with typical HI masses of a few times 10^5 solar masses and sizes of
the order of 1 kpc. Estimates of the dynamical and virial masses of some of the
HVCs indicate that they are likely gravitationally dominated by additional mass
components such as dark matter or ionised gas. Twelve of the clouds are
concentrated in an area of only 1 by 1 degree at a projected separation of less
than 15 kpc from the disk of M31. This HVC complex has a rather complicated
morphological and kinematical structure and partly overlaps with the giant
stellar stream of M31, suggesting a tidal origin. Another detected feature is
in close proximity, in both position and velocity, with NGC 205, perhaps also
indicative of tidal processes. Other HVCs in our survey are isolated and might
represent primordial, dark-matter dominated clouds.Comment: 18 pages, 16 figures, accepted for publication in Astronomy &
Astrophysic
Evaporation and condensation of spherical interstellar clouds. Self-consistent models with saturated heat conduction and cooling
Shortened version: The fate of IS clouds embedded in a hot tenuous medium
depends on whether the clouds suffer from evaporation or whether material
condensates onto them. Analytical solutions for the rate of evaporative mass
loss from an isolated spherical cloud embedded in a hot tenuous gas are deduced
by Cowie & McKee (1977). In order to test the validity of the analytical
results for more realistic IS conditions the full hydrodynamical equations must
be treated. Therefore, 2D numerical simulations of the evolution of IS clouds
%are performed with different internal density structures and surrounded by a
hot plasma reservoir. Self-gravity, interstellar heating and cooling effects
and heat conduction by electrons are added. Classical thermal conductivity of a
fully ionized hydrogen plasma and saturated heat flux are considered. Using
pure hydrodynamics and classical heat flux we can reproduce the analytical
results. Heat flux saturation reduces the evaporation rate by one order of
magnitude below the analytical value. The evolution changes totally for more
realistic conditions when interstellar heating and cooling effects stabilize
the self-gravity. Evaporation then turns into condensation, because the
additional energy by heat conduction can be transported away from the interface
and radiated off efficiently from the cloud's inner parts. I.e. that the
saturated heat flux consideration is inevitable for IS clouds embedded in hot
tenuous gas. Various consequences are discussed in the paper.Comment: 16 pages, 24 figures, accepted in Astronomy and Astrophysic
Faint Fuzzy Star Clusters in NGC1023 as Remnants of Merged Star Cluster Complexes
In the lenticular galaxy NGC1023 a third population of globular clusters
(GCs), called faint fuzzies (FFs), was discovered next to the blue and red GC
populations by Larsen & Brodie. While these FFs have colors comparable to the
red population, the new population is fainter, larger (R_eff > 7 pc) and, most
importantly, shows clear signs of co-rotation with the galactic disk of
NGC1023. We present N-body simulations verifying the hypothesis that these
disk-associated FFs are related to the young massive cluster complexes (CCs)
observed by Bastian et. al in M51, who discovered a mass-radius relation for
these CCs. Our models have an initial configuration based on the observations
from M51 and are placed on various orbits in a galactic potential derived for
NGC1023. All computations end up with a stable object containing 10 to 60% of
the initial CC mass after an integration time of 5 Gyr. A conversion to visual
magnitudes demonstrates that the resulting objects cover exactly the observed
range for FFs. Moreover, the simulated objects show projected half-mass radii
between 3.6 and 13.4 pc, in good agreement with the observed FF sizes. We
conclude that objects like the young massive CCs in M51 are likely progenitors
of the FFs observed in NGC1023.Comment: Accepted for publication in Ap
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