47 research outputs found
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
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
Low-column density gas clumps in the halo of the Milky Way
We report on the detection of low-column density neutral hydrogen clumps in
the halo of the Milky Way. Using high-resolution (FWHM~7 km/s) optical spectra
obtained with the VLT/UVES spectrograph we detect narrow interstellar
absorption by CaII and NaI at high radial velocities near -150 km/s toward the
quasar PKS 1448-232 (l=335.4, b=+31.7). Follow-up HI 21cm observations with the
VLA unveil the presence of a complex of small neutral hydrogen clumps with HI
column densities <8x10^18 cm^-2. The measured HI line widths imply that the gas
is relatively cold with temperatures T<900 K. Although the line of sight
towards PKS 1448-232 does not pass immediately through a known large
high-velocity cloud (HVC), the sky position and the measured radial velocities
suggest that these clumps are associated with HVC cloud complex L. An
inspection of other UVES quasar spectra shows that weak, narrow CaII absorption
at high velocities is a common phenomenon, even in directions where
high-velocity HI 21cm emission is not detected. This suggests that the Milky
Way halo contains a large number of high-velocity neutral gas clumps with low
HI column densities. If such clumps are typical for halos of spiral galaxies,
they should contribute significantly to the population of strong MgII absorbers
and Lyman-Limit Systems (LLS) seen in the circumgalactic environment of other
galaxies.Comment: 4 pages, 3 figures; accepted for publication in A&A Letter
CaII K observations of QSOs in the line-of-sight to the Magellanic Bridge
We describe medium-resolution spectroscopic observations taken with the ESO
Multi-Mode Instrument in the CaII K line (3933.661 Angstroms) towards 7 QSOs
located in the line-of-sight to the Magellanic Bridge. At a spectral resolution
R = 6,000, five of the sightlines have a signal-to-noise ratio of 20 or higher.
Definite Ca absorption due to Bridge material is detected towards 3 objects,
with probable detection towards two other sightlines. Gas-phase CaII K Bridge
and Milky Way abundances or lower limits for the all sightlines are estimated
by the use of Parkes 21-cm HI emission line data. These data only have a
spatial resolution of 14 arcminutes compared with the optical observations
which have milli-arcsecond resolution. With this caveat, for the three objects
with sound CaII K detections, we find that the ionic abundance of CaII K
relative to HI, A=log(N(CaK)/N(HI)) for low-velocity Galactic gas ranges from
-8.3 to -8.8 dex, with HI column densities varying from 3-6x10^20 cm^-2. For
Magellanic Bridge gas, the values of A are 0.5 dex higher, ranging from -7.8 to
-8.2 dex, with N(HI)=1-5x10^20 cm^-2. Higher values of A correspond to lower
values of N(HI), although numbers are small. For the sightline towards
B0251--675, the Bridge gas has two different velocities, and in only one of
these is CaII K tentatively detected, perhaps indicating gas of a different
origin or present-day characteristics (such as dust content), although this
conclusion is uncertain and there is the possibility that one of the components
could be related to the Magellanic Stream. Higher signal-to-noise CaII K data
and higher resolution HI data are required to determine whether A changes with
N(HI) over the Bridge and if the implied difference in the metalicity of the
two Bridge components towards B0251-675 is real.Comment: Astronomy and Astrophysics, in pres
External Mass Accumulation onto Core Potentials: Implications for Star Clusters, Galaxies and Galaxy Clusters
Accretion studies have been focused on the flow around bodies with point mass
gravitational potentials, but few general results are available for non-point
mass distributions. Here, we study the accretion flow onto non-divergent, core
potentials moving through a background medium. We use Plummer and Hernquist
potentials as examples to study gas accretion onto star clusters, dwarf and
large galaxy halos and galaxy clusters in a variety of astrophysical
environments. The general conditions required for a core potential to
collectively accrete large quantities of gas from the external medium are
derived using both simulations and analytic results. The consequences of large
mass accumulation in galaxy nuclei, dwarf galaxies and star clusters are
twofold. First, if the gas cools effectively star formation can be triggered,
generating new stellar members in the system. Second, if the collective
potential of the system is able to alter the ambient gas properties before the
gas is accreted onto the individual core members, the augmented mass supply
rates could significantly alter the state of the various accreting stellar
populations and result in an enhanced central black hole accretion luminosity.Comment: 24 pages, 15 figures, accepted to Ap
Multi-wavelength analysis of the dust emission in the Small Magellanic Cloud
We present an analysis of dust grain emission in the diffuse interstellar
medium of the Small Magellanic Cloud (SMC). This study is motivated by the
availability of 170 microns ISOPHOT data covering a large part of the SMC, with
a resolution enabling to disentangle the diffuse medium from the star forming
regions. After data reduction and subtraction of Galactic foreground emission,
we used the ISOPHOT data together with HiRes IRAS data and ATCA/Parkes combined
HI column density maps to determine dust properties for the diffuse medium. We
found a far infrared emissivity per hydrogen atom 30 times lower than the Solar
Neighborhood value. The modeling of the spectral energy distribution of the
dust, taking into account the enhanced interstellar radiation field, gives a
similar conclusion for the smallest grains (PAHs and very small grains)
emitting at shorter wavelength. Assuming Galactic dust composition in the SMC,
this result implies a difference in the gas-to-dust ratio (GDR) 3 times larger
than the difference in metallicity. This low depletion of heavy elements in
dust could be specific of the diffuse ISM and not apply for the whole SMC dust
if it results from efficient destruction of dust by supernovae explosions.Comment: 11 pages, 10 figures. Accepted for publication in Astronomy &
Astrophysic
Stationary models for the extra-planar gas in disc galaxies
The kinematics of the extra-planar neutral and ionised gas in disc galaxies
shows a systematic decline of the rotational velocity with height from the
plane (vertical gradient). This feature is not expected for a barotropic gas,
whilst it is well reproduced by baroclinic fluid homogeneous models. The
problem with the latter is that they require gas temperatures (above K)
much higher than the temperatures of the cold and warm components of the
extra-planar gas layer. In this paper, we attempt to overcome this problem by
describing the extra-planar gas as a system of gas clouds obeying the Jeans
equations. In particular, we consider models having the observed extra-planar
gas distribution and gravitational potential of the disc galaxy NGC 891: for
each model we construct pseudo-data cubes and we compare them with the HI data
cube of NGC 891. In all cases the rotational velocity gradients are in
qualitative agreement with the observations, but the synthetic and the observed
data cubes of NGC 891 show systematic differences that cannot be accommodated
by any of the explored models. We conclude that the extra-planar gas in disc
galaxies cannot be satisfactorily described by a stationary Jeans-like system
of gas clouds.Comment: 14 pages, 7 figures, accepted for pubblication in MNRA
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&
Gaseous Galaxy Halos
Galactic halo gas traces inflowing star formation fuel and feedback from a
galaxy's disk and is therefore crucial to our understanding of galaxy
evolution. In this review, we summarize the multi-wavelength observational
properties and origin models of Galactic and low redshift spiral galaxy halo
gas. Galactic halos contain multiphase gas flows that are dominated in mass by
the ionized component and extend to large radii. The densest, coldest halo gas
observed in neutral hydrogen (HI) is generally closest to the disk (< 20 kpc),
and absorption line results indicate warm and warm-hot diffuse halo gas is
present throughout a galaxy's halo. The hot halo gas detected is not a
significant fraction of a galaxy's baryons. The disk-halo interface is where
the multiphase flows are integrated into the star forming disk, and there is
evidence for both feedback and fueling at this interface from the temperature
and kinematic gradient of the gas and HI structures. The origin and fate of
halo gas is considered in the context of cosmological and idealized local
simulations. Accretion along cosmic filaments occurs in both a hot (> 10^5.5 K)
and cold mode in simulations, with the compressed material close to the disk
the coldest and densest, in agreement with observations. There is evidence in
halo gas observations for radiative and mechanical feedback mechanisms,
including escaping photons from the disk, supernova-driven winds, and a
galactic fountain. Satellite accretion also leaves behind abundant halo gas.
This satellite gas interacts with the existing halo medium, and much of this
gas will become part of the diffuse halo before it can reach the disk. The
accretion rate from cold and warm halo gas is generally below a galaxy disk's
star formation rate, but gas at the disk-halo interface and stellar feedback
may be important additional fuel sources.Comment: 50 pages, 9 figures (1 in 3D, view with a current version of Adobe),
to appear in ARA&A, 50, 49
High-velocity clouds as streams of ionized and neutral gas in the halo of the Milky Way
High-velocity clouds (HVC), fast-moving ionized and neutral gas clouds found
at high galactic latitudes, may play an important role in the evolution of the
Milky Way. The extent of this role depends sensitively on their distances and
total sky covering factor. We search for HVC absorption in HST high resolution
ultraviolet spectra of a carefully selected sample of 133 AGN using a range of
atomic species in different ionization stages. This allows us to identify
neutral, weakly ionized, or highly ionized HVCs over several decades in HI
column densities. The sky covering factor of UV-selected HVCs with |v_LSR|>90
km/s is 68%+/-4% for the entire Galactic sky. We show that our survey is
essentially complete, i.e., an undetected population of HVCs with extremely low
N(H) (HI+HII) is unlikely to be important for the HVC mass budget. We confirm
that the predominantly ionized HVCs contain at least as much mass as the
traditional HI HVCs and show that large HI HVC complexes have generally ionized
envelopes extending far from the HI contours. There are also large regions of
the Galactic sky that are covered with ionized high-velocity gas with little HI
emission nearby. We show that the covering factors of HVCs with 90<|v_LSR|<170
km/s drawn from the AGN and stellar samples are similar. This confirms that
these HVCs are within 5-15 kpc of the sun. The covering factor of these HVCs
drops with decreasing vertical height, which is consistent with HVCs being
decelerated or disrupted as they fall to the Milky Way disk. The HVCs with
|v_LSR|>170 km/s are largely associated with the Magellanic Stream at b<0 and
its leading arm at b>0 as well as other large known HI complexes. Therefore
there is no evidence in the Local Group that any galaxy shows a population of
HVCs extending much farther away than 50 kpc from its host, except possibly for
those tracing remnants of galaxy interaction.Comment: Submitted to MNRAS (19 pages, 11 figures). Comments are welcom