84 research outputs found
High-resolution 21-cm observations of low-column density gas clumps in the Milky Way halo
We study the properties of low-column density gas clumps in the halo of the
Milky Way based on high-resolution 21-cm observations.
Using interferometric data from the WSRT and the VLA we study HI emission at
low-, intermediate- and high radial velocities along four lines of sight
towards quasars. Along these sightlines we previously detected weak CaII and
NaI absorbers in their optical spectra.
The analysis of the high-resolution HI data reveals the presence of several
compact and cold clumps of neutral gas at velocities similar to the optical
absorption. The clumps have narrow HI line widths in the range of 1.8 to 13
km/s, yielding upper limits for the kinetic temperature of the gas of 70 to
3700 K. The neutral gas has low HI column densities in the range of 5E18 to
3E19 1/cm^2. All clumps have angular sizes of only a few arcminutes.
Our high-resolution 21-cm observations indicate that many of the CaII and NaI
absorbers seen in our optical quasar spectra are associated with low-column
density HI clumps at small angular scales. This suggests that next to the
massive, high-column density neutral gas clouds in the halo (the common 21-cm
LVCs, IVCs, and HVCs) there exists a population of low-mass, neutral gas
structures in the halo that remain mostly unseen in the existing 21-cm all-sky
surveys of IVCs and HVCs. The estimated thermal gas pressures of the detected
HI clumps are consistent with what is expected from theoretical models of gas
in the inner and outer Milky Way halo.Comment: 12 pages, 7 figure
Radio-Continuum study of the Nearby Sculptor Group Galaxies. Part 1: NGC 300 at lambda = 20 cm
A series of new radio-continuum (lambda=20 cm) mosaic images focused on the
NGC 300 galactic system were produced using archived observational data from
the VLA and/or ATCA. These new images are both very sensitive (rms=60 microJy)
and feature high angular resolution (<10"). The most prominent new feature is
the galaxy's extended radio-continuum emission, which does not match its
optical appearance. Using these newly created images a number of previously
unidentified discrete sources have been discovered. Furthermore, we demonstrate
that a joint deconvolution approach to imaging this complete data-set is
inferior when compared to an immerge approach.Comment: 13 pages, 12 figures, accepted to APSS, new version to correct the
missing reference
Warp or lag? The ionized and neutral hydrogen gas in the edge-on dwarf galaxy UGC 1281
The properties of gas in the halos of galaxies constrain global models of the
interstellar medium. Kinematical information is of particular interest since it
is a clue to the origin of the gas. Until now mostly massive galaxies have been
investigated for their halo properties. Here we report on deep HI and H{\alpha}
observations of the edge-on dwarf galaxy UGC 1281 in order to determine the
existence of extra-planar gas and the kinematics of this galaxy. This is the
first time a dwarf galaxy is investigated for its gaseous halo characteristics.
We have obtained H{\alpha} integral field spectroscopy using PPAK at Calar Alto
and deep HI observations with the WSRT of this edge-on dwarf galaxy. These
observations are compared to 3D models in order to determine the distribution
of HI in the galaxy. We find that UGC 1281 has H{\alpha} emission up to 25"(655
pc) in projection above the plane and in general a low H{\alpha} flux. Compared
to other dwarf galaxies UGC 1281 is a normal dwarf galaxy with a slowly rising
rotation curve that flattens off at 60 km/s and a central depression in its HI
distribution. Its HI extends 70" (1.8 kpc) in projection from the plane. This
gas can be explained by either a warp partially in the line-of-sight warp or a
purely edge-on warp with rotational velocities that decline with a vertical
gradient of 10.6 \pm 3.7 km/s/kpc. The line-of-sight warp model is the
preferred model as it is conceptually simpler. In either model the warp starts
well within the optical radius.Comment: Accepted for publication in MNRAS. 16 pages, 14 figure
The Vela Cloud: A Giant HI Anomaly in the NGC 3256 Group
We present Australia Telescope Compact Array (ATCA) observations of a
galaxy-sized intergalactic HI cloud (the Vela Cloud) in the NGC 3256 galaxy
group. The group contains the prominent merging galaxy NGC 3256, which is
surrounded by a number of HI fragments, the tidally disturbed galaxy NGC 3263,
and several other peculiar galaxies. The Vela Cloud, with an HI mass of 3-5 *
10**9 solar masses, resides southeast of NGC 3256 and west of NGC 3263, within
an area of 9' x 16' (100 kpc x 175 kpc for an adopted distance of 38 Mpc). In
our ATCA data the Vela Cloud appears as 3 diffuse components and contains 4
density enhancements. The Vela Cloud's properties, together with its group
environment, suggest that it has a tidal origin. Each density enhancement
contains ~10**8 solar masses of HI gas which is sufficient material for the
formation of globular cluster progenitors. However, if we represent the
enhancements as Bonnor-Ebert spheres, then the pressure of the surrounding HI
would need to increase by at least a factor of 6 in order to cause the collapse
of an enhancement. Thus we do not expect them to form massive bound stellar
systems like super star clusters or tidal dwarf galaxies. Since the HI density
enhancements have some properties in common with High Velocity Clouds, we
explore whether they may evolve to be identified with these starless clouds
instead.Comment: 47 pages, 13 figures (incl. a & b), accepted by AJ, changes are minor
additions, rearranging, and clarifications esp. in sections 6 &
Molecular gas in the inner 0.7kpc-radius ring of M31
The study of the gas kinematic in the central 1.5kpc x 1.5kpc region of M31
has revealed several surprises. The starting point of this investigation was
the detection at the IRAM-30m telescope of molecular gas with very large line
splittings up to 260km/s within the beam (40 pc). In this region, which is
known for its low gas content, we also detect an ionised gas outflow in the
circumnuclear region (within 75pc from the centre) extending to the whole area
in X-ray. Relying on atomic, ionised, and molecular gas, we account for most
observables with a scenario that assumes that a few hundreds Myr ago, M31
underwent a frontal collision with M32, which triggered some star-formation
activity in the centre, and this collision explains the special configuration
of M31 with two rings observed at 0.7kpc and 10kpc. The inner disc (whose
rotation is detected in HI and ionised gas ([NII])) has thus been tilted
(inclination: 43deg, PA: 70deg) with respect to the main disc (inclination:
77deg, PA: 35deg). One of the CO velocity components is compatible with this
inner disc, while the second one comes from a tilted ring-like material with
40deg inclination and PA=-35deg. The relic star formation estimated by previous
works to have occurred more than 100Myr ago could have been triggered by the
collision and could be linked to the outflow detected in the ionised gas. Last,
we demonstrate that the amplitude of the line splittings detected in CO centred
on the systemic velocity with a relatively high spatial resolution (40pc)
cannot be accounted for by a possible weak bar that is roughly aligned along
the minor axis. Although M31 has a triaxial bulge, there are no bar indicators
in the gas component (photometry, no strong skewness of the isovelocities,
etc.).Comment: 20 pages, 20 figures, Astronomy and Astrophysics, accepte
an overview of the MHONGOOSE survey: Observing nearby galaxies with MeerKAT
© Copyright owned by the author(s). MHONGOOSE is a deep survey of the neutral hydrogen distribution in a representative sample of 30 nearby disk and dwarf galaxies with H I masses from ∼ 106 to ∼ 1011 M, and luminosities from MR ∼ 12 to MR ∼ −22. The sample is selected to uniformly cover the available range in log(MHI). Our extremely deep observations, down to H I column density limits of well below 1018 cm−2 — or a few hundred times fainter than the typical H I disks in galaxies — will directly detect the effects of cold accretion from the intergalactic medium and the links with the cosmic web. These observations will be the first ever to probe the very low-column density neutral gas in galaxies at these high resolutions. Combination with data at other wavelengths, most of it already available, will enable accurate modeling of the properties and evolution of the mass components in these galaxies and link these with the effects of environment, dark matter distribution, and other fundamental properties such as halo mass and angular momentum. MHONGOOSE can already start addressing some of the SKA-1 science goals and will provide a comprehensive inventory of the processes driving the transformation and evolution of galaxies in the nearby universe at high resolution and over 5 orders of magnitude in column density. It will be a Nearby Galaxies Legacy Survey that will be unsurpassed until the advent of the SKA, and can serve as a highly visible, lasting statement of MeerKAT’s capabilities
Possible detection of the M31 rotation in WMAP data
Data on the cosmic microwave background (CMB) radiation by the Wilkinson
Microwave Anisotropy Probe (WMAP) had a profound impact on the understanding of
a variety of physical processes in the early phases of the Universe and on the
estimation of the cosmological parameters. Here, the 7-year WMAP data are used
to trace the disk and the halo of the nearby giant spiral galaxy M31. We
analyzed the temperature excess in three WMAP bands (W, V, and Q) by dividing
the region of the sky around M31 into several concentric circular areas. We
studied the robustness of the detected temperature excess by considering 500
random control fields in the real WMAP maps and simulating 500 sky maps from
the best-fitted cosmological parameters. By comparing the obtained temperature
contrast profiles with the real ones towards the M31 galaxy, we find that the
temperature asymmetry in the M31 disk is fairly robust, while the effect in the
halo is weaker. An asymmetry in the mean microwave temperature in the M31 disk
along the direction of the M31 rotation is observed with a temperature contrast
up to about 130 microK/pixel. We also find a temperature asymmetry in the M31
halo, which is much weaker than for the disk, up to a galactocentric distance
of about 10 degrees (120 kpc) with a peak temperature contrast of about 40
microK/pixel. Although the confidence level of the signal is not high, if
estimated purely statistically, which could be expected due to the weakness of
the effect, the geometrical structure of the temperature asymmetry points
towards a definite effect modulated by the rotation of the M31 halo. This
result might open a new way to probe these relatively less studied galactic
objects using high-accuracy CMB measurements, such as those with the Planck
satellite or planned balloon-based experiments, which could prove or disprove
our conclusions.Comment: 6 pages, 5 figures, in press on Astronomy and Astrophysics Letters,
201
The Herschel Exploitation of Local Galaxy Andromeda (HELGA). I: Global far-infrared and sub-mm morphology
We have obtained Herschel images at five wavelengths from 100 to 500 micron
of a ~5.5x2.5 degree area centred on the local galaxy M31 (Andromeda), our
nearest neighbour spiral galaxy, as part of the Herschel guaranteed time
project "HELGA". The main goals of HELGA are to study the characteristics of
the extended dust emission, focusing on larger scales than studied in previous
observations of Andromeda at an increased spatial resolution, and the obscured
star formation. In this paper we present data reduction and Herschel maps, and
provide a description of the far-infrared morphology, comparing it with
features seen at other wavelengths. We use high--resolution maps of the atomic
hydrogen, fully covering our fields, to identify dust emission features that
can be associated to M31 with confidence, distinguishing them from emission
coming from the foreground Galactic cirrus. Thanks to the very large extension
of our maps we detect, for the first time at far-infrared wavelengths, three
arc-like structures extending out to ~21, ~26 and ~31 kpc respectively, in the
south-western part of M31. The presence of these features, hosting ~2.2e6 Msol
of dust, is safely confirmed by their detection in HI maps. Overall, we
estimate a total dust mass of ~5.8e7 Msol, about 78% of which is contained in
the two main ring-like structures at 10 and 15 kpc, at an average temperature
of 16.5 K. We find that the gas-to-dust ratio declines exponentially as a
function of the galacto-centric distance, in agreement with the known
metallicity gradient, with values ranging from 66 in the nucleus to ~275 in the
outermost region. [Abridged]Comment: 15 Pages, 9 Figures. Accepted for publication in Astronomy and
Astrophysics. A high resolution version of the paper can be found at
http://wazn.ugent.be/jfritz/HelgaI_final.pd
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
An Introduction to Gas Accretion onto Galaxies
Evidence for gas accretion onto galaxies can be found throughout the
universe. In this chapter, I summarize the direct and indirect signatures of
this process and discuss the primary sources. The evidence for gas accretion
includes the star formation rates and metallicities of galaxies, the evolution
of the cold gas content of the universe with time, numerous indirect indicators
for individual galaxies, and a few direct detections of inflow. The primary
sources of gas accretion are the intergalactic medium, satellite gas and
feedback material. There is support for each of these sources from observations
and simulations, but the methods with which the fuel ultimately settles in to
form stars remain murky.Comment: 14 pages, 5 figures, Invited review to appear in Gas Accretion onto
Galaxies, Astrophysics and Space Science Library, eds. A. J. Fox & R. Dav\'e,
to be published by Springe
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