69 research outputs found
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 Gas Phase in a Low Metallicity ISM
Original article can be found at: http://journals.cambridge.org/ Copyright International Astronomical Union. DOI: 10.1017/S1743921308024927We present several results from our analysis of dwarf irregular galaxies culled from The HI Nearby Galaxy Survey (THINGS). We analyse the rotation curves of two galaxies based on âbulkâ velocity fields, i.e. velocity maps from which random nonâcircular motions are removed. We confirm that their dark matter distribution is best fit by an isothermal halo model. We show that the star formation properties of dIrr galaxies resemble those of the outer parts of larger, spiral systems. Lastly, we study the large scale (3âD) distribution of the gas, and argue that the gas disk in dIrrs is thick, both in a relative, as well as in an absolute sense as compared to spirals. Massive star formation through subsequent supernova explosions is able to redistribute the bulk of the ISM, creating large cavities. These cavities are often larger, and longerâlived than in spiral galaxies.Peer reviewe
26Al kinematics: superbubbles following the spiral arms? : Constraints from the statistics of star clusters and HI supershells
High energy resolution spectroscopy of the 1.8 MeV radioactive decay line of 26Al with the SPI instrument on board the INTEGRAL satellite has recently revealed that diffuse 26Al has large velocities in comparison to other components of the interstellar medium in the Milky Way. 26Al shows Galactic rotation in the same sense as the stars and other gas tracers, but reaches excess velocities up to 300 km sâ1Peer reviewe
The Carina Flare: What can fragments in the wall tell us?
CO(J=2--1) and CO(J=2--1) observations of the molecular cloud
G285.90+4.53 (Cloud~16) in the Carina Flare supershell (GSH287+04-17) with the
APEX telescope are presented. With an algorithm DENDROFIND we identify 51
fragments and compute their sizes and masses. We discuss their mass spectrum
and interpret it as being the result of the shell fragmentation process
described by the pressure assisted gravitational instability - PAGI. We
conclude that the explanation of the clump mass function needs a combination of
gravity with pressure external to the shell.Comment: 19 pages, 14 figures, accepted by A&
The structure of HI in galactic disks: Simulations vs observations
We generate synthetic HI Galactic plane surveys from spiral galaxy
simulations which include stellar feedback processes. Compared to a model
without feedback we find an increased scale height of HI emission (in better
agreement with observations) and more realistic spatial structure (including
supernova blown bubbles). The synthetic data show HI self-absorption with a
morphology similar to that seen in observations. The density and temperature of
the material responsible for HI self-absorption is consistent with
observationally determined values, and is found to be only weakly dependent on
absorption strength and star formation efficiency.Comment: 12 pages, 7 figures. Accepted for publication in MNRA
The non-thermal superbubble in IC 10 : the generation of cosmic ray electrons caught in the act
Superbubbles are crucial for stellar feedback, with supposedly high (of the order of 10 per cent) thermalization rates. We combined multiband radio continuum observations from the Very Large Array (VLA) with Effelsberg data to study the non-thermal superbubble (NSB) in IC 10, a starburst dwarf irregular galaxy in the Local Group. Thermal emission was subtracted using a combination of Balmer Hα and VLA 32 GHz continuum maps. The bubbleâs nonthermal spectrum between 1.5 and 8.8 GHz displays curvature and can be well fitted with a standard model of an ageing cosmic ray electron population. With a derived equipartition magnetic field strength of 44 ±8 ÎŒG, and measuring the radiation energy density from Spitzer MIPS maps as 5±1Ă10â11 erg cmâ3, we determine, based on the spectral curvature, a spectral age of the bubble of 1.0 ± 0.3 Myr. Analysis of the LITTLE THINGS HI data cube shows an expanding HI hole with 100 pc diameter and a dynamical age of 3.8 ± 0.3 Myr, centred to within 16 pc on IC 10 X-1, a massive stellar mass black hole (M > 23 Mâ). The results are consistent with the expected evolution for a superbubble with a few massive stars, where a very energetic event like a Type Ic supernova/hypernova has taken place about 1 Myr ago. We discuss alternatives to this interpretationPeer reviewe
The M81 Group Dwarf Irregular Galaxy DDO 165. II. Connecting Recent Star Formation with ISM Structures and Kinematics
We compare the stellar populations and complex neutral gas dynamics of the
M81 group dIrr galaxy DDO 165 using data from the HST and the VLA. Paper I
identified two kinematically distinct HI components, multiple localized high
velocity gas features, and eight HI holes and shells (the largest of which
spans ~2.2x1.1 kpc). Using the spatial and temporal information from the
stellar populations in DDO 165, we compare the patterns of star formation over
the past 500 Myr with the HI dynamics. We extract localized star formation
histories within 6 of the 8 HI holes identified in Paper I, as well as 23 other
regions that sample a range of stellar densities and neutral gas properties.
From population synthesis modeling, we derive the energy outputs (from stellar
winds and supernovae) of the stellar populations within these regions over the
last 100 Myr, and compare with refined estimates of the energies required to
create the HI holes. In all cases, we find that "feedback" is energetically
capable of creating the observed structures in the ISM. Numerous regions with
significant energy inputs from feedback lack coherent HI structures but show
prominent localized high velocity gas features; this feedback signature is a
natural product of temporally and spatially distributed star formation. In DDO
165, the extended period of heightened star formation activity (lasting more
than 1 Gyr) is energetically capable of creating the observed holes and high
velocity gas features in the neutral ISM.Comment: The Astrophysical Journal, in press. Full-resolution version
available on request from the first autho
The Fine-Scale Structure of the neutral Interstellar Medium in nearby Galaxies
We present an analysis of the properties of HI holes detected in 20 galaxies
that are part of "The HI Nearby Galaxy Survey" (THINGS). We detected more than
1000 holes in total in the sampled galaxies. Where they can be measured, their
sizes range from about 100 pc (our resolution limit) to about 2 kpc, their
expansion velocities range from 4 to 36 km/s, and their ages are estimated to
range between 3 and 150 Myr. The holes are found throughout the disks of the
galaxies, out to the edge of the HI; 23% of the holes fall outside R25. We find
that shear limits the age of holes in spirals (shear is less important in dwarf
galaxies) which explains why HI holes in dwarfs are rounder, on average than in
spirals. Shear, which is particularly strong in the inner part of spiral
galaxies, also explains why we find that holes outside R25 are larger and
older. We derive the scale height of the HI disk as a function of
galactocentric radius and find that the disk flares up in all galaxies. We
proceed to derive the surface and volume porosity (Q2D and Q3D) and find that
this correlates with the type of the host galaxy: later Hubble types tend to be
more porous. The size distribution of the holes in our sample follows a power
law with a slope of a ~ -2.9. Assuming that the holes are the result of massive
star formation, we derive values for the supernova rate (SNR) and star
formation rate (SFR) which scales with the SFR derived based on other tracers.
If we extrapolate the observed number of holes to include those that fall below
our resolution limit, down to holes created by a single supernova, we find that
our results are compatible with the hypothesis that HI holes result from star
formation.Comment: 142 pages, 55 figures, accepted for publication in the Astronomical
Journa
Galactic winds - how to launch galactic outflows in typical Lyman-break galaxies
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2013 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We perform hydrodynamical simulations of a young galactic disc embedded in a hot gaseous halo using parameters typical for Lyman-break galaxies (LBGs). We take into account the (static) gravitational potentials due to a dark matter halo, a stellar bulge and a disc of stars and gas. Star formation is treated by a local KennicuttâSchmidt law. We simplify the structure of the interstellar medium (ISM) by restricting the computational domain to a 25th of the full azimuthal angle, effectively assuming large-scale axisymmetry and neglecting any effects of spiral structure and focus on the large-scale ISM drivers, the superbubbles. Supernovae are triggered randomly and have preset event sizes of several tens to hundreds. We further investigate different halo gas pressures and energy injection methods. Many of our simulated galaxies, but not all, develop bipolar outflows. We characterize the strength of the outflow by mass and energy outflow rates, and investigate the effect of changes to the details of the model. We find that supernovae are more effective if comprised into larger superbubbles. The weight and the pressure of the halo gas is able to quench galactic outflows. The wind emerges from a series of superbubbles in regions where a critical star formation density is exceeded. The superbubbles expand into the gaseous halo at slightly supersonic speed, producing radiative shock waves with similar characteristics as the absorption systems observed around LBGs.Peer reviewe
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