52 research outputs found
The Violent Interstellar Medium of Nearby Dwarf Galaxies
High resolution HI observations of nearby dwarf galaxies (most of which are
situated in the M 81 group at a distance of about 3.2 Mpc) reveal that their
neutral interstellar medium (ISM) is dominated by hole-like features most of
which are expanding. A comparison of the physical properties of these holes
with the ones found in more massive spiral galaxies (such as M 31 and M 33)
shows that they tend to reach much larger sizes in dwarf galaxies. This can be
understood in terms of the galaxy's gravitational potential. The origin of
these features is still a matter of debate. In general, young star forming
regions (OB-associations) are held responsible for their formation. This
picture, however, is not without its critics and other mechanism such as the
infall of high velocity clouds, turbulent motions or even gamma ray bursters
have been recently proposed. Here I will present one example of a supergiant
shell in IC 2574 which corroborates the picture that OB associations are indeed
creating these structures. This particular supergiant shell is currently the
most promising case to study the effects of the combined effects of stellar
winds and supernova-explosions which shape the neutral interstellar medium of
(dwarf) galaxies.Comment: 8 pages, 4 figures, accepted for publication in PASA, in press.
Online version: http://www.atnf.csiro.au/pasa/16_1/walter/paper
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
The Star Formation Law in Nearby Galaxies on Sub-Kpc Scales
(Abridged) We present a comprehensive analysis of the relationship between
star formation rate surface density (SFR SD) and gas surface density (gas SD)
at sub-kpc resolution in a sample of 18 nearby galaxies. We use high resolution
HI data from THINGS, CO data from HERACLES and BIMA SONG, 24 micron data from
the Spitzer Space Telescope, and UV data from GALEX. We target 7 spiral
galaxies and 11 late-type/dwarf galaxies and investigate how the star formation
law differs between the H2-dominated centers of spiral galaxies, their
HI-dominated outskirts and the HI-rich late-type/dwarf galaxies.
We find that a Schmidt-type power law with index N=1.0+-0.2 relates the SFR
SD and the H2 SD across our sample of spiral galaxies, i.e., that H2 forms
stars at a constant efficiency in spirals. The average molecular gas depletion
time is ~2*10^9 yrs. We interpret the linear relation and constant depletion
time as evidence that stars are forming in GMCs with approximately uniform
properties and that the H2 SD may be more a measure of the filling fraction of
giant molecular clouds than changing conditions in the molecular gas.
The relationship between total gas SD and SFR SD varies dramatically among
and within spiral galaxies. Most galaxies show little or no correlation between
the HI SD and the SFR SD. As a result, the star formation efficiency (SFE = SFR
SD / gas SD) varies strongly across our sample and within individual galaxies.
We show that in spirals the SFE is a clear function of radius, while the dwarf
galaxies in our sample display SFEs similar to those found in the outer optical
disks of the spirals. Another general feature of our sample is a sharp
saturation of the HI SD at ~9 M_sol/pc^2 in both the spiral and dwarf galaxies.Comment: Accepted for publication in the AJ special THINGS issue. For a
high-resolution version visit: http://www.mpia.de/THINGS/Publications.htm
Time-resolved diffraction with an optimized short pulse laser plasma X-ray source
We present a set-up for time-resolved X-ray diffraction based on a short
pulse, laser-driven plasma X-ray source. The employed modular design provides
high flexibility to adapt the set-up to the specific requirements (e.g. X-ray
optics, sample environment) of particular applications. The configuration
discussed here has been optimized towards high angular/momentum resolution and
uses K-radiation (4.51 keV) from a Ti wire-target in combination
with a toroidally bent crystal for collection, monochromatization and focusing
of the emitted radiation. Ti-K photons per pulse
with relative bandwidth are delivered to the sample at 10 Hz
repetition rate. This allows for high dynamic range () measurements of
transient changes of the rocking curves of materials as for example induced by
laser-triggered strain waves.Comment: 29 pages, 8 figure
Estimating the star formation rate at 1 kpc scales in nearby galaxies.
Using combinations of H alpha, ultraviolet (UV), and infrared (IR) emission, we estimate the star formation rate (SFR) surface density, Sigma(SFR), at 1 kpc resolution for 30 disk galaxies that are targets of the IRAM HERACLES CO survey. We present a new physically motivated IR spectral-energy-distribution-based approach to account for possible contributions to 24 mu m emission not associated with recent star formation. Considering a variety of "reference" SFRs from the literature, we revisit the calibration of the 24 mu m term in hybrid (UV+IR or H alpha+IR) tracers. We show that the overall calibration of this term remains uncertain at the factor of two level because of the lack of wide-field, robust reference SFR estimates. Within this uncertainty, published calibrations represent a reasonable starting point for 1 kpc-wide areas of star-forming disk galaxies, but we re-derive and refine the calibration of the IR term in these tracers to match our resolution and approach to 24 mu m emission. We compare a large suite of Sigma(SFR) estimates and find that above Sigma(SFR) similar to 10(-3)M(circle dot) yr(-1) kpc(-2) the systematic differences among tracers are less than a factor of two across two orders of magnitude dynamic range. We caution that methodology and data both become serious issues below this level. We note from simple model considerations that when focusing on a part of a galaxy dominated by a single stellar population, the intrinsic uncertainty in H alpha- and FUV-based SFRs is similar to 0.3 and similar to 0.5 dex.Peer reviewe
Smooth HI Low Column Density Outskirts In Nearby Galaxies
This is an author-created, un-copyedited version of an article published in The Astronomical Journal. The Version of Record is available online at https://doi.org/10.3847/1538-3881/aabbaa.The low column density gas at the outskirts of galaxies as traced by the 21 cm hydrogen line emission (H i) represents the interface between galaxies and the intergalactic medium, i.e., where galaxies are believed to get their supply of gas to fuel future episodes of star formation. Photoionization models predict a break in the radial profiles of H i at a column density of âź5 Ă 10 19 cm -2 due to the lack of self-shielding against extragalactic ionizing photons. To investigate the prevalence of such breaks in galactic disks and to characterize what determines the potential edge of the H i disks, we study the azimuthally averaged H i column density profiles of 17 nearby galaxies from the H i Nearby Galaxy Survey and supplemented in two cases with published Hydrogen Accretion in LOcal GAlaxieS data. To detect potential faint H i emission that would otherwise be undetected using conventional moment map analysis, we line up individual profiles to the same reference velocity and average them azimuthally to derive stacked radial profiles. To do so, we use model velocity fields created from a simple extrapolation of the rotation curves to align the profiles in velocity at radii beyond the extent probed with the sensitivity of traditional integrated H i maps. With this method, we improve our sensitivity to outer-disk H i emission by up to an order of magnitude. Except for a few disturbed galaxies, none show evidence of a sudden change in the slope of the H i radial profiles: the alleged signature of ionization by the extragalactic background.Peer reviewedFinal Accepted Versio
The M81 Group Dwarf Irregular Galaxy DDO 165. I. High Velocity Neutral Gas in a Post-Starburst System
We present new multi-configuration VLA HI spectral line observations of the
M81 group dIrr post-starburst galaxy DDO 165. The HI morphology is complex,
with multiple column density peaks surrounding a large region of very low HI
surface density that is offset from the center of the stellar distribution. The
bulk of the neutral gas is associated with the southern section of the galaxy;
a secondary peak in the north contains ~15% of the total HI mass. These
components appear to be kinematically distinct, suggesting that either tidal
processes or large-scale blowout have recently shaped the ISM of DDO 165. Using
spatially-resolved position-velocity maps, we find multiple localized
high-velocity gas features. Cross-correlating with radius-velocity analyses, we
identify eight shell/hole structures in the ISM with a range of sizes (~400-900
pc) and expansion velocities (~7-11 km/s). These structures are compared with
narrow- and broad-band imaging from KPNO and HST. Using the latter data, recent
works have shown that DDO 165's previous "burst" phase was extended temporally
(>1 Gyr). We thus interpret the high-velocity gas features, HI holes, and
kinematically distinct components of the galaxy in the context of the immediate
effects of "feedback" from recent star formation. In addition to creating HI
holes and shells, extended star formation events are capable of creating
localized high velocity motion of the surrounding interstellar material. A
companion paper connects the energetics from the HI and HST data.Comment: The Astrophysical Journal, in press. Full-resolution version
available on request from the first autho
Little Things
We present LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes,
The HI Nearby Galaxy Survey) that is aimed at determining what drives star
formation in dwarf galaxies. This is a multi-wavelength survey of 37 Dwarf
Irregular and 4 Blue Compact Dwarf galaxies that is centered around HI-line
data obtained with the National Radio Astronomy Observatory (NRAO) Very Large
Array (VLA). The HI-line data are characterized by high sensitivity (less than
1.1 mJy/beam per channel), high spectral resolution (less than or equal to 2.6
km/s), and high angular resolution (~6 arcseconds. The LITTLE THINGS sample
contains dwarf galaxies that are relatively nearby (less than or equal to 10.3
Mpc; 6 arcseconds is less than or equal to 300 pc), that were known to contain
atomic hydrogen, the fuel for star formation, and that cover a large range in
dwarf galactic properties. We describe our VLA data acquisition, calibration,
and mapping procedures, as well as HI map characteristics, and show channel
maps, moment maps, velocity-flux profiles, and surface gas density profiles. In
addition to the HI data we have GALEX UV and ground-based UBV and Halpha images
for most of the galaxies, and JHK images for some. Spitzer mid-IR images are
available for many of the galaxies as well. These data sets are available
on-line.Comment: In press in A
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