13 research outputs found
The 21-SPONGE HI Absorption Survey I: Techniques and Initial Results
We present methods and results from "21-cm Spectral Line Observations of
Neutral Gas with the EVLA" (21-SPONGE), a large survey for Galactic neutral
hydrogen (HI) absorption with the Karl G. Jansky Very Large Array (VLA). With
the upgraded capabilities of the VLA, we reach median root-mean-square (RMS)
noise in optical depth of per
channel for the 31 sources presented here. Upon
completion, 21-SPONGE will be the largest HI absorption survey with this high
sensitivity. We discuss the observations and data reduction strategies, as well
as line fitting techniques. We prove that the VLA bandpass is stable enough to
detect broad, shallow lines associated with warm HI, and show that bandpass
observations can be combined in time to reduce spectral noise. In combination
with matching HI emission profiles from the Arecibo Observatory (
angular resolution), we estimate excitation (or spin) temperatures ()
and column densities for Gaussian components fitted to sightlines along which
we detect HI absorption (30/31). We measure temperatures up to for individual lines, showing that we can probe the
thermally unstable interstellar medium (ISM) directly. However, we detect fewer
of these thermally unstable components than expected from previous
observational studies. We probe a wide range in column density between
and for individual HI clouds. In addition,
we reproduce the trend between cold gas fraction and average found by
synthetic observations of a hydrodynamic ISM simulation by Kim et al. (2014).
Finally, we investigate methods for estimating HI and discuss their
biases.Comment: Accepted for publication in ApJ; 24 pages, 14 figure
Detection of the diffuse HI emission in the Circumgalactic Medium of NGC 891 and NGC 4565
We present detections of 21-cm emission from neutral hydrogen (HI) in the
circumgalactic medium (CGM) of the local edge-on galaxies NGC 891 and NGC 4565
using the Robert C. Byrd Green Bank Telescope (GBT). With our 5
sensitivity of cm calculated over a 20 km s
channel, we achieve detections out to kpc along the minor
axes. The velocity width of the CGM emission is as large as that of the disk
km s, indicating the existence of a diffuse component
permeating the halo. We compare our GBT measurements with interferometric data
from the Westerbork Synthesis Radio Telescope (WSRT). The WSRT maps the HI
emission from the disk at high S/N but has limited surface brightness
sensitivity at the angular scales probed with the GBT. After convolving the
WSRT data to the spatial resolution of the GBT (FWHM = 9.1), we find that
the emission detected by the WSRT accounts for %
(%) of the total flux recovered by the GBT from the CGM of NGC
891(NGC 4565). The existence of significant GBT-only flux suggests the presence
of a large amount of diffuse, low column density HI emission in the CGM. For
reasonable assumptions, the extended diffuse HI could account for %
and % of the total HI emission of NGC 891 and NGC 4565.Comment: 14 pages, 5 figures, published in Ap
Data Combination: Interferometry and Single-dish Imaging in Radio Astronomy
Modern interferometers routinely provide radio-astronomical images down to
subarcsecond resolution. However, interferometers filter out spatial scales
larger than those sampled by the shortest baselines, which affects the
measurement of both spatial and spectral features. Complementary single-dish
data are vital for recovering the true flux distribution of spatially resolved
astronomical sources with such extended emission. In this work, we provide an
overview of the prominent available methods to combine single-dish and
interferometric observations. We test each of these methods in the framework of
the CASA data analysis software package on both synthetic continuum and
observed spectral data sets. We develop a set of new assessment tools that are
generally applicable to all radio-astronomical cases of data combination.
Applying these new assessment diagnostics, we evaluate the methods' performance
and demonstrate the significant improvement of the combined results in
comparison to purely interferometric reductions. We provide combination and
assessment scripts as add-on material. Our results highlight the advantage of
using data combination to ensure high-quality science images of spatially
resolved objects.Comment: 29 pages, 20 figures. Accepted for publication in PASP. Code
repository available at: github.com/teuben/DataCom
A new method for spatially resolving the turbulence driving mixture in the ISM with application to the Small Magellanic Cloud
Turbulence plays a crucial role in shaping the structure of the interstellar
medium. The ratio of the three-dimensional density contrast
() to the turbulent sonic Mach number () of
an isothermal, compressible gas describes the ratio of solenoidal to
compressive modes in the turbulent acceleration field of the gas, and is
parameterised by the turbulence driving parameter:
. The turbulence driving parameter ranges
from (purely solenoidal) to (purely compressive), with
characterising the natural mixture (1/3~compressive, 2/3~solenoidal) of the two
driving modes. Here we present a new method for recovering
, , and , from observations on galactic
scales, using a roving kernel to produce maps of these quantities from column
density and centroid velocity maps. We apply our method to high-resolution HI
emission observations of the Small Magellanic Cloud (SMC) from the GASKAP-HI
survey. We find that the turbulence driving parameter varies between and within the main body of the SMC, but the median value
converges to , suggesting that the turbulence is overall driven more
compressively (). We observe no correlation between the parameter
and HI or H intensity, indicating that compressive driving of HI
turbulence cannot be determined solely by observing HI or H emission
density, and that velocity information must also be considered. Further
investigation is required to link our findings to potential driving mechanisms
such as star-formation feedback, gravitational collapse, or cloud-cloud
collisions.Comment: 20 pages, 16 figures, accepted to MNRA
Where do stars explode in the ISM? -- The distribution of dense gas around massive stars and supernova remnants in M33
Star formation in galaxies is regulated by turbulence, outflows, gas heating
and cloud dispersal -- processes which depend sensitively on the properties of
the interstellar medium (ISM) into which supernovae (SNe) explode.
Unfortunately, direct measurements of ISM environments around SNe remain
scarce, as SNe are rare and often distant. Here we demonstrate a new approach:
mapping the ISM around the massive stars that are soon to explode. This
provides a much larger census of explosion sites than possible with only SNe,
and allows comparison with sensitive, high-resolution maps of the atomic and
molecular gas from the Jansky VLA and ALMA. In the well-resolved Local Group
spiral M33, we specifically observe the environments of red supergiants (RSGs,
progenitors of Type II SNe), Wolf-Rayet stars (WRs, tracing stars 30
M, and possibly future stripped-envelope SNe), and supernova remnants
(SNRs, locations where SNe have exploded). We find that massive stars evolve
not only in dense, molecular-dominated gas (with younger stars in denser gas),
but also a substantial fraction (45\% of WRs; higher for RSGs) evolve in
lower-density, atomic-gas-dominated, inter-cloud media. We show that these
measurements are consistent with expectations from different stellar-age tracer
maps, and can be useful for validating SN feedback models in numerical
simulations of galaxies. Along with the discovery of a 20-pc diameter molecular
gas cavity around a WR, these findings re-emphasize the importance of
pre-SN/correlated-SN feedback evacuating the dense gas around massive stars
before explosion, and the need for high-resolution (down to pc-scale) surveys
of the multi-phase ISM in nearby galaxies.Comment: 34 pages, 14 figures. Submitted to ApJ. Comments welcome! The density
distributions will be made publicly available after journal acceptance of
manuscript. Please feel free to contact us in the meantime if you would like
to use the
A High Resolution Study of the HI-H2 Transition across the Perseus Molecular Cloud
To investigate the fundamental principles of H2 formation in a giant
molecular cloud (GMC), we derive the HI and H2 surface density (Sigma_HI and
Sigma_H2) images of the Perseus molecular cloud on sub-pc scales (~0.4 pc). We
use the far-infrared data from the Improved Reprocessing of the IRAS Survey and
the V-band extinction image provided by the COMPLETE Survey to estimate the
dust column density image of Perseus. In combination with the HI data from the
Galactic Arecibo L-band Feed Array HI Survey and an estimate of the local
dust-to-gas ratio, we then derive the Sigma_H2 distribution across Perseus. We
find a relatively uniform Sigma_HI ~ 6-8 Msun pc^-2 for both dark and
star-forming regions, suggesting a minimum HI surface density required to
shield H2 against photodissociation. As a result, a remarkably tight and
consistent relation is found between Sigma_H2/Sigma_HI and Sigma_HI+Sigma_H2.
The transition between the HI- and H2-dominated regions occurs at N(HI)+2N(H2)
~ (8-14) x 10^20 cm^-2. Our findings are consistent with predictions for H2
formation in equilibrium, suggesting that turbulence may not be of primary
importance for H2 formation. However, the importance of a warm neutral medium
for H2 shielding, an internal radiation field, and the timescale of H2
formation still remain as open questions. We also compare H2 and CO
distributions and estimate the fraction of "CO-dark" gas, f_DG ~ 0.3. While
significant spatial variations of f_DG are found, we do not find a clear
correlation with the mean V-band extinction.Comment: updated to match the final version published in April 201
Molecular Gas in the Outflow of the Small Magellanic Cloud
We report the first evidence of molecular gas in two atomic hydrogen (H I) clouds associated with gas outflowing
from the Small Magellanic Cloud (SMC). We used the Atacama Pathfinder Experiment to detect and spatially
resolve individual clumps of 12CO(2 → 1) emission in both clouds. CO clumps are compact (∼10 pc) and
dynamically cold (line widths - 1 km s 1
). Most CO emission appears to be offset from the peaks of the H I
emission, some molecular gas lies in regions without a clear H I counterpart. We estimate a total molecular gas
mass of M M mol 10 10 –
3 4 in each cloud and molecular gas fractions up to 30% of the total cold gas mass
(molecular + neutral). Under the assumption that this gas is escaping the galaxy, we calculated a cold gas outflow
rate of ˙ – M M - gas 0.3 1.8 yr 1 and mass loading factors of b 3 1– 2 at a distance larger than 1 kpc. These
results show that relatively weak starburst-driven winds in dwarf galaxies like the SMC are able to accelerate
significant amounts of cold and dense matter and inject it into the surrounding environment