COLD MILKY WAY H i GAS IN FILAMENTS Kalberla, P. M. W.; Kerp, J.; Haud, U. ...
The Astrophysical journal,
04/2016, Letnik:
821, Številka:
2
Journal Article
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ABSTRACT We investigate data from the Galactic Effelsberg-Bonn H i Survey, supplemented with data from the third release of the Galactic All Sky Survey (GASS III) observed at Parkes. We explore the ...all-sky distribution of the local Galactic H i gas with v LSR < 25 km s−1 on angular scales of 11′-16′. Unsharp masking is applied to extract small-scale features. We find cold filaments that are aligned with polarized dust emission and conclude that the cold neutral medium (CNM) is mostly organized in sheets that are, because of projection effects, observed as filaments. These filaments are associated with dust ridges, aligned with the magnetic field measured on the structures by Planck at 353 GHz. The CNM above latitudes b > 20 ° is described by a log-normal distribution, with a median Doppler temperature TD = 223 K, derived from observed line widths that include turbulent contributions. The median neutral hydrogen (H i) column density is NH i 1019.1 cm−2. These CNM structures are embedded within a warm neutral medium with NH i 1020 cm−2. Assuming an average distance of 100 pc, we derive for the CNM sheets a thickness of 0.3 pc. Adopting a magnetic field strength of Btot = (6.0 1.8) G, proposed by Heiles & Troland, and assuming that the CNM filaments are confined by magnetic pressure, we estimate a thickness of 0.09 pc. Correspondingly, the median volume density is in the range 14 n 47 cm−3.
Context. The Effelsberg-Bonn Hi Survey (EBHIS) is a new 21-cm survey performed with the 100-m telescope at Effelsberg. It covers the whole northern sky out to a redshift of z ~ 0.07 and comprises Hi ...line emission from the Milky Way and the Local Volume. Aims. We aim to substitute the northern-hemisphere part of the Leiden/Argentine/Bonn Milky Way Hi survey (LAB) with this first EBHIS data release, which presents the Hi gas in the Milky Way regime. Methods. The use of a seven-beam L-band array made it feasible to perform this all-sky survey with a 100-m class telescope in a reasonable amount of observing time. State-of-the-art fast-Fourier-transform spectrometers provide the necessary data read-out speed, dynamic range, and spectral resolution to apply software radio-frequency interference mitigation. EBHIS is corrected for stray radiation and employs frequency-dependent flux-density calibration and sophisticated baseline-removal techniques to ensure the highest possible data quality. Results. Detailed analyses of the resulting data products show that EBHIS is not only outperforming LAB in terms of sensitivity and angular resolution, but also matches the intensity-scale of LAB extremely well, allowing EBHIS to be used as a drop-in replacement for LAB. Data products are made available to the public in a variety of forms. Most important, we provide a properly gridded Milky Way Hi column density map in HEALPix representation. To maximize the usefulness of EBHIS data, we estimate uncertainties in the Hi column density and brightness temperature distributions, accounting for systematic effects.
Context. The H i halo clouds of the Milky Way, and in particular the intermediate-velocity clouds (IVCs), are thought to be connected to Galactic fountain processes. Observations of fountain clouds ...are important for understanding the role of matter recycling and accretion onto the Galactic disk and subsequent star formation. Aims. Here, we quantify the amount of molecular gas in the Galactic halo. We focus on the rare class of molecular IVCs (MIVCs) and search for new objects. Methods. The H i-FIR correlation was studied across the entire northern and southern Galactic hemispheres at Galactic latitudes | b | > 20° to determine the amount and distribution of molecular gas in IVCs. We used the most recent large-scale H i and FIR data, the Effelsberg Bonn-H i Survey, the Parkes Galactic All-Sky Survey, and the Planck FIR surveys. Results. We present a catalogue of 239 MIVC candidates on the northern and southern Galactic hemispheres. Among these candidates, all previously known MIVCs are recovered except for one single source. The frequency of candidates differs significantly between the northern and southern Galactic hemispheres and between negative and positive LSR velocities as well. Conclusions. In our approach we analyse the local Galactic environment. Extrapolating our results to the entire Galaxy, the global inflow of atomic and molecular IVC gas onto the Milky Way may account for the major fraction of the gaseous mass that is required to sustain the current Galactic star formation rate.
WALLABY – an SKA Pathfinder H i survey Koribalski, Bärbel S.; Staveley-Smith, L.; Westmeier, T. ...
Astrophysics and space science,
07/2020, Letnik:
365, Številka:
7
Journal Article
Recenzirano
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The Widefield ASKAP L-band Legacy All-sky Blind surveY (
wallaby
) is a next-generation survey of neutral hydrogen (H
i
) in the Local Universe. It uses the widefield, high-resolution capability of ...the Australian Square Kilometer Array Pathfinder (ASKAP), a radio interferometer consisting of
36
×
12
-m dishes equipped with Phased-Array Feeds (PAFs), located in an extremely radio-quiet zone in Western Australia.
wallaby
aims to survey three-quarters of the sky (
−
90
∘
<
δ
<
+
30
∘
) to a redshift of
z
≲
0.26
, and generate spectral line image cubes at ∼30 arcsec resolution and ∼1.6 mJy beam
−1
per 4 km s
−1
channel sensitivity. ASKAP’s instantaneous field of view at 1.4 GHz, delivered by the PAF’s 36 beams, is about 30 sq deg. At an integrated signal-to-noise ratio of five,
wallaby
is expected to detect around half a million galaxies with a mean redshift of
z
∼
0.05
(∼200 Mpc). The scientific goals of
wallaby
include: (a) a census of gas-rich galaxies in the vicinity of the Local Group; (b) a study of the H
i
properties of galaxies, groups and clusters, in particular the influence of the environment on galaxy evolution; and (c) the refinement of cosmological parameters using the spatial and redshift distribution of low-bias gas-rich galaxies. For context we provide an overview of recent and planned large-scale H
i
surveys. Combined with existing and new multi-wavelength sky surveys,
wallaby
will enable an exciting new generation of panchromatic studies of the Local Universe. — First results from the
wallaby
pilot survey are revealed, with initial data products publicly available in the CSIRO ASKAP Science Data Archive (CASDA).
Context. The local Galactic Hi gas was found to contain cold neutral medium (CNM) filaments that are aligned with polarized dust emission. These filaments appear to be dominated by the magnetic field ...and in this case turbulence is expected to show distinct anisotropies. Aims. We use the Galactic Effelsberg-Bonn Hi Survey (EBHIS) to derive 2D turbulence spectra for the Hi distribution in direction to 3C 196 and two more comparison fields. Methods. Prior to Fourier transform we apply a rotational symmetric 50% Tukey window to apodize the data. We derive average as well as position angle dependent power spectra. Anisotropies in the power distribution are defined as the ratio of the spectral power in orthogonal directions. Results. We find strong anisotropies. For a narrow range in position angle, in direction perpendicular to the filaments and the magnetic field, the spectral power is on average more than an order of magnitude larger than parallel. In the most extreme case the anisotropy reaches locally a factor of 130. Anisotropies increase on average with spatial frequency as predicted by Goldreich & Sridhar (1995, ApJ, 438, 763), at the same time the Kolmogorov spectral index remains almost unchanged. The strongest anisotropies are observable for a narrow range in velocity and decay with a power law index close to −8/3, almost identical to the average isotropic spectral index of −2.9 <γ< −2.6. Conclusions. Hi filaments, associated with linear polarization structures in LOFAR observations in direction to 3C 196, show turbulence spectra with marked anisotropies. Decaying anisotropies appear to indicate that we witness an ongoing shock passing the Hi and affecting the observed Faraday depth.
Context. A recent discovery of two stellar clusters associated with the diffuse high-latitude cloud HRK 81.4–77.8 has important implications for star formation in the Galactic halo. Aims. We derive a ...plausible distance estimate to HRK 81.4-77.8 primarily from its gaseous properties. Methods. We spatially correlate state-of-the-art HI, far-infrared and soft X-ray data to analyze the diffuse gas in the cloud. The absorption of the soft X-ray emission from the Galactic halo by HRK 81.4-77.8 is used to constrain the distance to the cloud. Results. HRK 81.4-77.8 is most likely located at an altitude of about 400 pc within the disk-halo interface of the Milky Way Galaxy. The HI data discloses a disbalance in density and pressure between the warm and cold gaseous phases. Apparently, the cold gas is compressed by the warm medium. This disbalance might trigger the formation of molecular gas high above the Galactic plane on pc to sub-pc scales.
Context. Because isolated high-velocity clouds (HVCs) are found at great distances from the Galactic radiation field and because they have subsolar metallicities, there have been no detections of ...dust in these structures. A key problem in this search is the removal of foreground dust emission. Aims. Using the Effelsberg-Bonn H i Survey and the Planck far-infrared data, we investigate a bright, cold, and clumpy HVC. This cloud apparently undergoes an interaction with the ambient medium and thus has great potential to form dust. Methods. To remove the local foreground dust emission we used a regularised, generalised linear model and we show the advantages of this approach with respect to other methods. To estimate the dust emissivity of the HVC, we set up a simple Bayesian model with mildly informative priors to perform the line fit instead of an ordinary linear least-squares approach. Results. We find that the foreground can be modelled accurately and robustly with our approach and is limited mostly by the cosmic infrared background. Despite this improvement, we did not detect any significant dust emission from this promising HVC. The 3σ-equivalent upper limit to the dust emissivity is an order of magnitude below the typical values for the Galactic interstellar medium.
Context.
The atomic phase of the interstellar medium plays a key role in the formation process of molecular clouds. Due to the line-of-sight confusion in the Galactic plane that is associated with ...its ubiquity, atomic hydrogen emission has been challenging to study.
Aims.
We investigate the physical properties of the “Maggie” filament, a large-scale filament identified in H
I
emission at line-of-sight velocities,
v
LSR
~−54 km s
−1
.
Methods.
Employing the high-angular resolution data from The H
I
/OH Recombination line survey of the inner Milky Way (THOR), we have been able to study H
I
emission features at negative
v
LSR
velocities without any line-of-sight confusion due to the kinematic distance ambiguity in the first Galactic quadrant. In order to investigate the kinematic structure, we decomposed the emission spectra using the automated Gaussian fitting algorithm G
AUSS
P
Y
+.
Results.
We identify one of the largest, coherent, mostly atomic H
I
filaments in the Milky Way. The giant atomic filament Maggie, with a total length of 1.2 ± 0.1 kpc, is not detected in most other tracers, and it does not show signs of active star formation. At a kinematic distance of 17 kpc, Maggie is situated below (by ≈500 pc), but parallel to, the Galactic H
I
disk and is trailing the predicted location of the Outer Arm by 5−10 km s
−1
in longitude-velocity space. The centroid velocity exhibits a smooth gradient of less than ±3 km s
−1
(10 pc)
−1
and a coherent structure to within ±6 km s
−1
. The line widths of ~10 km s
−1
along the spine of the filament are dominated by nonthermal effects. After correcting for optical depth effects, the mass of Maggie’s dense spine is estimated to be 7.2
−1.9
+2.5
× 10
5
M
⊙
. The mean number density of the filament is ~4 cm
−3
, which is best explained by the filament being a mix of cold and warm neutral gas. In contrast to molecular filaments, the turbulent Mach number and velocity structure function suggest that Maggie is driven by transonic to moderately supersonic velocities that are likely associated with the Galactic potential rather than being subject to the effects of self-gravity or stellar feedback. The probability density function of the column density displays a log-normal shape around a mean of ⟨
N
H
I
⟩ = 4.8 × 10
20
cm
−2
, thus reflecting the absence of dominating effects of gravitational contraction.
Conclusions.
While Maggie’s origin remains unclear, we hypothesize that Maggie could be the first in a class of atomic clouds that are the precursors of giant molecular filaments.
Context. Intermediate-velocity clouds (IVCs) are HI halo clouds that are likely related to a Galactic fountain process. In-falling IVCs are candidates for the re-accretion of matter onto the Milky ...Way. Aims. We study the evolution of IVCs at the disk-halo interface, focussing on the transition from atomic to molecular IVCs. We compare an atomic IVC to a molecular IVC and characterise their structural differences in order to investigate how molecular IVCs form high above the Galactic plane. Methods. With high-resolution HI observations of the Westerbork Synthesis Radio Telescope and 12CO(1 → 0) and 13CO(1 → 0) observations with the IRAM 30 m telescope, we analyse the small-scale structures within the two clouds. By correlating HI and far-infrared (FIR) dust continuum emission from the Planck satellite, the distribution of molecular hydrogen (H2) is estimated. We conduct a detailed comparison of the HI, FIR, and CO data and study variations of the XCO conversion factor. Results. The atomic IVC does not disclose detectable CO emission. The atomic small-scale structure, as revealed by the high-resolution HI data, shows low peak HI column densities and low HI fluxes as compared to the molecular IVC. The molecular IVC exhibits a rich molecular structure and most of the CO emission is observed at the eastern edge of the cloud. There is observational evidence that the molecular IVC is in a transient and, thus, non-equilibrium phase. The average XCO factor is close to the canonical value of the Milky Way disk. Conclusions. We propose that the two IVCs represent different states in a gradual transition from atomic to molecular clouds. The molecular IVC appears to be more condensed allowing the formation of H2 and CO in shielded regions all over the cloud. Ram pressure may accumulate gas and thus facilitate the formation of H2. We show evidence that the atomic IVC will evolve also into a molecular IVC in a few Myr.
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