66 research outputs found
Cold gas in the Milky Way's nuclear wind
The centre of the Milky Way is the site of several high-energy processes that
have strongly impacted the inner regions of our Galaxy. Activity from the
super-massive black hole, Sgr A*, and/or stellar feedback from the inner
molecular ring expel matter and energy from the disc in the form of a galactic
wind. Multiphase gas has been observed within this outflow, from hot
highly-ionized, to warm ionized and cool atomic gas. To date, however, there
has been no evidence of the cold and dense molecular phase. Here we report the
first detection of molecular gas outflowing from the centre of our Galaxy. This
cold material is associated with atomic hydrogen clouds travelling in the
nuclear wind. The morphology and the kinematics of the molecular gas, resolved
on ~1 pc scale, indicate that these clouds are mixing with the warmer medium
and are possibly being disrupted. The data also suggest that the mass of
molecular gas driven out is not negligible and could impact the rate of star
formation in the central regions. The presence of this cold, dense,
high-velocity gas is puzzling, as neither Sgr A* at its current level of
activity, nor star formation in the inner Galaxy seem viable sources for this
material.Comment: Published in the August 19 issue of Nature. This is the authors'
version before final edits. Published version is available at
http://www.nature.com/articles/s41586-020-2595-
High velocity clouds in the Galactic All Sky Survey I. Catalogue
We present a catalogue of high-velocity clouds (HVCs) from the Galactic All
Sky Survey (GASS) of southern-sky neutral hydrogen, which has 57 mK sensitivity
and 1 km/s velocity resolution and was obtained with the Parkes Telescope. Our
catalogue has been derived from the stray-radiation corrected second release of
GASS. We describe the data and our method of identifying HVCs and analyse the
overall properties of the GASS population. We catalogue a total of 1693 HVCs at
declinations < 0 deg, including 1111 positive velocity HVCs and 582 negative
velocity HVCs. Our catalogue also includes 295 anomalous velocity clouds
(AVCs). The cloud line-widths of our HVC population have a median FWHM of ~19
km/s, which is lower than found in previous surveys. The completeness of our
catalogue is above 95% based on comparison with the HIPASS catalogue of HVCs,
upon which we improve with an order of magnitude in spectral resolution. We
find 758 new HVCs and AVCs with no HIPASS counterpart. The GASS catalogue will
shed an unprecedented light on the distribution and kinematic structure of
southern-sky HVCs, as well as delve further into the cloud populations that
make up the anomalous velocity gas of the Milky Way.Comment: 21 pages, 14 figures, accepted for publication in ApJ
Broad-band radio circular polarization spectrum of the relativistic jet in PKS B2126-158
We present full Stokes radio polarization observations of the quasar PKS B2126−158 (z = 3.268) from 1 to 10 GHz using the Australia Telescope Compact Array. The source
has large fractional circular polarization (CP), mc ≡ |V|/I, detected at high significance across the entire band (from 15 to 90σ per 128 MHz subband). This allows us to construct the most robust CP spectrum of an active galactic nucleus (AGN) jet to date. We find mc ∝ ν+0.60 ± 0.03 from 1.5 to 6.5 GHz, with a peak of mc ∼ 1 per cent before the spectrum turns over somewhere between 6.5 and 8 GHz, above which mc ∝ ν−3.0 ± 0.4. The fractional linear polarization (LP;p) varies from 0.2 to ∼1 per cent across our frequency range and is strongly anticorrelated with the fractional CP, with a best-fitting power law giving mc ∝ p−0.24 ± 0.03. This is the
first clear relation between the observed LP and CP of an AGN jet, revealing the action of Faraday conversion of LP to CP within the jet. More detailed modelling in conjunction with high spatial resolution observations are required to determine the true driving force behind the conversion (i.e. magnetic twist or internal Faraday rotation). In particular determining whether the observed Faraday rotation is internal or entirely external to the jet is key to this goal. The simplest interpretation of our observations favours some internal Faraday rotation, implying that Faraday rotation-driven conversion of LP to CP is the dominant CP generation mechanism. In this case, a small amount of vector-ordered magnetic field along the jet axis
is required, along with internal Faraday rotation from the low-energy end of the relativistic electron energy spectrum in an electron–proton-dominated jet
The australia telescope compact array HI survey of the galactic center
We present a survey of atomic hydrogen (H I) emission in the direction of the Galactic Center (GC) conducted with the CSIRO Australia Telescope Compact Array (ATCA). The survey covers the area -5° ≤ l ≤ +5°, -5° ≤ b ≤ +5° over the velocity range -309 km s -1 ≤ v LSR ≤ 349km s -1 with a velocity resolution of 1km s -1. The ATCA data are supplemented with data from the Parkes Radio Telescope for sensitivity to all angular scales larger than the 145″ angular resolution of the survey. The mean rms brightness temperature across the field is 0.7K, except near (l, b) = 0°, 0° where it increases to ∼2K. This survey complements the Southern Galactic Plane Survey to complete the continuous coverage of the inner Galactic plane in H I at ∼2′ resolution. Here, we describe the observations and analysis of this GC survey and present the final data product. Features such as Bania's Clump2, the far 3kpc arm, and small high-velocity clumps are briefly described
Low-Mach-number turbulence in interstellar gas revealed by radio polarization gradients
The interstellar medium of the Milky Way is multi-phase, magnetized and
turbulent. Turbulence in the interstellar medium produces a global cascade of
random gas motions, spanning scales ranging from 100 parsecs to 1000
kilometres. Fundamental parameters of interstellar turbulence such as the sonic
Mach number (the speed of sound) have been difficult to determine because
observations have lacked the sensitivity and resolution to directly image the
small-scale structure associated with turbulent motion. Observations of linear
polarization and Faraday rotation in radio emission from the Milky Way have
identified unusual polarized structures that often have no counterparts in the
total radiation intensity or at other wavelengths, and whose physical
significance has been unclear. Here we report that the gradient of the Stokes
vector (Q,U), where Q and U are parameters describing the polarization state of
radiation, provides an image of magnetized turbulence in diffuse ionized gas,
manifested as a complex filamentary web of discontinuities in gas density and
magnetic field. Through comparison with simulations, we demonstrate that
turbulence in the warm ionized medium has a relatively low sonic Mach number,
M_s <~ 2. The development of statistical tools for the analysis of polarization
gradients will allow accurate determinations of the Mach number, Reynolds
number and magnetic field strength in interstellar turbulence over a wide range
of conditions.Comment: 5 pages, 3 figures, published in Nature on 13 Oct 201
Direct observations of the atomic-molecular phase transition in the Milky Way's nuclear wind
Hundreds of high-velocity atomic gas clouds exist above and below the
Galactic Centre, with some containing a molecular component. However, the
origin of these clouds in the Milky Way's wind is unclear. This paper presents
new high-resolution MeerKAT observations of three atomic gas clouds and studies
the relationship between the atomic and molecular phases at pc scales.
The clouds' atomic hydrogen column densities, , are less than
a \mbox{few}\times 10^{20} cm, but the two clouds closest to the
Galactic Centre nonetheless have detectable CO emission. This implies the
presence of H at levels of at least a factor of ten
lower than in the typical Galactic interstellar medium. For the cloud closest
to the Galactic Centre, there is little correlation between the
and the probability that it will harbour detectable CO
emissions. In contrast, for the intermediate cloud, detectable CO is heavily
biased toward the highest values of . The cloud most distant
from the Galactic Centre has no detectable CO at similar
values. Moreover, we find that the two clouds with detectable CO are too
molecule-rich to be in chemical equilibrium, given the depths of their atomic
shielding layers, which suggests a scenario whereby these clouds consist of
pre-existing molecular gas from the disc that the Galactic wind has swept up,
and that is dissociating into atomic hydrogen as it flows away from the Galaxy.
We estimate that entrained molecular material of this type has a Myr lifetime before photodissociating.Comment: 11 pages, 6 figures, 2 tables. Submitted to MNRA
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