Abstract
Radio, X-ray, and infrared observations of the inner few hundred parsecs of the Galactic Centre have highlighted two characteristics of the interstellar medium. The cosmic-ray ionization ...rate derived from molecular ions such as H$^+_3$ is at least two to three orders of magnitude higher than in the Galactic disc. The other is bipolar X-ray and radio emission away from the Galactic plane. These features are consistent with a scenario in which high cosmic-ray pressure drives large-scale winds away from the Galactic plane. The interaction of such a wind with stellar wind bubbles may explain the energetic non-thermal radio filaments found throughout the Galactic Centre. Some of the implications of this scenario is the removal of gas driven by outflowing winds, acting as a feedback to reduce the star formation rate in the central molecular zone (CMZ), and the distortion of azimuthal magnetic field lines in the CMZ to vertical direction away from the plane. The combined effects of the wind and the vertical magnetic field can explain why most magnetized filaments run perpendicular to the galactic plane. This proposed picture suggests our Milky Way nucleus has recently experienced starburst or black hole activity, as recent radio and X-ray observations indicate.
ABSTRACT Class I methanol masers are collisionally pumped and are generally correlated with outflows in star-forming sites in the Galaxy. Using the Very Large Array in its A-array configuration, we ...present a spectral line survey to identify methanol emission at 36.169 GHz. Over 900 pointings were used to cover a region 66′ × 13′ along the inner Galactic plane. A shallow survey of OH at 1612, 1665, 1667, and 1720 MHz was also carried out over the area covered by our methanol survey. We provide a catalog of 2240 methanol masers with narrow line-widths of ∼1 km s−1, spatial resolutions of , and rms noises ∼20 mJy beam−1 per channel. Lower limits on the brightness temperature range from 27,000 to 10,000,000 K, showing that the emission is of non-thermal origin. We also provide a list of 23 OH (1612), 14 OH (1665), 5 OH (1667), and 5 OH (1720 MHz) masers. The origin of such a large number of methanol masers is not clear. Many methanol masers appear to be associated with infrared dark clouds, though it appears unlikely that the entire population of these masers trace the early phase of star formation in the Galactic center.
Abstract
Magnetized radio filaments are found in abundance in the inner few hundred parsecs of our galaxy. Progress in understanding this population of filaments has been slow over the last few ...decades, in part due to a lack of detection elsewhere in the galaxy or in external galaxies. Recent highly sensitive radio continuum observations of radio galaxies in galaxy clusters have revealed remarkable isolated filamentary structures in the intracluster medium (ICM) that are linked to radio jets, tails, and lobes. The origin of this class of filaments is not understood either. Here, we argue that the underlying physical mechanisms responsible for the creation of the two populations are the same because of their similarities in morphology, spacing between the filaments, aspect ratio, and magnetic energy densities to the thermal pressure of the medium and that both populations have undergone synchrotron aging. These similarities provide an opportunity to investigate the physical processes in the interstellar medium (ISM) and ICM for the first time. We consider that the origin of the filaments in both the Galactic center and ICM is a result of the interaction of a large-scale wind with clouds, or the filaments arise through the stretching and collection of field lines by turbulence in a weakly magnetized medium. We examine these ideas in four radio galaxy filaments associated with four radio galaxies—IC 40B, IC 4496, J1333–3141, ESO 137–006—and argue that much can be understood in the future by comparing these two populations of filaments.
Abstract
We have examined the distribution of the position angle (PA) of the Galactic center filaments with lengths
L
> 66″ and <66″ as well as their length distribution as a function of PA. We find ...bimodal PA distributions of the filaments, and long and short populations of radio filaments. Our PA study shows the evidence for a distinct population of short filaments with PA close to the Galactic plane. Mainly thermal, short-radio filaments (<66″) have PAs concentrated close to the Galactic plane within 60° < PA < 120°. Remarkably, the short filament PAs are radial with respect to the Galactic center at
l
< 0° and extend in the direction toward Sgr A*. On a smaller scale, the prominent Sgr E H
ii
complex G358.7-0.0 provides a vivid example of the nearly radial distribution of short filaments. The bimodal PA distribution suggests a different origin for two distinct filament populations. We argue that the alignment of the short-filament population results from the ram pressure of a degree-scale outflow from Sgr A* that exceeds the internal filament pressure, and aligns them along the Galactic plane. The ram pressure is estimated to be 2 × 10
6
cm
−3
K at a distance of 300 pc, requiring biconical mass outflow rate 10
−4
M
⊙
yr
−1
with an opening angle of ∼40°. This outflow aligns not only the magnetized filaments along the Galactic plane but also accelerates thermal material associated with embedded or partially embedded clouds. This places an estimate of ∼6 Myr as the age of the outflow.
We report the discovery of a widespread population of collisionally excited methanol J = 4 sub(-1) to 3 sub(0) E sources at 36.2 GHz from the inner 66' x 18' (160 x 43 pc) of the Galactic center. ...This spectral feature was imaged with a spectral resolution of 16.6 km s super(-1) taken from 41 channels of a Very Large Array continuum survey of the Galactic center region. The revelation of 356 methanol sources, most of which are maser candidates, suggests a large abundance of methanol in the gas phase in the Galactic center region. There is also spatial and kinematic correlation between SiO (2-1) and CH sub(3)OH emission from four Galactic center clouds: the +50 and +20 km s super(-1) clouds and G0.13-0.13 and G0.25 + 0.01. The enhanced abundance of methanol is accounted for in terms of induced photodesorption by cosmic rays as they travel through a molecular core, collide, dissociate, ionize, and excite Lyman Werner transitions of H sub(2). A time-dependent chemical model in which cosmic rays drive the chemistry of the gas predicts CH sub(3)OH abundance of 10 super(-8) to 10 super(-7) on a chemical timescale of 5 x 10 super(4) to 5 x 10 super(5) years. The average methanol abundance produced by the release of methanol from grain surfaces is consistent with the available data.
ABSTRACT
We study the environment of Sgr A* using spectral and continuum observations with the Atacama Large Millimeter/submillimeter Array and Very Large Array. Our analysis of subarcsecond H30 α, ...H39 α, H52 α , and H56 α line emission towards Sgr A* confirms the recently published broad-peak ∼500 km s−1 spectrum towards Sgr A*. We also detect emission at more extreme radial velocities peaking near −2500 and 4000 km s−1 within 0.2 arcsec. We then present broad-band radio continuum images at multiple frequencies on scales from arcseconds to arcminutes. A number of elongated continuum structures lie parallel to the Galactic plane, extending from ∼0.4 arcsec to ∼10 arcmin. We note a non-thermal elongated structure on an arcminute scale emanating from Sgr A* at low frequencies between 1 and 1.4 GHz where thermal emission from the minispiral is depressed by optical depth effects. The position angle of this elongated structure and the sense of motion of ionized features with respect to Sgr A* suggest a symmetric, collimated jet emerging from Sgr A* with an opening angle of ∼30○ and a position angle of ∼60○ punching through the medium before accelerating a significant fraction of the orbiting ionized gas to high velocities. The jet with an estimated mass flow rate of ∼1.4 × 10−5 M⊙ yr−1 emerges perpendicular to the equatorial plane of the accretion flow near the event horizon of Sgr A* and runs along the Galactic plane. To explain a number of east–west features near Sgr A*, we also consider the possibility of an outflow component with a wider angle launched from the accretion flow at larger radii.
Recent MeerKAT radio continuum observations of the Galactic Centre at 20 cm show a large population of non-thermal radio filaments (NRFs) in the inner few hundred pc of the Galaxy. We have selected a ...sample of 57 radio sources, mainly compact objects, in the MeerKAT mosaic image that appear to be associated with NRFs. The selected sources are about four times the number of radio point sources associated with filaments than would be expected by random chance. Furthermore, an apparent correlation between bright IR stars and NRFs is inferred from their similar latitude distributions, suggesting that they both co-exist within the same region. To examine if compact radio sources are related to compact IR sources, we have used archival 2MASS, and Spitzer data to make spectral energy distribution of individual stellar sources coincident or close to radio sources. We provide a catalogue of radio and IR sources for future detailed observations to investigate a potential three-way physical association between NRFs, compact radio and IR stellar sources. This association is suggested by models in which NRFs are cometary tails produced by the interaction of a large-scale nuclear outflow with stellar wind bubbles in the Galactic Centre.
We have carried out Very Large Array (VLA) continuum observations to study the variability of Sgr A* at 43 GHz (l = 7 mm) and 22 GHz (l = 13 mm). A low level of flare activity has been detected with ...a duration of 62 hr at these frequencies, showing the peak flare emission at 43 GHz leading the 22 GHz peak flare by 620-40 minutes. The overall characteristics of the flare emission are interpreted in terms of the plasmon model of van der Laan by considering the ejection and adiabatic expansion of a uniform, spherical plasma blob due to flare activity. The observed peak of the flare emission with a spectral index, v super(-a), of a= 1.6 is consistent with the prediction that the peak emission shifts toward lower frequencies in an adiabatically expanding self-absorbed source. We present the expected synchrotron light curves for an expanding blob, as well as the peak frequency emission, as a function of the energy spectral index constrained by the available flaring measurements in near-IR, submillimeter, millimeter, and radio wavelengths. We note that the blob model is consistent with the available measurements; however, we cannot rule out the jet of Sgr A*. If expanding material leaves the gravitational potential of Sgr A*, the total mass-loss rate of nonthermal and thermal particles is estimated to be ,2 x 10 super(-8) M sub( )yr super(-1). We discuss the implication of the mass-loss rate, since this value matches closely the estimated accretion rate based on polarization measurements.
Abstract
We present high-pass-filtered continuum images of the inner 3.°5 × 2.°5 of the Galactic center at 20 cm with 6.″4 resolution. These mosaic images are taken with MeerKAT and reveal a large ...number of narrow filaments, roughly an order of magnitude increase in their numbers compared to past measurements. For the first time, we carry out population studies of the spectral index and magnetic field of the entire region. The mean spectral indices of the filaments are steeper than supernova remnants (SNRs) (−0.62) with a value of
α
∼ −0.83. The variation in
α
is much larger than for the SNRs, suggesting that these characteristics have a different origin. A large-scale cosmic-ray-driven wind has recently been proposed to explain the origin of filaments and the large-scale 430 pc bipolar radio and X-ray structure. This favors the possibility that the large-scale bipolar radio/X-ray structure is produced by past activity of Sgr A* rather than a coordinated burst of supernovae. A trend of steeper indices is also noted with increasing distance from the Galactic plane. This could be explained either by synchrotron cooling or weak shocks accelerating cosmic-ray particles in the context of the cosmic-ray-driven wind. The mean magnetic field strengths along the filaments range from ∼100 to 400
μ
G depending on the assumed ratio of cosmic-ray protons to electrons. Given that there is a high cosmic-ray pressure in the Galactic center, the large equipartition magnetic field implies that the magnetic filed is weak in most of the interstellar volume of the Galactic center.
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