Arched Filaments Research Articles

Multiscale magnetic fields in the central molecular zone: inference from the gradient technique

ABSTRACTThe central molecular zone (CMZ) plays an essential role in regulating the nuclear ecosystem of our Galaxy. To get an insight into magnetic fields of the CMZ, we employ the gradient technique (GT), which is rooted in the anisotropy of magnetohydrodynamic turbulence. Our analysis is based on the data of multiple wavelengths, including molecular emission lines, radio 1.4 GHz continuum image, and Herschel $70\, {\mu }{\rm m}$ image, as well as ionized [Ne ii] and Paschen-alpha emissions. The results are compared with the observations of Planck 353 GHz and High-resolution Airborne Wideband Camera Plus (HWAC+) $53\, {\mu }{\rm m}$ polarized dust emissions. We map the magnetic fields orientation at multiple wavelength across the central molecular zone, including close-ups of the Radio Arc and Sagittarius A West regions, on multiscales from ∼0.1 pc to 10 pc. The magnetic fields towards the central molecular zone traced by the GT are globally compatible with the polarization measurements, accounting for the contribution from the galactic foreground and background. This correspondence suggests that the magnetic field and turbulence are dynamically crucial in the galactic center. We find that the magnetic fields associated with the Arched filaments and the thermal components of the Radio Arc are in good agree with the HAWC+ polarization. Our measurement towards the non-thermal Radio Arc reveals the poloidal magnetic field components in the galactic center. For Sagittarius A West region, we find a great agreement between the GT measurement using [Ne ii] emission and HWAC+ $53\, {\mu }{\rm m}$ observation. We use the GT to predict the magnetic fields associated with ionized Paschen-alpha gas down to scales of 0.1 pc.

Warm ISM in the Sgr A complex

Context. The Arches Cluster – Arched Filaments (AF) system is our Galaxy’s prime example of the complexity involved in the interaction between the strong radiation field of numerous OB stars and their surrounding ISM in extremely harsh environments such as the Galactic center (GC) of the Milky Way. It offers a unique opportunity to study the close relationship between photon-dominated regions (PDRs) and H II regions and their relative contributions to the observed [CII] emission. Aims. We aim to investigate the I([CII]) versus I([NII]) integrated intensity behavior in the AF region in order to assess the [CII] emission contribution from the H II region, which is traced by [NII] line observations, and PDR components in the high-metallicity environment of the GC. Methods. We used [CII] 158 μm and [NII] 205 μm fine structure line observations of the AF in the literature to compare their observational integrated intensity distribution to semi-theoretical predictions for the contribution of H II regions and adjacent PDRs to the observed [CII] emission. We explored variations in the [C/N] elemental abundance ratio to explain the overall behavior of the observed relationship. Based on our models, the H II region and PDR contributions to the observed [CII] emission is calculated for a few positions within and near to the AF. Estimates for the [C/N] abundance ratio and [N/H] nitrogen elemental abundance in the AF can then be derived. Results. The behavior of the I([CII]) versus I([NII]) relationship in the AF can be explained by model results satisfying 0.84 < [C/N]AF < 1.41, with model metallicities ranging from 1 Z⊙ to 2 Z⊙, hydrogen volume density log n(H) = 3.5, and ionization parameters log U from −1 to −2. A least-squares fit to the model data points yields log I([CII]) = 1.068 × log I([NII]) + 0.645 to predict the [CII] emission arising from the H II regions in the AF. The fraction of the total observed [CII] emission arising from within PDRs varies between ~0.20 and ~0.75. Our results yield average values for the carbon-to-nitrogen ratio and nitrogen elemental abundances of [C/N]AF = 1.13 ± 0.09 and [N/H]AF = 6.21 × 10−4 for the AF, respectively. They are a factor of ~0.4 smaller and ~7.5 larger than their corresponding Galactic disk values. Conclusions. The large spatial variation of the fraction of [CII] emission arising either from H II regions or PDRs suggests that both contributions must be disentangled before any modeling attempt is made to explain the observed [CII] emission in the AF. We suggest thatsecondary production of nitrogen from low- to intermediate-mass stars in the Galactic bulge is a plausible mechanism to explain the large abundance differences between the GC and the Galactic disk. The mass loss of such stars would enrich the GC ISM with nitrogen as the gas falls into the inner GC orbits where the AF are located. Overall, our results show that tight constraints are needed on the [C/N] abundance ratio for the GC, significantly tighter than previous abundance measurements have discerned.

SOFIA/FORCAST Galactic Center Legacy Survey: Overview

Abstract The Galactic Center contains some of the most extreme conditions for star formation in our Galaxy, as well as many other phenomena that are unique to this region. Given our relative proximity to the Galactic Center, we are able to study details of physical processes to a level that is simply not yet possible for more distant galaxies, yielding an otherwise inaccessible view of the nuclear region of a galaxy. We recently carried out a targeted imaging survey of mid-infrared bright portions of the Galactic Center at 25 and 37 μm using the FORCAST instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA). This survey was one of the inaugural Legacy Programs from SOFIA cycle 7, observing a total area of 403 arcmin2 (2180 pc2), including the Sgr A, B, and C complexes. Here we present an overview of the survey strategy, observations, and data reduction as an accompaniment to the initial public release of the survey data. We discuss interesting regions and features within the data, including extended features near the circumnuclear disk, structures in the Arched Filaments and Sickle H ii regions, and signs of embedded star formation in Sgr B2 and Sgr C. We also feature a handful of less well studied mid-infrared sources located between Sgr A and Sgr C that could be sites of relatively isolated star formation activity. Last, we discuss plans for subsequent publications and future data releases from the survey.

Impact of Modified Spacer on Flow Pattern in Narrow Spacer-Filled Channels for Spiral-Wound Membrane Modules

A modified spacer, which was constructed with arched filaments and zigzag filaments, was designed to improve vortex shedding and generate a directional change in flow patterns of membrane modules, especially in the vicinity of the feed spacer filament, which is most affected by fouling. A unit cell was investigated by using a three-dimensional computational fluid dynamics (CFD) model for hydrodynamic simulation. The results of CFD simulations were carried out for the fluid flow in order to understand the effect of the modified spacer on vortices to the performance of arched filaments at different distances. From 2D velocity vectors and shear stress contour mixing, the flow pattern and dead zone flushing were depicted. The ratio of low shear stress area to the total area increased with the inlet velocity closed to 20%. The energy consumption with respect to flow direction for the arched filament was 80% lower than that in the zigzag filament. Compared with previous commercial spacers’ simulation, the friction factor was lower when the main flow was normal to the arched filament and the modified friction factor was close to the commercial spacers. The homogenization was realized through the flow pattern created by the modified spacer.

An Infrared Study of the Dust Properties and Geometry of the Arched Filaments H ii Region with SOFIA/FORCAST

Abstract Massive stellar clusters provide radiation ( ) and winds (∼1000 km s−1) that act to heat dust and shape their surrounding environment. In this paper, the Arched Filaments in the Galactic center were studied to better understand the influence of the Arches cluster on its nearby interstellar medium (ISM). The Arched Filaments were observed with the Faint Object InfraRed CAMera for the SOFIA Telescope at 19.7, 25.2, 31.5, and 37.1 μm. Color–temperature maps of the region created with the 25.2 and 37.1 μm data reveal relatively uniform dust temperatures (70–100 K) over the extent of the filaments (∼25 pc). Distances between the cluster and the filaments were calculated assuming equilibrium heating of standard-size ISM dust grains (∼0.1 μm). The distances inferred by this method are in conflict with the projected distance between the filaments and the cluster, although this inconsistency can be explained if the characteristic grain size in the filaments is smaller (∼0.01 μm) than typical values. DustEM models of selected locations within the filaments show evidence of depleted abundances of polycyclic aromatic hydrocarbons (PAHs) by factors of ∼1.6–10 by mass compared to the diffuse ISM. The evidence for both PAH depletion and a smaller characteristic grain size points to processing of the ISM within the filaments. We argue that the eroding of dust grains within the filaments is not likely attributable to the radiation or winds from the Arches cluster, but may be related to the physical conditions in the Galactic center.

Kinematics and properties of the central molecular zone as probed with [C ii

The Galactic Central Molecular Zone (CMZ) is a region containing massive and dense molecular clouds, with dynamics driven by a variety of energy sources including a massive black hole. It is thus the nearest template for understanding physical processes in extragalactic nuclei. The CMZ's neutral interstellar gas has been mapped spectrally in many neutral atomic and molecular gas tracers, but the ionized and CO-dark H2 regions are less well traced spectroscopically. We mapped the fine structure line of C+ at 158 microns, [C II], to identify and characterize features of the ionized gas in the CMZ, including UV irradiated neutral gas, photon dominated regions (PDRs), CO-dark H2 gas, and highly ionized gas. We observed the [C II] 158-micron fine structure line with high spectral resolution using Herschel HIFI with two perpendicular On-the-Fly strip scans, along l = -0.8 to +0.8 and b = -0.8 to +0.8, both centered on (l,b) = (0,0). We analyzed the spatial-velocity distribution of the [C II] data and compared them to those of [C I] and CO, and to dust continuum maps, in order to determine the properties and distribution of the UV irradiated gas and its dynamics within the CMZ. The longitude- and latitude-velocity maps of [C II] trace portions of the orbiting open gas streams of dense molecular clouds, the cloud G0.253+0.016, also known as the Brick, the Arched Filaments, and the ionized gas near Sgr A and Sgr B2. We use the [C II] and auxiliary data to determine the physical and dynamical properties of these CMZ features. The [C II] emission arises primarily from dense PDRs and highly ionized gas, and is an important tracer of the kinematics and physical conditions of this gas.

The warm ISM in the Sgr A region: mid-J CO, atomic carbon, ionized atomic carbon, and ionized nitrogen line observations with the Herschel/HIFI and NANTEN2/SMART Telescopes

AbstractWe present Herschel/HIFI sub-mm atomic carbon ([Ci] 3P1 – 3P0 and [Ci] 3P2 – 3P1), ionized carbon ([Cii] 2P3/2 – 2P1/2), and ionized nitrogen ([Nii] 3P1 – 3P0) line observations obtained in the frame of the Herschel Guaranteed Time HEXGAL (Herschel EXtraGALactic) key program (P. I. Rolf Güsten, MPIfR), and NANTEN2/SMART carbon monoxide (CO(J = 4 – 3)) observations of the warm gas around the Sgr A region. The spectrally resolved emission from all lines, and the corresponding line intensity ratios, show a very complex morphology. The determination of spatial and spectral (anti)correlation with known sources in the Sgr A region such as the Arched Filaments, NTF filaments, the Sickle, Quintuplet cluster, CND clouds, is ongoing work.

Sagittarius A Molecular Cloud Complex in H13CO+ and Thermal SiO Emission Lines

Abstract We present results of a high-resolution wide-field mapping observation of the Sagittarius A (Sgr A) molecular cloud complex in H$^{13}$CO$^+$$J$$=$ 1–0 and thermal SiO $J$$=$ 2–1 emission lines with the 25-beam receiver of the 100 GHz band operating on the Nobeyama Radio Observatory 45-m telescope. The mapping area covers a 0$^\circ\!\!\!.$5 $\times$ 0$^\circ\!\!\!.$5 area involving several named molecular clouds, for example GCM$-$0.02$-$0.07, GCM$-$0.13$-$0.08, GCM 0.11$-$0.11, the Sickle molecular cloud, the Arched filaments molecular cloud, and so on. The data have an effective angular resolution of 26$''$. The H$^{13}$CO$^+$ emission line is a famous tracer of molecular gas mass because the line is optically thin, even in the Galactic center region, and is not emphasized by shock. The emission line presents a clumpy distribution of the molecular cloud. The averaged fractional abundance in the Sgr A complex is $N$(H$^{13}$CO$^+$)$/N_{\mathrm{H_2}}$$\simeq$ (1.8$\ \pm\ $0.4) $\times$ 10$^{-11}$, comparing the LTE mass and the virial theorem mass. The SiO emission line is a famous and reliable tracer of shocked molecular gas. We find many molecular clouds that are remarkable only in the SiO emission line. Such molecular clouds have a large velocity width of up to 60 km s$^{-1}$. The brightness temperature ratio is up to $T_{\mathrm{B}}$(SiO)$/T_{\mathrm{B}}$(H$^{13}$CO$^+$) $\lesssim$ 8. The features are dominated by shock SiO-enriched gas. In such clouds, the ratio of the fractional abundance of SiO and H$^{13}$CO$^+$ molecules is $X$(SiO)$/X$(H$^{13}$CO$^+$) $\sim$ 100. The features are presumably made by supernova remnants. We found a prototypical example in GCM$-$0.02$-$0.07. It has two distinct structures. One is a ridge-like structure contacting with the Sgr A East shell; another is an expanding shell-like structure. There is a wide-velocity width ridge of SiO-enriched gas in GCM 0.11$-$0.11, which is adjacent to Vertical filaments. This suggests that the collision with the molecular cloud accelerates relativistic electrons, which illuminate the Vertical filaments–Polarized Plumes complex.

An analysis of infrared emission spectra from the regions near the Galactic Centre

We present consistent modelling of line and continuum IR spectra in the region close to the Galactic center. The models account for the coupled effect of shocks and photoionization from an external source. The results show that the shock velocities range between 65 and 80 km/s, the pre-shock densities between 1 cm-3 in the ISM to 200 cm-3 in the filamentary structures. The pre-shock magnetic field increases from 5. 10^{-6} gauss in the surrounding ISM to ~8. 10^{-5} gauss in the Arched Filaments. The stellar temperatures are ~38000 K in the Quintuplet cluster and ~27000 K in the Arches Cluster. The ionization parameter is relatively low (<0.01) with the highest values near the clusters, reaching a maximum >0.01 near the Arches Cluster. Depletion from the gaseous phase of Si is found throughout the whole observed region, indicating the presence of silicate dust. Grains including iron, are concentrated throughout the Arched Filaments. The modelling of the continuum SED in the IR range, indicates that a component of dust at temperatures of ~100-200 K is present in the central region of the Galaxy. Radio emission appears to be thermal bremsstrahlung in the E2-W1 filaments crossing strip, however a synchrotron component is not excluded. More data are necessary to resolve this questions.

SpitzerIRS Observations of the Galactic Center: Shocked Gas in the Radio Arc Bubble

We present Spitzer IRS spectra (R ~600, 10 - 38 micron) of 38 positions in the Galactic Center (GC), all at the same Galactic longitude and spanning plus/minus 0.3 degrees in latitude. Our positions include the Arches Cluster, the Arched Filaments, regions near the Quintuplet Cluster, the ``Bubble'' lying along the same line-of-sight as the molecular cloud G0.11-0.11, and the diffuse interstellar gas along the line-of-sight at higher Galactic latitudes. From measurements of the [O IV], [Ne II], [Ne III], [Si II], [S III], [S IV], [Fe II], [Fe III], and H_2 S(0), S(1), and S(2) lines we determine the gas excitation and ionic abundance ratios. The Ne/H and S/H abundance ratios are ~ 1.6 times that of the Orion Nebula. The main source of excitation is photoionization, with the Arches Cluster ionizing the Arched Filaments and the Quintuplet Cluster ionizing the gas nearby and at lower Galactic latitudes including the far side of the Bubble. In addition, strong shocks ionize gas to O^{+3} and destroy dust grains, releasing iron into the gas phase (Fe/H ~ 1.3 times 10^{-6} in the Arched Filaments and Fe/H ~ 8.8 times 10^{-6} in the Bubble). The shock effects are particularly noticeable in the center of the Bubble, but O$^{+3}$ is present in all positions. We suggest that the shocks are due to the winds from the Quintuplet Cluster Wolf-Rayet stars. On the other hand, the H_2 line ratios can be explained with multi-component models of warm molecular gas in photodissociation regions without the need for H_2 production in shocks.

Spitzer IRS Observations of the Galactic Center

We present Spitzer IRS R ∼ 600 spectra (10 - 38 μm) of 38 positions in the Galactic Center (GC), all at the same Galactic longitude of and including the Arches Cluster. Our positions include the Arched Filaments, regions near the Quintuplet Cluster, the ‘‘Bubble’’ lying along the same line-of-sight as the molecular cloud MO.13-0.13, and the diffuse interstellar gas along the line of sight at higher Galactic latitudes. From measurements of the [O IV], [Ne II], [Ne III], [Si II], [S III], [Fe II], [Fe III], and H2 S(0), S(1), and S(2) lines we determine the gas excitation and ionic abundance ratios. The main source of excitation is photoionization, with the Arches Cluster ionizing the Arched Filaments and the Quintuplet Cluster ionizing the gas nearby and at lower Galactic latitudes including the far side of the Bubble. In addition, strong shocks ionize gas to O+3 and destroy dust grains, releasing iron into the interstellar medium (ISM); the shock effects are particularly noticeable in the center of the Bubble but the highly ionized gas is present in all positions. The H2 lines are formed both in photodissociation regions in the GC and along the line of sight to the GC.

The Interaction of the Galactic Center Arches Cluster and the Thermal Filaments

We present Infrared Space Observatory (ISO) measurements of the far-infrared (FIR) fine-structure lines [S IV] (10.5 μm), [S III] (33.5 μm), [O III] (51.8 and 88.4 μm), [N III] (57.3 μm), [N II] (121.9 μm), [O I] (63.2 and 145.5 μm), and [C II] (157.7 μm) at five positions on the thermal ``Arched'' Filaments in the Galactic center region in order to investigate the influence of the massive, young Arches cluster on the interstellar medium. Serendipitous measurements of OH (119.3 μm) are also presented. Radio recombination line and ISO spectroscopic measurements provide strong evidence that the Arches cluster is responsible for the excitation of the Arched Filaments. The observations reported here were selected to provide measurements close to the Arches cluster and at uniformly increasing projected distances over the range 0-9 pc. Within the measurement uncertainties, the diagnostic line ratios most sensitive to excitation and least sensitive to density fluctuations, [O III]/[S III] and [S IV]/[S III], appear to decrease with projected distance from the Arches cluster. In agreement with other studies, this provides additional evidence that the cluster is the primary ionization source for the Arched Filaments. The total FIR luminosity also generally falls with projected distance from the Arches cluster. The best predictions from photoionization models for the rapid decrease in the measured line flux ratios with projected distance from the Arches cluster, however, are only accurate to a factor of 1.5, including allowance for additional offset along the line of sight. The measurements of lines formed predominantly in photodissociation regions are not well matched by current models. High-resolution spectra indicate gas moving at -70 km s-1 only at the location of the Arches cluster, significantly different from the predominant -30 km s-1 gas seen in the underlying molecular cloud.

The Molecular Component of the Galactic Center Arched Filaments H [CSC]ii[/CSC] Complex: OVRO Observations of the CS [ITAL]J[/ITAL] = 2–1 Line

The Owens Valley Radio Observatory (OVRO) millimeter array was used to make observations of the CS (2–1) line (at 97.981 GHz) arising from the G0.07+0.04 region of the km molecular cloud near the Galactic center, with a spatial resolution of ~8''. The ionized edge of this cloud forms the Arched Filaments H II regions, which are ionized by the adjacent Arches stellar cluster. The OVRO data were combined with single-dish data from 1986 obtained at the 30 m IRAM telescope by Serabyn & Gusten. A comparison of these CS (2–1) data and our recent H92α recombination line data reveals that the ionized and molecular gas are physically related, but that their velocities in this region differ by up to 35 km s-1. This difference in velocity can be understood if the gas that gave rise to the G0.07+0.04 H II region has been fully ionized. An overall comparison of the molecular and ionized gas across the entire -30 km s-1 cloud based on the single-dish CS (2–1) data and the H92α line data illustrates that such differences in velocity between the ionized and molecular gas are common and that the geometric arrangement of these components is complicated. Much of the ionized gas resides on the near side (to the observer) of the molecular cloud; however, in several regions some molecular material must lie in front of the H II region. The Arches stellar cluster therefore appears to be located in the midst of the molecular cloud, such that some of the near-side cloud surfaces along our line of sight have not been exposed to the ionizing radiation.

A VLA H92α Recombination Line Study of the Arched Filament H [CSC]ii[/CSC] Complex Near the Galactic Center

The Very Large Array has been used at 8.3 GHz in the DnC and CnB array configurations to carry out an H92??recombination line study of the ionized gas in the Arched Filaments H II complex, which defines the western edge of the Galactic center Radio Arc. The Arched Filaments consist of a series of curved filamentary ridges which extend over 9' ? 6' (22 ? 16 pc) and are intersected in two places by linear, nonthermal, magnetic filaments. The H92??observations cover the entire Arched Filaments region using a four-field mosaic, with an angular resolution of 128 ? 810; an additional higher resolution (36 ? 27) field was imaged in the SW portion of the H II complex. High-resolution continuum images are also presented. The H92??line properties of the ionized gas (line-to-continuum ratio, FWHM line width, T) are consistent with photoionization from hot stars and consistent with the physical properties of other Galactic center H II regions. The LTE electron temperatures vary only slightly across the entire extent of the source and have an average value of 6200 K. The velocity field is very complex, with velocities ranging from +15 to -70 km s-1?and the majority of velocities having negative values. Large velocity gradients (2?7 km s-1?pc-1, with gradients in some regions larger than 10 km s-1?pc-1) occur along each of the filaments, with the velocities becoming increasingly negative with decreasing distance from the Galactic center. The negative velocities and the sense of the velocity gradients can be understood in terms of the orbital path of the underlying molecular cloud about the Galactic center. The magnitudes of the velocity gradient are consistent with the cloud residing on an inner, elongated orbit which is due to the Galaxy's stellar bar, or with a radially infalling cloud. The ionization of the Arched Filaments can be accounted for completely by the massive Arches stellar cluster, which consists of more than 150 O stars and produces a few ?1051 photons s-1. This cluster is likely to be located 10?20 pc from the Arched Filaments, which can explain the uniformity of ionization conditions in the ionized gas.

Neutral Hydrogen in the Direction of the Vela Supernova Remnant

?????We have carried out a study of the distribution and kinematics of the neutral hydrogen in the direction of the Vela supernova remnant (SNR). A field of 68 ? 54 centered at l = 2641, b = -16 was surveyed using the Parkes 64 m radio telescope (half-power beamwidth 147 at 21 cm). Nearly 2300 H I profiles were obtained with a grid spacing of 75. The presence of a thin, almost circular H I shell, centered at v = 1.6 ? 0.8 km s-1, is revealed. This shell delineates the outer border of the X-ray emission as shown in the ROSAT observations of Aschenbach, Egger, & Tr?mper and wraps around the receding part of the remnant. In addition, two higher velocity features possibly associated with Vela are observed at about -30 and 30 km s-1. These features are interpreted as gas accelerated by the expansion of the supernova shock. The low systemic velocity observed suggests a distance shorter than 500 pc for the Vela SNR. The H I shell is ~7? in diameter and expands at v ~ 30 km s-1. By assuming a distance of 350 pc, we calculate for this shell a linear radius of 22 pc, a swept-up mass of ~1200?2300 M?, and an atomic preshock density of ~1?2 cm-3. The kinetic energy transferred by the supernova shock into the interstellar medium is ~(1?2) ? 1049 ergs, while the initial energy of the explosion is estimated to be ~(1?2.5) ? 1051 ergs. We present the distribution of the column density of the neutral material absorbing the X-radiation, an essential parameter in the analysis of X-ray data. A comparison between the H I and H? emission suggests that the H I shell contains embedded dust that might be responsible for increased optical absorption in this region. On the other hand, the brightest arc-shaped optical filaments associated with the western side of Vela show good correspondence with the H I features. From a comparison between the H I and Molonglo Observatory Synthesis Telescope 843 MHz radio continuum emission, we find that the outermost arched radio filaments correlate well with the main ridge of the H I shell. No strong inhomogeneities were found in the ambient H I medium in the direction of Vela X (the central nebula, powered by the pulsar PSR B0833-45).

Far-Infrared Observations of the Radio Arc (Thermal Arches) in the Galactic Center

view Abstract Citations (23) References (56) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Far-Infrared Observations of the Radio Arc (Thermal Arches) in the Galactic Center Timmermann, Ralf ; Genzel, Reinhard ; Poglitsch, Albrecht ; Lutz, Dieter ; Madden, Suzanne C. ; Nikola, Thomas ; Geis, Norbert ; Townes, Charles H. Abstract We report far-IR spectroscopy in [O III] 52 and 88 μm, [N III] 57 μm, and [O I] 63 μm of the "sickle" (GO. 18-0.04) in the Galactic center radio arc. We also mapped the entire eastern region of the arched filaments, including the "banana" (GO. 10+0.02) in [O III] 88 μm. These observations and the recent detection of three near-IR He I/H I emission-line stars near the "sickle" strongly support the concept of a combined H II/photodissociation region (PDR). G0.18-0.04 is a low-density H II region with ne ≍ 600 cm-3 that is most likely excited by stars of type earlier than O8 located in the Quintuplet cluster. Ionization of the arched filaments and the "sickle" by fast shocks or the "critical ionization mechanism" can be ruled out. We derive a fractional abundance ratio for [N]/[O] = 0.30-0.35 consistent with observations of other H II regions in the vicinity of the Galactic center. Unusually wide [O III] 88 μm emission lines of widths FWHM = (70-110) km s-1 have been detected in the mapped region of GO.18-0.04. The large intrinsic line widths may be related to the interaction of the plasma with strong magnetic fields that are known to be present in the radio arc. Publication: The Astrophysical Journal Pub Date: July 1996 DOI: 10.1086/177506 Bibcode: 1996ApJ...466..242T Keywords: GALAXY: ABUNDANCES; ISM: INDIVIDUAL ALPHANUMERIC: G0.10+0.02; ISM: INDIVIDUAL ALPHANUMERIC: G0.15-0.05; ISM: INDIVIDUAL ALPHANUMERIC: G0.18-0.04; GALAXY: CENTER; ISM: H II REGIONS full text sources ADS | data products SIMBAD (11)

VLA Radio Recombination Line Observations of Ionized Gas in the -30 Kilometers per Second Molecular Cloud (G0.04+0.03) near the Galactic Center. I. The Discrete Radio Sources

view Abstract Citations (32) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA Radio Recombination Line Observations of Ionized Gas in the -30 Kilometers per Second Molecular Cloud (G0.04+0.03) near the Galactic Center. I. The Discrete Radio Sources Zhao, Jun-Hui ; Desai, K. ; Goss, W. M. ; Yusef-Zadeh, F. Abstract Multifrequency spectral line and continuum observation of ionized gas in the -30 km s-1 molecular cloud (G0.04+0.03) have been carried out using the VLA to study the nature of the discrete radio sources located between Sgr A West (l = 0°) and the arched filaments of the continuum arc (l = 0°.2). Spectral indices determined from 6 and 20 cm continuum data indicate that the spectra of these sources are flat or rising with frequency. Radio morphologies and the detection of radio recombination lines (H110α and H92α) confirm that these sources are H II regions. The line emission arises at negative radial velocities (-60 to -20 km s-1), forbidden at these longitudes in the sense of Galactic rotation. The kinematics of the ionized gas suggest a close relationship with the molecular cloud observed in CS and 13CO at a mean radial velocity of -30 km s1 (G0.04+0.03). We derive LTE electron temperatures in the range 3500-7200 K. The low inferred electron temperatures may indicate relatively high metal abundances in these regions. The thermal sources are spatially correlated with [C II] 158 microns emission peaks and with the distribution of warm dust as indicated by the color temperature, Tc(55/125 μm). Overall, their properties and environments indicate that these H II regions are ionized by UV radiation from recently formed early-type stars in the molecular cloud at -30 km s-1. Publication: The Astrophysical Journal Pub Date: November 1993 DOI: 10.1086/173385 Bibcode: 1993ApJ...418..235Z Keywords: GALAXY: CENTER; ISM: H II REGIONS; ISM: MOLECULES; ISM: STRUCTURE; RADIO CONTINUUM: INTERSTELLAR full text sources ADS | data products SIMBAD (13)

Polarization of the far-infrared emission from the thermal filaments of the Galactic center arc

The polarization of the 100 micron continuum emission has been measured at 14 positions in the dense, warm molecular cloud associated with the arched filaments, or the 'bridge', of the radio arc near the Galactic center. At all positions the percent polarization is found to be quite large, ranging up to 6.5 percent. The polarization is interpreted in terms of thermal emission by magnetically aligned dust grains. The directions of the polarization vectors then indicate that the magnetic field is (1) parallel to the long dimension of the thermal radio filaments, and (2) very uniform on scales of 1-10 pc. Of several explanations for the inferred field geometry, the simplest is that it results from the unusually large dynamical shear in the emitting cloud.

Radio studies of the Galactic Centre - II. The arc, threads, and related features at 90 CM (330 MHz)

We present 90-cm VLA observations of a |$\sim 2^\circ \times 2^\circ$| field around the Galactic Centre. The full field was mapped with a resolution of |$100\times64 \,\text{arcsec}^{2},$| and smaller regions with resolutions as high as |$14\times9 \,\text{arcsec}^{2}.$| The resulting images contain most of the unique features associated with the Galactic Centre, which include the Arc and its linear filaments, the arched filaments, the ‘threads’, the thermal ‘spiral’ Sgr A West, the non-thermal source Sgr A East and Sgr C. When compared to higher-frequency observations there are, however, significant differences which we mostly attribute to the increasing optical depth of the thermal components. We show that most of the linear filaments of the Arc in the vicinity of G0.16 – 0.15, although non-thermal, have an intrinsically positive 90/20-cm spectral index |$(\alpha \sim + 0.3,$| where flux density |$S \propto v^{\alpha}),$| although at least one of the filaments shows a 90/20-cm spectral index of −0.4. The thermal feature G0.18 –0.04 is seen in absorption, showing clearly that it is in front of the Arc. The positive latitude portion of the Arc is embedded in an extended region of ionized gas with an emission measure |$\gt{10}^{5} \,\text{pc} \,\text{cm}^{-6}$| The ‘threads’ show up as prominently as the Arc at 90 cm. Other isolated linear features in this region, e.g. G359.54 + 0.18, G359.80 + 0.17 and Sgr C, appear similar to the threads. The 90/20-cm spectral index of three of these features is relatively steep |$(\alpha \sim -0.6)$| and the filament in Sgr C has |$\alpha = 0.55 \pm 0.4,$| whereas the ‘thread’ passing just north of Sgr A (G359.96+0.09) has a flat 90/20-cm spectral index. Hence there appear to be two classes of non-thermal filaments. The first class, the majority of which occur in the Arc, have flat or inverted spectra in which relativistic electrons must be produced, possibly in situ, on time-scales of less than 104 yr. The other class, which includes most of the isolated linear features, have relatively steep spectra and need not be directly associated with sources of relativistic particles. Two features observed at wavelengths longer than 1 m, namely the northern galactic lobe and the low-frequency jet, do not appear as distinct features at 90 cm. We suggest that these emission areas are caused by ‘windows’ in an otherwise optically thick thermal gas.

The thermal, arched filaments of the radio arc near the Galactic center - Magnetohydrodynamic-induced ionization?

view Abstract Citations (105) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Thermal, Arched Filaments of the Radio Arc near the Galactic Center: Magnetohydrodynamic-induced Ionization? Morris, Mark ; Yusef-Zadeh, Farhad Abstract This paper presents a detailed observational study of the most prominent thermal component of the radio Arc located near the Galactic center, the 'arched filaments'. Images produced from data gathered at 6 and 20 cm reveal that the arched filaments have a wispy, flocculent character that is quite different from the uniform, continuous, and almost straight filaments of the nonthermal portion of the Arc. The two systems of filaments show clear signs of interaction at their intersection. There is no evidence from the radio maps that the arched filaments are linked to the Galactic nucleus. Various ideas for the ionization mechanism of the arched filaments are considered. It is suggested that the ionization is caused by a magnetohydrodynamic interaction between a molecular cloud with a large peculiar velocity and an ambient poloidal magnetic field of milligauss strength. Publication: The Astrophysical Journal Pub Date: August 1989 DOI: 10.1086/167742 Bibcode: 1989ApJ...343..703M Keywords: Galactic Nuclei; Interstellar Magnetic Fields; Interstellar Matter; Milky Way Galaxy; Radio Sources (Astronomy); Astronomical Maps; Electric Current; Gas Ionization; Magnetohydrodynamics; Molecular Clouds; Nebulae; Astrophysics; GALAXIES: THE GALAXY; GALAXIES: NUCLEI; PLASMAS; NEBULAE: H II REGIONS; RADIO SOURCES: EXTENDED; INTERSTELLAR: MAGNETIC FIELDS; INTERSTELLAR: MOLECULES full text sources ADS | data products SIMBAD (32)