Galactic Center Radio Arc Research Articles

Evidence for an Interaction between the Galactic Center Clouds M0.10–0.08 and M0.11–0.11

We present high-resolution (∼2–3″; ∼0.1 pc) radio observations of the Galactic center cloud M0.10−0.08 using the Very Large Array at K and Ka band (∼25 and 36 GHz). The M0.10−0.08 cloud is located in a complex environment near the Galactic center Radio Arc and the adjacent M0.11−0.11 molecular cloud. From our data, M0.10−0.08 appears to be a compact molecular cloud (∼3 pc) that contains multiple compact molecular cores (5+; <0.4 pc). In this study, we detect a total of 15 molecular transitions in M0.10−0.08 from the following molecules: NH3, HC3N, CH3OH, HC5N, CH3CN, and OCS. We have identified more than sixty 36 GHz CH3OH masers in M0.10−0.08 with brightness temperatures above 400 K and 31 maser candidates with temperatures between 100 and 400 K. We conduct a kinematic analysis of the gas using NH3 and detect multiple velocity components toward this region of the Galactic center. The bulk of the gas in this region has a velocity of 51.5 km s−1 (M0.10−0.08) with a lower-velocity wing at 37.6 km s−1. We also detect a relatively faint velocity component at 10.6 km s−1 that we attribute to being an extension of the M0.11−0.11 cloud. Analysis of the gas kinematics, combined with past X-ray fluorescence observations, suggests M0.10−0.08 and M0.11−0.11 are located in the same vicinity of the Galactic center and could be physically interacting.

Analyzing the Intrinsic Magnetic Field in the Galactic Center Radio Arc

Abstract The Radio Arc is a system of organized nonthermal filaments (NTFs) located within the Galactic center (GC) region of the Milky Way. Recent observations of the Radio Arc NTFs revealed a magnetic field that alternates between being parallel and rotated with respect to the orientation of the filaments. This pattern is in stark contrast to the predominantly parallel magnetic field orientations observed in other GC NTFs. To help elucidate the origin of this pattern, we analyze spectro-polarimetric data of the Radio Arc NTFs using an Australian Telescope Compact Array data set covering the continuous frequency range from ∼4 to 11 GHz at a spectral resolution of 2 MHz. We fit depolarization models to the spectral polarization data to characterize Faraday effects along the line of sight. We assess whether structures local to the Radio Arc NTFs may contribute to the unusual magnetic field orientation. External Faraday effects are identified as the most likely origin of the rotation observed for the Radio Arc NTFs; however, internal Faraday effects are also found to be likely in regions of parallel magnetic field. The increased likelihood of internal Faraday effects in parallel magnetic field regions may be attributed to the effects of structures local to the GC. One such structure could be the Radio Shell local to the Radio Arc NTFs. Future studies are needed to determine whether this alternating magnetic field pattern is present in other multi-stranded NTFs, or is a unique property resulting from the complex interstellar region local to the Radio Arc NTFs.

A VLA Polarimetric Study of the Galactic Center Radio Arc: Characterizing Polarization, Rotation Measure, and Magnetic Field Properties

Abstract The Radio Arc is one of the brightest systems of nonthermal filaments (NTFs) in the Galactic Center, located near several prominent H ii regions (Sickle and Pistol) and the Quintuplet stellar cluster. We present observations of the Arc NTFs using the S, C, and X bands of the Very Large Array interferometer. Our images of total intensity reveal large-scale helical features that surround the Arc NTFs, very narrow subfilamentation, and compact sources along the NTFs. The distribution of polarized intensity is confined to a relatively small area along the NTFs. There are elongated polarized structures that appear to lack total intensity counterparts. We detect a range of rotation measure values from −1000 to −5800 rad m−2, likely caused by external Faraday rotation along the line of sight. After correcting for Faraday rotation, the intrinsic magnetic field orientation is found to generally trace the extent of the NTFs. However, the intrinsic magnetic field in several regions of the Arc NTFs shows an ordered pattern that is rotated with respect to the extent of the NTFs. We suggest this changing pattern may be caused by an additional magnetized source along the line of sight, so that we observe two field systems superposed in our observations. We suggest that the large-scale helical segments near the Radio Arc could be components of such a source causing these changes in the intrinsic magnetic field, and some variations in the polarization and rotation measure values along the NTFs.

M0.20–0.033: An Expanding Molecular Shell in the Galactic Center Radio Arc

Abstract We present high-frequency Karl G. Jansky Very Large Array (VLA) continuum and spectral line (NH3, H64α, and H63α) observations of the Galactic Center Radio Arc region, covering the Sickle H ii region, the Quintuplet cluster, and molecular clouds M0.20−0.033 and M0.10−0.08. These observations show that the two velocity components of M0.20−0.033 (∼25 and 80 km s−1), previously thought to be separate clouds along the same line-of-sight, are physically connected in position–velocity space via a third southern component around 50 km s−1. Further position–velocity analysis of the surrounding region, using lower-resolution survey observations taken with the Mopra and ATCA telescopes, indicates that both molecular components in M0.20−0.033 are physically connected to the M0.10−0.08 molecular cloud, which is suggested to be located on stream 1 in the Kruijssen et al. orbital model. The morphology and kinematics of the molecular gas in M0.20−0.033 indicate that the two velocity components in M0.20−0.033 constitute an expanding shell. Our observations suggest that the M0.20−0.033 expanding shell has an expansion velocity of 40 km s−1, with a systemic velocity of 53 km s−1, comparable to velocities detected in M0.10−0.08. The origin of the expanding shell is located near the Quintuplet cluster, suggesting that the energy and momentum output from this massive stellar cluster may have contributed to the expansion.

The nature and origin of the Galactic center radio arc: a VLA Faraday study

Despite their discovery almost 30 years ago, the origin of the Galactic center nonthermal filaments (NTFs) remains poorly understood. The improved capabilities of the VLA offer a fantastic opportunity to make a multi-frequency, full spectropolarimetric study of the radio arc at high angular resolution. Observations presented here are from DnC and CnB array configuration data taken at S, C and X band (coverage between 2-12 GHz; continuum only). In addition there are also Ka and Q band (continuum and spectral line coverage) observations that are part of the study but not shown in this short proceedings. These data will allow us to make the first high angular resolution Faraday study of the Galactic center radio arc.

Thermal SiO and H13CO+Line Observations of the Dense Molecular Cloud G0.11−0.11 in the Galactic Center Region

We obtained the first view in H13CO+ J = 1-0 and a high-resolution map in thermal SiO lines of G0.11-0.11, which is a molecular cloud situated between the Galactic Center radio arc and Sgr A. From a comparison with previous line observations, we found that the H13CO+ J = 1-0 line is optically thin, whereas the thermal SiO lines are optically thick. The line intensity in H13CO+ J = 1-0 shows that the cloud has a large column density, up to N(H2) = (6-7) × 1023 cm-2, which corresponds to about 640-740 mag in AV or 10-12 mag in A25 μm. The estimated column density is the largest known of any even in the Galactic center region. We conclude from the intensity ratio of SiO J = 1-0 to CS J = 1-0 that emitting gas is highly inhomogeneous for SiO abundance on a scale smaller than the beam width ~35''.

New Nonthermal Filaments at the Galactic Center: Are They Tracing a Globally Ordered Magnetic Field?

New high-resolution, wide-field 90 cm VLA observations of the Galactic center (GC) region by Nord and coworkers have revealed 20 nonthermal filament (NTF) candidates. We report 6 cm polarization observations of six of these. All of the candidates have the expected NTF morphology, and two show extended polarization, confirming their identification as NTFs. One of the new NTFs appears to be part of a system of NTFs located in the Sgr B region, 64 pc in projection north of Sgr A. These filaments cross the Galactic plane with an orientation similar to the filaments in the Galactic center radio arc. They extend the scale over which the NTF phenomena is known to occur to almost 300 pc along the Galactic plane. Another NTF was found in the Galactic plane south of the Sgr C filament but with an orientation of 45° to the Galactic plane. This is only the second of 12 confirmed NTFs that is not oriented perpendicular to the Galactic plane. An additional candidate in the Sgr C region was resolved into multiple filamentary structures. Polarization was detected only at the brightness peak of one of the filaments. Several of these filaments run parallel to the Galactic plane and can be considered additional evidence for nonpoloidal magnetic fields at the GC. Together the 90 and 6 cm observations indicate that the GC magnetic field may be more complex than a simple globally ordered dipolar field.

Mapping Magnetic Fields in the Cold Dust at the Galactic Center

We report the measurement of 158 new 350 μm polarimetry vectors in the central 30 parsecs of the Galactic center. These data were obtained at the Caltech Submillimeter Observatory using Hertz. Morphologically, these results show a consistency with previously published far-infrared and submillimeter results. We find that the angle of the magnetic field inferred from these observations is related to the 350 μm flux as obtained by SHARC/CSO in the following way. At low fluxes, the magnetic field angle is consistent with that of a poloidal field as seen in nearby features such as the Galactic Center Radio Arc and the Northern and Southern Threads. At high fluxes, the magnetic field is oriented parallel to the plane of the Galaxy. This relationship suggests a model in which an initially poloidal field is sheared out in dense regions which are dominated by gravity. If this model is correct, it implies a characteristic field strength for the region of 3 mG.

X‐Ray Thread G0.13−0.11: A Pulsar Wind Nebula?

We have examined Chandra observations of the recently discovered X-ray thread G0.13-0.11 in the Galactic center Radio Arc region. Part of the {\sl Chandra} data was studied by Yusef-Zadeh, Law, & Wardle (2002), who reported the detection of 6.4-keV line emission in this region. We find, however, that this line emission is {\sl not} associated with G0.13-0.11. The X-ray spectra of both G0.13-0.11 and the point-like source embedded are well characterized by a power law. The 2 -- 10 keV band luminosities of these two components are $\sim 3.2\times 10^{33}{\rm ergs s^{-1}}$ (G0.13-0.11) and $\sim 7.5 \times10^{32} {\rm ergs s^{-1}}$ (point source) at the Galactic center distance of 8 kpc. The morphological, spectral, and luminosity properties strongly indicate that G0.13-0.11 represents the leading-edge of a pulsar wind nebula, produced by a pulsar (point source) moving in a strong magnetic field environment. The main body of this pulsar wind nebula is likely traced by a bow-shaped radio feature, which is apparently bordered by G0.13-0.11 and is possibly associated with the prominent nonthermal radio filaments of the Radio Arc. We speculate that young pulsars may be responsible for various unique nonthermal filamentary radio and X-ray features observed in the Galactic center region.

The ionized gas in the Galactic Center Radio Arc

We present Infrared Space Observatory (ISO) observations of fine structure lines toward the Radio Arc region, close to Sgr A, in the Galactic center region (GCR). The ionization at large scale is dominated by the clusters of massive stars in this region: the Quintuplet and the Arches cluster. Any other possible mechanism as interaction with the magnetic fields revealed by the Non-Thermal Filaments (NTFs) must play a minor role. The radiation in this area is relatively hard ($T_\mathrm{eff}\sim 33-35$ kK) but diluted. It reaches large distances due to the inhomogeneity of the medium.

A New System of Parallel Isolated Nonthermal Filaments near the Galactic Center: Evidence for a Local Magnetic Field Gradient

We report the discovery of a system of isolated nonthermal filaments approximately 0.5 deg. northwest (75 pc in projection) of Sgr A. Unlike other isolated nonthermal filaments which show subfilamentation, braiding of subfilaments, and flaring at their ends, these filaments are simple linear structures and more closely resemble the parallel bundled filaments in the Galactic center radio arc. However, the most unusual feature of these filaments is that the 20/90 cm spectral index uniformly decreases as a function of length, in contrast to all other nonthermal filaments in the Galactic center. This spectral gradient may not be due to simple particle aging but could be explained by a curved electron energy spectrum embedded in a diverging magnetic field. If so, the scale of the magnetic gradient is not consistent with a large scale magnetic field centered on Sgr A* suggesting that this filament system is tracing a local magnetic field.

A Molecular Cloud and an Expanding Cavity Adjacent to the Nonthermal Filaments of the Galactic Center Radio Arc

Abstract Large-scale CO images taken with the $2 \times 2$ focal-plane array receiver on the 45 m telescope of Nobeyama Radio Observatory reveal parsec-scale structure of molecular gas around the Galactic center radio arc. The LVG analysis shows that the gas kinetic temperature is considerably enhanced ($T_\mathrm{k} \geq 70 \,\mathrm{K}$) in CO 0.13–0.13, a cloud which seems to be in contact with the nonthermal filaments of the Galactic center radio arc. This lends support for an interaction between CO 0.13–0.13 and the nonthermal filaments. The interaction with the strong magnetic field in and around the nonthermal filaments may heat up the molecular gas in CO 0.13–0.13 through shock dissipation and/or magnetic viscosity. We also find an expanding cavity adjacent to the nonthermal filaments. The shell has a radius of $\sim 5 \,\mathrm{pc}$ and an expansion velocity of $25 \,\mathrm{km} \,\mathrm{s}^{-1}$. This expanding cavity might have been formed by several hundred supernovae or a single hypernova within $3 \times 10^5 \,\mathrm{yr}$. Magnetic tubes in this region could have been swept up by the expanding cavity, and compressed by interactions with CO 0.13–0.13. An interaction between the cloud and magnetic field may accelerate electrons to relativistic energies through shock acceleration or magnetic field reconnection.

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.

4.12. A molecular cloud interacting by the vertical filaments of the Galactic center radio arc

Surely the most striking radio feature in the Galactic center may be a sheaf of straight vertical filaments (VFs, e.g., Yusef-Zadeh, Morris, &amp; Chance 1984) of the radio arc. The VFs are believed to be the manifestations of strong magnetic field lines (≥1mG) which have been illuminated by some local relativistic particle source.

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)

An infrared quintuplet near the Galactic center

A compact cluster of five luminous infrared sources has been found in the vicinity of the Galactic center radio arc. All of the sources have large polarizations in the near-infrared and deep silicate absorptions at 10 microns. Strong absorption lines of CO are seen at 4.6 microns toward each source. The observed polarization and spectral features are likely to be interstellar in origin. The temperatures of the sources, based on infrared photometry and corrected for extinction, are in the range 600-900 K. None of the sources show either infrared recombination line emission from atomic hydrogen, lines of molecular hydrogen, or overtone band of CO at 2.3 microns. Because the objects are tightly clustered, all are likely to be very young objects; however, accurate classifications cannot be made at the present time. 31 refs.

H I absorption measurements over the Galactic center Radio Arc region

The compact array of the VLA was used to study H I absorption in a 100-pc region in the center of the Galaxy. An absence of gas was observed at 40-50 km/s across the Radio Arc, suggesting that the '40 km/s molecular cloud' possibly associated with Sgr A is placed behind the Arc. At +20 km/s, the observations show absorption by material which could be depolarizing much of the synchrotron emission from the Arc filaments. The kinematic structure revealed by high-resolution molecular and radio recombination line observations is confirmed by the distribution of H I gas at -10 to -60 km/s.