5,412 research outputs found
Manifestations of the Galactic Center Magnetic Field
Several independent lines of evidence reveal that a relatively strong and
highly ordered magnetic field is present throughout the Galaxy's central
molecular zone (CMZ). The field within dense clouds of the central molecular
zone is predominantly parallel to the Galactic plane, probably as a result of
the strong tidal shear in that region. A second magnetic field system is
present outside of clouds, manifested primarily by a population of vertical,
synchrotron-emitting filamentary features aligned with the field. Whether or
not the strong vertical field is uniform throughout the CMZ remains
undetermined, but is a key central issue for the overall energetics and the
impact of the field on the Galactic center arena. The interactions between the
two field systems are considered, as they are likely to drive some of the
activity within the CMZ. As a proxy for other gas-rich galaxies in the local
group and beyond, the Galactic center region reveals that magnetic fields are
likely to be an important diagnostic, if not also a collimator, of the flow of
winds and energetic particles out of the nucleus.Comment: To appear in "LESSONS FROM THE LOCAL GROUP" - A Conference in Honour
of David Block and Bruce Elmegreen, eds: Freeman, K.C., Elmegreen, B.G.,
Block, D.L. & Woolway, M. (SPRINGER: NEW YORK
Evidence for A Parsec-scale Jet from The Galactic Center Black Hole: Interaction with Local Gas
Despite strong physical reasons that they should exist and decades of search,
jets from the Galactic Center Black Hole, Sgr A*, have not yet been
convincingly detected. Based on high-resolution Very Large Array images and
ultra-deep imaging-spectroscopic data produced by the Chandra X-ray
Observatory, we report new evidence for the existence of a parsec-scale jet
from Sgr A*, by associating a linear feature G359.944-0.052, previously
identified in X-ray images of the Galactic Center, with a radio shock front on
the Eastern Arm of the Sgr A West HII region. We show that the shock front can
be explained in terms of the impact of a jet having a sharp momentum peak along
the Galaxy's rotation axis, whereas G359.944-0.052, a quasi-steady feature with
a power-law spectrum, can be understood as synchrotron radiation from
shock-induced ultrarelativistic electrons cooling in a finite post-shock region
downstream along the jet path. Several interesting implications of the jet
properties are discussed.Comment: 33 pages, 7 figures; Accepted for publication in The Astrophysical
Journa
A Nonthermal Radio Filament Connected to the Galactic Black Hole?
Using the Very Large Array, we have investigated a non-thermal radio filament
(NTF) recently found very near the Galactic black hole and its radio
counterpart, SgrA*. While this NTF -- the Sgr A West Filament (SgrAWF) --
shares many characteristics with the population of NTFs occupying the central
few hundred parsecs of the Galaxy, the SgrAWF has the distinction of having an
orientation and sky location that suggest an intimate physical connection to
SgrA*. We present 3.3 and 5.5 cm images constructed using an innovative
methodology that yields a very high dynamic range, providing an unprecedentedly
clear picture of the SgrAWF. While the physical association of the SgrAWF with
SgrA* is not unambiguous, the images decidedly evoke this interesting
possibility. Assuming that the SgrAWF bears a physical relationship to SgrA*,
we examine the potential implications. One is that SgrA* is a source of
relativistic particles constrained to diffuse along ordered local field lines.
The relativistic particles could also be fed into the local field by a
collimated outflow from SgrA*, perhaps driven by the Poynting flux accompanying
the black hole spin in the presence of a magnetic field threading the event
horizon. Second, we consider the possibility that the SgrAWF is the
manifestation of a low-mass-density cosmic string that has become anchored to
the black hole. The simplest form of these hypotheses would predict that the
filament be bi-directional, whereas the SgrAWF is only seen on one side of
SgrA*, perhaps because of the dynamics of the local medium.Comment: 9 pages, 4 figures, accepted for ApJ Letter
Uniform Silicon Isotope Ratios Across the Milky Way Galaxy
We report the relative abundances of the three stable isotopes of silicon,
Si, Si and Si, across the Galaxy using the transition of silicon monoxide. The chosen sources represent a range in
Galactocentric radii () from 0 to 9.8 kpc. The high spectral
resolution and sensitivity afforded by the GBT permit isotope ratios to be
corrected for optical depths. The optical-depth-corrected data indicate that
the secondary-to-primary silicon isotope ratios
and vary much less than predicted on the basis of
other stable isotope ratio gradients across the Galaxy. Indeed, there is no
detectable variation in Si isotope ratios with . This lack of an
isotope ratio gradient stands in stark contrast to the monotonically decreasing
trend with exhibited by published secondary-to-primary oxygen
isotope ratios. These results, when considered in the context of the
expectations for chemical evolution, suggest that the reported oxygen isotope
ratio trends, and perhaps that for carbon as well, require further
investigation. The methods developed in this study for SiO isotopologue ratio
measurements are equally applicable to Galactic oxygen, carbon and nitrogen
isotope ratio measurements, and should prove useful for future observations of
these isotope systems.Comment: 18 pages, 12 figures, 2 tables. Published in The Astrophysical
Journal, Volume 839, Issue
A New Perspective of the Radio Bright Zone at The Galactic Center: Feedback from Nuclear Activities
New observations of Sgr A have been carried out with the VLA using the
broadband (2 GHz) continuum mode at 5.5 GHz, covering the central 30 pc region
of the RBZ at the Galactic center. Using the MS-MFS algorithms in CASA, we have
imaged Sgr A with a resolution of 1", achieving an rms 8 Jy/beam, and a
dynamic range 100,000:1.The radio image is compared with X-ray, CN
emission-line and Paschen- images obtained using Chandra, SMA and
HST/NICMOS, respectively. We discuss several prominent radio features. The "Sgr
A West Wings" extend 5 pc from the NW and SE tips of the ionized "Mini-spiral"
in Sgr A West to positions located 2.9 and 2.4 arc min to the NW and SE of Sgr
A*, respectively. The NW wing, along with several other prominent features,
including the "NW Streamers", form an elongated radio lobe (NW lobe), oriented
nearly perpendicular to the Galactic plane. This radio lobe, with a size of
14.4 pc x 7.3 pc, has a known X-ray counterpart. A row of three thermally
emitting rings is observed in the NW lobe. A field containing numerous
amorphous radio blobs extends for a distance of ~2 arc min beyond the tip of
the SE wing; these features coincide with the SE X-ray lobe. Most of the
amorphous radio blobs in the NW and SE lobes have Paschen-
counterparts, suggesting that a shock interaction of ambient gas concentrations
with a collimated nuclear wind (outflow) that may be driven by radiation force
from the central star cluster within the CND. Finally, we remark on a prominent
radio feature located within the shell of the Sgr A East SNR. Because this
feature -- the "Sigma Front" -- correlates well in shape and orientation with
the nearby edge of the CND, we propose that it is a reflected shock wave
resulting from the impact of the Sgr A East blast wave on the CND.Comment: 18 pages, 9 figures, ApJ accepte
Star Formation in the Extreme Galactic Center Environment
Copious star formation occurs in the dense Central Molecular Zone (CMZ) of
our Galaxy, but at a much smaller rate than occurs in a comparable mass of
molecular gas in the Galactic disk. The combination of large turbulent velocity
dispersions, a relatively strong magnetic field, and a strong tidal field all
contribute to inhibiting star formation (SF) in different ways in different CMZ
locations. Nonetheless, there are spectacular displays of recent and ongoing SF
in the CMZ, including massive young stellar clusters, sites of abundant SF in
progress, and numerous spots of protostellar or YSO activity. The presence of
giant molecular clouds in the CMZ that are almost entirely devoid of SF
indicates that SF requires a trigger that is not present everywhere. The
dominant provocation of SF is likely to be cloud compression, either by
large-scale shocks or by orbital motion of clouds into a region of enhanced
tidal compression and/or enhanced external pressure. Recent hypotheses for
where and how SF takes place in the CMZ are constrained by the recent orbital
determinations of the massive Arches and Quintuplet clusters. Star formation in
the central parsec is subject to a very different set of physical conditions,
and is less well understood, but is important for the co-evolution of the
central black hole and the nuclear star cluster.Comment: 5 pages, Proceedings of the 7th Chile-Cologne-Bonn-Symposium: Physics
and Chemistry of Star Formatio
- …