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\markboth{J. Staguhn et al.}{G359.54+0.18}
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\title{The interaction of the G359.54+0.18 Nonthermal Filaments with the
Ambient Medium}

\author{J. Staguhn and J. Stutzki}
\affil{Universit\"at zu K\"oln, 1. Physikalisches Instit\"ut\,
Z\"ulpicher Str. 77, 50937 K\"oln, Germany}
\author{F. Yusef-Zadeh}
\affil{Dearborne Observatory, Northwestern University, 2131 Sheridan
Road, Evanston, IL 60208, USA}
\author{K. I. Uchida}
\affil{Max Planck Institut f\"ur Radioastronomie, Auf dem H\"ugel 69,
53121 Bonn, Germany}


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%\altaffiltext{1}{Visiting Astronomer, Cerro Tololo Inter-American Observatory. 
%CTIO is operated by AURA, Inc.\ under cooperative agreement with the National
%Science Foundation} 
%\altaffiltext{2}{Society of Fellows, Harvard University} 
%\altaffiltext{3}{Patron, Alonso's Bar and Grill}

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\begin{abstract}
We present a study of the Galactic Center nonthermal filament 
system  G359.54+0.18 located to the north of the Sgr C region.  
We find evidence in support of the suggestion by Serabyn \& 
Morris that the nonthermal filaments are the manifestations 
of large-scale vertical magnetic field lines 
%(found only in the  GC region) 
illuminated by collisions with molecular clouds.  
Included in the study are observations of, (1) the 3 mm emission 
lines of CS, HCO$^+$ and other molecular species with the SEST, 
(2) several lines of $^{12}$CO and $^{13}$CO with the 3-m KOSMA 
antenna, (3) 5 GHz radio continuum emission with the VLA, and (4) 
H79$\alpha$ recombination line emission with the 100-m antenna at 
Effelsberg.  
\end{abstract}

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\keywords{molecular clouds,galactic center,radio filaments,magnetic fields}

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The G359.54+0.18 nonthermal filament system  and its potentially associated 
molecular clouds are well suited for this study: 
It is the system furthest off the Galactic plane and thus suffers least 
from source confusion.
Our high resolution 5 GHz continuum image of the G359.54+0.18 system 
(Fig. 1, contours) reveals a diffuse and 
somewhat clumpy emission structure near the easternmost tip of the 
filaments.  Additionally, the molecular maps of the region, made with 
the SEST
\footnote{The SEST telescope is 
operated by the Swedish National Facility for Radio Astronomy, Onsala 
Space Observatory and by ESO.}, show a localized cloud 
(identified as ``Cloud A'', seen east of the
radio feature in  Fig. 1 and in Fig. 2)
east of, and directly adjacent to, the clumpy continuum component.  
H79$\alpha$ recombination 
line emission, at velocities 
(v$_{lsr}$ = 106 km s$^{-1}$) consistent with the velocity gradient of the 
nearby molecular gas, is detected at the position of the diffuse continuum 
component ($\alpha$,$\delta$ = 17:41:10.0, -29:14:00), indicating that it is 
thermal in nature and linking it kinematically to the adjacent 
molecular Cloud A.  A strong velocity gradient in Cloud A, directed 
towards the interface region, implies interaction between 
the two (Fig. 2). 
The cloud emitts stronger in the high density 
tracing transition of HCO$^+$(1-0) than in CS(2-1).  
At the central position of Cloud A we have detected emission from 
the HNCO 5(0,5)-4(0,4) transition, a transition possibly pumped by 
the IR emission from warm dust or tracing high density clumps within 
the cloud with n$_{crit}\sim 10^5$ cm$^{-3}$ (Armstrong \& Barret, 1985).  

\input userdsk:[staguhn.dr.procs_chile]figures1

On larger scales, both the position and velocity of 
Cloud A suggests that it is part of the northern edge of the 
``negative velocity feature'' (Bally et al., 1988). The negative 
velocity feature has highly ``forbidden'' velocities and displays 
one of the steepest velocity gradients in the Galactic Center 
region.  

A second molecular cloud (identified as ``Cloud B'', lower center of Fig. 1
and Fig. 3) is 
observed along the curved portion of nonthermal filament. This cloud also
exhibits a velocity gradient towards the nonthermal filaments. The 
CS(2-1) emission from Cloud B is relatively strong.  In fact, 
the brightness ratios  of CS to HCO$^+$ are very different in the two
clouds described here. Keeping the slightly lower critical
density of HCO$^+$(1-0) in comparison to CS(2-1) in mind, this finding could 
indicate a different chemistry 
in both clouds. D. Jansen (1995) e.g. argues that HCO$^+$ emission in 
comparison to most other neutral molecules, appears to be more enhanced in 
diffuse molecular clouds which can be penetrated by ambient UV fields.
Sternberg (1995) points out that HCO$^+$ is efficiently produced in the 
hot HI/H$_2$ regions of PDRs whereas the formation of CS is more efficient in 
the more inner cold SI layers of PDRs in which the molecules are more shielded 
from the ambient UV field.
Cloud A, with the smaller of the CS to HCO$^+$ intensities, is  
situated closely adjacent to the diffuse HII region 
at the filaments tip, whereas Cloud B is located near the 
nonthermal filaments itself.
\input userdsk:[staguhn.dr.procs_chile]figures2

Cloud B also appears to be physically associated with the 
filaments.  It has been previously suggested 
by Bally \& Yusef-Zadeh (1989) that the bend in the nonthermal filaments 
is in response to a collision with a molecular cloud.  The location 
of Cloud B within the bend of the filaments makes it a prime 
candidate for the colliding partner.  As with the case of 
Cloud A, the molecular line data towards this region shows 
possible kinematic evidence of a collision --- a steep velocity 
gradient, directed toward the nonthermal filaments, is observed 
within Cloud B (Fig. 4).  The role that Cloud B possibly plays
in the illumination of the nonthermal filaments, however, 
is yet to be explored.

Our observations of the G359.54+0.18 filament system 
support the scenario, proposed by Serabyn \& Morris (1994), 
whereby the nonthermal filaments are illuminated by synchrotron 
emitting electrons accelerated by the process of magnetic field 
line reconnection instigated by colliding molecular clouds.  HII 
regions are also involved in this process --- as 
is observed toward at least three other potential cloud/filaments 
pairs.  Some degree of preionization is possibly needed in order 
that the cloud is able to effectively interact with the magnetic 
field component and/or, perhaps, a nearby HII region is required 
to provide enough free electrons for subsequent acceleration 
along the filaments.  We find evidence of all three components 
requisite for the cloud-collision/magnetic-reconnection scenario: 
(1) a set of nonthermal filaments supposedly delineating large 
scale GC magnetic field lines, (2) two molecular clouds adjacent 
to the filaments, and (3) an HII region at the tip of the filaments, 
evidenced by structure in the radio continuum images and by the 
detection of H79$\alpha$ recombination line emission.  Moreover, 
a direct association between molecular Cloud A and the HII region 
is suggested by their similar H79$\alpha$ recombination line 
and molecular emission line velocities.

\begin{references}
\reference J.T. Armstrong \& A.H. Barret, 1985, APJS, 57, 535;
\reference J. Bally, A.A. Stark, R.W. Wilson, \& C. Henkel, 1987, APJS, 65, 13;
\reference J. Bally \& F. Yusef-Zadeh, 1989, APJ, 336, 173;
\reference{} D. Jansen, Thesis, Leiden 1995; 
\reference A. Sternberg, 1995, in ''The Physics and Chemistry of Interstellar
                 Molecular Clouds', Proceedings of the 2nd Zermatt Conference,
                 ed. by G. Winnewisser and G.C. Pelz, Springer Verlag,
                 Heidelberg 1995;

\reference E. Serabyn \& M. Morris,  1994, APJ, 424, L91;  

\end{references}

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