------------------------------------------------------------------------ From: Wolfgang Reich p098wre@mpifr-bonn.mpg.de Date: Thu, 9 Mar 2000 14:45:26 +0100 (MET) To: gcnews@aoc.nrao.edu Subject: submit nobar.tex to appear in PASJ Vol. 52, No. 2 \documentstyle[PASJadd]{PASJ95} % % PASJ LaTex example3 % %\draft \input {psfig.sty} \markboth{W.\ Reich et al.} {150~GHz NOBA observations} \newcommand{\vol}{47} \newcommand{\no}{3} \newcommand{\lett}{} \newcommand{\spage}{1} \newcommand{\rdate}{1999 August 6} \newcommand{\adate}{} \begin{document} \setcounter{page}{1} \title{150~GHz NOBA observations of the Galactic Center Arc} \author{Wolfgang {\sc Reich},$^1$ Yoshiaki {\sc Sofue},$^2$ and Hiroshi {\sc Matsuo},$^3$} \institute{ $^1$ {\it Max-Planck-Institut f\"ur Radioastronomie, Auf dem H\"ugel 69 D-53121 Bonn, Germany}\\ {\it E-mail(WR): wreich@mpifr-bonn.mpg.de}\\ % colon was missing $^2$ {\it Institut of Astronomy, The University of Tokyo, Mitaka, Tokyo 181, Japan}\\ {\it E-mail(YS): sofue@mtk.ioa.s.u-tokyo.ac.jp}\\ % colon was missing $^3$ {\it Nobeyama Radio Observatory, Minamimaki, Minamisaku, Nagano 384-13, Japan}\\ {\it E-mail(HM): matsuo@nrosv1.nro.nao.ac.jp} } \abst{We have used the seven beam 150~GHz bolometer NOBA installed at the Nobeyama 45-m telescope to map the central section of the Galactic Center Arc. In addition we mapped Sgr~A, which shows Sgr~A* and the thermal spiral structure. The results agree with previous mm-observations. South of the thermal "sickle" feature (G0.18-0.04) we observe three regions of enhanced emission along the Arc, which are located slightly offset relative to the most intense vertical filaments seen at low frequencies. This 150~GHz emission is observed at the apparent interacting areas of dense molecular material with the Arc. Another structure is seen south of the molecular cloud, where the Arc's vertical filaments apparently cross a weak filamentary structure running orthogonally in the direction of Sgr~A. The 150~GHz results are unexpected in view of previous 32~GHz and 43~GHz results, which indicate a fading of the Arc towards higher frequencies. Cold dust can be ruled out as the origin of the 150~GHz emission. Synchrotron emission from quasi-monoenergetic electrons or an electron distribution with a low energy cut-off seems compatible with the available data. The coincidence of enhanced emission with regions of interacting molecular gas strongly suggests that high energetic electrons are accelerated in those places where the magnetic field is compressed and subsequently enter and illuminate the Arc. } \kword{Galaxy:center-radio continuum-particle acceleration-magnetic fields} \maketitle \thispagestyle{headings} \section{Introduction} The Galactic Center Arc is a unique radio continuum feature located just about 13 arcminutes apart from the $\rm Sgr~A^{*}$. Morphologically it consists of numerous thin filaments running approximately perpendicular to the Galactic plane as seen at arcsec angular resolution (Yusef-Zadeh et al., 1984). The filaments are embedded in diffuse emission. Its non-thermal nature has been established by missing recombination lines and linear polarization, which amount is close to the intrinsic value at high frequencies (Lesch and Reich, 1992). Nevertheless the formation and illumination of the Arc with relativistic particles is not fully understood. Reich et al. (1988) have shown that the Arc's spectrum is slightly inverted and compatible with the existence of a quasi-monoenergetic electron population or an electron spectrum with a low energy cut-off. Various models have been proposed for the Arc as reviewed by Morris (1996) and Serabyn (1996). Strong magnetic fields of the order of mGauss have been proposed for the nearly straight Arc filaments, which are up to 40~pc long. These high magnetic fields are required to resist the pressure of the ambient interstellar medium (Yusef-Zadeh and Morris, 1987). However, such high magnetic fields imply a short synchrotron life-time of the radiating electrons and reacceleration might be required. Sofue et al. (1992) have made high resolution 43~GHz observations and failed to see the thin synchrotron filaments near the thermal "sickle" feature (G0.18-0.04), while lower resolution 43~GHz observations clearly show the diffuse emission from the Arc. This implies a lower magnetic field strength in the diffuse Arc component and in consequence a longer life-time for the electrons. 32~GHz and 43~GHz observations with the Effelsberg 100-m telescope indicate the spectral turn-over in this frequency range (Sofue et al., 1999). Sofue et al. (1992, 1999) conclude that the filaments should not be elder than about 4000 years. Tsuboi et al. (1997) found evidence for the Arc interacting with an expanding molecular shell and calculated the resulting ram pressure on the Arc's filaments. They came up with about 5~mGauss for the magnetic field strength along the Arc's filaments to be in pressure equilibrium implying an electron life time of a few hundred years only. To further study the spectral behaviour of the Arc towards short mm-wavelength we have performed 150~GHz observations. \section{Observations and Reduction} We have made 150~GHz observations of the Arc and Sgr~A in March 1999 using the Nobeyama 45-m telescope, which is equipped with a sensitive seven element bolometer array (NOBA) with about 30~GHz bandwidth. It has been described in detail by Kuno et al. (1993). In brief, six feeds are symmetrically arranged around a central feed with a projected distance on the sky of 16". Total intensity data are recorded from the central feed and the six differences between the offset feeds and the central feed. The algorithm to restore the total intensity for all seven beams has been described in detail by Kuno (1993), where simulations proof the reliability of this method. >From the six differences measured a plane is fitted to the differential data. By appropriate integration along all fitted intensity gradients of a single scan the atmospheric corrected emission for the central beam is restored. With this value the six difference measurements for the offset beams are converted into individual intensities. At 150~GHz the beam size of the 45-m telescope is a circular Gaussian of about 12" HPBW. An extended error beam is seen when mapping strong sources, which peaks at about -12dB and decreases to about -22dB at about 1' distance from the source center. The error beam needs to be taken into account when the dynamic range exceeds about 16, but this does not affect the results for the Arc and marginally the map of Sgr~A, which we decided not to clean. Wind pressure on the telescope exceeding about $\rm 5~m~s^{-1}$ causes a beam broadening and we have regularly observed the nearby source OH 5.89-0.39 to check the beam size, the pointing and the atmospheric extinction. Sky dips have been regularly performed in addition. Useful observations could be made in nights with opacities between 0.06 and 0.15. OH~5.89-0.39 also served as a flux density calibrator assuming 8.8~Jy. This value is uncertain by about 10\%. Mapping has been performed in the equatorial coordinate system by moving the telescope in right ascension or declination direction, respectively, and accordingly the bolometer was rotated in such a way that the seven beams simultaneously observe seven rows or columns of a map separated by 5.3". One observation consists of several coverages, which were subsequently processed and added to one map. The total integration time for one observation of a 3' x 3' field was about 30 minutes and that for a 5.5' x 5.5' field was about one hour. We mapped a field of 3' x 3' centered on Sgr~A* and two slightly overlapping fields along the Arc of 5.5' x 5.5', respectively. The data have been reduced with the standard continuum reduction system for observations with the 45-m telescope, which includes the restoration algorithm for NOBA observations. This reduction package is incorporated in the IDL image processing software. The raw data of one observation have been carefully edited before they were added into a map. The relative zero-level was set to both ends of each single scan. The maps have been calibrated and pointing corrected. The final combination of maps has been made using the NOD2 based reduction package, where in particular the PLAIT program (Emerson and Gr\"ave, 1989) was applied to destripe a set of maps observed at different scanning directions. Due to varying atmospheric conditions and different wind speeds during the observations, we finally got the best result for the Sgr~A field by combining maps from five observations. For the northern field of the Arc also five maps have been combined. For the southern field the combination of two maps gave the lowest signal-to-noise ratio, which have been observed at quite favourable atmospheric conditions with an opacity of 0.10. However, the wind speed of about $\rm 5~m~s^{-1}$ increases the beam width to about 18" (HPBW). \section{Results} \subsection {Sgr~A} We show a 150~GHz contour map of Sgr~A in Fig.~1 at an effective HPBW of about 13.5". The rms-noise is measured to be about 20~mJy/beam~area. We have separated the compact source Sgr~A* from the thermal spiral by applying a background-filtering (unsharp-masking) technique (Sofue and Reich, 1979) and measured the flux density of Sgr~A* by fitting a two dimensional elliptical Gaussian to the source. At our angular resolution Sgr~A* confuses with the weak thermal sources IRS~2 and IRS~13 ($<$200~mJy) as discussed by Falcke et al. (1998). The peak flux density is about 3.6~Jy with an absolute error below 15\%. The underlying spiral structure of Sgr~A West is optically thin thermal emission. We have compared our map with the 230~GHz map of Zylka and Mezger (1988) observed with the IRAM 30-m telescope (kindly provided by R. Zylka in numerical form). Doing the same unsharp masking operation to separate the thermal spiral from Sgr~A*, we measure at 13.5" angular resolution a 230~GHz flux density of 2.6~Jy. This is in agreement with the 2.5~Jy quoted by Zylka and Mezger (1988) for the original 12"~beam, although they use a different method to account for the contribution from the thermal spiral structure. The 230~GHz observations are from 1986/1987. The intensity of the thermal spiral structure is almost identical and vanishes when subtracting both maps. Formally about 4\% difference in intensity are expected for optically thin thermal emission. A slight excess of up to 300~mJy/beam~area of the 230~GHz emission is seen at the western side of the southern arm, where dust emission from the circum-nuclear-disk shows up (Mezger et al., 1989). The total flux density in the field shown in Fig.~1 is about 32~Jy, when Sgr~A* is excluded. This is larger than the 15~GHz flux density of 22.6~Jy as measured by Zylka and Mezger (1988) from the VLA-map by Brown and Liszt (1984), which seems to miss some large scale structure. The 150~GHz flux density measured from Sgr~A* in March 1999 is about 0.5~Jy higher than that observed with NOBA in October 1997 (Falcke et al., 1998). Although variability seems possible we note that this excess is within the errors of both measurements. \begin{figure} %14cm \centerline{\psfig{figure=sgra150r.ps,height=8cm,% bbllx=60pt,bblly=10pt,bburx=585pt,bbury=500pt,clip=}} \caption{Total intensity map of Sgr~A. The HPBW of 13.5" is indicated in the lower left corner. Contours run in steps of 100~mJy/beam~area up to 1.5~Jy/beam~area (labeled contour) and from 1.75~Jy/beam~area in steps of 0.5~Jy/beam~area. The maximum is at 5.0~Jy/beam~area.} \label{PI-fig} \end{figure} \begin{figure} [hbt] %14cm \centerline{\psfig{figure=arc1.4.150.21.5s.ps,height=11cm,% bbllx=48pt,bblly=117pt,bburx=400pt,bbury=610pt,clip=}} \caption{The Galactic Center Arc at 150~GHz. The angular resolution is 21.5" (HPBW). Contours are 75~mJy/beam~area apart. A section of a 1.4~GHz VLA-map (Yusef-Zadeh, 1986) is shown grey-scale coded at the same angular resolution.} \label{1.4-fig} \end{figure} \subsection{The Galactic Center Arc} Our result for the Arc is shown in Galactic coordinates in Fig.~2 at an angular resolution of 21.5"~(HPBW). Both of the observed fields have been combined into a single map, where some zero level adjustment at the overlapping region was necessary. The rms noise is about 15~mJy/beam~area in the northern field and about 20~mJy/beam~area in the southern area, which has less coverages. A grey-scale coded 1.4~GHz VLA-map from Yusef-Zadeh (1986), which we have convolved from 10.7"~x~10.1" to 21.5"~(HPBW), is shown overlaid. \begin{figure} [hbt] \centerline{\psfig{figure=arccut.ps,height=9cm,% bbllx=48pt,bblly=37pt,bburx=540pt,bbury=540pt,clip=}} \caption{A 1' wide slice at b = $-0.13^{\circ}$ across the Galactic Center Arc at 1.4~GHz, 43~GHz and 150~GHz. The angular resolution is 21.5" (HPBW).} \label{cut-fig} \end{figure} As seen in Fig.~2 the emission at 1.4~GHz and 150~GHz from the thermal "sickle" (G0.18-0.04) is quite similar. Differential intensity plots (TT-plots) give an average spectral index of $\rm \alpha = -0.1 \pm 0.05~(S \sim \nu^{\alpha})$. Thermal emission is also seen from the small "pistol" feature (l,b = $0.16^{\circ}$,$-0.07^{\circ}$). Its peak flux density is about 265~mJy/21.5"~beam at 1.4~GHz and about 170 mJy/21.5" beam at 150~GHz. A spectral index close to $\rm \alpha = -0.1$ is calculated as expected for optically thin thermal emission. The strong vertical filament at 1.4 GHz disappeared at 150~GHz, while three distinct emission features are seen near the filament. Their maxima are slightly shifted to the west and some extended emission runs vertically away from the Arc direction. The peak flux densities are about 200~mJy/21.5"~beam at l,b = $0.17^{\circ}$,$-0.09^{\circ}$, 380~mJy/21.5"~beam at l,b = $0.16^{\circ}$,$-0.13^{\circ}$ and 300~mJy/21.5"~beam at l,b = $0.15^{\circ}$,$-0.18^{\circ}$. The morphology of the Arc at 150~GHz is also different from the single dish maps with comparable angular resolution obtained at 32~GHz (Lesch and Reich, 1992) and in particular at 43~GHz (Sofue et al., 1999), where the Arc appears as a smooth continuous bar like structure, which is significantly weaker than the thermal emission visible from the "sickle" area. In contrast the maximum emission at 150~GHz is in the central section of the Arc. The distinct 150~GHz emission features along the Arc clearly have an inverted spectrum and are up to a factor of 2 stronger than the emission seen at 43~GHz (Sofue et al., 1999). This corresponds to a spectral index of up to $\rm \alpha = +0.5$, although the error is large. We show a slice across the Arc at the central 150~GHz emission maximum in Fig.~3. Outside of the distinct emission features the spectrum at the Arc is flat between 43~GHz and 150~GHz. However, we note that the diffuse background emission is known more accurately at 43~GHz due to the Nobeyama Galactic Center survey (Sofue et al., 1986). We do not have similar information on the large scale structure at 150~GHz at present and it remains open if the large scale structure exceeding about 3' is represented correctly. Therefore the low level diffuse emission just south of the "sickle" and close to the "pistol" feature needs to be checked. A more detailed spectral analysis is in preparation. \section {Molecular gas in interaction with the Arc} Fig.~4 shows an overlay of the Arc with the Nobeyama CS~(J=1-0) observations by Tsuboi et al. (1997), which clearly shows that the enhanced 150~GHz emission is seen in those regions, where the molecular gas approaches the Arc most closely. The CS-data shown in Fig.~4 are for the velocity range between $\rm 15~km~s^{-1}$ and $\rm 45~km~s^{-1}$, where Tsuboi et al. (1997) found strongest morphological evidence for an interaction with the Arc (see their Fig.~2). The molecular gas has also been traced in the course of line surveys in CO as discussed by Oka et al. (1997) and also seen in $\rm C^{18}O$ and HNCO by Lindquist et al. (1996). \begin{figure} [htb] %14cm \centerline{\psfig{figure=cs150.ps,height=11cm,% bbllx=48pt,bblly=117pt,bburx=400pt,bbury=610pt,clip=}} \caption{The Arc at 150~GHz (HPBW = 21.5") is shown grey-scale coded with contours of the CS~(J=1-0) emission (HPBW = 34") as observed by Tsuboi et al. (1997) overlaid. The velocity range is from $\rm 15~km~s^{-1}$ to $\rm 45~km~s^{-1}$. } \label{PI-fig} \end{figure} The enhanced emission seen at l,b = $0.15^{\circ}$,$-0.18^{\circ}$ does not seem to coincide well with intense CS-emission from interacting molecular gas. In this area the vertical Arc structure is seen to cross other weak structures, which run almost perpendicular to the Arc towards Sgr~A. This is clearly visible at 1.4~GHz (Fig.~2). These features, which are much fainter than the emission from the Arc, are seen on all sensitive large scale observations of the Galactic Center region, but have not been studied in detail. The spectrum appears to be flat. Polarization is missing and it is likely that they are thermal. We note that these structures run along the southern boundary of the molecular cloud (Fig.~4), although their relation is unclear by now. The crossing of these features and their possible interaction with the Arc was already discussed by Benford (1988), where a current running in a closed circle across the Arc, the filamentary structures, and the Sgr~A region was suggested. \section {On the nature of the 150~GHz emission} The present observations show enhanced emission regions with an inverted spectrum between 43~GHz and 150~GHz. This enhanced emission is observed at the apparent interacting regions of dense molecular gas with the Arc and thus supports ideas where molecular clouds compress the poloidal large scale magnetic field in the Galactic Center to a field strength of up to a few mGauss. Particle acceleration will take place in these regions of interaction. However, 150~GHz is a high frequency where cold dust emission might show up and this possible contribution needs to be clarified before considering other possibilities. Although it seems quite unlikely that the interaction process leeds to enhanced cold dust emission, this needs a check by comparing with higher frequency data. In this frequency range optically thin emission from cold dust should scale with about $\rm \nu^{4}$. The IRAM 230~GHz survey by Zylka and Mezger (in prep., Mezger et al., 1996, Fig.~21) does not show any significant emission along the Arc. The weak patchy structure seen in this area does not exceed about 100~mJy/11"~beam. This corresponds to an emission of less than 400~mJy/21.5"~beam and is in agreement with the slightly inverted spectrum between 43~GHz and 150~GHz. The low 230~GHz emission definitely rules out emission from cold dust, where we expect an increase by about a factor of 5 between 150~GHz and 230~GHz. We have also checked the IRAS~100~$\mu$m and 60~$\mu$m low resolution maps for this region, which show a depression in intensity along the Arc (see the IRAS~60~$\mu$m map shown by Reich et al., 1987). Moreover, a higher-resolution FIR map at 16-26~$\mu$m from the MSX experiments (Shipman et al., 1997) shows no signature of dust emission at the 150~GHz peak, but the peak appears to be located rather in the center of a hole of dust emission. To summarize, available data rule out a significant contribution from cold dust to the observed 150~GHz emission. Absorption mechanisms in the Galactic center have been looked at by Lesch et al. (1988). Their calculations show that neither synchrotron self-absorption nor thermal absorption of optically thin synchrotron emission is able to explain the inverted Arc spectrum up to 43~GHz (Reich et al., 1988). The size of the emitting region is by some orders of magnitude too large that synchrotron self-absorption becomes relevant. This is even more extreme for the case of the 150~GHz emitting regions. The same holds for thermal absorption, where Lesch et al. (1988) showed that the required column densities are several orders of magnitude too large to invert the spectrum of the Arc. The 150~GHz emission we observe requires even larger column densities. A decomposition of the spectrum of the Arc by Reich et al. (1988) results in $\rm \alpha = +0.3$ for the filaments and a diffuse flat spectrum component with $\rm \alpha = -0.2$. High resolution VLA-observations by Anantharamiah et al. (1991) have confirmed the inverted $\rm \alpha = +0.3$ spectrum for the filaments. Such an inverted spectrum is expected to originate from a quasi-monoenergetic electron distribution or an electron distribution with a low energy cut-off. The emission visible at 150~GHz has a similar spectrum as the Arc shows at lower frequencies, thus a similar electron distribution is required. However, physically the 150~GHz emission is separated from the filaments as seen from Fig. 2. It seems likely that the interacting regions between the molecular cloud and the Arc are the acceleration region for the electrons, which subsequently enter the Arc and its filaments. The Arc's filaments are most intense in this area and have about the same electron spectrum. The evolution of the inverted radio spectrum away from the point of injection, up to about 400~pc out of the Galactic plane, was discussed by Pohl et al. (1992). With increasing distance from the injection point the intensity decreases and the spectrum steepens. \section{Origin and life-time of the 150~GHz emission.} Tsuboi et al. (1997) have calculated a magnetic field strength of about 5~mGauss to keep the Arc in pressure equilibrium with the massive expanding molecular shell, which is partly shown in Fig.~4. This shell has a diameter of about 7.5~pc and is centered at l,b~=$0.11^{\circ}$,$-0.11^{\circ}$. Its density is about $\rm n(H_{2}) = 6~10^{4}~cm^{-3}$ and the expansion velocity is $\rm 20~km~s^{-1}$. The shell rotates with about $\rm 5~km~s^{-1}$. As already calculated by Tsuboi et al. (1997) a magnetic field of 5~mGauss results in a synchrotron life-time for 0.9~GeV electrons of about 400~years. Assuming that 150~GHz is close to the maximum frequency $\nu_{max}$ of the emission, a critical frequency of $\nu_{c}$ = 450~GHz ( $\nu_{max} = 0.3~\nu_{c}$) results. With $\rm \nu_{c}~[MHz] = 16.08~10^{6}~B_{\bot}~[Gauss]~E^{2}~[GeV]$ we calculate an energy of $\rm E \sim~2.4~GeV~(\gamma=4600)$ for the electrons in the 150~GHz emission regions. The life-time of the electrons reduces, because of $\rm t_{1\prime2} = (119.7~B_{\bot}^{2}~[Gauss]~E~[GeV])^{-1}$, to about 140 years. Taking the radius of the strongest 150~GHz emission region at l,b = $0.16^{\circ}$,$-0.13^{\circ}$ to about 1.5' or 3.7~pc (for a Galactic Center distance of 8.5~kpc), the electrons need a drift velocity of about 0.08~c to cross the region within their life-time. Pohl et al. (1992) calculated from modelling the synchrotron emission of the entire Arc structure a bulk velocity of about 0.1~c. In this model a magnetic field strength for the Arc's filaments of 1~mGauss was assumed, where the accelerated electrons are injected. The drift velocity is close to the minimum velocity required here. Therefore we conclude that a magnetic field strength of 5~mGauss exceeds the maximum field strength to allow accelerated particles to escape the region of acceleration and to enter the Arc. The time the expanding molecular shell needs to cross the emission area is of the order of $2~10^{5}$ years, about two or three orders of magnitude larger than the synchrotron life-time of the 150~GHz emitting electrons or the Arc's filaments. The crossing time is larger or similar to the life-time of the entire Arc structure. Therefore the Arc appears as a short living structure, just supplied with freshly accelerated electrons at the surface of the molecular cloud and might disappear or deformed when the molecular cloud has crossed. In that case the assumption of pressure equilibrium is no longer needed and smaller magnetic fields with longer electron life-times will result. Using the above equation we get for a magnetic field strength of 1~(0.3)~mGauss a life-time of 1500~$(10^{4})$~years. However, in the case of weaker magnetic fields the acceleration process has to be more efficient, since higher electron energies are required to radiate at 150~GHz. For a field strength of 1~(0.3)~mGauss electrons with energies of 5.4~(10)~GeV ($\gamma$ = $10^{4}~(1.9~10^{4}))$ are needed. Various mechanisms to accelerate high energetic electrons showing up in the Arc have been discussed as reviewed by Morris (1996) or Serabyn (1996). Shock acceleration or magnetic reconnection seem the most likely processes. Both mechanisms are able to result in a quasi-monoenergetic electron distribution. Schlickeiser (1984) has shown that a monoenergetic electron distribution by shock acceleration is provided, if the particle escape time is much longer than the acceleration time. Magnetic reconnection gives about the same energy for all runaway electrons. This process converts the magnetic field energy into particle energy and thus reduces the magnetic field strength. However, the efficiency of magnetic reconnection is just up to 10\%, which seems too low to reduce the magnetic field strength substantially. Magnetic reconnection is a local process, where magnetic fields of opposite directions merge. The electric field strength and the length of the merging magnetic fields determine the energy of the accelerated runaway electrons (Lesch and Reich, 1992). In this case the efficiency depends very much on the details of the interaction as well as the clumpiness of the molecular material, its degree of ionization and its magnetic field strength. The molecular clumps encounter and compress the magnetic field traced by the diffuse emission which surrounds the filaments of the Arc. Its strength was estimated to be of the order of 0.1~mGauss (Reich, 1990). Lesch and Reich (1992) calculate a maximum Lorentz factor of $\gamma_{max} \sim 4~10^{4}$~${\rm [B/1~mGauss]^{4}}$ in the case of magnetic reconnection and reasonable assumptions for the thermal electrons and the movement of the molecular cloud in the Arc region. $\gamma_{max}$ will be too low for a magnetic field strength substantially below 1~mGauss to account for the observed 150~GHz emission and we conclude that a 1~mGauss field seems to be a good estimate for both requirements, a long enough life-time and a high enough electron energy. \section{Summary} 150~GHz observations with the bolometer array NOBA installed at the Nobeyama 45-m telescope have resulted in maps at 13.5" angular resolution for Sgr~A and at 21.5" for a large section of the Galactic Center Arc. We measure a peak flux density for Sgr~A* of 3.6 Jy in March 1999. This confirms previous measurements of an excess emission of Sgr~A* above 100 GHz (Falcke et al., 1998). The integrated flux density of the mini-spiral underlying Sgr~A* was measured to be 32~Jy and its thermal nature is confirmed. We observe intense emission features associated with the Galactic Center Arc, but slightly shifted towards the west from its most intense filaments. The spectrum of the emission is inverted relative to 43~GHz and compatible with an origin from quasi-monoenergetic electrons or an electron spectrum with a low energy cut-off, but not with optically thin emission from cold dust. The 150~GHz emission coincides with areas with apparent interaction of the Arc's magnetic field and dense molecular gas takes place. An estimated magnetic field strength of about 5~mGauss in order to keep the Arc in pressure equilibrium results in problems with the synchrotron life-time of the radiating electrons seen at 150~GHz. However, the time the molecular material needs to cross the emitting regions is much larger than the synchrotron life-time of the Arc's filaments or the 150~GHz emitting electrons and the assumption of pressure equilibrium might not be valid. A lower magnetic field strength requires a larger electron energy to be emit at 150~GHz. Using previous estimates for magnetic reconnection as a possible acceleration mechanism, a magnetic field strength of about 1~mGauss seems to be most likely. \par \vspace{1pc} \par We are grateful to the staff of the Nobeyama Radio Observatory for support during the observations. We like to thank M. Tsuboi and R. 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