------------------------------------------------------------------------ archedfil.tex AJ, May 2001, in press Content-Length: 92924 %astro-ph/0102130 \newcommand{\gusten}{G\"{u}sten} \newcommand{\kms}{km s${}^{-1}$} \newcommand{\al}{$\alpha$} \newcommand{\ab}{$\sim$} \newcommand{\yz}{Yusef-Zadeh} \newcommand{\p}{$\pm$} \newcommand{\ad}{$\alpha$,$\delta$$_{(J2000)}$} \newcommand{\beam}{beam$^{-1}$} \newcommand{\x}{$\times$} \newcommand{\T}{T$_e^*$} \newcommand{\pc}{pc$^{-1}$} \newcommand{\phot}{photons s$^{-1}$} \documentclass[preprint]{aastex} \begin{document} \title{A VLA H92$\alpha$ Recombination Line Study of the Arched Filament \\H II Complex Near the Galactic Center} \author{Cornelia C. Lang\altaffilmark{1,2}, W. M. Goss\altaffilmark{2}, Mark Morris\altaffilmark{1}} \altaffiltext{1}{Department of Physics \& Astronomy, 8371 Math Sciences Building, University of California, Los Angeles, CA 90095-1562; CCL's current address: Astronomy Program, LGRT B-517O, University of Massachusetts, Amherst, MA 01003, email: clang@ocotillo.astro.umass.edu} \altaffiltext{2}{National Radio Astronomy Observatory, Box 0, Socorro, NM 87801} \begin{abstract} The Very Large Array has been used at 8.3 GHz in the DnC and CnB array configurations to carry out an H92\al~recombination line study (at 8.3 GHz) 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\arcmin\x6\arcmin~(22 \x~16 pc) and are intersected in two places by linear, nonthermal, magnetic filaments. The H92\al~observations cover the entire Arched Filaments region using a four-field mosaic, with an angular resolution of 12\farcs8 \x~8\farcs10; an additional higher resolution (3\farcs6 \x~2\farcs7) field was imaged in the SW portion of the H II complex. High resolution continuum images are also presented. The H92\al~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 \kms~to $-$70 \kms~and the majority of velocities having negative values. Large velocity gradients (2$-$7 \kms~\pc, with gradients in some regions $>$ 10 \kms~\pc) 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 $>$ 150 O-stars and produces a few \x~10$^{51}$ 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. \end{abstract} \keywords{Galaxy: center -- ISM: HII regions -- ISM: individual (G0.10+0.08)} \section{Introduction} Several of the most unusual H II regions in the Galaxy are found within 30 pc of the Galactic center. High resolution radio observations made with the Very Large Array (VLA) of the National Radio Astronomy Observatory\footnotemark\footnotetext{The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under a cooperative agreement with the Associated Universities, Inc.} over the past 15 years have revealed the remarkable filamentary morphology of H II regions such as the Sickle and Pistol (\yz~\& Morris 1987a; Lang, Goss \& Wood 1997), the Arched Filaments (\yz~1986), and SgrA West (Ekers et al. 1983; Schwarz, Bregman, \& van Gorkom 1989, Roberts \& Goss 1991, 1993). The largest and most prominent of these regions is the ``Arched Filament'' H II complex, comprised of a series of curved, narrow ridges of radio emission which define the western edge of the well-known Galactic center Radio Arc (Yusef-Zadeh et al. 1984). The Arched Filaments extend for 9\arcmin~$\times$ 6\arcmin~(or 22 $\times$ 15 pc at the assumed Galactic center distance of 8.0 kpc (Reid 1993)) and are located 10\arcmin~(25 pc) in projection from the center of the Galaxy, SgrA$^*$. The thermal nature of these filamentary ridges was first revealed by recombination line observations (Pauls et al. 1976; Pauls \& Mezger 1980; Yusef-Zadeh 1986; Yusef-Zadeh, Morris \& van Gorkom 1987). In addition to the peculiar morphology of these thermal filaments, the recombination line studies have shown that the kinematics of the Arched Filaments are also very striking, with large velocity gradients along their lengths and predominantly negative velocities in a positive velocity quadrant of the Galaxy, i.e., counter to Galactic rotation for circular orbits. A large molecular cloud complex was discovered in CS (J=2$-$1) at the position of the Arched Filaments and extends southward over 16\arcmin~(or 40 pc) near SgrA. This cloud exhibits emission over a range of velocities similar to those of the ionized gas (i.e., 5 to $-$55 \kms) and is therefore known as the ``$-$30 \kms~cloud'', its name representing the average velocity of the gas in this cloud (Serabyn \& \gusten~1987). In addition to these unusual properties of the Arched Filaments, the surrounding interstellar environment is remarkable for several reasons. First, at their location near the Galactic center, the strong differential gravitational forces of this region are likely to influence both the morphology and kinematics of the interstellar gas within the inner kiloparsec (\gusten~\& Downes 1980; Serabyn \& \gusten~1987) and may well play an important role in understanding the Arched Filaments. Second, the Arched Filaments are intersected by two prominent systems of non-thermal filaments (NTFs). These NTFs are unique to the central 250 pc of the Galaxy; the long (up to 50 pc) and narrow ($<$0.1 pc) synchrotron filaments show strong linearly polarized radio emission and have magnetic field orientations aligned with their long axes (Yusef-Zadeh \& Morris 1987b; Yusef-Zadeh, Wardle \& Parastaran 1997; Lang et al. 1999ab). The NTFs are understood as evidence for a large-scale poloidal magnetic field which pervades the central 250 pc of the Galaxy (Morris 1994). The NTFs in the Radio Arc intersect the northern edge of the Arched Filaments, and to the southwest, the Northern Thread NTF crosses the southern end of two of the Arched Filaments (Morris \& Yusef-Zadeh 1989, hereafter MYZ; Lang et al. 1999b). The nature of the intersection between the thermal gas and NTFs remains a major outstanding issue in understanding the interstellar medium at the Galactic center. The origin and acceleration of relativistic particles in the NTFs may be due to magnetic reconnection at positions where the NTFs are interacting with associated ionized and molecular gas (Serabyn \& Morris 1994). Finally, one of the most exceptional, massive stellar clusters in the Galaxy is located at the eastern edge of the Arched Filaments. The brightest sources in this cluster were first revealed by medium resolution, near-infrared observations (Nagata et al. 1995; Cotera et al. 1996). The near-infrared spectroscopy of Cotera et al. (1996) showed that 13 of the stars can be classified as highly-evolved massive stars such as Wolf-Rayet (WR) and Of stellar types. The ionizing flux generated by these 13 stars may provide part of the ionization of the Arched Filaments. More recent, high resolution observations of the ``Arches Cluster'' made at near-infrared wavelengths at the W.M. Keck Observatory and with the NICMOS camera on the {\it Hubble Space Telescope (HST)} show that it is very densely packed, harboring more than 150 O-stars (Serabyn et al. 1998; Figer et al. 1999). In addition, Figer et al. (1999) estimate that the cluster has a total mass of \ab10$^{4}$ M$_{\sun}$, with an age of only \ab2 Myr. A number of far-infrared (FIR) studies have revealed strong FIR line and continuum emission arising from the Arched Filaments (Genzel et al. 1990; Erickson et al. 1991; Morris et al. 1995; Davidson et al. 1994; Colgan et al. 1996). Before this luminous stellar cluster was discovered, initial explanations for heating of the Arched Filaments relied on shocks between the $-$30 \kms~molecular cloud and the interstellar medium in the Galactic center (Bally et al. 1988; Hayvaerts, Norman \& Pudritz 1988). In addition, MYZ proposed that the source of heating of the ionized filaments was MHD-induced ionization resulting from the large relative velocity between the molecular cloud and the strong, ambient magnetic field. So far, none of these models has been able to account for all aspects of the radio and FIR observations. In particular, Colgan et al. (1996) reported that the FIR luminosity, [OIII] line fluxes, and low electron densities cannot be explained by either shocks or MHD models for ionization. Instead, these authors have concluded that the Arched Filaments must be uniformly photoionized by a distribution of massive stars. In this paper, new VLA observations of the H92\al~recombination lines arising from the Arched Filaments are presented. These observations were carried out in order to make a detailed study of the ionized gas in this unusual source. These data cover the entire region of the Arched Filaments using a four-field mosaic, with higher spatial resolutions (2$-$12\arcsec) than the previous VLA H110$\alpha$ study of Yusef-Zadeh (1986), which was centered only on the western filaments with a resolution of 22\arcsec. The goals of these observations are to understand: (1) the peculiar morphology of the Arched Filaments, (2) the complicated kinematics and velocity field, (3) the interaction of the ionized gas with the massive and luminous stellar cluster and (4) the nature of intersections between NTFs and ionized gas. $\S$2 provides a summary of the observations and data reductions; results from the 8.3 GHz continuum images are presented in $\S$3; results from the H92$\alpha$ line observations are presented in $\S$4, and $\S$5 includes a discussion of the morphology, kinematics and ionization of the Arched Filaments. \section{Observations and Data Reduction} VLA continuum and recombination line observations of the Arched Filaments were made at 8.3 GHz in the DnC and CnB array configurations. Details of the observations are summarized in Tables 1 and 2. Calibration and editing were carried out using the {\it AIPS} software of NRAO. Line-free channels were used to determine the continuum, and continuum subtraction was done in the (u,v) plane using the {\it AIPS} task UVLSF. The four fields observed in DnC configuration were mosaicked using a maximum entropy deconvolution algorithm (mosmem) in the {\it MIRIAD} software package. The H92$\alpha$ line and corresponding continuum mosaics were imaged with natural weighting to achieve the best possible signal-to-noise ratio in the recombination line. An additional continuum mosaic was created with uniform weighting in order to obtain higher spatial resolution. The southeastern field (Arches1) was also observed in the CnB array, and these data were combined with the corresponding DnC observations using the {\it AIPS} task DBCON. The combined uv-data were imaged using the {\it AIPS} task IMAGR for both the 8.3 GHz continuum and the H92\al~line with natural weighting. A higher resolution 8.3 GHz image of a portion of this field was made with uniform weighting to highlight the fine-scale structure. The parameters of all images discussed in this paper are summarized in Table 3. The recombination line analysis was done using {\it GIPSY}, the Groningen Image Processing SYstem (van der Hulst et al. 1992). Spatially-integrated, continuum-weighted line profiles were made for selected regions of the Arched Filaments using PROFIL, and Gaussian models were fitted to these averaged profiles using PROFIT. Single-component Gaussian functions were also fitted to the line data for each pixel having a signal-to-noise ratio $>$ 4. \section{Continuum Images at 8.3 GHz} Figure 1 is a schematic of the sources near the Arched Filament H II complex which are discussed in this paper and depicted in Figures 2 and 3. The 8.3 GHz continuum mosaic of the Arched Filaments is shown in greyscale and contours in Figures 2 and 3, with a resolution of 7\farcs8 $\times$ 6\farcs6, PA=$-$1.0. Both images have been corrected for primary beam attenuation. The 8.3 GHz continuum image is very similar to the 1.4 and 4.8 GHz continuum images of MYZ. The Arched Filaments are comprised of two pairs of filamentary structures, the ``eastern'' and ``western'' filaments: E1, E2, W1, and W2 (after MYZ). The filaments are long and very narrow structures, extending up to \ab9\arcmin~(22 pc) in the north-south direction, with widths of only 20\arcsec~(0.8 pc) on average. The four Arched Filaments cover an area of \ab6\arcmin~(15 pc) from east to west, but most of the emission is concentrated in the narrow ridges, which have an areal filling factor of \ab10\%. The similarity in the curvature of all four Arched Filaments is especially striking, and gives the appearance of the western half of a set of concentric circles, the apparent center of which would be located 1$-$2\arcmin~to the East. Across the Arched Filaments, the brightness is unevenly distributed, and has a diffuse and tenuous nature. In particular, along W1 and W2, the filaments appear to be comprised of multiple filamentary strands, with the edges of these regions markedly brighter than the central portions of the filaments. The brightest peaks, known as G0.07+0.04 and G0.10+0.02, have relatively uniform brightness over their 60$-$90\arcsec~extents, whereas the brightness over the rest of the filaments is much less uniform. Although it appears that G0.10+0.02 may be connected to E1 or E2, there is a dramatic decrease in brightness between G0.10+0.02 and the southern extent of the E1 and E2 filaments (at \ad=17 45 47.0, $-$28 49 12). In this region, the emission drops to 15 mJy \beam, whereas to the North (in E1), the intensity is $\geq$ 25 mJy \beam, and the intensity in G0.10+0.02 exceeds 85 mJy \beam. However, this is not the case for the G0.07+0.04 region and the W1 filament. Along W1, the emission does not vary as substantially as in the E1, E2, and G0.10+0.02 region. Moving northward along W1, the intensity increases fairly constantly from 50 to 80 mJy \beam~for \ab3\farcm5, followed by a decrease in intensity to \ab50 mJy \beam~starting at \ad=17 46 35.0, $-$28 50 00. There are no dramatic changes or discontinuities in the intensity along W1, whereas G0.10+0.02 has a sharper boundary that separates it from the E1 and E2 filaments. Therefore, in this paper, G0.10+0.02 will be considered a separate source from E1 and E2, whereas the W1 filament will include G0.07+0.04. The Radio Arc NTFs (to the North) and Northern Thread NTF (to the SW) are apparent in Figure 2 where they intersect (in projection) the Arched Filaments. Yusef-Zadeh \& Morris (1988) have pointed out that the morphology and discontinuity of the NTFs where they meet the Arched Filaments suggests that there may be a physical connection. On the western side of this image, the Northern Thread NTF is just perceptible where it crosses, in projection, both W1 and W2. A collection of diffuse, extended sources is located to the south of the W2 filament. Several of these features are nearly linear and are oriented parallel to the NTFs, including the adjacent Northern Thread NTF. These ``quasi-linear'' features were first pointed out by MYZ. Based on the continuum flux density at 8.3 GHz, physical parameters of the ionized gas in the Arched Filaments have been derived using the formulation of Mezger \& Henderson (1967) and the corrections to this formulation (Viallefond 1991). In using these formulae, we assume a uniform density, spherical, ionization-bounded H II region with \T=6200 K, and Y$^+$=0.06 (see $\S$4). Although the Arched Filaments are obviously not spherically-symmetric, we divide the region into segments which can be better approximated by a spherical H II region in order to derive the standard parameters for comparison with other H II regions. The ionization-bounded assumption is also valid for the regions we are considering, as the radio emission arises in the the Arched Filament complex from the ionization-bounded side of the nebula. Part of the nebula (to the east of the Arches cluster) is likely to be density-bounded, as the distribution of underlying molecular material falls off completely (Serabyn \& \gusten~1987) and there is no detectable free-free emission arising from this region. Table 4 lists the derived parameters for these regions: total flux density (S$_\nu$), measured angular size, linear size of the equivalent sphere, electron density (n$_e$), emission measure (EM), ionization parameter (U), mass of ionized hydrogen (M$_{HII}$), number of Lyman continuum ionizing photons, (N$_{Lyc}$), and the 8.3 GHz continuum optical depth ($\tau_c$). Figure 5 shows a higher resolution (2\farcs3~\x~1\farcs6, PA=64\arcdeg) 8.3 GHz continuum image of a single field in the southeastern portion of the Arched Filaments. As in Figure 2, W1 and W2 appear comprised of multiple filamentary strands and narrow ridges of projected width $<$ 3\arcsec~(0.125 pc). G0.07+0.04 is prominently bifurcated at an orientation nearly perpendicular to the Galactic plane. Several of the point-like sources to the south of G0.07+0.04 were catalogued by Yusef-Zadeh (1986) as H5-H7. Four additional sources appear in this image, and following the above nomenclature, they are labelled as H9-12 (see Figure 1) and the parameters are listed in Table 5. \section{H92\al~Recombination Line Observations} Spatially-integrated, continuum-weighted H92\al~profiles were made over each of the four Arched Filaments in order to characterize their global H92\al~properties. For these profiles, G0.10+0.02 was assumed to be a member of the E1 filament. The profiles are shown in Figure 6, and properties of the Gaussian fits to these profiles (line-to-continuum ratio, T$_l$/T$_c$; central LSR velocity, V$_{LSR}$; and FWHM line width, $\Delta$V) are given in Table 6. Single component profiles were fitted to the H92$\alpha$ line in the E2, W1, and W2 filaments; region E1 was fitted with a double peaked profile. A possible detection of the He92\al~line was made in E2, and the intensity ratio of helium to hydrogen, Y$^+$, was calculated to be 0.04\p0.015. The central velocities in the W1 and W2 filaments (V=$-$27 and $-$44 \kms) are more negative than those in E1 and E2 (V=$-$12 \kms), and the double profile in the E1 filament suggests that the velocity field in this region may be complex and comprised of multiple components. The line widths of the integrated profiles in the Arched Filaments range from 27$-$38 \kms. Line widths for integrated regions within other Galactic center H II regions, such as G0.18-0.04, G0.15-0.05, and SgrA West were found to be much broader (typically \ab50 \kms), although smaller integrated regions in these regions have an average line width of \ab30 \kms~(Lang et al. 1997; Roberts \& Goss 1993). The increased line widths for the larger integrated regions in all cases can be attributed to a combination of the large velocity gradients across each filament and the large areas over which the profiles were integrated. The H92\al~line was also sampled on smaller scales at 25 positions within the Arched Filaments, representing the well-defined emission complexes illustrated in Figure 7. These 25 H92\al~profiles, also spatially-integrated and continuum-weighted, are presented in Figure 8, and the corresponding Gaussian properties of these profiles are listed in Table 7. In order to examine the spatial variation in the H92$\alpha$ line properties in further detail, single-component Gaussians were fit to each pixel having a flux density above a 4$\sigma$ level. The resulting spatial distribution of line amplitude, FWHM line width, and LTE electron temperature (derived from the measured values of T$_l$/T$_c$ and $\Delta$V), are presented in Figure 9. The H92$\alpha$ recombination line flux densities have peak values in the range of 3 to 25 mJy \beam. The strongest H92$\alpha$ emission (25 and 23 mJy \beam) occurs at the continuum peaks G0.07+0.04 and G0.10+0.02. As shown in Figure 9a, the H92$\alpha$ line emission follows the continuum emission closely, including the substructure and multiplicity of filaments apparent along the length of W1. The northern boundary of the H92$\alpha$ emission in the Arched Filaments coincides with the NTFs in the Radio Arc. The emission abruptly declines toward the southernmost NTF in the Radio Arc, with the exception of an H92$\alpha$ line arising from a small region in the Radio Arc north of E2 (Profile 1). To the South, the H II region known as H5 (E \& W) (Profile 16) represents the southernmost source of H92$\alpha$ line emission in our data; this source has been studied in detail by Zhao et al. (1993). Detections of the He92$\alpha$ line have been made at the 2$-$3$\sigma$ level in several regions of the Arched Filaments (Profiles 7, 9, 16 \& 19). Values of Y$^+$ range from 4$-$8\% in these regions, consistent with other radio recombination line studies of Galactic center H II regions, which typically show Y$^+$\ab5\% (Mehringer et al. 1993; Roberts \& Goss 1993; Lang et al. 1997). The only known enhancement of He92$\alpha$ in this region is the detection of Y$^+$=14\p6\% in portions of the Pistol nebula (Lang et al. 1997), thought to be metal-enriched ejecta from an earlier evolutionary stage of the Pistol star (Figer et al. 1998). \subsection{Physical Conditions of the Ionized Gas} The line-to-continuum ratios across the Arched Filaments range from 0.04 to 0.17, with an average value of 0.11 (Table 7), consistent with a typical line-to-continuum ratio in the H92$\alpha$ line of 0.1 (calculated by assuming typical LTE conditions for Galactic center H II regions: $\Delta$V\ab30 \kms~and T$_e$$^*$\ab6000 K). The FWHM line widths in the Arched Filaments range from 15 to 44 \kms, with an average value of 28 \kms, similar to the radio recombination line widths observed for other Galactic center H II regions ($\Delta$V\ab 27 \kms~in a survey by Downes et al. 1980; $\Delta$V\ab33 \kms~in SgrB1 by Mehringer et al. (1992); and $\Delta$V\ab35 \kms~in the Sickle and Pistol by Lang et al. (1997)). Typically, the line widths in Table 7 with values $>$ 35 \kms~(corresponding to Profiles 10, 13, 19, \& 20) are due to the substantial velocity gradients present over the integration regions. Figure 9b also illustrates that over most of the Arched Filaments, the line widths are 20$-$30 \kms. In a few places, the line widths exceed 50 \kms. These positions are the sites of double profiles, and therefore the broad line widths are likely due to a poorer fit by a single Gaussian component. \subsubsection{Electron Temperatures} LTE electron temperatures, T$_e^*$, are calculated for the 25 regions, based on the measured values of T$_l$/T$_c$, $\Delta$V, and an assumed value of Y$^+$=0.06 (eq. [22]; Roelfsema \& Goss 1992). Although the He92$\alpha$ line was only detected in some regions of the source, this average value was used for all the regions, since \T~depends only weakly on Y$^+$; assuming a value of Y$^+$=~0 only increases \T~by a few percent. To determine the importance of non-LTE effects, the LTE departure coefficients (b$_n$,$\beta$$_n$) are calculated based on average values of the electron density (n$_e$\ab~300 cm$^{-3}$) and continuum optical depth ($\tau$$_c$\ab0.0015) (see Table 4). The departure coefficients are used to derive non-LTE temperatures for several regions (eq. [23] of Roelfsema \& Goss 1992). At most, the non-LTE temperatures are decreased by 2\% from the LTE values. Therefore, the non-LTE corrections can be considered to be a negligible effect in the Arched Filaments, and we can assume that the emission occurs under LTE conditions. In addition, Shaver (1980) points out that for a given emission measure (EM), there is an observing frequency where the non-LTE effects can be considered negligible ($\nu$ = 0.081 EM$^{0.36}$ (GHz)). Following Shaver (1980), for the H92$\alpha$ line at 8.3 GHz, the EM corresponding to LTE conditions has a value of 3.8\x10$^5$ pc cm$^{-6}$. The values of EM listed in Table 4 are in the range of 1.2 to 3.0\x10$^5$ pc cm$^{-6}$, compatible with the assumption that the H92\al~lines in the Arched Filaments are emitted under LTE conditions. The values of T$_e^*$ in the Arched Filaments range from 5000\p300 to 8900\p900 K, with an average value for the LTE electron temperature of 6200 K. Similar T$_e^*$ have been measured in other H II regions in the Galactic center: 7000 K in SgrA West (Roberts et al. 1993), 6400 K in the ``H'' regions (Zhao et al. 1993), and 5500 K in the Sickle (Lang et al. 1997). Several of the regions in the Arched Filaments (Profiles 4, 6, 11, \& 12) show very narrow FWHM line widths ($\Delta$V$<$20 \kms) and represent some of the narrowest lines observed in Galactic center H II regions. Such narrow lines can place upper limits on the electron temperatures of the ionized gas. The Doppler temperature, T$_D$, is defined by the line width for thermal motion in the absence of turbulence and pressure broadening (T$_D$=21.8~($\Delta$V)$^2$ K, where $\Delta$V is in \kms). Narrow line widths have been observed in only a small number of sources and provide an important demonstration that electron temperatures as low as 4000$-$5000 K do exist in some nebulae (Shaver et al. 1979, 1983; Kantharia et al. 1998). Values of T$_D$ in regions where the lines are very narrow can be compared with \T~to check for consistency. For this comparison, values for T$_D$ in the 25 regions are listed in Table 7. The narrowest lines in the Arched Filaments (15.8, 16.7, and 17.5 \kms) place upper limits on the electron temperatures in these regions of 5400\p400, 6600\p400, and 6000\p600 K respectively. In most of the regions of the Arched Filaments, the measured electron temperatures are consistent with the Doppler temperatures within the errors. In two regions (4 and 6) there is a discrepency (only two sigma) between the Doppler temperature and the measured \T. Figure 9c shows the distribution of \T~across the Arched Filaments. Over most of the source, the \T~are \ab6000 K, but along the northern edges of E2 and W1, and along the middle of the E1 filament (Regions 4, 5, 6, and 7), the \T~appear to increase up to 10,000 K. A likely explanation for the increased \T~in the northern portion of E2 and W1 is that the continuum emission in these regions is contaminated by the non-thermal contribution of the NTFs in the Radio Arc; the line-to-continuum ratio is therefore underestimated, and T$_e$ is therefore overestimated, since T$_e$ $\propto$ (T$_l$/T$_c$)$^{-0.87}$. By measuring the continuum emission in a region of the Arched Filaments adjacent to the Radio Arc NTFs, we estimate that 40\% of the continuum at the positions of the NTFs is nonthermal, and we correct the values of T$_l$/T$_c$ and \T~accordingly. Similar corrections were made to the line-to-continuum ratios in the Sickle H II region, where the values were significantly reduced due to the non-thermal contribution from the NTFs in the Radio Arc, which intersect the H II region at several positions (Lang et al. 1997). \subsection{Velocity Field} Figure 10 shows the distribution of central velocities in the Arched Filaments, which range from $-$70 to +15 \kms. The most impressive feature of Figure 10 is the presence of remarkable velocity gradients along the extent of each of the Arched Filaments. Figures 11 a-e show position-velocity diagrams for each filament (W1, W2, E1, E2, and G0.10+0.02). These diagrams were created to illustrate velocity as a function of position following closely the ridges of emission in each case. The velocities were sampled starting at the northernmost point (see captions for Figures 11a-e) and continuing southward along each filament. The sense of the velocity gradient in W1 and W2 can be characterized by increasingly negative velocities southward along both filaments. Figures 11 a and b show that the velocities steadily decrease from $-$10 \kms~to $-$60 \kms~in both cases. The most negative H92$\alpha$ line emission in the Arched Filaments (V $<$ $-$50 \kms) occurs at the southern extents of W1 and W2, in the vicinity of \ad=17 45 36.0, $-$28 51 30. The velocity structure in E1, E2, and G0.10+0.02 is more complex, and the velocity gradients do not have the same sense, or the same degree of continuity, as those in the W filaments. In the northernmost region of E1, a small spur of line emission which is oriented nearly perpendicular to the eastern edge of the filament (at \ad=17 45 54.3, $-$28 48 10) has velocities which are more negative than in the rest of the filament (V\ab$-$40 \kms~in the spur compared with $-$20 \kms~across E1). The velocity in this spur decreases southward along its length, similar to the sense of the gradient in W1 and W2. In the middle of E1 the velocities can be characterized with values of $-$15 to 0 \kms, which become only slightly more negative (V\ab$-$20 \kms) in some regions to the south. Over parts of E1, the gradient has the same sense as W1 and W2, but most of the ionized gas has velocities which are less negative than those in the W filaments. The sense of the velocity gradient in E2 is nearly opposite to that in the W filaments; the velocities become more positive southward along E2 (Figure 11d). In fact, there are several concentrations of {\it positive} velocity emission in E1 and E2, which are not present in the W filaments. The most extreme positive velocities in the Arched Filaments (V\ab+10 \kms), occur along the southern edge of E2. At this position (\ad=17 45 42.6, $-$28 49 30), there is also a spur of negative velocity emission located on the west side of E2 and extends over \ab2\arcmin~(5 pc). The velocity gradient along this component has the same sense as in the W1 and W2 filaments. G0.10+0.02 has velocities which range from about $-$20 \kms~near its northern portion, to values which are increasingly more negative ($-$40 \kms) along its southern extent, also resembling the gradients in W1 and W2. In addition, G0.10+0.02 has an unusual spur of positive velocity emission located at its southeastern edge. The magnitudes of the velocity gradients vary across the Arched Filaments and represent some of the most extreme and coherent gradients in the Galaxy. In W1, the velocity smoothly decreases from N to S, equivalent to a change of 40 \kms~over 7\arcmin, or 2.3 \kms~\pc. Along W2, a gradient of \ab5 \kms~\pc~is present in the N-S direction. In E2, the velocity ranges from $-$30 \kms~to +20 \kms, corresponding to a velocity gradient of 7 \kms~\pc, whereas in the E1 filament, the velocity varies by 2.4 \kms~\pc~in the N-S direction. The velocity gradients with the largest magnitude in E1 and E2 occur in a direction {\it perpendicular} to the long axis of the filament. Figures 11f and g show position-velocity diagrams for slices of 1\arcmin$-$1\farcm5~(2.5$-$3.8 pc) length taken across E1 and E2 in a direction nearly perpendicular to the long axis of the filament where it is apparent that the velocities are changing rapidly. In both cases the gradients in this direction are \ab16 \kms~\pc~, several times larger than the gradients observed in the N-S direction in the other filaments. The velocity gradients in the Arched Filaments are only surpassed in magnitude by the nearby ionized streamers of SgrA West, which surround the nuclear black hole, SgrA$^*$, and have velocities varying from $-$200 \kms~to 200 \kms~over \ab3 pc (Roberts \& Goss 1993). \subsubsection{Double-Peaked H92$\alpha$~Profiles} Two of the profiles in Figure 8 have double-peaked structure (Regions 12 and 14), as well as a a double profile between Regions 20 and 22 (at \ad=17 45 33.5, $-$28 47 30). At each of these positions there are large differences in the velocities of adjacent filaments as can be seen in Figure 10. The double profiles in the Arched Filaments do not have a systematic or symmetric distribution across any part of the source, such as might be indicative of an expanding, H II shell. Therefore, it appears that the double profiles simply occur at the interfaces of two gas components which have different velocities. These velocities, however, are consistent with the previous velocity results presented above. Figure 12 illustrates that a simple superposition of ionized gas components which have largely different velocities can explain the double profiles in all three regions. Figure 12 shows three integrated profiles from each of these double-peaked regions; each profile has been sampled at \ab15\arcsec~intervals from east to west across the region (corresponding to left to right in Figure 12). The central profile in Region 12 (Figure 12 b) shows a double profile with velocities of +5 \kms~and $-$41 \kms. In the east and west parts of Region 12 (Figures 12 a and 12 c), the dominant components of velocity are +11 \kms~and $-$33 \kms, respectively, which appear to correspond to the two velocity components apparent in Figure 12. The positive velocities likely arise from the ionized gas in the adjacent part of E2, which is characterized by a similar velocity range (+5 to $-$5 \kms). The negative velocity emission arises from a negative velocity spur located to the south and east of E2 (Region 11) which was previously discussed and found to have velocities in the range of $-$30 to $-$45 \kms. A similar pattern is detected in the double components of Regions 14 and between W1 and W2. The eastern portion of Region 14 (Figure 12d) is characterized by a positive velocity (V=18 \kms), in contrast to the negative velocity ($-$40 \kms) in the western part of this region (Figure 12e). At this position, the superposition of the negative-velocity emission in G0.10+0.012 and a positive-velocity component produces the two peaks in Profile 14. The position-velocity diagram of G0.10+0.02 (Figure 11e) also shows the two velocity components which differ by \ab60 \kms. There is a large velocity gradient in this positive-velocity component and the velocities range from +20 to $-$10 \kms. The two peaks in Figure 12 h have velocities of $-$4 and $-$35 \kms, corresponding closely to the central velocities of the W1 and W2 filaments (Figures 12g and 12i). {\it The presence of such double-peaked profiles illustrates that this H II complex consists of a series of independent filaments of ionized gas having different kinematics and different displacements along the line of sight.} \subsection{High Resolution H92\al~Line Observations} The CnB and DnC array observations of a single field in the southwestern portion of the Arched Filaments were combined to produce a higher resolution (3\farcs6 $\times$ 2\farcs7, PA=45\arcdeg) H92$\alpha$ line cube. This region was chosen since it has strong emission and contains the intersection of the Northern Thread NTF with both the W1 and W2 filaments. Figure 13 shows the distribution of H92$\alpha$ emission in this region in greyscale, with 8.3 GHz continuum contours. The strongest emission arises from along the W1 filament, in a series of ``knots'', with most of the emission concentrated at G0.07+0.04. The emission along the rest of W1 is tenuous, and along portions of W1, the line emission is dramatically edge-brightened, so that the center of the filament appears hollow. The velocity field of the high resolution data (Figure 14) resembles the overall structure of the lower resolution image (Figure 10). The velocity gradients in the W1 and W2 filaments are consistent with those measured in the lower resolution data: 2 to 4 \kms~pc$^{-1}$. At the locations of intersection between W1, W2 and the Northern Thread NTF, there do not appear to be any significant changes or discontinuities in the line properties (FWHM line width or velocity field). If there were significant discontinuities, this would indicate that a physical interaction is taking place between the ambient magnetic field and the ionized gas. However, the velocity along the southern portion of W1 varies as smoothly as in the lower resolution data (Figure 10), becoming increasingly negative toward the southern extent of the filament. In fact, the central velocity changes more abruptly in the northern portion of W2, where values range from \ab0 \kms~to $-$35 \kms~over several pc. Thus, large disturbances in the velocity field at the positions of projected intersection with the Northern Thread NTF are not observed. \section{Discussion} \subsection{Morphology} The close correspondences between the velocities and morphology of the ionized and molecular gas in the Arched Filaments that Serabyn \& \gusten~(1987) illustrated indicate that the Arched Filaments represent the ionized edge of the $-$30 \kms~cloud. The Arched Filaments therefore derive their morphology in part from the distribution of molecular gas. Tidal disruption due to the large differential gravitational forces near the Galactic center is likely to have influenced the morphology of the $-$30 \kms~cloud. In fact, Serabyn \& \gusten~(1987) propose that this molecular cloud is just at the stability limit for tidal disruption (n\ab5 $\times$10$^4$ cm$^{-3}$ for a radius of \ab30 pc) and has become tidallly unstable as it has fallen in toward the Galactic center from an outer radius. The elongated morphology of the Arched Filaments therefore can be ascribed in part to such tidal shearing of the cloud, given that the overall length of the filaments (\ab20 pc) is comparable to their projected radial displacement from the Galactic center (\ab25 pc). The CS emission in the inner 0.5-1\arcdeg~from the survey of Bally et al. (1988) shows that the clouds in this region are elongated along the Galactic plane, whereas at 1\fdg5, the clouds are oriented orthogonal to the plane. Other evidence for shearing of the cloud includes the orientation of the magnetic field in the Arched Filaments, traced by the far-IR polarization vectors (Morris et al. 1992; Morris et al. 1995). The inferred magnetic field orientation is aligned along the length of the filaments, suggesting that shearing occurs in a direction parallel to the filament long axis. This conclusion can be drawn for any arbitrary initial magnetic field geometry as long as the kinetic energy density in the shear motions exceeds the energy density in the magnetic field, in which case the magnetic field will be deformed until it is oriented in a direction parallel to the shearing (Morris et al. 1992). Of course, the location of the ionizing source in relation to the molecular cloud will also affect the morphology of the Arched Filaments. Several clues suggest that the ionized gas in the Arched Filaments is likely to be on the near edge of the molecular cloud. Observations of the Brackett-$\gamma$ line at 2.166 $\mu$m reveal an extinction toward the Arched Filaments consistent with the general extinction toward the Galactic center, indicating that the ionized gas is on the near side of the molecular cloud (Figer 1995; Cotera et al. 2000). These results are consistent with the fact that the observed H92\al~velocities are blueshifted at some locations relative to the velocities of the molecular gas at the same positions (Serabyn \& \gusten~1987). A further, more detailed study of the relative locations and velocities of ionized and molecular gas in the Arched Filaments is the subject of Paper II (Lang et al., in prep.). \subsection{Kinematics} The H92$\alpha$ line observations provide several important clues for understanding the kinematics and velocity field of the ionized edge of this peculiar molecular cloud. From these data we have deduced the sense and the magnitude of the velocity gradients at several positions across the source. The velocity gradients along the W1 and W2 Arched Filaments are well-organized and remain coherent over extents as large as 20 pc. It therefore seems appropriate to describe the velocity field