HIGHLY POLARIZED COUNTERPART OF THE DOUBLE HELIX NEBULA

The Galactic center region is the nearest nucleus of a spiral galaxy. Extensive observations over two decades ago revealed the magnetic fields in this region (e.g., Yusef-Zadeh et al. 1984; Tsuboi et al. 1985, 1986; Seiradakis et al. 1985). The large-scale structure of the magnetic field is approximately perpendicular to the Galactic plane, shown as the vertical filaments (hereafter VF) and the polarized plumes (hereafter PP). The PP are twin linear-polarization-bright and off-plane components located on both ends of the VF. These form a large-scale poloidal magnetic field extending over 150 pc, which we refer to as the VF–PP complex (hereafter VFPP). The "threads," which are filamentary features seen in a radio polarization map, are also signs of large-scale magnetic fields in the Galactic center region (Lang et al. 1999). However, the VFPP is much brighter than the "threads" at centimeter wavelength. The VFPP is different from them in supply of relativistic electrons emitting synchrotron radiation. Recently, the Spitzer Space Telescope revealed fascinating features in the Galactic center region (Stolovy et al. 2006). In the course of the observations, the double helix nebula (hereafter DHN) was found as an off-plane structure at 24 μm in the region (Morris et al. 2006). The DHN is apparently double helical long filaments seen in IR, which suggest that the DHN is physically related to the circum-nuclear disk (hereafter CND) of the Galactic center, and the magnetic torsional wave induced by the CND makes the twisting appearance of the DHN. However, there is no clear connection between the CND and the DHN in any radio continuum maps. The DHN is located near the positive latitude or galactic northern end of the north PP. It is suggestive of a physical interaction between the DHN and the PP. To confirm the interaction, the polarization properties of the DHN and the PP may be important. Throughout this Letter, we will adopt 8.5 kpc as the distance of the Galactic center. We show the direction on the sky with respect to galactic coordinates.

Single-dish observations easily observe widely extended features. Interferometric observations are less sensitive to detect such features because the features are resolved-out, although they are superior to single-dish observations for calibration accuracy and homogeneity of the data. Therefore, we investigate the linear polarization of the less bright region extended northward from the end of the north PP and the DHN using the observation data of the Galactic center region at 10 GHz with the Nobeyama Radio Observatory (NRO) 45 m telescope from 1985⁴ (Tsuboi et al. 1986). The observation of the Galactic center region was performed in polarization mapping mode using a four-frequency-channel polarimeter (BW = 500 MHz × 4 ch) which was equipped in the NRO 45 m telescope. The total frequency bandwidth of the observation is 2 GHz from 8.8 to 10.8 GHz. At present, such wide frequency band polarimetry at 10 GHz is unavailable with the Nobeyama 45 m telescope, because interference due to city radio noise has increased. The beam size of the NRO 45 m telescope is FWHM = 26 at 10 GHz. The aperture efficiency is η_a = 65%. The side lobe level is less than −25 dB of the peak of the beam. The polarimeter consists of a rotatable λ/2 phase shifter followed by an orthomode transducer. The two outputs of the orthomode transducer are switched by a diode switch to derive differential polarizations, Stokes parameters Q and U. The mapping is done in a raster scan mode. Each scan is almost perpendicular to the Galactic plane. The scan length and the scan interval were 30 and 002 (12), respectively. Both ends of each scan are assumed to be emission free. The resultant noises of the total intensity and the polarized intensity are 36 and 7 mJy beam⁻¹, respectively. Typical statistical error of the degree of polarization including weather conditions was Δp ≃ 2% around the end of the north PP, although the instrumental polarization of the NRO 45 m telescope was calibrated to less than 0.5% at 10 GHz. Figure 1 shows reproduced linear polarized intensity and linear polarization degree maps at 10 GHz including the VFPP. Because of the faintness of the PP, the four-frequency-channel data are combined. The PP are clearly seen as a pair of prominent highly polarized structures in Figure 1. The local peaks in the linear polarization degree are at b = 025 and b = −045, respectively. The maximum of the polarization degree is p ⩾ 45% at the northern local peak. The area between these peaks presumably suffers from Faraday depolarization caused by ionized gas and the magnetic field near the Galactic plane. The north PP can be traced up to b = 075. A red square in Figure 1 indicates the area around the DHN. The north end of the north PP is slightly bending east around b = 045 and is two-forked around b ⩾ 05. The western forked feature apparently corresponds to the DHN.

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Figure 2(a) shows the distribution of B vector around the DHN overlaid on the linearly polarized intensity map at 10 GHz. This area is shown by the red square in Figure 1. The position angles of the B vectors are corrected for Faraday rotation, estimated with comparison among the directions of polarization of four frequency bands. The counterpart of the DHN is identified as a tangled feature located near the north end of the north PP in the polarized intensity map. Typical error of the position angle of B vector is Δθ = 15° around the DHN. The counterpart branches around l = 009, b = 053 from the north PP and is extending approximately perpendicular to the north PP. Figure 2(b) shows the same distribution of the B vector overlaid on the map of the linear polarization degree. This figure also shows the IR image at 24 μm of the DHN (Morris et al. 2006). The position angles of B vectors are presumably ordered along the twisting filaments of the DHN seen in the IR map.

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Figure 3(a) shows the difference between position angles of B vector and IR filaments of the DHN. Figure 3(b) shows the enlarged and contrast enhanced map of Figure 2(b) for exploring the relation between the B vectors and the IR filaments, which are labeled as A and B. The position angles of IR filaments are determined from the local orientations on Figure 3(b) by eye fit. The observed B vectors are not always on the filaments. In this case, we used the nearest portion of the filaments. The error of the relative position angle between the B vector and the IR filament is difficult to quantitatively estimate. The error bars show only the error of B vector. The data points are distributed in the range of −40° to 40°. It suggests that magnetic fields in the DHN trace the IR filaments.

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Although the angular resolution of the map is as large as 26, the local peak of the linear polarization degree of the DHN is p = 15% ± 2% at l = 003, b = 065. Meanwhile, the DHN can also be identified in the linearly polarized intensity map at 8.6 GHz with the Parkes 64 m telescope (Haynes et al. 1992). The angular resolution of the map is also 3'. The peak of the linear polarization degree at 8.6 GHz is p = 10% approximately at the same position. The high polarization degree means that there is highly ordered magnetic field with relativistic electrons in the DHN. The B vectors in the DHN trace the IR filaments in the DHN, and the DHN is located on the extension of the magnetic field in the PP. In addition, there is no clear boundary between the DHN and the north end of the north PP in radio continuum at 10 GHz (Handa et al. 1987). The DHN is not an isolated feature from the PP. It is plausible that the continuous magnetic field penetrates both the DHN and the VFPP. A recent study of radio continuum emission shows that faint radio filaments are adjacent to the IR filaments in the DHN and seem to be along their west periphery (see Figure 24 in Law et al. 2008).

The magnetic field penetrating through the DHN and the VFPP is probably connecting to the large-scale magnetic field out of the Galactic center region. If so, high-energy electrons can move along the magnetic field easily and can result in an outflow. Previous observations show that the spectral index, α, along the VFPP (S_ν ∝ ν^−α) decreases with increasing distance from b ≃ −01 (Tsuboi et al. 1995). Although the origin of relativistic electrons is still unknown, it is proposed that the relativistic electrons in the VFPP are accelerated around b ≃ −01 (e.g., Tsuboi et al. 1997). The lifetime of relativistic electrons in the magnetic field is given by T_1/2(s) = 1/2 × C₁₂H^−3/2(G) assuming a power-law energy distribution of synchrotron-emitting electrons between the upper and lower cutoff energies (Pacholczyk 1970, Chapter 7). Here, C₁₂ is a factor depending on the spectrum shape and H is the strength of the magnetic field. The relativistic electrons are expected to be moving along the magnetic field as in a normal plasma. The velocity of the bulk motion must be less than Alfvén velocity, which is given by $V_A\,\mathrm{(m\, s}^{-1})=2\times 10^9 \frac{H\,\mathrm{(G)}}{\sqrt{n_H\,\mathrm{(cm^{-3})}}}$. The drift range of the relativistic electrons emitting at 10 GHz is less than V_A × T_1/2. We assume that the spectrum between 10 MHz and 10 GHz is flat, because this component is intense even at 10 GHz. The strength of the magnetic field in the VFPP is estimated to be H ⩽ 0.1 mG from the elongation of molecular clouds observed by the Nobeyama Millimeter Array (Tsuboi et al. 2009). If the electrons in the DHN are transported for 140 pc from the acceleration area which is expected to be adjacent to the molecular cloud G0.11-0.11 (Tsuboi et al. 1997), the upper limit of gas density, n_H, is as low as n_H ⩽ 0.25(cm⁻³). This density is too small for the DHN, because the DHN is clearly seen as a dust cloud at 24 μm. Therefore, we can rule out a plasma outflow with the relativistic electrons from the acceleration area near the Galactic plane as the origin of the DHN.

We analyze linear polarization observations around the DHN of the Galactic center region at 10 GHz with the Nobeyama 45 m telescope. The B vectors in the DHN are presumably along the twisting filaments. The high linear polarization degree up to p = 15% ± 2% indicates highly ordered magnetic field in the DHN. We hypothesize that the polarization counterpart of the DHN is a faint extension of the PP.

The authors thank A. Miyazaki of the National Astronomical Observatory Japan for useful discussion and also thank referee M. Morris at UCLA for constructive comments.

Facilities: No:45m - Nobeyama 45m Telescope