The Astrophysical Journal, 578:224–228, 2002 October 10 # 2002. The American Astronomical Society. All rights reserved. Printed in U.S.A.
MAGNETIC FIELDS IN GAS FLOWS NEAR THE GALACTIC CENTER J. S. Greaves,1 W. S. Holland,1 and W. R. F. Dent UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK;
[email protected],
[email protected],
[email protected] Received 2001 November 30; accepted 2002 June 16
ABSTRACT Polarization of molecular lines at 1.3 mm wavelength has been used to trace magnetic field directions between 1 and 3 pc north of Sgr A*. This region connects the Galactic center and the circumnuclear disk (CND) and contains the northern arm of the ionized minispiral. Previous observations using polarized thermal dust emission have shown a rather uniform north-south magnetic field but with uncertainty about the relative contributions (and confusion) of the rotating disk and infalling streamers. By separating these components by velocity in carbon monoxide (CO) spectra, we find that the 20 km s1 streamer has a different magnetic field from that of the CND, and there is no clear correlation with gas flow directions. However, the dust data do trace the CND magnetic field direction within a few degrees. The CO polarization levels demonstrate the theoretically expected variations with line optical depth. Subject headings: Galaxy: center — ISM: magnetic fields — ISM: molecules — polarization — submillimeter and 850 lm (Novak et al. 2000; A. C. Chrysostomou et al. 2002, in preparation) shows similar polarization patterns, but the emission of cold dust clouds may be even more important in the submillimeter range, with Novak et al. (2000) estimating that a third of the 350 lm signal is not from the CND. To separate the several clouds that may have magnetic fields, we have used millimeter spectropolarimetry of the CO J ¼ 2 1 transition at 1.3 mm wavelength. The advantage of this approach is that magnetized clouds may be separated by velocity even when they overlap spatially. The technique uses population imbalances among the magnetic sublevels of molecular rotational levels, which can produce polarization either parallel or perpendicular to the net plane-of-the-sky magnetic field and is known as the Goldreich-Kylafis effect (Goldreich & Kylafis 1981). These observations are difficult because each molecular line spectrum must have stable calibration to better than 1%, but the techniques have recently been proved (Glenn, Walker, & Jewell 1997; Greaves et al. 1999; Girart, Crutcher, & Rao 1999). Greaves et al. (1999) presented some detections for the Galactic center region that were in good agreement with the dust polarimetry results of Hildebrand et al. (1993), but the line polarization data were velocity-averaged and the two points observed were on the periphery of the CND. This paper presents new CO polarization detections toward the northern streamer, lying between Sgr A* and the north end of the CND, and discusses the magnetic field morphology.
1. INTRODUCTION
A magnetic field near the center of our Galaxy is likely to be important for the gas dynamics in general and, in particular, for material orbiting within the circumnuclear disk (CND) and the infall of gas onto the massive black hole candidate Sgr A*. Understanding these processes within the central few parsecs of our Galaxy may also contribute to a better understanding of how gas is channeled onto extragalactic black holes and starburst nuclei (Toniazzo, Hartquist, & Durisen 2001; Beck 2000; Greaves et al. 2000; Mezger, Duschl, & Zylka 1996). Two direct methods of probing the Galactic center magnetic field are polarization of thermal dust grain emission (at submillimeter to far-infrared wavelengths) and Zeeman effects (in the centimeter to millimeter regime). Polarimetry maps the orientations of the magnetic field components in the plane of the sky, while Zeeman measurements give the strength and direction of the line-of-sight magnetic field. Polarimetry thus gives more geometrical data, but for dust continuum emission has no velocity information, so clouds lying along the same line of sight are blended. This is a particular concern for the Galactic center region, which includes (Plante, Lo, & Crutcher 1995; Novak et al. 2000) the CND at radii of approximately 1.5–3 pc, several gas streamers inside the disk, and at least two massive giant molecular clouds connected to the outside of the disk—plus the foreground emission from clouds in Galactic spiral arms and the 5 kpc ring. Initial observations at 100 lm (Hildebrand et al. 1993) showed a roughly north-south polarization pattern superposed on the ringlike far-infrared emission of the CND, so foreground clouds appear to make little contribution. However, inside the northern side of the highly inclined ring, a far-infared streamer was observed that may contain non-CND material and includes and extends the northern arm of the minispiral of atomic and ionized gas (Jackson et al. 1993). Recent polarimetry at 350
2. OBSERVATIONS
The data were obtained at the James Clerk Maxwell Telescope (JCMT) in Hawaii, with the RxA2 receiver, DAS autocorrelator, and UKT14 polarimeter on 1997 August 1. Polarized spectra were obtained at the frequency of the CO J ¼ 2 1 line, 230.538 GHz, and the telescope beam size was 2100 FWHM. Observing conditions were wet with high local humidity, but the sky was very stable, with seeing estimated at about 100 . Sky emission was subtracted by switching the telescope to a reference position roughly 1
1 Formerly at Joint Astronomy Centre, 660 North A‘ oho ¯ ku¯ Place, University Park, Hilo, HI 96720.
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GALACTIC CENTER MAGNETIC FIELDS northwest of Sgr A*. The data reduction procedures have since been fully developed and are described by Greaves et al. (1999). Repeatability of results has been established by comparing data taken with different observational setups and 2–4 yr apart (Greaves et al. 1999; Greaves, Holland, & Ward-Thompson 2001). The polarimeter consists of a half-wave plate, which is stepped to a series of different angles, thus rotating the incoming source plane of polarization so that the singlepolarization receiver sees modulating signals. Each wave plate cycle included 16 s of integration at 16 equally spaced angles. For each of the four positions observed, either four or five complete cycles were obtained, totaling approximately 20 minutes of integration. Including extensive calibration (after each 16 s spectrum) the total period of observation was 4 hr, during which the sky rotated by 70 of parallactic angle. This assists in reducing the level of systematic errors, since effects that are fixed in the receiver cabin coordinate frame rotate with respect to the sky frame and thus tend to self-cancel with time. The polarization parameters were obtained by subtracting pairs of spectra taken at wave plate angles 45 apart, to directly estimate the magnitudes of the two orthogonal components of linear polarization (see, e.g., Fig. 1 in Stewart 1985). Polarized spectra have been binned to a resolution of 20 km s1 to improve the signal-to-noise ratio. The source positions were chosen to coincide with points for which dust polarization data exist (Hildebrand et al. 1993) and are given in arcsecond offsets from (B1950.0) 17h 42m 29 94, 28 590 1900 (which is approximately 100 west of Sgr A*). Instrumental polarization (IP) of 0.6% at angle of 110 (in the receiver cabin frame of reference) was subtracted from the data. This value was measured in 1996 December using Saturn (assumed to be unpolarized) and has a variation of 0.25% across the passband: this was the dominant systematic and is taken to be the error on the IP subtraction. A second measurement, using Jupiter, was made in 1997 July but was of lower quality, being affected by a baseline ripple in the spectra. However, the range of IP values across the passband was 0:7% 0:4% at 70 30 , which is quite consistent with the Saturn result. An extra instrumental effect can arise because the beam shape is slightly elliptical, with FWHM of about 2200 and 2000 . Rotating the wave plate effectively rotates this ellipse over the source, so in a highly structured region such as the Galactic center, changes induced in the measured signal can mimic a polarization. This effect has been estimated using the map shown in Figure 1 by taking sets of four points in a 4000 wide square and weighting them by the beam power in different orientations. That is, a simulated spectrum is formed with two opposite corners having weights of 0.31 and the other two corners having weights of 0.24. These weights are calculated from the Airy beam profile and ellipticity and are relative to the central point in the beam. A second simulated spectrum is formed in the same way, but the weights are swapped. This effectively simulates rotating an elliptical beam over the sky, and the difference between the two net spectra yields the false polarization. The maximum value found in any one 20 km s1 spectral channel at an individual position was 1%, but after averaging different spatial points (see below), the intensity gradients in the map tend to cancel, and the final effect is only 0.15% at any one velocity. Adding this in quadrature to the IP subtraction error yields a typical total systematic of 0.3%, which is slightly
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larger than the measurement errors of the source polarizations and will introduce extra position angle uncertainties of 7 –8 (Wardle & Kronberg 1974). 3. RESULTS
The various clouds identified with the Galactic center region can be identified from CO channel maps, which are shown in Figure 1. The CND is a highly inclined rotating ring with i 70 and oriented with the major axis about 25 east of north, and it appears at +60 km s1 at our (þ1900 ; þ1900 ) and (000 ; þ7500 ) positions, increasing to a velocity of +100 km s1 at the (þ1200 ; þ3400 ) and (þ1900 ; þ5600 ) positions (Jackson et al. 1993). The ring emission appears indistinct at +60 km s1 because the gray scale is dominated by a brighter cloud feature to the east (seen in full in Fig. 5d of Zylka, Mezger, & Wink 1990). There are also two prominent gas streamers at approximately 20 and +80 km s1, both of which pass near Sgr A. Other emission at velocities less than +50 km s1 is from foreground clouds not directly associated with the Galactic center. An example of the polarization spectrum for the (þ1900 ; þ5600 ) position is shown in Figure 2. The CND appears as a clear, high-velocity peak at +100 to +110 km s1, and the two streamers can also be identified as peaks in the intensity spectrum. Polarization is detected at the higher velocities, but the signal-to-noise ratio is low. Because different cloud features are separated in velocity by only of the order of the 20 km s1 spectral bin, we cannot do any further velocity averaging. Instead, we have averaged the data for the four spatial positions to obtain good detections, and the results are listed in Table 1. The increase in the signal-to-noise ratio is generally consistent with common polarization directions among the four positions and is discussed below. CO polarization is detected at the 5–6 level for the 20 km s1 streamer and the CND, with similar percentages but with angles differing by 70 . The second streamer, at +80 km s1, is not detected, but as shown in Figure 1, the positions observed lie somewhat to the east of the streamer. If the (þ1900 ; þ1900 ) point is excluded, the average polarization of the three remaining positions is 1:6% 0:1% at an angle of 5 1 . The (þ1900 ; þ1900 ) point may be unpolarized, since the value obtained was p 0:5% 0:9%. If the remaining three-point average is valid, the polarization direction for the +80 km s1 streamer is similar to that of the CND. Dust polarimetry for these points shows a net direction of +85 (Novak et al. 2000) or +88 (Hildebrand et al. 1993). The former data were taken with the same beam size as the JCMT observations but at points offset by 100 – 700 , while the latter were at identical positions but with a larger beam. The continuum results are consistent, however, and the dispersion in angle among the four positions is small, only 6 . These authors found the magnetic field direction (perpendicular to the E plane of emission from aligned grains) to be north-south and interpreted it to be largely from the CND. Table 1 shows that the CND line emission is polarized at +3 , which is 82 –85 from the dust direction. Both the dust and CO polarimetry results are consistent with a north-south magnetic field in the northern region of the CND (assuming the CO polarization direction is parallel to the field). This is a rather surprising
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Fig. 1.—CO J ¼ 2 1 channel maps of the CND, from the JCMT data archive. Velocity bins are 20 km s1 wide, centered around +100, +80, +60, and 20 km s1, and the grid point spacing was 2000 . Crosses: Positions observed. Star: Position of Sgr A*; the coordinate frame is B1950.0. Contours: Average intensity in steps of 2.5 K from 2.5 to 22.5 K on the TA (atmosphere-corrected) intensity scale. Gray scale: Low intensity (black) to high intensity (white). Dashed lines: Guide to regions of the CND (left panels) and the streamers (right panels).
outcome, since the position angle of the inclined ring is about 25 east of north, not north-south. An axially symmetric magnetic model in which the field lines are dragged around by differential rotation (Hildebrand et al. 1993; Wardle & Ko¨nigl 1990) produces a largely northsouth field in this region of the CND, but only if the major axis of the structure is oriented north-south, which is somewhat at odds with molecular maps (Jackson et al. 1993). The four positions observed are representative of the gas flow directions within the ring.
Although streamers were visible in dust emission (Hildebrand et al. 1993), their magnetic fields do not appear to contribute much to the net polarization, since the dust data and that from CO in the CND agree well. It is not clear whether one or both of the two streamers might be seen in the dust maps, but the 20 km s1 streamer does not have the same polarization direction as the CND (Fig. 1). The inferred magnetic field direction is at either +73 or 17 , neither of which agrees with the streamer orientation, which lies somewhat positive from north in Figure 1.
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TABLE 1 CO Polarization Results Streamers Measurements
Foreground
20 km s1
+80 km s1
CND
p (%) ................... h (deg) .................
0.3 ...
1.20.2 +736
0.70.6 8
1.00.2 +36
Note.—CO polarization results for foreground clouds [vðLSRÞ ¼ 10 to þ 50 km s1], streamers at 20 and +80 km s1, and the circumnuclear disk (velocities of +60 or +100 km s1, see text). Percentage errors have been corrected for positive-definite bias (Wardle & Kronberg 1974).
4. DISCUSSION
and A. C. Chrysostomou et al. (2002, in preparation) are generally consistent with those found for the gas. Therefore, the dust data can be used to model the CND magnetic fields with reasonable accuracy. However, there is at least one additional magnetic field component in a diverging direction in the –20 km s1 streamer, and this is detected only when the various clouds can be separated by velocity. A more extensive CO data set combined with three-dimensional reconstructions of the various clouds (Vollmer & Duschl 2000) would allow us to produce a threedimensional magnetic field map of the entire region within a few parsecs of Sgr A*. The relatively new technique of millimeter line polarization is also shown to be producing results in good agreement with theory. As discussed by Kylafis (1983), the dust and line polarization directions should be either parallel or perpendicular. Intermediate cases indicate either that the data are suspect or that they do not both trace the same region. Perpendicular directions are found here for the Galactic center, consistent with one of the two theoretical expectations when the same magnetic field is observed. In fact, the parallel case has also been seen for a point at the northeast edge of the CND (Greaves et al. 1999). The outcome is highly sensitive to the gas flow and magnetic geometry. As shown by Figure 5 of Kylafis (1983), there is a switch between line polarization being parallel and perpendicular to the magnetic field on changing the gas flow structure from a one-dimensional to a two-dimensional case. Such changes may be common around the Galactic center, where streaming and expansion effects could occur in the same vicinity. In addition, the polarization percentages for CO are expected to be inversely related to the optical depth of the gas, for moderately bright lines. This is because both planes of polarization saturate to the maximum intensity for 41, so p tends to zero. (For faint lines with 5 1, p also falls because radiative transitions are outweighed by collisional ones, which do not create the level imbalances necessary for polarization.) The predicted behavior for moderately large opacities is seen here. The CND and Galactic center streamers have CO J ¼ 2 1 optical depths of 1–3 and average polarization of 1%. In contrast, the foreground gas clouds at 10 to +50 km s1 have opacities 10 and polarization limits of 0.3%.2 These results strengthen the possibility
The aim of this study was to check whether dust polarization gives reliable results in a region of such complex source structure as the Galactic center. The comparison with CO results shows that the magnetic fields deduced with the dust technique by Hildebrand et al. (1993), Novak et al. (2000),
2 The optical depths were estimated from the 12CO:13CO line intensity ratio, using 13CO J ¼ 2 1 spectra in the JCMT archive and assuming a 12C:13C abundance ratio of 25 for the Galactic center and 60 for the outer Galaxy (Langer & Penzias 1990).
Fig. 2.—Spectra for the (+1900 , +5600 ) position. Top: Intensity spectrum I at the original resolution of 0.2 km s1, while the polarization percentage ( p, middle) and direction (h, bottom) have been regridded to 20 km s1 channels. The polarization percentage and angle error have been corrected for positive-definite bias (Wardle & Kronberg 1974). Error bars are the standard error of the mean of the data.
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that the basic physics of line polarization from molecules is correctly understood. The JCMT is operated by the Joint Astronomy Centre on behalf of the UK Particle Physics and Astronomy Research
Council, the National Research Council of Canada, and the Netherlands Organization for Pure Research. We would like to thank the staff of the JCMT for their help and patience with these difficult observations and the two referees for insightful comments.
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