The 870 μm continuum emission and its polarized components were detected (Figure 2; Table 1 & 2). The results are presented in this section.
2.3.1 Continuum Emission
In e2, a compact 870 μm continuum emission structure with a radius of ∼ 1
(0.03 pc) is centered at ∼0.7 east of e2. Extending to the north-west of this compact emission, a fainter structure with an overall length of ∼ 2 (0.07 pc) is detected.
The H2O (Genzel et al. 1981) and (J,K)=(9,6) NH3 (Pratap et al. 1991) masers are located in this north-west extension,∼ 2away from the continuum peak. Associated with the continuum peak, there are OH masers detected within 0.5 to the east and
∼1 to the south of e2 (Gaume & Mustel 1987; Fish et al. 2006), suggesting that it is an active star forming site.
In e8, the 870μm continuum peak is centered at 0.3 west of e8. e4, e1 and e3 are at the periphery of the 870 μm continuum emission. There is an extension toward the south-west with an overall length of∼3. Associated with e8, an NH3 maser spot was detected 0.8 south of the 870 μm continuum peak by Pratap et al. (1991). The OH (Gaume & Mustel 1987) and H2O (Genzel et al. 1981) masers are also associated with e8 and the 870 μm continuum peak, suggesting again that this is an active star formation site.
When fitted with a Gaussian, the deconvolved size of the 870 μm emission in e2 is 0.9×0.8, slightly larger than the synthesized beam, and therefore, the e2 core has been resolved. For e8, the deconvolved size is 0.9×0.3 with a P.A. of 12◦. Therefore, e8 has been resolved along the major axis of the dust ridge but not along the minor axis. In both e2 and e8, the 870 μm continuum emissions are associated with the NH3 cores (Ho et al. 1983; Zhang & Ho 1997), suggesting that they are also tracing the dense regions.
The measured 870 μm flux densities within the upper and lower boxes in Figure 2(b), associated with e2 and e8, are 9.3 and 4.0 Jy, respectively. The flux
densi-ties of the free-free continuum Ff f at 1.3 cm in e2 and e8 are 300 mJy (Gaume &
Johnston 1993) and 17 mJy (Zhang & Ho 1997), respectively. In order to estimate theFf f contribution at 870 μm, we extrapolate from 1.3 cm, assuming Ff f ∝ ν−0.1. Although this assumption of optically thin emission is crude, it has been shown that the resultant Ff f roughly agrees (within a factor of 3) with the estimate from the radio recombination line at 2 mm (Zhang, Ho, & Ohashi 1998), suggesting that the assumed Ff f ∝ ν−0.1 is reasonable. The extrapolated Ff f at 870 μm is ∼230 and 13 mJy for e2 and e8, respectively. As compared to the 870 μm flux densities, Ff f
contributes∼2% for the e2 region and 0.3% for the e8 region. Therefore, the 870 μm continuum is dominated by dust emission. Hereafter, the structures traced by the 870 μm emission in the e2 and e8 regions are named as e2 dust ridge and e8 dust ridge, respectively.
Assuming a dust temperature of 100 K (Zhang, Ho, & Ohashi 1998), a dust grain emissivity Q(λ) ∝ λ−β with β = 1, and the normal gas to dust ratio of 100, we estimate gas massesMgasof 245 and 106 Mfor the e2 and e8 dust ridges, respectively (cf. Tang et al. 2009). Note that theMgasgiven here is highly affected by the assumed β. If the assumed β is 2, the estimated Mgas will be 14 times larger. Assuming the extents along the line of sight are equal to the diameters of the emission area in the e2 and e8 dust ridges, the average gas number densitiesnH2 are 3.4×106 and 2.2×106 cm−3, respectively. By using the same equation and the same assumed values of β and dust temperature, the Mgas estimated from the 2 mm dust continuum (Zhang, Ho, & Ohashi 1998) for the e2 and e8 dust ridges are 1100 and 590 M, respectively.
The difference in the estimated Mgas at 2 mm and 870 μm is most likely due to the missing flux from the extended component, which is not recovered with our SMA observations. In comparison, with the same assumptions, theMgas of the envelope is 1834 M as traced at 1.3 mm by BIMA (Lai et al. 2001). The Mgas associated with the e2 and e8 dust ridges recovered with our SMA observations is∼ 19% of the Mgas
in the envelope.
The main conclusion from the dust continuum data is that the associated mass is large. The morphology of the dust continuum is elongated. The positional offsets between the various embedded sources are significant, such as between the positions
of the 870 μm peaks and the UCHII regions. These results are consistent with the formation of a cluster of stars.
2.3.2 Dust Polarization
The polarization in the e2 and e8 dust ridges is detected and resolved (Figure 2 (c) and (d)). Throughout the paper, P.A. is defined from the north to the east. In the e2 dust ridge, the bulk of the polarization vectors form a ring around the 870μm peak with a radius of ∼1 and with the geometric center near the continuum peak instead of e2. In the north-west extension of the dust ridge, the polarization appears to be perpendicular to the major axis of the extension.
The e8 dust ridge is ∼7 away from the phase center. Even though the antenna response is 15% less efficient than at the phase center, the polarization revealed is clearly also not as uniform as previously seen with BIMA. The polarization vectors again form a ring like structure around the continuum peak. The polarization is weaker in e8 with more vectors between 2 to 3 σIp.
In comparison, the polarization in the envelope of the e2 and e8 regions, as revealed with an angular resolution of 3 (0.1 pc) with BIMA, shows a relatively uniform distribution in P.A. and therefore, a fairly uniform B⊥ field at 1.3 mm (Figure 2(a);
Lai et al. 2001). In their results, the polarization in the e2 region is weak and resolved into e2 main and e2 pol NW, named in the same paper, according to the P.A. of the polarization vectors. The component e2 pol NW is at 3 to the north-west of e2.
There is a gap where no polarized emission is detected between e2 and e2 pol NW. In the e8 region, the polarization in the BIMA results is nearly uniform with a decrease in polarization percentage near the peak position.
In order to test if the differences in polarization properties from SMA and BIMA are due to their different angular resolutions, we smoothed our SMA results to the BIMA resolution, as shown in Figure 3. Wherever the polarized emission was both detected at 1.3 mm and 870 μm, the resultant P.A.s of the polarization differed by
∼ 30◦ on average. This significant difference can be due to the different sampling of the visibilities, which are in the range of 6 to 170 kλ (λ=1.3 mm) for the BIMA
and in the range of 30 to 262 kλ (λ=870 μm) for the SMA. Specifically, the SMA filtered out the more extended and uniform component which is larger than 8. At the same angular resolution, the derived global B⊥field directions in e2 and in e8 are therefore consistent in the regions where both the SMA and BIMA have polarization detections. Most importantly, the smoothed SMA polarization map shows that the polarization percentage has decreased significantly, especially near the continuum peak positions, where the field geometry is more complex at the resolution of 0.7.
This demonstrates that the low polarization percentage at the emission peaks is due to the limited angular resolution when a more complex underlying B field morphology has not been resolved. This effect can also be due to the decrease of the alignment efficiency of the dust grains in denser regions (Lazarian & Hoang 2007) or due to geometrical effects, such as the differences in the viewing angles (Gon¸calves et al.
2005). However, in this case, the complex B field structure is the dominant effect.
The polarization percentage P (%) decreases with increasing continuum intensity I in both e2 and e8 even for the higher resolution SMA results (Figure 4). Since the BIMA results come from a resolution effect, the same might be true for the SMA results at the emission peaks. Away from the emission peaks, the general increase in P (%) is somewhat misleading. Figure 2(a) shows that this effect is not symmetrical on either side of the elongated envelope, i.e. the P (%) differs with positions on the same contour level of I. This is reflected by the large dispersion in P (%) at any value of I/Imax. Several effects, including B field geometry related to the line of sight, need to be disentangled. That the P (%) ranges mainly between 1% to 10%
(Figure 4), seems to agree with the model of grain growth in the dense regions where grain alignments are via radiative torques (see Figure 11 in Pelkonen et al. 2009).
However, based on our results, the effects of angular resolution and geometry must first be taken into account.