Extended emission around Ultra Compact HII regions

G004.417$+$0.126

Figure 6.6: The left panel shows the NVSS image of the Ultra Compact H II region G004.4$+$0.1 at 1420 MHz. The RMS noise in this image is $\sim 0.5$ mJy and the resolution of $\sim45\times45 {\mathrm{arcsec^2}}$. The right panel shows the 327-MHz image. The resolution in this image is $\sim15\times 11 {\mathrm{arcsec^2}}$ and the RMS noise of $\sim3.5$ mJy.

This source, visible in the 327-MHz GMRT image as well as in the 1420-MHz NVSS image (Fig. 6.6), coincides with a an UC H II region G004.417$+$0.126 (Becker et al.1994) (classified on the basis of its high frequency flux densities and IR colour selection criteria (Wood & Churchwell1989b; Wood & Churchwell1989a)). Images of this source at 327 MHz from the GMRT and 1420 MHz from the NVSS presented here are the first resolved images of this source. The extended emission around a compact core seen these images is similar to that detected for other UC H II regions using the VLA in D-array configuration (Kim & Koo2001; Kurtz et al.1999).

The peak flux densities measured at 327 and 1420 MHz, from images smoothed to the same resolution, are 0.49 and 0.51 Jy respectively. The spectral index between 327 and 1420 MHz corresponding to these values is close to zero and is consistent with this being a flat spectrum thermal source. The integrated flux densities from the 5 and 1.4 GHz Galactic plane surveys (Becker et al.1994) however corresponds to a negative spectral index between 5 and 1.4 GHz (the 1.4 GHz flux density from their measurement is in fact underestimated due to missing flux for sources larger than 120 arcmin; inclusion of the missing flux will make the spectral index more negative). Cuts taken across the spectral index map made using the images at 327 and 1400 MHz are shown in Fig.6.7. The spectral index of the compact core is reasonably flat between 1.4 GHz and 327 MHz. Here also, away from the core, the spectral index is negative, indicative of non-thermal component of emission (neither of these images suffer from missing flux). The average spectral index measured from the resolved images at these frequencies also show a gradient from nearly zero for the core to $\sim-0.3$ for the nebula.

Figure 6.7: Plot of vertical and horizontal slices of the spectral index map between 327 and 1400 MHz taken across the compact core of G004.417$+$0.126. The compact core exhibits flat spectrum while the extended emission around the core has a steeper spectrum.

In the far infrared colour-colour plot of $\log(S_{25\mu m}/S_{12\mu m})$ vs. $\log(S_{60\mu m}/S_{12\mu m})$ (Wood & Churchwell1989a), UC H II regions are concentrated in the upper left quadrant of the plot (around $\log(S_{25\mu m}/S_{12\mu m}) \approx 1.0$ and $\log(S_{60\mu m}/S_{12\mu m}) \approx 2.0$). The IRAS flux densities for this source are 16.07, 132.7, 1010 and 2748 Jy at 12, 25, 60 and 100$\mu$ m respectively (Becker et al.1994). On the IR colour-colour plot, this source lies at $\log(S_{25\mu m}/S_{12\mu m}) \approx 0.9$ and $\log(S_{60\mu m}/S_{12\mu m}) \approx 1.8$, which indicates that this is an UC H II region. H85$\alpha$ RRL transition at $V_{LSR}=4.1$ km sec$^{-1}$ has also been detected towards this direction (Lockman1989). This puts a lower limit on the linear size of a few pc corresponding to the observed angular size of $\sim 4$ arcmin and a distance corresponding to systemic velocity of the RRL towards this source. Again, this is large compared to the typical size for the UC H II regions ($<0.1$ pc). EM of $\sim11\times10^{7}$ pc cm$^{-6}$ for this source, using the peak flux density at 5 GHz and assuming $T_e=10^4$ filling the resolution element, is consistent with this source being a UC H II region (Wood & Churchwell1989b).

Recent detection of associated extended emission around many of the so called UC H II regions (Kim & Koo2001; Koo et al.1996; Kurtz et al.1999) is on a similar scale as the extended emission seen for this source. The extended emission seen in the 327 and 1400-MHz images is therefore not surprising; the advantage of high resolution provided by the GMRT simultaneously with sensitivity to large angular scales is apparent. However, it is unclear what ramifications this extended emission might have on the models that attempt to explain the morphology of UC H II regions (Kurtz2000). Scaled versions of current models are unlikely to explain the emission at arcmin scales. Similarly, the spectral index variation across the source (from the compact core to the extended component) is harder to explain.

G003.349$-$0.076

Figure 6.8: Sub-image showing extended emission around two catalogues Ultra Compact H II regions located at ${\mathrm{RA}_{J2000}}=17^h53^m41^s$, ${\mathrm{Dec}_{J2000}}=-26{^\circ}06{^\prime}08{^{\prime\prime}}$ and ${\mathrm{RA}_{J2000}}=17^h53^m41^s$, ${\mathrm{Dec}_{J2000}}=-26{^\circ}06{^\prime}04{^{\prime\prime}}$. The resolution in the image is $\sim 20\times 10 {\mathrm{arcsec^2}}$ and the RMS noise $\sim 5$ mJy/beam.

Two UC H II regions, namely G003.349$-$0.076 and G003.351$-$0.077 (Becker et al.1994) lie at the edge of the field containing the barrel shaped SNR G003.6$-$0.2. The location of these objects coincides with the northern most compact peak of emission in the sub-image of this region shown in Fig. 6.8. Extended emission in the immediate vicinity of these compact sources on the scale of several arc-seconds to several arc-minutes is also clearly visible in this image. The quality of the image for this region is, however, not very good, possibly due to primary beam attenuation as well as due to antenna tracking errors on some of the antennas due to which sources on the edge of the beams suffer from effective differential short time scale gain changes. The precise morphology of this extended emission as well as the flux density of this emission, therefore, cannot be reliably determined from this image. High resolution observations, centred on this region at a few frequencies, using the GMRT will be required to determine the nature of this extended emission.