5 Latitude extent of carbon line emission

We examined the data collected along galactic latitude toward two longitudes (l = 0 $.\!\!^\circ$0, Fig. 6 and l = 13 $.\!\!^\circ$9, Fig. 7) in the low-resolution survey. The beam center was shifted by half the beamwidth (i.e. 1$^\circ$) along the galactic latitude and data was collected up to $b = \pm$ 4$^\circ$ toward l=0 $^\circ$ and up to $b = \pm$ 3$^\circ$ toward l=13 $.\!\!^\circ$9. The carbon feature is clearly seen in the spectrum averaged over the entire latitude range observed toward l=0$^\circ$ (see Fig. 6c). The data toward the Galactic center was excluded since the carbon line emission in this direction is fairly strong and hence likely to dominate the averaged spectrum. Carbon lines are also detected when the data from positions separated by the beamwidth (i.e. 2$^\circ$) over the entire sampled region are averaged, confirming the presence of carbon line emission over several degrees in galactic latitude.

Our data indicates that the carbon line emission extends from $b\sim -$2$^\circ$ to $b \sim +$4$^\circ$ toward the galactic longitude l = 0$^\circ$. The widths of the carbon lines seen in the positive latitude spectra and the negative latitude spectra (Figs. 6b and d) differ by a factor of $\sim$2.5. The line parameters are listed in Table 2. The difference in line width may indicate the presence of distinct line emitting regions along the latitude extent, maybe with different physical properties.

Figure 6: Spectra of data averaged over different galactic latitude ranges toward l = 0 $.\!\!^\circ$0 are shown in the top plot. The bottom schematic shows the stacking of the ORT beams along positive latitudes toward l = 0 $.\!\!^\circ$0; similar data was also obtained for negative galactic latitudes. The spectra in the panels: a) G0.0+3.5avg was obtained by averaging data from beams centered at b = 3$^\circ$ and 4$^\circ$. b) G0.0+2.5avg was obtained by averaging data over 5$^\circ$ from beams centered at b = 1$^\circ$, 2$^\circ$, 3$^\circ$ and 4$^\circ$. c) G0.0+0.0avg was obtained by averaging data over 10$^\circ$ centered at b = 0$^\circ$. d) and e) are similar to b) and a) except that the data is averaged over negative latitudes. The spectrum toward the galactic centered has been excluded from all the averaged spectra. The carbon line emission is seen to extend from $b\sim -$2$^\circ$ to $\sim$4$^\circ$.

Figure 7: Spectra obtained by averaging data over different galactic latitude range toward l = 13 $.\!\!^\circ$9. Data is obtained up to $b=\pm 3$$^\circ$. a) G13.9-2.0avg is obtained by averaging the data over 4$^\circ$ in latitude about b = -2$^\circ$. b) G13.9+2.0avg is same as a) but about b = +2$^\circ$. c) G13.9|b|>1 is the spectrum obtained by averaging all the data at latitude |b| > 1$^\circ$. The spectrum toward l= 13 $.\!\!^\circ$9, b= 0 $.\!\!^\circ$0 has been excluded in all the averaged spectra shown in the figure.

Figure 7d shows the spectrum toward the longitude l =13 $.\!\!^\circ$9 obtained by averaging the line emission along latitude between $b = \pm$ 3$^\circ$. The spectrum toward l=13 $.\!\!^\circ$9, b=0$^\circ$ has been excluded in the average spectra shown in Fig. 7. A narrow feature ( $\Delta V \sim$ 9 km s^-1) is clearly detected near 18.5 km s^-1. A weak, broad carbon line feature also seems to be present in the spectrum. The spectra averaged over the positive and negative latitude extents are shown in Figs. 7a and b. The narrow feature is clearly evident in these spectra; however the signal-to-noise ratio of this feature is low. This feature is also present in the spectrum shown in Fig. 7c, which is the average of all data at |b| > 1$^\circ$. The line parameters are listed in Table 2. The narrow carbon line emission is extended over, at least, the latitude range -3$^\circ$ to +3$^\circ$ suggesting the presence of a single large diffuse C  II region. (see Sect. 6.2.1 for further discussion on the narrow carbon line emission.)

From the above two cases, we believe that the carbon line emitting gas in the inner Galaxy is spread over a galactic latitude extent of at least $b \sim \pm3$$^\circ$.