All Figures

$\begin{figure} \par\mbox{\includegraphics[width=6.3cm,clip]{8479f1-1.eps} \hspac... ...mm}\includegraphics[width=6.3cm,clip]{8479f1-7.eps}\hspace*{32mm}} \end{figure}$ Figure 1: Molecular line emissions in NGC 1068. The velocity resolution was set to 25 ${\rm km~s}^{-1}$ for HNC and HCN, and to 15 ${\rm km~s}^{-1}$ for CO and CN. The spectra are centered with respect to the heliocentric systemic velocity $v_{\rm sys} = 1137$ ${\rm km~s}^{-1}$ . Emission from the spiral arms are detected in the CO 1-0 line. The CO 2-1 line is dominated by the emission coming from the CND. The two main spingroups of CN are detected in both J=1-0 and J=2-1 transitions. In the CN 1-0 the second spingroup is corrupted by noise in the spectrum. The two spingroups are blended in the CN 2-1 line. The different line shapes (profiles) of the HCN and HNC spectra seem to indicate that their emissions emerge from different regions.
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In the text

$\begin{figure} \par\mbox{ \includegraphics[width=6.4cm,clip]{8479f2-1.eps} \hspa... ...mm}\includegraphics[width=6.4cm,clip]{8479f2-5.eps}\hspace*{35mm}} \end{figure}$ Figure 2: Molecular line emissions in NGC 1365. The spectra are centered with respect to the heliocentric systemic velocity $v_{\rm sys} = 1636$ ${\rm km~s}^{-1}$ . The velocity resolution was set to 25 ${\rm km~s}^{-1}$ for HNC and HCN, and to 15 ${\rm km~s}^{-1}$ for CO and CN. A double peak line shape is observed in all the spectra. This structure is related with the double peak emission coming from the center of the galaxy. In the CN spectra both spingroups are detected. The second spingroup of the CN 2-1 line overlaps the double structure itself.
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In the text

$\begin{figure} \par\mbox{\includegraphics[width=6.4cm,clip]{8479f3-1.eps} \hspac... ...s}\hspace*{4mm}\includegraphics[width=6.4cm,clip]{8479f3-8.eps} } \end{figure}$ Figure 3: Molecular line emissions in NGC 3079. The velocity resolution was set to 20 ${\rm km~s}^{-1}$ for HNC and HCN, and to 10 ${\rm km~s}^{-1}$ for CO and CN. The spectra are centered with respect to the heliocentric systemic velocity $v_{\rm sys} = 1116$ ${\rm km~s}^{-1}$ . Emission from four structures are observed in the CO lines. Only the main spingroup is observed in the CN 1-0 line. The second spingroup is on the right edge of the spectrum, out of the bandwidth. Instead, both spingroups are observed in the CN 2-1 line, although the second spingroup overlaps the double structure of the CN emission. The both transitions of HCN and HNC have similar line shapes, which indicates that their emissions emerge from the same region.
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In the text

$\begin{figure} \par\mbox{\includegraphics[width=6.7cm,clip]{8479f4-1.eps} \hspac... ...} \hspace*{6mm}\includegraphics[width=6.7cm,clip]{8479f4-4.eps} } \end{figure}$ Figure 4: Molecular line emissions in NGC 2623 (left) and NGC 7469 (right). The velocity resolution was set to 25 ${\rm km~s}^{-1}$ for HNC and to 15 ${\rm km~s}^{-1}$ for CO. The spectra are centered with respect to the heliocentric systemic velocities $v_{\rm sys} = 5535$ and $v_{\rm sys} = 4892$ ${\rm km~s}^{-1}$ for NGC 2623 and NGC 7469, respectively. We do not detect HCN 3-2 emission in NGC 2623 nor HNC 3-2 in NGC 7469. Their observed intensities were less than $2\sigma$ .
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In the text

$\begin{figure} \par\includegraphics[width=18cm]{8479f5.eps} \end{figure}$ Figure 5: (Top-left panel) Position-velocity ( p-v) map of HCN 1-0 emission in NGC 1068, adapted from Tacconi et al. (1994). The spectra below are the respective emission lines of Fig. 1, re-rescaled to fit the velocity scale of the HCN p-v map. (Top-middle panel) Position-velocity map of CO 3-2 emission in NGC 1365, adapted from Sandqvist (1999). The spectra below are the re-scaled versions of the ones shown in Fig. 2. (Top-right panel) Position-velocity map of CO 1-0 emission in NGC 3079, adapted from Koda et al. (2002). The spectra below are the re-scaled versions of the ones shown in Fig. 3.
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In the text

$\begin{figure} \par\includegraphics[width=6.7cm,clip]{8479f6-1.eps}\par\vspace*{... ...\par\vspace*{2mm} \includegraphics[width=6.7cm,clip]{8479f6-3.eps} \end{figure}$ Figure 6: Excitation conditions modelled for the 3-2/1-0 line ratios of HCN (top) and HNC (middle) observed in NGC 1068. The conditions required for the HCN and HNC molecules overlap in a narrow region. The relative column densities in the overlap zone of the excitation conditions of these molecules is shown in the bottom plot. The optical depth in the whole region explored for HCN ranges between 0.03 and 10 in the J=1-0 line, and between 0.32 and 30 in the J=3-2 line. In the case of HNC the optical depth ranges between 0.01 and 30 in the J=1-0 line, and between 0.003 and 30 in the the J=3-2 line. The optically thin limit of both molecules and lines is depicted by the right edge of the excitation conditions, whereas the optically thick limit correspond to the left edge of the figures above.
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In the text

$\begin{figure} \par\includegraphics[width=6.7cm,clip]{8479f7-1.eps}\par\vspace*{... ...\par\vspace*{2mm} \includegraphics[width=6.7cm,clip]{8479f7-3.eps} \end{figure}$ Figure 7: Excitation conditions modelled for the 3-2/1-0 line ratios of HCN (top) and HNC (middle) observed in NGC 3079. The excitation conditions required for these molecules overlap in most of the range explored. This suggest that their emissions emerge from gas with the same physical conditions. The bottom plot shows the overlap zone. There is a large zone of excitation conditions where the column density of HNC is between 3 and 10 times larger than that of HCN. The contour lines of -0.5 depict the zone of the physical conditions for which the column density of HNC is about 3 times larger than that of HCN. For both molecules and transition lines the optical depth ranges between 0.001 and 30. The optically thick limit is basically defined by the limit of convergence of RADEX ( $\tau _{\rm max}\sim 100$ ).
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In the text

$\begin{figure} \par\includegraphics[width=6.7cm,clip]{8479f8-1.eps}\par\vspace*{2mm} \includegraphics[width=6.7cm,clip]{8479f8-2.eps}\par\end{figure}$ Figure 8: Excitation conditions modelled for the 3-2/1-0 line ratio of HNC (top) and HCN (bottom) observed in NGC 2623 and NGC 1365, respectively. For NGC 2623 the optical depth of HNC 3-2 ranges between 0.003 and 10 in the J=1-0 line, and between 0.32 and 30 in the J=3-2 line. In the case of NGC 1365 the optical depth in the HCN 3-2 line ranges between 0.1 and 30, and between 0.01 and 10 in the J=1-0 line.
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In the text