Astronomy & Astrophysics

All Figures

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{1488fg01.ps} \end{figure}$ Figure 1: 8.3 GHz continuum emission from the central disk of NGC 253 using VLA A configuration data. The rms in the image is 0.04 mJy/beam and the peak flux density is 63 mJy/beam. The contour levels are (-5, 5, and higher in steps of 1.4) times the rms. The synthesized beam is 0.50 $^{\prime \prime }$ $\times$ 0.28 $^{\prime \prime }$ , PA = 0 $^{\circ }$ (6 pc $\times$ 3.4 pc). The sources 5.49-42.3, 5.59-41.6 and the 15 pc region marked in the figure refer to H92 $\alpha$ line emitting regions, discussed in Sect. 4. The two remaining line sources 5.72-40.1 and 5.75-39.9 cannot be seen clearly in the figure.
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In the text

$\begin{figure} \par\includegraphics[width=8.5cm,clip]{1488fg02.ps} \end{figure}$ Figure 2: 15 GHz continuum emission from the central disk of NGC 253 using VLA A configuration data. The rms in the image is 0.12 mJy/beam and the peak flux density is 44 mJy/beam. The contour levels are (-3, 3, 5, and higher in steps of 1.35) times the rms. The synthesized beam is 0.31 $^{\prime \prime }$ $~\times~$ 0.14 $^{\prime \prime }$ , PA = -20 $^{\circ }$ (3.7 pc $\times$ 1.8 pc). The sources 5.49-42.3, 5.59-41.6 and the 15 pc region marked in the figure refer to H92 $\alpha$ line emitting regions, discussed in Sect. 4. The two remaining line sources 5.72-40.1 and 5.75-39.9 cannot be seen clearly in the figure.
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In the text

$\begin{figure} \par\includegraphics[width=8.4cm,clip]{1488fg03.ps}\par\vspace*{10mm} \par\includegraphics[width=8cm,clip]{1488fg04.ps} \end{figure}$ Figure 3: The four extranuclear H92 $\alpha$ sources, discussed in Sect. 4.1, and the central extended emission are detected. Top: the H92 $\alpha$ line emission (thick contours) overlayed on the 8.3 GHz continuum emission (thin contours). Two compact sources of line emission (5.49-42.3 and 5.59-41.6) are seen towards the SW of the emission surrounding the radio nucleus. The synthesized beam is 0.50 $^{\prime \prime }$ $\times$ 0.28 $^{\prime \prime }$ , PA = 0 $^{\circ }$ and rms of the continuum image is 0.04 mJy/beam. Bottom: a close-up view of the H92 $\alpha$ line emission towards the 25 pc region (grey scale) overlayed on a high resolution 8.3 GHz continuum image (contours). The line emission consists of two additional extranuclear sources (5.72-40.1 and 5.75-39.9) and a contiguous 15 pc sized central line emitting region. The synthesized beam of the continuum and line images are 0.35 $^{\prime \prime }$ $\times\;$ 0.19 $^{\prime \prime }$ , PA = -3 $^{\circ }$ and 0.45 $^{\prime \prime }$ $~\times\;$ 0.24 $^{\prime \prime }$ , PA = -3 $^{\circ }$ respectively. The rms of the continuum image is 0.07 mJy/beam and the contours are in steps of 1.4 starting from 10 $\sigma$ .
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm,clip]{1488fg05.ps}\vspace*{3mm} ... ...06.ps}\hspace*{2mm} \includegraphics[width=4.2cm,clip]{1488fg07.ps} \end{figure}$ Figure 4: a) VLA A configuration "naturally weighted'' images of H75 $\alpha$ emission in individual channels from the central 15 pc region of NGC 253. The central heliocentric velocity (in km s^-1) of each channel is marked in each frame. The synthesized beam is 0.35 $^{\prime \prime }$ $\times$ 0.22 $^{\prime \prime }$ , PA = -10 $^{\circ }$ and the rms in the channel images is 0.36 mJy/beam. The contour levels are (3.5, 3.8, 4.1, 4.4, 4.7, 5, 5.3) times the rms. b) H75 $\alpha$ spectrum towards the continuum peak (radio nucleus). c) H75 $\alpha$ spectrum towards the line source 5.76-38.9.
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In the text

$\begin{figure} \par\includegraphics[angle=-90,width=8.8cm,clip]{1488fg08.ps} \end{figure}$ Figure 5: 1.4 GHz continuum emission from the inner disk of NGC 253 using the VLA BnA configuration data. The rms in the image is 0.13 mJy/beam and the contour levels are from 1.3 to 664 mJy/beam in steps of 1.7. The synthesized beam is 5.1 $^{\prime \prime }$ $\times$ 3.8 $^{\prime \prime }$ , PA = 51 $^{\circ }$ .
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{1488fg09.ps} \end{figure}$ Figure 6: Co-added H166 $\alpha$ and H168 $\alpha$ spectra towards the continuum peak of NGC 253 using the VLA BnA configuration data for the natural weighted image. Synthesized beam is 5.1 $^{\prime \prime }$ $\times$ 3.8 $^{\prime \prime }$ , PA = 50 $^{\circ }$ and the rms noise in the line cube is 0.26 mJy/beam.
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In the text

$\begin{figure} \par\includegraphics[width=4cm,clip]{1488fg10.ps}\hspace*{2mm} \i... ...g12.ps}\hspace*{2mm} \includegraphics[width=4cm,clip]{1488fg13.ps} \end{figure}$ Figure 7: H92 $\alpha$ VLA spectrum towards the four compact extranuclear line sources using the A configuration data, identified by the coordinates of the associated/nearest continuum source: a) 5.49-42.3, b) 5.59-41.6, c) 5.72-40.1 and d) 5.75-39.9. The synthesized beam is 0.50 $^{\prime \prime }$ $\times$ 0.28 $^{\prime \prime }$ , PA = -2 $^{\circ }$ . The rms in the line cubes is 0.12 mJy/beam and the velocity resolution is 56.4 km s^-1 .
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In the text

$\begin{figure} \par\includegraphics[width=13.7cm,clip]{1488fg14.ps} \end{figure}$ Figure 8: VLA A configuration images of the H92 $\alpha$ line emission in individual spectral channels in the central "15 pc region'' of NGC 253, at higher resolution. The central heliocentric velocity of each channel is marked in each frame. Line emission in every alternate channel is independent due to Hanning smoothing. The synthesized beam is 0.41 $^{\prime \prime }$ $\times$ 0.22 $^{\prime \prime }$ , PA = 10 $^{\circ }$ and the rms in the channel images is 0.13 mJy/beam. The contour levels are (-3, 3, 3.5, 4, 4.5, 4.75, 5, 5.5, 5.8, 6, 6.5, 7, 7.5) times the rms.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1488fg15.ps}\par\vspace*{3mm} \includegraphics[width=7.5cm,clip]{1488fg16.ps} \end{figure}$ Figure 9: a) High resolution integrated VLA H92 $\alpha$ line emission from the 15 pc region, summed over velocities 316.6 to 90.8 km s^-1. The synthesized beam is 0.38 $^{\prime \prime }$ $\times$ 0.20 $^{\prime \prime }$ , PA = -2 $^{\circ }$ . The contour levels are (1, 1.4, 1.8, 2.2, 2.6, 3. 3.4, 3.8, 4.2, 4.6, 5) times 10³ Jy/beam $\times$ Hz. The "star'' symbols mark the derived positions of the four line sources, as listed in Table 3. b) The positions of the four H92 $\alpha$ sources are marked as "star symbols'' on the contours of 15 GHz continuum emission imaged at high resolution. The 15 GHz continuum image is aligned using the continuum and line emission from the radio nucleus. The synthesized beam of the continuum image is 0.22 $^{\prime \prime }$ $\times$ 0.10 $^{\prime \prime }$ , PA = -15 $^{\circ }$ .
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In the text

$\begin{figure} \par\includegraphics[width=4.2cm,clip]{1488fg17.ps}\hspace*{2mm} ... ...19.ps}\hspace*{2mm} \includegraphics[width=4.2cm,clip]{1488fg20.ps} \end{figure}$ Figure 10: The VLA A configuration H92 $\alpha$ spectra towards the line emitting sources in the 15 pc region, obtained towards the positions listed in Table 3. The synthesized beam is 0.51 $^{\prime \prime }$ $\times$ 0.28 $^{\prime \prime }$ , PA = -2 $^{\circ }$ . The rms noise in the channel images is 0.12 mJy/beam.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1488fg21.ps} \end{figure}$ Figure 11: Integrated line flux density vs. frequency for NGC 253 for low resolution RRL data. The error bars are marked and the solid line is the best linear fit through the data. 1.4 GHz data: our work; 4.8 GHz and 8.3 GHz data: Anantharamaiah & Goss (1990); 99 GHz data: Puxley et al. (1997).
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In the text

$\begin{figure} \par\includegraphics[width=4.3cm,clip]{1488fg22.ps}\hspace*{2mm} \includegraphics[width=4.3cm,clip]{1488fg23.ps} \end{figure}$ Figure 12: The expected dependence of the line spectral index $\alpha$ (defined as $F_{\nu }$ $\propto$ $\nu ^{\alpha }$ , where $F_{\nu }$ is the integrated flux density of the RRL at frequency $\nu$ ) between the 1.4 GHz H166 $\alpha$ and the 99 GHz H40 $\alpha$ line flux densities as a function of density and emission measure under a) LTE conditions and b) non-LTE conditions. The boxes mark the solution space for Components I (open box) and II (hatched box) of the ionised gas, derived in Sects. 5.2 and 5.3.
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In the text

$\begin{figure} \par\includegraphics[width=6cm,clip]{1488fg24.ps}\par\vspace*{3mm} \includegraphics[width=6.8cm,clip]{1488fg25.ps} \end{figure}$ Figure A.1: a) The 1.4 GHz continuum emission from NGC 253 taken using the VLA C configuration data, showing sources identified with the galaxy. The synthesized beam is 35 $^{\prime \prime }$ $\times$ 17 $^{\prime \prime }$ at a PA of 26 $^{\circ }$ . The contour levels are in steps of (-20, -10, -7, 7, 10, 20, 40, 60, 80, 100, 200, 400, 600, 800, 1000, 2000, 4000, 6000, 8000) times the rms, 0.2 mJy/beam. b) Higher resolution BnA configuration image. The area shows the sources detected by Ulvestad (2000). The synthesized beam is 3.8 $^{\prime \prime }$ $\times$ 2.6 $^{\prime \prime }$ at a PA of 61 $^{\circ }$ . The rms of the image is 0.1 mJy/beam and the contour levels are in steps of (5, 7, 10, 20, 40, 60, 80, 100, 200, 400, 600, 800, 1000, 2000, 3000, 4000) times the rms. Negative contours are below -5 $\sigma$ .

In the text

$\begin{figure} \par\includegraphics[angle=-90,width=8.8cm,clip]{1488fg08.ps} \end{figure}$	Figure 5: 1.4 GHz continuum emission from the inner disk of NGC 253 using the VLA BnA configuration data. The rms in the image is 0.13 mJy/beam and the contour levels are from 1.3 to 664 mJy/beam in steps of 1.7. The synthesized beam is 5.1 $^{\prime \prime }$ $\times$ 3.8 $^{\prime \prime }$ , PA = 51 $^{\circ }$ .
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$\begin{figure} \par\includegraphics[width=8.3cm,clip]{1488fg09.ps} \end{figure}$	Figure 6: Co-added H166 $\alpha$ and H168 $\alpha$ spectra towards the continuum peak of NGC 253 using the VLA BnA configuration data for the natural weighted image. Synthesized beam is 5.1 $^{\prime \prime }$ $\times$ 3.8 $^{\prime \prime }$ , PA = 50 $^{\circ }$ and the rms noise in the line cube is 0.26 mJy/beam.
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$\begin{figure} \par\includegraphics[width=4.2cm,clip]{1488fg17.ps}\hspace{2mm} ... ...19.ps}\hspace{2mm} \includegraphics[width=4.2cm,clip]{1488fg20.ps} \end{figure}$	Figure 10: The VLA A configuration H92 $\alpha$ spectra towards the line emitting sources in the 15 pc region, obtained towards the positions listed in Table 3. The synthesized beam is 0.51 $^{\prime \prime }$ $\times$ 0.28 $^{\prime \prime }$ , PA = -2 $^{\circ }$ . The rms noise in the channel images is 0.12 mJy/beam.
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$\begin{figure} \par\includegraphics[width=7.5cm,clip]{1488fg21.ps} \end{figure}$	Figure 11: Integrated line flux density vs. frequency for NGC 253 for low resolution RRL data. The error bars are marked and the solid line is the best linear fit through the data. 1.4 GHz data: our work; 4.8 GHz and 8.3 GHz data: Anantharamaiah & Goss (1990); 99 GHz data: Puxley et al. (1997).
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