Calculation of equivalent width

The calculation of the equivalent width, Weq, of the absorption lines of NaiD are completed as a part of the absorption fitting codeifsfitusing the method outlined in Rupke et al. (2005b). The method used in Rupke et al. (2005b) also assumes resolved but blended absorption lines in the doublet, which also applies to our observations, and requires a

Figure 4.6: The comparison of the ionised (y-axis) and neutral (x-axis) gas velocities as defined through emission line analysis and the fitting of the Na_iD absorption lines, respectively. The panels indicate the limits of the ionised gas between±200 km/s. The neutral gas velocities show groupings shifted slightly from the systemic velocity as has been indicated in the histograms of Figure 4.9. The third component of the ionised velocity has some components that may correlate with the NaiD velocities at -200 km/s, suggesting the third ionised gas component might also trace the outflow along the minor axis of IRAS F10257-4339. However, there are not enough points to say this is definitively the case.

Figure 4.7: Equivalent width, Weq, values from fitting multiple components to NaiD absorption features. Left presents the total Weqcalculated at each spaxel, centre presents the Weqof the first component and right presents the Weqof the second component. The contours represent Gaussian smoothed ionised velocity where dashed contours represent negative velocities and solid contours represent positive velocities (inclusive of zero).

Maps of Weq shown in Figure 4.7 were calculated through theifsfitcode. This calcula- tion used a signal to noise ratio of 3σ to remove components of low quality. The contours are derived from the first ionised velocity components. The Weq values range from zero to>7 ˚A. This figure shows both the first and second components of the NaiD absorption

fitting as well as the total Weq.

The highest values are concentrated along the edge-on disk of the southern galaxy, which can be seen in the HST observations, highlighted by the red and yellow spaxels in Fig- ure 4.7. In comparison to the HST image of this galaxy there does not appear to be a direct relation to the morphology of the northern galaxy. However, Sakamoto et al. (2014) identified the possible orientation of the southern galaxy which indicated that the disk of the southern galaxy is perpendicular to the line of the two nuclei. The absorption of NaiD

seen in high Weq may be tracing the absorption due to the disk of the southern galaxy. The lower total Weq is present towards the region of the system where the highly blue- shifted neutral gas velocities are observed. This is indicative of a low column density or

Figure 4.8: Left panel - map of the number of components fit to the Na_iD absorption feature in each spaxel. Right panel - the SNR in the total Weqsplit by the spaxel with 1 (red) and 2 (grey) components.

A map of where the 2-components are in the observations is presented in Figure 4.8 (left panel). In the case of Rupke & Veilleux (2015) the addition of a second component decreased the systemic uncertainty of the Weq values, when compared to dust. However, it was found in this chapter that the addition of a second component neither improved nor worsened the systematic or systemic uncertainty of the Weq. This indicates that the addition of another component is required to describe the absorption feature. For all spaxels a signal-to-noise cut of 3σ was used to remove the most uncertain fits from the analysis.

In order to explicitly show the velocity to Weq structure of IRAS F10257-4339 in the neutral gas, a histogram of the velocities was created (Figure 4.9, left panel) and also a histogram of Weq (right panel). A rudimentary velocity cut-off was defined, using the velocity distribution in Figure 4.5, of -200 km/s to separate the bulk of the NaiD

absorption due to the southern galaxy and the outflow from the northern nucleus. This cut-off was applied in Figure 4.9 to identify the Weq associated with the outflow from the northern nucleus. It is apparent from the right panel that the outflows associated Weq does not differ significantly from the rest of the gas in Weq in the remainder of the galaxy. A tail appears in the Weq in the red as the cut-off has not completely excluded all points not associated with outflow or included all points associated with the outflow.

Figure 4.9: Left panel presents a histogram of the NaiD velocities in km/s. The peak of the distribution is around -50 km/s, an off-set from the systemic velocity of the system. The dashed line is the cut-off assumed for where a substantial number of components are due to the outflow along the minor axis of the northern galaxy. Right panel presents the histogram of the Weq for each component. The red histogram shows the Weq for components with velocities below the cut-off for the outflow. The peak of the red histogram shows the smaller Weqas seen in the maps of Figure 4.7.

The study of this galaxy shows two unrelated physical processes occurring; one possibly due to the disk and spiral arms of the southern galaxy and the other due to an outflow along the minor axis of the northern galaxy. In order to confirm the Weq that was observed is due to dust in the southern galaxy (and investigate how Weq correlates across the whole galaxy) it was necessary to compare this to the dust of this galaxy. The dust content of this galaxy was calculated through the HST bands F450W-F814W using archival HST data, and is presented in colour-colour maps in Figure 4.10. The top left panel is the HST image with the WiFeS FoV overlaid in black. Top right panel is the HST image in the WiFeS FoV at the HST resolution (0.0500/pix). Bottom left panel is the colour map adjusted to the resolution of the WiFeS observation (100/pix). Bottom right panel is the IFS continuum image for comparison of the galaxy structure to the other three panels in Figure 4.10.

Figure 4.11 compares the Weq to the HST colour, or dust attenuation, obtained through the above process of convolving the HST images to our WiFeS spatial resolution. There is a correlation between the colour and total Weq as is often seen in U/LIRGs (Veilleux et al., 1995a; Kim et al., 1998). See also a similar resolved relation (with spatially-resolved observations) in ULIRGs (Rupke & Veilleux, 2013). To understand this correlation phys- ically N(H), hydrogen column density, is needed in order to fully understand the dust observed for IRAS F10257-4339.