EDP Sciences
Free Access
Volume 584, December 2015
Article Number A113
Number of page(s) 25
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201526613
Published online 01 December 2015

Online material

Appendix A: Comparison with archival VLA data and single-dish observations for NGC 7252

For a sanity check, we retrieved from the VLA archive the H I  observations of NGC 7252 from Hibbard et al. (1994) and compare them with the new ones. We reduced the archival observations following standard procedures in AIPS and created a datacube with spatial resolution of 25′′ (robust weights) and velocity resolution of 10.6 km s-1. The D-array data were interpolated to match the spectral resolution of the C-array data.

Figure A.1 shows a PV diagram obtained by collapsing the final datacube along the declination axis, giving a complete overview of the kinematic structure of the system. The comparison between the new (greyscale) and archival (red contour) data shows some differences on small spatial scales due to different noise levels in the two data sets. Overall the new VLA data are consistent with the archival ones.

thumbnail Fig. A.1

PV diagram for NGC 7252 obtained by integrating the H I  emission along the declination axis. The new data are shown in greyscale. Contours are at 18, 36, and 72 mJy/beam. The archival data (published by Hibbard et al. 1994) are represented with a red contour at 18 mJy/beam. Both datasets were spatially smoothed to 25′′ resolution.

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From the new VLA data we derive a total H I  flux of 5.0 Jy km s-1. This is in good agreement with the single-dish measurement by Richter et al. (1994), who find a total H I  flux of 4.6 Jy km s-1 using the Green Bank Telescope (FWHM = 21′ × 21′). Other single-dish measurements (who rely on single pointings) reported lower fluxes: Huchtmeier (1997) find 3.8 Jy km s-1 using the Effelsberg telescope (FWHM = 9′ × 9′), while Dupraz et al. (1990) find 3.6 Jy km s-1 using the Nançay telescope (FWHM = 4′ [ E−W ] × 22′ [ N−S ]). Most likely, the latter single-dish observations miss part of the H I  emission in the tidal tails, extending for ~12 in the E-W direction. We conclude that no significant emission is missing from the new VLA data.

Appendix B: Comparison with B07 for NGC 5291

thumbnail Fig. B.1

Disc models for NGC 5291N (top), NGC 5291S (middle), and NGC 5291SW (bottom) adopting the parameters from B07. Left panels: total H I  maps from the observed cube (solid contours) and the model cube (dashed ellipse). The cross and dashed line illustrate the kinematical centre and major axis, respectively. The circle to the bottom-left corner shows the H I  beam. Right panels: PV diagrams along the major axis obtained from the observed cube, model cube, and residual cube. Solid contours range from 2σ to 8σ in steps of 1σ. Dashed contours range from 2σ to 4σ in steps of 1σ. The horizontal and vertical lines correspond to the systemic velocity and dynamical centre, respectively.

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For the TDGs around NGC 5291, we find significantly lower values of Mdyn/Mbar than B07 because we estimate both higher values of Mgas (by factors of ~1.5 to ~2) and lower values of Vrot (by a factor of ~2). There are two reasons for the different estimates of Mgas: (i) we integrate the H I  flux out to larger radii than B07, according to the results of our 3D disc models; this is the dominant effect; and (ii) we consider all the H I  emission that is consistent with a rotating disc, whereas B07 assumed that some H I  gas at the high/low velocity end was in the background/foreground (instead of being associated with the TDG potential well). The different estimates of Vrot are also driven by two separate reasons: (i) we find that line-of-sight velocities at the edges of the PV diagrams (employed by B07) tend to over-estimate the “true” rotation velocity because they do not consider the line-broadening due to turbulent motions; and (ii) we find higher disc inclinations than assumed by B07. Hereafter, we discuss these two effects in details.

B07 derived rotation curves using the envelope-tracing method, which considers line-of-sight velocities near the edges of PV diagrams. Specifically, for a given radius along the major axis, they took the velocity towards the far edge of the PV diagram at 50% of the H I  peak level. For poorly resolved rotating discs, this is a rough way to consider beam-smearing effects, which are well-known to artificially broaden the emission-line profiles and systematically skew them towards the systemic velocity. The details of beam-smearing effects, however, depend on the intrinsic gas distribution, rotation curve shape, gas velocity dispersion, and disc inclination. Thus, full 3D models are required to take them into account (Swaters et al. 2009; Lelli et al. 2010). In particular, the velocity at 50% of the H I  peak may systematically over-estimate the rotation velocity in discs with low values of Vrot/σH I (as dwarf galaxies), given that the broadening of the line profile is driven by both σH I and resolution effects. Our 3D disc models take all these observational effects into account and suggest that the rotation velocities of these TDGs are closer to the H I  peak than adopted by B07. This is demonstrated in Fig. B.1.

For the three TDGs near NGC 5291, we built additional 3D models following the same procedures of Sect. 5.1 and adopting the kinematical parameters of B07 (i = 45°, σH I = 10 km s-1, and Vrot from 50 to 70 km s-1 depending on the TDG). The right panels of Fig. B.1 compare PV diagrams along the major axis obtained from the observed cubes and these model cubes. The kinematic parameters from B07 produce PV diagrams that are too extended in the velocity direction. This leads to large negative residuals at high/low line-of-sight velocities and large positive residuals near the systemic velocity, indicating that Vrotsin(i) is overestimated and σH I is underestimated. It is clear that our new kinematic parameters provide a much better description of the data (cf. with Figs. 57).

Regardless of the method employed to estimate the rotation velocities, these projected quantities must be corrected for inclination by multiplying by 1/sin(i). B07 assumed that the TDGs around NGC 5291 have the same inclination angle as the collisional H I  ring (i = 45°), which was estimated using numerical simulations. B07 pointed out that this value of i is uncertain because (i) the modelled ring is not exactly circular and its morphology depends on the details of the collision; and (ii) some simulated TDGs show a discrepancy between their rotation axis and the ring axis (up to 18° in a few cases). Using 3D disc models, we directly estimate i by comparing observed H I  maps with model H I  maps (before pixel-to-pixel renormalization). In left panels of Fig. B.1, the dashed ellipses correspond to disc models with i = 45°. This value of i is roughly acceptable for NGC 5291N and NGC 5291SW, whereas it is clearly underestimated for NGC 5291S. The dashed ellipses in the top-middle panels of Figs. 57 illustrate our adopted inclinations, which are slightly larger than 45° (from 55° to 65°, as given in Table 7). Thus our estimates of Vrot are further decreased with respect to those of B07 by a factor sin(i = 45°)/sin(inew), i.e. by ~15% to ~30%. Clearly, inclination plays a minor role here.

Appendix C: Channel maps of individual TDGs

Figures C.1 and C.2 show H I  channel maps from the observed cubes (red contours) and model cubes (blue contours). Contours are at −3σ (dashed), 3σ, 6σ, and 9σ. The channel maps are superimposed on an optical image of the TDG. The cross marks the kinematical centre. Line-of-sight velocities are indicated to the top-left corner. The H I  beam is shown to the bottom-left corner. The model cubes are described in detail in Sect. 5.1. Here we provide a concise description for each TDG. NGC 5291N is surrounded by H I  emission belonging to the underlying tidal debris. At Vl.o.s. ≃ 4145 to 4100 km s-1, there is

extended H I  emission to the west, which may correspond to another, more uncertain TDG candidate. NGC 5291S is nicely reproduced by our disc model. At approaching velocities (Vl.o.s. ≃ 4645−4635km s-1), there is some anomalous gas on the receding side of the disc. NGC 5291SW is a complex case due to the low signal-to-noise ratio. The H I  emission at Vl.o.s. ≃ 4730−4720km s-1 does not show a continuous, coherent kinematical structure, hence it has not been considered in the disc model. NGC 7252E is well reproduced by our disc model despite it is a poorly resolved case with low signal-to-noise ratio. NGC 7252NW is closely reproduced by our disc model. VCC 2062 is characterised by two distinct kinematic components, which overlap in both space and velocity (see PV diagrams in Fig. 10). The second, irregular component extends to the south-west at Vl.o.s. ≃ 1155−1125km s-1 and does not show a coherent velocity structure, hence it is not considered in our disc model.

thumbnail Fig. C.1

H I  channel maps for NGC 5291N (top), NGC 5291S (middle), and NGC 5291SW (bottom). See Appendix C for a detailed description of this image.

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thumbnail Fig. C.2

H I  channel maps for NGC 7252E (middle), and NGC 7252NW (bottom), and VCC 2062 (top). See Appendix C for a detailed description of this image.

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Appendix D: Additional 3D disc models

Figures D.1 and D.2 compare 3D disc models based on a flat rotation curve (middle panel) and a solid-body rotation curve (right panel). The models are described in detail in Sect. 5.1.

The PV diagrams are obtained along the disc major axis. Solid contours range from 2σ to 8σ in steps of 1σ. Dashed contours range from 2σ to 4σ in steps of 1σ. The horizontal and vertical lines correspond to the systemic velocity and dynamical centre, respectively.

thumbnail Fig. D.1

Major axis PV diagrams for NGC 5291N (top), NGC 5291S (middle), and NGC 5291SW (bottom). See Appendix D for a detailed description of this image.

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thumbnail Fig. D.2

Major axis PV diagrams for NGC 7252E (top), NGC 7252NW (middle), and VCC 2062 (bottom). See Appendix D for a detailed description of this image.

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© ESO, 2015

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