One observational problem hindering the acceptance of the existence of the dark matter indicated by the Hi data concerned the presence of large-scale non-circular motions in the outer parts, as frequently emphasized by sceptical observers (e.g., Baldwin 1974, 1975). Einasto et al. (1974) simply rejected this hypothesis — without stating a reason — even though the one galaxy they show data for, IC 342, was found to be clearly warped (Newton 1980).
Rogstad et al. (1974) came up with a novel approach to model the velocity field of M83, using new data obtained with the two-element Owens Valley interferometer. They introduced the notion of what later was called the “tilted ring model”, by modelling a galaxy with a set of concentric annuli each having a different spatial orientation. Although they were forced to use a model rotation curve on account of M83's low inclination, later on rotation curves were derived by determining the rotation velocity for each annulus separately (Bosma 1978, 1981a). It is of interest to display the results of Rogstad et al. (1974) side by side with more modern results, as has been arranged in Fig. 2. Since the outer contour in the 1974 Hi image is 1.37 × 1020 cm−2, the presence of extended Hi disks in M83-like systems is detectable with the upcoming large Hi surveys, even for those with shorter integration times such as planned for the APERTIF-SNS and WALLABY surveys.
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Figure 2. Left two panels: Hi distribution and tilted ring model of M83 (reproduced with permission from Rogstad et al. 1974); next to it a deep optical image obtained by Malin and Hadley (1997) with a shallower colour image superimposed (credit: D. Malin). The rightmost image was constructed from GALEX UV data, optical images, and the Hi distribution obtained in the Local Volume HI Survey conducted with CSIRO's Australia Telescope Compact Array (ATCA; credit: B. Koribalski and A. R. López-Sánchez). All images are on the same scale. |
The first study of Hi warps in spiral galaxies seen edge-on was done by Sancisi (1976), whose clearest case was NGC 5907. This galaxy has subsequently been studied extensively in the optical regime, in order to find the presence of extraplanar light. A “faint glow” was found by Sackett et al. (1994a), whose report made the cover of Nature, but later work by Zheng et al. (1999) indicated the presence of an arc. A deeper picture was published by Martínez-Delgado et al. (2008), and is reproduced in Fig. 3. Further modelling of the warp has been done in Allaert et al. (2015). The surmise in Sancisi (1976) that warps are occurring in galaxies seemingly free of signs of interactions thus turns out to be not justified, but the stellar mass involved in the stream appears minor compared to that in the main galaxy. As is the case for M83, the streamer indicates a past interaction or merging event, a process which could have caused the warp as well.
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Figure 3. Left: two Hi channel maps at either side of the systemic velocity of NGC 5907, superimposed on an optical image of the galaxy, indicate clearly the presence of a warp in the Hi distribution (reproduced with permission from Sancisi 1976). Right: at the same scale, a deep optical image showing the presence of a diffuse tidal stellar stream around this galaxy (reproduced with permission from Martínez-Delgado et al. 2008). |
Rogstad et al. (1976; 1979) found warps in M33 and NGC 300, and I found several more during my thesis work, as discussed below. Briggs (1990) outlined a set of “rules of behaviour” for warps by studying Hi images of a number of warped galaxies studied by the end of the 1980s. They start in the region beyond the optical radius (ropt), and become prominent at the Holmberg radius (rHo), while the line-of-nodes turns in such a manner that the direction in which it is turning is leading with respect to the spiral arms. This behaviour is due to differential precession, as already noted in Kahn and Woltjer (1959) and was privately discussed in 1976 between Rogstad and Bosma for five cases then known. Newton (1980) shows clearly that the data of IC 342 do not support this picture, but his work was not considered by Briggs (1990). Note that for the giant low surface brightness disk galaxy Malin 1, the Hi velocity field observed by Lelli et al. (2010) shows clearly that this galaxy also violates Briggs's rule no. 3, and that the faint giant low surface brightness disk imaged recently by Galaz et al. (2015) is in the warped part of the galaxy.