Hi rotation curves dark matter In my thesis work (Bosma 1978, 1981a, 1981b) I collected extensive Hi data on a number of galaxies using the WSRT and its 80-channel receiver. I produced warp models using tilted ring models, and derived mass models from rotation curves ignoring possible vertical motions associated with the warps. For the photometry, new observations were obtained (cf. van der Kruit 1979), and the resulting plot of the local M / L ratios was reported in Bosma (1978) and Bosma and van der Kruit (1979). I augmented the sample with literature data so as to arrive at a comprehensive figure of 25 rotation curves split over six panels of different morphological types. This Figure was reproduced in an influential review article by Faber and Gallagher (1979), and gained widespread attention. The local M / L ratios in the outer parts of galaxies were found to be very large in quite a number of cases, establishing the presence of dark matter.
At present, such data are considered as primary evidence for the presence of dark matter in spiral galaxies. Contemporary data by Rubin et al. (1978), followed by a systematic study of galaxies of type Sc (Rubin et al. 1980), Sb (Rubin et al. 1982) and Sa (Rubin et al. 1985) show also flat and rising rotation curves, but it has been convincingly shown, first by Kalnajs (1983) and later by Kent (1986, 1987, 1988), as well as by Athanassoula et al. (1987), that for most of the galaxies in Rubin's survey the data do not go out far enough in radius to unambiguously demonstrate the need for dark matter (see also Bertone and Hooper 2016).
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Figure 4. At left, the Hi distribution in the galaxy NGC 2841 observed by Bosma (1978), overlaid on a deep IIIaJ image provided by H.C. Arp; the inset shows the Hubble Atlas image (Sandage 1961). The middle panels show the warp model. At right a mass model of the galaxy adjusted to the Hi rotation data in Bosma (1978), calculated as described in Athanassoula et al. (1987), using surface photometry data from the Spitzer Survey of Stellar Structure in Galaxies (S4G; Muñoz-Mateos et al. 2015), and overlaid on scale on a Sloan Digital Sky Survey (SDSS) colour image. The orange ellipse around the galaxy outlines the Holmberg (1958) dimensions. Montage as in Roberts (1988), suggested in this form by Bernard Jones (private communication). |
For this review, I will concentrate on the most extended Hi disks found in Bosma (1978, 1981a, 1981b), and show as example the galaxy NGC 2841 (Fig. 4). The Hi extends out to ∼2.5 times the Holmberg radius, i.e., ∼ 3.5 × ropt. The tilted ring model describing the warp leaves out the northernmost feature, an asymmetry further emphasized in Baldwin et al. (1980), although of much smaller amplitude than that in the Hi distribution in M101. A similar extended Hi disk was found for NGC 5055 (Fig. 5), which became later a prototype of a Type I extended UV disk (Thilker et al. 2007), and for which Martínez-Delgado et al. (2010) found extensive streamers, some of which are not associated with the Hi gas or the young stars. In Bosma (1978, 1981b) I summarize data on the extent of the Hi disks in my sample, and found a wide variety of values of the ratio rHI / ropt, where rHI is defined as the isophote where the Hi column density is 1.82 × 1020 cm−2, with a mean of 2.2 ± 1.1.
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Figure 5. Top left, a SDSS colour image of NGC 5055, with next to it a representation of the Hα velocity fiel from the GHASP survey (reproduced with permission from Blais-Ouellette et al. 2004), and just above it the Hα rotation curve of Burbidge et al. (1960). The top right panel shows the velocity field of NGC 5055 shown on the front cover of Bosma (1978). The colour lookup table used there is such that the boundaries of each coloured patch correspond to equispaced isovelocity contours. Bottom left is the GALEX near-UV image (data taken from the NASA/IPAC Extragalactic Database, NED) and bottom right a deep optical image from Martínez-Delgado et al. (2010) (credit: D. Martinez-Delgado and T. Chonis). All images are on the same scale. The star-forming patch west of the centre of NGC 5055, UGCA 342, can already be seen on the deep IIIaJ image (van der Kruit 1979) also shown as Fig. 4.3.1b in Bosma (1978). |