Ken Freeman Research School of Astronomy & Astrophysics The Australian National University

1. Rotation Curves and Dark Matter Ken Freeman Research School of Astronomy & Astrophysics The Australian National University NGC 300 (ESO) Panel session: The Birth of the Cold dark matter Theory Scaling Relations and the Nature of dark Matter: Kingston, Monday, July 16, 2018

2. Before ~1960 The idea of DM was around from Galactic models at the time of Kapteyn (1922), Jeans (1926), Oort (1932). Zwicky (1933, 1937) estimated M/L ~ 1000 for the Coma cluster. Similar results for Virgo (Smith 1936). It got limited attention. Kahn & Woltjer’s (1959) timing estimate for the mass of M31 + MW was a very strong indication that individual galaxies contain a large fraction of non-luminous matter. They found a total mass of the Local Group > 1.8 x 1012 M⦿ , about 6 x higher than the apparent stellar mass of M31 + MW. Again, not much interest. Some not very strong indications from early rotation curves that there could be non-luminous matter in galaxies (e.g. Babcock M31, Oort NGC 3115)

3. Then came the M31 21 cm observations by Roberts (1966). In two of the four quadrants of M31, the velocity field was consistent with a rotating disk in near-circular motion. The rotation curve was declining at R = 25 kpc, but much more slowly than Keplerian. The M31 rotation data were presented at several conferences in the 1960s. I believe Roberts was thinking about faint/dark matter in the context of M31. We talked a few times, and I think the possibility came up in conversation. In hindsight, M31 is not the easiest system in which to detect DM, because it has a large bulgein addition to its disk.From more recent data, the dark halo does not contribute much to the rotation for R < 30 kpc: Carignan et al (2006), Bosma (2016)

4. M31 21-cm rotation curve, from Effelsberg / GBT (Carignan et al 2006) The dark halo becomes significant only in the outermost parts

5. M31 rotation curve Optical data from Rubin and Ford (1970) plus outer 21 cm data Blue curve shows shape of the rotation curve for the disk alone. Bosma, in Bertone & Hooper 2016

6. NGC 300 M33 Having worked with de Vaucouleurs, I was interested in the exponential disk, common in late-type disk galaxies, and calculated the rotation curve of a thin exponential disk in centrifugal equilibrium under its own gravity. I wanted to compare this rotation curve with data for late-type (Scd and later) galaxies which had insignificant bulges. For most of these galaxies, the optical rotation data did not go out far enough, and the radio data had low spatial resolution. The only two galaxies I could find were NGC 300 and M33. For those, the rotation curves turned over at larger radius than expected from the surface photometry. (The data for those two were consistent with later higher-resolution 21 cm observations). Benintende ESO

7. Here’s what I said (1970): If [the data] are correct, then there must be in these galaxies additional matter which is undetected, either optically or at 21 cm. Its mass must be at least as large as the mass of the detected galaxy, and its distribution must be quite different from the exponential distribution which holds for the optical galaxy. The importance of 21 cm rotation curves (RCs) for probing the gravitational field in the outer parts of disk galaxies was already obvious. For a pure disk galaxy, the RC turns over at 2.15 scalelengths. Optical RCs mostly do not extend beyond ~ 3 scalelengths. 21 cm RCs often go out to radii of many scalelengths. This was perhaps not fully appreciated at the time. Looking ahead a few years …Kalnajs made this point very clearly at the 1982 Besancon conference. The stellar distribution alone can give a flat rotation curve out to ~ 3 scalelengths.

8. Four optical rotation curves: rotation curve computed for constant M/L. Flat optical rotation curves need not mean dark halos (Kalnajs1982)

9. Going back to ~ 1970 … Was there really a problem ? Is low-luminosity matter needed in the outer regions of spirals ? Argument continued for several years Rogstad & Shostak’s (1972) data from Owen’s Valley interferometer for 5 Scd galaxies confirmed the need for low-luminosity matter in the outer regions. | shows Radius = 2.15 scalelengths, where rotation curve for disk alone would turn over Data for two galaxies in Rogstad & Shostak and in VLA THINGS sample (2008) agree well.

10. Gottesman & Davies (1970) found an increase in M/L with radius in the outer parts of M31 from their Jodrell Bank 21cm data. 21 cm data for M31 and M33 from the Cambridge (UK) Half-mile Telescope - 2 arcmin beam (Baldwin, 1973) indicated no need for extra matter: no increase of M/L with radius.

11. Roberts & Rots 1973 Rotation curves for M31 M81 (WSRT) M101 (Rogstad & Shostak) to radii = 25 to 30 kpc Rotation curves decline slowly at large R: indicates significant mass density at large R.

12. Roberts & Rots (1973) believed that the higher M/L ratio at large R would easily be “compatible with a reasonable variation in the [local] luminosity function”. Around this time, Ostriker & Peebles (1973), Ostriker, Peebles & Yahil (1974), and Einasto et al (1974) proposed that most of the mass of giant spirals is in a giant corona [of faint stars], with masses ~ 1012 M⦿ extending out to several x 100 kpc. The mass increases linearly for 20 < R < 500 kpc, and M/L ~ 200 within the Holmberg radius. Whalosand Wbaryonswere believed to be comparable, and the idea that the dark halos were made of faint stars persisted until well into the 1990s. The halo microlensing experiments of the 1990s were expecting to find brown dwarfs ESO

13. Freeman, Carrick & Craft (1975) digitally stacked deep images of the nearby large spiral NGC 253 on IIIa-J film (blue), taken with an 8-inch Schmidt camera with a 12 degree field. The surface photometry on the minor axis of this near-edge-on galaxy reached below 29 mag sq. arcsec. For an Ostriker, Peebles & Yahil halo in NGC 253 with a near-isothermal King model distribution, the M/LB > 600. ESO

14. Salpeter & Krumm (1977) observed flat 21 cm rotation curves for some large edge-on spirals with Arecibo, but there were worries about the effect of sidelobes. 45 kpc

15. In my opinion, Bosma’s rotation curves ended the argument. 25 galaxy rotation curves from Bosma’s thesis 1978 (Westerbork: 25” beam)