Caption: Galaxy rotation curve for M33 (Triagulum Galaxy, NGC 598)

Features:

1. M33 is in the constellation Triangulum---which is why it is also called the Triangulum Galaxy).

   It is 0.73--0.94 Mpc away and has radial velocity -179(3) km/s.

   It is type SA(s)cd in the de Vaucouleurs galaxy morphological system which is a generalization of the Hubble sequence (of galaxy morphological classification.

2. M33, the Andromeda Galaxy (M31, NGC 224), and the Milky Way are the only large galaxies in the Local Group of Galaxies.

3. The graph's horizontal axis is radius from the galaxy center in kiloparsecs.

4. The graph's vertical axis is circular orbital velocity in kilometers per second (km/s) (which is the natural unit for most astronomical-object velocities).

   The assumption made in both observations and modeling is that all orbiting material are in circular orbits. This assumption is considered a good approximation for disk galaxies (i.e., spiral galaxies lenticular (S0) galaxies).

5. The curves:
   1. The black curve is a fit to the observations shown as data points with error bars. The data points for < 5 kps are from observations of stars and those for > 5 kpc are from measurements of the hydrogen 21-centimeter line (21.1061140542 cm, 1420.4057 5176 67(9) MHz ≅ 1420 MHz). In both cases, the observations make use of the Doppler effect of spectral lines to determine velocity along the line of sight and simple modeling to derive the circular orbital velocities. The black curve is the actual galaxy rotation curve.
   2. The blue curve is inferred from the observed mass in stars and Newtonian physics. The blue curve would be the galaxy rotation curve if the only mass were the mass in stars.
   3. The green curve is inferred from the observed mass in gas and Newtonian physics. The green curve would be the galaxy rotation curve if the only mass were the mass in gas.
   4. The red curve is inferred from the actual galaxy rotation curve and the observed mass in stars and gas. It would the galaxy rotation curve if the only mass were the mass in dark matter. The red curve allows one to determine to some degree the structure of the dark matter halo of M33.
   5. Note, yours truly cannot verify the graph quantitatively.

6. The inferred dark matter halo is qualitatively the same for almost all galaxies and the dark matter halos prove at least for almost all large galaxies that there must ⪆ 30 times more mass than observed in baryonic matter (i.e., ordinary matter made of protons, neutrons, and electrons) in almost all large galaxies (e.g., Dekel et al. 2019, Fig. 1, p. 2 or directly Dekel et al. 2019, Fig. 1, p. 2).

7. Dark matter has so far ONLY been detected by its gravitation effect on galaxies, the large-scale structure of the universe, and cosmology. Other than gravitation, its interactions with itself and baryonic matter must be very weak and possibly non-existent. For a further explication of dark matter, see Galaxies file: galaxy_rotation.html and Wikipedia: Dark matter.