1926

Lundmark Knut, "Internal Motions of Messier 33," ApJ, 63, 67 (1926) - NADS
    [T]he motion of translation--the total proper motion of the nebula--was found to be = -0.0015 arcsec/year;
= -0.0050 arcsec/year, which is in good agreement with the corresponding values of van Maanen, namely +0.0034 arcsec/year and -0.0044 arcsec/year, respectively. Internal motions were found to be: rotation = +0.0016 arcsec/year (NESW), and radial = - 0.0057 arcsec/year (outward). [The negative outward measurement may be interpreted as representing an inflowing motion.]

Hubble, E.P., "No. 310. A spiral nebula as a stellar system. Messier 33," Contributions from the Mount Wilson Observatory / Carnegie Institution of Washington, 310, pp. 1-39 (1926) - NADS
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Hubble, E. P., "The Spiral M33 as a Stellar System," ApJ, 63, 236-274 (1926) - NADS
    The present contribution to the subject consists of observational data concerning the particular spiral, Messier 33. The data lead to a conception of the object as an isolated system of stars and nebulae, lying far outside the limits of the galactic system. To this extent the evidence favors the island-universe hypothesis, but, in respect to dimensions and luminosity, the spiral is more closely comparable with the Magellanic Clouds than with the galactic system itself. [p. 237. Also, see pp. 273-274.]
    The ratio of minor to major axis appears to be about 2:3; the tilt is therefore about 42°, and the differential velocity of 200 km/sec can be interpreted as the radial component of a linear velocity of rotation about the nucleus of

[See: Messier 33 Internal Motions on this website.

1927

Lundmark, Knut, "Studies of Anagalactic Nebulae - First Paper," Nova Acta Regiae Societatis Scientiarum Upsaliensis, Volumen Extra Ordinem Editum, (1927).
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Van Maanen, A., "Investigations on Proper Motion - Twelfth Paper: The Proper Motions and Internal Motions of Messier 2, 13, and 56," Contributions from the Mount Wilson Solar Observatory, 66, 89-112, (1927); ApJ, 66, 89 (1927) - NADS
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Lindblad, Bertil, "On the Nature of the Spiral Nebulae," MNRAS, 87, 420-426 (1927) - NADS
    In his Leçons sur les Hypothèses Cosmogoniques, p. 262, Poincaré attempts to draw a parallel between the figure of a rotating stellar system and the figure of equilibrium of a rotating incompressible liquid. The stellar system is considered in the picture of a gaseous mass with stars as molecules. ...
[p. 420]
    The possibility of explaining the spiral arms rests in this case on principles quite different from those suggested by Poincaré. Our point of departure is simply a theorem on the motion of a material particle under gravitation in the equatorial plane outside of a homogeneous spheroid. ... [p. 422]
    A comparison between such orbits [described above] and the spiral arms of the nebulæ Messier 33 and 81 was made in the paper cited. ...[p. 423] [This may be Hubble, E., ApJ, 64, 349 (1926) or it may refer to an unpublished statistical investigation by Lundmark. Checking.]
    It may be remarked that the arms evidently wind out form the "mother-system" in the same direction as the rotation of the system, while the opposite is the case according to the suggestion by Poincaré.
[p. 425]
    The present theory connects a multitude of facts concerning our stellar system [the Milky Way galaxy] and the spirals, which have only been touched upon very incompletely in this exposition of the general principles. With full appreciation of the beautiful* theory of Jeans, I have therefore ventured to give the present account of the theory which treats the problem from an older, though considerably modified, point of view. [p. 426]
    * [Beware of unrequired adjectives!] [This seems to be one of the seminal papers in the development of one of cosmology's missing mass problems.]

1928

Brown, E. W., "Gravitational Motion in a Spiral Nebula," Obs, 51, 277-286 (1928) - NADS [No abstract or article.]
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Curtis, Heber D., "The Unity of the Universe," JRASC, 22, 399 (1928) - NADS

1929

Hubble, E. P., "A Spiral Nebula as a Stellar System, Messier 31," ApJ, 69, 103-158 (1929) - NADS
    Resolution. -- The outer regions of the spiral arms are partially resolved into swarms of faint stars, while the nuclear region shows no indications of resolution under any conditions with the 100-inch reflector. Intermediate regions show isolated patches where resolution is pronounced or suggested.
    Distance of M31 derived from Cepheid criteria. -- Comparisons of period-luminosity diagrams indicate that M31 is about 0.1 mag. or 5 percent more distant than M33, and about 8.5 times more distant than the Small Magellanic Cloud. Using Shapley's value for the cloud, we find the distance of M 31 to be 275,000 parsecs.
    Relative dimensions of M31 and the galactic system. -- A tentative comparison of sizes, masses, luminosities, and densities suggests that the galactic system is much larger than M31 but that the ratio is not greater than that between M31 and other known extra-galactic systems.
    Early visual observers of the spectrum reported bright lines on a continuous background. In 1899, however, Scheiner photographed the now familiar solar-type absorption spectrum and announced emphatically that the nebula must be a system of stars. Radial velocities of the order of -300 km/sec. have since been measured by several observers. ... The linear velocity of rotation as indicated by the measures is of the order of 0 .48x km/sec., where x is the distance from the nucleus in seconds of arc. The measures extend to about 150" from the nucleus, and the rotation is in the sense that the south preceding end of the nebula is approaching us relative to the nucleus. [pp 104-105]
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Hubble, Edwin P., "A Relation Between Distance and Radial Velocity among Extra-Galactic Nebulae," PNAS, 15, 168-173 (1929) - NADS - [PNAS PDF. Subscription needed.]

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Markov, A., "On the Nature of Spiral and Gaseous Nebulae," AN, 234, 329 (1929) - NADS
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Markov, A., "Verbesserungen zu dem Artikel (On the Nature of Spiral and Gaseous Nebulae)," AN, 235, 143 (1929) - NADS [In German.]
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Perrine, C. D., "The Motions and Status of the Spiral Nebulae and Globular Clusters," AN, 236, 329 (1929) - NADS

1930

Lemaître, Georges, "On the Random Motion of Material Particles in the Expanding Universe. Explanation of a Paradox," BAN, 5, 273 (1930) - NADS
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Perrine, C. D., "The High Velocities of the Spiral Nebulae," AN, 240, 319 (1930) - NADS
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Van Maanen, A., "Investigations on Proper Motion - Sixteenth Paper: The Proper Motion of Messier 51, NGC 5194," Contributions from the Mount Wilson Solar Observatory, 408, 311-314 (1930) - NADS