Julius Scheiner – 19th century extragalactic spectroscopy

Juluis Scheiner published a lot in German language and made a lot of solar and stellar research. I’m concentrating on his English production on extragalactic issues.

Scheiner (1898) studied the reason why nebular spectra sometimes showed Hβ lines while showing little or no Hα lines, which seemed to go against the common knowledge back then that Hα lines are brighter than Hβ lines. He had trouble of studying the phenomenon with spectrophotometers because they weren’t able to measure so faint light as needed in the study. He then made a remarkable experimental setup:

The Geissler tube was set up at the distance of distinct vision (or at a distance somewhat greater), and viewed with a direct-vision system of prisms, the capillary bore of the tube serving as a slit. Between the tube and the prism-system two Nicol prisms were introduced, one of which could be turned and its angular displacement measured. By turning this prism the hydrogen lines could be made to vanish.

With this setup, he found an interesting thing:

Then on weakening the light, there occurred, at a certain intensity, an apparent equality of the two lines, after which Hα disappeared and then Hβ.

Scheiner & Wilsing (1902) also studied some issues relating to different spectral lines and their intensities in nebulae.

Scheiner (1899) discussed the spectrum of the Andromeda nebula (Messier 31) and showed that it consisted of stars. He then proceeded to discuss the Milky Way in the light of that evidence:

The irregularities of the Milky Way, especially its streams, can be quite well accounted for, as Easton attempted to do, if they are regarded as a system of spirals and not as a ring system.

And further:

In spite of the unfavorable projection under which we see the Milky Way, it does not seem impossible to establish the spiral character of the principal forms, and, furthermore, to bring the proper motions of the stars of the Milky Way into relation with this.

Apparently, this spectrum of Scheiner of M31 was the first succesful spectrum of a galaxy (Rubin, 1995). There seemed to be some minor dispute with Edward Fath and Scheiner over the spectrum of M31 as discussed in Scheiner (1909).

Julius Scheiner links

MNRAS: Obituary
Wikipedia: Julius Scheiner

References

Rubin, 1995, ApJ, 451, 419, “A Century of Galaxy Spectroscopy”

Scheiner, 1898, ApJ, 7, 231, “On the Spectrum of Hydrogen in the nebulæ”

Scheiner, 1899, ApJ, 9, 149, “On the spectrum of the great nebula in Andromeda”

Scheiner, 1909, ApJ, 30, 69, “Note on the Spectrum of the Andromeda Nebula”

Scheiner & Wilsing, 1902, ApJ, 16, 234, “Determination of the intensity-ratios of the principal lines in the spectra of several gaseous nebulae”

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George Paddock – early work on cosmological redshift

In addition to the work on extragalactic objects, George Paddock did some work on planets, stars, and galactic nebulae. He also worked on the radial velocity equations of binary stars. Here, I will concentrate on his extragalactic work (which contains only couple of papers).

Paddock (1916) discussed spiral galaxies in their relation to the galactic stellar system (this was well before it was established that spiral galaxies are not part of our own galaxy). He made an observation from the radial velocities of different objects:

The average radial velocities except the spirals range in increasing magnitude from zero to fifty kilometers per second. But a considerable jump is noticed from the fifty kilometers to 400 kilometers for the average of the spirals.

Based on this he presented a question:

Are the spirals dissociated from the star system?

Paddock then mentioned some Slipher’s arguments of the radial velocities of spiral galaxies. Paddock also discussed solar motion and its possible effect to the radial velocities of spiral galaxies. He noted that the spirals having measured radial velocities by that time were distributed in two groups and the Magellanic clouds were a third group, but he also said:

These objects, however, can hardly be considered to form a unitary system of associated objects, for it must be noticed that the average velocity of each of the three groups of objects is decisively positive, which means that they are receding not only from the observer or star system but from another.

What he describes here is an expanding motion. He continued:

Accordingly a solution for the motion of the observer thru space should doubtless contain a constant term to represent the expanding or systematic component whether there be actual expansion or a term in the spectroscopic line displacements not due to velocities. This brings up the question whether these large displacements are to be interpreted as due entirely to velocities.

13 years before Hubble’s redshift-luminosity relation, Paddock was already pondering similar questions. He brought up NGC 1068 with its fuzzy and broad spectroscopic lines as a possible example showing that all of the redshift might not be due to velocity (note that later there has been lot of discussion on the possible discordant redshifts in NGC 1068 system). He suggested that there might be a constant term resembling the K-term of stellar radial velocities and went on to quantify the term from the solar motion derived from the radial velocities of galaxies. He got a rather large value for the K-term (about 250-350 km/s) but he concluded that it is likely be due to small sample size and that he expected it to diminish with larger sample.

Campbell & Paddock (1918) discussed their spectroscopy on NGC 4151. They first mentioned that according to a photograph by Curtis, they thought that NGC 4151 was a planetary nebula. They then descibed their spectroscopy. They mentioned not finding the expected spectrum of a planetary nebula, and determined the radial velocity of 940 +/- 40 km/s for NGC 4151. They also noted that a new photograph by Curtis clearly showed a spiral structure, and that the character of the spectrum resembled the spectrum of NGC 1068.

References

Paddock, 1916, PASP, 28, 109, “The Relation of the System of Stars to the Spiral Nebulæ”

Campbell & Paddock, 1918, PASP, 30, 68, “The Spectrum and Radial Velocity of the Spiral Nebula N. G. C. 4151”

Links

(University of California: in memoriam) George Frederic Paddock: Lick Observatory

Knut Lundmark – extragalactic distance scale

Lundmark & Lindblad (1917) studied the spectral types of spiral galaxies. For NGC 3031 (Messier 81) they noted that Edward Fath had earlier determined that the spectrum resembles that of a K star, and their analysis also showed that if the spectral types of stars were applied to NGC 3031 spectrum, it would belong to spectral class K. They proceeded to analyse some other galaxies in the same manner. They ended their analysis by studying the differences in calculated and observed spectral types:

Hence it follows that the spectral type calculated by us should on an average differ from those determined in the usual way, where the spectral lines have been observed, by an interval at least twice as large as A-K. This not being the case, it seems to us that our investigation can be considered as a confirmation of the result found by Shapley, Hertzsprung and others, that no sensible absorption exists in space.

In a follow-up paper Lundmark & Lindblad (1919) continued these studies.

Lundmark (1921) discussed Messier 33 and wondered about possible distance indicators:

Another question is: As the only difference between the rifts in Messier 33 and those in Milky Way seems to be that the former have dimensions about 1/100 of the latter’s, will that mean that the objects in the spiral are 100 times as far away as the corresponding objects in the Milky Way?

Lundmark then noted that M33 seemed to have nearby background galaxies:

A long exposure Crossley photograph by Sanford shows that some of the nebulae apparently belonging to Messier 33 must have spiral structure. It is too early to speculate about spirals of different order, primary and secondary systems. The most natural explanation is perhaps that in this region we must expect to see several far away small spirals mixed up with nebular objects belonging to the great spiral.

Then follows what I think is quite remarkable thought from the point of view of the subject here in this blog. Lundmark had earlier noted that there has been some nebulous objects found near M33 that seemingly are extensions of M33’s spiral arms, then he said:

If the spaces between the spiral arms are filled with absorbing dark matter we get the impression of an arrangement in the extension of the spiral arms also of these background objects.

(Note that dark matter here doesn’t refer to the modern concept of dark matter, instead it refers just to regular matter that is not bright and therefore not visible to us, and absorbs the background light.) Remarkable thing here is that it is an example of how alignments between unassociated objects can occur sometimes with quite natural explanations. At the end of the paper, Lundmark gave some arguments of the large distance of M33; size of star clusters compared to Milky Way and the presence of apparent foreground stars.

Lundmark (1922) addressed some of the questions raised by parallax measurements made by van Maanen that differed from Lundmark’s measurements. Lundmark argued that the measured proper motions in that time only represented an upper limit. He also presented a calculation of parallax based on assumed systematic motions of spiral galaxies based on their measured radial velocities. He then mentioned a method to determine distance:

Parallaxes obtained by assigning to the brightest resolved stars in spirals an absolute magnitude equal to that of the brightest stars of our stellar system give still larger distances.

He didn’t specify any distances but he did give a range:

To sum up: different methods give for spiral nebulae distances ranging from about 10,000 light-years to 1,500,000 light-years.

He also suggested that diameters of galaxies could be distance indicators:

We have very likely to deal with millions of spirals, and it would be strange if we should have the largest of the spirals in our neighborhood. It is more natural to assume the spirals to have roughly the same linear dimensions, and that the smaller angular diameters in the mean indicate the more distant object.

He then estimated that visible universe extends out to 2,000,000 lightyears. He returned to van Maanen’s measurements, first discussing the extent of the Milky Way briefly and then using van Maanen’s measurements to derive masses for a few spiral galaxies. He got enormous masses as result, larger than the estimates of our own galaxy by that time. He then proceeded to discuss the motions in galaxies and made an interesting remark, showing how spiral galaxies was thought to work back then:

The matter we see in the measured spirals, if moving with a rather constant velocity, as indicated by the measures, must have been ejected during an interval of time of about 100,000 to 300,000 years.

He then made some arguments, based on this, about development stage of spiral galaxies and about the stellar ages. He also noted that amount of stars and supposed young ages of the galaxies meant that star production must be very rapid. But he ended the discussion with a note of doubt of the correctness of it.

Lundmark (1924) discussed the problem of redshifts and specifically the high redshifts of galaxies. He stated the problem:

Another question is, whether such a large Doppler shift represents motion in the line of sight alone or is caused in other ways? The validity of the Doppler principle has been proved by laboratory experiments only for velocities smaller than 1 km./sec. or so. The measures of stellar spectrograms giving such velocities as can be computed from the laws of gravitational astronomy… …have proved the correctness of the Doppler formula for velocities as high as 100 km./sec., and thus it seems allowable to assume that the displacements found for globular clusters and spiral nebulae are due to motions of the objects in the non-relativistic sense or to motions and the above mentioned effect of the curvature of the space-time.

He then proceeded to discuss the apex of the solar motion derived from the redshifts of globular clusters and spiral galaxies. He noted that they gave a different motion than nearby stars and hypothesized that our local system has a motion as a whole relative to the globular clusters and sipral galaxies. He also noted that our own motion seemed to suggesting that we are revolving around galactic centre, but he calculated the orbital period to be 3 billion years (3 x 109 years).

He then turned to de Sitter’s suggestions of the curvature of the space. He studied if there’s relation between the radial velocity of objects and their distance. He first compared the radial velocities of globular clusters to their distance estimations, and found no correlation. He did the same with different type of stellar objects (cepheids, novae, O stars, eclipsing variables, R stars, N stars). He then started analysing spiral galaxies in same manner. He started with a discussion of the situation on their distance estimates. As a sidenote, he argued that nearest spiral galaxies cannot be at distances of many millions of lightyears because some of them had shown to be resolved into stars and that novae and variable stars had been observed in them.

He used a distance scale based on the angular dimensions and magnitudes of the spiral galaxies assuming that they only depend on their distance. He plotted the resulting distance estimates against the radial velocities of spiral galaxies and concluded:

Plotting the radial velocities against these relative distances (fig. 5), we find that there may be a relation between the two quantities, although not a very definite one.

Lundmark was very close here to establish the redshift-distance relation five years before Hubble, probably only restricted by his distance indicators which were not very good ones. He also derived the value for the curvature radius of space-time, and got R = 2.4 x 1012 km as result.

Lundmark (1924b) studied the distance to Large Magellanic Cloud (LMC). He first argued that LMC was in many ways similar as spiral galaxies but decided to call objects like LMC as “nebulae of the Magellanic Cloud type”. He then determined the parallax of LMC with different methods. From the mean of these parallaxes, he determined the distance to the LMC to be 100,000 lightyears.

Lundmark (1924c) derived solar motion based on spiral galaxy measurements and the mean parallax of the spiral galaxies, and finally derived the mean distance to spiral galaxies. He got two values, 76,000 and 61,000 lightyears. Lundmark (1925) reviewed the distance determination methods to spiral galaxies. He noted that spiral galaxies seem to be out of reach of parallax measurements. Proper motion measurements seemed to be too noisy at the time. He then started discussing radial velocities. He first briefly noted that redshift doesn’t seem to correlate with the inclination of the spiral galaxy, indicating that they don’t “move like a discus thrown through space”. There were no correlation with redshift and galactic position either, but there was a correlation between the redshift and the dimensions of the spiral galaxies.

Lundmark then noted a kind of redshift-type relation. He assumed an evolutionary sequence where redshift got smaller when objects get older. “Globular” nebulae were youngest and had highest mean redshift, sequence then continued: “early spirals”, “late” spirals, Magellanic cloud nebulae, Magellanic clouds. This is of course interesting in the context of this blog because here we have the first suggestion of age dependent redshift. Lundmark interpreted this as a sort of K-effect (calling it “Campbell shift”):

The most characteristic feature of the radial velocities of spirals is the presence of a very large Campbell shift of the same nature as is found in most classes of giant stars.

Lundmark then proceeded to derive a value for the Campbell shift of spiral galaxies. Very interesting thing here is that his result included distance. His result is:

VCs = 513 + 10.365r – 0.047r2 km/s

Here r has unit of Andromeda distance multiples. He interpreted the result:

According to the above expression the shift reaches its maximum value, 2250 km./sec. at some 110 Andromeda units, which, according to results given later on, corresponds to a distance of 108 light-years. As the peculiar velocities of spirals seems to be smaller than 800 km./sec. one would scarcely expect to find any radial velocity larger than 3000 km./sec. among the spirals.

The last comment is of course wrong, but it is worth emphasizing that Lundmark gave a redshift-distance relation here. Whether it was the first one ever made, I don’t know, but this was four years before Hubble published his redshift-distance relation.

Lundmark then discussed some details on our own motion in space and the efforts to determine parallax of spiral galaxies. Then he discussed novae as standard candles for measuring distance to spiral galaxies. He reviewed the evidence that novae really occur in spiral galaxies, and then he described the research of Curtis on the subject and how he had arrived to a conclusion that closest spiral galaxies are millions of lightyears away from us based on the magnitude difference of novae in our galaxy and novae in spiral galaxies.

Lundmark then gave results of his studies of distances to the novae in our own galaxies, determined by their parallax. He determined the absolute magnitude of novae in our own galaxy, and did the same with the novae in Andromeda galaxy (M31). He also presented arguments for the similarity of the novae in Andromeda galaxy to the novae in our own galaxy, and for the Andromeda galaxy being a galaxy of its own instead of a stellar system in our own galaxy. Finally, he used the absolute magnitudes he had derived to calculate the distance to the Andromeda galaxy, and got 1.4 million lightyears, a very good estimate by that time (Hubble published his famous result when Lundmark was writing this paper, Hubble’s result was 930,000 lightyears, current value is about 2.7 million lightyears). Lundmark repeated this to NGC 4486 and got a distance of 8 million lightyears (current value is about 53 million lightyears).

Following Hubble’s lead, Lundmark determined the distance to Andromeda galaxy also by using Cepheids. He got few distance estimates; 620,000, 880,000, and 1,500,000 lightyears. Lundmark also used “Oepik’s method” to derive the distance to NGC 4594. The method uses rotation velocity of the spiral galaxy, so it seems to have some similarity to Tully-Fisher relation. The resulting distance to NGC 4594 was 56 million lightyears (current value is about 35 million lightyears). Lundmark mentioned having determined the distance to Messier 33 in 1920 as 1.5 million light years (current value is about 3 million lightyears) using the absolute magnitudes of regular stars. Lundmark closes this remarkable paper by presenting rather mathematically heavy discussion of the extent of the universe.

Lundmark (1930) studied the question if globular clusters and elliptical galaxies are related. Based on some similar features, he suggested that elliptical galaxies are made of stars just like spiral galaxies. He then mentioned the difference of the mean radial velocity between spiral and elliptical galaxies. He also argued that the two elliptical galaxies near Andromeda galaxy were associated with it because they were practically in same direction and had almost the same radial velocity. He then compared the absolute magnitudes of elliptical galaxies and globular clusters and found:

[The absolute magnitude of brightest globular cluster] is a considerably lower value for M than the value of the Andromeda companion, but, on the other hand, there seems to be no real cleft between the absolute magnitudes of several elliptical anagalactic nebulae and those of the brightest globular clusters.

Overall, he built a case where globular clusters are slightly outside of our own galaxy. Elliptical galaxies seemed generally to be companions to spiral galaxies, and as their appearance was also quite similar, it was natural to suggest that globular clusters and elliptical galaxies are related objects. This goes against current thinking, though. He closed the paper by saying:

If the sequence of globular clusters here suggested exists and if the smaller ones have a rapid motion, it might very well be that the globular clusters keep up the relations between the stellar systems and travel from Galaxy to Galaxy. These clusters are then something like what the comets were thought to be in the cosmogonies of Laplace and Schiaparelli – they are “the wandering boys of the Universe”.

In addition to the works mentioned here, Lundmark worked on different properties of stars and nebulae, and made a galaxy catalog. He also published in German and in Swedish, which papers were not considered here due to my poor skills in those languages. Lundmark (1956) would be very interesting paper with apparently a thorough historical overview on extragalactic research and distance indicators, but not freely available. I’ll just finish with the abstract of that paper:

First, an historical outline is given of the “Island-Universe” conception (Galilei, 1609), and of the development of our knowledge of the nebulae. The cosmological views of the eighteenth century are surveyed, and in particular the developments in England during the Restoration Period (1660-1700), the Augustan Age (1700-1745), and the era of Rationalism and Neo-Romanticism (1750-1820), due to Newton, Halley, Hooke, Bradley, Thomas Wright, and John mitchell. The latter’s work founded on stellar-statistical principles resulted in 1767 in the derivation of an average distance of nebulae. Herschel’s work, and Herbert Spencer’s dictum of 1858 are discussed. Bolin’s attempt of 1907 referring to the parallax of the Andromeda nebula, and other work by Curtis in 1917 and Lundmark in 1919 are described. The various distance-indicators are introduced ( e.g. the use of novae since 1919, of supergiants since 1920, of Cepheids since 1924, and of globular clusters since 1931), and absorption effects are considered. On the basis of these indicators a distance of the Andromeda nebula of 1.23 × 106 light-years is derived. The importance of supernovae in this connection is indicated, and also the facts pointing towards a necessary increase in the metagalactic distance-scale.

Links

1959, MNRAS, 119, 342, “Obituary Notices : Knut Emil Lundmark”.

Hetherington, 1976, JHA, 7, 73, “New Source Material on Shapley, Van Maanen and Lundmark”

There seems to have been a dispute between Lundmark and Hubble about their galaxy classification systems published in 1926:
Hart & Berendzen, 1971, JHA, 2, 200, “Hubble, Lundmark and the Classification of Non-Galactic Nebulae”. A brief note on the subject.
Teerikorpi, 1989, JHA, 20, 165, “Lundmark’s Unpublished 1922 Nebula Classification”. See this article for the new piece of information about Lundmark’s unpublished work on galaxy classifications in 1922.

Wikipedia: Knut Lundmark

References

Lundmark & Lindblad, 1917, ApJ, 46, 206, “Photographic effective wavelengths of some spiral nebulae and globular clusters”

Lundmark & Lindblad, 1919, ApJ, 50, 376, “Photographic effective wavelengths of nebulae and clusters”

Lundmark, 1921, ApJ, 50, 376, “The Spiral Nebula Messier 33”

Lundmark, 1922, PASP, 34, 108, “On the Motions of Spirals”

Lundmark, 1924, MNRAS, 84, 747, “The determination of the curvature of space-time in de Sitter’s world”

Lundmark, 1924b, Obs, 47, 276, “The distance of the Large Magellanic Cloud”

Lundmark, 1924c, Obs, 47, 279, “Determination of the apices and the mean parallax of the spirals”

Lundmark, 1925, MNRAS, 85, 865, “Nebulæ, The motions and the distances of spiral”

Lundmark, 1930, PASP, 42, 23, “Are the Globular Clusters and the Anagalactic Nebulae Related?”

Lundmark, 1956, VA, 2, 1607, “On metagalactic distance-indicators”

Updates

– November 22: Changed the “radius of the curvature of the universe” to “curvature radius of space-time”. Added the missing names and characters of the abstract of Lundmark (1956), the abstract has parts missing in ADS too (probably due to careless copy-pasting), so it wasn’t exactly my mistake.

Edward Fath – early spectroscopy of galaxies

Although I’m concentrating on Fath’s extragalactic work, I’ll mention a paper among his early works as a curiosity; Steppins & Fath (1906), titled “The Use of Astronomical Telescopes in Determining the Speeds of Migrating Birds”.

Fath (1909) measured spectra of spiral galaxies, but he wasn’t able to determine their redshifts. He noted that by that time, general concensus was that spiral galaxies have continuous spectrum and that there were only two studies casting doubt on that. Both of them were inconclusive according to Fath. Fath then discussed the spectra of individual objects. He noted on the Andromeda galaxy (M31):

It contains little more than the spectrum of nucleus, which is not of a stellar character.

Some other objects he measured were NGC 1068 (of which there seemed to be a debate ongoing if it even is a spiral galaxy, or “spiral nebula” as they were called back then), NGC 3031 and NGC 4736. On NGC 5194, he said:

Photographically it is faint. Because of this it is not a promising object for spectrographic analysis, but it seemed best to make an attempt. … The plate showed nothing.

Well, you can’t succeed every time… He summed up his research on the question of the spectrum of spiral galaxies:

No spiral nebula investigated has a truly continuous spectrum.

Fath then proceeded to interesting discussion on the possible explanation for the spectrum of spiral galaxies. He notes that the spectrum usually comes only from the nuclei of spiral galaxies and resembles stellar spectrum, then he says:

The hypothesis that the central portion of a nebula like the famous one in Andromeda is a single star may be rejected at once unless we wish to modify greatly the commonly accepted ideas as to what constitutes a star.

He says that a possible explanation would be a star cluster at the center but asks:

Is it reasonable to assume that in a condensed cluster we should have stars of one spectral type strongly predominating?

He mentioned that there weren’t enough spectral measurements of star clusters to determine the answer to this question, so he proceeded to measure a few star clusters himself. He found out that one spectral type can predominate strongly. He then considered a parallax measurement of Andromeda galaxy and noted that the value of the parallax leads to the impossibly small sizes of hypothesized star cluster members. So, Fath is left with a hypothesis that to his knowledge is only one that can explain the spectrum, but other evidence rather conclusively show that the hypothesis is not likely to be valid.

Fath (1910) discussed some aspects of the distribution of spiral galaxies in the sky. Fath (1911) continued with spiral galaxy spectra. He gave some new spectral information on individual objects, and then suggested a preliminary spectral type division for all nebulae. Fath (1913) was his third paper on the subject, and here he only discussed individual objects.

Although Fath showed that the spectrum of spiral galaxies is not continuous but that it contains spectral lines, he didn’t discuss the possibility of measuring the redshifts of galaxies.

Links

Not much information is available on Edward Fath, but here’s at least something:

A Science Not Earthbound: A Brief History of Astronomy at Carleton College, page 11

References

Fath, 1909, LicOB, 5, 71, “The spectra of some spiral nebulae and globular star clusters”

Fath, 1910, PA, 18, 544, “The Distribution of Nebulae and Globular Star Clusters”

Fath, 1911, ApJ, 33, 58, “The spectra of spiral nebulae and globular star clusters”

Fath, 1913, ApJ, 37, 198, “The spectra of spiral nebulae and globular star clusters”

Stebbins & Fath, 1906, Sci, 24, 49, “The Use of Astronomical Telescopes in Determining the Speeds of Migrating Birds”

Francis Pease – Dark matters

Francis Pease is known for his work on measuring star diameters, but here I will have a look at his work on spectroscopy of extragalactic objects. Francis Pease’s first extragalactic works were simply publishing measured radial velocities of galaxies and communicating some details on their spectra (Pease, 1915a, 1915b, 1915c, 1916a). Pease (1916b) was closing in on a redshift anomaly, although Pease didn’t seem to notice it then. He reported observations on NGC 4594 (a.k.a. Messier 104) and noted a strange feature:

Light was thus received from points at different distances from the nucleus. An extraordinary feature is that the relation between radial velocity and distance is sensibly linear, thus:
    Velocity = -2.78 x + 1180
in which x is the distance from the center in seconds of arc.

So, it seems that Pease might have published first account here of the flat rotation curves which currently are thought to suggest the presence of dark matter, but the level of detail is so poor in Pease’s paper that it is hard to say for sure. However, in the next paper, Pease (1916c) clearly recognizes that something is not correct there. This paper is basically the same observations reported with plenty more detail. Pease gives a table of his observations, and gives the same equation for the velocity curves. He notes that the equation was obtained from least squares fit, but he also says:

Within the limits of accuracy of the measures the change of rotational velocity is linear, although there may be some variation in individual parts of the nebula.

While interpreting this result, he says:

In any event the results seem to be inconsistent with a system involving planetary motion about a central nucleus, since this would require an increase of linear velocity toward the center of the nebula.

But Pease doesn’t give any comments about possible reasons for this. Pease (1918) studied the rotation curve of the Andromeda Galaxy, M31, and found it to have a linear rotation curve as well. But whether we should start to campaign for Pease as a founder of dark matter instead of Zwicky based on these studies presented here, I’m not so sure.

References

Pease, 1915a, PASP, 27, 133, “The Radial Velocity of the Nebula N. G. C. 1068”

Pease, 1915b, PASP, 27, 134, “Radial Velocity of the Andromedæ Nebula”

Pease, 1915c, PASP, 27, 239, “Radial Velocities of Six Nebulæ”

Pease, 1916a, PASP, 28, 33, “The Spiral Nebula Messier 33”

Pease, 1916b, PASP, 28, 191, “The Rotation and Radial Velocity of the Spiral Nebula N. G. C. C. 4594”

Pease, 1916c, PNAS, 2, 517, “The Rotation and Radial Velocity of the Spiral Nebula N. G. C. 4594”

Pease, 1918, PNAS, 4, 21, “The Rotation and Radial Velocity of the Central Part of the Andromeda nebula”

Links

Wikipedia: Francis Pease

Vesto Slipher – the redshift anomaly pioneer

Vesto Slipher was the first to measure the redshift of an extragalactic object. Slipher (1913) noted that the spectroscopic faintness of extragalactic objects were the main reason for the lack of redshift measurements by that time. The object he chose as the target was the “Andromeda nebula” (today known as “Andromeda galaxy”), M31. He gave a detailed description of his spectral measurements and gave a redshift of cz = -300 km/s. This was the first redshift anomaly he found. He said:

The magnitude of this velocity, which is greatest hitherto observed, raises the question whether the velocity-like displacement might not be due to some other cause, but I believe we have at the present no other interpretation for it. Hence we may conclude that the Andromeda Nebula is approaching the solar system with a velocity of about 300 kilometers per second.

Even if wasn’t very difficult problem, back then such high redshift velocities had not been observed in celestial objects, so to him it was a redshift anomaly. I think the fact that the first ever observed extragalactic redshift immediately raised a redshift anomaly nicely highlights the remarkable history of redhifts as problem raisers and solvers. After noting the high apparent velocity and having made a rather peculiar (or so it seems today at the least) remark about M31 having encountered a “dark “star””, he made a comment that showed great intuition (while at the same time being based on a rather small sample):

That the velocity of the first spiral observed should be so high intimates that the spirals as a class have higher velocities than do the stars and that it might not be fruitless to observe some of the more promising spirals for proper motion.

Slipher (1914) discovered that galaxies rotate and inferred from that observation that some nebulae are edge-on spirals. However, he’s rather vague on the details on the edge-on conclusion, he just seems to suggest that because there is rotation observed in the “spindle” type nebula, it suggests that they are edge-on spirals. I don’t see why they couldn’t have been other types of nebulae, also rotating.

Slipher (1915) gave a summary of galaxy redshifts known at that point. He noted an anomaly he had faced but which seemed to have been solved:

As far as the data go, the average velocity is 400 km. It is positive by about 325 km. It is 400 km on the north side and less than 200 km on the south side of the Milky Way. Before the observation of N.G.C. 1023, 1068, and 7331, which were among the last to be observed, the signs were all negative on one side and all positive on the other, and it then seemed as if spirals might be drifting across the Milky Way.

And right after that, another redshift anomaly:

N.G.C. 3115, 4565, and 5866 are spindle nebulae – doubtless spirals seen edge-on. Their average velocity is about 800 km, which is much greater than for remaining objects and suggests that the spirals move edge forward.

That sounds rather funny in the context of the today’s knowledge, but back then it was perfectly sound remark.

Slipher (1917a) analyzed the galaxy redshifts known at that point. He first discussed the general features of spectral features of all nebulae. He then pointed out that the faintness of galaxies was still a problem for their spectroscopy, followed by a discussion on the measurements techniques. He then proceeded to analyze the known redshifts. By that time, he noted, it had become clear that (spiral) galaxies were a separate class of objects (remember that it was not yet clear if galaxies were objects of our own galaxy or extragalactic objects):

Referring to the table of velocities again: the average velocity 570 km is about thirty times the average velocity of the stars. And it is so much greater than that known of any other class of celestial bodies as to set the spiral nebulae aside in a class to themselves. Their distribution over the sky likewise shows them to be unique – they shun the Milky Way and cluster about its poles.

He mentioned that as the redshift velocity of most spiral galaxies seemed to suggest that they are receding from us, it might imply that they are scattering (an early reference to expanding space perhaps?), but that their tendency to cluster seemed to argue against it. He then returned to the spirals move edge forward hypothesis. He showed a table where he had divided the spiral galaxies to three groups; face-on, inclined, and edge-on. Face-on spirals had generally lowest velocities, inclined spirals had generally higher velocities, and edge-on spirals had highest average velocity (but it seems to me that the inclined and edge-on difference is not very clear).

He then discussed the evidence of rotation in spiral galaxies. He specifically adressed the problem of the direction of the rotation. There was a problem of not knowing which side of inclined spiral galaxy is closer to us. He suggested an indirect way to find that out by looking at the dark bands that are seen in edge-on spiral galaxies:

If now we imagine we view such a nebula from a point somewhat outside its plane the dark band would shift to the side and render the nebula unsymmetrical – the deficient edge being of course the one nearer to us.

And when that was checked in practice, it turned out the spirals were rotating all to the same direction:

The central part – which is all of the nebulae the spectrograms record – turns into the spiral arms as a spring turns in winding up.

He mentioned also the possibility he had discussed in his previous work, that spirals in one side of the sky are receding and in one side are approaching (which we know today was just accidental due to his small sample). He suggested that it might be due to our own motion, and even calculated the direction and speed of the motion from the available evidence. However, as the other stars didn’t seem to show this kind of motion. He suggested that this was evidence to the theory that spiral galaxies are extragalactic objects, far away galaxies like our own Milky Way.

In these early days there were some concerns about the accuracy of the redshift measurements, and Slipher (1917b) shows an example where he answers one critic showing that the accuracy is indeed sufficient.

Slipher (1921) announced that a new redshift record had been made; the NGC 584 had been measured to recede from us with a redshift velocity of 1800 km/s.

References

Slipher, 1913, LowOB, 2, 56, “The radial velocity of the Andromeda nebula”

Slipher, 1914, LowOB, 2, 66, “The detection of nebular rotation”

Slipher, 1915, PA, 23, 21, “Spectrographic observations of nebulae”

Slipher, 1917a, PAPhS, 56, 403, “Nebulae”

Slipher, 1917b, Obs, 40, 304, “Radial velocity observations of spiral nebulae”

Slipher, 1921, PA, 29, 128, “Two nebulae with unparalleled velocities”

Links

Short biography from the Lowell Observatory
Wikipedia – Vesto Slipher
John Peacock’s page on some of Slipher’s work on galaxies