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The results of the Galaxy Zoo project force us to revise the theory of the formation of spiral arms of galaxies

In 1924, Edwin Hubble finally proved that there are other galaxies in the Universe besides our Milky Way. In 1926, he published the first version of the classification of galaxies according to their appearance and structure. This classification was refined throughout the XX century and eventually became generally accepted. However, a new study that used the results of the work of the civil science project Galaxy Zoo, whose participants manually classified thousands of galaxies, showed that it is far from complete clarity. It turned out that the relationship between the mass of the galaxy bulge and the density of spiral arms is actually not direct, as previously thought: for galaxies with a small bulge, the sleeves are both weakly and tightly wound around the center. Because of this, apparently, it is necessary to revise the theory of the formation of arms in spiral galaxies.

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Jiya Saini
Jiya Saini is a Journalist and Writer at Revyuh.com. She has been working with us since January 2018. After studying at Jamia Millia University, she is fascinated by smart lifestyle and smart living. She covers technology, games, sports and smart living, as well as good experience in press relations. She is also a freelance trainer for macOS and iOS, and In the past, she has worked with various online news magazines in India and Singapore. Email: jiya (at) revyuh (dot) com

In the last decade of the 19th century, astronomers began to suspect that the “spiral nebulae” discovered in the middle of that century (it must be said that several nearby galaxies — the Large and Small Magellanic Clouds, the Andromeda Nebula and the Triangle Galaxy — were known in the Middle Ages, but Scientists did not guess their true nature), which at that time could be observed only in a few of the largest telescopes, can be the same star systems (they were not yet called galaxies), like our Milky Way. To prove this, it was necessary to find out the distance to these structures, since a small planetary nebulawithin our galaxy and a huge spiral galaxy, millions of light-years distant from us, will have the same angular size and look very similar.

Discussions were held for more than a dozen years (see, for example, the Great Controversy), and Edwin Hubble put the final point in them in 1924 . Having access to the most perfect at the moment the telescope – the 2.5-meter telescope in Hawker Mount Wilson Observatory, he first saw some variable stars in these nebulae, and used fixed for several years before the relationship between the luminosity of the star and its pulsation period, was able to reliably establish that these stars and nebulae cannot be located in the Milky Way, which means that they are separate, very distant structures from us, which we now call galaxies. This amazing result increased the size of the universe known to mankind so much that it was first published – an unheard of case – not in a scientific journal, but in The New York Times newspaper in the form of a small note Finds Spiral Nebulae Are Stellar Systems. Dr. Hubbell confirms that they are ‘Island Universes’ similar to our own. It’s funny that the reporter made a mistake in writing the Hubble name.

In 1926, the famous Hubble article was published in the Astrophysical Journal, in which the first classification of galaxies was proposed (E. P. Hubble, 1926. Extragalactic nebulae). He divided the galaxy in appearance into elliptical and spiral. He classified elliptical galaxies according to the degree of elongation (from spherical, E0 to cigar-like, E7), and spiral, in turn, into two subtypes: with a jumper (in English bar, SB) and without it (S). This article also expressed the strong idea that there are still transitional galaxies that are not open at the time – lenticular.(denoted by S0). The elliptical branch and two branches of spiral galaxies met just at the point S0, forming the famous Hubble tuning fork (Fig. 1).

Fig. 1. The Hubble tuning fork, illustrated by the galaxies that Hubble himself used. These beautiful images were obtained, of course, much later: they are taken from the SDSS survey data. The Sa class is the famous Sombrero galaxy, the Sc class is the Pinwheel Galaxy. Lenticular galaxies (class S0) at the time of Hubble were not yet known, therefore the galaxy NGC 6278 was placed on the image, which was open at that time but not studied enough. Figure from the discussed article in MNRAS

Spiral galaxies (with and without a bar) were divided into 3 classes depending on the size of the inner, brightest part, which is spherical and called the bulge, as well as on the degree of twist of the spirals around it: the Sa and SBa galaxies have the largest the bulge and the densest winding of the spiral arms, then the Sb and SBb classes, and for the galaxies from the Sc and SBc classes, the bulge is usually difficult to distinguish, and the arms are weakly twisted.

As can be seen, already at the very beginning of the study of spiral galaxies, a connection was noticed between the size of the bulge and the shape of the spirals: the more pronounced the bulge, the stronger the spirals wound around it.

Despite the fact that many of Hubble’s assumptions about the evolution of galaxies (for example, about the transition of elliptical galaxies into spirals as they develop) were not confirmed later and were rejected by the scientific community, the classification itself turned out to be very convenient. In the second half of the 20th century, clearer classification criteria were established in the works of Allan Sandige and Gerard de Voucler, another class of spiral galaxies with the dimmest bulge (Sd) was added, and the Hubble tuning fork was finally approved as a convenient and universal classification of galaxies that entered all astronomy textbooks.

Further accumulation of our knowledge of galaxies has shown a surprisingly close connection between the morphology (in fact, the orbits of stars) of the galaxy and its evolutionary way. This means that thanks to the classification, much can be said about the history of the galaxy (how star formation took place in it, whether there were collisions with other galaxies, how the gas content and total stellar mass, etc.) varied when there is no other information. As a result, over time, some properties of galaxies began to be determined simply on the basis of its classification: first, after visual classification, to attribute it to one of the types (for example, to the SBc type), and then derive relations from the masses of its various components (bulge, spiral arms, halo) and other characteristics (even the average age of its stellar population).

The question arises: how well is such an approach? How trustworthy is the classification, which was compiled from the results of research of several hundred galaxies (in the original work Hubble analyzed only about three hundred galaxies)?

Now, in the era of Big Data, the number of known galaxies has exceeded several tens of millions and it is very tempting to use this amount of information to refine and recheck some old hypotheses and theories that are commonly relied upon in the astronomical community. Despite the fact that automatic classifiers using machine learning already exist and help in work, people are still needed for accurate and reliable classification – many people, and even astronomers from the entire planet are not enough here. The so-called civil science comes to the rescue – attracting volunteers, many of whom may not have special education, to solve scientific problems: in 2007, the Galaxy Zoo project was founded, the participants of which were to help with the visual classification of the galaxies of Sloanovsky digital sky survey (SDSS).

Fig. 2. Randomly selected galaxies from among the proposed for the analysis of the participants of the project Galaxy Zoo. Top row – galaxies that have any features (spirals, bulges, bright clusters nodes, etc.). The bottom row – elliptical galaxies, in which it is impossible to distinguish individual bright signs. These eight galaxies are at the same distance from us (400 million light years), which means the difference in apparent size and brightness is due to the different physical sizes of these galaxies (the smallest galaxies are on the left, the largest are on the right ). Figure from the discussed article in MNRAS

For them, prepared special color images of galaxies, obtained by overlaying the original images taken in three different filters (Fig. 2). These pictures should be described by answering questions like “Elliptical or spiral galaxy?” , “Do you see signs of a jumper?” , “Does the image show signs of spiral arms?” , “How big is the bulge compared to the whole galaxy?”one of the suggested answer choices. Despite the seeming simplicity of the questions, the project participants, in fact, repeated the work of Edwin Hubble, only on a much larger number of objects: instead of the three hundred galaxies included in Hubble’s original work, this time almost 300,000 galaxies were classified – including much dimmer ones. and smaller in size. Since any person, no matter if he is astronomer or not, may be mistaken, each galaxy was independently classified by 40 people and its final description was determined taking into account the range of opinions.

The work of the participants in the Galaxy Zoo project was analyzed by a group of astronomers led by Karen L. Masters from Haverford College. To their surprise, it was found that the existing Hubble classification is not accurate, and therefore determining the physical characteristics of a galaxy according to its belonging to one or another class can lead to errors.

As mentioned above, the separation of spiral galaxies into Sa, Sb, Sc, Sd classes was tied simultaneously to the size of the bulge and the degree of twist of the spiral arms. In ambiguous cases, the classification was determined precisely by spinning: for example, if the bulge is not very large and, rather, corresponds to the type of Sc, but the spiral arms are tightly wound, then the galaxy was classified as Sa. It has now become clear that the twist of the sleeves cannot be used as a reliable criterion and it is necessary to rely only on the size of the bulge.

To show this, the authors of the article described the degree of “winding” of the spiral arms through the numerical parameter avg : when avg  = 1.0, the sleeves fit very closely to the bulge, and when avg  = 0.0, they diverge from it quite spaciously. Bulge size is determined by the parameter avg, which varies from zero (for galaxies without a bulge) to unity (for galaxies in which the bulge is the most significant part). Masters and colleagues significantly thinned the initial catalog of galaxies, leaving only those images where the galaxies are not hidden behind areas of gas or dust, and also arranged so that the structure of the sleeves and bulges is clearly visible, and therefore the possibility of error during classification is minimal. Only 4830 galaxies were included in the final sample, but this number is quite enough to see the discrepancies between the existing theory and current observations (Fig. 3).

Fig. 3. The main results of the work under discussion. The vertical axis represents the parameter w avg , which reflects the twisting of the spirals of the galaxy, and on the horizontal axis, the parameter B avg, which expresses the size of the bulge. The black dashed line qualitatively illustrates the previous ideas about the structure of galaxies: weak bulges correspond to weak winding, massive bulges – tight winding. However, the actual distribution of galaxies on this graph (each point is a separate galaxy, the gray areas from the darkest to the lightest ones correspond to 0.5, 1, 1.5 and 2 standard deviations) testifies that if large bulges really tend to have tightly wound spirals, then for small bulges the weak winding predominance is not even close. The red and blue lines show the median values of the winding density and the size of the bulges, respectively. Figure from the discussed article in MNRAS

The fact that galaxies with massive bulges can only have tightly wound spirals has been known before. This is not surprising, because usually such bulges appear in massive and bright galaxies that could be observed with confidence even in the times of Hubble. Galaxies with a small bulge (that is, obviously less bright), as it turned out, can have all possible types of spirals. This observation can be interpreted as the discovery of the variable speed of evolution of spirals depending on the size of the bulge (the more massive it is, the faster the spirals, which initially can have any shape, will be tightly wound and pressed to the bulge) and, probably, it will lead to a change in the basic theory explaining the formation of spirals – the theory of static density waves.

This theory, proposed in 1964 by the astrophysicists Jia-Jiao Lin and Frank Shu, for the first time gave a mathematical description of spiral arms as fixed regions of increased stellar density, into which individual luminaries periodically fall during their multimillion-year revolution around the galaxy center. An analogy of this theory can be a traffic jam, which itself does not move, although the machines first approach it at normal speed, then slowly crawl in traffic, and then accelerate again (such effects are discussed in the Problem Wave effects in traffic jams). Mathematical equations describing the sleeves in the theory of density waves correlated the angles at which the spirals are wound on the bulge with the mass of the galaxy (which directly depends on the size of the bulge). A little awkward, this theory was nevertheless able to give a harmonious explanation of the long-term spirals: all previous models (as well as our life experience) showed that in any dynamic system the spirals should disappear, either quickly moving away from the center, or vice versa, winding up on the central region.

Over the next 50 years, observations and accumulated computer simulations were piling up, from which it followed that Lin and Shu’s theory is incomplete and, probably, needs to be revised. However, the results of simulations are not considered evidence in astrophysics, and observations were based either on specific galaxies that do not fit into the theory, or on small samples of such galaxies, so this theory remained afloat.

Over time, theories appeared in which the behavior of spiral arms is explained by tidal forces: in them the arms are independent objects — they gravitationally bind the stars and rotate around the center of the galaxy together with the stars, changing with time. If any of these theories is correct, then the observations presented in fig. 3, it is easy to explain: in young spiral galaxies, spiral arms of different shapes and twist may initially form. In this case, the arms are not static formations, they evolve and wind up more tightly on the galaxy under the influence of the mass of its central part, and the bulge is more massive, the faster it will happen.

It is also possible that, additionally, jumpers (bars) that exist in a number of galaxies play on the formation of spirals – according to the observations of the authors of the article under discussion, the more pronounced the jumper, the weaker the spiral winding in the galaxy.

The authors notice that, surprisingly, but 170 years after the discovery of the first spiral structures in “star nebulae”, we still do not fully understand how they are formed. In fact, there is nothing terrible in clarifying the classification: a situation where new details are discovered as knowledge accumulates, and the old classification is no longer relevant, it is not new – it’s enough to recall Pluton’s “degrading” from planets to dwarf planets because some of which are relatively close to Pluto’s orbit.

Source: Karen L. Masters et al. Galaxy zoo: wind galaxies are a winding problem of the galaxies are winding  // MNRAS . 2019. DOI: 10.1093 / mnras / stz1153.

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