Question B1: What is a galaxy?

Question B1: “What is a galaxy?  – vote here!”


Answer: The astronomical object we commonly call a “galaxy” has no formal definition yet. This issue is now raised to a more formal problem by Forbes & Kroupa (2011)(F&K).

Here is the associated press release.

Science and New Scientist also report on this question.

Your vote is of interest: Being motivated by the vote at the General Assembly of the International Astronomical Union in Prague in the year 2006 on Pluto’s status in the Solar System and given the lack of a formal definition of what constitutes a “galaxy”, Prof. Duncan Forbes from Swinburne University in Melbourne has organised a poll to seek if a consensus may emerge how a galaxy could perhaps be defined. To contribute to the poll, feel free to cast your vote here as to what you think a galaxy is. But please read the above F&K paper first.

And, feel free to post your own views on what a galaxy is in the comments section below.

The results of the poll and of the discussion will be reported at conferences.




As introduced in the previous contribution to The Dark Matter Crisis, Question A: Galaxies do not work in LCDM, sociology and majority views, PK was recently contacted by a few people, and here are excerpts from some of the questions asked and the replies. These help to illustrate some of the issues at hand. The questions are

A) So the LCDM model fails on scales smaller than about 8 Mpc?

B1) What is a galaxy? (this contribution)

B2) What is a galaxy? (Addendum on the relaxation time) 

C) What are the three best reasons for the failure of the LCDM model?

I: Incompatibility with observations

II: MOND works far too well !

III: Fundamental theoretical problems


D) What about the Bullet cluster?  And what about the Train-Wreck cluster Abell 520?

E) Why is the main stream community so reluctant to  go along with accepting the failure of LCDM?

This contribution deals with Question B1, while upcoming contributions will concentrate on the remaining questions.


Traditional ideas seen from a modern perspective:

The word “galaxy” comes from the Greek word for “milky circle”. The discovery of galaxies as extra-galactic objects is a fascinating part of science history and is outlined for example in the Wikipedia article on Galaxy.

In the standard LCDM model galaxies are the central typically 0.2-40kpc large region weighing a few per cent of mass of a vast dark matter halo which is about ten times larger in extend. Each galaxy has a distinct and typically violent formation history as a result of the coalescence and merging of smaller dark-matter halos. It is part of a complex dynamic environment. A large range of properties of galaxies is thus expected, with the more massive galaxies being expected to appear later in time and in denser environments.

Modern research however is indicating galaxies to be more like true island universes, as originally thought by Immanuel Kant in the middle of the 18th century. Indeed, the large fraction of all major galaxies are large thin disk galaxies, in which stars and gas move about a common center of gravity on near-circular orbits. In disk galaxies new stars can form from the cold gas. In the seminal paper “Galaxies appear simpler than expected“, Disney et al. (2008, Nature) demonstrate the followig:

Disk galaxies are  observed to look las if they formed and evolved largely isolated, one galaxy loooking more or less like another (thin disk, nobulge, typical profile with a scalelength that can be predicted approximately if the luminosity is known). Ony the minority show evidence for strong interactions and important influence from the environment.  Galaxies form a one-parameter family, meaning that there is one parameter which defines most of the appearance of the galaxy. Based on the sample of about 560 galaxies in the Local Volume Peebels & Nusser (2010, Nature), in their paper on “Nearby galaxies as pointers to a better theory of cosmic evolution“, also describe galaxies to be “island universes”.

But, as one goes to lower masses, at which point does one enter the regime of star clusters? Can a galaxy be composed of merely 1000 stars? Can a star cluster be composed of ten million stars? A disc galaxy today consists mostly of stars with about 10-20 % of mass in gas. But there are also dwarf and massive elliptical galaxies, in which star formation is not proceeding any longer. They look red as they are composed mostly of older stars which orbit on randomly oriented orbits about a common center of mass.

It is the low-mass, faint end which is now challenging our understanding of what a galaxy actually is.

Does a stellar population consisting of say, 100,000  stars, which is called a “galaxy” have different  properties to a “star cluster” which consists of 100,000 stars? Or is there a regime where such a distinction between galaxy and star cluster is no longer possible?


Some modern ideas:

How to define a galaxy is becoming an issue given that the large modern telescopes and the simultaneous spectroscopic observations of many sources in large fields of view have lead to the discovery of small stellar-populations, which can be called large star clusters or small galaxies.

The observational break-through came with the PhD thesis of Dr. Michael Hilker (PhD thesisHilker et al. 1999), who was then working in Bonn with Dr. Tom Richtler and Prof. Klaas S. de Boer. Nearly simultaneously an Autralian-British-American team made the same discovery as documented by their paper “Compact stellar systems in the Fornax Cluster: Super-massive star clusters or extremely compact dwarf galaxies?” (Drinkwater et al. 2000). They were later named “ultra compact dwarf galaxies” (UCDs) by Phillips et al. (2001).

Tradionally, an astronomer would refer to a star cluster as being a population of stars which are gravitationally bound to each other and all of which have the same age and chemical composition. But, today we know that “star clusters” exist that contain stars which do not have the same chemical composition, nor the same age. A prime example is Omega Centauri, which is traditionally known as the most massive star cluster in the Milky Way weighing about 3 million solar masses. Hilker & Richtler (2000) have done a seminal study of this object demonstrating that it seems to contain stars with ages that differ by up to 6 Gyr. That is, star formation seems to have gone on for 6 Gyr in this “star cluster”. Until then it was thought that only galaxies could do this. Furthermore some of the satellite “dwarf spheroidal (dSph) galaxies” around the MilkyWay appear to have only a few thousand stars (Strigari et al. 2008, Nature). Remember that the star cluster Pleiades contains only about a thousand stars.

That the distinction between star clusters and spheroidal dwarf galaxies was becoming blurred was actually realised somewhat earlier by Pavel Kroupa. Having seen the amasing HST images of the interacting pair of Antennae galaxies where the inner parts of the extensive tidal arms  are littered with thousands of newly formed star clusters, it was immediately apparent that many of the clusters were clustered into larger aggregates, groups, complexes or superclusters of star clusters. The star clusters in the cluster complexes must be interacting gravitationally. What sort of object evolves if the many clusters in a complex merge? Kroupa (1998) quantified the resulting objects showing that spheroidal dwarf galaxies emerge from the merging star clusters.

This research lead to many follow-on papers dealing with the evolution of star-cluster complexes under various conditions (see PhD thesis of Michael Fellhauer, and Fellhauer et al. 2002). It has emerged that Omega-Centauri-type objects, that dSph-galaxy-type objects and UCD-type objects can be formed, depending on the mass, size and tidal field. Also, “star clusters” which also have a large radius can be explained as merged superclusters (Fellhauer & Kroupa 2002, Bruens & Kroupa 2010).

A formal definition between a galaxy and a star cluster was introduced by Kroupa (1998) who defined a galaxy to be a stellar-dynamical system of stars which has a median two-body relaxation time longer than a Hubble time. Mathematically this is equivalent to stating that a galaxy has a smooth potential, that is, that there are no irregularities in the potential for the stars to scatter on significantly over the age of the universe.

What is a two-body relaxation time?  In any self-gravitating stellar population the stars constantly exchange gravitational and kinetic energy with each other. When two stars pass each other, they give each other tiny gravitational nudges. This is also happening in the Solar System between its planets – as a result of the ongoing gravitational pulls the planets are changing their orbital eccentricities and periods all the time. In a stellar system, these tiny nudges between the stars cause the system to evolve towards energy equipartition as a result of which it changes its overall morhpology (its radius, and the radial distribution of its stars). The time-scale over which the system re-arranges itself significantly is measured in terms of the median two body relaxation time. One can say that after a relaxation time, a stellar system has “relaxed”, that is, that it has significantly evolved towards energy equipartition.

Note that defining a galaxy to be a stellar-dynamical system with a median two-body relaxation time longer than the age of the universe is the same as defining it to be a system in which the time-scale for energy-equipartition is longer than a Hubble time. Energy equipartition between stars is therefore not an important process in a galaxy. A star cluster, on the other hand, has an energy-equipartition time-scale shorter than the age of the universe. Thus, energy equipartition plays a significant role in the evolution of a star cluster.

Note that a system of stars being “relaxed” is not synonymous with the system being in dynamical equilibium. A system achieves dynamical equilibrium within a few crossing times, but it may not be relaxed. The crossing time is the time a typical star needs to cross through the system (e.g. it is about 200 Myr for the Milky Way near the Solar radius) and it is much shorter than the two-body relaxation time which is many times 13 Gyr for the Milky Way. A system in dynamical equilibrium means that the star cluster or galaxy does not “wobble”, that is, its radius for example, does not evolve or change, unless one is looking at it over time-scales similar to a relaxation time.

Thus, according to Kroupa’s definition, a galaxy has not had enough time to relax. A star cluster, on the other hand, being defined to be a stellar system with a median two-body relaxation time which is shorter than a Hubble time, does relax. And, in the process of relaxation, a star cluster evaporates its stars, and thus looses mass. If the relaxation time is sufficiently short, e.g. if it is only a small fraction of the age of the universe, then a star cluster will completely evaporate leaving not more than a few stars in orbit about each other. Globular clusters have a two-body relaxation time of roughly 100 Myr to a few Gyr, so they are still around. Open star clusters, such as the Pleiades or Hyades, have a two-body relaxation time of a few tens Myr such that they dissolve as they age, both having in fact already lost the majority of their stars which they were born with.

Now, it emerges that Kroupa’s definition of a galaxy implies that every known object which has so far been called a galaxy remains a galaxy, while Omega Centauri would formally be a star cluster, but can also be viewed as a low-mass UCD galaxy. Omega Cen is a transition-type object, with properties between those of a star cluster (two-body relaxation time slightly shorter than a Hubble time) and those of a true galaxy (complex stellar population).

Noteworthy is however that dark matter only appears in systems with a median two-body relaxation time longer than a Hubble time, as is demonstrated by Dabringhausen, Hilker & Kroupa (2008). That is, according to Kroupa, dark matter only appears  in “galaxies”.

But why should dark matter care about the relaxation time of a stellar system?

Or, is a “galaxy” simply a system which behaves according to Milgrom’s law of dynamics?

What do you think constitutes a galaxy? Vote here! and discuss below.

By Pavel Kroupa and Marcel Pawlowski (19.01.2011): “Question B1: What is a galaxy?” on SciLogs. See the overview of topics in  The Dark Matter Crisis.


Author: Prof. Dr. Pavel Kroupa

I am a Czech-Australian teaching and researching at the University of Bonn on dynamics and stellar populations. After studying physics at The University of Western Australia, Perth, I obtained my PhD from Cambridge University, UK, as an Isaac Newton Scholar at Trinity College. After spending eight years in Heidelberg I habilitated at the University of Kiel, Germany. I then took up a Heisenberg Fellowship and later accepted the position as a professor at Bonn University in 2004. I was awarded a Leverhulme Trust Visiting Professorship (2007, Sheffield, UK) and a Swinburne Visiting Professorship (2007, Melbourne, Australia). In 2013 I received the Silver Commemorative Medal of the Senate of the Czech Republic, and I took-up an affiliation with the Charles University in Prague in 2016. Pure innovative science can only truly thrive in non-hierarchical societies in which competition for resources is not extreme. Therefore I see the need for the German academic system to modernise (away from its hierarchies) and warn of academic systems that are based on an extreme competition for resources (USA), as these stifle the experimentation with new ideas.

23 thoughts on “Question B1: What is a galaxy?”

  1. Bound dark matter haloNone of these options above seem to satisfy as a good definition for a galaxy. Therefor, I’d like to define a galaxy as “a bound dark matter halo.”
    And if dark matter doesn’t exist then a galaxy can be defined as a bound stellar system in which Newtonian gravity doesn’t work. i.e. if Newtonian gravity plus the baryonic mass can not be used to make a self-consistent model of the dynamics and the potential.
    These two definitions are functionally equivalent. But both are hard to measure observationally

  2. on the classification criteriaIn my opinion, none of the 5 proposed properties in the online poll can be used as a criterion.
    1) The present age of the Universe cannot be a special timescale as this is an anthropocentric argument. Increasing the globular star cluster size and mass will at some point result in the dynamical relaxation time larger than T_hubble. The opposite situation, namely the low mass star depletion as a result of the dynamical evolution is observed among the least massive UCDs (see Chilingarian et al. 2011, arXiv:1011.1852)
    2) A half-light radius: why 100pc? M32 is very close to it (105pc), and some UCDs as well. However, M32 is a rapidly rotating system with a central CMBH not detected in UCDs so far.
    3) The presence of complex stellar populations doesn’t seem to be the strict criterion as one cannot exclude the possibility of the star cluster merger as a way to create such a system.
    4) The diagnostics of the dark matter presence in compact stellar systems is very challenging – we never exceed the 50% DM fraction threshold given the factor of 2 uncertainty from the IMF shape. The same situation (very little dark matter) is observed in much brighter and larger dwarf elliptical galaxies in clusters (dE/dS0). At the same time, in extremely low-luminosity “dark matter dominated” systems, in my opinion, we have to study possible observational biases and physical phenomena (e.g. binary stars) that may result in the overestimated dynamical masses.
    5) Satellites might have fallen on-to the central galaxy in a group a very long time ago, and examples of luminous isolated galaxies are well known since several decades.
    Other alternatives that I can propose (also not universal) are: (a) the presence of internal angular momentum — every rotating stellar system can be considered as a galaxy, however the absence of rotation does not mean that it is a star cluster; (b) position of a pressure supported system in the Fundamental Plane and its projections (or in the Fundamental Manifold) — it is well known that GCs and “galaxies” occupy distinct regions joining at the UCD locus; (c) presence of a central black hole or a central stellar nucleus, however, some galaxies are known not to host a nucleus.
    I would like to pose a different question: DO WE REALLY NEED TO MAKE THIS “ZOOLOGICAL” CLASSIFICATION? A similar problem is to distinguish between a rich group and a poor cluster of galaxies. We should study stellar systems regardless of their classification and to try to understand their origin and evolution. Nowadays, biologists classify the species based on genetic information. Maybe we should follow this approach and distinguish between galaxies and “non-galaxies” using their formation and evolution mechanisms.

  3. Galaxy definitionA galaxy is an isolated stellar system.
    I heard this question debated at a couple of recent conferences. At one, Simon White said “I know one when I see one.” I agree. This is not as flippant as it sounds. What, after all, was Hubble doing when he classified galaxies? How was he able to recognize objects as galaxies at all? Clearly he did manage that much.
    I do not think we should place too much emphasis here on quantitative measures, be they relaxation times or gravitational binding. We can look at the sky and recognize galaxies without recourse to such things. Certainly our definition should not include dark matter – that is not what we see!
    I worry a bit more about distinguishing between galaxies and star clusters, as the ultrafaint galaxies have forced us to consider. Here, the word “isolated” carries a lot of weight. It intentionally recalls the old phrase “island universe.” And it allows us to be intentionally vague.
    Consider a globular cluster. In the halo of the Milky Way, it is not isolated – clearly a star cluster. Take that same cluster and transplant it to an isolated spot in the middle of a void. If we observed such a beast, would we call it a galaxy? I think we would. If so, there can be no hard and fast quantitative distinction between star clusters and galaxies based only on their physical properties: their environment matters.
    To be fair, some things disturb me about my simple definition. One is that I would consider an isolated, rotating gas cloud devoid of stars to be a galaxy. So “stellar system” is inadequate, much as “island universe” now sounds archaic. I suppose we could replace “stellar” with “baryonic”, though it sounds ugly. Also, a rotating cloud implies stability and permanence as opposed to a whisp of gas that is not in equilibrium. So the notion of gravitational binding is implicit. Of course, that is true for any object that has been around for the better part of a Hubble time – we do not need to measure its rotation curve to recognize it as bound.
    The other thing that disturbs me are the ultrafaint dwarfs. Are these isolated systems? Clearly not, in the sense of being proximate to the Milky Way like the example of a halo globular cluster. But they are pretty isolated on the sky. Certainly the classical dwarfs count as galaxies (and as isolated stellar systems) in spite of orbiting the Milky Way. So here I think I am comfortable with a relaxed interpretation of “isolated.” We know it when we see it.

  4. Galaxy definitionI agree with some of the reservations in the comments above. One thing that Stacy’s comments make me think about is: what is *a* galaxy? as opposed to two galaxies about to merge? That’s generally accepted to be long before the central BHs merge in some cases (probable example Arp 220), and intuitively is something to do with being able to describe them dynamically as separate potentials. So I feel dynamical properties are key to a galaxy’s self identity.
    “I know one when I see one” – at large masses this is true… except.. Another thought experiment: does a bound rich cluster of galaxies satisfy the criterion? The individual stars have long two-body relaxation times, yes? So how do you tell when something is a cluster and something is a galaxy? (for a cD, where does the galaxy end and the cluster begin?)
    At small masses, which is the emphasis of your paper, things are less clear. I think this is indeed like the planet problem: we have some things in the intermediate mass range which everyone agrees are ‘proper’ galaxies. We have some Pluto analogs at the low mass end – dwarf gals, globulars, etc. and perhaps some ambiguity at the high end – in analogy to the planet/star boundary.
    At the low mass end, perhaps it’s more productive to ask what the star formation physics is that gives an upper bound to the mass of super star clusters? Anything more massive than that is a galaxy, anything less than that is a ‘small bound stellar system’, including some dwarf galaxies.

  5. It happens galaxy mystery is solved: a galaxy is a life
    Rational Galaxy Structure and its Disturbance
    Why is there little dust in elliptical galaxies? Here is a promising answer.
    Firstly, galaxies are rational. Rationality means that the density distribution of stars is proportional. Secondly, galaxy arms are linearly-shaped and irrational. The presence of arms is the disturbance to the rational disks and bars. Therefore, any disturbance to rational structure
    produces cosmic dust.
    Is an Algebraic Cubic Equation the Primitive Instinct Beyond Electromagnetic and Nuclear World?
    Everyone lives his or her life instinctively. Does the instinct originate from the natural world? If the instinct is a rational process, is the natural world rational? Unfortunately, people have not found any rational principle behind the natural world. Because human activities are realized directly through electromagnetic and nuclear forces of entropy-increase, people are difficult to recognize the principle. Compared to the large-scale structure of galaxies, human bodies and their immediate environment are the “microscopic” world. The electromagnetic and nuclear forces which rule the world, however, disappear in the formation of large-scale galaxy structures. Similarly they disappear in the formation of the solar system. My previous papers found many evidences that galaxies are rational. This paper shows that large-scale galaxy structure must originate from an algebraic cubic equation.
    New Human Civilization Glittering Under Galaxy “snowflakes”
    What is human life? From the material sense, it is a material structure composed of oxygen, carbon and other atoms. From the biological point of view, it is a kind of advanced animal that understands the natural world, recognizes and creates products. In the last tens of thousands of years, human beings created languages and tools, and achieved a near-perfect understanding of the microscopic world of elementary particles. However, in the 21st century, mankind has experienced irreversible crises such as environmental pollution, erratic weather, food shortage, and population explosion. However, the crisis is also an opportunity. In the complexity of this world, a Chinese scientist opened a window for the understanding of human beings ourselves as well as the universe. The Earth is the direct environment for the survival of human life, but the root cause of human creation is Milky Way. Surprisingly enough, the life of galaxies is determined by a cubic algebraic equation. Therefore, the general public all have the potential to understand the lives of galaxies. Coincidentally, with human invention of computers, the general public has the potential to run the simple computer program (See Appendix of this paper) to generate and study galaxy snowflake chart (a simple graph expressing the internal structure of galaxies). Therefore, we see the hope of mankind: A harmonic general-public administrated society of new civilization rather than controlled by a few elites, will inevitably be born!

  6. Definition of a galaxy– The definition of a galaxy may depend on the background and expertise of the researcher:
    a cosmologist may call a galaxy any DM halo, as this is (or for long has been) how galaxies were modeled in their numerical simulations (which initially did not include any baryonic matter)
    – other possible additional definition:
    + A definition depending on the large scale environment: a globular cluster kicked out far from his host galaxy, and independent from it might become a galaxy when in the IGM. A satellite galaxy bound to its host is then a stellar system… Like the definition of planets vs moons.
    + There is evidence for global scaling relations linking the massive ellipticals to the faint dwarf ellipticals:
    surface brightness vs size, luminosity, mass vs SFR, etc …
    Not only the size matters. Then different families of objects may be determined
    + Presence or not of a central black hole
    – it is mentionned in the paper “if being gravitationally bound is a requirement to be a galaxy, then collections of tidal material are not galaxies”
    -> this may be a bit confusing as TDGs are gravitationally bound systems (this is at least my definition of such systems).
    – absence of dark matter in TDGs:
    the story is a bit more complex, as ther is evidence for a missing mass component in TDGs, confirmed in several objects. However, this missing component exceeds by a factor of 2-3 the visible one, and not 10 like for the DM dominated galaxies.
    It most likely consists of baryonic matter, the presence of which is extremely difficult to assess with direct observations (like cold molecular gas)
    – other special cases: W3 in the merger NGC 7252, extremely massive GC, UCD about to be kicked out?
    A lot of literature and debate on this object
    – VCC 2062: the main criteria to think it is a Tidal Dwarf Galaxy vs a pre-existing disrupted system is the high metallicity and high molecular gas content.

  7. Biology has (maybe) solved thisModulo the “I know it when I see it” iteration, which also creeps into the paper, on the cutoff of small systems:
    I [a layman; but we seems invited] is always looking at biology for this, because they have a serious problem in classifications (naming), relationships (here, phylogeny) and populations (species problem).
    What they do is to separate classifications, which may be artificial and convenient, from natural relationships which follows from the processes the populations obey.
    Having that analog, and being clear that it is an analogy which so always a risky business, much of the same happens in astronomy. Dynamics of systems (i.e. processes) could and, looking at how it works rather well in biology perhaps should, decide.
    First, we can look at the more clear (I take it) status of planets. Obviously from the analogy a sufficiently good choice out of the possible ones is currently used. The “clearing the neighborhood” criteria separates out a natural population (planets) from others (belt objects). This is in the same way that a “biological species” definition would pick out eukaryote species of the 26+ possible species conceptualization candidates.
    And as for species conceptualizations there is a fuzziness since the processes which separates populations act gradually, so there _should_ be funny cases or it isn’t nature. Say, planets-to-be while undergoing clearing.
    Second, we can look at that list of galaxy criteria for processes naturally resulting in (for the purpose) distinct populations. Gravitation results in several populations, so isn’t enough. A dark matter halo is a possible criteria, it fails in some cases as expected. And so on.
    The definition of “clearing/dominating the neighborhood” particularly seems like a keeper, since galaxies interact and coalesce while growing; and it fails in some cases as it should.
    The latter criteria seems to me then like a candidate for a more general astronomical relationship classification, looking at behavior of, and processes resulting in, sufficiently isolated systems to work with as examples of individuals out of a population. Using satellites as a practical realization of such a criteria may be a good compromise.

  8. What is a galaxy?The way I reason about galaxy as defining the crux of the matter is this:
    The gravitational core of the galaxy has greater influence on the entire stellar structure than other gravitational foci within or outside it. It should be qualified as self-bound (self-binding) for any substructures within it.

  9. UNA GALAXIA ES UN CICLÓN CÓSMICO QUE EVOLUCIONA LA MATERIAThis comment has been deleted but we kindly suggest the author (Justo Ruiz Ortis) to consider submitting the same comment in English.

  10. The definition should be chosen so that it is most fruitful. You don’t want to see papers saying “Galaxies, except for X ones, ” all the time.

  11. A Galaxy is the result of an evolutionary processA new Galaxy is formed from the ejection of energy and mass from the Galactic Core Object at
    the centre of an existing galaxy. (in fact, as the energy and mass ejection is usually from
    both sides of such a galaxy), then we must expect to see two galaxies form from such
    ejection; one at each side. This is an evolutionary event that marks the point of maximum
    mass for any galaxy; the point where one galaxy must thereafter lose energy and mass to
    permit the evolution of the new. The surrounding disc of stars form the accretion disc for
    the Galactic Core Object at the centre of the galaxy. So a galaxy has to have both a core
    object and an accretion disc.
    As we must also expect that the revolving Galactic Core Object will precess; then there will
    be times when such precession will throw out smaller energy and mass ejections, out of line
    of the main ejection, to form smaller globular clusters scattered around the eventual
    galaxies so formed.
    The energy, (from the sometimes billions of stars that have formed the full mass of the
    Galactic Core Object), is stored in the balanced gravity band between the inner and outer
    event horizons inside, INSIDE, the structure of the Galactic Core Object. The breakup of the
    outer ring of mass, (and thus the process that releases the energy and mass), is described
    in great detail in chapter 42, The Whirlpool Galaxy in a new light, The Universe is a Cloud
    of Surplus Proton Energy
    This is exactly the same process, but on a very much larger scale, that forms the energy
    ejections from Planetary Nebula and is the well known, but never publicised, “big (and often
    disorienting) leaps forward” that was written into the final paragraph of “The Extraordinary
    Deaths of Ordinary Stars”, Bruce Balick et. al. Scientific American, July 2004, page 35.
    A galaxy is thus an object formed from energy and mass ejected from an older galaxy where,
    over a very great period of time, the central mass of the newly ejected energy and mass has
    come together to form a galactic Core Object sufficiently massive to bring the rest of the
    energy and mass into an accretion disc. The stars within a galaxy cannot “bump” into each
    other because they are all gravitationally attached to each other and balanced gravity
    effects within such attachments; balance out the forces and stabilise the whole entity.

  12. Apologies, try this instead.A new Galaxy is formed from the ejection of energy and mass from the Galactic Core Object at the centre of an existing galaxy. (in fact, as the energy and mass ejection is usually from both sides of such a galaxy), then we must expect to see two galaxies form from such ejection; one at each side. This is an evolutionary event that marks the point of maximum mass for any galaxy; the point where one galaxy must thereafter lose energy and mass to permit the evolution of the new. The surrounding disc of stars form the accretion disc for the Galactic Core Object at the centre of the galaxy. So a galaxy has to have both a core object and an accretion disc.
    As we must also expect that the revolving Galactic Core Object will precess; then there will be times when such precession will throw out smaller energy and mass ejections, out of line of the main ejection, to form smaller globular clusters scattered around the eventual galaxies so formed.
    The energy, (from the sometimes billions of stars that have formed the full mass of the Galactic Core Object), is stored in the balanced gravity band between the inner and outer event horizons inside, INSIDE, the structure of the Galactic Core Object. The breakup of the outer ring of mass, (and thus the process that releases the energy and mass), is described in great detail in chapter 42, The Whirlpool Galaxy in a new light, The Universe is a Cloud of Surplus Proton Energy
    This is exactly the same process, but on a very much larger scale, that forms the energy ejections from Planetary Nebula and is the well known, but never publicised, “big (and often disorienting) leaps forward” that was written into the final paragraph of “The Extraordinary Deaths of Ordinary Stars”, Bruce Balick et. al. Scientific American, July 2004, page 35.
    A galaxy is thus an object formed from energy and mass ejected from an older galaxy where, over a very great period of time, the central mass of the newly ejected energy and mass has come together to form a galactic Core Object sufficiently massive to bring the rest of the energy and mass into an accretion disc. The stars within a galaxy cannot “bump” into each other because they are all gravitationally attached to each other and balanced gravity effects within such attachments; balance out the forces and stabilise the whole entity.

  13. Previous four commentsDefining a galaxy as an object with a central super massive black hole would imply that the large number of pure-disk “galaxies” would not be galaxies. For example, our neighbour Triangulum (M33) would then not be a galaxy.
    There is no convincing evidence nor any theoretically known process for new galaxies being formed as ejected objects from the cores of older galaxies. This hypothesis would need to explain how the matter within the new galaxies emerges. Also, there would be galaxies that are much older than others, which is not observed.

  14. What is a Galaxy Name?How much does it really matter how we define “galaxy”? Yes, we often assign distinct boundaries to sometimes arbitrary divisions in what is essentially a continuum which nonetheless has, for lack of a better term, basins of attraction. However, when you are near the boundary, you cannot see which basin you belong to, whether this is because of fractal blurring or because the boundary is not precisely defined. For example, you can easily distinguish the back of your head from your face, but there is no distinct boundary. Yet it is very reasonable to give distinct names. When you are dealing with a spot on the side of your head, you resolve possible confusion be using a locally relevant name, such as “ear” or “mastoid process”.
    By analogy, it seems no more fruitful to define the boundary of “galaxy” than it would to say exactly where your face ended.
    Perhaps this urge to name and impose an order of categories, with a precision that does not obtain, arises from some primitive instinctual drive that we as scientists and rational people should strive to overcome rather than indulge.
    The implication is that it doesn’t matter where exactly you place the edge of the set of all galaxies. If your research results depend strongly on the exact domain of your subject, then you had best define it precisely anyway. It seems unlikely that many results would be so dependent. If they were, then it also seems unlikely that the dependence would coincide exactly with the definition of galaxy, no matter what that definition was.
    I hope that participants in the next astronomy meetings will have better ways to spend their time, unless of course, it is in the evening over beers, in which case, I hope they invite me!

  15. Food for thought: Life cycle of a galaxyTo determine what is and wnat is not a galaxy, we should examine the lifecycle of a galaxy. This leads to a number of questions:
    o How are galaxies formed (born) & how do they die.
    o When does a star cluster become a galaxy & why.
    o Once a galaxy always a galaxy? Is a “dead galaxy” not a galaxy any more.
    This is akin to stars:
    o Is a gas cloud a star? (Not yet, but could be)
    o Is a star a star even if it has no planets orbiting it? (Yes)
    o Is a small star not a star? … Are only big stars stars? (No)
    o Is a dwarf or dark star still a star? (Yes)
    o Is a black hole formed by the death of a star still a star? (No it metamorphed)
    o Is a nebula formed by a super nova a star? (Not any more)
    o Is the projected longevity of a star used to determine whether it is, or was a star or not? (No, not until it morphs into something else)
    The real questions are then:
    o What are the stages in the galactic lifecycle.
    o It would be convenient if there a single observable event that signifies the birth of a galaxy.
    – Is there one?
    – Is it a minimum critical Mass?
    – Is it the formation of a galactic nucleus
    – Is it when the nucleus becomes a massive black hole
    o What are the different sub-classes of galaxies. Can or do they die in different ways?
    o Is there such a thing as a juvinile galaxy? If so, what is it. Is it required to have a galaxy nucleus, a minimum size, or are there other criteria?
    o A juvinile galaxy may be an environment which will not allow it to mature. Since it will always be or die as a juvinal galaxy Is it still a galaxy?
    o If a galaxy dies, is it still a galaxy? (Did it morph? if so, into what did it morph? is it now a UCD?)

    My vote:
    o A relaxation time that is greater than the age of the Universe (No – Why longevity – Just a different class of galaxy)
    o A half-light radius greater than 100 pc (No – Big or small, still a galaxy)
    o Hosts a satellite stellar system (No – Satelite or no, still a galaxy)
    o The presence of complex stellar populations (This could be part of it)
    The presence of dark matter (This could be part of it)
    o Are there other criteria like:
    – Formation or existance of a galactic nucleus
    – Minimum mass – Could this be related to wheter or not it has a nucleus?
    – Presence of a massive black hole in the center?
    – Was it ever a galaxy and then morphed to a dwarf or other form?
    – ????

  16. What is a galaxy?To answer this question you MUST first ask a more important question : What is gravity, inertia, empty space and how do they all interact to generate the conditions that form a galaxy, star cluster or collision of two galaxies that form very large black holes.
    The answer to gravity and inertia can be explained in the following 3 videos:
    Artificial Gravity | Cosmic Matrix Theory Pt. 1 (Full Movie)
    Artificial Gravity | Cosmic Matrix Theory Pt. 2 (Full Movie)
    Artificial Gravity | Cosmic Matrix Theory Pt. 3 (Full Movie)
    At least this is a start.

  17. Conventions in nomenclatureI would argue that naming conventions are starting to follow a pattern. Where at all possible, fundamental properties are used. In the case of galaxies, there really aren’t any fundamental properties – at least that anyone knows of.
    Next up, useful definitions often draw heavily on theory, such that what the theory includes the definition includes and what the theory excludes the definition excludes.
    Our current theory of galaxies is that they form around a supermassive nucleus, such as a supermassive black hole. This would make a galaxy more like a gigantic accretion disk rather than a cluster.
    If we follow this argument, then a galaxy could be defined as an accretion disk or stably orbiting dust cloud of such size and density as to allow the formation of stars, the full stellar lifecycle and stable solar systems for those stars.
    If theory diesn’t really offer any answers, or not enough of any, then you can look for statistical anomolies that are universally present in what is observed as a galaxy and universally absent in anything that is definitely not a galaxy.
    Statistical clustering tends to be how genetic genealogy and genetic archaeology works.
    I’d argue that a globular cluster need not have any central object. There’s no actual requirement for it. However, every definite galaxy that has been studied in any depth has definitely got an object that is both at the centre of mass and the centre of rotation for both itself and the galaxy.
    Finally, the morphological classification system. Direct comparison of forms. Largely abandoned by many disciplines (way too many errors, the most recent to be discovered being the misclassification of the Egyptian jackal) and problematic where it is still used (astronomy and paleantology being the two most notable holdouts) because it creates massive problems.
    The definition of a planet is morphological. There’s no fundamental property that I can think of, but we’ve plenty of theories of planetary formation and our knowledge of geology is adequate. It would be possible to create unique definitions for planets, comets and asteroids that “just worked” and could be stepwise refined as the theory was refined rather than ever being replaced wholesale for reasons of convenience.
    (The new definition was not written to handle exoplanets at all and cannot be trivially adjusted to work in many such cases. As definitions go, this puts it somewhere between useless and stupid.)
    Likewise, a definition of a galaxy that “just worked” rather than being politically convenient and only really suitable for some specific galaxies and not others would seem to make much more sense.
    A universal definition that draws from theory and therefore is refined as the theory is refined and applies everywhere the theory applies is the logical, scientific, rational approach.

  18. A chemical answerIt is several years that I find the following definition as very intriguing:
    A galaxy is a self-gravitating system that was/is the site of chemical evolution driven by Supernovae.
    This is (a) relatively easy to check observationally, since a spread in IRON abundance must be observed, (b) it is related to the deepness of the ORIGINAL potential well, not the the present-day one that could gave been strongly modified by evolution/environment, (c) it is dark-matter independent but it is sensitive to the presence of dark matter: a system with only 100 stars showing a spread in Iron must have an unseen mass sufficient to retain SN ejecta, so the criterion includes “dark” galaxies – on an observational basis (d) excludes ordinary globular clusters whose “multiple populations” are traced by spread in light elements (Na,O,Mg,C,N,He) that are likely due to chemical evolution driven by much less energetic polluters (AGB or FRMS). On the other hand UFG are galaxies (as far as real Fe spread is observed), Omega~Cen and G1 are nuclear relics of galaxies (see Bellazzini et al. 2008, AJ, 136, 1147 and Carretta et al. 2010ApJ…714L…7). I put the above ideas into a relatively detailed scheme that I presented at a meeting lat year.

  19. A rose, by any other nameI think the idea of categorizing has been powerful, but also sometimes deceptive in the past, and we should be careful when using it today. Are “transition-type” galaxies really transient, or merely objects with intermediate properties in some parameter space? Are there really five distinct species of dwarf galaxies, or are there perhaps only three, or maybe six? Rather than defining multiple sub-categories, it would often make more sense to talk about objects simply in terms of their physical properties, such as their stellar mass, sizes, stellar ages, gas content, dynamical mass etc., whichever is relevant to the question posed.
    But of course, some abstraction is useful when we’re trying to get a qualitative understanding. What we call a galaxy then depends on our concept of galaxy formation. While there does not seem to be a lower limit for stellar mass in galaxies, there appears to be a lower limit for dynamical mass. It is therefore not unreasonable to assume that galaxies form in dark matter haloes, according to processes which either become very inefficient (e.g. cooling), or strongly self-regulating (e.g. feedback) at very low masses. The concept that a galaxy is an assembly of gas and stars which lives in a dark matter halo has yet to be falsified. In this sense, all galaxies are created equal, meaning in the same way, and with the common property of galaxyness. Galaxy interactions apparently complicate the picture (i.e. when does a satellite galaxy become a stellar stream), but at this point, we fortunately have more than one word at our disposal to describe the situation.
    If any of these assumptions change, my definition of a galaxy also changes. But that’s just to say that words have meanings.
    As to the proposed five criteria:
    1) Galaxies should be relaxed (apart from mergers), but there are many other relaxed objects in the universe that aren’t galaxies.
    2) The half-light radius is a concept that does not scale to very small systems, which can still be galaxies in the formation-sense.
    3) The question of whether a galaxy must contain multiple stellar populations is moot; the equivalent of asking whether life begins at, or only after self-replication.
    4) The presence of dark-matter in itself is not sufficient; but as I described above, its role in the formation is important.
    5) The presence of satellite galaxies becomes self-referencing.

  20. @ Till Sawala: A rose, by any other nameTo point 1) “Galaxies should be relaxed”:
    Please do note that a galaxy cannot be relaxed, because the two-body relaxation time is longer than a Hubble time.
    A typical open or globular star cluster, on the other hand, has evolved significantly towards a relaxed state, which, however, can never be achieved in a self-gravitating stellar dynamical system (negative specific heat capacity).
    See also Question B2.
    A galaxy can be phase-mixed and virialised, i.e. it is a solution to the time-independent collisionless Boltzmann equation. This is the case because the time it takes a star to orbit through a galaxy is typically much shorter than a Hubble time (e.g. in our Milky Way it is a few hundred million years).

  21. big spaceWhat if space is big, really really big. Some time in the past an event occurred in our part of this really really big space and eventually became our detectable universe. Since one event occurred it is just as likely that many events occured. Perhaps there are galaxies of universes and clusters of galaxies of universes in the cosmos. They would be undetectable.
    What if gravity just is, everywhere all at once. A distant universe would exert some minute gravitational pull, but lots and lots of effects add up.

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