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.
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
B1) What is a galaxy? (this contribution)
C) What are the three best reasons for the failure of the LCDM model?
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 thesis, Hilker 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.