What are the criteria for the classification of galaxies?

Question

There are lots of galaxies, for example, the Milky Way and so on. These galaxies consist of lots of stars. I want to know how galaxies are classified. Is it only by their shape that is caused by gravity? Or are there some other criteria?

Answer 1

There are three main classes of galaxies: Irregulars, Ellipticals, and Spirals. Irregular galaxies, as their name suggests, do not fit into the "normal" classification scheme.

So, how do we distinguish between elliptical and spiral galaxies?

Brighness profile

The radial brightness profile of an elliptical galaxy follows a deVaucouleur law ($r^{1/4}$).

Spiral galaxies have an exponential radial brightness profile, although their central regions ("bulge") also follows a deVaucouleur law.

Star formation

Stars are formed in the spiral arms of spiral galaxies (and can be formed in irregulars), while elliptical galaxies tend to only have old, and consequently low mass, stars.

Components

As far as we can tell, all galaxies consist of a dark matter halo and stars. In addition, spiral galaxies also have clouds of dust and gas. If conditions are right, these can form new stars. (Some ellpticals have a very thin, very hot gas component as well, but there is a lot less of it than in a spiral galaxy).

Kinematics

Spiral galaxies are rotationally supported, while elliptical galaxies are mainly pressure-supported (i.e. they act like an ideal gas, with stars as gas molecules). There are some rotational features present in ellipticals, but they tend to be minor compared to the overall random motion.

A graphical overview of the various galaxy types is usually shown in the Hubble tuning fork diagram. Note that this does not indicate an evolutionary progression from one type to the next.

Answer 2

Alex answers nicely how galaxies can be classified according to their morphology. In astronomy, galaxies are detected using a variety of detection techniques. Especially in the high-redshift (i.e. distant) Universe, galaxies are not easily detected and are only visible using specific methods (although some galaxies show up with multiple techniques). These methods each probe different things, and galaxies belonging to one class will thus have other parameters than galaxies belong to other classes, although there will always be ome overlap.

The physical properties defining whether a galaxy may be selected by a given technique is hence not only morphology, but also stellar mass, star formation rate, dust mass, size, clumpiness, kinematics, luminosity, the presence of active galactic nuclei, and many others.

Accordingly, we ofted classify galaxies from the method and the selection criterion used (and preferably use a three-letter acronym to describe them):

Some of these are:

Lyman-break galaxies (LBGs)

The technique used to find these galaxies revolutionized the field in the mid-90'es (Steidel et al. 1996). The reason is that a large field of view can be investigated, allowing to detect many galaxies at the same time. The idea is to observe the same field in several different wavelength bands. If large amounts of neutral hydrogen is present, wavelengths shortward of the "Lyman-break" at 912 Å, or 91.2 nm, needed to ionize hydrogen are absorbed, effectively making the galaxy invisible in all bands shortward of this. And because the light is redshifted as it travel through the Universe toward us, galaxies at different redshifts will drop out of different band (the method is also called the "drop-out technique"). An example is seen here, where the galaxy spectrum (black line) drops steeply so that flux enters the red $R$ band and the green $G$ band, but not the ultraviolet $U$ band:

^{Credit: J. Fynbo.}

In the above figure, the break has been redshifted to somewhere between the $G$ band and the $U$ band, constraining its redshift to roughly $z = 3$-$4$. To further constrain the redshift, spectroscopic follow-up is needed.

Since large amounts of neutral hydrogen is needed, this technique will tend to select massive, and hence rather evolved galaxies.

Lyman alpha emitters (LAEs)

When an electron decays from the first excited state to the ground state, a s-called Lyman alpha (Ly$\alpha$) photon is emitted. This can happen either when a hydrogen atom is perturbed (in a collision) and excited, or if hydrogen is ionised and recombines. Both mechanisms are at play when galaxies are born, where gas accretes onto a central potential (causing collisions), and young, massive star ionize the surrounding gas.

Galaxies found from their ability to emit Ly$\alpha$ are called LAEs. They can be either found either spectroscopically — where are strong emission line will be seen at $\lambda = 1216$ Å — or photometrically by observing the field in a broadband and a narrowband centered at $\lambda = 1216$ Å and looking for excess flux in the narrowband.

Since this techniques tends to probe young galaxies, they will often be relatively small, but with a high star formation rate. And because dust absorbs Ly$\alpha$ more easily than other wavelengths, LAEs tend to be rather dust-free.

Sub-millimeter galaxies (SMGs)

If a galaxy, on the other hand, is very dusty, it can be difficult to detect in the optical and, especially, in the ultraviolet. The reason is that dust has a strong preference to absorb light with shorter wavelengths. But the energy absorbed must go somewhere, and is thus emitted again, although at longer wavelengths, i.e. in the infrared and in the sub-mm region. Galaxies found this way are referred to as SMGs. Because it takes some time for the dust mass to build up, this technique tends to probe evolved, massive galaxies.

Damped Lyman alpha absorbers (DLAs)

The three techniques described above all have in common that they detect galaxies from their emission. A complimentary technique is looking for absorption features in the spectrum of a bright background source, e.g. a quasar. Because the light is redshifted on its way, especially hydrogen but also metals such as iron and magnesium produce absorption lines at various places in the spectrum corresponding to the wavelength that the quasar light has been redshifted to at a given point in space. Diffuse hydrogen filaments make narrow absorption line known as the Lyman $\alpha$ forest, and when a large pocket of gas is present — which indicates the presence of a galaxy — a broad ("damped") absorption line is produced.

An example is seen in this spectrum of the quasar Q2348-011 lying at $z=3.0$. An intervening galaxy at $z=2.6$ causes the broad absorption at $\lambda\sim4400$ Å.

^{Credit: Laursen (2010).}

Only in few cases is the galaxy responsible for the absorption found. This is partly because the light from the quasar outshines everything in its (projected) vicinity, but possibly also because the huge hydrogen cloud is a galaxy in the making, that perhaps hasn't form many stars yet. And since the probabiliy for sightlines toward quasars of hitting a small galaxy is larger than hitting a large galaxy (due to the total cross section of small galaxies being larger), galacitc counterparts of DLAs should tend to be small. Thus, DLAs are thought to probe young galaxies in the process of forming.

Other types

Other types includes distant red galaxies (DRGs), (ultra)luminous infrared galaxies (LIRGs and ULIRGs), and gamma-ray burst host galaxies (GHGs). One of the big challenges of astronomy is to ascertain how the galaxies of the different group fit in some big picture. Is there for instance an evolutionary sequence from DLA→LAE→LBG→SMG→DRG (see e.g. Gawiser 2005)?

Answer 3

A galaxy is just a very large collection of stars (and interstellar matter such as dark matter, gas and dust) held together by gravity. Galaxies are classified mainly as elliptical, spiral, and irregular. There is not a specific criterion other than the fact that the stars all all bound by their mutual gravitational attraction. It is speculated that most galaxies have a black hole at its center. This is not necessary however for a collection of stars to be considered as a galaxy.

Answer 4

The primary, traditional classification for galaxies in the local universe is based on "morphology" -- in other words, on their optically visible shape; this goes back to the Hubble Sequence.

I'll list the main categories and the defining shape, and then some other characteristics which are not part of the main criteria.

Elliptical Galaxies: These are circular or elliptical in projected shape (ellipsoidal or triaxial in 3D shape), with no visible disk and very little gas or dust, and little or no evidence for young stars.

The stars are almost all old, and tend to orbit in random directions. Very luminous/massive ellipticals tend to have centrally concentrated radial profiles in the stellar density (now usually described using Sersic profiles with high values of the index $n$); faint, low-mass "dwarf ellipticals" have more exponential stellar profiles.

S0 (or Lenticular) Galaxies: These have a prominent disk of stars, but one which lacks visible spiral arms and has little or no gas or dust, and little or no evidence for young stars. The disk may, however, have one (or sometimes two) stellar bars, and sometimes rings as well.

The stars are mostly old and almost all orbit in the same direction within the disk, but the orbits may be somewhat elliptical rather than circular. They almost always have a prominent "bulge" of stars dominating the middle of the galaxy; the bulge may be a very centrally concentrated part of the disk, the vertically thickened part of a stellar bar, or a round collection of old stars with mostly random orbits (somewhat like a small elliptical galaxy) -- or a combination of all three.

Spiral Galaxies: These have a prominent disk of stars, gas and dust; the disk has spiral arms in it (hence the name). The subclassifications within this category (e.g., Sa vs Sb vs Sc vs Sd) are based on a combination of three factors: the relative prominence of a central bulge (if any); how tightly or loosely wound the spiral arms appear to be; and the degree to which the spiral arms are smooth versus being broken up into fragments and stellar clusters.

The stars and gas almost all rotate in the same direction, with orbits that are relatively circular. They are almost always a mix of young and old stars, with new stars being formed in the disk. They may have a bulge in the center, but some do not; the bulges may be as diverse and complicated as those in S0 galaxies.

Irregular Galaxies: As the name suggests, these are more raggedy, lopsided, and generally "shapeless". They are usually rich in gas, and are almost always lower in mass than the other types; they are, like spirals, often forming stars at the present time.

There are a number of different kind of dwarf (= faint, low-mass) galaxies which may or may not fall neatly into the above categories. For example, dwarf spheroidal galaxies are very faint and low-mass; in terms of structure, stellar orbits, and the absence of gas or current star formation, they resemble ellipticals, but are very diffuse rather than centrally concentrated. Recent and still somewhat mysterious discoveries include "ultracompact dwarf" (UCD) galaxies and "ultradiffuse galaxies".