Galaxy Morphological Classification

**Galaxy morphological classification** is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin Hubbleand later expanded by Gérard de Vaucouleurs and Allan Sandage.

Some information has been received from the Wikipedia Article of the same name.

## **Galaxy Types**

### **Elliptical Galaxies**

> Please see IC 2006 or NGC 3597 for examples.
>
>

**Elliptical galaxies** have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter "E", followed by an integer *n* representing their degree of ellipticity on the sky. Unlike flat spiral galaxies with organization and structure, elliptical galaxies are more three-dimensional, without much structure, and their stars are in somewhat random orbits around the center. They make up the third most amount of all galaxies.

Elliptical galaxies range in size from tens of millions to over one hundred trillion stars. Originally, Edwin Hubble hypothesized that elliptical galaxies evolved into spiral galaxies, which was later discovered to be false, although the accretion of gas and smaller galaxies may build a disk around a pre-existing ellipsoidal structure. Stars found inside of elliptical galaxies are on average much older than stars found in spiral galaxies. Most elliptical galaxies are composed of older, low-mass stars, with a sparse interstellar medium and minimal star formation activity, and they tend to be surrounded by large numbers of globular clusters.

They vary greatly in both size and mass with diameters ranging from 3,000 lightyears to more than 700,000 lightyears, and masses from 105 to nearly 1013 solar masses. This range is much broader for this galaxy type than for any other. The smallest, the dwarf elliptical galaxies, may be no larger than a typical globular cluster, but contain a considerable amount of dark matter not present in clusters.

The Hubble classification of elliptical galaxies contains an integer that describes how elongated the galaxy image is. The classification is determined by the ratio of the major (*a*) to the minor (*b*) axes of the galaxy's isophotes.

10 x (1 - b/a) [*Elliptical Ratio*]

A spherical galaxy will have a ratio of 1:1, which is equal to a galaxy type of E0. In literature, the max is E7, though most E4 to E7 galaxies have been found to be misclassifications due to being highly inclined. But they can still exist for the most part.

All ratios below are with a first and b second.

#### **E0**

Ratio of **1:1**. Basically a near perfect circle/sphere.

### **Lenticular Galaxies**

> Please see NGC 2787 or NGC 6861 for examples.
>
>

**Lenticular galaxies** (denoted S0A or S0B) are a type of galaxy intermediate between an elliptical and a spiral galaxy in galaxy morphological classification schemes. They contain large-scale discs but they do not have large-scale spiral arms. Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. They may, however, retain significant dust in their disks. As a result, they consist mainly of aging stars (like elliptical galaxies). They make up the least amount of galaxies.

Lenticular Galaxies are usually classified by whether or not they have a bar in the bulge and the dust absorption. Subscripts on the 0 are used when referring to the absorption, with 1 being the least (basically with no characteristics) and 3 being the most (with large swaths being dominated by dust).

#### **S0A**

**S0A**, sometimes just called S0, is the more common form of the Lenticular. It is characterized by not having a central bar within the bulge region.

#### **S0B**

**S0B** is the less common form of the Lenticular. It is characterized by having a central bar within the bulge region.

### **Spiral Galaxies**

> Please see Messier 101 or UGC 12158 for examples.
>
>

**Spiral galaxies** consist of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge, which is similar in appearance to an elliptical galaxy. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters. They are given the symbol "S". Roughly two-thirds of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol "SB". Non-barred are denoted "SA". They make up the most amount of galaxies.

The lists below show whether the type has a bar, the common star population type (CSPT) in the bulge, the maximum amount of arms it can have (the minimum is always 2), and the usual density of the arms.

### **Irregulars**

> Please see NGC 1427A or ESO 486-21 for examples.
>
>

**Irregular galaxies** are galaxies that do not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. They make up the second most amount of galaxies.

Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force. Irregular galaxies may contain abundant amounts of gas and dust. This is not necessarily true for dwarf irregulars.

Irregular galaxies are commonly small, about one tenth the mass of the Milky Way galaxy. Due to their small sizes, they are prone to environmental effects like crashing with large galaxies and intergalactic clouds.

#### **Irr-I**

An Irr-I galaxy (**Irr I**) is an irregular galaxy that features some structure but not enough to place it cleanly into the Hubble sequence.

* Subtypes with some spiral structure are called **Magellanic spiral galaxies** (**Sm**) galaxies. They are galaxies with one to three spiral arms, and are named after their prototype, the Large Magellanic Cloud, an SBm galaxy. They can be considered to be intermediate between dwarf spiral galaxies and irregular galaxies.
	+ SAm - Irregular Spiral with no Central Bar.
	+ SABm - Irregular Spiral with intermediate Central Bar.
	+ SBm - Irregular Spiral with Central Bar.

* Subtypes without spiral structure are called **Im** galaxies. The prototype for it is the Small Magellanic Cloud, a barred Im galaxy. They may or may not contain the Bar-structure, but contain no visible evidence of a spiral.

#### **Irr-II**

An Irr-II galaxy (**Irr II**) is an irregular galaxy that does not appear to feature any structure that can place it into the Hubble sequence. These are generally very chaotic looking, and have absolutely no uniform look.

#### **dIrr**

A dI-galaxy (or **dIrr**) is a dwarf irregular galaxy. This type of galaxy is now thought to be important to understand the overall evolution of galaxies, as they tend to have a low level of metallicity and relatively high levels of gas, and are thought to be similar to the earliest galaxies that populated the Universe. They may represent a local (and therefore more recent) version of the faint blue galaxies known to exist in deep field galaxy surveys.

#### **dSph**

A **dwarf spheroidal galaxy** (**dSph**) is a term in astronomy applied to small, low-luminosity galaxies with very little dust and an older stellar population. They are found typically found as companions to larger galaxies quite often. While similar to dwarf elliptical galaxies in appearance and properties such as little to no gas or dust or recent star formation, they are approximately spheroidal in shape and generally have lower luminosity.

## **Nuclear Bars**

**Nuclear Bars** are small Bars that can be found within galaxies. They are located solely in the nucleus, and this are difficult to view in low resolution. It forms in the very core, typically looking like an offset cluster of stars. There are two types, **Leading** and **Trailing**. They orbit at roughly the same speed, making them clearly visible forever until something changes it.

Leading Nuclear Bars lead forward of the main structure of the galaxy by up to 45°. Trailing Nuclear Bars lag behind the main structure of the galaxy by up to 45°. They are believed to be caused by the core forming in an opposite direction of the rest of the galaxy, otherwise they are completely unknown.

## **Rings**

**Rings** are a special shape that happens within a few galaxies, reported in about 2% of all galaxies, though are more commonly seen in spirals. Ring-like structures can occur in three places, though can have multiple instances in certain areas.

**Nuclear Rings** (**nr**) can only occur within the nuclear bulge of the galaxy. They are extremely small and compact and are only able to occur once due the general amount of room in the nucleus there is.

**Inner Ring** (**r**) are rings that occur within the context of the body of the galaxy. They can form anywhere just beyond the nucleus out to the edge of the disk (if spiral) or central area (if other).

**Spiral Ring** (**rs**) are rings that occur within the spiral structure of a spiral galaxy and have spirals itself present. Normally, these are seen in barred spiral galaxies, just at the end of the bar and surrounding it, with the spirals coming off of it instead of the bar. If that didn't make sense, it's basically the ring has a spiral structure within it that is tight enough to form a ring-like structure.

**Outer Ring** (**R**) are rings that occur just beyond the disk / ellipse in the halo of stars surrounding the galaxy. They do not have to have the same inclination as the rest of the galaxy. These can also occur as far out as the gravitational and dark matter influence of the galaxy allows. They are generally really faint, and may not even be visible in visible light.

## **Prominence**

**Prominence** is the general color of a galaxy.

**Prominent A Stars** (**a**) indicates a galaxy has a predominantly *Blue* color to it.

**Prominent A-F Stars** (**af**) indicates a galaxy has a predominantly *Blue-White* color to it.

**Prominent F Stars** (**f**) indicates a galaxy has a predominantly *White* color to it.

**Prominent F-G Stars** (**fg**) indicates a galaxy has a predominantly *White-Yellow* color to it.

**Prominent G Stars** (**g**) indicates a galaxy has a predominantly *Yellow* color to it.

**Prominent G-K Stars** (**gk**) indicates a galaxy has a predominantly *Yellow-Orange* color to it.

**Prominent K Stars** (**k**) indicates a galaxy has a predominantly *Orange* color to it.

Yes, I am aware that there are O, B, and M Class stars, but the first two do not form enough to cause a shift in the coloration of a galaxy and the last one is too dim and is overpowered by the rest. K and M Class have really long lifespans, so a galaxy solely populated by M Class won't happen for almost another half a trillion years, possibly undershooting.