Normal Galaxies

Reading: Chapter 24: Normal Galaxies

Most Distant Galaxy Seen by HST with redshift z ~ 7.6

Galaxies are clusters of stars, gas, dust, and dark matter which serve as markers in our study of the structure of the Universe. Shown below are two clusters of galaxies, the Coma Cluster and Abell 2218. Each contains thousands of galaxies and, interestingly, the Abell cluster acts a gravitational lens amplifying some galaxies located at greater distances behind it.

Coma Cluster

Abell 2218

Several kinds of galaxies can be seen in the two clusters. Here, we consider the types of galaxies and then present several of their properties.

Hubble developed the following morphological classification scheme for galaxies. Basically, Hubble considered ellipticals, and spirals and barred spirals. However, in addition, there were galaxies which had disks but no arms, S0's (lenticulars) , and amorphous looking things which he referred to as Irregulars.

The Hubble diagram for galaxy classification is not thought to represent an evolutionary scheme. There may, however, be evolution between the different Hubble classes due to collisions between galaxies. The Milky Way galaxy is classified as an Sb galaxy in the Hubble scheme. Despite the fact that the Hubble Sequence is based only on the appearance of galaxies (morphology of galaxies), several physical properties of galaxies vary smoothly along the sequence. We have,

little gas and dust <----------------------> lots of gas and dust
mainly Pop II stars <----------------------> Pop I & II stars
Reddish <----------------------------------> Bluish
little ongoing star formation <------------> star formation
large bulge <------------------------------> small bulge
                  tight,smooth arms <---------> open, loose arms
Mass: 108-1013 MSun (Ellipticals) <--> 1012-109MSun (Spirals)

Origin of Elliptical and Spiral Galaxies

The basic idea is that either an elliptical galaxy or spiral galaxy will form depending upon when star formation occurs in the galaxy formation process. Galaxies are thought to form from the collapse of low-density gas clouds. If the gas turns into stars during the early stages of the process, then we essentially have a bunch of BBs collapsing to form the galaxy. Because stars are small and they are far apart, they don't collide in the formation process. This allows the stars to maintain roughly their initial shape and to settle into a roughly spherical form. In this case, they become elliptical galaxies.

If the gas does not turn into stars quickly, then we have a system of collapsing gas clouds. The gas clouds are much larger than are stars and collide much more readily during the formation stage. The collapsing material thus runs into opposing material as it tries to pass through the equatorial plane. The collisions do not allow the collapsing material to pass through each other and the material is forced to settle into a disk. After the disk forms, star formation begins in earnest and a spiral galaxy is produced. (for an analogous situation, consider the star formation process).

Distribution of Galaxies In Space

Clustering Scales

• Planetary Systems (Marcy & Butler site) ===> ~ 0.5 light day

• Globular Clusters ===> ~ tens of light years

• Galaxy Sizes (Hickson Compact Group 87, Abell 2218) ===> size ~ 0.1 million light years to 1 million light years

• Local Group
  • The Milky Way galaxy and M31 (Andromeda) are the most massive members of the Local Group. Other prominent members are M33 (Triangulum) and the Large and Small Magellanic Clouds (LMC and SMC). Outside of the Milky Way, Andromeda (M31), and Triangulum (M33), the Local Group galaxies are dwarf irregulars or dwarf ellipticals. All told, there are ~45 members in the Local Group. (e.g., Andromeda, M31, Triangulum, M33), Extended Local Group of galaxies ===> size ~ 3 million light years

• Clusters (Virgo cluster, Coma cluster) ===> size ~ 10 million light years
  • The Virgo cluster with its > 2,500 members, at distance ~55 Mpc from the Milky Way galaxy is the closest large cluster. In fact, the Local Group is bound to the Virgo cluster and it is conceivable that someday the Local Group will have a close encounter with Virgo and may, in fact, merge or be subsumed by Virgo. The largest galaxy in Virgo is the giant elliptical galaxy M87 (Hubble type E0 or E1) which sits near the center of the cluster. M87 is ~120,000 light years in diameter and shows strong activity (M87 is an AGN [Active Galactic Nucleus]).
  • The Coma cluster another large cluster contains over 1,000 galaxies and lies ~305 Mpc from the Milky Way galaxy. Interestingly, the central region of the Coma Cluster is dominated by giant elliptical galaxies (NGC 4874 and NGC 4889). Both NGC 4874 and NGC 4889 with diameters of 260,000 light years and 330,000 light years are much larger than the visible disk of the Milky Way galaxy (~100,000 light years)!
    Galaxy Collisions: Galaxies Gone Wild

    Although galaxies are far apart (the Milky Way and Andromeda are around 2,200,000 light years apart), the separations are not that great when compared to the diameters of the galaxies (the Milky Way may have a size as large as 300,000 light years) and with their speeds of approach (the Milky Way and Andromeda approach each other at the rate ~500,000 km/hours--at this rate in 3 or 4 billion years, the Milky Way and Andromeda will be close enough together to interact). It is not far-fetched to suggest that collisions between galaxies have significantly affected galaxy evolution and hence galaxy populations and the appearances of galaxies. For example,

    • Milky Way/Andromeda interaction.
    • the collision of two disk galaxies [Video 1] could produce a structure similar to the Antennae Galaxy
      
      and Mice Galaxy
      
      

    • A collision between a disk galaxy and a spheroidal galaxy could produce a structure very similar to the Cartwheel Galaxy
      

    • Close encounters between galaxies could also lead to mergers. Such mergers could lead to the development of the giant elliptical galaxies found in the central regions of rich galaxy clusters.

  Clusters of Clusters ( Local Super-Cluster) ===> ~ 300 million light years

• Great Wall and Voids ===> 600 & 300 million light years (~ 0.6 and 0.3 billion light years)

• Universe ===> ~ 50 billion light years

Computer simulations of structure formation in the Universe and of a galaxy cluster.

Comment--the CMBR is very smooth ---> the Universe was very smooth at the time of formation of the CMBR (very nearly at the birth of the Universe). The Universe did, however, show structure even at the time of the formation of the CMBR ( BOOMERANG result). This means that the Universe generated all of the observed structure in a short amount of time.