Lecture 23: Galaxies

Galaxies Beyond the Milky Way

1. Suppose other star systems similar to the Milky Way exist, so that they are confined to a disk.
2. Suppose that this other star system is separated from the Milky Way by a distance which is larger than the diameter of the Milky Way.
3. This far away star system will look like a circular fuzzy blob when we observe it.

The Spiral Nebulae, Messier Catalogue

• Faint, fuzzy patches of light (which stay in a fixed position relative to the stars) have been known for centuries.
• Charles Messier catalogued these fuzzy patches of light from 1758 to 1782.
• The objects in Messier's list can be put into two groups:
  1. Objects inside of the Milky Way
     • Open clusters (or galactic cluster): groups of stars found in the plane of the Milky Way. (Eg. the Pleiades)
     • Globular clusters: groups of stars found in the spherical halo of the Milky Way.
     • Emission nebulae: cloud of glowing gas
     • Reflection nebulae: cloud of gas and dust shining from light reflected from stars.
     • Planetary nebulae: the gas ejected from a Red Giant star
     • Supernova remnants: the glowing gas left over after a supernova.
  2. Objects outside of the Milky Way (in other words other galaxies):
     • Spiral Galaxies
     • Elliptical Galaxies
     • Irregular Galaxies

In 1845 Lord Rosse built a telescope with a 6 foot primary mirror.

• Lord Rosse was the first astronomer to see details on some of Messier's objects and to identify spiral arms.
• The objects with spiral arms were called spiral nebulae.

• For the next 80 years there was a great deal of debate about the spiral nebulae.
• Some people thought that these were spiral shaped gas clouds which were inside of the Milky Way galaxy. This view suggested that the Milky Way galaxy is the entire universe and that nothing exists outside of the Milky Way.
• Other people agreed with Kant's idea that the Universe is filled with galaxies similar to the Milky Way, and that the spiral nebulae are other galaxies far from us.
• Since the distances to the spiral nebulae were unknown, it was impossible to resolve the debate. Famous public Shapley-Curtis debate about "the scale of the Universe" took place in 1920.

Hubble's Measurement of the distance to Andromeda

• In 1923 Edwin Hubble photographed the fuzzy object in the constellation Andromeda which we now call the Andromeda galaxy (M31).

• Hubble discovered that he could resolve Cepheid Variable stars in his photographs.

• He made use of the Cepheid variable Period-Luminosity relationship to find the distance to Andromeda.

• Hubble found that the distance to Andromeda is larger than the size of the Milky Way (which Shapley had measured).

• This proved that Andromeda is another galaxy and that the Milky Way is just one of many galaxies in the Universe.

• The modern distance to Andromeda is 770 kpc. (Diameter of Milky Way is only about 30 kpc.)

Hubble's photo of M31	Modern photo of M31

Modern Observations of Distant Galaxies

• Hubble Space Telescope was pointed to a patch on a sky in 1996, and then in 2003 and observed for very long time, obtaining Very and Ultra "deep" (high sensitivity) image

Hubble Ultra Deep Field 11 square arcmin (one tenth of full moon) 11 days exposure (400 orbits around Earth) 4 colours ~ 10 thousands galaxies

Hubble's Classification of the Galaxies

• Hubble created a classification system based on the shape of the galaxy to describe the different types of galaxies observed.

• Galaxies are classified as either: Spiral, Barred Spiral, Elliptical, Lenticular or Irregular.
• Further subgroups exist within each type of classification.

A Spiral Galaxy	A Barred Spiral Galaxy
An Elliptical Galaxy	A Lenticular Galaxy	An Irregular Galaxy

Spiral Galaxies

M 65: Type Sa	M 63: Type Sb	NGC 2997: Type Sc

• Hubble classified the spiral galaxies based on the relative brightness of the central bulge and the disk and on the tightness of the spiral windings.
• All spiral galaxies are given the letter "S" and are followed by the lower-case letter a, b, or c.
• Sa galaxies have the brightest central bulges and very tightly wound arms which are often not very prominent.
• Sa galaxies have I_bulge/I_disk ~ 0.3
• Sb galaxies have less bright central bulges and more prominent arms than Sa galaxies.
• Sc galaxies have the dimmest central bulges and have loosly wound arms which are very prominent.
• Sc galaxies have I_bulge/I_disk ~ 0.05
• Galaxies which are intermediate between Sa and Sb are labelled Sab. Similarly Sbc are between Sb and Sc.

A Grand Design Galaxy Sc spirals have very prominent spiral arms. They are often called "Grand Design Galaxies". Note that groups of young blue-coloured stars, red ionized hydrogen regions and dust lanes are distinctly shown in this photo.

Barred Spiral Galaxies

• The classification of barred spirals is similar to the classification of regular spirals.
• Barred Spirals are denoted: SB followed by a, b, or c.
• SBa galaxies have the brightest nuclear bulges.
• SBc galaxies have the least bright nuclear bulges.
• In order of brightest nuclear bulge to least bright: SBa, SBab, SBb, SBbc, SBc.

• About half of all spiral galaxies are barred spirals.
• All spirals (barred or regular) have young stars and signs of star formation in the arms of the galaxy.
• The star formation makes the arms look sort of blue in colour.
• The nuclear bulges typically have older stars which are redder in colour.

Elliptical Galaxies

• Ellipticals are classified by their shape.
• Ellipticals are denoted by: E followed by a number from 0 to 9
• The number is given by 10x(1-b/a) where
  b = the apparent size of the short axis
  a = the apparent size of the long axis

• It should be remembered that the shape which we see may not be the true shape.
• A football shaped galaxy will look like a circle if viewed from the right direction.

• Elliptical galaxies have much less gas and dust than spiral galaxies.
• Elliptical galaxies have very few young stars.
• Most stars in ellipticals are old red stars.
• The stars in elliptical galaxies formed long ago, and there has not been a new episode of star formation.

Lenticular Galaxies

NGC 5866: Type S0	NGC 4251: Type S0	NGC 4340: Type SB0

• Lenticular galaxies are disk-like galaxies with no obvious spiral structure.
• S0 denotes a regular lenticular.
• SB0 denotes a barred lenticular.

Irregular Galaxies

Large Magellenic Cloud: Type Irr I	Small Magellenic Cloud: Type Irr I	The Cigar Galaxy: Type Irr II

• Any galaxy that isn't a spiral, barred spiral, elliptical or lenticular is called irregular.
• Type I irregulars are denoted: Irr I and have a hint of spiral structure.
• Type II irregulars are denoted: Irr II and correspond to everything else.
• The irregular galaxies often show signs of star formation.
• The Large and Small Magellenic clouds are nearby galaxies which orbit the Milky Way.
• The tides caused by the Milky Way might be deforming the Magellenic clouds.
• The Magellenic clouds can be seen without a telescope from the Southern hemisphere.

Properties of the Galaxies

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Distances to the Galaxies

• Determinning distance to far away object requires sequence of steps of using "standard sticks", "standard candles" or established relation between properties that depended on distance. We speak about distance ladder For the Galaxies:
  1. Determine distance to nearby stars using parallax.
  2. Identify one which can serve as standard candles - e.g variable stars
  3. Calibrate period - luminosity relation for nearby Cepheids and RR-Lyrae by establishing their absolute luminosity, since distance have been measured.
  4. Use RR Lyrae to determine distance to globular clusters
  5. Use Cepheids (much rarer) to establish distance to nearby galaxies
  6. Having distance to nearby galaxies, calibrate Tully-Fisher relation - proportionality between luminosity of spiral Galaxies and how fast spiral galaxy rotates
  7. Measure rotation of far away spiral galaxies (Doppler shift) and use Tully-Fisher relation to find their absolute luminosity, and therefore the distance from brightness.
  8. Use theoretical knowledge of SNIa as standard candles, calibrate luminosity - decay time based on nearby (very rare) supernovae, and use SNIa to measure very far distances.
• Errors in early steps propagate throughout the ladder. E.g error in parallaxes of 10% means we will know all the distances in the Universe only up to 10%.

Groups and Clusters of Galaxies

Galaxies do not exist in isolation. The Milky Way has a number of small satellite galaxies (including the Large and Small Magellenic Clouds) which orbit the Milky Way. Together with Andromeda galaxy (M31) and some small galaxies in between it forms The Local Group groups of galaxies have several to several dozen members. Masses of groups are ~1013 MSun

This photo of the Andromeda Galaxy shows 2 small elliptical galaxies which orbit Andromeda.

The Milky Way and Andromeda are the two largest galaxies in a group of about 30 galaxies called the Local Group. The Local Group is about 1 Mpc = million pc in radius.

Clusters of Galaxies

At large scales some galaxies are forming clusters of galaxies Galactic clusters are high concentrations of galaxies kept together by mutual gravity. Galactic clusters contain thousands of galaxies, in the volume of 2-3 Mpc across (the distance to Andromeda is 0.7 Mpc) Masses of galactic clusters are 1014-1015 MSun The Virgo Cluster of Galaxies The Virgo Cluster is 30 Mpc from the Local Group There are about 2000 galaxies (of all types) The radius of the cluster is about 2 Mpc

The Coma Cluster of Galaxies The Coma Cluster is about 100 Mpc from the Local Group Almost every object in this picture is a galaxy There are more than a 1000 galaxies The radius of the cluster is about 1 Mpc This cluster is much denser than the Local Group