Galaxies on Parade! Hubble Types Edwin Hubble, American astronomer extraordinaire, is responsible for much of modern astronomy, including his distance/redshift relationship (called the Hubble Law). He also developed the classification system for galaxies used today. Hubble’s system identifies four types of galaxies based on their shapes and numerous subtypes based on the galaxy’s deviation from that standard shape. Elliptical Galaxies M32 M87 M60 Centaurus A Elliptical Galaxies are, well, elliptical in shape. •About 1/3 of known galaxies •Range from 6000-300,000 ly in diameter; from 106-1013 solar masses •Smaller ones (dwarf ellipticals; eg. the Sagittarius Dwarf, a satellite of the MW discovered in 1994) are more common •Subtypes range from very spherical (called E0) to elongated (E7). Note the lack of blue in these galaxies. What does that imply? Spiral Galaxies M31 (Andromeda)* M51 (Whirlpool) M104 (the Sombrero) Spiral (S) galaxies come in 3 flavors: •Sa - tightly wrapped arms •Sb - less tightly wrapped (Andromeda) •Sc - loosely wrapped arms (M51) •All are characterized by dust lanes separating bluish arms containing young stars. •Bright •Commonly found •Rotate •Span 80,000-250,000 ly in diameter and contain 109-1012 solar masses Barred Spirals •The “bar” refers to a line of stars/dust/gas in the middle instead of a nice, round nuclear bulge. • These are designated like spirals, but with a B: SBa, SBb, SBc. M91 •The a, b, and c refer to increasing openness of the arms (M91 = SBb, M83 = SABc (SA = intermediate between barred and spiral) M95 M83 Irregular (Irr) Irregular galaxies are just that. LMC M82 –the only irregular Messier Object. Peculiar and Starburst Galaxies Hey! It’s M82 again! Isn’t it irregular? Yes, but its irregularity is due to gas shooting out of its center. This may be due to a large mass of stars forming at the same time near galactic center, hence the term “starburst”. At the right you see a group called “Stephen’s Quintet”. These galaxies have altered their shapes through gravitational interaction with each other. Their shapes are not irregular, but they also don’t fit the other categories. Hence the term “peculiar”. Halton Arp cataloged peculiar galaxies, so many of these are called Arp objects. The Hubble Tuning Fork Diagram Hubble created a diagram to represent how the galaxy types SEEM to be related. Some may be, but there is no evidence to support an evolution of sorts from E to S. Repeat: this does not show any relationships between galaxies, just a pictorial representation of the (Stolen from SDSS) different types. Interactions Galaxies interact gravitationally, with long-lasting consequences: Here is Stephen’s Quintet again, showing X- rays (blue) produced by gas being accelerated between the galaxies. plume The “tadpole” galaxy. The long tail is the result of the tiny bright blue object at the upper left corner of the disk having swept through the larger spiral. The tiny galaxy at the bottom right is losing stars to the tadpole, as shown by the plume coming from the smaller satellite. Measurements show these two galaxies are the same distance from the earth. Ring Galaxies Ring galaxies are the result of collisions between two galaxies. NGC4560A, a polar ring galaxy: two perpendicular rings. This is rare. Hoag’s object (above) and the Cartwheel Galaxy (right) are both the result of a small galaxy passing through a big one. Active Galactic Nuclei AGNs are seemingly normal galaxies whose cores hide very bright radio sources: Centaurus A (optical) Centaurus A (radio) How does that work? Centers of AGNs are often thought to contain black holes. The radio signals are polarized, which suggests synchrotron radiation. Other types of AGNs include Seyfert Galaxies and Quasars. Galaxy Arrangement •The Local Group: Andromeda (M31), MWG, M33 (spiral in Triangulum). •95% of local group’s mass in these 3. •What else? Well…Sagittarius Dwarf, LMC, SMC, the recently discovered and widely disrupted Canis Major Dwarf, and the dwarf galaxies in Ursa Minor, Draco, Carina, Sextans (dwarf), Sculptor, Fornax, Leo I, Leo II, and perhaps the Phoenix Dwarf, and Leo A – all satellites of MWG. •Then M31 has satellites, too: M32 and M110 as well as fainter and more far-out NGCs 147 and 185, the very faint systems And I, And II, And III and possibly And IV, And V, And VI (Pegasus dwarf), And VII (Cassiopeia dwarf), and And VIII. •M33 isn’t alone either. Only one galaxy, GR8, seems not to be a satellite. Other Groups: the Maffei 1 group, which besides the giant elliptical galaxy Maffei 1 also contains smaller Maffei 2, and is associated with nearby IC 342 and a number of smaller galaxies. Highly obscurred by dark dust near the Milky Way's equatorial plane. the Sculptor Group or South Polar Group (with members situated around the South Galactic pole), dominated by NGC 253 the M81 group the M83 group Clusters •Smaller galaxies formed first and then combined to form larger spirals like the MWG. •As galaxies interact, their sizes change •Dynamic friction: when a star (or small galaxy) passes through a galaxy it is slowed down by gravitational interactions and its kinetic energy is transferred to the gas of the larger galaxy, heating it up and expanding it. This is the process by which one galaxy “eats” another one. •It is thought that MWG and Andromeda will collide (they are moving toward each other). The result would be a giant elliptical galaxy. •Size: local group is about 3 Mly in diameter The Virgo Cluster (one part, at least) •About 50 Mly away •Contains 16 Messier galaxies and a total of 200 galaxies altogether. •Local group is an outlying part but moving toward the cluster •Too massive to avoid, our group will eventually merge with it Other Clusters The Coma cluster, as seen from Hubble. Nearly every object is a galaxy. Other clusters… Types of Clusters The Coma cluster is regular, meaning it has a concentrated central core and a spherical shape. Clusters are usually hot X- ray sources. Gas in between the galaxies is heated strongly. The Virgo Cluster is irregular. (Virgo in X-ray) Large-Scale Structure A slice of the sky. Galaxies are plotted as white dots. Large scale structures include •Superclusters (tens of clusters arranged in chains. Virgo Supercluster is poor, about 15 Mpc in diameter) •Filaments •Voids •Sheets •The Great Wall •The Great Attractor? Map is from the CfA Redshift Survey.