|Types of Galaxies
Key points: Basic types of galaxy - spiral, barred, elliptical, irregular; types vs. distance (how young galaxies differ from those around us)
Spiral galaxies are the most intriguing visually,
But in fact galaxies come in many shapes and sizes.
Galaxy shapes have 3 broad categories, based on the role of the bulge (the round distribution of stars at the center) and the disk (the flat distribution that includes the spiral arms).
spirals, ellipticals, irregulars
Ellipticals are basically all bulge with no disk. They can range from spherical to elongated, football-like shapes
|Picture of an elliptical galaxy (M87, the giant elliptical at the center of the nearby Virgo cluster of galaxies (about 15 million parsecs away):||A more "elliptical" elliptical, M86|
Irregulars -- catchall type for galaxies without symmetric shapes, and no clear bulge or disk
|Picture of an irregular galaxy, the Large Magellanic Cloud, at 55 thousand parsecs one of the two closest galaxies to the Milky Way (and possibly in the process of merging with us!) APOD, W.-H. Wang, IfA, Hawaii, http://antwrp.gsfc.nasa.gov/apod/ap060510.html|
1) Interstellar material ranges from essentially none in ellipticals to substantial quantities in some irregulars
2) Ellipticals have low to zero rotation rates while spirals have relatively high rotation rates; that is, the stars in ellipticals have a wide variety of orbits - a little like a swarm of gnats - whereas in spirals the stars have orbits that lie in the plane of the disk
3) The most massive galaxies are ellipticals with some irregulars being very small ("dwarfs"). Spirals tend to be intermediate in size.
We use the local group spiral galaxy M31 to help visualize galaxies across the electromagnetic spectrum (Images are from Beck et al. (radio), Thilker et al. (HI), S. Muller (CO), Deevereux (H alpha), MIPS team (far and mid infrared), 2MASS (near infrared), APOD - J. Ware (visible) GALEX (ultraviolet), and ROSAT (x-ray).
|Here, we run through the entire spectrum starting with the radio and ending in the X-ray. The spiral arms are strong for radio through mid-infrared, and in the ultraviolet, and in ionized gas, all of which respond strongly to recent star formation.They also show clearly in molecular gas, which is the fuel for forming stars. In the near infrared, the image is dominated by old stars in the bulge, and the spiral arms are almost invisible. In the x-ray, we see massive double stars left over from previous episodes of star formation. The atomic gas and to some extent the far infrared spread far beyond the regions of the spiral arms and galaxy disk. There is also a component of ionized gas in the nuclear region. Thus, the types of gas have very different distributions, and most of the star formation is well separated from the older stars. (animation by G. Rieke)|
The most distant galaxies - are they the same types as nearby ones?
|Under the leadership of former UA professor Bob Williams, the Hubble Space Telescope spent many days observing a small region just above the Big Dipper. The "Hubble Deep Field", or "HDF", is just a point to our unaided eyes. Here we zoom in by a factor of 1000 to get a look. The total area of the HDF is about 1% that of the full moon. (From STScI)|
|At first glance, the HDF shows more and more faint galaxies just like the nearby ones. If we look at the ones at the highest redshift, they seem subtly different -- many are small, like pieces of the large nearby galaxies. This behavior reminds us of the models of the early Universe, where galaxy fragments formed first and only later merged into large galaxies.|
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hypertext G. H. Rieke
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