Types of Galaxies         Spiral galaxies are the most intriguing visually, But in fact galaxies come in many shapes and sizes.

Galaxy shapes have 3 broad categories:

  spirals, ellipticals, irregulars

Ellipticals -- 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

Picture of an irregular galaxy, the Large Magellanic Cloud, at 100 thousand parsecs one of the two closest galaxies to the Milky Way (and possibly in the process of merging with us!)

Note the following systematic trends:

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

3) The most massive galaxies are ellipticals with some irregulars being very small ("dwarfs"). Spirals tend to be intermediate in size.

Galaxies across the spectrum

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. However, there is no sign of an "end" to the galaxies -- we have found them to redshift z nearly 6!

Although the galaxies look different, another theory is that we are seeing only regions of hot, young stars (since at the high redshift the "optical" images for the HDF are probing the ultraviolet wavelengths from the galaxies, redshifted into the optical). At least in a few cases where the Hubble Telescope has taken deep near infrared images (which would probe the intrinsic optical region, redshifted), this idea gains support. See the images to the left -- the top one is the original HDF image, showing peculiar scraps of galaxies, and the bottom one shows a merger with the infrared image, which all together looks like a spiral studded with regions where bright, young stars have recently formed. (From STScI)

In the 18th Century, Thomas Wright proposed that the Universe was filled with groupings of stars like the Milky Way, from http://homepage.mac/com/kvmagruder/bcp/milky/shape.htm		Mixtec personification of the Milky Way, from http://garnet.acns.fsu.edu/~dco2511/
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