Peter Schneider - Extragalactic Astronomy and Cosmology

The spiral galaxy NGC1232 may resemble our Milky Way if it would be observed from above (face-on). This image, observed with the VLT, has a size of 6.8 6.8, corresponding to a linear size of 60kpc at its distance of 30Mpc. If this was our Galaxy, our Sun would be located at a distance of 8.0kpc from the center, orbiting around it at a speed of220kms. A full revolution would take us about 230 106yr. The bright knots seen along the spiral arms of this galaxy are clusters of newly-formed stars, similar to bright young star clusters in our Milky Way. The different, more reddish, color of the inner part of this galaxy indicates that the average age of the stars there is higher than in the outer parts. The small galaxy at the lower left edge of the image is a companion galaxy that is distorted by the gravitational tidal forces caused by the spiral galaxy. Credit: European Southern Observatory

We see the spiral galaxy NGC4565 from the side (edge-on); an observer looking at the Milky Way from a direction which lies in the plane of the stellar disk (from the side) may have a view like this. The disk is clearly visible, with its central region partly obscured by a layer of dust. One also sees the central bulge of this galaxy. As will be discussed at length later on, spiral galaxies like this one are surrounded by a halo of matter which is observed only through its gravitational action, e.g., by affecting the velocity of stars and gas rotating around the center of the galaxy. Credit: European Southern Observatory

This image of the galaxy M82 illustrates very clearly that any given waveband provides a rather restrictedand biasedview of cosmic objects. Shown is a composite image, obtained from three different telescopes. Blue color shows the X-ray radiation of this galaxy, as recorded by the X-ray satellite Chandra. The infrared light is shown in red , and was observed with the Spitzer Space Telescope. The optical light from M82 was recorded with the Hubble Space Telescope and is shown in yellow-green . Finally, line emission from hydrogen gas is displayed in orange . The distributions of radiation from different wavelengths is obviously very different; only the joint set of observations can provide us with an understanding of this galaxy. In fact, M82 is a rather special object, a so-called starburst galaxy, named because this galaxy forms new stars at a rate much higher than this happens in the Milky Way and other normal spiral galaxies. The stars of the galaxy are distributed in a disk, as seen from the optical light, and most of the newly formed stars are located close to the center. The most massive of the stars explode in a supernova; these gigantic explosions can heat, and drive substantial amounts of gas and dust out of the galactic plane. The hot gas radiates X-rays and is clearly seen on both sides of the stellar disk, as well as the dust which emits in the infrared light. The image size is 7.9, corresponding 8.5kpc for distance of 3.7Mpc. Credit: X-ray: NASA/CXC/JHU/D.Strickland; IR: NASA/JPL-Caltech/C. Engelbracht (University of Arizona); optical: NASA, ESA, and The Hubble Heritage Team