In shape, the Milky Way is a spiral galaxy, possibly with a bar. Hubble’s “tuning fork” depicts the range of possible shapes: round balls of billions of stars grade into sausage-shaped ellipticals, known as E0’s to E7’s. Lenticular (S0) galaxies form a juncture. Then, the scheme splits into two arms, one for the ordinary spirals (S), the other for spirals with a bar of stars in the middle (SB). A lower case letter suffix denotes the relative importance of the bulge (a, if dominant) versus spiral arms (c). At first this classification was thought to be developmental or evolutionary. It was as if a round ball elongates into an elliptical galaxy, then flattens in to a pancake, ultimately sprouting spiral arms. Nice idea, though that turns out to be far from true. But the morphological classification and its abbreviations have stuck with astronomers.
A galaxy's shape can tell its story though. Arms of blue giants surround yellow halos, contrasting generations of young and old stars. Irregular galaxies hint at something amiss in their formation, if not a violent history. Brighter than expected nuclei in Seyfert galaxies and quasars seem possible only when unimaginable rates of mass are turned to energy. Galaxies seen together are likely to be gravitationally bound, and evidence of past is in the warp of the disk, a band of dust or stream of stars. Deeper, clusters of thousands of galaxies tell of the early history of the universe. The brightest in a cluster has been used as a "standard candle," and the fluctuations in brightness across the disk is another distance measure. And when plotted against the redshift of spectral lines, the true scale of the space and time is revealed.
All the way back, as it turns out. Astronomers at Bell Labs1 have just reported on cosmic shear. Gravitational lensing is the bending of light by the gravity of matter along the way. Bending also distorts the images of distant galaxies. Just like looking at your face reflected in a spoon reveals the shape of the spoon, round or oval or flat. The distorted room reflected in the other shiny surfaces in the kitchen tells you as much about your pots and pans, silverware and faucets as their arrangement in the room.
Likewise, distortions in the images of galaxies represent the distribution of large concentrations of matter in the universe. Though a tiny effect, the “cosmic shear” or correlation in apparent shapes can be mapped to the ripples of matter in a once hot and dense early universe. Thus at the farthest reaches, galactic shape and size tell the whole cosmic story in a sense, one that we are only just now able to read. It can be used to constrain cosmological theories: the result to date is consistent with an open universe or a flat universe with a dominant cosmological constant. Cold dark matter theories seem to be inconsistent with these results.
This is amazing stuff, a tour de force of telescope instrumentation, image processing and data crunching. This one study used images of 145,000 deep galaxies spread over three, half-degree fields, a window not so different than the view in a low power eyepiece. Galaxies, however pretty they appear, seem static, although unusual things do happen from time to time. No matter how long you stare at them, it is only a galactic glimpse. Mostly though they change far too slowly. Their stories can only be read in their sizes and shapes.
1Wittman, D.M.,Tyson, J.A., Kirkman, D., Dell’Antonio, I., and Bernstein, G. 2000. Detection of weak gravitational lensing distortions of distant galaxies by cosmic dark matter at large scales. Nature 405 (6783): 143-149, 11 May 2000.
Moondark is written by Doug Miller and published on the web, in the Delmarva Star Gazers'Star Gazer News and in the Delaware Astronomical Society's FOCUS. Please address comments and suggestions to dmiller@udel.edu. This document was last revised on 25 July '00. All text and images copyright © 2000 Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.
