Galaxies: A Very Short Introduction (Very Short Introductions) by Gribbin John

Author:Gribbin, John [Gribbin, John]
Language: eng
Format: epub, mobi
Publisher: Oxford University Press
Published: 2008-03-26T16:00:00+00:00

12. The central region of the galaxy M100, imaged by WFPC2 on the HST

There are practical difficulties that have to be overcome. Among other things, you have to make sure that all of the diameters have been measured in the same way, that the sample is restricted to galaxies which have the same overall structure as the galaxies in our local sample, and that the observations are indeed picking up all of the relevant galaxies. One of the most important factors to allow for is that it is easier to see bigger galaxies, so for larger redshifts there will be fewer small galaxies than there should be in the sample because they have been overlooked. This is an effect known as Malmquist bias. Fortunately, by comparing the numbers of galaxies of different sizes at different redshifts it is possible to work out the statistics of this effect – the way small galaxies drop out of the sample as redshift increases – and correct for it. In a further complication, nearby galaxies have to be left out of the calculation, because their random Doppler shifts are comparable to their cosmological redshifts and confuse the picture. But the technique works for galaxies out to about 100 Megaparsecs, and even with all these restrictions one of the standard catalogues, known as RC3, provides a sub-set of well over a thousand suitable galaxies that satisfy all these criteria. This is ample to provide a statistically reliable sample. When all of the work is done, the value for the Hubble Constant based on comparison of galaxy diameters comes out in the high 60s, if the Milky Way is indeed just an average spiral. This value agrees with the other measurements.

This is far from being the best or most accurate way to measure the Hubble Constant, but it is valuable for two reasons. First, it is a nice, physical technique which can be understood in terms of our everyday experience, where we know that a cow standing on the other side of a large field only looks small because it is so far away. It does not require any deep understanding of physics or mathematics. Second, the argument can be turned on its head. The first real proof that the Milky Way is just an average spiral came from comparing its size with the sizes of just 17 relatively nearby galaxies. But if H is close to 70, as the more sophisticated observations and analyses indicate, then we can use that value to calculate the average size of the 1,000-plus galaxies in our sample, some of them 100 Mpc away, and we find that it is indeed very close to the size of the Milky Way and the average size of our nearby sample. At the very least, our Galaxy is typical of the kind of disc galaxy found in a ‘local’ region of space some 200 Mpc across with a volume of more than 4 million cubic Megaparsecs.

But this is indeed still a local bubble compared with the size of the observable Universe.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.