![]() |
Department of Physics | University of Durham |   | Level One |
Having completed your catalogue of galaxies in Cl0016+16, we now wish to fit to the colour-magnitude relation of the early-type cluster members - the strong ridge-feature which is apparent in your colour-magnitude diagram. We list below the main steps you'll have to complete to derive the relationship between colour and magnitude for the early-type galaxies.
Now, we compare the predicted colour of a galaxy with an apparent magnitude of I=21 from your linear fit to the C-M relation with that observed for a galaxy with this luminosity in the local Universe. We take into account just the shifts in the filter passbands for observations of galaxies at high redshift. This predicts that an early-type galaxy with an apparent magnitude of I=21 in Cl0016+16 should have a colour of (V-I)=2.68±0.03. What is the difference between this colour and that derived from your fit? In what sense is the difference (are your observations bluer or redder than the prediction) and what might be the cause of this? Remember that you are observing the galaxies in Cl0016+16 at high redshift and hence seeing them as they appeared at substantially earlier epochs, due to the finite travel time for light, and that younger stellar populations tend to be bluer (as they have more young, massive blue stars in them).
The rate of change of (V-I) colour with age can be estimated from theoretical models of the evolution of simple stellar populations (based on observations of globular clusters and models of stellar structure). These indicate that the (V-I) colour of a galaxy (a significant time after the formation of the stars) should become redder at a rate of d(V-I)/dt = 0.05 magnitudes per Gyr. Taking the colour difference which you derived above and this rate of change of colour, estimate the how much younger the stars in Cl0016+16 appear to be compared to those in local Universe, this is the lookback time to Cl0016+16.
Finally, use your fit to the C-M relation to determine the magnitude of a galaxy which has an apparent colour of (V-I)=2.4. The equivalent rest-frame colour in a local cluster, corrected for the evolutionary effects discussed above, corresponds to an absolute magnitude of -21.3±0.1 in the I-band. Using the apparent (m) and absolute magnitudes (M) of the galaxy estimate the distance modulus (µ) for Cl0016+16 and hence the distance (r) to the cluster in parsecs (for Ho = 50 km/sec/Mpc).
At this point you should have noted down the following information:
The following sections of course text books will provide background information on the astronomy discussed in this exercise.
The Extragalactic and Cosmology group at Durham is one of the leading research groups in Europe investigating the growth and evolution of structure in the Universe, including rich clusters. The observational work undertaken in Durham on clusters focuses on the evolution of their galaxy populations, searches for clusters in the early Universe and the X-ray emission from clusters. Theoretical research into clusters includes work on their use in cosmological tests and predictions of the results from the observational investigations. More details of the research into clusters of galaxies undertaken in Durham can be found here. Finally, here is a Further exercise illustrating galaxy morphology.
Thanks to Richard Bower for the original concept for this web lab. Also thanks to the Royal Society and University of New South Wales who paid enough for me to be able to sit and write this listening to the rain in Sydney while drinking good scotch.