The Colour-Magnitude Relation for Elliptical Galaxies


In the following experiment you will be measuring the colours and magnitudes of galaxies in a distant rich cluster from a CCD image taken with Hubble Space Telescope (HST). By plotting this information on a colour-magnitude diagram you can study the properties of the galaxy population in the cluster. Including information about the morphologies of the galaxies (from the high resolution HST image) into this analysis you can measure the relationship between colour and apparent magnitude for early-type (elliptical and S0) galaxies in the cluster. Finally, you can compare the predicted colour of a galaxy of a known luminosity from the local Universe with what you observed in the distant cluster to estimate how much bluer early-type galaxies were in this distant cluster. Using the theoretical rate of change of colour for a mix of stars as a function of age you can then convert this observed colour difference into an estimate of the lookback-time to the epoch when the cluster is observed. This section starts with a brief overview of the properties of clusters of galaxies, before introducing two concepts which are necessary for completion of the lab: galaxy morphology and colour-magnitude diagrams.

Clusters of Galaxies

Cluster of galaxies are the most massive collapsed structures in the Universe, the largest ones have central masses in excess of 10,000 × the mass of our galaxy. These very rich clusters are intrinsically rare objects, and most galaxies in the Universe inhabit the field, crudely defined as the lower-density regions outside clusters and voids. The most massive nearby cluster is the Coma cluster (left) the central regions of which are dominated by two super-massive elliptical galaxies. The extreme conditions found in such rich clusters including very high densities of galaxies, as well as large amounts of very hot gas which emit at X-ray wavelengths, making clusters some of the most luminous X-ray sources in the sky. More information about clusters can be found here.

The image to the right illustrates a view of the central regions, a 1 Mpc across, of a distant rich cluster. This is a true colour image, constructed from individual exposures through blue (B), visual (V) and infrared (I) filters, and the large numbers of yellow galaxies are the luminous cluster galaxies (the large interacting spiral in the upper-right hand corner is foreground of the cluster). The strong central concentration of yellow, elliptical and S0 (see below) galaxies in the cluster can be readily seen, and the cluster centre is further highlighted by the massive dominant elliptical galaxy lying at the bottom of the cluster's potential well.

Galaxy Morphology and Elliptical Galaxies

A galaxy's ``morphology'' is a description of the structure of the galaxy, e.g. spiral or elliptical, and is typically estimated by eye from optical images. A number of schemes have been constructed to classify galaxy morphology into different classes and in this way to attempt to understand the physical processes which define galaxy morphology and from this gain a deeper understanding of galaxy formation and evolution.

The major visible components of giant galaxies are the bulge and disk. The bulge is a roughly spherical cloud of stars in the central parts of the galaxy, this cloud is mostly supported by the random motions of the stars within it. The disk component is a rotationally supported, usually quite thin and extending to larger radii than the bulge component. The disk can also show spiral arms resulting from on-going star-formation in the gas-rich disk material. The bulge and disk are thus the morphological features which are typically used to classify galaxies. Specifically, the relative luminosities of the disk and bulge components of the galaxy and the degree of contrast of the arms in the disk are used to separate galaxies into different classes. The presence of a linear bar-like feature in the galaxy is also used to classify galaxies. The figure below shows the classical tuning fork classification diagram of the Hubble galaxy morphology scheme. In the Hubble scheme galaxies are ranked on the relative strength of the bulge and disk components: galaxies with a massive bulge, but no visible disk are termed ``Elliptical'' (E), those with large bulges and a small disk are ``S0'' galaxies, after this come the various sub-types of spiral galaxy - Sa, Sb, Sc, Sd - a sequence of decreasing bulge luminosity compared to the disk light (this sequence is also described as one from early- to late-type spirals). There is a parallel sequence of barred spiral galaxies, and Elliptical galaxies are further catagorised on the basis of their shapes: E0 (circular) to E7 (highly elliptical).

Below are postage stamp images of each of the main morphological classes. These images have been taken from the digitised version of the Sky Survey.

The description above applies to massive luminous galaxies. There are also a wide range of dwarf galaxies which share some of the same characteristics of the luminous galaxies, as well as classes of low-surface brightness galaxies and other peculiar galaxies, leading to a wide and rich variety of morphological classes. Furthermore, morphologies of galaxies in the distant Universe acquired with the Hubble Space Telescope indicate that an increasing number of galaxies at earlier epochs are hard to place within the confines of the standard Hubble scheme set out above. This implies that a galaxy's morphology can change over the course of its lifetime. Nevertheless, galaxy morphologies remain some of the most basic and most useful information which can be gleaned for a galaxy. The importance of morphology is shown by the good correlation between a galaxy's morphology and the rate of star-formation within the galaxy, with later-type spirals showing stronger star-formation. Another strong morphological correlation is between galaxy morphology and galaxy density - with early-type galaxies (E/S0/Sa) being preferentially found in regions of higher galaxy density (the cores of rich clusters of galaxies) and later-type spiral galaxies inhabiting the lower density surrounding regions (termed the ``Field'').

Colour-Magnitude Diagrams

The remainder of this lab will concentrate on the properties of the galaxy population of a massive cluster at high redshift. The majority of the bright galaxies in this cluster are early-type galaxies (E/S0). These galaxies show a strong correlation between their colours and their luminosities (or masses), with brighter/more massive galaxies being redder and fainter/less massive ones bluer. This can be seen in the figure below which shows the colours and magnitudes of galaxies in the cluster shown in this image. The galaxy colours are measured from the apparent magnitudes of the galaxies in two different regions of their spectra: through a filter in the blue (B) around 4500Å and one in the infrared centred close to 8100Å, called the I band. The colour is expressed as simply the difference between the magnitudes in B and I: hence, (B-I). The strong linear feature between I=18-22 with a colour of around (B-I)=3 is formed by the E and S0 galaxies within the cluster. At fainter magnitudes this relation fades and a population of faint blue galaxies becomes apparent. The linear relation for the brighter galaxies indicates that most of the E and S0 galaxies within the cluster were formed via the same mechanism and that this mechanism couples the colour of the stars formed within the galaxy to the final mass of the galaxy.

One mechanism which can produce such an effect is the collapse of a single, massive gas cloud forming all the stars in the galaxy in a short period of time. The first supernovae from the initial burst of star-formation produce large quantities of hot gas which is enriched in heavy elements. The deeper potential wells of the massive galaxies can contain this hot gas and it is therefore incorporated into the next generation of stars which are formed. These stars then have higher metal contents and this tends to make them redder. The hot gas expelled by the supernovae in less massive galaxies can easily escape the galaxy and hence it is not used in the formation of the next generation of stars - which are therefore metal-poor and hence blue in colour.

The same relation between luminosity and colour appears to hold in different clusters at the same epoch. The important feature of the colour-magnitude (C-M) relation of early-type galaxies for the purpose of the remainder of this exercise is thus that in the local Universe a specific luminosity of early-type galaxy has a well-defined colour. We can use this relation to investigate the change in the colour of early-type galaxies in distant clusters resulting from the younger ages of the stars in these galaxies, which are observed as they were several billion years ago.

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