What Contributes to The Spectra of Galaxies?
A typical galaxy contains stars, gas, dust, and Dark Matter. Although Dark Matter claims a huge portion of the galaxy, it doesn’t influence a spectroscopic analysis of the galaxies. The only components that create a spectrum are stars, gas, and sometimes dust. So, it is a satisfactory guess that a typical galactic spectrum should contain a ‘mixture’ of emission spectra from stars and absorption spectra from dust.
Stellar Radiation, due to its nature, can be compared to that of an ideal Black Body Radiation. Its flux keeps on rising until it reaches a particular wavelength and then slowly dips off:
A spectrum like that, tells us a wealth of information. Looking at the sharp dips (absorption lines) in the curve tells us what element is potentially present at the stellar surface and what its ionization state is. It can inform us about its temperature and Luminosity. By observing its shift in wavelength we can calculate the radial velocity and if the shift is periodic, we can detect its binary nature.
The gas in a galaxy gives rise to absorption spectra. The gas in a galaxy is partly visible in the form of hot clouds known as HII regions. Such regions are usually only seen where there is ongoing star formation, and so are prominent in the spiral and irregular galaxies. The optical spectrum of an HII region consists of just a few emission lines. As this HII region arrives from star formation regions, they are intensely bright:
The Dust Components usually don’t contribute to the spectra in the optical regions but it radiates a huge amount of infrared light at wavelength of 100 micro-meters. However, in this article we shall only remain concerned in the optical region.
The Optical spectra of a normal galaxy (we are considering every galaxy other than AGNs to be normal) is a composite of Absorption lines from the stars and emission lines from the gas clouds. However, while summing up the two lines, we have to remember 2 important factors:
1. Different types of star have different absorption lines in their spectra. When the spectra are added together, the absorption lines are ‘diluted’ because a line in the spectrum of one type of star may not appear in the spectra of other types.
2. Doppler shifts can affect all spectral lines. All lines from a galaxy share the red-shift of the galaxy, but Doppler shifts can also arise from motions of objects within the galaxy. As a result, the absorption lines become broader and shallower.
Spectra of Spiral and Irregular Galaxies:
1. As mentioned before, Spiral and Irregular Galaxies contain a huge amount of gas clouds as they are often active places for stellar formation. Therefore, for such a galaxy, we should expect strong and intensely bright emission lines of HII, which can be observed clearly, even when amidst an abundance of Stellar Spectra.
2. The Spiral Galaxies and the irregular ones have a high amount of rotational speed. They rotate fast enough to create spiral arms which are relatively younger sources of stellar formation. So, while observing the spectra of such galaxies we observe the ‘broadening’ of spectra lines due to ‘Doppler Shift’ effect.
A typical spectrum of Spiral and Irregular Galaxies might look like the following:
The spectrum, collected from the galaxy NGC 2276, clearly shows sharp HII lines and a closer look shows line broadening too.
So, how does the spectrum of an elliptical galaxy look like? The elliptical galaxies are old and are not that ‘active’ for enormous stellar formation. Consequently, there is a lack of gas clouds responsible for producing HII lines. Moreover, these galaxies are elliptical and circular in shape which means they don’t show rapid motion. For this reasons we can conclude that the spectra of elliptical galaxies should like that of a M-type star stellar spectra and indeed this is what we see:
