Elliptical galaxies appear redder in color compared to spiral galaxies. This reddish hue is due to the older stellar populations that exist in elliptical galaxies. As stars age, they become redder and emit light more strongly at redder wavelengths. In elliptical galaxies, star formation largely ceased billions of years ago. This means their stars are overwhelmingly older and redder. Spiral galaxies have more active star formation, containing young, bluer stars along with older redder ones. The mixture of stellar ages and colors gives spiral galaxies their more blueish-white appearance.
What are elliptical galaxies?
Elliptical galaxies are spheroidal collections of stars that have very little gas and dust compared to spiral galaxies. They exhibit very little new star formation activity. Stars move in more random orbits rather than rotating in a disk like in spirals. Ellipticals range from nearly spherical to quite elongated shapes, giving rise to their classification into E0, E3, E5, E7 based on their projected ellipticity. Giant ellipticals are often found in the centers of galaxy clusters.
What determines a galaxy’s color?
A galaxy’s overall color is determined by the mixture of colors of its constituent stars. The color of stars, in turn, is primarily a function of their surface temperature. Hotter stars appear bluer or white, while cooler stars appear redder. A galaxy’s color therefore depends on the distribution of stellar temperatures and luminosities within it.
The strongest emission from stars is in the visible part of the spectrum. Blue light has shorter wavelengths while red light has longer wavelengths. Blue stars are hotter with surface temperatures exceeding 10,000 Kelvin while red stars are cooler with temperatures of Why are elliptical galaxies reddish in color?
Elliptical galaxies appear red because they contain mostly old, low-mass stars. As stars age, they exhaust their core hydrogen fuel and evolve into red giants which are cool, redder stars. Massive, hotter burning O and B type stars which appear blue or white have lifetimes of just millions of years. In elliptical galaxies, star formation ended billions of years ago so very few hot, blue stars remain.
The oldest red giants emit most of their light in the red and infrared. Per unit mass, these red giants emit up to tens of thousands of times more light than main sequence stars. Even though elliptical galaxies contain some hotter stars left over from their initial burst of star formation, the light emitted is overwhelmed by the abundance of old, cool red giants.
Why do spiral galaxies appear bluer?
Spiral galaxies have ongoing star formation in their spiral arms and central regions. This produces young, massive, hot stars which emit blue and ultraviolet light. OB associations, HII regions, and nebulae all trace sites of new star birth in spirals. Short-lived blue stars and clusters give spirals their distinctive, bluer hue compared to the reddish ellipticals.
The mixture of light from both hot, younger stars and cooler, older stars gives spiral galaxies more of a blueish-white appearance. Different portions can appear bluer or redder depending on concentrations of star formation activity or older vs younger stellar populations. Grand design spiral galaxies with well-defined arms appear bluer on average than fragmented, irregular spirals.
Spectral Energy Distributions
We can examine the full spectral energy distributions (SEDs) of elliptical and spiral galaxies to reveal their detailed differences. SEDs plot the full emitted light as a function of wavelength or frequency. Ellipticals peak at redder, longer wavelengths while spirals peak at bluer, shorter wavelengths corresponding to hotter average stellar temperatures.
The following table compares the SEDs of typical elliptical and spiral galaxies:
| Wavelength | Elliptical SED | Spiral SED |
|---|---|---|
| Ultraviolet | Weak | Strong |
| Optical | Peaks in red | Peaks in blue/green |
| Infrared | Strong | Moderate |
The SEDs confirm that ellipticals are dominated by cool, old red stars while spirals contain a larger population of hot, young blue stars.
Redshift effects
The observed colors of galaxies also appear redder due to their cosmological redshift. The farther a galaxy is, the more its overall spectrum is shifted to longer wavelengths by the expansion of the universe. Intrinsically bluer galaxies will have their light shifted into the visible red and infrared as their distance increases.
This redshift effect has to be accounted for when interpreting the intrinsic stellar populations and colors of galaxies. For example, very distant ellipticals can appear quite blue in visible light observations but this is only because their redshifted ultraviolet light has been shifted into the observable window. Their stellar population remains old and red.
Dust effects
Dust extinction from interstellar dust grains can also redden the light from both spiral and elliptical galaxies. However, ellipticals contain very little cold gas or dust overall. The effects of dust extinction and reddening are more pronounced in spiral galaxies.
Dust grains preferentially absorb and scatter blue light. This attenuates and reddens starlight passing through spiral arms where dust is concentrated. The dust extinction effects have to be modeled and removed to study the underlying stellar populations. With corrections for dust, spiral galaxies appear even bluer intrinsically.
Gas content
The amount of gas present in elliptical vs spiral galaxies also explains their differing colors. Ellipticals are gas-poor “red and dead” systems while spirals are gas-rich with active star formation.
Gas provides the raw fuel for star formation. Spirals have abundant HI (neutral hydrogen) regions and molecular gas reservoirs which fuels ongoing star formation. The hot, young stars produce the bluer light. Ellipticals lost their gas long ago or expelled it via supernovae winds and AGN feedback. With no gas left, they cannot form new stars.
Metallicity effects
Stellar metallicity also affects the colors of galaxies. Ellipticals tend to be more metal-rich compared to spirals. Increased metallicity produces redder giant stars with more complex spectra.
The first generation of stars formed (Pop III stars) were metal-poor. Younger stellar populations in spirals and the outer regions of ellipticals formed out of less enriched gas and thus appear bluer. The metal-rich cores of giant ellipticals harbor redder stars.
Environmental influences
A galaxy’s environment can impact its star formation history and color. Ellipticals dominate in dense clusters where mergers and gas stripping shut down star formation early. Field ellipticals outside of clusters can exhibit bluer colors if some residual star formation continued.
Spirals in clusters can lose gas due to ram pressure stripping by the hot intracluster medium. This quenches star formation, reducing the blue light. Spiral galaxies in the field evolve more isolated and retain their gas longer, appearing bluer.
Conclusions
In summary, elliptical galaxies appear redder than spirals primarily due to:
– Ellipticals containing older stellar populations. As stars age and evolve off the main sequence, they become red giants.
– Spirals undergoing continuous star formation which produces young, hot, massive blue stars.
– Ellipticals being gas-poor systems. Lack of gas prevents new star formation to generate blue stars.
– Metallicity effects making elliptical stars redder on average.
– Environmental effects stripping gas in ellipticals and spirals in dense clusters.
Observed colors are also affected by redshift and dust. Correcting for these effects further enhances the intrinsic color differences between elliptical and spiral galaxies based on their stellar ages, gas content, and star formation histories.
