UNIVERSITY PARK, Pa. — Extremely faint planetary nebulae — colorful cosmic objects that appear at the end of a star's life — have been detected in distant galaxies using data from the Multi-Unit Spectroscopic Explorer (MUSE) instrument. The research team, led by the Leibniz Institute for Astrophysics Potsdam (AIP) and including Penn State scientists, succeeded in detecting the nebulae using a filter algorithm in image data processing.
The method, which appears in a paper published July 22 in the Astrophysical Journal, opens up new possibilities for measuring cosmic distances, and thus also for determining the Hubble constant, a unit that describes how fast the universe is expanding.
Planetary nebulae are dying stars. They appear as a star evolves from the red giant stage into a white dwarf. When a star has used up its fuel for nuclear fusion, it blows off its gas envelope into interstellar space, contracts, becomes extremely hot, and excites the expanding gas envelope, causing it to glow.
“Regular stars like the sun emit at a continuum of wavelengths of light, so if you were to take a spectrum of them, you would see a rainbow,” said Robin Ciardullo, professor of astronomy and astrophysics at Penn State and a member of the research team. “But, for a planetary nebula, the gas surrounding the star absorbs the star’s light and re-emits it only at a few, choice wavelengths. It’s like taking all the light of a rainbow and squeezing it down so that it all comes in a single color — green. So green ends up very bright and all the other colors are not there at all.”
In distant galaxies, planetary nebulae appear so small as to be just another point-like light source. But, if you take a spectrum of the nebula, you would see a lot of light in the re-emitted wavelength emission lines and nothing else. This makes them easy for researchers to distinguish from regular stars with special optical filters tuned to these wavelengths.
Planetary nebulae are known in the neighborhood of the sun, one of the closest and brightest in our Milky Way galaxy is the Helix Nebula, 650 light years away. As their distance increases, their apparent diameter in an image shrinks and their apparent brightness decreases even further. A planetary nebular like the Helix Nebula in our neighboring galaxy, the Andromeda Galaxy, at a distance almost 4000 times greater would only be visible as a dot, and its apparent brightness would be almost 15 million times fainter. But, with modern large telescopes and long exposure times, such objects can nevertheless be imaged and measured using optical filters or imaging spectroscopy.
“With the PMAS instrument developed at AIP,” said Martin Roth, first author of the new study and head of the innoFSPEC department at AIP, “we succeeded in doing this for the first time spectroscopically for a handful of planetary nebulae in the Andromeda Galaxy in 2001 to 2002 on the 3.5m telescope of the Calar Alto Observatory using the method of integral field spectroscopy. However, the relatively small PMAS field-of-view did not allow yet to investigate a larger sample of objects.”
It took 20 years to develop these first experiments further using a more a powerful instrument with a more than 50 times larger field-of-view on a much larger telescope. MUSE at the Very Large Telescope in Chile was developed primarily for the discovery of extremely faint objects at the edge of the universe currently observable to scientists and has produced spectacular results for this purpose since the first observations. It is precisely this property that also comes into play in the detection of extremely faint planetary nebula in a distant galaxy.