Needle in a haystack: Finding planetary nebulae in distant galaxies

Sam Sholtis and Sarah Hönig
July 26, 2021

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.

Ring galaxy NGC 474 with inset of MUSE images showing detection of planetary nebula

The ring galaxy NGC 474 (top) at a distance of about 110 million light years with MUSE fields in the ring structure marked by boxes. The ring structure was formed by merging processes of colliding galaxies. MUSE image data in the two marked fields (bottom). Left: Image in the continuum of light wavelengths with the band of unresolved stars as well as globular clusters marked by circles. Right: filtered image in the redshifted oxygen emission line, from which the planetary nebulae emerge as point sources from the noise. The artefacts created by instrumental effects have completely disappeared.

IMAGE: DES/DOE/Fermilab/NCSA & CTIO/NOIRLab/NSF/AURA (top), AIP/M. Roth (bottom)

The galaxy NGC 474 is an example of a galaxy that, through collision with other, smaller galaxies, has formed a conspicuous ring structure from the stars scattered by gravitational effects. It lies roughly 110 million light years away, which is about 170,000 times further than the Helix Nebula. The apparent brightness of a planetary nebula in this galaxy is therefore almost 30 billion times lower than that of the Helix Nebula and is in the range of cosmologically interesting galaxies for which the team designed the MUSE instrument.

The research team developed a method for using MUSE to isolate and precisely measure the extremely faint signals of planetary nebulae in distant galaxies with high sensitivity using a particularly effective filter algorithm in image data processing. By applying this data processing to archival data for the ring galaxy NGC 474, based on two very deep MUSE exposures with 5 hours of observation time each, a total of 15 extremely faint planetary nebulae became visible.

This highly sensitive procedure opens up a new method for distance measurement that is suitable for contributing to the solution of the currently discussed discrepancy in the determination of the Hubble constant, said the researchers.

“In 1989, George Jacoby and I figured out a way of using the luminosity function of planetary nebulae within a galaxy — the relative number of very bright, medium bright, and not-so-bright nebulae — sto measure the distance to a galaxy,” said Ciardullo. “We measured the distances to about 50 galaxies, which is just about every suitable target that was close enough for us to see with the telescopes available at the time. But since the turn of the millennium, telescopes have gotten bigger and a new generation of high-tech instruments have greatly improved our ability to detect and measure faint objects.”

With MUSE, the “Planetary Nebula Luminosity Function (PNLF)” technique could be pushed to much greater distances, according to the researchers. Penn State undergraduate student Owen Chase and Penn State graduate student Brian Davis performed an initial analysis of MUSE’s archival data and developed prototype measurement software, which was further improved by Roth at AIP. This greatly improved the detectability of planetary nebulae in distant galaxies. The new study shows that MUSE can measure the distance to far-off galaxies at more than twice the range previously achievable and will allow an independent measurement of the Hubble constant.

“In a sense, this is a ‘how-to’ paper, describing the best way to use MUSE for the project, and the best way to analyze the MUSE data,” said Ciardullo. “The plan is to go into production mode in future papers.”

In addition to Ciardullo, Roth, Chase and Davis, the research team includes George H. Jacoby at the National Science Foundation’s NOIRLab and Peter Weilbacher at AIP.

More information can be found at the MUSE website.

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Last Updated August 09, 2021