Despite being one of the most venerable and prominent objects in the night sky, the Andromeda Galaxy (M31) still has surprises. And a group of amateur astronomers have uncovered the latest: a previously unknown emission nebula lying just southeast of Andromeda and spanning half the width of the galaxy itself.
The feature was discovered in images taken last year with an Oxygen-III (OIII) filter by French astroimager Yann Sainty, who worked with Marcel Drechsler and Xavier Strottner to process and analzye the data. They have designated the feature Strottner-Drechsler-Sainty Object 1.
They then worked with a team of professional astronomers and other astroimagers to confirm the find. The team published their results in Research Notes of the AAS last month — as well as a stunning, highly-processed image on the astroimaging site Astrobin (reproduced above).
A side project
The observations of Andromeda began as a side project for the trio, who had originally teamed up for another reason: Drechsler and Strottner maintain a catalog of planetary nebulae, and had asked Sainty to capture several known and candidate objects.
Sainty traveled all across France in search of the darkest sites he could find for his mobile observing setup, which includes a 4.2-inch Takahashi refractor and a CMOS astronomical camera from ZWO. After concluding this months-long project, Sainty “decided to focus on a relaxing and easy project — the Andromeda Galaxy,” Drechsler said in a statement shared with media, including Astronomy and ZWO.
“While working on the Andromeda project, Yann Sainty did something that few astrophotographers before him have done — he used an OIII filter to better bring out the faint HII regions,” said Drechsler. “Since an OIII filter is relatively new territory in astrophotography, Yann sent the data to [me] and Xavier for review. Yann’s secret hope, perhaps, was to have a previously unknown planetary nebula or supernova remnant in the data.”
When Drechsler and Strottner looked at the OIII images, they noticed “an extremely faint nebulosity … at the edge of the image that seemed to continue outside the photo.” At first, the team considered whether it was an artifact, like a gradient introduced through a faulty flat-field calibration image. But Drechsler “urged Sainty to collect more OIII data, thinking he spotted finer sub-structures in the barely-visible nebula.”
Sainty collected more images through the fall of 2022, eventually totaling 111 hours of exposure. As he did, the team began increasingly sure they had found something real — and previously unreported.
The team reached out to professional astronomers to aid in verifying their discovery, including Robert Fesen of Dartmouth College in Hanover, New Hampshire. In an interview with Astronomy at last month’s meeting of the American Astronomical Society (AAS) in Seattle, Fesen pointed to the arc in an image and summed up his initial reaction: “What the hell is that?”
“When they sent it to me, I said, ‘There’s something wrong with your camera, and go fix it and leave me alone,’” he quipped. “[Dreschler] came back a couple of weeks later: ‘Rob, it’s real.’ And I said, ‘Look, you haven’t tried hard enough to kill it.’”
To confirm it, other astroimagers joined the hunt: Bray Falls working with two remote telescopes in California, Christophe Vergnes and Nicolas Martino in France, and Sean Walker (associate editor at Sky and Telescope magazine) observing with a remote telescope in New Mexico. Their results convinced Fesen: “Five different telescopes see stuff there? At different levels of resolution, but it’s in the same spot of the sky off M31? I decided it’s real.”
Remarkably, the nebula had been missed by previous OIII surveys of M31 on professional-grade telescopes, including one by the 3.6-meter Canada-France-Hawaii Telescope (CFHT) on Mauna Kea. That’s because many instruments designed for research simply aren’t well-suited to spot such a faint and extended nebula.
CFHT’s MegaCam instrument has a field of view of 1° — wide by professional standards, but still not wide enough to capture the full extent of the new object, which spans 1.5°. The MegaCam survey of M31 also used a filter that allowed a relatively wide range of wavelengths to pass through — over 10 nanometers. Sainty used an off-the-shelf Antlia filter with a bandwidth of just 3 nm, which better isolated the OIII signal from background noise.
Here or there?
The find has set the astronomical community ablaze with speculation about the object’s nature, including whether it is physically next to Andromeda, which is 2.5 million light-years away. It is entirely possible that the newfound object is part of the Milky Way and simply lies along our line of sight to our galactic neighbor.
One possibility that the team considered was that the feature is caused by Andromeda beginning to interact with the Milky Way. But, they wrote, “the arc seems much too close to M31 to fit that picture. More likely, it lies within M31’s halo and is related to the numerous stellar streams, especially the Giant Stellar Stream whose eastern edge lies close to the OIII arc.”
However, Fesen tells Astronomy that since then, “I have started to think it less likely to be a feature of M31, but, instead, a Milky Way nebula much closer. But who knows.”
To settle the issue, Fesen and his colleagues hope to obtain a spectrum with a professional-grade observatory. From this, they can measure any Doppler shift in the light caused by motion toward or away from the Milky Way — and whether it matches the motion of Andromeda itself.
Whether or not the arc is ultimately associated with Andromeda, the discovery highlights the role that amateur astronomers and imagers with widely available high-quality narrowband filters are playing in discovering faint, extended emission nebulae.
Fesen expressed admiration for the imagers, who, he notes, are taking data that totals exposures of “fractions of a day or more.” He pointed to one of the confirmation images: “That one picture’s 86 hours. Are you kidding me? [Sainty’s image] was taken over 22 nights over three months of clear weather. This is insane.”