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Discovery short paper
RNAAS Research notes of the AAS (short paper)

IMPORTANT NOTE

The [OIII] emission arc appears very bright in the images - but it is an extremely faint object that can only be adequately visualized by special subtraction techniques, since the signal in the unprocessed condition is almost completely outshone by the light from the galactic halo of M31.
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The research on STROTTNER-DRECHSLER-SAINTY OBJECT 1 is still in its early stages. The existence of the [OIII] emission arc was confirmed beyond doubt. All data, hypotheses and conclusions are based on the state of knowledge as of January 9, 2023.

AMATEUR ASTRONOMERS IN MODERN SCIENCE

Optical sky surveys with emission lines are particularly useful for identifying various types of emission nebulae, including H II regions, planetary nebulae, supernova remnants, stellar wind bubbles, and other phenomena. Most of these surveys have focused on detecting H-alpha emission along the Galactic plane. The [O III] emission lines are particularly important for studying nebulae. However, with the advent of affordable but sensitive large-format CMOS detectors combined with high-quality narrow-band filters, amateur astronomers are playing an increasingly important role in the discovery of faint emission nebulae.

The discovery shown here was possible because amateurs and established scientists worked hand in hand - a lively exchange across numerous national boundaries. Many months of intensive research, analysis, and photography were required to announce this discovery.

THE DISCOVERY

In an amateur photograph of the Andromeda galaxy (Messier 31), a prominent arc-shaped [O III] structure was discovered in August 2022. The arc has an extension of about 1.5 x 0.45 degrees, a distance of only 1.2 degrees from the center of M31 and is located in the southeast of the galaxy. Messier 31 is undoubtedly one of the most photographed deep sky objects ever. All the more surprising was the discovery of a structure in the immediate vicinity of the galaxy. To exclude the possibility of errors (reflections in the optics, artifacts of the detector, amplifier glow, etc.), four separate photographs were taken with [O III] filters from different manufacturers by four different photographers at three different locations around the world at different times. The existence of the object was thus confirmed beyond doubt.

The exact nature of the M31 [O III] emission arc is still unknown, but given the circumstantial evidence, likely scenarios can be assumed.
It's possible that Strottner-Drechsler-Sainty Object 1 is an arc segment of a galactic supernova remnant, or a bright part of an ancient galactic planetary nebula and thus far in the foreground as part of our galaxy.

However, researchers on the team believe other scenarios are more likely.
In it, the arc is in close proximity to M31 and is formed by violent interactions of stellar and tidal streams due to galactic mergers around M31. Up to the possibility that the arc is halfway between us and M31, and it is an interaction shock of the galactic halos of the Milky Way with that of M31. This would therefore be a gigantic structure between the galaxies or in the vicinity of M31 itself.

correctly oriented [O III] sum image
Yann SAINTY , 382mm focal length 24h, Antlia 3nm [O III], location: Lorraine, France
Continuum subtraction (for coordinate determination the stars were inserted in this version)

A NEW FEATURE OF THE ANDROMEDA GALAXY

The Andromeda Galaxy is the closest large spiral galaxy to the Milky Way, at a distance of about 2.5 million light-years, and is our largest cosmic neighbor.
Under good conditions and without technical aids, it is the most distant object that can be permanently observed with the naked eye.

More than 130 years have passed since the very first photograph of the Andromeda Galaxy.

The beginnings of astrophotography date back to the late 19th century. It was pioneers like the British astronomer Isaac Roberts ( 1829 - 1904) who connected light-sensitive photographic plates to telescopes to produce photographs of astronomical nebulae and galaxies for the first time. On December 29, 1888, Roberts made what was probably the first long exposure of the Andromeda galaxy; the exposure time was 4 hours at the time. Since then, the galaxy, also named Messier 31, has become one of the most photographed deep sky objects.
But these were only the first steps in the new discipline of astrophotography.

Through modern digital sensors, the quality of astronomical photos has improved at a breathtaking pace. While until a few years ago Messier 31 was photographed almost exclusively with broadband filters to show the light of the entire visible spectrum, ambitious astrophotographers increasingly started to expose the galaxy also with line filters, such as an H-alpha filter, to show the HII regions and the star forming regions in the spiral arms of the galaxy.

Increasingly powerful sensors and more sophisticated filters allowed astrophotographers to become more experimental. It was only a matter of time before the light of the doubly ionized oxygen, the so-called [O III] line, of Messier 31 at 500.7 nm was captured by amateurs for the first time.
The turquoise glowing light of the [O III] line, is found in all spiral arms and far beyond the galactic disk. Tiny turquoise dots, far from the Galactic center, reveal Andromeda's most distant supernova remnants and planetary nebulae.

This tiny image section of a peripheral region of M31 shows the OIII signal of several dozen planetary nebulae and supernova remnants.
In the entire image, the team was able to identify several hundred PNs and SNRs.

When the wavelengths of hydrogen and oxygen are combined, Andromeda reveals a stunning array of colors, revealing details never before seen.
For example, a flower-shaped nebula, glowing in [O III], which has the incredible dimensions of the Small Magellanic Cloud. Or a Pillar measuring many thousands of light-years, which seems to rise from the core of the Andromeda Galaxy. (see image below)

The area around the companion galaxy M32, showing the flower-shaped [O III] nebula as well as the [O III] pillar

But also many known and unknown supernova remnants are more clearly visible with the combination of wavelengths.
But the galaxy has kept one secret for a very long time ...

A NEVER BEFORE SHOWN [O III] STRUCTURE AROUND MESSIER 31

In August 2022, French astrophotographer Yann SAINTY made a photograph of our neighboring galaxy Messier 31. What made it special, he used an Antlia 3nm [O III] filter in addition to the usual broadband RGB and narrowband H-alpha filters to show the galaxy's HII regions and supernova remnants. Using an [O III] filter to image M31 has been extremely rarely done by amateur astronomers. The team found a few examples of this, but all of these photos were only relatively briefly imaged with [O III] filters, or the field of view was limited to the galactic disk due to a higher focal length of the optics used.

Yann SAINTY asked the amateur astronomers Marcel Drechsler and Xavier Strottner for an estimation if the exposure time with his [O III] filter would be already sufficient to show the [O III] emissions of the HII regions of M31 well in a photo. From their first look at the data, the two amateur astronomers noticed an arc-shaped structure in the southeast of M31.
A continuum subtraction of the [O III] narrow band data revealed an isolated structure that appeared to be completely independent of the galactic halo of M31. To eliminate the first possible sources of error at this stage of the discovery, the US astrophotographer Bray Falls was called in to make a second [O III] exposure. His image field was larger and showed the [O III] arc completely for the first time. The arc in both photos was congruent, making artifact less likely. After the three discoverers the arc was named "Strottner-Drechsler-Sainty Object 1" (HASH database of the University of Hong Kong), planetarynebulae.net

[O III] Sum image, Bray Falls, 3nm [O III] Astrodon 2", continuum subtraction, false colors
Location: Sierra Remote Observatory, Auberry California

NO KNOWN M31 STELLAR STREAM

Considering the position of the [O III] arc, it seems obvious that this structure could be one of the numerous tidal or stellar streams which are almost always formed by the merging of smaller structures during the formation of large galaxies throughout their evolution over billions of years. However, a comparison with scientific work showed that the M31 [O III] emission arc does not match any of the known stellar streams and is completely unknown to science so far. On comparison images of luminance photos and [O III] photos, the arc appears much brighter than all known stellar streams of the galaxy.

Top left: Illustration of the 50 kPc region around M31, the mosaic of the Isaac Newton Telescope with the 5 Known Large Structures.
top right: Overlay with the [O III] image by Bray Falls (false color representation). The figure shows that the [O III]-arc is in a 90 degree angle to the known "Giant Stream E"

Luminance representation of the 150 kPc region of M31

OBSERVATIONS

To our knowledge, this emission feature has not been previously reported in the literature. No significant Hα emission from the hydrogen was found to coincide with the [O III] filaments, suggesting a flux ratio I([O III])/I(Hα) > 5. No apparent match to the emission was found in other online surveys either: e.g., X-ray (ROSAT), UV (GALEX), infrared (IRAS/IRIS, Planck), optical (DSS, SDSS), and radio (VLA FIRST, 408 MHz).
Why this [O III] emission has remained undetected until now is an obvious question that needs to be addressed. Faint [O III] emission nebulae, such as Strottner-Drechsler-Sainty Object 1, are virtually invisible in photographs taken with broadband filters or color cameras. The team also initially considered the [O III] emission to be an artifact caused by stray light, internal reflections, faulty image processing, or glow from the detector amplifier.

The reason for the late discovery of the emission arc around M31 appears to be the combination of the nebula's extremely low surface brightness and its unusually large angular size. Many imaging systems are not capable of detecting such a faint and large line emission nebula. For example, although the recent 2019 CFHT (Canadian French Hawaii Telescope) MegaCam [O III] survey of the PNe population in the halo of M31 published by Bhattacharya's team covered the sky region of the arc, it failed to detect the extensive [O III] emission. These images were taken with a 10.2 nm relatively wide [O III] 500.7 nm filter with a low pixel resolution (0.187" per pixel). This is unsuitable for detecting faint, diffuse, and extended nebulae that appear just above the background and detector noise.

THE NATURE OF THE EMISSION ARC IS STILL UNCLEAR

Because of the recombination of O++ in a cosmically short period of hundreds to a few thousands of years, only a few phenomena can be considered. Curved, filamentary structures like this arc are often observed in planetary nebulae (PN). The absence of infrared and ultraviolet signals could fit a particularly close PN scenario. However, the models show that electron temperatures in excess of 60000 K are required for the ionized oxygen to hydrogen line ratio found. To such high heating, all potential white dwarfs in the region are too cold.
A filamentary nebula that shines particularly brightly in [O III] could also be a galactic supernova remnant (SNR) high above the Galactic disk. However, the lack of coincident radio or ultraviolet emission makes an SNR scenario very unlikely.

The vector of M31's proper motion measured by GAIA points exactly in the direction of the [O III] emission arc, suggesting a possible interaction of M31 with the Milky Way. But the arc seems to be a bit too close to M31 to fit into this picture, despite the fact that the motion of M31 is directed almost exactly towards us. It is very likely that the arc is inside the halo of M31 and is related to the numerous stellar streams, especially the "Giant Stellar Stream", whose eastern edge is close to the structure. A spectrum of the [O III] emission arc would provide radial velocity information that could link it to M31 and its structures in the halo.

AUTHORS

Marcel Drechsler 1, Xavier Strottner 2, Yann Sainty 3, Robert A. Fesen 4, Stefan Kimeswenger 5, 6, J. Michael Shull 7, Bray Falls 8, Christophe Vergnes 9, Nicolas Martino 9, Sean Walker 10

1 Èquipe StDr, Bärenstein, Feldstraße 17, 09471 Bärenstein, Germany
2 Èquipe StDr, Montfraze, 01370 Saint Etienne Du Bois, France
3 54000 Nancy, Lorraine, France
4 6127 Wilder Lab, Department of Physics and Astronomy, Dartmouth College, Hanover, New Hampshire, 03755, USA
5 Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstr. 258, 6020 Innsbruck, Austria
6 Instituto de Astronomia, Universídad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
7 Department of Astrophysical and Planetary Sciences and CASA, University of Colorado, 389-UCB, Boulder, CO 80309, USA
8 Sierra Remote Observatories, 42120 Bald Mountain Road, Auberry, California, 93602, USA
9 Various Amateur Observatory Sites, Lorraine, France
10 MDW Sky Survey, New Mexico Skies Observatory, Mayhill, NM 88339, USA

PUBLICATION

Drechsler, M. ,Strottner , X., Sainty Y., et al., Research Notes of the American Astronomical Society, Vol. 7, id. 1, (2023) DOI: 10.3847/2515-5172/acaf7e