ATLAS - THE TITAN THAT BEARS THE WORLD ON ITS SHOULDERS
Since the early days of astronomy, celestial objects have been named after mythological figures, deities and other cultural elements - a long tradition that is maintained to provide continuity and a link to the history of astronomy. By using such names for new discoveries, the public's attention is drawn to these objects and their significance is emphasized. In Greek mythology, Atlas was a Titan who held up the celestial vault at the westernmost point of the then known world. He is also the personification of the Moroccan Atlas Mountains, where most of the data for G209.9-8.2 was acquired. Due to this fact and its resemblance to a bluish globe, the team unanimously decided to name this previously unknown supernova remnant ATLAS.




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RIGHT ON THE DOORSTEP
After the Nereides SNR G107.5-5.2 (https://www.astrobin.com/towmdb/) in the constellation Cassiopeia, the Atlas SNR G209.9-8.2 is the second large supernova remnant that has been discovered, analyzed and photographed by our team of amateurs and professional astronomers. And once again, the location of the remnant is extremely unexpected, being halfway between Messier 42 and the Rosette Nebula in the constellation Orion.


An accurate illustration of the Atlas SNR within the constellation Orion, showing its size and exact location. (c) Bray Falls


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THE TEAM
Yann Sainty, France  ·  Bray Falls, US  ·  Marcel Drechsler, Germany  ·  Professor Robert Fesen, US  ·  Curtis Morgan, US  ·  Aziz Ettahar Kaeouach, Morocco  ·  Richard Galli, France


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THE DISCOVERY OF THE ATLAS - ONCE AGAIN UNEXPECTED
The first trace of this giant supernova remnant dates back to 2022, when team member Marcel Drechsler examined various radio surveys for conspicuous structures that could indicate previously unknown galactic SNR. In the NRAO VLA Sky Survey (NVSS) at 1.4 GHz, a conspicuous structure appeared that seemed to consist of a faint eastern and western arc. The two arcs encompassed an area of exactly 2 degrees in the night sky, equivalent to 120 arcminutes, or four times the diameter of the Moon.
Such a large emission shell about eight degrees from the galactic plane was quite unexpected. A first test with an H-alpha filter along the strongest radio signal was supposed to bring clarity, but nothing could be seen in the images. So the "Orion SNR", as the team had named it for the time being, went dormant for a year.

In 2023, Bray Falls discovered a faint optical [O III] emission in images taken by him and Curtis Morgan, corresponding to the position and size of the faint radio shell discovered in 2022. The remnant, which has a diameter of about 1.8° in the visual spectrum and is centered at l = 209.9°, b = -8.2°, was then photographed by the team with over 320 hours of exposure time, with the goal of detecting another giant supernova remnant in a prominent northern constellation.
The result was an almost complete emission shell, which appeared brightest in the north (top of the image) and east (left of the image).




The first hints of optical signal from Atlas were identified during a large survey campaign this winter, spanning from the constellations of Fornax and Eridanus all the way to Gemini and Canis Major. Using a Rokinon 135mm lens and a dual narrowband filter at Hakos in Namibia, the survey ran from West to East at a depth of 1.5hrs per panel. Requiring a total of 65hrs of exposure time to arrive at panel number 43 which contains the Atlas SNR (image below). 



Next image is field number 43, and at first glance there is nothing obvious in this field. Atlas has extremely low surface brightness, and so it barely stands out from the image background.




The only trace of the Atlas SNR visible here is the brightest northern filament section near TaWe1. It is not uncommon for planetary nebulae to harbour previously unknown oiii shells. Therefore, we initially expected this signal to be a halo of TaWe1, but the fact that it appears to be moving away from the PN gave hope that it could be something independent. 
A six-hour integration of the potential filament region with a Delta Rho 350 performed by Bray Falls and Curtis Morgan confirmed that it is indeed an interesting structure that likely represents an SNR (image below).




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A TRUE SUPERNOVA REMNANT?
Although we do not yet have optical spectra that definitively confirm that this nebula with a diameter of almost 2 degrees is a true SNR, there is little doubt about its SNR nature. Given its large angular size, its location eight degrees from the galactic plane, and its strong filamentary nebular structure, especially in [O III], it exhibits many of the expected characteristics of an SNR. There is no OB association within or near the shell that would indicate an H II region. On the other hand, if it is a planetary nebula (PN), it shows an unexpectedly strong ratio of [O III] to Hα emission and an inverted morphology where [O III] is strongest at larger radii with little Hα. It would also be the largest known PN with a diameter of 2 degrees, surpassing the current record holder SH2-216 with a diameter of 1.6 degrees. 

To get an idea of the enormous size of G209.9-8.2 and considering it as a possible PN, we can take a look at the planetary nebula G208.9-8.0 (TaWe1) located exactly north of G209. 9-8.2, whose bright inner region has a diameter of 145 arcseconds. Together with the oval halo, the PN reaches a full diameter of 330 arc seconds, which is almost as large as the famous Dumbbell Nebula (M27, NGC 6853). Despite the fact that TaWe1 is a comparatively large PN, G209.9-8.2 makes it look downright tiny.


Image above: the first color image of the planetary nebula G208.9-8.0 (TaWe1)
The image is upscaled by an AI.


Another planetary nebula in the image is the bubble-shaped PHR 0615-0025, which is located on the periphery of the northwestern filament group.
Even in comparison with this PN, it is unlikely that G209.9-8.2, which dwarfs PHR 0615-0025 in size by a factor of many hundreds, is a PN.


Image above: the first color image of the planetary nebula PHR 0615-0025
The image is upscaled by an AI.


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THE ATLAS - ALSO A RADIO GIANT
As mentioned at the beginning, the first trace leading to G209.9-8.2 was a weak radio signal showing two faint arcs at 1.4 Ghz. The eastern periphery (left in the image) fits perfectly with the data in the visual range.
However, the situation is different with the western periphery (right in the image). Here we observe that the radio shell extends 0.2° degrees further west than the detectable visual signal.




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A CLOSER LOOK
In [O III] the remnant shows a bright, flat northern edge, the eastern edge of which is unusually sharp and well defined. This sharp eastern edge is also brightest on the 1.4 GHz NVSS map. Although much of the western edge is faint and/or absent, there is diffuse emission near the center of the remnant. As we have seen in other remnants, the Hα emission from G209.9-8.2, in contrast to that in [O III], is much less pronounced. While we find some filaments along the northern and western edges of the remnant, most of the Hα emission is diffuse. Consequently, we are not sure if all of the diffuse Hα we see here is actually related to the remnant.

Here we see that a Hα filament along the western edge is consistent with the [O III] filament and that there is a large diffuse Hα emission field at the southern tip of the remnant. There is also a small curved filament at the northern edge of the remnant and a very faint Hα emission outside (north of) the bright east-west line of the [O III] filament. 




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MORE THAN 320 HOURS OF OBSERVATION
The data acquired in Morocco was collected remotely by Yann Sainty, Aziz Ettahar Kaeouach and Richard Galli at the High Atlas Observatory at the Oukaimeden Observatory in the High Atlas in Morocco.
In the United States, Bray Falls and Curtis Morgan imaged the object in the Sierra Nevada mountains at the Sierra Remote Observatories (SRO). In total, over 320 hours of exposure were taken at the two stationary sites in Morocco and in the Nevada mountains from December 2023 to February 2024 - at the darkest locations accessible to amateur astronomers today.


Image above: a first test with an OIII filter and a few hours of exposure time, which should make the exact positioning of all the team members' image fields possible in the first place. The biggest concern of team members Yann Sainty and Marcel Drechsler was that important details or structures of the remnant would not be imaged.


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GOOD PLANNING IS ESSENTIAL
The preparatory work for such a project is very important. It would be a nightmare to invest hundreds of hours of work only to realize that not all setups cover the entire field of the object. Therefore, we first had to estimate the size of the object and retrieve the information from each setup to determine the fields.
Team member Yann Sainty planned and calculated all the coordinates of each panel in each setup to provide to each team member according to their setups.

Once the data acquisition was complete, a major challenge was the complicated combination of all the data, which differed not only in their image dimensions - due to the different optics and camera sensors - but also in the north orientation of the respective data sets. However, team member Yann Sainty was able to overcome all of these initially insurmountable problems after weeks of work.


Image above: colored representation of all optics used and the mosaics created for each one


The result is a high-resolution mosaic image that not only includes the images in the red, green and blue spectral range, but also those in the hydrogen line H-alpha and in the line of ionized oxygen O III. In addition, there is a so-called luminance image, which covers the entire optical spectral range. In this way, even the extremely faint dust nebulae could be isolated and visualized.

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AN ARTISTIC PRESENTATION WITH A SCIENTIFIC BACKGROUND
The image of G209.9-8.2 shown by our team is an artistic representation based on the data we collected. We have taken care to ensure that the processing is true to the original, but we have also attached great importance to creating a visually appealing image. Please bear this in mind when viewing the image.

The two small planetary nebulae in the image were processed separately and inserted into the final image, as both PNs have a very high surface brightness that far exceeds that of G209.9-8.2. In order to create a harmonious overall image, the brightness of the PN was therefore significantly reduced. 
The brightness of individual areas of G209.9-8.2 was also adjusted, as the slightly flattened northern area is the brightest part of the SNR, but we also wanted to show the fainter southern parts clearly. The northern part was therefore also dimmed.

The brightness of the cosmic dust, on the other hand, was increased, as this is of great importance for understanding G209.9-8.2 - parts of the cosmic dust block emissions from the remnant (especially from OIII) and thus explain the special appearance of the Atlas SNR.

In order to obtain an unobscured view of the SNR, the apparent size of the stars was also reduced. This processing technique is now common in astrophotography, but we would still like to mention it here.

When looking at our detailed overall image of the Atlas SNR, you might think that artificial intelligence (AI) was used here. And indeed, various AI applications were used for image processing. However, we made sure that the AI did not add any artificial details to the image. We used AI applications that reduced noise, moderately sharpened the image and corrected slightly distorted stars. After using an AI, the details were compared with those of the raw image and checked for accuracy. All other processing techniques are of a traditional nature and were implemented using the Photoshop and PixInsight software.

The comparison in the images below shows the OIII raw data in grayscale and the finished image at 100% with the same image section below.
The raw data was non-linearly stretched, continuum subtracted and the star artifacts were removed.




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OUR SPECIAL THANKS
The team would like to thank Professor Robert Fesen for his confidence in us and for the scientific follow-up,  Benjamin Martino for producing the superb video presenting the new Atlas SNR,and the whole team of the Oukaimeden Observatory and  Zouhair Benkhaldoun for providing us with incredible shooting conditions.

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