Preface

After the DSC’s first image release of the Whirlpool galaxy in August 2023, the group has made considerable progress - be it by implementing new data-splits, stacking techniques or experience in handling these data-sets. Despite the image being a big success for us at the time, this year we felt that with this newly gained knowledge, a much better and detailed image of M51 can be created. With this in mind, our goals were set on improving with respect to our old image and potentially achieve results similar to those of professional observations.
Not only is this project a testament of how much a group can grow and learn within just over one year, but also yet again highlights how a group of amateurs with modest equipment can contribute to science.
By being transparent and giving insight into how we do these things we hope to inspire others to try and do the same - that being said, the first tutorial on pre-processing data on this scale will hopefully release within a month, with other changes on the website coming soon as well.

Note that the technical aspects of the following write-up largely repeats the content of the original—since the principles of physics haven’t changed (thankfully)— but it does include quite a lot of new insights and details

Write-up

The Whirlpool galaxy, also known as Messier 51, is a galaxy located in the constellation Canes Venatici. It lies at a distance of roughly 23 million lightyears and spans some 80.000 ly in diameter. M51 is a pretty popular target amongst astrophotographers as it has some very distinctive spiral arms which were first discovered by William Parson in 1845, also making it the first object where spiral arms were detected.
Another unique attribute to this galaxy is the tidal stream which shows even at low integration times. The full extent of the tidal stream on the other hand is rarely shown as it requires dark skies and lots of integration time - using our new group imaging split, we were able to reveal the streams in even more detail than in our original version. These streams are a result of the tidal and gravitational interaction between M51a (left) and M51b (right). So in fact M51 refers to the ensemble of the two galaxies rather than just one individual one.
As is the case for many interacting galaxy-pairs, M51 has been studied excessively to better understand what’s happening between two merging galaxies. However, it wasn’t until 2018 that A. Watkins et al. discovered a large ionised gas cloud north of the galaxy system, which was the main focus for this collaboration.
You can spot this recently discovered ionised gas cloud, or how we like to call them, “Ha cliffs”, on the right hand side of our image - knowing how difficult it is to reveal the cliffs from our original project, we used all of our tricks and went with super long (average of ~1300 seconds) subs and a total of 383h00m of integration in Ha alone.

As noted earlier, the main focus with this project was the Ha, however, with a larger FOV as in the original, we also decided to go deep on LRGB to reveal both the tidal stream and potential IFN in the region.
With the small FOV group of the DSC we started gathering data in late April and imaged for about 3 months, followed by 3 months of pre- and post-processing the data. We had a total of 17 people working on the project - 1 Editor, 1 Stacker , 16 photographers and myself coordinating (Stacker and myself also got data for the project) - in the end we managed to get a total of 531h47m of integration - the longest ever on this field.

Before getting into any details in the picture, I want to thank everybody who contributed to this project. It was a pleasure working together with everybody!

Tim Schaeffer - @Tim Schaeffer – Coordinator and Photographer
Carl Björk - @Palmito – Stacker and Photographer
Steeve Body - https://steevebody.com/ – Editor

And our photographers:
Fabian Neyer - http://www.starpointing.com/
Aki Jain - @Meeps51
Ryan Wierckx - @Ryan Wierckx
Paul Kent - @Paul Kent
Brian V. - @Brian Valente
Antoine and Dalia Grelin - www.galactic-hunter.com
Nicolas Puig - @Nicolas PUIG
Stephen Guberski - @Stephen Guberski
Mike Hamende - @Mike Hamende
Julian Shapiro - @Julian Shapiro
John Dziuba - @John Dziuba
Mikhail Vasilev - @Mikhail Vasilev
Bogdan Borz - @Bogdan Borz
Adrien Keijzer - @Adrien Keijzer
Special thanks to:

Carl, who, as always, put in a remarkable effort stacking nearly 3000 files.
Steeve, for another super impressive and well-rounded edit.

Interesting features in our image

The Hydrogen alpha cliff formation

The Ha cliff formation is a remarkable yet rarely imaged feature found within the Whirlpool Galaxy, and simply put is likely a result of the interaction between the two galaxies - the exact nature is a topic of ongoing research and will be added to this post once we can give reliable information and sources.
As we’ve learned from our first go at this target, long sub-exposures are an absolute must in order to actually pick up any signal from these faint clouds - putting extra emphasis on this, we collectively reached our highest-yet average sub-exposure length for any project, for a staggering ~1300s per exposure, effectively helping us in achieving the goals of maximising SNR. This, along with nearly 400h of pure Ha data allowed us to accurately and properly extract the cliffs, unlike in the first project where the data simply wasn’t good enough, resulting in merely a blotch of Ha which - by comparison - is real signal, i.e. the brightest part of the cliffs, but lacks any definition.

Below is a comparison between (in this order) the first project, this new one and professional data from its discovery by Watkins et al..

© (left to right) DSC’s old Ha stack (+continuum), DSC’s new Ha stack (+continuum) and Watkin’s discovery picture

Note that using wider bandpass Ha filters (5-7nm) will in fact be better than using a super narrow (3nm) one as they allow for Nii to pass through, which is roughly twice as bright as the Ha in these cliffs. For this, ideally also have access to dark skies.
Of course, for the data to be of any use to science and for the presented signal to be actual Ha/Nii, the resulting stack was continuum subtracted - a practice everyone should do. The resulting image can be found below.

© DSC - M51 Ha/Nii continuum subtracted

An interesting thing to note is the little arc the Ha makes when it leaves M51b - this is a so-called “AGN-blown bubble”, so it is the result of the active galaxy nuclei in M51.

Full tidal stream and IFN

Tidal streams are diffuse, elongated structures of stars, gas, and dust, often extending beyond the boundaries of galaxies. As M51a and M51b are currently engaged in a violent gravitational interaction, the smaller galaxy M51b (or NGC 5195) experiences tidal forces that disrupt its shape and provoke the formation of new stars. Conversely, NGC 5195 also influences M51, leading to the formation of spiral arms, making this galaxy pair so unique.
These tidal streams allow researchers to reconstruct the two galaxies’ orbital history as well as trace past movements of diverse stars or gas in the galaxy, allowing for a more exact understanding of interactions between galaxies.
In order to get this entire stream, long integration times and dark skies are required, which is why only people from our group from B3 and below got LRGB data.

In the end, we had 109h52m of Luminance and close to 40h of RGB data, which together with the low light pollution it was shot in allowed us to reveal them even better than the first time. As a result of this high quality Broadband data, our team was also able to reveal the super faint IFN in this region, making this region seem even more busy.Below, a picture of the raw Luminance stack as well as the final LRGB starless image can be found.

© DSC - Starless LRGB image showcasing the Broadband nebulosity in this region

Blue stellar stream

Similar to M82’s blue stellar stream, M51 also has one that can be found to the north of the galaxy (right hand side of the picture) but is rarely noticed.
As for the cigar galaxy, the exact origin of this stream is yet to be exactly determined. The two leading explanations are that it’s either an interaction remnant or a shocked starburst stream, though most likely it is a secondary result of a galactic low-velocity shock caused by the interaction between M51a and M51b. The resulting shock heats and compresses the gas to optimal star-forming conditions, giving the stream its signature blue colour.
In the picture below you can see a comparison between our Old M51 image and the new one - unfortunately we did not get the same definition in the blue stellar stream this time, which might be due to the lower integration time in B, at only 12h or the emphasis on Ha drowning the Blue out.

© DSC - Blue stellar stream in the old and new image

Background galaxies

As most regions in the night sky, the background is truly packed with background galaxies, with a wide variety in size. Having a close look at our image, the immense number of galaxies can be seen, most as just red dots or as part of bigger clusters. Given the depth and wide field captured, zooming in truly is a surreal experience and we invite everyone to go check out the full-resolution version on our website!
It is also worth mentioning the galaxy on the very left with its elongated tidal stream, giving it a very special look.

For any further questions about the project, feel free to leave a reach out to us using the contact form or by leaving a comment!

If you want to see our image in greater detail, feel free to go to our gallery, where the image is uploaded in full resolution ( 5619 × 4151 px), enabling you to explore the picture by yourself and being able to zoom in on every tiny detail!
Link to our gallery: https://deepskycollective.com/gallery

Integration overview

Here you find a list of integration contribution from all 16 photographers

We hope that you enjoy this image!

Text written by Tim @xSky_Watcher, organiser and co-ordinator of the project.