New Image of M51--Better (I think)

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Jared Willson Senior Member • Posts: 1,151
New Image of M51--Better (I think)

A couple weeks ago I posted an image of M51, the Whirlpool Galaxy. It's a lovely "grand design" face-on spiral galaxy and also part of an interacting pair along with NGC 5195. Hard to beat that. There were a couple things that disappointed me in my image, though. The first was that the image wasn't clearly better than at least one of my previous attempts at M51. The colors were nice, and it was surprisingly deep considering I took it from Oakland, CA, but it was lacking some punch.

I decided to do two things. First, I collected a bunch of H-alpha data in order to enhance the star forming regions in M51. I hoped that would add the punch that I thought was lacking in the first version. Second, I took my telescope to a dark sky location to see what I could do in a single night. This meant less data than with the last image (which was taken over four different nights), but better quality data. I hoped this would let me get just a touch deeper.

The result was super interesting. First, after I calibrated my luminance data I was surprised to find some artifacts towards the south west of my image (the upper right corner). Was there something wrong with my flats? It looked, frankly, like integrated flux nebula (IFN)--gas and dust lit up by the combined light of the Milky Way galaxy, but I had never seen or heard of IFN around M51. It's definitely in the right general part of the sky, though--well above the galactic plane where it isn't obscured by the Milky Way itself. I did a little research, and, sure enough, there is some IFN around M51, and the samples I found matched my photo, so it wasn't simply an artifact from bad flats. Cool! It's pretty faint, though, so it mostly disappears when I process the image to avoid noise overpowering the rest of the subject. I'll include a high-noise version of the image that emphasizes the IFN.

Second, when looking at the IFN I also saw what looked like a tidal tail streaming out of NGC 5198, the bright elliptical galaxy to the south (right) of the Whirlpool. A tidal tail is a stream of stars, gas, and dust that have been disrupted from the gravitational tidal forces of interacting galaxies. Basically, NGC 5198 probably had a dwarf galaxy pass through it, and in the process the dwarf galaxy got ripped apart and spread across 300,000 light years of space. Awesome! Again, I had never seen an image of this feature, but when I went looking for it there are a few out there. The oldest reference I could find of the feature in the professional literature was from the 1980's, so it wasn't found till the CCD era when low surface brightness objects become easier to detect. By the way, if you see Hubble images of NGC 5198, those are mis-tagged. They are of a planetary nebula, not of this galaxy. Not certain which planetary nebula Hubble photographed, but NGC 5198 is definitely an elliptical galaxy.

I was hoping the H-alpha data would add some vibrance to the image--they did. The HII regions really stand out in this new picture. I was also hoping for added depth. It's there. Just one night under dark skies let me go deeper than four nights from home. I was also hoping for a touch more resolution. That part is questionable. The seeing was decent, but nothing special. Most of my luminance frames measured around 2.2" FWHM, so good but not great. The tracking of the mount was excellent, though. I measured that at about 0.3" RMS which is about as good as the seeing conditions would allow. Equipment all worked well.

Anyway, here are the images. First, a wide field version showing M51 and its surroundings.

M51 and NGC 5195, Widefield

Next, a crop of the above image image.

M51 and NGC 5195, Crop

Here is the annotated version showing surrounding galaxies and quasars. This is a quasar rich portion of the sky, due primarily to the fact that we are looking outside the plane of the Milky Way, so relatively little to obscure the view. Anything labeled SDSS (Sloan Digital Sky Survey) is a 20th magnitude or brighter quasar. Anything labeled PGC, IC, or NGC is a galaxy. Not all NGC's or IC's are galaxies, but in this picture they happen to be. On interesting thing is that deep images like this one start to show the shortcomings in the most common databases for faint galaxies. There are at least four obvious galaxies to the left of NGC 5195 that aren't labeled and should have been as well as one PGC above M51 that is labeled, but there is no object visible. The fainter you get, the more mistakes you find in the automated searches.

M51, Annotated

Finally, here is a waaaayyy over-stretched luminance frame showing the IFN to the upper right in the image as well as the tidal tail coming out of NGC 5198.

IFN and Tidal Tail

All sub exposures were three minutes long. A total of 4.5 hours (under suburban skies) of H-alpha were used to enhance the HII (red/pink regions) in M51. I took 2.5 hours of luminance data, and one hour each of red, green, and blue data. Aside from the H-alpha, all data were captured on the night of April 10/11th from near Lake Berryessa, California. The skies there are about as dark as anything within two hours of my house. I measured 21.7 MPSAS which is just un the edge between Bortle 4 and Bortle 3. The telescope is a 12.1" Riccardi-Honders. The mount is an AP1100GTO with absolute encoders (no guiding required for short exposures like these). The camera is a QHY 600 monochrome which has the same chip as a Sony A7mk4.

Overall, I'm super happy with the result.

- Jared

 Jared Willson's gear list:Jared Willson's gear list
Leica Q2 Leica SL2 Hasselblad X1D II 50C Leica SL 90-280mm F2.8–4 Hasselblad XCD 30mm F3.5 +8 more
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