M51 plus and minus HA

[timh]

Took advantage of a couple of weather gaps to capture RGB galaxy images at long focus (f = 1000mm) around the new moon (13/02) - and HA in a clearsky gap earlier in January.

Skywatcher 200 PDS Newtonian F 5.0 plus Baader field flattener
Ioptron CEM70 mount
PHD2 guiding via 10 x 50 finder and ASI 120mm
Pegasus Cube focuser
ASI 294 MC PRO for RGB images at -10C
ASI 294 MM PRO and Optolong 7 nm HA filter for HA images at -10C

Gray-scale flats (100 frames) and darks (40 frames) at -10 C created by SC

160 x 40s (106 min) RGB images -(total accepted after many rejected by the SC filters and then a few more by PixInsight)
9 x 180s HA images
Gain 124, offset 10
Bortle 6 skies

Preprocessing and postprocessing in Pixinsight (DBE, Photometric colour calibration, pixelmath, MLT, Histransform, Curves) and tidied up in Affinity.

The image on the left is without addition of the H alpha. A balanced HA - red channel difference image was added back to the red channel to create the image on the right. The HA nicely shows up the very high rate of star formation going on in M51 - presumptively due to the disruption of the spiral arms due to the relatively recent passage of the associated galaxy, NGC 5195. I also like the fact that the HA signal removes any ambiguity about which points of light are associated with the galaxy and which are foreground stars. A couple at least that I would have taken to be foreground are actually massively bright HA regions seen at some 23 M ly. Must be a good galaxy to live in for spectacular deep sky astronomy!

For galaxy images is interesting to note at which point hubble red shift (plus any local recessional velocity) will move the HA signal outside of the bounds of the filter. My 7 nm filter should be OK out to about 100 M ly but beyond that a wider bandwidth filter or even an SII filter might be called for?

Tim

[oopfan]

Hi Tim,

Excellent work! Another thing to consider regarding the bandwidth of an Ha filter, there are two Nitrogen lines on either side of Ha. A 3nm filter will nicely isolate Ha, but a 6nm filter will pass some NII. Not all emission nebulae have strong NII, but enough to make life interesting. I've seen spectra where NII outshines Ha.

Brian

[Menno555]

Nice one!! Did you ever try this with M33? It has some cray regions in it.

Menno

[timh]

Thanks Menno,

M33 is on my list but it will have to wait til autumn comes around again. Have collected data on 9 galaxies thus far. M101 is probably most spectacular wrt HA. The contrasts in activity are really interesting and something that I am pursuing at the moment. Naively I had imagined that all the galaxies that show a lot of blue would also show a corresponding level of HA but, on reflection, I suppose that the HA level depends on how much H is still close by or has been blown away or stripped off since the burst of star formation (evidenced by the knots of bluish light) occurred? - need to look this up. M94 and M63 for example show bluish inner rings, strikingly bright inner rings in the case of M94, but associated with only a a few discrete points of strong HA. M51 and M81 on the other hand - and especially the former - show many points of high HA intensity. I am guessing that this might mean that the star-forming activity in M81 and M51 was more recent than that in the other two galaxies? It was also interesting to be able to also detect points of HA in M109 since, at its distance (~60Mly?) and recessionary velocity the HA band would have been red-shifted to ~ 658.5 nm and at the edge of my filter's ability to detect it. Next opportunity I want to try using my SII filter (672 nm) to see if I can detect any red-shifted HA light in any of the spiral galaxies in the Coma galaxy cluster (350M ly). Probably a long shot to detect anything but if it works then a nice backyard-accessible demonstration that the Universe is expanding ... not that I doubt it .

Tim