M51 Whirlpool Galaxy (broad- and narrowband merge)

[Menno555]

Tim, keep on throwing words like that in. My English is okay but more words to learn are always welcome

I really don't know. My logic says that if blue-ish star forming light would pass, it would be all over (so to speak) M51 cause it has so many star forming regions. But it isn't and there is only that spot. Mind you, I don't know if it is part of M51 or that it is in front of it. The NED catalogue didn't had any redshift info on the source, so also no distance mentioned.
But also, the L-eXtreme is, for a color camera filter, very narrow with 7nm for both lines. So maybe it's better in "pin pointing" sources?
But no matter, it's just cool to see this 1 blue spot in that see of red

Menno

[timh]

Menno,

Your English is outstanding -- whereas my Dutch is non-existent....although some English words (e.g maelstrom - which describes the core of M51 very well) do I believe originate from Dutch ? However I will henceforth try and work at least one obscure English words into my replies to you

Back to the science though - I am pretty confident that my assertion that the blue/ green that you are seeing is purely of stellar origin is correct. As far as I know (someone may correct me?) the only types of narrowband object that are brighter in OIII than in HA are planetary nebulae. They shine brighter in OIII because a) their central dwarf stars are so ridiculously hot (> 100K), b) OIII light emission (unlike HA) is driven by a temperature dependent collision process and c) that when first formed planetaries are small enough that the O is not too dilute and so plenty of collisions happen. Obviously planetary nebulae are so dispersed and short-lived that they couldn't feasibly account for a spot of light visible at 20Mly in another galaxy. One problem is probably with the way narrow band filters are described? The L-extreme is described as an HA/ OIII dual band filter -- but that description is only apposite when it is pointed at a purely narrow band emitting object (e.g. like the regions of an emission nebula or a planetary nebula). For objects like stars that shine across a continuous spectrum narrowband filters equally pass light at their set wavelengths - but in that case the light passed is of course neither HA nor OIII.
Best wishes
Tim