How do you know if a galaxy will respond to Ha?

[swag72]

I prefer capturing nebula (I think!!) but as Galaxy season is on us, I thought I'd work on galaxies and improve data capture, processing etc. So, with the sky clear, but the moon coming up, that leads me to Ha - But how do Iknow what galaxies will respond to Ha?

When I get a galaxy in the FOV on the camera and with the Ha filter in I can make something out - Is that an indication? Or is there some inexhaustable resource somewhere that will give you that type of information?

[swag72]

Any takers on this question?

[Merlin66]

Check your catalogues first...

Barred spiral galaxies have active Ha regions...ellipticals and globular have none.

[RobH]

Face on or near face on spirals tend to have regions that show up in Ha, but edge on ones don't as they are usually obscured by dust.

Rob

[FraserClarke]

It basically depends if there is ongoing star formation in the galaxy. You'll find that most Sprial galaxies (e.g. M31) and Irregular galaxies (e.g. M82) will have star formation and hence H-alpha regions. Elliptical galaxies (e.g. M87) are basically made up of old stars with no (or very very few) new ones being formed, so there won't be much H-alpha signal from them.

So, H-alpha is probably worth it on spirals and irregulars, but not on ellipticals. There are always exceptions of course

The other slight complication is the redshift of the galaxies. For objects beyond ~100 million light years, the H-alpha line will be redshifted outside of your H-alpha filter, so you won't see it! Galaxies at that distance are pretty faint though, so probably not the ones you're worried about.

[swag72]

Good info, thanks

[ollypenrice]

Interaction between galaxies tends to trigger star fromation. NB M82.

Olly