by JeanTate » Mon Jan 20, 2014 1:28 am
cosmo_uk wrote:rstevenson wrote:Well, there's a
Green Pea in it. And it looks like either a merging pair of ellipticals, or perhaps a foreground lensing effect splitting the one elliptical galaxy into two.
I can't tell if all these are in one galaxy cluster or not. If so, I'd guess the Green Pea is not part of the cluster, since GPs are a form of star burst galaxy, meaning it'd be a lot brighter and therefore more distant in order to seem about the same brightness in this pic. That red galaxy may also be in the background, from its colour, but would have to be intrinsically
much larger and brighter to appear about the same size and brightness here, if that is true, so maybe not -- but what else would make it so red?
Rob
Actually the spectra is quite interesting now I've looked at it properly as it is 2 objects superimposed on each other: 1. The BCG as you can see by the continuum emission and strong 4000A break (redshift to 4800A ie redshift=0.2ish) and Ca HK absorption lines ie its an old red galaxy. The emission lines however belong to a galaxy at redshift=0.35ish as the OII emission line for example (3727A rest) has been redshifted to 5050A ish.
So maybe the green object has been gravitationally lensed by the cluster (or is just behind the cluster) and so we're seeing a composite spectra of an old red galaxy at z=0.2 and a starforming/AGN (accreting black hole containing galaxy) at z=0.35, interesting as it means SDSS got the redshift wrong for this red galaxy!
Thank you cosmo_uk!
If you look closely, you'll see all the usual suspects in that spectrum, at z=0.209: H&K, G, Mg, Na, even a hint of H-alpha (in absorption).
The emission lines - of the background object - may be good enough (high enough S/N) to plot on a BPT diagram; anyone care to guess what it'll show? AGN or starforming or composite?
How could you tell if it's a gravitational lens, and not just an overlap?
[quote="cosmo_uk"][quote="rstevenson"]Well, there's a [url=http://en.wikipedia.org/wiki/Pea_galaxy]Green Pea[/url] in it. And it looks like either a merging pair of ellipticals, or perhaps a foreground lensing effect splitting the one elliptical galaxy into two.
I can't tell if all these are in one galaxy cluster or not. If so, I'd guess the Green Pea is not part of the cluster, since GPs are a form of star burst galaxy, meaning it'd be a lot brighter and therefore more distant in order to seem about the same brightness in this pic. That red galaxy may also be in the background, from its colour, but would have to be intrinsically [i]much[/i] larger and brighter to appear about the same size and brightness here, if that is true, so maybe not -- but what else would make it so red?
Rob[/quote]
Actually the spectra is quite interesting now I've looked at it properly as it is 2 objects superimposed on each other: 1. The BCG as you can see by the continuum emission and strong 4000A break (redshift to 4800A ie redshift=0.2ish) and Ca HK absorption lines ie its an old red galaxy. The emission lines however belong to a galaxy at redshift=0.35ish as the OII emission line for example (3727A rest) has been redshifted to 5050A ish.
So maybe the green object has been gravitationally lensed by the cluster (or is just behind the cluster) and so we're seeing a composite spectra of an old red galaxy at z=0.2 and a starforming/AGN (accreting black hole containing galaxy) at z=0.35, interesting as it means SDSS got the redshift wrong for this red galaxy! :)[/quote]
Thank you cosmo_uk!
If you look closely, you'll see all the usual suspects in that spectrum, at z=0.209: H&K, G, Mg, Na, even a hint of H-alpha (in absorption).
The emission lines - of the background object - may be good enough (high enough S/N) to plot on a BPT diagram; anyone care to guess what it'll show? AGN or starforming or composite?
How could you tell if it's a gravitational lens, and not just an overlap?