What do you think this object is?

[JeanTate]

Its name is SDSS J075137.19+325447.3, and here is its SDSS DR7 spectrum:

[cosmo_uk]

Looks like a Brightest Cluster/Group Galaxy to me. Largest galaxies in the Universe sit in the middle of huge potential wells. From the spectra its showing line emission (particularly all that OIII) so it probably has an accreting black hole in it (AGN) and maybe some residual star formation

[rstevenson]

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

[neufer]

An approaching automobile checking out a XMAS display on a foggy night.

[Beyond]

Wow! Responses almost a year later. Is that some sort of a record for the Asterisk*

[geckzilla]

I think Jean gave up on us as a serious source of information months ago. I remember having no idea at the time what to write in response to this but it does look like just an odd grouping of distant galaxies to me now. I think a little more context might help too.

[cosmo_uk]

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!

[JeanTate]

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?

[JeanTate]

I think Jean gave up on us as a serious source of information months ago. I remember having no idea at the time what to write in response to this but it does look like just an odd grouping of distant galaxies to me now. I think a little more context might help too.

That's true, and it wasn't just this thread that proved rather, um, disappointing.

But the number of hits, for almost any thread here (in this section) is HUGE! And cosmo_uk cracked the case (partly at least), so how about I post some more of my questions? Each in a separate thread, of course!

[geckzilla]

I feel like the forum has more knowledgeable people hanging around now than it did a couple of years ago. For a while it seemed lacking in participation after we banned pseudoscience discussions but I think things are picking back up again now and in a much more positive manner rather than being a place to simply debunk anti-science. It'd be great if you started more threads. We still can't promise any answers, though.