2 objects or one?


I’d like to be a better user of this data set, and I’d like to learn how to look up the redshift of what I see in the image, if it is possible. Here I’ve come across what I guess are 2 discrete objects, but which struck me at first as being something like an erupting galactic core, or maybe a blue galaxy interacting with another that isn’t.

Might anyone tell me how I may look up the the redshifts in these images? Is it available right from the decals browser, or do I need to take RA/Dec to a different site and do a lookup?

I think I’ve been able to answer my own question, and if I’ve got it right, the blue galaxy is in fact more distant.

The process I used for this is as follows:

  1. On the plate in Decals, click the radio button for SDSS Spectra
  2. Click on the galaxy link that appears on the image. You are redirected to the DR12 Science Archive Server (SAS) page.
  3. Click open in Cas link, and look up the z value for the galaxy in question and zoom in to nearby objects to do the same.

If this is sufficient, I’ve got z=15.54 for the yellow galaxy, and z=21.44 for the blue. I know little about how z values indicate distance, but it doesn’t sound like these birds are flocking together. I suppose the other thing to do is use ra/dec and look up directly in skyserver.sdss.org.


I sent this to our team and they commented,

Strong emission line cone, perhaps? “A spectrum is worth a thousand images.”


as in https://en.wikipedia.org/wiki/Hanny's_Voorwerp ?
OIII would be in g-band.

so that’s pretty cool.

You can check whether SDSS has a spectrum for an object in the viewer by clicking the “SDSS Spectra” overlay. You’ll see that this galaxy and its nearby yellowish neighbour are both at redshift z ~ 0.09. No spectrum for your object though.

There is an unfortunate naming problem – SDSS includes images in the u,g,r,i, and z bands, so the “z” you found in CAS, with values z between 15 and about 23, are z-band magnitudes. These are not redshifts, also called “z”. The max redshifts for SDSS galaxies are like z~1. So that z=21.44 value is the z-band magnitude (brightness) of the blue object.


Really?! It might be a Voorwerp?