redshift
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by
suelaine
Where do I find the information of "redshift" of a particular galaxy, This one for instance.
Posted
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JeanTate
in response to suelaine's comment.
Click on the Galaxy Zoo examine link, then on the View on SkyServer one. That should bring up a separate tab/window for SDSS J161546.52+295253.1. In the right-hand panel, in the section headed "SpecObjID = 1583023838487144448", there's a column heading "z", and below it 0.008. That's the redshift for this galaxy, as determined from its SDSS spectrum.
It's a pretty neat object, a blue elliptical! 😃
Posted
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majortim
So the redshift gives me a hint how far away the galaxy is. So is there rough guide for estimating the distance? Moreover how can I determine the redshift of a nearby object, when I'd like to know if the two objects are merging or if it just appears as if?
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ElisabethB
moderator
Not all the objects have their redshift measured.
We are asked to classify these images purely on a visual basis. So, for me a merger is when there is clearly interaction between the galaxies. Ie, when they are clearly distorted.Happy hunting 😄
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JeanTate
in response to majortim's comment.
That's a big topic, which I can't do justice to in this post. 😦
Other than for galaxies in the Local Group - such as M31 and M33 - and many in the nearby Virgo cluster, the Hubble distance-redshift relationship can be used to get a rough estimate of the distance, given the redshift.
If two galaxies appear close, on the sky, and if you know their redshifts, then it's possible that they may be interacting if the difference in redshift is no more than ~a few hundred km/sec (you can convert redshift to line-of-sight velocity by multiplying the redshift by the speed of light).
Redshifts accurate enough to estimate distance - using the Hubble relationship - need to be measured spectroscopically.
All too brief, but if you'd like to know more about any of this, please just ask!
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majortim
So as the redshifts here are rarely measured spectroscopically, I couldn't estimate the distance anyway even if I knew how?
But if I had such data, could I just enter this into my spreadsheet app of my choice (with the necessary formula) and get the distances?Posted
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by
JeanTate
in response to majortim's comment.
As far as I know, the UKIDSS involves only imaging, so if there are spectra of the galaxies, they'll have been obtained by different programs or observations. By far the biggest such separate program - for parts of the sky which UKIDSS covers - is the SDSS. Collectively, the various phases of SDSS have well over a million galaxy spectra. Off the top of my head, I don't know what percentage of UKIDSS objects we get to classify here in GZ are also SDSS objects with spectra, but I'd guess that it's pretty high, maybe even over 90%. Also, any particular galaxy may have spectra from other, independent, observations or programs; these can be found from the associated NED webpages (holler if you don't know how to find these).
In addition to pretty accurate estimates of redshift - from SDSS spectra - you can get what are called photometric redshift estimates, z_ph. Depending on what you intend to do with redshift estimates, these z_ph may be good enough, most of the time.
if I had such data, could I just enter this into my spreadsheet app of my choice (with the necessary formula) and get the distances?
Yes ... with all the usual caveats that come with extra-galactic astronomy, starting with the 'mandatory' warning: 'it depends ...' on what values of the key cosmological parameters you choose to use; on the estimated error in the stated redshift value; on ....
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