PhotoZ Error Bar
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by ChrisMolloy
This is AGZ00047hl, which is the target galaxy in the image.
AGZ00047hl has a PhotoZ of z=0.210344 +/-0.01863.
The object at the top edge, with a slight overlap of AGZ00047hl is SDSS J012011.71-205354.8. This has a PhotoZ of z=0.119722 +/--1000.
What does the error bar of +/--1000 mean for SDSS J012011.71-205354.8. The error bar only decreases?
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by Capella05 moderator in response to ChrisMolloy's comment.
Hi Chris,
I believe it means that the photoZ for the 2nd object is unusable.
There appears to be a lot of confusion about photoZ going around, and I have been meaning to write a glossary entry about it. PhotoZ is an estimate calculated by an algorithm, using the colour of other objects / galaxies in the vicinity. It needs several variables in order to make the calculation, so there is a error margin.
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by Budgieye moderator
Here is what SDSS says about PhotoZ. Warning: lots of big words!
http://skyserver.sdss.org/dr7/en/help/docs/algorithm.asp?key=photoz
To me, it looks like the photoz error of -1000 looks like a math operation error.
With the huge error margins in photoz, I think anyone who is not colour blind can do an equally good job. I look at the target galaxy and think, " fuzzy detail so at least z=0.1, turning orange so z=0.2, but too too orange, so not 0.3. With the smaller galaxy, I think "Well, it looks about the same"
The accurate way of estimating the distance of a galaxy is by doing a spectral chart, which is time-consuming and costs money. Only a million and half spectra have been done !!! but there are a billion objects to be looked at. http://www.sdss3.org/dr9/
So no spectral data is available for the less interesting galaxies.But if someone is going to do a computer search of yellow-orange galaxies, they can't put "yellow-orange" in the search parameters, they need a number. So for a computer
yellow-orange is photoz 0.1-0.3
galaxy is "a source which is not round"
Unfortunately, it might be mean two orange stars close together, or a extremely distant orange quasar near something else.
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by ChrisMolloy
Thanks Capella05, Budgieye,
With the huge error margins in photoz, I think anyone who is not colour blind can do an equally good job. I look at the target galaxy and think, " fuzzy detail so at least z=0.1, turning orange so z=0.2, but too too orange, so not 0.3. With the smaller galaxy, I think "Well, it looks about the same"
Very informative Budgieye. Very similar to what you'd posted in the GZ forums on the Colours of Galaxies in SDSS: Redshift Chart. I'd completely forgotten your post. Will use it as a cross reference/sanity check next time for PhotoZ.
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by Budgieye moderator
Thank you !! 😃 😃
updated version of the chart is
Astrophysics for Galaxy Zoo Talk - redshift z PhotoZ spectra emissions AGN
Galaxy Redshift Chart http://talk.galaxyzoo.org/#/boards/BGZ0000001/discussions/DGZ0000ulp?page=2
The links work better than in the forum version.
I'll put this reply into it.
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