Give peas a chance!

[starry nite]

Though I came across it navigating, this was previously-posted as 'magenta galaxy?' by ilrobi.

ra=138.43953811, dec=40.94117359, ObjId = 588013382192726039
http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=588013382192726039

[FermatsBrother]

Though I came across it navigating, this was previously-posted as 'magenta galaxy?' by ilrobi.

Hi Starry - This is an excellent example of this colour shift that I've just described. According to the spectrum, it should be a dull grey, but the near IR is superimposed as red which gives it it's colour.
Fermats Brother

[FermatsBrother]

So does that mean some of the purple peas are green?

Hi Starry - Basically it indicates that blue peas and magenta (blue + red) peas on the screeen may actually be green.
There's probably some way of manipulating your screen software to see the actual colours on screen.
Fermats Brother

[FermatsBrother]

Blue pea, no spectrum. (filter responses: g=18.44, r=18.74 & i=18.99)
If you look hard, you can see a reflection of the SDSS telescope in it !
Fermats Brother
http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=587732703944572950

[FermatsBrother]

Another blue pea; no spectrum. But according to the responses in the colour filters (g=19.13, r=19.12 & i=19.13) it should show as white !
Fermats Brother
http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=588017726542905486

[Rick Nowell]

Nothing is obligatory, but when your list is all posted objects and then you add unposted ones to it, how is anyone supposed to know?

They could look at the list or post it again. It's not really a problem with a small number of potential suspects- especially
since the people doing the postings are few in number and seemingly obsessed with lists anyway.

"Peas" are emission line galaxies with powerful [OIII] emissions that fall into the filter that is presented as "green" by SDSS, making them green peas.

It's interesting as to why these ELGs appear green, whereas others do not. Is it solely because of the [OIII] emissions? Indeed
why are there such massive [OIII] emissions?
It would be good to have a definitive colour, as seen by us. Red seems completely wrong though- these are galaxies with
extreme physics going on. Red to me suggests a more elderly population.

[quarkspin]

Absolutely fascinating - thanks for the lists SN.

I must be useless at hunting peas, I never seem to find them (unlike SO's which are kind of hard to miss)

I follow a bit about the colours but the colour our eyes see depends entirely on whatever filter the light has gone through (essentially blocking certain wavelengths) so unless we know the filters, we don't actully know much about the visible spectrum of the 'pea'.... have I got that right?

I do remember seeing some amazing Roman glass that was  blue on reflected light but looked red on transmitted light. This could end up in existentialism:)

Q

[FermatsBrother]

I follow a bit about the colours but the colour our eyes see depends entirely on whatever filter the light has gone through (essentially blocking certain wavelengths) so unless we know the filters, we don't actully know much about the visible spectrum of the 'pea'.... have I got that right?
Q

Here are the transmission charateristics of the filters on the CCDs in the camera:

u' = detector "sees" actual ultraviolet.   Screen display as nothing 
g' = detector "sees" actual blue/green.  Screen display as blue
r' = detector "sees" actual red.             Screen display as green
i' = detector "sees" actual near infrared. Screen display as red
z' = detector "sees" actual infrared.       Screen display as nothing
The imaging timing sequence is r' i' u' z' g'. This is why asteroids show up as green-red-space-space-blue.

Fermats Brother
                                                                                                             http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=588007004163801311

[FermatsBrother]

Once again, the comprehensive list of all OIII galaxies found on SDSS has outgrown its boundaries, thanks to all the efforts of the Pea-hunters out there. I'm reposting it here in four parts that will hopefully last a little longer.

Hi Starry - Looking at the spectra of your peas, and from what I've just pointed out about actual colours v. colours on the display screen, the following might be of interest.
Your break-points for classification fits neatly into the movement of the OIII peak (with increasing z values) into the different colour filters on the cameras.

  z                  wavelength of OIII        actual colour of OIII      view on screen
0.00                      5007                          green                      blue
0.02                      5107                          green                      blue
0.04                      5207                          green                      blue
0.06                      5307                          green                      blue
0.08                      5407                          green                      blue
0.09                      5457                          yellow                     blue+green
0.10                      5508                          yellow                     blue+green
0.11                      5558                          yellow                     blue+green
0.12                      5608                          orange                    green
0.15                      5758                          orange                    green
0.17                      5858                          orange                    green
0.18                      5908                          orange                    green
0.20                      6008                          red                         green
0.22                      6108                          red                         green
0.24                      6209                          red                         green
0.26                      6309                          red                         green
0.28                      6409                          red                         green
0.30                      6509                          red                         green
0.32                      6609                          red                         green
------------------------------------------------------------------------
0.40                      7000                          nearIR                     red
0.60                      8000                          nearIR                     red
0.80                      9000                          IR                           not seen


At a wavelength of 7000 angstroms the OIII line will start to register on the IR camera and will be displayed as red on the screen. So expect to see some red OIII galaxies !!
Fermats Brother

 

[starry nite]

Very interesting. I wonder if there are some images from somewhere else in true color. I'd like to see if the "nearby" ones are really a bright green.

[quarkspin]

Absolutely fascinating Fermat, thanks.

Q

[Galaxy Hunters Inc]

Thats very interesting, FermatsBrother.
I haven't looked yet, but can you tell if the asteroids are coming or going by where the gaps are or the way the colours show?

[Rick Nowell]

In the posting by Nightwatch "What is this green colored thingy" that started this all off http://www.galaxyzooforum.org/index.php?topic=8926.msg88388 Zookeeper Kevin says much the same thing as Fermats Brother.

The fact that the image is green comes from the fact that the powerful [OIII] line sits in the r-band (which is the green in RGB).

This makes no sense to my tired brain, as I thought red would be the opposite colour to how it should be. Very interesting.
Why do galaxies with such strong emissions appear red, when, say, a W-R galaxy is usually bright blue. Confused....Our sun
is orange-red, yet this does not have the enormous emission lines. I think I am barking up the wrong tree. It seems very
contradictory.

Thanks FB for your definitive colour table. It leaves me none the wiser though, but that's not your fault...

[starry nite]

Purple haze....

ra=172.8923687, dec=13.81349001, ObjId = 587738409261334613
http://cas.sdss.org/astro/en/tools/explore/obj.asp?id=587738409261334613

[FermatsBrother]

Thats very interesting, FermatsBrother.
I haven't looked yet, but can you tell if the asteroids are coming or going by where the gaps are or the way the colours show?

Hi Galaxy Hunters Inc - I'm always pointing this out on the asteroid thread !
If you understood posting # 821, you can work out that the direction of travel is from green to blue, so the asteroid in that posting is heading downwards (in the celestial frame).
So green-blue, right to left is normal direction.
Green to blue, left to right is "apparent retrograde motion" i.e. they appear to be going backwards for a time (2.5 - 3.5 Earth months), just like Mars and the other outer planets do.
Cheers - Fermats Brother