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View Full Version : Galaxy Zoo Discovers New Group of Galaxies: 'Green Peas'



Fraser
2009-Jul-27, 09:00 PM
Citizen scientists from the Galaxy Zoo project have discovered*rare galaxies they're calling the “Green Peas.” They're small in size, bright green in color, and proficient at churning out new stars — plus, they could reveal*unique insights into how galaxies form stars in the early universe.The newly discovered galaxies appear in the image at left, from [...]

More... (http://www.universetoday.com/2009/07/27/galaxy-zoo-discovers-new-group-of-galaxies-green-peas/)

George
2009-Jul-28, 12:37 AM
So why are they green? Excluding ionized oxygen emissions, not many things are green out there.

Are these distant blue galaxies (z = 0.2) that have redshifted to green?

ngc3314
2009-Jul-28, 01:24 AM
So why are they green? Excluding ionized oxygen emissions, not many things are green out there.

Are these distant blue galaxies (z = 0.2) that have redshifted to green?

Just so - their distinguishing feature, aside from compact images, is extremely strong [O III] emission, which has redshifted into the SDSS r band at z=0.2 or so. That makes them show up as green in the standard SDSS color mapping, gri --> bgr, which works well for most non-emission-line galaxies and gives the best signal for a simple color composite. At lower redshift, they are blue peas, and at higher redshift, they shade into orange.

George
2009-Jul-29, 01:07 AM
I should of guessed a southern pea-picker would know all about these critters. ;) But I'm still a little slim on the pickin's...


Just so - their distinguishing feature, aside from compact images, is extremely strong [O III] emission,... So, does this make them a bit green or does it blend to a more blue-green (cyan) coloring with no redshift?


... which has redshifted into the SDSS r band at z=0.2 or so. That makes them show up as green in the standard SDSS color mapping, gri --> bgr, Are you saying that gri --> bgr means green red IR is the observed shift from bgr?


... which works well for most non-emission-line galaxies and gives the best signal for a simple color composite. At lower redshift, they are blue peas, and at higher redshift, they shade into orange. What, no yellow peas? ;)

ngc3314
2009-Jul-29, 04:24 AM
I should of guessed a southern pea-picker would know all about these critters. ;) But I'm still a little slim on the pickin's...

Maybe that's better than my usual catfish self-identification.


So, does this make them a bit green or does it blend to a more blue-green (cyan) coloring with no redshift?

To my eye, [O III] emission at low redshift is a very pure emerald green (based mostly on seeing one planetary nebula with what was at the time a Really Big Telescope).



Are you saying that gri --> bgr means green red IR is the observed shift from bgr?
Galactochromologically speaking.. the standard "color" images delivered by the SDSS image server (and used in Galaxy Zoo) use this slightly odd mapping, first because the SDSS doesn't have a blue filter (u is too short and has pretty low signal) and second because most galaxies are dominated by continuum light and are close enough to a one-parameter family in color that using gri gives a result visually similar to a more exact RGB. As a nice bonus, this color mapping just about undoes the effects of redshift for z~0.2 or so, so peas were picked up as green in the images when [O III] redshifted into the r filter.


What, no yellow peas? ;)

Not so's I've noticed; I think it's because H-alpha shows up in the red for the redshift range where [O III] would map to yellow, and makes the whole thing more orange.

George
2009-Jul-29, 01:49 PM
Maybe that's better than my usual catfish self-identification. I suppose I'll always be a "pea pickin'" Tennessee Ernie Ford fan and assume all those in the South are also. :)


To my eye, [O III] emission at low redshift is a very pure emerald green (based mostly on seeing one planetary nebula with what was at the time a Really Big Telescope). The first color I saw was with the McDonald's 87" that revealed a blue ring in the Eskimo planetary nebula. I haven't seen green, but I recall Phil's front page "Alien Skies" mag. article that claimed some OIII regions would appear green, yet he gave no examples. Do you have a nice green planetary that I can share with the asterochromology department (t&c*)? :)

One of the early heliochromology studies (t&c) showed that there is great variation in the assignment of color to wavelength. I compiled several data sets and made a loose fit. OIII at 500.7 nm seems to be on the border between the transition from cyan to green (no redshift). Green lasers (532nm), in comparison, are close to the middle of the green band and seem emerald green to me.


Galactochromologically speaking.. [How you're talkin' !! :)]


...the standard "color" images delivered by the SDSS image server (and used in Galaxy Zoo) use this slightly odd mapping, first because the SDSS doesn't have a blue filter (u is too short and has pretty low signal) and second because most galaxies are dominated by continuum light and are close enough to a one-parameter family in color that using gri gives a result visually similar to a more exact RGB. As a nice bonus, this color mapping just about undoes the effects of redshift for z~0.2 or so, so peas were picked up as green in the images when [O III] redshifted into the r filter. Ah, thanks. So SDSS at no redshift is reshifted (in effect) and at z= 0.2 it is not and relatively true in color. :)


Not so's I've noticed; I think it's because H-alpha shows up in the red for the redshift range where [O III] would map to yellow, and makes the whole thing more orange. I suppose, for a yellow result, a strong blue galaxy that has a lower than normal level of IR might map to yellow, but yellow is such a very narrow band in the spectrum, the odds are small. Yet, the number of galaxies is not. :)

* t&c -- tounge and check

George
2009-Jul-29, 07:54 PM
As a nice bonus, this color mapping just about undoes the effects of redshift for z~0.2 or so, so peas were picked up as green in the images when [O III] redshifted into the r filter.
That is an interesting point. This suggests that any object (with a reasonable Planck distribution) at z ~ 0.2 will be mapped by SDSS into a color close to the color that it sees the object with its filters.

KaiYeves
2009-Jul-30, 02:08 PM
Stupid genie. I wished for galactic peace, not peas... he needs a hearing aid.

George
2009-Jul-30, 06:11 PM
:) I'm sure he was just trying to a peas you.

ngc3314
2009-Jul-30, 07:42 PM
Next time, be careful not to ask for a Big Lunch, or any gamma-ray burps. Hmm - Evil Nebula, cold interstellar crowds...