One of the main Galaxy Zoo attractions, for many zooites, is the colors.
Sometimes the colors are intense:
Sometimes they are subtle:
Today's Object of the Day is about Post-quench galaxies, the white ones.Galaxy Zoo Quench
is the most recent of the Galaxy Zoo projects, and in some ways the most ambitious.
It makes use of
Into the pool, a very cool tool, ZooTools
That's a scatterplot I created using Tools; it plots the colors of (almost) all the 3002 objects in the Quench Sample catalog , one blue point for each object.
Huh? How can you 'plot colors'?
Well, to an astronomer, a 'color' is a number; specifically, it's a magnitude. Even more exactly, it's the difference between two magnitudes, which are the brightnesses of an object - a star, say - in two different 'bands' (or through two different 'filters'). The SDSS camera
has five filters, so it
took (the camera has retired
) images in five bands, and - after processing - produced five magnitudes for each object it identified, one for each band (more on what astronomers mean when they say 'color', check out this Quench Talk
discussion: What is "color"?
The SDSS images Galaxy Zoo has been using make use of just three of the five bands, the ones called g (for 'green'), r (for 'red'), and i (for 'infrared'). Each band is mapped to a different color channel, to produce an RGB image; g is mapped to B, r to G, and i to R. For display on your computer screen, the intrinsic colors are exaggerated in a very specific (and very clever) way, so that even very subtle differences in color are made obvious.  In terms of information (or data), and leaving aside brightness (or intensity), the GZ SDSS images contain just two unique colors, 'astronomer colors' that is
. And what are they? They are (g-r) and (r-i), or the difference in brightness between the g-band and the r-band, and the r-band and i-band.
To illustrate, here's another 2-color plot of the Quench Sample (one blue diamond per object in the Quench Sample catalog):
Most of the Quench Sample galaxies are within a triangle-shaped area, roughly bounded by the points (0.38, 0.25), (1.00, 0.32), and (1.00, 0.60). Like this:
But what color do the galaxies at these three points have, when viewed the way we saw them when we were asked to classify them? See for yourself ('zsp' stands for spectroscopic redshift)!
So, in some sense, the colors of most Quench Sample galaxies are 'between' these three.
Time to take a look at a spectrum.
Post-quench galaxies have very distinctive spectra; they look an awful lot like those of A stars, stars with an 'A-type' spectrum (long historical story; ask an astronomer about "Oh, be A fine ...
). Except that the spectrum of a post-quench galaxy is - mostly - flat, not rising towards a peak at the far blue end. What really stands out are the strong Balmer (hydrogen) absorption lines, which start off - at H-alpha - fairly modest, and get deeper and deeper. Like this: 
And here's a spectrum of one of the Quench Sample galaxies:
Yes, it has some emission lines (including H-alpha? It's hard to say; can you tell why?) - which you certainly won't see in any A-type star! - but look at those Balmer absorption lines!
And what does this galaxy look like? What color is it? It's the middle one of these three:
And where are these three, on the 2-color plot? Here:
The middle one (AGS00000mc) is represented by the yellow diamond; the other two are red diamonds; can you work out which is which?
And the white diamond? What it that?
Well, it's the star
in my Quench "White" Collection
. Here is its spectrum:
Can you guess what color it appears, in the SDSS Explore image?
white, didn't you?
 posted by galaxybabe, on July 26, 2007
(click the link to get the galaxy's details), in the Gotta love these blues
 posted by Doomy, on August 20, 2007
(click the link to get the galaxy's details), in the Give peas a chance!
 posted by Tyrre, on August 11, 2008
(click the link to get the galaxy's details), in the Red Spirals
 posted many times; for example by c_cld on November 22, 2010
(click the link to get the galaxy's details; yes, it's a possible overlap), in the Wanted! Galaxy pairs which overlap but are not merging
 first posted by starry nite (I think), but I took this from Budgieye's excellent thread, Colours of Galaxies in SDSS : Redshift chart
; the galaxy is DR7 ObjId 587727179003723785
 posted by carol tye, on August 20, 2010
(click the link to get the galaxy's details), in the Powder puff galaxies
thread. This thread is devoted, as carol writes, to "lilac and pink galaxies they remind me of powder puffs
 This is a screenshot of part of one of my Dashboards; you can see the whole Dashboard here
. One of the great things about Tools is the ability to share Dashboards
 for more on this, see this 2003 paper by Lupton et al.: Preparing Red-Green-Blue (RGB) Images from CCD Data