Understanding a plot obtained from the 2dF redshift survey The diagram below (taken from here) represents the distribution of Galaxies obtained from 2dF redshift survey. I have a few very naive questions about this.   


*

*Which point does the centre correspond to? Is it the location of Earth?

*What do the numbers on the circular arcs mean?

*Why is it a two-dimensional plot when the survey is three-dimensional (over some solid angles I expect)?

*What does the $3^\circ$ slice mean?
From my questions, it should be pretty clear that I'm no expert. So an answer using simple language that avoids jargon would be helpful.
 A: *

*Yes. (Or at least approximately). 

*It is the right ascension celestial coordinate of the galaxies. The 2dF redshift survey took place in limited regions in the sky. Galaxies can be labelled in terms of a coordinate consisting of a redshift, a right ascension and a declination. The galaxies here lie within a narrow strip in declination and the declination coordinate has been "collapsed" (i.e. ignored) to produce this plot (see #3).

*If you only consider galaxies from within a subset of the survey, consisting of a relatively narrow strip, then the galaxies are distributed over what approximates to a 2-dimensional space. i.e. The third dimension of height (or declination) in space has been collapsed here.

*I believe this simply refers to the (small) angular width of the slice that has been considered.
A: *

*Most redshift surveys correct for the Doppler shift caused by the Earth's orbit around the sun, and so they call them "heliocentric" redshifts. No actual effort to shift the origin off of the Earth is made, I believe, because a difference of $1\operatorname{au}$ is well within the margin of error for these typically $\operatorname{Mpc}$ distances. For a lot of them, the distance to the center of the Milky Way ($\sim 10 \operatorname{kpc}$) wouldn't be a detectable shift, either.

*The numbers along the circular arcs are right ascension angle, measured in hours:minutes:seconds. Declination is, naturally, somewhat projected off. They're just a coordinate system on a sphere, exactly like longitude and latitude, measured in funny units that were more convenient for observational purposes before computers. For more details, see: celestial coordinates.

*The plot is $2$-d because the 2dF survey, and this plot, is old (final data release: 2003). You can generate a $3$-d plot yourself using a program like TOPCAT and the data from the survey yourself.

*The "$3^\circ$-slice" bit I'm pretty certain means they only used a $3$ degree wedge in declination to produce this plot; the survey has more sources than this.
More redshift surveys you can look at: 6dFGS, SDSS, GAMA, DEEP2, AGES, VVDS, and zCOSMOS. Those are just some of the the "general purpose galaxy" surveys available.
