Michael Blanton explains the purpose of the NYU Value-Added Galaxy Catalog built from SDSS data: it is not just a list of detected galaxies, but a careful record of what parts of the sky and what distances the survey could have detected a galaxy if one were there. Because galaxies are only visible down to a certain brightness, surveys naturally see more nearby objects and fewer distant ones, so this selection effect has to be corrected for before galaxies can be used to measure statistical correlations over large cosmic distances. The catalog was built to put that kind of large-scale statistical analysis on a firm footing.
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The name of the catalog itself, the NYU Value-Added Galaxy Catalog, drew some self-deprecating jokes for sounding like a business term.
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10:52keeping on that trajectory, your career has also been one of making large data sets available to people, and have those data sets be such that it's easier for astronomers to analyze. Right? And one of the things that I'm referring to here is the NYU value-added galaxy catalog. >> Yeah. >> Um it sounds like a venture capital thing, you know, what what is the value you're adding to my portfolio? But um if I were to ask you, >> Yeah, yeah. >> what is the value that you added to the SDSS with >> Yeah, in retrospect, we would have handled that name differently, But,
11:32>> [laughter] >> Well, you're coming to California, so >> Yeah. Yeah. >> It's the right wording here. >> I mean, really in that case, um one of one of my personal obsessions about doing these large uh uh you know, analyzing the and taking these large data sets is that for a lot of things you want to do, you don't only want to study you don't only only want to say, "Oh, I've detected these galaxies. They have these red shifts, whatever." You also need to know exactly what you did, like where you could have seen a galaxy. If there was a galaxy there, would you have seen it? Right? Because you can only see the
12:14galaxies down to a certain brightness. >> Right. >> And that means for a galaxy that is of a certain luminosity, like it's intrinsic brightness, that means there's a distance out to which you can see it. And so so when you observe the sky, you're mostly seeing nearby things and fewer faraway things. >> Yeah. >> And you know, the things we were trying to do uh with SDSS at that time was map the universe on the largest scales. And if you just took that map, you would be like, "Wow, everything's near us." >> Right. >> Uh and you have to correct for that and correct for the fact that we looked in this part of the sky and not that part of the sky. So so most of the work of
12:55that catalog was very carefully, you know determining what parts of the s- universe we we we uh had actually been able to see things so that we could then do things like measure the statistical correlations between galaxies over very large distances. So that was that was sort of the main reason for that catalog. It was sort of uh so that statistical analyses, you know, had a had a firm basis. >> Yeah, that that totally makes sense. And I might be going off on a limb here, but you know, one of the things that I hear in cosmology all the time, um at
13:35Princeton I actually took an undergrad course with Neta Bahcall >> Oh, yeah. >> on cosmology. It was great. She's She's an amazing professor and I learned a lot. Um and you know, one of the things that we learned in that course that was kind of a central tenant was that the universe is isotropic. You know, whichever way you look, it's about the same. Was your Was your, you know, value-added galaxy catalog part of that conclusion? >> So So, yes. Uh
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