Dr. Michael Blanton on Open Data, Galaxy Surveys, and the Future of Astronomy (EP 43)

Hosted by Lester Nare and Krishna Choudhary, this episode is the second interview in our ongoing collaboration series with Carnegie Observatories. Krishna sits down with Dr. Michael Blanton, the new Director of the Carnegie Observatories, for a wide-ranging conversation on how astronomy became one of the most data-rich sciences, how the Sloan Digital Sky Survey helped change the culture around open data, what the next era of astronomical data science and AI could look like, and one of the galaxy mysteries Blanton still wants to solve: why the most massive galaxies in the universe stop forming stars. The conversation starts with Blanton’s Princeton roots and his work connected to the Sloan Digital Sky Survey, then moves into the culture of public astronomical data, the NYU Value-Added Galaxy Catalog, Vera Rubin Observatory, Carnegie’s role in the future of astronomy, the Magellan telescopes, astronomical archives, MaNGA and eBOSS, galaxy formation, dark matter, and even the science behind the black hole visualizations in Interstellar. Audio note: this was one of our first out-of-studio interviews, and there are a few minor audio issues in parts of the conversation. We appreciate your patience, and we’ll be better prepared for future field interviews. Also, if you’re in Los Angeles, Krishna will be giving a talk at Exploring Physics at UCLA, hosted by UCLA’s physics outreach organization Continuum, on Saturday, June 6 at the Fowler Museum. His talk runs from 9:30–10:30 AM. Register here: https://luma.com/3al1hj5h

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:00Lester Nare?

  Hello, Internet. This is your captain speaking, Lester Nare. We have begun preparations for our return to regularly scheduled programming with our resident PhD, Krishna Choudhary, expected back on the pod later this June after some well-deserved family leave. But today, we have our second interview in our ongoing collaboration series with Carnegie Observatories. In this interview, Krishna sat down with Michael Blanton, the new director of the Carnegie Observatories, for a conversation about open data, the Sloan Digital Sky Survey, the future of astronomical data science, and the mystery of why the

• 0:42Lester Nare?

  universe's most massive galaxies stop forming stars. And one quick announcement before we jump in. For those of you who are here, local to Los Angeles, Krishna will be giving a talk at Exploring Physics at UCLA, hosted by UCLA's Physics Outreach Organization Continuum on Saturday, June 6 at the Fowler Museum. This talk will run from 9:30 to 10:30, and we will include a registration link in the description. So if you're in the neighborhood, come stop by. I will be in the audience. It's sure to be a great talk as we warm back up to our regular episodes. But as always, we are going to learn about the science from

• 1:27Lester Nare?

  the ground up today, because this is From First Principles.

• 1:47Lester Nare?

  I am joined today by Michael Blanton, the new director of the observatories here at Carnegie Science. Michael, thank you so much for joining us.

• 1:56Krishna Choudhary?

  Thanks for having me.

• 1:58Lester Nare?

  You know, I wanted to start by taking a trip down memory lane, because both you, me, and Lester were...

• 2:09Krishna Choudhary?

  Part of the institution at Princeton University. You got your PhD there in Peyton Hall. I was in the neighboring Jadwin Hall and I was always very jealous of all the astronomers for having such a beautiful building. Whereas we got, you know, whatever monstrosity Jadwin Hall was. So, one question from one Princeton alum to another Princeton alum always, how often do you go back for reunions?

• 2:35Speaker 02

  I never go back.

• 2:37Krishna Choudhary?

  You never go back?

• 2:38Speaker 02

  I was a PhD student there and, and so we existed in a slightly different culture than undergraduates.

• 2:47Krishna Choudhary?

  Yeah.

• 2:48Speaker 02

  You know, eating clubs for us.

• 2:50Krishna Choudhary?

  No, yeah, but Peyton Hall was pretty close to all the eating clubs. So that was, I always.

• 2:56Speaker 02

  I walked by them on my way to Hoagie Haven.

• 2:58Krishna Choudhary?

  Oh, boy. I'm sure you didn't think very highly of the undergrads there, but we promise.

• 3:03Speaker 02

  Actually, they were great. I TA'd while I was there.

• 3:06Krishna Choudhary?

  Right.

• 3:07Speaker 02

  And they were amazing undergraduates, so.

• 3:10Krishna Choudhary?

  Well, just as a pitch, you know, there are graduate student alumni reunions at the same time, so you should, you should, you should.

• 3:18Speaker 02

  They, I did go to the reunions when I was a PhD student there. They were really great parties, but I haven't been back for any since then.

• 3:27Krishna Choudhary?

  Yeah, of course. Well, so you got your PhD there in astrophysics. One of your advisors was Jeremiah Ostraker.

• 3:36Speaker 02

  Yeah.

• 3:36Krishna Choudhary?

  And he was seminal in starting, or at least initiating, the concept of the Sloan Digital Sky Survey, which was this monumental effort to map out a large swath of the universe. And one of the things that I've read about the SDSS is that it completely sort of changed the culture of astronomy from, you know, now you guys were just putting out the data for free on this newly formed internet. Could you talk a little bit about what astronomy was like back then and how it's changed today?

• 4:11Speaker 02

  Yeah, so, so Jerry was one of my two advisors. And indeed, at that, at that time, he was putting together the Sloan Digital Sky Survey. You know.

• 4:25Speaker 02

  That's, you know, if you look back at astronomy in the, you know, before, around that time was the mid 90s, you know, a lot of the data was accumulated in non-digital forms. So there were still people collecting data on photographic plates.

• 4:44Krishna Choudhary?

  Wow.

• 4:45Speaker 02

  Like glass plates that

• 4:46Krishna Choudhary?

  Right.

• 4:46Speaker 02

  had emulsion on them.

• 4:47Krishna Choudhary?

  Yeah.

• 4:49Speaker 02

  And digital detectors had been introduced. And other electronic detectors before, you know, before that. But they, they really hadn't been as large format. You couldn't do like a large, like if you're imaging the sky, you can get like a really big CCD to actually do that. And, and so for that reason, a lot of things were still photographic.

• 5:20Speaker 02

  You know, the, the, you know, starting around 1990, it became possible to conceive of things which were bigger and digital. And once you are doing something digitally, you are then able to share it in, in a much, you know, much, you know, much more broadly, much more easily.

• 5:41Krishna Choudhary?

  Right.

• 5:43Speaker 02

  And so a few people recognized that that was going to be a possibility. And for example, the people at, you know, the Hubble Space Telescope, which obviously was sending data down digitally.

• 5:56Krishna Choudhary?

  Of course.

• 5:57Speaker 02

  They, they had, they had a lot of, you know, data distribution to the community. And STSS was this other big digital thing that was happening. And partly in order to help fund the survey and motivate the National Science Foundation and the Sloan Foundation to fund it, they promised somewhat, somewhat rationally maybe to make it all public and freely available.

• 6:27Krishna Choudhary?

  Yeah.

• 6:29Speaker 02

  And, and that was very new, just because, both because of the technology, but also just because of the culture there. And there was a genuine concern among people that like, oh, you're really giving away the store here, you're scooped.

• 6:44Krishna Choudhary?

  Yeah, yeah.

• 6:45Speaker 02

  Which is

• 6:46Krishna Choudhary?

  I remember.

• 6:46Speaker 02

  You know, which is a term I normally associate with, you know, you know.

• 6:53Speaker 02

  You know, characters in the front page or something, uh, rather than scientists. But, uh, but, but some scientists are afraid of being scooped.

• 7:02Krishna Choudhary?

  Okay.

• 7:02Speaker 02

  Um, and, uh, but STSS really from the beginning had a very different philosophy of, of eventually making the data fully public. This, I was a graduate student of the time. I was doing theory.

• 7:15Krishna Choudhary?

  Right.

• 7:15Speaker 02

  So I wasn't, I wasn't part of conceiving of it that way. But when I heard about it, it seemed like so intuitive, like that's the right thing to do. You, you know, you make the data available, you make it, uh, uh, so any astronomer can get to it. And you're going to get a lot more out of the data. They'll, people will be able to collaborate freely. They will do things, you know, you hadn't thought to do.

• 7:38Krishna Choudhary?

  Yeah.

• 7:38Speaker 02

  Um, and there's just too much science with the data set like that for everybody to do. Um, I think what's been surprising to me over the years is that, so to do that right, you have to really spend enough money to make it good.

• 7:56Krishna Choudhary?

  Mm hmm.

• 7:56Speaker 02

  I think the genius there was Alex Zelay at Johns Hopkins, who actually, you know, made the, the Sky server and later Sci server, which, which really made things accessible, uh, to everybody, to like middle school students and,

• 8:11Krishna Choudhary?

  Right.

• 8:12Speaker 02

  and, and as well as astronomers.

• 8:16Speaker 02

  And you, you have to spend the time and effort to do that. And I think what's been surprising to me over the years is you have to keep telling people that lesson. People still have an instinct to keep their data close and to not spend the effort to make it freely available.

• 8:34Speaker 02

  Um, and, uh, and that sort of argument, I thought that that argument was won once and for all.

• 8:41Krishna Choudhary?

  Right.

• 8:41Speaker 02

  Uh, but it was not, it has, it has to be reargued, you know, almost every project.

• 8:48Krishna Choudhary?

  Yeah.

• 8:48Speaker 02

  So that was surprising to me because it seemed so natural. I don't know if that answered your question.

• 8:53Krishna Choudhary?

  No, totally, totally. And I think, you know, modern astronomy has definitely changed a lot, right? Because, I mean, with the Vera Rubin that's coming online, that's very similar in that sense. The, the data is going to get fed to whatever server very, very quickly as soon as the telescope takes that photo, right?

• 9:11Speaker 02

  Uh, uh, yes. So the, so for Rubin.

• 9:15Speaker 02

  You know, as soon as they, you know, you know, as soon as they get the data, they will be sending out alerts publicly.

• 9:22Krishna Choudhary?

  Right.

• 9:23Speaker 02

  Sometime this year, they'll probably start. And those will be fully public. And then the data itself eventually becomes

• 9:32Speaker 02

  public and and will be distributed to the partners.

• 9:37Krishna Choudhary?

  Yeah, yeah. I think that that's a really cool thing about astronomy that I think other sciences have not totally gotten to where astronomy has, you know, in terms of just public distribution of data. I think that's that's something that other scientists can probably learn from.

• 9:53Speaker 02

  I I I think so. It's helpful. And this is one of the reasons I think that people who work in data science or machine learning and what have you, I think, really like working in astronomy as sort of a sandbox.

• 10:06Krishna Choudhary?

  Yeah.

• 10:06Speaker 02

  I mean, it's a sandbox, Stan. It's like our livelihood. But I think I think the reason they they like that is actually there is a lot of public data and there's no privacy issues. Right?

• 10:18Krishna Choudhary?

  Yeah.

• 10:18Speaker 02

  So it's it's sort of a that's there are many, you know, it's like there are many sciences where it's a legitimate problem if the data becomes available, like in the social sciences, it's not so easy.

• 10:31Krishna Choudhary?

  Totally, yeah. In the neurosciences too, if you're dealing with human data, but you know, there's a place called Allen Brain Institute in Washington. They're publishing a lot of, you know, mouse and RODIT data freely available. And that's sort of changed.

• 10:47Speaker 02

  The mice are unlikely to sue.

• 10:49Krishna Choudhary?

  Yes, yes, very much so. You know, keeping 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.

• 11:08Krishna Choudhary?

  Analyze, right? And one of the things that I'm referring to here is the NYU Value Added Galaxy Catalog.

• 11:16Krishna Choudhary?

  It sounds like a venture capital thing. What is the value you're adding to my portfolio? But if I were to ask you, what is the value that you added to the SDSS?

• 11:28Speaker 02

  Yeah, in retrospect, we would have handled that name differently, but.

• 11:33Krishna Choudhary?

  Well, you're coming to California, so. It's the right wording here.

• 11:37Speaker 02

  I mean, really, in that case, one of my personal obsessions about doing these large,

• 11:48Speaker 02

  you know, analyzing 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 want to say, oh, I've detected these galaxies. They have these redshifts, 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 galaxies down to a certain brightness. And that means for a galaxy that is of a certain luminosity, like its intrinsic brightness, that means there's a distance out to which you can see it.

• 12:27Speaker 02

  And so, so when you observe the sky, you're mostly seeing nearby things and fewer far away things. And, you know, the things we were trying to do 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.

• 12:47Krishna Choudhary?

  Right.

• 12:47Speaker 02

  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 that catalog was very carefully, you know, determining what parts of the universe we 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.

• 13:28Krishna Choudhary?

  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 Princeton, I actually took an undergrad course with Nita Bakal and cosmology was great.

• 13:40Speaker 02

  She's a

• 13:41Krishna Choudhary?

  She's an amazing professor and I, I learned a lot. Um, and you know, one of the things that we learned in that course that was kind of a central tenet 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?

• 14:01Speaker 02

  So, so, yes. Uh, uh, I think that the strongest evidence for that comes from the cosmic microwave

• 14:10Krishna Choudhary?

  Oh, okay, fair.

• 14:11Speaker 02

  Where it's, where it's just very clear that like

• 14:14Krishna Choudhary?

  Yeah.

• 14:14Speaker 02

  Art of the sky and that part of the sky, which have never talked to each other.

• 14:18Krishna Choudhary?

  Right.

• 14:18Speaker 02

  Are

• 14:19Krishna Choudhary?

  The same

• 14:19Speaker 02

  Have the same temperature.

• 14:20Krishna Choudhary?

  Yeah.

• 14:21Speaker 02

  Um, so, so I think that, that i- i- you know, is the strongest evidence, but it is true that, um, with, with the largest volume sample of, uh, SCSS, we were able to do that for the galaxies as well. We could answer the question, like, how homogeneous is the universe? Um, and, uh, and, and we did, we did have a, um, a quantitative paper, you know, based on the volume we surveyed, which, which, uh, showed that it's very, very uniform.

• 14:58Krishna Choudhary?

  Yeah, it is very, very

• 14:59Speaker 02

  On those large scales, right?

• 15:01Krishna Choudhary?

  Right.

• 15:01Speaker 02

  On the small scales, everything's clumped, but once you're at hundreds of millions of light years or a billion light year scales, then

• 15:08Krishna Choudhary?

  Yeah.

• 15:08Speaker 02

  Everything's pretty much the same.

• 15:10Krishna Choudhary?

  Yeah. It's like soup.

• 15:12Speaker 02

  Yeah.

• 15:12Krishna Choudhary?

  Yeah. Very cool. Um, so now I want to talk about your transition to Carnegie. You've got a new job now. Congratulations, by the way.

• 15:22Speaker 02

  Thanks.

• 15:23Krishna Choudhary?

  Um, you left New York for the best city on the planet, Los Angeles.

• 15:28Speaker 02

  Okay.

• 15:29Krishna Choudhary?

  Me and the Leshra and Nater.

• 15:30Speaker 02

  It's out, but you know.

• 15:32Krishna Choudhary?

  Not to us. So I grew up here and, uh, you know, I've always loved it. So welcome to Southern California. This is an institution, Hubble walked these halls, you know. Why, why make the move? What about Carnegie brought you here?

• 15:48Speaker 02

  The astronomy?

• 15:49Krishna Choudhary?

  Yeah.

• 15:50Speaker 02

  So I was in a department, a department of physics at NYU.

• 15:53Krishna Choudhary?

  That's right.

• 15:54Speaker 02

  And one of the things I've, I've, even in this first month is just really clear is, is just,

• 16:03Speaker 02

  just every morning there, you know, there is such a range of experts here talking about every single aspect of astronomy. You know, yesterday someone's talking about very large scale structure. People are talking about exoplanets. It's really like that. So one of the main draws for me was just that environment where, where, uh, where I'm learning something new every day from the people around here.

• 16:37Speaker 02

  And, and so, so that's actually the fundamental thing.

• 16:43Speaker 02

  Another thing is just simply the resources in terms of the engineering resources. Laskin's Companis Observatory is an amazing facility. And, uh, and, and SDSS actually partnered with Lask and Ponis. We actually,

• 17:01Krishna Choudhary?

  Oh, yeah.

• 17:02Speaker 02

  Now, uh, you know, uh, in SDSS five, they're completely using the hundred inch telescope. And that's, that's a, uh, you know, uh, so I've had a, you know, a long, you know, a decade or so of experience working with Carnegie already through that.

• 17:21Krishna Choudhary?

  Right.

• 17:23Speaker 02

  And then in terms of the job itself, like a director job, I was, I, you know, I started in SDSS as like, you know, Writing software, which actually I kind of still do.

• 17:36Krishna Choudhary?

  Right.

• 17:37Speaker 02

  But in in a previous phase of SDSS, I was the director.

• 17:43Speaker 02

  And I sort of learned at that time that I I kind of liked enabling science. I got a satisfaction from making it possible for people, as you said, get at their data, but also get the science they wanted done out of the project. And so it's an admit like that was an administrative and leadership role, but very close to the science, like not like, you know, fans to any deans, but not like being a dean. Like, like, really, like, very close to the scientific decisions. And and I think and this job, being director here, I I think has that same flavor.

• 18:25Speaker 02

  You're very close to what people are doing and the, you know, the sorts of decisions they have to make that that require scientific judgment. And and so I like that place where it's a leadership role and you're really, it's enabling science, but you're you're sort of close to, you know, what's actually happening.

• 18:46Krishna Choudhary?

  Right, yeah. I mean, just before we were filming in this room, there was, I think, a journal club meeting.

• 18:52Speaker 02

  Yeah.

• 18:53Krishna Choudhary?

  Yeah, they were discussing the bullet cluster data from the James Webb. And I was just eavesdropping and it was such a cool conversation that, you know, everybody was just gathered around talking about one paper. So it is an incredible institution.

• 19:05Speaker 02

  Well, this is amazing thing about astronomy, which which is different than other fields is that, well, I don't know how different it is from other fields, but I think it's different is that, you know, when you're an astronomer, you kind of have to know all the bits of astronomy. Like, they actually all end up being relevant in some way. So it is relevant to the people who study stars. What's happening in the bullet cluster? Like that turns out there are enough connections to like how galaxies form and then how stars form. And the elements that are in stars, that, that those things are connected enough that you really have to, you know, have some knowledge of what's happening across the field.

• 19:48Krishna Choudhary?

  Yeah. So it's not just like dark matter guys or girls, but kind of the bullet cluster. It's like everyone.

• 19:54Speaker 02

  Yeah, no, no, I think it's certainly very relevant to, to the people who study galaxies.

• 20:01Krishna Choudhary?

  Right.

• 20:01Speaker 02

  Then that is connected to the people who study stars, which is connected to the people who study planets. And, and, and it's very hard to, you know, you're kind of expected as an astronomer to have some handle on all of that. Uh, and, and that I think is, and partly it's because it's, you know, there's only whatever tens of thousands of, of professional astronomers in the world. So it just is a small field. But, um, but that I really like about it, that you're, you, you sort of have a connection to almost every astronomer that you meet.

• 20:32Krishna Choudhary?

  Right. Yeah, that makes sense. Um, so you've been, you're, you're a very new director here.

• 20:38Speaker 02

  Very new.

• 20:39Krishna Choudhary?

  What are you looking forward to in your tenure here?

• 20:43Speaker 02

  So there's, um, you know, the, the things that are going on here. The, really the main observational event right now is, is Magellan to six and a half meter telescopes. And, uh, and the SDSS program at the DuPont. Uh, and what I think, you know, our challenge, one of our challenges is, you know, with any, you know, any large telescope like this, really any telescope, you want to make sure that the things that are happening at it are continuing to be cutting edge.

• 21:26Speaker 02

  So I think one of the things we really have to, to figure out is the strategy for, for, for keeping the telescope functioning. It was built in the nineties. It's got all of the, you know, it's got some nineties technology still in there. There are things like that, which are, which, which really have to be done. There are sort of more on the technical. You know, infrastructure side.

• 21:50Krishna Choudhary?

  Right.

• 21:51Speaker 02

  But then there's also, there are new opportunities for instrumentation now. There is, there's instrumentation that, that you could build better now and be able to look at fainter things, look at things over, you know, wider swaths of sky. And I think that, you know, in the, in the, in that, those sorts of time scales, I think we have to figure out how to build those instruments. And, and one of the things about that is that, and I think this is borne out by some of the successful efforts, recent efforts, is that you really have to do this collaboratively.

• 22:28Krishna Choudhary?

  Right.

• 22:28Speaker 02

  Like, you know, Carnegie has to be working with, you know, and this, this, this is part of like the way Carnegie has worked with SDSS, which has brought a lot of new instrumentation and energy to certain parts of Carnegie. And I think this is true at Magellan as well, the six and a half meters is, I think you have to be working with other, other parts of the community to build the things that are cutting edge. You don't do it alone anymore. So that, that's one aspect. I think there's another aspect, which is that I really think that, you know, we are sitting on a pile of data.

• 23:10Krishna Choudhary?

  Yes.

• 23:10Speaker 02

  It's a huge pile of data and, and facilities that, you know, that can create them. And I think that there, there's a lot of work in, you know, I sort of think of it as astronomical data science, but that would include everything from, from soft, you know, telescope software to, to, you know, data reduction software, to machine learning, to, you know, AI applied to, to, to the data. Like, I think that, that is something where I think Carnegie should have like a really strong position in.

• 23:53Speaker 02

  And so that, that's something I would really like to, to see happen.

• 23:59Krishna Choudhary?

  Yeah. Do you have, uh, do you have any plans with all the photographic plates in your basement?

• 24:05Speaker 02

  I think, no, the answer to that is no.

• 24:09Krishna Choudhary?

  Yeah, I mean, it's a hassle.

• 24:11Speaker 02

  So, so there, there, there are program, like Harvard has a program for their library where they, they're digitizing all of these things. I, I think that is in principle, interesting. I think you have to, I think you have to do it in a science driven way. I think, because it's such a big problem. I mean, you've seen.

• 24:30Krishna Choudhary?

  Yeah.

• 24:30Speaker 02

  Yeah, I know. There was all sorts of things, all sorts of different data, all sizes in G apps. And so you really would, you want to approach that with, I really want to study X and I'm going to do something.

• 24:45Krishna Choudhary?

  Yeah.

• 24:45Speaker 02

  Yeah, I think that's the way you approach that.

• 24:47Krishna Choudhary?

  Yeah. And, and for what it's worth, I mean, any astronomer can request Carnegie for like a specific plate and things like that, right? You have some way of doing it?

• 24:58Speaker 02

  I don't know the answer to that question. That was one of the things I.

• 25:01Krishna Choudhary?

  Yeah.

• 25:02Speaker 02

  I should probably know.

• 25:03Krishna Choudhary?

  Okay. Well, I might be misremembering, but last time when we got a, when we got a tour, they were like, you know, people come in and they say, oh, I want to look at this part of the sky. You know, and there was some, somebody.

• 25:12Speaker 02

  I hope we say yes to that. That would be great.

• 25:14Krishna Choudhary?

  I think, I think you do, but maybe, I don't know, maybe.

• 25:19Speaker 02

  Yeah, no, absolutely. We should. And there, you know, there are definitely things in astronomy where people have studied this star for.

• 25:29Krishna Choudhary?

  Right.

• 25:30Speaker 02

  Hundreds of years and, and, and this is a hundred year period where especially if it's something significant enough that people are doing that. Yeah, there probably is information in our archive about it.

• 25:42Krishna Choudhary?

  Yeah.

• 25:44Krishna Choudhary?

  Definitely. I mean, your archives here are just spectacular. So, um, one of the things that you talked about was that, you know, as director, you're not like, you're not someone like just pure administration. You're making scientific decisions. You were the director of SDSS four.

• 26:03Speaker 02

  Yeah.

• 26:03Krishna Choudhary?

  Right? So could you give an example of like a scientific decision that, that you, you know, it was the buck stops with you and you had to make that decision? And what, what, what were the things that you weighed when making that decision?

• 26:19Speaker 02

  So, um, I think there.

• 26:26Krishna Choudhary?

  And it doesn't have to be from SDSS.

• 26:28Speaker 02

  Yeah, I, I think SDSS probably has, has the examples I could most easily talk about. You know, I, I, I think there was.

• 26:41Speaker 02

  Just tell me if this is too far in the weeds, but you know.

• 26:44Krishna Choudhary?

  We love the weeds on the podcast.

• 26:45Speaker 02

  Okay, you like the weeds, good. So in SSS4, we had two programs that were using what we call the dark time, which is when the, the moon is, is, is new.

• 26:55Krishna Choudhary?

  Oh, okay. It's on the other side of the earth.

• 26:57Speaker 02

  It, well, it's on, yeah, it's on, uh, it's, it's on the, uh.

• 27:02Krishna Choudhary?

  The day side.

• 27:03Speaker 02

  The sun.

• 27:03Krishna Choudhary?

  Yeah.

• 27:04Speaker 02

  Right? So it's, it's when the sky is dark.

• 27:06Krishna Choudhary?

  Yes.

• 27:06Speaker 02

  Right? So, so, uh, one, one was called Manga, which is another naming, uh, uh, fiasco anime? Yeah. Spelled the same way. It makes it hard to.

• 27:18Krishna Choudhary?

  What was the, what was the full form?

• 27:20Speaker 02

  Uh, the mapping, uh, nearby galaxies at Apache Point Observatory.

• 27:26Krishna Choudhary?

  Okay.

• 27:26Speaker 02

  Yeah.

• 27:27Krishna Choudhary?

  That's, yeah, that's, okay.

• 27:28Speaker 02

  Yeah, yeah. But it was observing nearby galaxies.

• 27:32Krishna Choudhary?

  Great.

• 27:32Speaker 02

  Um, and then there was EBOSS, which was like the last regship survey we did.

• 27:36Krishna Choudhary?

  Another great name.

• 27:37Speaker 02

  Yeah, yeah. EBOSS, yeah. That was the extended BOSS survey.

• 27:41Krishna Choudhary?

  Okay.

• 27:41Speaker 02

  Anyway, so, anyway, retrospectively. But, um, but what was happening, we, we had to make a decision between this, you know, how much time we were spending on each.

• 27:53Krishna Choudhary?

  Right.

• 27:53Speaker 02

  And, you know, there was a question of, uh, the competition for each of these two. And basically Manga, actually for a data set like that, the way, the, the amount of information we're getting per galaxy, uh, the number of galaxies. And actually, this program ended in 2021, but there's still nothing that's comparable to it, even now.

• 28:16Krishna Choudhary?

  Right.

• 28:16Speaker 02

  Whereas with EBOSS, it was our last regship survey. And another regship survey called DESI was starting up.

• 28:23Krishna Choudhary?

  Oh.

• 28:23Speaker 02

  Near the end of when we were, uh, ending, but not, but.

• 28:29Speaker 02

  You know, it was gonna be a small overlap.

• 28:32Krishna Choudhary?

  Right.

• 28:32Speaker 02

  And our eBoss team was like, you know what? Like, we all wanna work on this other project.

• 28:38Krishna Choudhary?

  Yeah.

• 28:38Speaker 02

  You know, and we really wanna close down eBoss sooner, but we wanna reach our goals.

• 28:44Krishna Choudhary?

  Yeah.

• 28:44Speaker 02

  And so the decision there was sort of, let's bring forward eBoss and just spend, you know, I forget exactly, but spend like a whole year just doing that. Don't do any manga. And then wait for manga to do in the last year of the survey. I'm getting the exact timing wrong.

• 29:03Krishna Choudhary?

  I know.

• 29:03Speaker 02

  So we'll back. There's a whole report to the Sloan Foundation about it. But, uh, but, but that was a decision where it's like, we're gonna put your data taking on hold so we can get these people done.

• 29:15Krishna Choudhary?

  Mm-hmm.

• 29:16Speaker 02

  And then we'll go back to you, which is quite risky because at that time, we didn't have enough money to finish the project.

• 29:21Krishna Choudhary?

  Oh, wow.

• 29:22Speaker 02

  Yeah, yeah, yeah, so.

• 29:23Krishna Choudhary?

  Okay.

• 29:23Speaker 02

  Yeah, yeah, yeah, so it was, um, we didn't have enough money to finish the project until maybe eighteen months before we finished.

• 29:30Krishna Choudhary?

  Wow.

• 29:30Speaker 02

  But a part of that, so that's a scientific decision. It's like, do you prioritize this thing and put the other thing at risk? And then you, you sort of say, well, we'll give you a little more time if you'll take it later.

• 29:44Krishna Choudhary?

  Yeah.

• 29:44Speaker 02

  And, and so you work it out that way. And it also became a, a selling point to the DOE to give us a little more money, the Department of Energy, to give us a little more money to finish eBoss.

• 29:58Krishna Choudhary?

  Right.

• 29:58Speaker 02

  Uh, uh, and so, so that's an example where it's a, a scientific decision because of the timing of these projects.

• 30:06Krishna Choudhary?

  Yeah.

• 30:07Speaker 02

  And, but it's somehow intimately tied to the funding decision. Like, that only works if you can actually get a little money that helps you close your budget. And so that's an example where, where it's like, it's not just an administrative decision or a scientific decision. They're, they're intimately connected.

• 30:25Krishna Choudhary?

  Right, and just to be clear, these are two programs that are asking different questions scientifically.

• 30:31Speaker 02

  Totally different.

• 30:32Krishna Choudhary?

  They're trying to use the same telescope.

• 30:33Speaker 02

  They're using exactly the same telescope.

• 30:35Krishna Choudhary?

  Right, and and EBOSS is your the redshift?

• 30:39Speaker 02

  Yeah.

• 30:39Krishna Choudhary?

  So that's the one that's just like charting galaxies far away?

• 30:42Speaker 02

  Wrapping the universe on the largest scales.

• 30:44Krishna Choudhary?

  Right, at the time, yeah, at the time, but I guess DESI was coming on and I think DESI is at in Tucson, right? In Kit Peak?

• 30:52Speaker 02

  Yeah, it's at Kit Peak.

• 30:53Krishna Choudhary?

  So it's like nearby?

• 30:54Speaker 02

  It's nearby and it was the same team of people, like the team of people at EBOSS. EBOSS were were the people, a key part of the team that had proposed DESI, so.

• 31:06Krishna Choudhary?

  And they'd already gotten time, so they're like, let's just finish up.

• 31:09Speaker 02

  Yeah, they were they they were sort of, you know, we're gonna, we really want to move on to this other thing and and and, you know, because for good reasons.

• 31:19Krishna Choudhary?

  Yeah.

• 31:19Speaker 02

  Um, and so so that was their motivation.

• 31:22Krishna Choudhary?

  Okay, very cool. And then, um, I guess MANGA was nearby galaxies.

• 31:28Speaker 02

  Yeah.

• 31:28Krishna Choudhary?

  What was the what was the question? Because I I imagine with redshift, you're trying to answer, you know, Hubble's constant expansion of the universe.

• 31:35Speaker 02

  Yep.

• 31:35Krishna Choudhary?

  Cosmology at the biggest scales kind of thing.

• 31:37Speaker 02

  Pretty much.

• 31:37Krishna Choudhary?

  What was MANGA's prerogative?

• 31:39Speaker 02

  MANGA was, MANGA's tagline was the galaxy survey for people who love galaxies.

• 31:44Krishna Choudhary?

  Okay.

• 31:45Speaker 02

  So so it was getting, so it was this much smaller number of galaxies. I think by the time we finished the SDSS redshift surveying at the end of EBOSS, we had maybe somewhere between three and four million galaxy redshifts. MANGA observed 10,000 galaxies.

• 32:03Krishna Choudhary?

  Okay.

• 32:03Speaker 02

  Which is much smaller number.

• 32:05Krishna Choudhary?

  Yeah.

• 32:05Speaker 02

  But for each galaxy, you are observing not just, you know, the the the redshift survey just takes the spectrum of the center of the galaxy.

• 32:15Krishna Choudhary?

  Right, it's like the mean.

• 32:16Speaker 02

  It's just get one redshift and get, and you get more information. You get something about the overall age of the stars, the metallicity of the stars, something about the gas in the galaxy. Uh, but MANGA, you had these fiber bundles, uh, where you put the whole fiber bundle, they're not this big, they're like that big.

• 32:35Krishna Choudhary?

  Yeah.

• 32:35Speaker 02

  But, um.

• 32:36Krishna Choudhary?

  Optic, these are optic fibers.

• 32:37Speaker 02

  Optical fibers.

• 32:38Krishna Choudhary?

  Not the string theory fiber bundles, just.

• 32:40Speaker 02

  That is, yeah, there's no Lee algebra here in the creation of these fiber bundles. Um, the uh, the, these fiber bundles are pieces of glass. Um, and, and, and they're put on the galaxy in the focal plane of the telescope. And we would do seventeen of them at a time. Uh, and get information across the face of.

• 33:02Krishna Choudhary?

  Oh, very cool.

• 33:03Speaker 02

  So you could get a rotation curve.

• 33:05Krishna Choudhary?

  Yeah.

• 33:06Speaker 02

  You could get, you know, the gradients of the gas distribution and the metallicity distribution. So it's a much more information about, uh, you like if there was an active galactic nucleus, which is like a supermassive black hole at the center of the galaxy. You could get that separately from the rest of the galaxy.

• 33:27Krishna Choudhary?

  Yeah, so before there was just one spectrum and the entire galaxy, all of its spectra was coming through. And you're getting this thing. Now you're sort of like pixelating a galaxy and tiling it.

• 33:37Speaker 02

  Yeah.

• 33:38Krishna Choudhary?

  Because the spectra over here, because it's moving, you know, the, maybe it's rotating this way is different from the spectra over here. Yeah. Is different from the center. If there's some black hole that's acting up.

• 33:47Speaker 02

  Yeah.

• 33:47Krishna Choudhary?

  Very cool. I see. Yeah. That's, that's, that's super fascinating.

• 33:53Krishna Choudhary?

  I wanted to wrap up with two questions.

• 33:56Speaker 02

  Okay.

• 33:57Krishna Choudhary?

  One was, you know, with your, with your tenure coming up in Carnegie, what is like one mystery of the universe? That you personally are super, you know, curious about and you hope that your efforts can, can lead to an answer?

• 34:12Speaker 02

  I mean, there are many.

• 34:15Krishna Choudhary?

  Yeah. If you were to, you know, one that.

• 34:18Speaker 02

  If I'm gonna pick one.

• 34:19Krishna Choudhary?

  Yeah, personally.

• 34:20Speaker 02

  Personally.

• 34:21Krishna Choudhary?

  Yeah.

• 34:21Speaker 02

  Then I, I grew up as a galaxy person.

• 34:25Krishna Choudhary?

  Yeah.

• 34:26Speaker 02

  Uh, and I'm still fascinated by galaxies and I, well, it's really two things. One, one is that we, which is really just about the galaxies, which is that, and this has been a mystery since I was a graduate student.

• 34:42Krishna Choudhary?

  Okay.

• 34:43Speaker 02

  Uh, and is still, as far as I'm concerned, still. Mysterious, if we, even if we have some better knowledge, which is that the most massive galaxies in the universe are not forming any stars. There are no stars forming in them. They are, they are, they are filled with old stars that mostly formed billions of years ago.

• 35:07Krishna Choudhary?

  Okay.

• 35:07Speaker 02

  And although you would think they would have available gas, in some cases you see the available gas.

• 35:13Krishna Choudhary?

  Mm hmm.

• 35:14Speaker 02

  Which is normally what new stars form out of, they're not actually doing that. And that problem, which existed in the nineties, is, is still not, as far as I'm concerned, explained today. And that has consequences, like those galaxies don't get as massive as your theoretical expectation would be. Now, there is a theory that is the, that is sort of, if there is a generally accepted theory, it would be this, which is that somehow the supermassive black holes that are accreting ma- matter

• 35:56Speaker 02

  at the centers of these very massive galaxies are preventing star formation from happening.

• 36:03Krishna Choudhary?

  Okay.

• 36:03Speaker 02

  And somehow.

• 36:06Krishna Choudhary?

  Somehow.

• 36:06Speaker 02

  Somehow, and so, so there are like, you know, at least half a dozen different numerical simulation methods where people incorporate this and successfully stop the stars from forming in these most massive galaxies. All of the models work.

• 36:26Krishna Choudhary?

  Okay.

• 36:26Speaker 02

  And they're all different.

• 36:28Krishna Choudhary?

  Okay, yeah, got it.

• 36:29Speaker 02

  So they all start with, there's a black hole at the center of the galaxy and, and it's doing something. And then they all do different things. And then the end result is that they get the galaxies to have the right mass.

• 36:43Krishna Choudhary?

  Okay.

• 36:45Speaker 02

  And, but, but the, the, the ways in which they do that in detail are all different.

• 36:51Krishna Choudhary?

  Okay.

• 36:52Speaker 02

  And, and that to me is, you know, I would like to know which of those answers is correct.

• 36:58Krishna Choudhary?

  Yeah, that's so interesting. So that seems counterintuitive. I mean, big galaxies should just have. Yeah, cuz the Milky Way is forming stars, right? The Milky Way is not one of these.

• 37:10Speaker 02

  The Milky Way is prob- is maybe ten times less massive than

• 37:15Krishna Choudhary?

  Okay, than these guys, yeah.

• 37:16Speaker 02

  Then then this happens, depending on exactly how you count.

• 37:19Krishna Choudhary?

  Understood.

• 37:20Speaker 02

  Um, but uh, but yeah, so the Milky Way is in this mass range where some some galaxies aren't forming stars, some are, and Milky Way is actually kind of in the middle.

• 37:30Krishna Choudhary?

  Uh huh.

• 37:31Speaker 02

  Um.

• 37:31Krishna Choudhary?

  But these guys, it's like no one is forming stars in this.

• 37:34Speaker 02

  It's very rare for for for them to be forming stars.

• 37:38Krishna Choudhary?

  Okay.

• 37:39Speaker 02

  And again, you you need a lot of energy to stop the gas from flowing in and forming stars. You need to like heat it up or blow it out. You need a lot of energy, so like people are like, these black holes, they can generate a lot of energy.

• 37:51Krishna Choudhary?

  Uh huh.

• 37:52Speaker 02

  Which is a really good argument, but the exact way in which that happens is

• 37:57Krishna Choudhary?

  Still a mystery.

• 37:58Speaker 02

  Is still a mystery.

• 38:00Krishna Choudhary?

  That's cool.

• 38:00Speaker 02

  So that that that's one thing, and I would say the other thing really for me, which is also related to galaxies, is that I really think dark matter is a gigantic mystery.

• 38:12Krishna Choudhary?

  Everybody is.

• 38:12Speaker 02

  And and I I I I, one of the things that motivates me about studying galaxies, although they're very

• 38:20Speaker 02

  complicated and annoying in the way that that that it's hard to make predictions about them.

• 38:26Krishna Choudhary?

  Uh huh.

• 38:26Speaker 02

  Uh, I do feel like if we really wanna know more about dark matter, we better understand the physics of galaxies a lot better than we do today. Otherwise,

• 38:34Krishna Choudhary?

  Yeah.

• 38:34Speaker 02

  I don't think we're making progress um, without without sort of understanding the way the regular material works.

• 38:42Krishna Choudhary?

  Yeah, yeah, all right. Totally, that's that's a fascinating problem. Um, well, all the best to you and Carnegie, there's one last question that I wanna ask. I wanna go back down memory lane to your undergrad days.

• 38:56Speaker 02

  Oh, boy.

• 38:56Krishna Choudhary?

  In at Cornell.

• 38:57Speaker 02

  Yes.

• 38:58Krishna Choudhary?

  You did engineering physics, right?

• 39:00Speaker 02

  Yeah.

• 39:01Krishna Choudhary?

  Um, but you actually were in the research group where they were doing numerical relativity.

• 39:06Speaker 02

  Yeah, right.

• 39:07Krishna Choudhary?

  Right? Um, so have you seen Interstellar? The movie?

• 39:12Speaker 02

  I have, I have.

• 39:14Krishna Choudhary?

  And what did you think about those visualizations? Because I imagine that's kind of, you know, the Superman, you were looking at numerical simulations around holes.

• 39:22Speaker 02

  Yeah, actually, yeah, we were.

• 39:23Krishna Choudhary?

  What did you think about that?

• 39:24Speaker 02

  We were doing visualization.

• 39:25Krishna Choudhary?

  Right, but with a much smaller computer, I bet.

• 39:28Speaker 02

  It was a much smaller computer. We did them, we would, we, in fact, we had to, we did most of our work on green terms. Like these, you know what these are? You don't want to.

• 39:39Krishna Choudhary?

  I don't know, actually.

• 39:40Speaker 02

  It's just, you've got a keyboard, it's got a monitor that's 24 by 80.

• 39:45Krishna Choudhary?

  Oh, my gosh, okay.

• 39:46Speaker 02

  Yeah, it's like, you know, like in War Games or whatever. Like, it was one of those.

• 39:50Krishna Choudhary?

  Yeah, all right.

• 39:51Speaker 02

  And then you were making these images. And then to see the images, you walk over to a room where the monitor was.

• 39:58Krishna Choudhary?

  Okay.

• 39:59Speaker 02

  Anyway, it was a while ago. And then you wanted to put that video. And then we would make videos for our professors to show at conferences.

• 40:05Krishna Choudhary?

  Right.

• 40:05Speaker 02

  And like, there was a whole nother room where it was like, you slowly, frame by frame, put this onto a VHS.

• 40:13Krishna Choudhary?

  Yeah.

• 40:13Speaker 02

  Anyway. It, so it was, so the visualization has gotten better.

• 40:19Krishna Choudhary?

  Yeah, definitely.

• 40:21Speaker 02

  So what Interstellar does, I believe, and, and which is, is a really neat thing, is showing you, like, how the light actually, how you would see it if you were in that space-time.

• 40:35Krishna Choudhary?

  Yeah.

• 40:36Speaker 02

  And so, yeah, like, do all the ray tracing.

• 40:39Krishna Choudhary?

  Yeah.

• 40:39Speaker 02

  So, so we did do ray tracing, like, similar to that. We were doing it, but we didn't do that for our visualization. We were doing that because there was a scientific question we were asking about. Basically, we're asking, like, if you take two things, well, you know, we had these simulations that the graduate students and postdocs had run of like, of matter collapses, forms a black hole. At that time, you couldn't make them do this.

• 41:06Krishna Choudhary?

  Okay.

• 41:06Speaker 02

  They could only come straight together. And they would form a black hole, but it's like, when does the black hole form?

• 41:13Krishna Choudhary?

  Right.

• 41:13Speaker 02

  You only, the, the definition of a black hole is, the event horizon is when light can't get out.

• 41:19Krishna Choudhary?

  Right.

• 41:21Speaker 02

  No matter what, like it forever can't get out. And, and if you think about it, that's not really a local thing. You can't just say, oh, in this frame of this simulation, at this particular moment, there's a black hole anywhere. You have to figure out if I shine light here, can it eventually get out? And if, if, if this, if the thing's in motion, if the stuff is still flowing, then it's not obvious.

• 41:46Krishna Choudhary?

  Right.

• 41:47Speaker 02

  Right? So, so, so which frame are you talking about? Well, yeah, it's like, well, it sort of depends on what these two clumps of matter do. If they're moving away from each other, maybe the light's fine. If they're coming together, there will be a black hole there and this light won't get out. So, so we were doing ray tracing to figure out the answer to that question, like where the black hole actually formed.

• 42:10Krishna Choudhary?

  Right.

• 42:11Speaker 02

  That was our job in the, in the undergraduate group. But it was way, like we would, we could do a few thousand traces through this space time. We, we did not have the computing to like render an image through ray tracing. That, that was, that was beyond our capability. So the stuff in Interstellar is, is amazing in that way. Where these days you can, you can literally make a picture of, of what, what you would see in, in these situations.

• 42:43Krishna Choudhary?

  Yeah, and I think Kaitor's group actually did publish a few scientific results through the, you know, visualization process of Interstellar, which I think is fascinating.

• 42:54Speaker 02

  Yeah.

• 42:55Krishna Choudhary?

  Well, it's been a great conversation with you. Thank you so much for taking the time. We really look forward to what you're gonna do here at Carnegie.

• 43:03Speaker 02

  So do I.

• 43:06Krishna Choudhary?

  Yeah, and, um, yeah, we'll, we'll, we'll see, we'll see what happens. Thank you.

• 43:09Speaker 02

  Great, thanks.