Friday, September 29, 2006

Journal Club Talk: Star Formation Rates

One of the "neat" things done here to help graduate students become accostomed to talking in front of a group of people is a weekly journal club. The idea is that every Friday at 12:30 (usually people bring their lunch) a grad student gives a half hour talk (assuming no interruptions, of which there are always plenty, so it's usually more like forty-five mintues or an hour) on some recent paper. Usually, this paper is completely unrelated to their own research, so that instead of the student "defending" their own work, they are learning new science.

Journal club is one of the aspects of the program here that I gladly tell prospective students about, but personally try to weasel my way out of. In principle, it's a great thing to force grad students to learn how to make effective slides, to become comfortable standing in front of a room full of people, to be able to easily field unsolicited questions. In practice, it's fantastic, as long as I'm not the student in question. I succeeded in not giving a talk last year, but when, on our way to the conference in Japan in June, Paul (who is in charge of scheduling and running journal club this year) found out I hadn't yet given one, I found myself first up on the list for this year, unable to sneak out.

Thus I found myself standing in front of the department today talking about "Radial variation of attenuation and star formation in the largest late-type disks observed with GALEX." I had resolved to not be one of those students who waits until the last minute to prepare, but well, there's this paper I've been procrastinating on for a while, and I didn't get around to mailing it out to the co-authors until Wednesday night... which meant that I didn't start, uhm, reading the paper until yesterday morning.

I felt like I was in Junior Lab all over again, only I actually found the material interesting, wasn't being graded (per se), and didn't have a lab partner to ask questions of. It took me all day to actually read the paper, and I didn't start even making slides until about 5p.m., when I realized that I haven't actually given a talk in over two years. Everyone these days seems to be using PowerPoint... which I haven't used since, I think, 1997? So I dallied for about half an hour, finally deciding that, in fact, LaTeX would be easier.

I finished the slides this morning, and then Paul suggested that I might want to practice. That was about when I started trying to crawl out of my skin. Normally, I don't get the slightest bit nervous when talking in front of a group, but then, normally, I actually know what the hell I'm talking about. This... this, I couldn't have given you a solid definition of "attenuation" or told you how astronomers ("we") estimate star formation rates 48 hours ago. So, I tried practicing, and it was disastrous: I couldn't stand still, I couldn't find any words not etymogically related to "uhm," and I definitely couldn't remember half of what I actually wanted to say.

But the talk wound up going fine, I think. Have you ever been so tired you can't fall asleep? It was like that: I think I was so nervous I forgot to be nervous. There was a plethora of interuptions (before I could even get off the title slide!), which, as I discovered, actually helped. If your audience keeps interupting you, it makes the "talk" more like a conversation than an information dump. Some people get really nervous and upset if their audience can't keep quiet, asks really difficult questions ("But isn't that just bogus?"), or talks amongst themselves. I find that not only do I not mind, but I actually kind of like it: it takes the pressure off me. Furthermore, if need be, I know I can always regain control of the room (since I am standing in front of it).

The paper itself is fairly interesting, and, from what I can tell, important. The most compelling result "concerns the notion of a threshold" of star formation. The rate at which galaxies, or more specifically, regions within galaxies, make stars has lots of implications for galaxy evolution and other areas of astronomy. Unfortunately, we can't just take a yearly (or million-years-ly) census of a galaxy and count up how many new stars we see. Instead, people model what the spectra of stars of different mass look like (how much light they put out at different wavelengths) and the relative numbers and ages of stars of different masses, which can then be turned into "we expect to see this much of this kind of light when stars are forming at this rate." One star formation tracer is quite popular because it involves looking at an emission line from hydrogen, called Hα, which is a very strong feature in red light, which we can see from the ground. People have noticed that in most galaxies, at some large radius, Hα just quits being seen. Since this radius corresponds to a surface mass density, people have also talked about a critical "threshold" density below which no star formation can occur. The star formation tracer used in this paper, on the other hand, is ultraviolet (UV) light, which cannot be seen from the ground (at least while we still have an ozone layer in the atmosphere!). So, the data are from a satellite, GALEX. The main problem with the notion of a star formation threshold is that it's not actually a star formation threshold; it's an Hα threshold. This paper shows pretty convincingly that UV radiation is still seen beyond the threshold at which Hα is observed, which, presumably, means that star formation is occuring. They argue that no more Hα is seen at large radii because really massive stars (more than about ten solar masses) are needed in order to have Hα, and these stars are very rare and die very quickly. When it becomes very improbable to have even one of these massive stars within a large region, then no more Hα will be seen. The UV light, on the other hand, is sensitive to slightly less massive (and therefore more common) stars, and can therefore be seen at larger radii.

Tuesday, September 26, 2006

AstroVino #1: Intro and Component Tasting

Tonight was the first class of the wine tasting class I'm taking this quarter. Offered by and at the home of one of the professors here, Rick, it's a ten week course covering over sixty wines from four continents. We've got a fun astronomy-filled group of graduate students, postdocs, professors, and a spouse or two. The basic goal of the course is for us to learn the language of wine, the kinds of wine we do and do not like, and how to pick which wines might go well with which dishes. (Given some of my past experiences with wines, this is probably a good thing.) Mostly for my own personal edification, I'm going to try to post my impressions of each week's class before I forget all the nice details.

This week we started off with a bit of a history lesson. I personally really like the idea of the first person to taste wine: grapes are stored in a jar, they ferment, some sad soul comes along and decides to fix everything by poisoning themselves, they take a drink, and hey!, everything seems a little bit better...

The first food of the evening was a handful of concord grapes. This was a real eye-opener for me. They tasted like artificial grape flavoring tastes... but in grape form!! Where have I been all these years?! Amazing, simply amazing. We examined how the juice tastes compared to the pulp (which is actually white), how the juice from the dark purple skin tastes compared to the skin and the pulp. And how bitter the stem is. The grape tasting was complemented by an Ohioan wine made from concord grapes.

The bulk of the class was dedicated to "component tasting." Did you ever do that thing in elementary school where you have to make a map of your tongue based on dipping cuetips into sugar water and salt water and bitter water and swabbing it in your mouth? It was like that, but with wine and no little kids going, ewwwwww. We were given a simple base wine (Almaden Mountain Chablis, a white box wine) which had been enhanced with six different flavors. The components were acidity (citric acid), sugar (sucrose), sweetness (glycerine), tannin (... grape tannin), oak (soaked oak chips in the wine), and simulated oxidation (adding dry Fino sherry). Most of the tastes were fairly subtle, but the tannin and the oak were quite strong. Apparently, by the way, if a wine smells like sherry (and thus not like "wine"), it is probably due to an overabundance of acetaldehyde, and means the wine has "gone bad."

The class finished off with an assignment: taste six different wines (three whites and three reds) and try to identify the components in each. The whites were an NV Gazela Vinho Verde (Portugal), a 2005 Reisling Piesporter Michelsberg Spatlese (Germany), and a 2004 Clos du Bois North Coast Chardonnay (California). The reds were a 2004 Milton Park Syrah (Australia), a 2004 Santa Rita Riserva Cabernet Sauvignon (Chile), and a 2004 Louis Drouhin La Foret Pinot Noir. Normally, I shy away from white wines as I have never really liked them, and I usually find even cheap red wines quite potable. I'm also not exactly known for trying new things, but I figure if I'm going to be doing this class, I should try everything.

And so I did. The Vinho Verde, a light fizzy chilled wine, was quite tart; it's main goal in life is to be a refreshing summer wine. The Riesling was sweet and fruity; apparently, wines from colder climates are usually more acidic, but 2005 was an unusually hot season (according to the German sitting across the table from me), which probably explains why this bottle was so sweet. And the Chardonnay ... tasted like oak. And so did the next one. And the one after that. Even after I had some goat cheese on bread, they all tasted like oak. Have you ever been in a wood-panelled sauna? This taste of oak is like that, but consuming your entire head. Even the wines that wouldn't recognize the inside of an oak barrel if they saw it were reeking of oak after the Chardonnay. It's hours later now and I've had ice cream and brushed my teeth multiple times and my nose and sinuses are still asking me what the hell I just put them through.

But at least I know now to shy away from Chardonnays and anything claiming to be oak-intensive ....

Sunday, September 24, 2006

Adventures with

Apparently, I'm a rampaging robot.

I wanted to look something up in a paper I've written, and obviously, finding it on astro-ph is easier than going to a terminal opening it from *gasp* the hard drive, or even worse, standing up and trying to find a paper copy on my desk. At least, this was indisputably clear to me, until I clicked on the convenient PDF button, and instead of being given the PDF, I was told:

Access Denied

Sadly, you do not currently appear to have permission to access

Mm. So this is amusing. It's also not the same error message you get if you're trying to access a paper that hasn't yet been made public. The page does include a link to their explanation of Access Denied, though:

Access Denied

Accesses from your site have triggered our automatic robot detection system. (Sometimes a block is caused by another user from behind the same proxy.) Blocks are usually removed automatically after about a week.

I sent them an email in response. I said,

I'm not a robot. I am a graduate student.
The meat of their automated reply was—and I paraphrase here—"blah blah blah orange elephants blah blah." They closed with—and this part is a direct quote—:
Message joins 79 other compelling messages current in queue.
Have a pure day.
Hopefully they'll be able to discern the stubtle different between the two worker classes, and I won't be forced to use a mirror site for a week.

Friday, September 22, 2006

The Undergrads Are Back

Well, Wednesday was the first day of classes (quarter system, hence, oddness, but that's another post). The most noticeable change between this week and last week is that the undergrads are back. The only upside of this I can see is that now the restaurants near campus will once again have reasonable operating hours; on the other hand, now there will be hordes of people at them as well ...

The astronomy department here is situated on the top floor of one of the chemistry buildings. One the first floor is a little coffee stand that also serves cold sandwiches and pastries. There are 40,000 goddamn undergrads at this school, and I swear, at certain times of day (like noon) they are all on the first floor, milling around the elevator. And then they look confused when the elevator doors open and people want to, you know, get out?

Most of them manage being lost and confused without actually moving, but some of them need a little nudge. These are the ones you find on the stairwell between the second and third floors looking for the basement (it's completely irrelevant that there are no classrooms above the 2nd floor, by the way). Or they wander into an office on the fourth floor (that no one can find if they are looking for it) trying to find a room in a different building.

Then there's the girl who visited our theoretical spectroscopy class this week. It's about one minute before the bell rings (yes, we have bells; yes, this is a "real college"; yes, it's demeaning; yes, I jump about ten feet every time it goes off and I'm standing near it; apparently a past president was a former high school principal and the university has never quite recovered). The professor is standing there at the front of the room, you know, in front of the chalkboard, talking about the densities and temperatures of tokamaks and the broad line regions of active galactic nuclei. And this girl, the type with the long hair that she probably spent an hour that morning trying to make look "natural," with the jeans that ride just a little too low and the shirt that's just a little too tight, walks in, completely oblivious to the fact that there's this man talking. "Hi guys," she says, smacking her gum. And she sits down on the far side of the room ... and right around then, the professor is done speaking (he had successfully completely ignored her entrance), and so we all get up and leave. She looks flabbergasted, like she can't figure out why we're all getting up and leaving ...

Then there are the ones who want me to join their clubs or sign their petitions or whatever the hell else it is they're trying to do. And I can't help but laugh when I'm just walking along, minding my own business, and someone comes up to me and asks if I want to enter to win $25,000 for grad school...

Tuesday, September 19, 2006

McArthur Fellowships

The McArthur Fellowships were announced today. One of the winners, Josiah McElheny, made a sculpture of the Universe entitled "An End to Modernity," with a good deal of scientific coaching from a professor here, David Weinberg. Art and cosmology simply aren't combined in an intelligent and beautiful way often enough.

Monday, September 18, 2006

Overdue Supernovae

We're overdue a supernova or two. (A supernova is the violently explosive death of a (usually) massive star.) The Milky Way is believed to have a supernova rate of one or two per century, and yet, there has not been a supernova observed in our home galaxy since the advent of the modern telescope (in round numbers, let's say that was 100 years ago).

There isn't really a problem here, other than the fact that this is kind of annoying and we astronomers will fall over ourselves in a melee of sheer giddiness while pointing every telescope known to mankind at the thing when it finally happens. The reason it's not a problem is thanks to "small number statistics"; there have been about half a dozen supernovae in our Galaxy in the last millenium. We've simply been unlucky.

The supernova rate for a given galaxy is usually based on a measurement of the star formation rate. This may seem a little counterintuitive, but supernovae precursors are (usually!) massive stars. Massive stars have a much shorter lifetime than lighter ones; the lifetime of our own Sun is about 10 billion years, while the lifetime of a star ten times heavier is more like 10 million years. If we see a young star die, we know that it was born recently. Turning this around, if we can measure the rate of star formation, we can estimate a supernovae rate.

Star formation rate measurements are usually based on measurements of the density of ionized gas, since star formation and young hot stars will ionize the gas they are sitting in. More locally (and more simply, and more sexily) we can watch radioactive isotopes decay. Aluminum-26, which has a half-life of 746,000 years, emits a gamma ray with an energy of 1.81keV when it decays. So, by looking at the strength of this emission in the Milky Way, an estimate can be made of the Galactic star formation, and thus supernovae rate. This has been done, estimating a supernovae rate of about two per century.

So we know we're due. It will happen; we just have to be ready. And while we're pretty damn sure we'll notice it by the fact that it'll signal thousands of events in sitting happy neutrino detectors,* it's an interesting question as to whether or not we'll actually be able to get all of the information feasible from it. As I see it, there are two basic problems. The first is noticing it. The Milky Way takes up a rather large fraction of the sky, but there are all-sky monitoring surveys either already in place or on the drawing board. For many of these, however, "all-sky" really means "all of the sky we can see without looking in the plane of the Galaxy because it's really messy down there." Unless the supernova goes off nearly opposite the other side of the center of the galaxy from where we are situated and we can't see it becaues there's just so much other junk between us and it, I doubt that detection will be a problem. Historical supernovae are just that: they were bright and noticeable enough to sometimes even be noted by the casual observer.

No, no, what is much more likely to be the real kicker is the fact that telescopes and instrumentation have been designed for observing really faint objects, or really far away objects. Not incredibly bright nearby ones, which is exactly what a Galactic supernova would be. If it goes off while we've still got working space telescopes, will any of them even be able to take a snapshot of it without it saturating the detector or worse? The large telescopes that the majority of the astronomy community is gravitating towards are not ideally suited for studying bright nearby stars. If our nearest neighbor, Andromeda (also known as M31), gets around to having a supernova (Andromeda is also stalling on killing off stars lately, it seems), though, we might be better prepared to stare at it with all of our lids open.

* On the other hand, apparently dropping a wrench near the big neutrino detecting water tank can cause thousands of events as well...

Wednesday, September 13, 2006

Today's Headaches

I've seen this blog advertised as "a good insight into life as a grad student." With that in mind, think of this post as more of a delicate description of day-to-day educational hazards than as the wry whining it is more likely to actually be.

So, my summer vacationing officially over, I came upon the surprising discovery today that September arrived without my noticing or approval. Unfortunately, I have a lot to do before the final day of September rolls around: I need to register for classes and sign up for health insurance, prepare and give a journal club talk (which is required to be on a paper or set of papers unrelated to my own research, which is to say, on a topic about which I am currently ignorant), and write up a conference proceedings. I would also really really like to be done and finished with this paper I have been stalling on for almost a month now. Clearly, the easiest-to-accomplish of these options is registering for classes.

Or so the naive would believe. I am starting my fifth quarter here, and as of yet, I have not managed to get through a quarter without some kind of registration snafu. First quarter, there was the issue of Ohio State deciding to issue me a completely random social security number (which, unfortunately, doubles as a student ID here). I, of course, did not have knowledge of this random set of nine digits, and therefore registering for classes with my own SSN was... tricky at best. This was easy enough to resolve, though.

Second quarter, I easily registered for classes, but about halfway through the term I started getting angry letters from the Bursar's office telling me they would kick me out of school if I didn't pay my tuition. Now, this was interesting, as I am not responsible for paying my tuition—my department is. Around this same time, I got a bill for one of my student loans, including a late fee. Uhm. I'm a student. My student loans aren't supposed to have payments due while I'm still in school. Turns out, one office had come to a full realization concerning my actual social security number, but some other office hadn't exactly gotten the message, and so my tuition had in fact not been paid, causing other offices to inform my lenders that I was no longer a fulltime student.

The problems involving registration for third quarter were actually all my fault, if I remember correctly. I was busy: I was observing in Arizona for over a week, and then there was spring break, and I just kind of forgot to register for classes. Whoops. And I was still trying to convince my lenders that I was a fulltime student, but I ... wasn't registered for any classes. So I got to spend a few hours running around a maze-like building writing checks of various amounts and calling my officemates for various passwords until I could actually register for classes.

Fourth quarter (summer quarter), I didn't register for "classes" until the very last day possible to do so without paying a fee. In the summer, we are still supposed to be fulltime students, and so we register for a full load of research with our current advisors; I waited until I knew who my new advisor would be before registering. No problem. Except I accidentally typed in "1" instead of "15" in the "number of hours" slot. It told me so; I went back and changed it, leaving the country thinking everything was hunky-dory. In Japan I got an email asking me why I had registered for only one hour, but that it would all be taken care of.

About a week ago, the graduate studies director for our department stopped by my office with a piece of paper, asking me why I was registered for no hours, and why I am a Master's student instead of a PhD candidate. Other than constant ill-luck with these sorts of things, I honestly didn't know, and said as much. I was, though, greatly amused at having yet another thing to add to my litany of registration snafus.

The amusement stopped abruptly around 11:26 this morning, when I tried to register for classes, and more importantly, sign up for student health insurance. The webpage claimed I am not eligible for health insurance because I am not a full-time student. This is unfortunate, as I would really like to have health insurance. After some increased back and forth with various parties, I learned that someone somewhere forgot to fill out some form, and everything should be taken care of shortly...

Tired, hungry, and grumpy, I then went to AGN lunch. AGN lunch is usually quite fun, not least because one selfless grad student takes Chipotle orders from people in the department beforehand, goes to Chipolte and brings back lunch for everyone. The burritos and burrito bowls are all numbered and laid out sequentially on the back table in the conference room. Today, however, my burrito was missing. The neatly ordered numbers simply skipped that everso important second odd prime. Tired, very hungry, and very grumpy, I went and fetched my own lunch instead.

The rest of the afternoon dragged on as I stared at the computer screen, each sentence I wrote looking suspiciously similar and as content-less as the one I had just deleted. At some point, the selfless Chipotle grad student brought me a bag of Werther's in apology for the lack of burrito, which cheered me up for maybe half an hour. A deep headache set in, the kind where anything hard and sharp (such as the edge of a desk) eyes me as if everything would be better if it rammed into my forehead at a high velocity. Coffee didn't work. Tylenol didn't work. Water didn't work. And worst of all, I was still wishing the weekend hadn't yet ended.

Now, I am a moody person. Those who know me will laugh at the understatement of this fact. The relevant aspect of said moodiness to this story is that many of my decisions are made based on my mood: what to wear, what music to listen to—and what to eat. And this particular mood dictated that a bag of Doritos was in order.

With barely enough change, I went over to the next building and put my quarters into the snack machine. A-7. The satisfying spiraling of the coils holding the bag—aaaand it just hung there, just barely, but most securely hanging there. In a heavy machine I could try with all my might to rattle, and never succeed.

About an hour and negative two sentences later, I mustered up the composure to borrow a dollar from someone, and tried my luck again at vending machine roulette.

I won.

I spent the next half hour or so happily downing half a bottle of Coke and one of the bags of Doritos, and even managed to squeak out three sentences or so of this paper. By then, of course, it was definitely time to go home.

Hopefully tomorrow will have less of a headache and more productivity.

Saturday, September 09, 2006

Free Wireless

I learn something every day. Apparently, the clustrmaps website is offensive, sexual, illegal, or harmful to my computer system. I know this because the Columbus airport blocks such sites, and said site is indeed blocked.

Driving here this morning, I also learned why a rear windshield wiper is useful. I then learned that US Airways apparently charges a $25 standby fee, though this fee doesn't apply to tickets "purchased" with frequent flyer miles.

Hopefully after this short sojourn, I'll be in the mood to learn about more... universally applicable sorts of things.

Thursday, September 07, 2006


I spent four hours and six hundred dollars at the Bureau of Motor Vehicles today, leaving with a piece of colorful paper, some painted metal, and a driver's licence that expires not only before I plan on moving out of Ohio, but also four years earlier than the licence I posessed when I woke up this morning. "I feel like I just made out with a meteor shower." And there's nothing like that look after the eye exam (taken after I've confirmed that I have to wear corrective lenses) and the lady asks, that was without corrective lenses? No, no no, I'm wearing contacts. Oh. Yes, that's the look, that one right there—worth every penny.

I am also now a registered voter of Franklin County, Ohio—the only free of charge accomplishment of the day.

Saturday, September 02, 2006

Vietnam's National Day

My father and I left Hanoi for the States a year ago today, after a full month touring southeast Asia. Coincidentally, September 2 is also Vietnam's National Day. We had managed to miss the Queen's Birthday in Thailand by a day, Malaysia's National Day by a few weeks, and Singapore's National Day by two days. But for Vietnam's National Day, we were in the capital, trying to get out, and staying within near spitting distance of the Vietnamese equivalent of the Esplanade on the Fourth of July. Both of us were in a horrendous mood; my father's wallet and glasses had been stolen as we squeezed through half a mile of ass-to-stomach thick crowd the night before, and I didn't know whether I more wanted American food or to walk down a hassle-free Walmart aisle.

Our flight out was shortly before midnight, so we took our last dinner at the cozy backpacker's place we had already frequented on a number of occasions. While my dad was shooting off a final email to my mother, I had a few drinks and made a final purchase of a 750mL bottle of Da Lat red wine. I hadn't actually had any on the trip, and we hadn't actually made it to Da Lat, which is in the mountains north of Ho Chi Minh City (also known as Saigon). Next time I'm in Vietnam, I'll definitely try to get there; we simply didn't have time on this trip.

We trudged back to the hotel, going well out of our way to this time avoid the burgeoning masses of people around the lake, getting back around 7:30 or so. The roads around our hotel were completely blocked off, and it only took about ten minutes to realize that "we ordered a taxi for you" really meant "I have no idea what you said, but I don't want you mad at me." So we walked away from the commotion for a few blocks, looking for someone to give us a ride to the airport.

Now, for those of you who haven't had the distinct pleasure of travelling somewhere where they can tell you're a foreigner by the color of your skin (please note that this is not an issue in the U. S. of A.) and where everyone knows that you can take advantage of a foreigner because they are endless fountains of money, let me explain something. One of the main reasons I was so ready for this slightly-too long trip to be over was that I had begun to cultivate an intense desire to be able to walk down the street without every other person yelling at me, Hey lady, want a taxi? Considering this, there is a strong level of irony to the fact that we had a hell of a hard time finding someone willing to give us a ride to somewhere else for the exchange of a few colorful pieces of paper.

We eventually found two guys on motorcycles, and realizing this was the best we were going to get, decided on a slightly too high price, and set off. There had been a bit of discussion over the word "airport," and then some arguing between the two of them as to, presumably, the best way to get there, but it was good to be on our way.

We arrived only about twenty minutes later. At the (closed) Vietnam Airlines head office. I honestly do not think that either of these gentlemen really knew what an airplane or an airport is. But they wanted their money. And explaining to them that they hadn't taken us where they told us they would didn't really work. And we really really needed to get to the airport.

At this point, the streets were completely packed, we were out a decent bit of cash (wallet stolen and all, we had a finite amount, see), and now no earthly clue as to our location, except that there probably wasn't a plane headed to Seoul within a mile.

One advantage to the street being at a standstill is it wasn't too difficult to find a cab. We agreed on a definitely too high price, and climbed in. And just sat there. And moved a little bit. And then sat there some more. Sometime between getting off of the motorcycles and into the cab and the cab not really going anywhere, the fireworks had started. Now, in the U.S., if you're in a car, and you're on the road, even if it is July 4th, if fireworks start going off, ... you keep driving, right? Right? You don't stop the car, get out, and watch the fireworks, even if you're in the middle of making a left-hand turn. This is probably because most people in the United States have actually seen fireworks before... as one kind woman about my age explained to us as we were trying to get the Red Traffic Sea to part, "But it's our National Day! Everyone is so excited!"

The only car that made any perceptible progress for the next half hour was the taxi (originally) next to us, with six people in it, most of whom were hanging out of the windows yelling and motioning at everyone else in the traffic jam. I think they were everso calmly explaining, "We've got a lady giving birth in the back seat, so please move over so we can get through."

Once the (rather brief, even if you are worried about missing an international flight) fireworks display was over, and we were definitively on our way, I recognized a familiar tune in the instrumental music on the radio. It was a forty-five minute ride out to the airport, we didn't miss the flight, and I was only mildly scolded by the immigration agent because my visa expired on the first of September. As I was moving shortly after returning, I even had the joy of going to Walmart almost daily for a week.

And now I've been here for almost a year. When my parents visited a couple of weeks ago, I marinated some steaks for them, and my dad opened up the bottle of Da Lat wine I had been saving for a special occasion. We toasted and sipped. Only one sip each. It was... earthy, in that thick muddy black dirt under your toenails kind of way. At least it'll make for good cooking wine...

Friday, September 01, 2006

Weak Lensing

All of the (physics) research I did as an undergrad related in some way to gravitational lensing. As papers with long author lists often take a while to go through the do-the-work-then-do-more-work-then-someone-writes-it-up-then-argue-over-what-got-written-up-then-try-to-publish-it process, it's no big surprise that a paper on "high precision weak lensing analyses" based in part on work I haven't touched in two years now showed up on astro-ph this week.

Gravitational lensing refers to the bending of the path of light due to gravity. If there is mass between us and some faraway source (which for something sufficiently far away, there always is) then that mass will act as a lens and distort how the source appears to us. If the source is very close to directly behind the lens, the source's image can be so distorted that a single galaxy is stretched out into an arc, or even have multiple images. This is known as strong lensing. In weak lensing, the background image is only slightly distorted; for instance, a perfect circle might be sheared to appear slightly elliptical. The neat—and useful—thing about gravitational lensing is that it is only susceptible to where the mass is—and that's it. Even if we can't "see" the mass. The other incredibly alluring aspect to graviational lensing is its (albeit, somewhat superficial) simplicity: all we need is a working theory of gravity. That's it. One theory. No chemistry, no convection modelling, no particle physics, no theory of galaxy evolution or anything else of the like.

We define the "ellipticity" of an object to be (1 - b/a), where b/a is the axis ratio. (If it's a circle, for instance, then the long axis and the short axis are the same length (the diameter), so a circle has zero ellipticity. A straight line, on the other hand, has an ellipticity of 1.) The effects of weak lensing are very very small—only on the order of a few percent—and since a typical galaxy has an ellipticity of tens of percent, it's impossible to measure distortions due to weak lensing for a single galaxy.

This doesn't stop us, however, from measuring the effects of weak lensing on lots of galaxies. Specifically, lensing doesn't happen haphazardly; the direction along which a galaxy is sheared tells us about where the mass is that is doing the lensing. The idea is that if you have some field of un-lensed galaxies, then they should be randomly oriented, and have an average shear of zero. On the other hand, say we have some lensing mass (like a cluster of galaxies) then the average shear of the background galaxies is not zero, and we can figure out how the mass is distributed in the cluster.

Now, this sounds all fine and dandy, but there are two very difficult steps. The obvious one is that "measuring the shear" of a galaxy isn't exactly the most straightforward procedure. Sure, you can just measure the axis ratio, but unfortunately, it's not nearly that simple. Galaxies are very complicated creatures, and deconvolving the galaxy's inherent properties from the changes due to lensing is... tricky. Is it better to directly measure the shape moments of the galaxy, or do you take an idealized model of a galaxy and stretch and squish it until it looks like the one in question? We don't really know.

The much more subtle problem is that of the PSF, or point spread function. See, data have this unfortunate feature of not being ideal. The PSF is one of the descriptions of how data can be annoying; it describes how a single point of light is spread out into a blob. When you see spikes on images, for instance, they are due to the PSF. The irritating part is that the PSF often has an ellipticity on the order of a few percent (just like the shear signal we want to measure!), varies with time (so not all of your images have the same PSF), and can vary across the detector itself (so that the PSF on the left-hand side of the image is different from on the right-hand side). The PSF must be accounted for before the shear can be measured, and a mis-estimation in the PSF's properties can lead to a mis-measurement of the shear signal. And then, assuming the PSF is perfectly modelled (unlikely), is it better to just subtract the PSF's ellipticity from that of the galaxies, or should we attempt to deconvolve the galaxy and the PSF? Again, there are arguments both ways, but no clear answer.

This recent work is part of a courageous attempt to try to determine which measurement methods work, which don't, and, more importantly, why. This is perhaps more effective than the scientific equivalent of lots of people saying, "mine's better than yours because I like mine and I don't like yours." What we have done is to make a set of simulated images, with realistic PSFs, realistic galaxies, and realistic noise, like what one would get from data taken at a large ground-based telescope. Most importantly, a shear was applied to each image which could then be measured. (My contribution to this work, by the way, was in making the simulated images.) Different people threw their weak lensing analysis pipelines at the images just like they would real data, and from their results, we can now quantitatively talk about why a certain method is more effective at correcting for the PSF than another method, or in what situations certain methods are more or less effective at measuring the shear. Other factors potentially affecting shear measurment, such as how galaxy evolution and—get this—pixel size, are also becoming clearer. The cautionary bit is that anyone doing or planning on doing a weak lensing survey should strongly test their measurements methods on simulated data. The good news, though, is that "high precision" weak lensing is in fact possible.