Monday, December 31, 2007

Clay Sculpture: a Bhuddhess

I mentioned in November that I took a clay sculpture class last summer (as in, I started this post on July 26, apparently), and now that the majikle day has passed, I can share the formerly Top Secret product (a Christmas present for my dad):
This is a Bhuddhess for my dad's massage parlor (formerly my bedroom). The inspiration was the weeks we spent touring Thailand (and other various places in southeast Asia), replete with daily Thai massages and many famous Bhuddhas. This Bhuddha is in the meditation position, which I figured was the most similar to the mental state achieved in a good massage. The base on which she is sitting is supposed to be a lotus flower, a la this picture from a lake near Battambang, Cambodia:
I'm also rather disturbed at how disturbingly similar this picture (which I took) is to one of the iPhone's (and, apparently, newer versions of Apple's OSX) default wallpapers.

Sunday, December 30, 2007

Hotel Mauna Kea

What is it like to observe on Mauna Kea, the tallest mountain in Hawaii? Hat tip to the Angry Astronomer:

Monday, December 24, 2007

Bismuth and Copper, on a Tree

(NB: I would have made this post earlier, but alas, my camera has broken. Santa?)

I have been taking a jewelry class for the last few months, and as those who know me in Real Life invariably ask upon hearing this tidbit: "Jewelry?!" Well, good point. But one of the projects I have spent the most time on was making bismuth crystal Christmas tree ornaments out of copper:

This particular crystal is one of the ones a friend gave me last year, shortly before I discussed making bismuth crystals here (see the crystal on the top-left in the first picture in that post). Next steps include getting better at making my own bismuth crystals (it's a lot of fun, but rather tricky as it turns out), and turning them into more pieces of art and jewelry: necklaces, earrings, pendants, and more ornaments.

Wednesday, December 19, 2007

More astro-ph Nuggets

Today's first piece of fun on astro-ph is a short paper by Travis Metcalfe on "The Production Rate and Employment of Ph.D. Astronomers," or: how difficult is it to get a tenure-track job given that you have a Ph.D. in astornomy? I haven't actually read this preprint, but Travis had a very interesting poster at the AAS meeting last May on the same topic, so I am guessing it is based on the same information.

The second is a short conference proceedings by Chris Stubbs on the Crisis in Fundamental Physics, or, what will happen if w really does equal -1. Translation: what if the properties of the dark energy really are the most boring vanilla stuff we can come up with ... and thus also boring both theoretically and observationally. This, combined with Simon White's screed from a few months ago, paint the question of dark energy as inherently interesting (there is something going on in the universe that we really don't understand, and it might be related to why our really-good-for-everything-else theories of quantum mechanics and gravity don't play nice together), but in the meantime it poses a scientifically and sociologically potentially crippling/stagnating problem.

Saturday, December 15, 2007

How to Measure the Masses of Galaxy Clusters

I've spent some time recently thinking about the usefulness of galaxy clusters in astronomy and cosmology (aside from them being interesting in their own right). As the most easily identified most massive structures in the universe, one obvious parameter that is often desired of them is their mass. But how does one go about weighing the most massive objects in the universe?

Stellar light: This is the simplest and most straightforward way to measure the mass of a cluster, but it also requires the most assumptions. The basic idea is that if you know how much light the stars in the cluster are emitting, and if you have some good guess at how much mass a cluster of a given luminosity is (i.e., the "mass to light ratio"), then you can estimate the cluster's mass. An even simpler version is to just count up the number of galaxies in the cluster, and say, "well, clusters with this many galaxies have on average about this mass ..." And, counting the number of galaxies in a cluster is actually rather trickier than it sounds because it takes a lot of telescope time to verify that individual galaxies are in fact cluster members.

Galaxy velocity dispersion (also stellar light): If you're going to go through all the trouble of verifying which galaxies are actually in the cluster, then you can actually measure the mass of the cluster rather than merely estimating it. By taking the velocity dispersion of the galaxies along the line of sight, and assuming that the cluster is relaxed and in virial equilibrium, you can measure the mass, which is essentially proportional to the square of the velocity dispersion.

X-rays: Galaxy clusters have a lot of hot ionized gas in them. This hot gas emits high energy photons due to what is known as "bremsstrahlung": when an electron changes course as it goes whizzing past a (positively charged) ion, it is accelerating, and therefore gives of radiation. We can measure the temperature of the gas from this radiation, and like with the galaxy velocity dispersion, when we assume the cluster is in virial equilibrium—not always an accurate assumption, especially if, say, the cluster is merging with another cluster, or just forming—then we can calculate the mass of the cluster. This is one of the most popular and straightforward way of measuring cluster masses; the only tricky part really is the fact that one has to go to space in order to get X-ray data.

Weak lensing: As light from galaxies passes near a cluster, the cluster's gravity causes the light's path to bend slightly, which in turn causes the shape of galaxies behind the cluster along our line of sight to appear slightly disorted on the sky. (If you want a less hand-wavey explanation, you can look at this post I wrote last year. The mass and the physics are the same, even if the regimes are slightly different.) By measuring the average shape change of background galaxies in different annuli around the cluster, we can measure the surface mass profile of the cluster. Many people will argue (including me, perhaps) that weak lensing is the only way of measuring the entire cluster mass—gravity only cares about where the mass is, not what is causing it, and so gravitational lensing is sensitive to the underlying dark matter profile of the cluster, not just where the gas, galaxies, or light happen to live. Weak lensing is also a nice technique because it can be done with ground-based telescopes using visible-wavelength (or near infrared) light, but it does not require a plethora of spectra like galaxie velocity dispersion measurements do. Converting a measured weak lensing profile to an actual cluster mass estimate, however, involves converting a surface mass density excess to a surface mass density (i.e., the "background" surface mass density must be well-estimated) and then converting the surface mass density to a mass (i.e., assumptions about the three-dimensional structure of the cluster must be made in order to turn a two-dimensional map into a three-dimensional mass).

Sunyaev-Zel'dovich effect: One of the most exciting and promising new ways of measuring cluster masses is through their imprint on the cosmic microwave background (CMB, or as "real cosmologists" call it, the "camb"). As a CMB photon goes through a cluster, it will interact with some of the high energy electrons in the cluster's hot gas (the same ones responsible for the X-ray emission). The result is that the CMB photon gains a little bit of energy, causing the CMB to appear hotter in the direction of the cluster than it would be in the cluster's absence. Through a conspiracy of math and physics, the change in CMB intensity is essentially due to the cluster's mass alone—and because the clusters are relatively close by compared to the surface of last scattering (the origin of the CMB photons), the SZ signal is basically independent of the cluster's redshift. The redshift independence is both a blessing and a curse: while we can theoretically detect high-redshift clusters with the SZ effect, we have no way of constraining their redshift using the SZ effect alone—and high redshift objects are exactly the ones which are more difficult to detect and study using the other techniques I've described here. The SZ effect has been observed for several clusters, but to date no clusters have been conclusively discovered from their SZ signature. This may change soon, however, as two telescopes (the South Pole Telescope and the Atacama Telscope) capable of detecting the SZ effect have come online in the past year. At the very least, the SZ effect promises to be a powerful technique for constraining the measurements from the other techniques described here.

Monday, December 10, 2007

Citations and

A couple of years ago, a paper came out saying that astronomy papers posted on astro-ph get about twice as many citations as those which do not. Plenty of people think that if you are going to post a paper to arXiv, then you should time it so that it, for example, shows up near the top of the daily astro-ph listings. The second paper on today's astro-ph list takes this a step further, showing that the papers appearing near the "top" of the daily astro-ph listings receive essentially twice as many citations as those further down on the list. The interesting question here—one which it's fairly difficult to disentangle—is whether this difference is really purely due to a positional bias (people glance at the first few papers on astro-ph in the morning and then stop, and thus are more likely to remember and cite those first few papers), or if it is a selection effect (people who care enough to make sure their paper is near the top of the list might simply have better science to share, or be better at self-promotion in other contexts, like conferences).

Saturday, December 01, 2007

Astronomers Stuck in a Cloud

I've been at the MDM observatory near Tucson since yesterday around sundown, and last night I started a blog post which looked something like this:

Well, I'm back on the mountain, and it is raining. A lot.
And then the power went out, and since the generator was down as well, we had no power, no heat, no internet, and—after a few hours—no phone either. We were in the middle of a cloud, a white windy rainy mass, wherein we had no connection to the outside world. It's an odd thing, being in a dark building with only a couple of flashlights in the middle of storm after everyone else has gone to sleep, and I (having been staying up late in this time zone for several days already) was the only one awake. So I sat around for a while thinking about galaxies and stars and clusters how everything is interrelated and how I'll never come up with a thesis topic.

Today has been more lighthearted—and more of an extended hurricane party, but with more astronomy and rampant silliness. The first year grad students are here this weekend, nominally to learn how to observe. (Does learning how to do a lightning shutdown count?) They have finals next week and so there has been a lot of questions buzzing around along the lines of "Why are metal poor stars bluer and fainter than metal rich stars?" and "What's the difference between the Tully-Fisher relation and the Fundamental Plane?" Between this and the conference last week (more on that later), I feel as though I've been walking in an astronomy-saturated fog for a week.

The power came back around 2:30pm and the internet and phone followed around 4. With the return of the outside world, there has been a lot of online Scrabble (yes, I've been converted) and now the watching of the Oklahoma-Missouri game on the small TV in the kitchen ... something about if Oklahoma wins then it is good for Ohio State and if Ohio State wins the national championship then alumni will want to give more money which will eventually be good for the astronomy department.