distant clusters

It was clusters all the time on Friday, as Moustakas's collaborators on the ESO Distant Cluster Survey have been in town all week working on Spitzer data. They discussed some of their results, past and future, at group meeting. In the afternoon, Markevich (CfA) told us about physical measurements of astrophysical plasma that are possible in clusters, particularly in clusters like the famous bullet cluster where there are fast-moving substructures creating shocks.


Sagittarius stream

Zolotov and I visited Kathryn Johnston (Columbia) this afternoon to talk Sagittarius stream. Johnston convinced us that what we have (in the public SDSS spectroscopy) may be interesting and constraining on models. We also discussed with Sanjib Sharma (Columbia) some of his very novel methods for finding substructure in N-dimensional data sets, using information-based metrics.


stellar mass and priors

I was at SUNY Stony Brook to give the astro seminar. I took the opportunity to spend some quality time with Pizagno, who is converting multi-band SDSS images into stellar-mass images by various methods, for the purposes of making stellar-mass-based structural properties and comparisons with kinematics measurements. The big issue is how to work in the low signal-to-noise parts of the images; everything would be fine if the conversion from CCD counts to stellar mass was a linear operation, but it is not (because components of stellar mass must add non-negatively and also because there is dust absorption). Pizagno's very nice solution—which he is testing against the other ideas you might have—is to pull the low s/n pixels towards a prior color determined by averaging the very low s/n pixels. His method is correct even if the low s/n parts of galaxies are a different color from the bright parts (which they are, in general), but it also makes use of the pixel-to-pixel information you do have at low s/n, and it correctly implements a gaussian prior in each pixel. I love the method technically, of course, but it also seems to give much better results than, say, doing adaptive smoothing or medianing at the outer parts.

BTW—for the nerds—non-negative linear optimization is awesome these days. Read Blanton & Roweis on K corrections for a non-trivial application in astronomy.


GALEX and environment

I spent a very enjoyable day at Johns Hopkins, crashing the GALEX science team meeting, where they asked me lead a discussion on galaxy environments. I worked through the usual issues: How do you measure environment and what are the trade-offs? (You either get well-defined physical scale or else good s/n but never both!) At what scale is the environment most connected to galaxy properties? (1 Mpc or so.) What is most closely connected to environment, in an information sense? (Star-formation rate.) How do you perform the most sensitive possible experiments? (By averaging low s/n quantities as a function of high s/n quantities and not the other way around!)

This all started some pretty lively discussion, and there are some great new directions from GALEX, including but not limited to: GALEX colors have much more dynamic range among red galaxies than optical colors, so you can hope to do more sensitive things in the green valley of galaxies that are neither completely red nor completely blue. GALEX can tell you about the spatial distribution and extent of star formation in low-redshift galaxies, which might show interesting environment dependences. Because GALEX is so sensitive to dust extinction, and dust is mixed with gas, in principle it might be possible to use GALEX to measure, maybe indirectly, the gas resevoirs in galaxies with low star-formation rates. In general, the fact that star-formation rate is so tied to environment means that GALEX is the right tool for the job.


not much

Between drafting the PRIMUS NOAO proposal, teaching, and committees, I didn't do much research today, although Michele Redi (NYU) demonstrated at lunch that, given exotic fields or large extra dimensions, Lorentz invariance can in principle be broken without affecting the cosmological model.



I spent a small chunk of today testing our pre-alpha functionality on astrometry.net. I filed a huge number of tickets. Sorry Dustin!


false positives, galaxy merging

[Just back from travel, hence intermittent posting. The following was composed on an airplane.]

I spent this morning (Thursday) in San Francisco (thank you, delayed flight) working with Roweis on what we call the verify step in astrometry.net. This is the step where we decide, for a given hypothesis about an image's pointing, rotation, or scale, whether that hypothesis is correct (or very close to correct). Roweis and I are proposing that we base this decision on an approximate but conservative estimate of the false-positive probability; that is, we consider a hypothesis correct if the match we observe between the image stars and the catalog stars is extremely unlikely to have occurred by chance. One interesting aspect of this is that we have to track all hypotheses we have tested so far: Your false positive rate goes up the more you look.

Philosophically, the reason that astrometry.net's internals work is that the verify step is very simple and fast. Our quad matching system is very unreliable; it generates thousands of hypotheses, only one of which is correct. But because the verify step is fast, we don't mind having many hypotheses to check. The least flattering description of our system is that it does brute-force search of every conceivable hypothesis (of which there are many billions), but with the quad system ordering the hypotheses so that we are very likely to hit the correct one in the first few thousand!

I spent yesterday (Wednesday) battling it out with Santa Cruzians about the red sequence of early-type galaxies and constraints on any processes of galaxy–galaxy merging that can cause it to evolve. That was lively! Also Croton (Berkeley) and Wechsler (Stanford) came to UCSC to join the battle.

It is hard to summarize the discussion here, but points that came up include: If you want to look at the stellar populations of a galaxy that is post-merger, you have to consider that some of the stars may have been thrown to large radius and are now part of the intragroup stellar population. In thinking about the evolution of the tilt of the red sequence, you have to correctly treat the differential evolution that comes from blue galaxies fading more into the L-star part of the sequence, percentage-wise, than into the high-mass part. There might be massive, star-forming progenitors of the massive red galaxies hiding among ULIRGs and luminous AGN. The observed evolution of the total mass on the red sequence does not require a lot of evolution at the most massive end, though that is in large part because the signal-to-noise of the observations is limited. And etc. Interestingly, I think we all agreed that merging ought to make the massive end of the red sequence become broader (in some metric) with cosmic time.


Google, Konidaris

[I am on travel in California this week.]

I spent yesterday at Google, visiting Roweis, who is moving there for a year. Google is everything it is cracked up to be! I met various heroes of the open-source world (does the name "Guido van Rossum" mean anything to you?), and, perhaps surprisingly, talked science all day. There is an enormous amount of interest in astronomy at Google, and a lot of things going on at Google that are relevant to astronomers, including especially work on open-source development and distribution of large, public datasets. In particular, there may be some possible relationship for Google in the distribution of the knowledge generated and aggregated by astrometry.net.

Today I was at UCSC for the qualifying exam of Nick Konidaris. He gave a very nice presentation on the importance of the galaxies in the "gap" between red and blue, and their implications for galaxy evolution. He has outlined a plan for figuring out what they are, how they evolve, and whether they explain the growth of the red sequence with cosmic time. It is a great project.


merger rates

It was all talk today. Among other things, Masjedi and I discussed his thesis (currently being written for a late April defense) on the growth of massive galaxies by merging. He uses small-scale clustering to put an absolute limit on the mean accretion rate. We realized that with Sheldon's fast clustering infrastructure and Masjedi's methodology, we could straightforwardly and rapidly generalize the merger rate limit results to all kinds of galaxies, not just the very massive ones.


AAS abstract, alpha

Zolotov and I worked on her abstract to submit for the AAS meeting this summer in Hawaii. It is on the properties of the Sagittarius stream.

The rest of my time was spent on more alpha-related testing, ticketing, and feedback.


critical tests of CDM, alpha

Spent all day yesterday at Princeton arguing with Peebles about critical tests of CDM; ie, observations of physical phenomena relating to galaxies where even if we marginalize over our uncertainties about galaxy formation a conflict between the observations and the theory could potentially rule out or non-trivially confirm the CDM model. We focused on three areas: (1) the Milky Way as an anomalous galaxy (in, eg, having an old, thin disk), (2) galaxy–galaxy merger rates, and (3) regularities among early-type galaxies (which are allegedly built from heterogenous mergers). We have had this conversation about every six months for the last eight years; maybe this year we will write a paper?

Spent all day today stress-testing our alpha-ready astrometry.net service, and kept finding that it is not quite alpha-ready.


going alpha

Worked on small things related to astrometry.net going alpha on the web, including testing the blind solver on some hard images, and inspecting input and output pages, writing tickets, etc. The alpha release will be to a limited group of people, and with limited functionality, but we would rather do a few things right than a lot of stuff badly.

I also made this image, matched to last week's image but with the HI contours overlayed:


galaxy images

After—but totally unrelated to—my rant about IGM interactions, Jacqueline van Gorkom (Columbia) emailed me regarding the possibility of detecting with SDSS data the tidal features in spiral galaxies that are undergoing IGM interactions in Virgo! So I made her some images, including this one (below), of a galaxy that is plausibly interacting with the IGM in Virgo.

The image is tiny not because the galaxy is tiny (it is several arcminutes in size) but rather because the image is matched in coordinates (pointing, rotation, and scale) to an HI map that van Gorkom and collaborators have made.


galaxies and the IGM

There is an old idea kicking around that galaxies must interact strongly with the inter-galactic medium, especially when they fall into rich clusters. This idea comes from theoretical considerations (of, say, ram pressure), from observational considerations (of, say, galaxies clearly distorted by IGM interactions, and from the simple fact that galaxies in clusters have lower star-formation rates. This idea is wrong!

Actually, it is not totally wrong, as I discussed at length with Schiminovich today. There certainly are examples of galaxies that are interacting strongly with their IGM. However, it is not a dominant process in galaxy formation and evolution. No statistical observations of galaxy star-formation rates or activity as a function of clustocentric distance show the virial radius (or any other radius) strongly, and (with minor exceptions) galaxies do not point the way to the centers of their nearest clusters. Most of these things I know from unpublished work (though some of it is here and here), but perhaps my long argument with Schiminovich today will convince me to dust some of that unpublished stuff off.


group galaxy luminosity distribution

I worked out one possible null hypothesis to test in my fossil groups project: I compare the distribution of magnitude differences (second brightest minus brightest) to the distribution you get if you randomize the galaxies at fixed group multiplicity. You have to randomize within / among groups of the same multiplicity or else you will find an effect just because the luminosity distribution is a strong function of environment.

I also worked out the most conservative possible assumptions to make about galaxies with no measured redshift (a few percent of the SDSS sample from which our group catalog is built).


coordinates on the sky

Zolotov is very close to a measurement of the radial velocity as a function of position along part of the Sagittarius stream, a large substructure in the Milky Way halo. In order to present her results she has to transform her data into the random and ill-documented coordinate systems used in this business. I failed to help her in this today.