2006-04-28

Princeton

I gave the Princeton Gravity Group talk today, about galaxy merger rates as observed in the nearby Universe.

Afterwards, I talked to Padmanabhan and Finkbeiner about SDSS-III, PANSTARRS, PRIMUS, and other big, new survey ideas. Finkbeiner argued that we should forget all that and look for direct evidence for dark matter (by annihilation to gamma rays or any other means available). Padmanabhan was skeptical.

I spoke with Strauss and Ho about high-redshift post-starburst galaxies found in the quasar target selection sample. It looks like there are enough (hundreds out to redshift 0.8 or so!) that we can ask about evolution and environments, in comparison with Quintero's sample.

I spoke with Gunn about the nascent galaxy atlas with his name on it. We are a long way off, but Blanton and I have made huge progress in the last few weeks.

Hester and I continued our discussion of ram-pressure stripping and how the observations in the optical speak to that.

2006-04-27

2006-04-25

ram-pressure stripping, surveys, neutrinos, talks

On Monday, Janice Hester (Princeton) showed us at group meeting theoretical and observational results on ram-pressure stripping of gas from galaxies as they fall into clusters or groups. At lunch, Uros Seljak (Trieste) showed us that the dark matter can't (basically) be sterile neutrinos, using the clustering of Lyman alpha forest. In the afternoon, Blanton and I had long discussions of future surveys, with the long-term goal of deciding what we should get ourselves into.

Today I gave two talks at Eastern Illinois University, including a well-attended public talk. Thanks to my hosts for a great time.

2006-04-23

talks

On the chinatown bus from Boston to NYC, I worked on the two Harlow Shapley lectures I am giving this week at Eastern Illinois University, one technical and one non-technical.

2006-04-21

PRIMUS spectral smoothing, NED mosaics

With Burles I worked on making smoothed versions of the PRIMUS spectra, with a smoothing kernel that is constant in km/s despite the fact that the spectra have very strongly non-linear dispersion.

I also worked on details of the NED SDSS mosaics; they are nearly ready for prime-time.

2006-04-20

NED imaging

I worked on subroutines for comprehensive SDSS imaging of NASA Extragalactic Database sources. The idea is that we ought to be able to turn any NED object query output into the instructions for creating an imaging mosaic of SDSS data. This is all in preparation for the nascent Gunn Atlas, discussion of which heated up this week.

2006-04-19

environment indicators

I worked on galaxy envionment indicators today.

If you are measuring galaxy environments, you can choose from some options: You can choose a measure of local density that either (1) has a fixed physical length scale (radius) or else (2) is high in signal-to-noise. The reason for this dichotomy is that the former are done, basically, by counting the (necessarily small) number of galaxies in the relevant local volume, while the latter uses something like the Nth closest or a clustocentric distance or tesselation, where the scale is set by what gives good signal-to-noise (and is therefore different in different environments). We have shown that a fixed scale of 1 Mpc is about the best possible scale, but you can't measure it at high signal-to-noise.

After this, you can choose a measure that either (a) uses redshift information or else (b) works with imaging data alone. The latter allows you to go fainter into the luminosity function, and work with cheaper (and more readily available) data, but it costs you a lot in signal-to-noise, because you have (even with good photometric redshifts) a lot of projected galaxies (either foreground or background). Purely imaging indicators also have the problem that they almost never have systematics that are constant with redshift, since foreground/background and photometric redshifts all vary strongly with redshift for reasonable (read: the usual) choices that can be made.

2006-04-18

prism spectra

Spent time with Burles looking through prism spectra taken as part of the PRIMUS project. Most of the spectra look nothing like what a spectroscopist is used to looking at, since the dispersion is totally non-linear, and we are (deliberately) not dividing out the system sensitivity (we are doing all fitting/modeling in the observed frame, so we multiply the models, not divide the data; there is a difference). But we found a few clear broad-lined quasars, one of which turned out to be a z=1.6 quasar with double-peaked emission lines incorrectly given a lower redshift in the DEEP2 redshift survey. Apparently PRIMUS is finding lots of redshift errors in the various large surveys, including DEEP2, VVDS, and COMBO-17. Not to worry, though, the vast majority of published redshifts are correct! We formulated a dumb project for me: Find all the SNe among the 18,000 or so extant PRIMUS spectra. I'm on it!

2006-04-17

photometry, spectroscopy, projects, inflation

In the late weekend (after taxes), I worked on fundamental photometric calibration, analyzing the contribution by Stubbs and Tonry. I don't think a great deal of what they say is truly new, but it is certainly about time someone wrote it down and got the conversation rolling. It is boring as heck, but it is required if precision cosmology is to move forward. My own contributions on this subject are here (my most-cited first-author paper) and here (my least).

This afternoon, Burles, Coil (Arizona), and I discussed the current and near-future steps with PRIMUS. Coil, Blanton, and Eisenstein are working on sample selection, mechanical collisions, and masks. Cool (Arizona) is working on extractions. Burles is working on getting the wavelength solutions right (his arc-fitting software is incredible). Masjedi is working on redshifts. What's next? Science. Note how I am not working on anything. Very clever, no?

At group meeting, Burles told us about the gravitational lenses he and Bolton (Harvard) have been finding in copious numbers (about 40?) from looking for two-redshift objects in the SDSS spectroscopy. They have one of the largest statistical collections of gravitational lenses in existence, and they have many uses for lensing, cosmology, and galaxy astrophysics.

After Burles, Coil told us about the clustering of quasars (from SDSS) with galaxies (from DEEP2) at redshift of unity. She finds that quasars are clustered like the galaxies, and have similar bias. She finds this at great signal-to-noise by using a cross-correlation (rather than auto-correlation). Indeed, as Eisenstein, I, and others have been arguing for many years, the cross-correlation is much higher in signal-to-noise than the auto-correlation function for rare populations, and you should almost never use the latter when you can use the former. Coil's result is a great advertisement for this fact, because the cross-correlation of DEEP2 with 17 (yes, 17) SDSS quasars has a higher signal-to-noise measurement of clustering at Mpc scales than the entire 2dF QSO survey auto-correlation function!

After Coil, Marla Geha (OCIW) told us about the gas fractions of dwarf galaxies, which show an enormous range, but one that is a very strong function of environment. Dwarf galaxies with very low gas fractions are almost always close to (ie, within hundreds of kpc of) more luminous galaxies. This effect has never been seen before because prior to Blanton et al (2005), there has not been a dwarf sample selected without regard to environment! The very nice thing is that Geha's results (with Blanton and Masjedi) rule out many ideas about dwarf galaxy evolution and support others—a rare thing in the world of galaxy astrophysics, filled as it is with soft predictions.

After group meeting, Mukhanov (Munich) gave a wonderful informal talk about what inflation naively and straightforwardly predicts (and what it does not). Nice!

2006-04-16

galaxies and the UV radiation budget

[No posts for a while as I have been out sick. I was only in for two hours since Tuesday, to see this talk:]

Alice Shapley (Princeton) gave a data-packed talk about galaxies at redshift around 2 and 3. She reminded us that there is no more redshift desert around 2, which was a major issue in my graduate thesis work. She spent most of her talk estimating the contribution of high-redshift galaxies to the ionizing ultraviolet radation, which we know is there from, eg, the lyman-alpha forest, and which at an earlier epoch reionized the Universe (but substantially after the Universe recombined from the primordial electron-positron plasma). Though there are huge uncertainties, and no sample of galaxies is large enough and well-enough studied to give a definitive answer, it does appear that the high-redshift galaxies release enough ultraviolet light into intergalactic space to make up most of the ionizing background radiation. In other news, she showed some reasonable (though admittedly not air-tight) evidence that the mass–metallicity relation is much less metal-rich at redshift 3 than at the present day (or than at redshift 1, which Moustakas, a few months ago, showed us was also less metal-rich than the present day).

2006-04-11

large surveys

Blanton and I spent a good chunk of time discussing large surveys. Most of the big, upcoming astrophysics surveys want new participants to buy in with cash plus work. The question is: Where's the biggest bang for the buck? (And, I guess: Can we get some bucks?)

2006-04-10

anomalous galaxies, anthropics, halo shapes

In group meeting, Ben Weiner (UMD) spoke about galaxies with anomalous rotation curves—really anomalous! Later in the day, he, Blanton, and I tried to find low-redshift (ie, SDSS) analogs to his "diffuse red galaxies" that he finds in substantial numbers at redshift of unity. We found a few among a clean sample of 30,000 nearby galaxies. Ben has been remarkably adept at finding galaxies at redshift unity that don't seem to exist today.

At lunch, Lenny Susskind (Stanford) gave a very nice talk about the large-scale structure bound on the shape of the inflaton potential. He calls it the anthropic bound, but I call it the observation that there are galaxies. Somehow, Susskind has convinced himself that there can't be astronomers if there aren't galaxies. Don't ask me how! But despite this problem of interpretation/philosophy (don't count on great physicists to be great methodologically), his talk was great and reminded me of all the cosmological physics connected to inflation.

After lunch, Andrew Zentner (Chicago) gave a great talk about the shapes (ie, triaxiality) of galaxy halos in CDM, and bounds thereon. He finds that the Milky Way halo is unusually spherical relative to what we expect of dark-matter-only CDM, but not so anomalous when you consider the effect of the baryon cooling on the halos (it makes them more spherical). Nonetheless, he predicts a strong Holmberg effect (concentrations of MW satellite galaxies along the axis perpendicular to the galaxy disk); we (Masjedi, Willman, and others) do not think there is a very strong effect around our own Galaxy or any others, although the literature is filled with claims and counter-claims. A rule of thumb: When the literature is filled with claims and counter-claims, and they are all a few sigma, then probably the real punchline is that there isn't much effect.

2006-04-08

clusters

[For yesterday:]

Mike Gladders (OCIW) gave a very nice talk on the use of optically selected clusters in cosmology, showing that the results of surveys like RCS-2 and its successors will be great for cosmological tests. From my point of view his most interesting result is that though the bivariate distribution of mass and redshift for clusters is consistent with standard-model cosmology, the same distribution for clusters selected to have strong-lensing arcs is totally wrong. There are issues in interpretation, but the effect is so strong, it suggests that the solution will be important and interesting.

I also worked on some pretty pictures.

2006-04-06

tidal interactions, physics education

Boring day today, writing text to distinguish non-merger tidal interactions of galaxies from the kind of strong tidal interactions that occur during major mergers.

More interesting was the NYU Physics Colloquium by Wieman (Colorado) on teaching undergraduate physics. He showed the main research results (use peer interactions, focus on concepts and beliefs, discuss the organizational structure, connect to the real world, force the students to interact and participate) that I already know, but also one I didn't: In some cases (building circuits was his example), students learn to build real circuits in the lab more quickly if they learn from a simulation than if they learn from building real circuits! This only works where the simulation is carefully constructed to be realistic but also free of irrelevant distractions (apparently students new to physics often spend time worrying about things like the color of the insulation on the wires).

2006-04-05

pretty pictures

I worked on things related to our nascent galaxy atlas, including some code and some general planning. No new pretty pictures to report.

In other (non-research-related) news, tonight, after my class, from the roof of 715 Broadway, Quintero and I took this picture:

2006-04-04

ultra-faint MW satellites

As my loyal readers both know, in 2005 Willman discovered the least luminous galaxy in the Universe: it is a tiny companion to the Milky Way, discovered initially as an over-density of about eight red-giant stars in a small patch of the sky in Ursa Major. Willman's objective, statistical search for such objects has put very strong limits on what else can be out there. Blanton, Willman, and I discussed some of the technical details of her forthcoming paper on these limits. This is a difficult project, but very important, given that dark-matter models predict enormous numbers of companions in the dark sector, and galaxy-formation models have very different mechanisms for explaining which are observable as galaxies.

2006-04-03

LHC, clustering, and weak lensing

There were two talks on the LHC today, one by Jenni (CERN) on the ATLAS experiment, in which NYU is about to become a partner, and the other by da Costa (Harvard) about the top. I embarassed myself by asking about the uncertainty principle: Unlike astronomers with photons, high-energy experimentalists do not try to detect particles at the fundamental limits of precision. Of course such precision would be insanely expensive (and these machines are expensive enough already). But in both talks, the LHC came across very well; I am very excited about it.

In group meeting, Molly Swanson (MIT) discussed her work on mangle (a toolkit for the description of piecewise constant functions on the sky using spherical polygons) and some very nice results on linear relative bias among different galaxy populations. She showed that linear bias is a pretty damn good model, even on few-Mpc scales!

Sheldon, Masjedi, and I discussed features in the weak lensing signal around LRGs that Sheldon is convinced (probably rightly) are evidence of significant systematic errors. But the error is hard to find and I got to wondering if the result could be real. It could be if a significant fraction of LRGs are tens of kpc from the centers of their dark-matter halos. Is that insane? Surely galaxy formation models would tell us if it were true? Time to call in the theorists.

2006-04-01

code bugs, LRG clustering, cosmic rays

The NYU Computer Science Colloquium was by Xie (Stanford) on automatic detection of bugs in code, through direct boolean modeling of the source code. He noted some remarkable statistics, eg: that it is believed that most commercial code has of order 10 bugs per thousand lines of code; that bugs cost society on the order of 0.6 percent of the GDP; that GM vehicles are expected to have 100 million lines of code by 2010 (they already have a million). Xie showed that security-critical bugs (eg, for buffer overflow) can be found automatically with few false positives, and he is responsible for finding hundreds of bugs in open-source software, such as openSSH, mySQL, and the Linux kernel.

Kazin and I discussed the possibility of explaining Majedi's correlation function by looking at the separation distribution of the two most massive subhalos inside halos massive enough to contain two LRGs. Wechsler's results (discussed here earlier) suggest that this project will work well.

Sigl (Paris 7) gave a nice overview of the current and future of very high-energy cosmic rays, which is bright, given the start of the Auger Observatory in Argentina. Auger is almost certain to see anisotropies at very high energies, because the GZK cut-off means that they come from nearby, and there just aren't that many possible sources, even if they come from gamma-ray bursts!