I made some ugly pictures of Willman 1, Beth's odd (and tiny) Galactic globular cluster, based on our KPNO Mosaic imaging data. I am not posting the pictures here because they aren't yet ready for prime-time. Morad helped me diagnose some problems with our pixel masking and now the pictures are re-making.
Sam and I had yet another conversation about geometry!
Sam and I discussed some of the steps involved in the "resolve" part of automated astrometry, where we have to decide among many computer-generated hypotheses about where each field is pointing and its rotation and scale. We think that a single coincidence among quads will be sufficient, but there are issues for finding even that coincidence (what's a coincidence?).
I got comments on the post-starburst environment paper from Tucker, Knaizev, Goto, Humboldt, and Morad. These comments were great; in particular they pointed out that I have completely ignored an important and relevant literature, much of it emanating from SDSS itself. I should hang my head in shame, but in fact I am happy that everyone is doing my work for me.
Preprints: You can either read them or you can write them
—Penny Sackett
talk, talk, talk!
I talked to Schlegel at length about the conceptual problems with drizzle, and reiterated my concerns that (a) it does not optimize a scalar objective function; ie, the image it produces is not the "best" in any conceivable sense; and (b) philosophically, it treats the telescope as a photon bucket, not a device that measures the intensity. This latter problem is subtle, but if you treat the detector as a flux collector, you have to worry about "flux conservation" and "gaps in the detector" etc, but when you think of it as device that measures the intensity field, you just have to determine each pixel's "beam" and then note that each measurement pixel constrains the beam-convolved intensity map. Of course, drizzle exists, and my ideas are vapor-ware, or maybe bar-ware, since they are just polemics shouted in bars.
I talked to Sam and Morad about the current status of the astrometry project, and we heard that the indexing job we started on Friday finished in an hour (which is good) and we checked some numbers relating to methodologies for determining, at test time, which index matches are correct, and which are flukes.
I discussed projects and drafts with Morad, Quintero, and Blanton.
Sam and I worked through the geometry—and symmetries and ambiguities—of the star quadrangles used for indexing in the current iteration of the blind astrometry project. We checked some code Sam wrote and started an indexing run on the USNO-B subset Morad made us.
Burles found a bug in my ACS WCS code. It was ugly. I fixed it. Then I wept tears of shame.
I discussed Lockman's polemic with Schechter (who is credited with pestering) and Blanton. I think most of Lockman's general, philosophical points are correct, but his conclusions are incorrect: The fact that you need to know the folklore to be a good astronomer is a reason not to maintain the folklore system, but to change it. Of course change requires good people and money. His argument that everyone should get cozy with the raw data is a good one, but it is on a slippery slope: He insists that you must know the raw data (I agree), but he doesn't insist that you build your own correlator. Sounds like an observer (as opposed to an experimentalist) talking.
I briefly discussed the environment- and luminosity-dependences of the merging rate with Morad, and high-density spectroscopy with Burles.
I reminded myself of the details of our future "Gunn Atlas" built from the SDSS data on low-redshift galaxies (it's a great idea), for my next attempt at NSF funding. Unfortunately, NSF funding proposals do not count as "research" for the purposes of this diary, so I am going to have to shut up about this now.
It may sound crazy, but the data are consistent with a picture in which the morphological and star-formation properties of galaxies are in fact set at a relatively early time, before infall, and the relationship with environment comes from the fact that the galaxies that find themselves inside clusters come from a special (ie, not representative) set of environments and initial conditions.
Today I worked on the problem of putting this argument, or a softened version of it, into the post-starburst environments paper.
I cranked out an unintelligible, unreferenced, but complete introduction to the post-starburst environments paper. This brings the current version up to what I call a "zeroth" draft—ie, all there but not yet in a form that is ready for the co-authors to read, let alone the public.
Sam and I spent all afternoon playing with SDSS run 745, camcol 3, field 101, looking at its astrometry, the number of USNO-B1.0 stars in the field, and the probability that the field will contain an astrometric index quadruple in our current plan for automated astrometry. It looks like our index ought to make quads with a maximum separation of between 2 and 7 arcmin to have good coverage and good precision when applied to the SDSS data. We realized that if the USNO-B1.0 and all data we ever see is of high precision, the best plan would be simply to index quads at the smallest possible angular scale; the only reason not to do this is that there is positional jitter in both the USNO catalog and any input image, which makes matching more difficult at the smallest scales. Of course at larger scales, camera distortions enter, so it still might be best always to be looking at the smallest available quads.
I wrote more text, including mentions of the fact that we can rule out tidal triggers for the (majority of the) K+A galaxies, and that we see no excess of K+As in cluster infall regions. This latter is in direct contradiction to many off-hand claims (based on data-free arguments) in the literature. I hope I can trace some of those down.
Wrote text in the post-starburst environment paper.
Helped Burles and Tongyan with color pictures of gravitational lenses.
Worked out (yes, it is obvious) that if the merger rate onto LRGs as a function of secondary magnitude is proportional to the luminosity function, then the mass-weighted merger rate will be dominated by galaxies around L-star. If it is going to diverge to the faint end, then the LRG-galaxy cross correlation will be a very strong function of the secondary galaxy magnitude, with dwarfs having much higher amplitude cross-correlation than giants. Doesn't seem likely; in fact I think we know this not to be true. The assertion in Murali et al 2002 that the mass build-up in galaxies is dominated by smooth accretion rather than merger events (is this the work upon which Romeel's assertion is based?) is not very robust, as it involves a huge extrapolation below the mass resolution of the simulation.
I gave feedback on recent progress on Quintero's clustocentric project, and Sam and Morad's work towards the first milestone on astrometry.net.
I put two more figures into the post-starburst environments paper, but unless I write some text for that paper tonight, I have failed today.
Morad has a great plan for the next project on the merger rate: determination of the mass accretion rate onto LRGs as a function of secondary mass. He will find out what galaxy masses dominate the mass accretion rate. One question: if the accretion onto galaxies is dominated by steady rather than impulsive (merger) events (as Romeel instructs us), does that mean that the mass spectrum of accretion events ought to "diverge" towards the low-mass end?
On day five of The Fabulous, a bit of heat broke out about the merger rate, with the van Dokkums and the Conselices arguing for high rates, and the Pattons and Hoggs for low. Basically, you get low rates if you assume all close pairs merge on a dynamical time (the maximal possible assumption), and you get high rates if you assume all high-asymmetry or faintly-tidal-featured galaxy had a merger in the last dynamical time. Which is more believable? Well, since the close pair rate is an absolute upper limit, I have to assume that either tidal features last for a long time, or they are raised by minor mergers. I don't think there are any other options for resolving this discrepancy.
In related news, it came up at coffee that although the galaxy-galaxy merger rate may or may not be expected to be low, the galaxy group-group merger rate has to be high, because halo mergers are frequent. Now that Berlind has a catalog, lets do it.
On day four of The Fabulous, many people showed believable galaxy luminosity functions for redshift 5, 6, 7, and maybe 8, or 10! Mannuci called redshift 4.5 "low redshift" without any hint of irony.
