More work yesterday on central stellar densities (which are effectively measures of bulge masses); I got everything to run on my full sample of 140,000 galaxies. I also got a reminder from Moustakas that I need to review the "secular evolution" literature, which claims that bulges can grow from disks without external drivers such as mergers, tidal impulses, or starbursts. I am skeptical!
I wrote code to measure central stellar densities on SDSS galaxies and tested it on a sample of 1000. I discussed the project briefly with Moustakas, who was reluctant to agree that the lack of red galaxies with low central stellar densities indicates that blue galaxies never quiescently stop forming stars. But I don't see any way around it. I guess the sense in which Moustakas and I agree is that my observation puts a constraint on the gas contents (or sources of gas) of blue galaxies.
I started extracting SDSS data for my project on young early-type galaxies. I need the aperture photometry (which almost no-one uses), so the extraction is taking its time. While I waited, I worked on images of Local Group members for Willman. Here's one (click on it to enlarge):
I started working on getting the young early-type galaxies out of the Sloan Digital Sky Survey. My plan is to do the simplest possible thing, ie, to simply find galaxies that, if
shut off and faded for a few Gyr, would have the structural properties of typical red galaxies of the same stellar mass. No by-eye or subjective hocus-pocus.
Mierle and I had a solid pair-coding session this morning, with Skype (tm) for voice interaction and old-school unix screen used as a tool for me to see what Mierle was typing in his office in Toronto. This is a standard operating procedure for us now, and is very efficient. Today was no exception, we found and fixed some bugs and think-os in astrometry tweak for astrometry.net. The conversion of all of our code from IDL (tm) to C is an ambitious project, to say the least!
Later in the day I made a set of fake data on which tweak is required to succeed. From these, Mierle will construct a set of functional tests that tweak is required to pass after any change in the code.
Spent quite a bit of the afternoon on the spectral extraction code mentioned previously. I hardened the algorithm a bit by making it fit to the detected spectral positions, and then re-centroid on the spectral images and re-fit, etc. I also formatted all my output to be consistent with our existing quick-look pipeline (although better, I hope).
I spent the last few days traveling; hence the lack of posts. The postable work includes figuring out the most robust methodology for associating nod-and-shuffle spectra in the PRIMUS 2d images with entries in the file used by the milling machine to cut the masks, and simultaneously figuring out which holes in the mask are primary and which are secondary, etc.
I also figured out what might be a fast and robust way to find and fit the satellite trails in calibrated astronomical images. Time to register "satellitetrail.net"! Along the same lines, I also worked on the fitting and subtraction of what in SDSS data are known as "filter reflections"—glare we get because the filter edges get exposed to the light of stars outside but near the field.
In group meeting, Wu gave a nice summary of Julianne Dalcanton's result that low-mass disk galaxies don't appear to have dust lanes. Dalcanton et al suggest that this has to do with gravitational instability in a rotating disk; the large disks are unstable to gravitational collapse of dust and gas, while the small disks aren't. This hypothesis links the presence of dust lanes with metallicity evolution in a non-trivial way, because you get higher metallicity when the gas from early generations of stars collapses efficiently to make new stars. Wu and I are interested in whether this relates to the trends of PAH emission with luminosity (since PAH emission comes from illuminated, dusty regions).
After Wu, Renbin Yan (Berkeley) showed beautiful results on post-starburst galaxies, LINERs, and Seyferts in DEEP2 and SDSS. He finds that a lot of red galaxies contain LINERs, and many post-starburst galaxies contain LINERs. He has many punchlines, but among them are that certain emission-line ratio selections can reliably select red galaxies (because some kinds of weak AGN—or whatever they are—appear only in early-type galaxies), and that there aren't enough post-starburst galaxies in DEEP2 and SDSS to explain the evolution in the red sequence since a redshift of unity. This is great, because other paths to the red sequence are hard to find (given that red galaxies do not look like faded versions of blue galaxies).
I had a great day today, because I spent virtually the whole day coding!
I worked on making my code to find the spectra in a PRIMUS CCD image more robust and more accurate (in its model of the nod and shuffle behavior). It seems to work on all of our data. What remains is for me to reformat the output to what is expected downstream in our two-d spectral reduction pipeline.
I spent a lot of the day babysitting the re-build of our RC3 page. Then, when it all completed, Blanton and I realized that I had made a significant mistake. I fixed the code, deleted everything, and re-started!
Now that Blanton has made a full estimate of the sky level for every image taken in the Sloan Digital Sky Survey, it has become time to re-make our images of very bright galaxies. I worked on that today. I provide an example below.
It has also become time to provide the world with images of low-redshift luminous red galaxies, where the default SDSS pipelines underestimate flux. This has been an embarrassing issue for SDSS, without a lot of sensible discussion. Any volunteers for re-measuring the fluxes, sizes, and other parameters of a few 104 galaxy images?
I conversed with Matt Kleban (NYU) for a while today about modifications to gravity that could remove the need for dark matter or for a cosmological constant. I mentioned my heresy about acceleration—that it is not demonstrated at high significance with SNe alone; it requires the combination of SNe and LSS or CMB constraints in the context of a physical cosmogonic model (ie, the many-sigma demonstration of acceleration is indirect). This encourages me to investigate a(t) in other models, but unfortunately almost all
theoretically allowed modifications to gravity create strongly coupled, long-range gravitational interactions. There is a conflict, in other words, between models that can be computed (and therefore constrained) and models that are theoretically possible/allowed. This is not a good situation, because the non-zero dark energy can only be said to be experimentally demonstrated if it beats some plausible competitors.
In the PRIMUS project, we are taking spectra of thousands of galaxies simultaneously. Because we know the properties of the instrument and of our slitmask, we know pretty well where those spectra are on the CCD images we read out of the IMACS instrument. However, we do not know exactly; there can be offsets, scale differences, rotations, and shear. I spent the day writing automated, robust code to find the spectra in the CCD images, given a slitmask file and a set of IMACS CCD readout images. The low dispersion of our spectra makes everything more difficult—except this, which is in fact made easier by low (and non-linear) dispersion.
Masjedi gave a nice talk at group meeting about merging at redshift around 0.6 from COMBO-17 as estimated by Bell, and Kathryn Johnston (Columbia) gave our astro seminar on the merging history of the Milky Way and M31. There are lots of informative observations, but I haven't yet figured out a hard test of CDM.