source detection, astrometry pipelines

Burles saved my life today by writing an extremely simple and elegant source detection system that will work on the HST and SDSS images we are using to get astrometry.net up and running.

I wrote down on the astrometry.net trac system wiki outlines of the source-detection and WCS-tweak pipelines. This will help organize my work for the next week or two. Unfortunately, I have to get both of these pipelines somewhat functional before I leave for my Christmas travel.


disk galaxies

Willman's one-week informal workshop (five participants?) on Local Group substructure and equivalents got off to a nice start today, with discussions about current and future surveys pertaining to the structure of the Milky Way and Local Group. There was pessimism about understanding anything about the Milky Way's accretion history from studies of the very nearby (<100 pc) stellar velocity field (something we have worked on a bit here).

In the afternoon, participant Lucio Mayer gave a nice talk in which he convinced me that a lot of the angular momentum problem and Tully–Fisher theory–observation discrepancies were highly resolution-dependent. Indeed, he showed that resolution is a much bigger effect than what is assumed about stellar feedback, at least for these problems, and at least for SPH simulations. That's news.


IDL, cosmic rays

Neal Weiner and I got working on his use of IDL and reading of FITS files. We spent a significant amount of time setting environment variables! He asked "is this what astrophysics research is like?". I answered "absolutely".

Burles took care of the cosmic rays in the HST/ACS data on the COSMOS field. It turns out that STScI flags them for you if you have a CR split or multiple exposures with (close to) the same pointing.


robust fitting

I worked on the problem of least-square fitting with a soft relationship between data and model; in particular the situation in which you are trying to get the best possible astrometric WCS for an image in the face of severe uncertainty about which source in the image ought to be identified with each astrometric standard star. Right now our WCS tweak code does a hard assignment and then updates it on each iteration, but I think there are better, continuous solutions.

I also worked on the problem of getting ultra-precise relative astrometry for, say, HST/ACS images of the COSMOS field, which is a slightly different problem from getting as-precise-as-possible absolute astrometry for each image using USNO-B1.0. It must be possible to align the HST images in a relative sense to much, much better than 0.05 arcsec, but we can't put things on the USNO-B1.0 system to better than about 1 arcsec (and the USNO-B1.0 system is not even this good, globally).


200th post

This is my 200th post (!), but it is just to note that yesterday and today comprise the US Thanksgiving holiday, so nothing was done (if cooking 90 lb of food for 22 people counts as nothing).



I finished and submitted my Marseille review. In the end, this is what I wrote about morphology–density:

In Figure 5 I show the variation of concentration (a measure of bulge/total ratio and therefore a morphology surrogate [citations]) on environment (clustocentric distance, a high-precision but variable-scale environment measure), in narrow color slices. In Figure 6 I show the variation of color on environment in narrow concentration slices. These Figures look very different: Color depends on environment independently of morphology, morphology does not independently of color. Of course I am over-stating the result by calling this morphology; take it as you wish, but it is clear that color and concentration are not on an equal footing when we ask what they can tell us about environment.

In current thinking about the reasons for the morphology–density relation (things like ram-pressure stripping, mergers, late accretion, tidal perturbations), these results are very difficult to understand. What physical processes can tell galaxy star-formation rates about their environments and tell morphologies to keep track of star-formation rates but not do much to the morphologies independently? I think the conclusion has to be that the processes that set morphology (or, really, concentraion or bulge/total ratio) are somehow internal to the galaxies.


iDM, morphology–density

Weiner and I discussed the inelastic dark matter results of Tucker-Smith and Weiner, and the (future) development of a physical model for the abundance that is consistent with cosmology and direct dark-matter detection experiments.

Quintero and I sharpened up his discussion of the morphology–density relation.




review, COSMOS astrometry

I worked on my Marseille review.

Burles assigned me the task of re-solving the astrometric WCS for all of the HST/COSMOS images, because (as he rightly pointed out) this is a perfect overlap of the PRIMUS and astrometry.net projects (see sidebar). I worked out the necessary changes to our system to make this happen. The big deal is that if you want high precision, you can't just fit to USNO-B1.0, you have to propagate faint sources in the HST imaging onto the celestial sphere, and simultaneously fit for (1) the pointing and rotation of each ACS exposure, (2) the pixel scale and camera distortions, and (3) the sky coordinates (RA, Dec) of all the faint sources that appear in any two exposures. Though some people have implemented such systems (it's not rocket science), no-one, to my knowledge, has a general, plug-and-play system. That's one of the long-term goals of astrometry.net, of course.


Milne universe, chi-squared

Rocky Kolb (Fermilab) gave a talk about the possibility that small-scale (tens of Mpc) order-unity density perturbations can mysteriously produce cosmic acceleration. When various of us (Dvali, Gruzinov, me) asked him how, physically, small-scale variations that average to zero could affect large scales, he pointed to terms that don't obviously sum to small totals in a series expansion of cosmic perturbation theory in GR. In my view, the fact that Kolb can't sum a divergent infinite series is not an argument that acceleration is caused by small-scale inhomogeneities! But in particular, and to be concrete, since the universe he considers is non-relativistic matter (dust in GR parlance), I asked Kolb for a physical reason that the Milne calculations (ie, the derivation of the FRW Universe in an entirely Newtonian gravity framework) are wrong, as he must assert that they are (since they agree with the conventional GR calculations and show no acceleration). He said simply that it was possible that they are.

Masjedi, Berlind, Blanton, and I discussed hypothesis testing (in the context of Masjedi's project to fit halo occupation models to his small-scale correlation function. For the Nth time, I figured out the answer to a question first asked of me by Scott Tremaine (Princeton): If you have two equally plausible models to explain a set of data, a chi-squared difference of just one or two (in total chi-squared, not per degree-of-freedom) is sufficient to prefer one model over the other. But if you have a model, and you want to show that the model is not allowed by the data, you have to show that the model has a chi-squared that is significantly larger than the number of degrees of freedom. Of course both of these statements are only rigorously justified when you have a linear problem and uniform priors on the parameters and exactly gaussian (and well-known) uncertainties.


halo occupation, morphology–density, gaps

Masjedi showed me his result that he cannot fit the LRG–LRG correlation function at small scales with a halo occupation model that uses the NFW profile for the galaxies. This is true even if he allows the concentrations of the halos to vary. This is a nice result and will motivate him to fit for the Masjedi profile. Look out Moore and NFW!

Quintero made this plot (below), which shows that although there is a relationship between Sérsic index (concentration) and clustocentric distance (environment) when we look at all SDSS galaxies (leftmost panels, top is color histogram, bottom is dependence of quantiles of concentration on environment), if we take narrow color slices, there is no concentration–environment relation within any slice. This is not true if we reverse things; ie, if we take narrow concentration slices, there is a color–environment relation even within each slice. This reflects on the relative fundamental-ness of the color–environment and concentration–environment relations. By the way, the latter is often called, in recent literature, the morphology–density relation.

I continued to work on coding the magnitude gaps.


Jackiw, gaps

Roman Jackiw gave a talk about symmetry-breaking modifications to E&M and GR.

Blanton and I refined a bit more my ideas about the most conservative possible magnitude gap calculation. I began the implementation of it within the NYU-VAGC.


fossil groups

As I have discussed earlier, there is a hypothesis out there that groups with a large magnitude gap between brightest and second-brightest galaxies are considered likely candidates to be fossil groups, in which multiple group members have (in the past) merged into a large, remnant galaxy. I say considered because this is far from demonstrated. In fact, the distribution of magnitude gaps is very close to what one would expect from a Poisson sampling of an envionment-dependent luminosity function. But we find that there are groups with anomalously large magnitude gaps. The question is, in the face of small redshift incompleteness (and usually spectrograph constraints cause the incompleteness to be higher in compact groups), what is the most conservative estimate of a group's magnitude gap? I think I figured that out today. I will implement it next week.


galaxy evolution

At group meeting, Blanton discussed his results regarding evolution from redshift unity to one tenth, comparing SDSS and DEEP2. He confirms the data result of Bell et al and Faber et al, but not the punchline, because he can show that the uncertainties in the modeling of stellar evolution (and the photometry of galaxies across redshift) are comparable to the evolutionary effects. So, at the moment, merging is not required to explain the evolution in the luminous part of the red sequence.

Jim Peebles (Princeton) gave a great talk about the issues, as he sees them, with extending CDM (well tested on large scales) down to very small scales. He put a lot of emphasis on voids (as usual) and merging. The merging predicted by CDM simulations is indeed hard to reconcile with observations; on the voids his argument is "morphological" rather than statistical. But all this was very timely, because it is exactly what I am working on with NSF proposals, review papers, and research papers, but it was also, as always, solid gold Peebles!

Masjedi, Peebles, and I, in separate pairwise interactions, discussed Masjedi's results and the interpretation of the correlation function in terms of steady-state merging (at small scales). We discussed the issue of whether or not the mean, pairwise, galaxy–galaxy infall velocity dr/dt can be reverse engineered from the correlation function.


Virgo members

I made pretty pictures of Virgo Cluster members (the members that confused the SDSS's photometric pipeline software) for Ronin Wu today.


Eric Bell, positronium

Masjedi and I discussed the relationships between his limit on the local massive-galaxy merger rate and the usual "pair fraction" that appears in the literature—not a very good statistic, for a variety of reasons, the best being it is never consulted or computed when calculating any good estimate of the merger rate! We also discussed the relationship between his results and those of Eric Bell (MPIA) who, inspired by Masjedi, has started to do a similar project with COMBO-17 data.

Blanton and I also discussed Blanton's apparent conflict with Bell on the evolution in the red sequence since a redshift of unity.

It doesn't count as research, but Quintero forced me to work out the energy spectrum of positronium, in the context of answering a (difficult) GRE question. We got it, and we got the right answer, but only after a somewhat embarassing amount of calculation and re-calculation!


Eddington-limited pulsars?

Here's a question for Steve Thorsett (UCSC), who I am hoping is a lurker here at Hogg's Research:

After a nice talk by Gruzinov here about pulsar emission (Gruzinov has recently discovered a force-free electrodynamics "solution" for a rotating dipole, with rotation and dipole axes aligned), he and I talked a bit about the Eddington limit. No pulsars appear to emit above the Eddington limit (even when you consider total nebular emission and/or implied spin-down luminosity). The Crab and one other are basically at the Eddington limit, and all others are below. Is this a coincidence? Part of me says "yes", because gravity doesn't enter in any of the usual considerations about the emission mechanism. But part of me says "no", because (a) nothing emits above Eddington, and (b) if the pulsar is above Eddington, the emission might significantly distort the plasma in the magnetosphere and "break it" or even distort the outer layers of the NS, and change the moments of inertia.

Interestingly, if people are right about spin-down, the Crab was hugely super-Eddington in the past, and it is a coincidence that we see the Crab only now just as it has passed into the sub-Eddington phase! That sounds like the kind of argument a cosmologist would make.

Of course I am talking about the total nebular emission, not the pulsed emission (which, I understand, is a small fraction of the total—either nebular or spin-down—luminosity).


trouble tickets

I spent quite a bit of time this weekend updating, organizing, and creating bug reports on the astrometry.net internal tracking system.


PRIMUS, environments, AO

Discussed PRIMUS extensively with Eisenstein (Arizona) and Blanton, and worked a bit on the Steward/Magellan proposal for the pilot project.

Quintero spoke at group meeting about his clustocentric results, and got a good discussion going about the lack of a morphology–density relation; ie, the result that concentration depends on environment only because it depends on color and color depends on environment, and not vice-versa.

Judy Cohen (Caltech) went through the basics of AO, and then showed incredible images of M31 globular clusters taken with the Keck AO system. Awesome! Even more awesome when compared side-by-side with HST images.


no evolution beyond "closed box"

I spent a lot of today in discussions with Judy Cohen (Caltech) and Blanton. We discussed the editorial position that all the data on low and high redshift galaxies (where by high I mean z=1) is consistent with no evolution in masses, sizes, angular momenta, etc, but only evolution in stellar populations. This is the position Peebles and I both basically hold: that it has been very hard to find incontrovertible evidence that galaxies accrete or merge significantly since redshift unity. There are many conflicting results in the literature, but this just bolsters this position, in my opinion. Until recently, I had been convinced by Bell et al that the galaxies on the red sequence must be growing by mergers, but in fact this study is now in direct conflict with the new (as yet unpublished) work on larger samples by Blanton.


elevator pitch

Inspired by a conversation yesterday with Yann LeCun (NYU CS), I wrote an "elevator pitch" (short description, possible to say during an elevator ride) for the possible "data mining" project we have imagined as the full generalization of Willman's search for Milky Way companions: Find all statistical anomalies in the distribution of stars on the sky, using positions and magnitudes in all available bands.


two kinds of dark matter

I realized today that any kinematic calculations one might do relating dark matter experimental data to constraints on the dark matter's physical properties are all non-relativistic. Duh! I started calculations relevant to reinterpretation of dark-matter experiments in terms of a dark-matter model in which there are two dark matter species with a small mass difference (the project assigned to me by Weiner). Apparently everything I am doing has been done before, but I am not yet discouraged.