Lame Duck Comparison

I am an expert in the Sextractor software. It's a great tool. Not perfect (the coding could be cleaner, better annotated etc etc) but a veriable swiss army knife of a piece of software. So people use it as a yardstick to compare their own code to. However, this often happens in cases where it has already been clearly stated that SE won't work too well (crowded fields for example).

So a paper like this one annoys me slightly:

http://arxiv.org/abs/1211.5805

Probabilistic Catalogs for Crowded Stellar Fields

Brendon J. Brewer, Daniel Foreman-Mackey, David W. Hogg

(Submitted on 25 Nov 2012)

We introduce a probabilistic (Bayesian) method for producing catalogs from images of crowded stellar fields. The method is capable of inferring the number of sources (N) in the image and can also handle the challenges introduced by overlapping sources. The luminosity function of the stars can also be inferred even when the precise luminosity of each star is uncertain. This is in contrast with standard techniques which produce a single catalog, potentially underestimating the uncertainties in any study of the stellar population and discarding information about sources at or below the detection limit. The method is implemented using advanced Markov Chain Monte Carlo (MCMC) techniques including Reversible Jump and Nested Sampling. The computational feasibility of the method is demonstrated on simulated data where the luminosity function of the stars is a broken power-law. The parameters of the luminosity function can be recovered with moderate uncertainties. We compare the results obtained from our method with those obtained from the SExtractor software and find that the latter significantly underestimates the number of stars in the image and leads to incorrect inferences about the luminosity function of the stars.

The issues I have with this paper are twofold. First, the comparison is in crowded fields, an area that every piece of SE documentation (including my own) states is not what to use SE for.
And the actual comparison is extemely weak. They only tweak the detection threshold, which is only one of the relevant parameters here. The other are the level of deblending that could be tweaked and the number of deblending thresholds. I bet I can retrieve quite a few more than 30% of the stars with just SE.

So the whole paper now comes down to making their own (computationally heavy) approach look good. The SE catalog is a straw man. Of course I have not been asked to referee it so they're probably fine.

IMF

I crashed the Initial Mass Function workshop at the Lorentz Center, organized by Scott Trager.

Very nice atmosphere. Excellent discussion. Too many open questions on this topic.

The IMF has been on my "need to learn more about" list for a while. We're off to a good start.

Some of my tweets from today:

Opening the hood on how to get from cluster function to stellar one good definition of a can of worms #imflc (8) #theory

Problem in M/L in high-mass, high-metallicity environments factor 10 worse. #imflc (7)

"All problems in extragalactic astrophysics can be solved by a suitable choice of the IMF" - Dave #imflc (6)

Many claims of different IMFs in extra-galactic systems, every whichaway #imflc (5)

Bastian: claims of bottom-heavy IMF never refuted, just ignored #imflc (4)

Bastian gave props to two (ex)ESA fellows: Nate Leigh & Morten Andersen #imflc (3)

Early types with high metallicity is where IMF gets weird #imflc (2)

Nate Bastian says IMF uniform in Galaxy according to MW SF community #imflc (1)

ResearchGate

Just signed up for researchgate.net and it is an interesting experiment. The Astronomy side of things is still a little too light. And I cannot for the life of me figure out what their RG score means. Probably another metric to stress about. Meanwhile, hardly anyone I work with or whose work I follow is on it yet. Then again, maybe too early to tell for this new thing.

Skinning the pigs

Several papers are in the final stages. Almost ready to go to journal, almost ready to show to coautors. The colloquialism for that is that "one has skinned the pig except the ear". They are so close to done it's maddening. Today I finished the Zoo paper on dust lanes and it's off to co-authors. Last of the quantified morphology papers is pretty much ready to go to MNRAS. Next up are the S4G morphology paper wrap-up and submission, the Milky Way in M-dwarfs paper and then there is the paper on VIMOS observations of the occulting galaxy pair.

Good thing I gave the last of the talks for this year, the Internal Colloquium at ESTEC. Chatted about HI morphology to everyone interested in galaxies at ESA.