primus data reduction

Burles came to NYU for the day and he, Blanton, and I discussed data reduction issues. We are evolving to a data modeling approach for the extremely non-trivial, low-resolution, nonlinear-dispersion spectral data we are taking; that is, we are building a two-dimensional model of each frame. What is usually thought of in astronomy as the extracted spectrum is—in this context—just parameters in that model. When I say the words we are, I really mean Burles is. Beautiful stuff! Maybe someday soon our 140,000 (or so) spectra will yield redshifts?


theoretical merging

I spent this evening reading carefully the important (though now somewhat out-of-date) paper on galaxy growth by merging and accretion in numerical simulations by Murali et al (2002). Unfortunately, the article gives its results entirely in terms of the volume average of the merger rate over all galaxy masses and types, when the per-galaxy growth rate might be a more stable (and definitely more easily testable) statistic. The paper finds that the merger rate as measured observationally is consistent with their findings, though the test would be more sensitive if they had produced and tested per-galaxy numbers. However, they find that for massive galaxies, the smooth accretion rate of intergalactic material is substantially larger than the galaxy merger rate (in mass per time units); I think this is not consistent with present-day observations (although it is hard to test definitively if the IGM material is ionized).

Despite the great improvement in both observations and simulations, there is a surprising paucity of good theoretical predictions for the merger rate since this nice paper.


image labeling

I spent quite a bit of time today with Apostol re-building the image labeling we use for SDSS images, like this. It might not sound like research but making a new, digital galaxy atlas is one of my research goals. (Click on the image for higher resolution.)


LSS requirements

I worked today on the requirements for our post-SDSS baryon acoustic feature large-scale structure project. I finally figured out that requirements should have a hierarchical form that flows from high-level science goals down to science requirements down to technical hardware and software specs.


strong gravitational lensing

[Sorry for the lack of posts; I have been on vacation since Monday.]

Last Monday (the 20th), I gave our informal lunch talk on the uses of strong gravitational lenses (multiply imaging or very high magnification systems) in cosmology. I am a bit suspicious that strong lensing will be crucial in the coming era of cosmology, but the talk gave me an opportunity to highlight some places where I think it might be important:

  • Strong lenses might put robust limits on the radial profiles and evolution of collapsed objects, when simulations and theory become good enough to make the relevant predictions. As it is, there is an interesting lack of low-redshift gravitationally lensed arcs from massive clusters.
  • High magnification lenses can be used to find very faint, very high-redshift sources. There are already some quasi-believable redshift 10 candidate galaxies out there, and some definite redshift 6 galaxies.
  • Well-understood lenses lensing variable quasars can be used to make measurements of distances (times distance ratios) to pin down the world model.

All of these projects are immature now, but I got the opportunity to quip that as we transition from precision cosmology to accurate cosmology (Ben Weiner's joke), we might need one or more of these techniques. Lensing is a blunt tool, but it does do some things very well.


baryon acoustic feature

Today was an all-day meeting regarding this new project (as yet un-named) to measure the baryon acoustic feature at multiple redshifts to determine the expansion history of the Universe and the physical properties of the dark energy, using the SDSS Telescope after SDSS-II operations complete. A large group from Princeton, LBNL, MIT, Harvard, CITA, CWRU, and NYU (plus Arizona, FNAL, and Portsmouth by phone) converged here at NYU to discuss the science, the technical requirements, funding (of course), and data reduction and management. It was wonderfully productive, but I was talked into shepherding the requirements document.


Blanton and I spent a good part of the day talking about and working on SDSS-III, an ambitious project to measure the baryon acoustic feature at redshift 0.6 or so, and its evolution to redshift 0.3 or so.


Moustakas astrometry

Worked through some astrometry issues with Moustakas, who is one of our pre-alpha testers; that is, he is actually fixing our code so it works on his images (and on his laptop!).


galaxy mass images

I made some SDSS images with good background subtraction for Jim Pizagno's project of converting galaxy images into mass images. Here's one:


public release of data

Productive and lively discussion at coffee today (Ben Weiner visiting from NOAO, Zabludoff, Moustakas, Blanton, myself) about good policies for the journals and large surveys to adopt in regards the public release of data and code.

My vision (with which no-one agreed) is that the journals should require all papers be released with a tarball that contains all the data and code, such that a reader can unpack the tarball, type make, and produce all of the analyses and figures for that paper! Of course there are implementation issues, but if we want to stay clamped to the manifold of repeatable science (Sam Roweis) we have no other options. I could write about this for hours, but I have an NSF proposal due. I hope to return to this later, because in the course of the discussion I discovered many new reasons to support my view.

We are very far from my vision right now, and it is hard to get there incrementally.


galaxies in clusters

It is great to have Zabludoff and Zaritsky here! We spoke briefly about the suggestion that galaxies in clusters are strongly affected by the cluster itself. If this happens it is pretty subtle: (1) The difference between the galaxy populations at high density and low appears not to be that disks have been removed at high density, but that bulges have been augmented. (2) Galaxies in clusters do not look like the centers of galaxies not in clusters; ie, they are not stripped field galaxies. You could say duh! to this, but there are still many papers appearing that treat severe stripping as a competetive hypothesis.


galaxy photometry

Discussed galaxy photometry with Wu. It is remarkably subtle, just the process of figuring out how much integrated intensity is coming from an extended source.


I spent all of yesterday on non-bloggable grant-proposal writing.


Zabludoff and Zaritsky

Some of my unbloggable (consult rules at right) activity of the last year has led to the arrival of Ann Zabludoff (Arizona) and Dennis Zaritsky (Arizona) at NYU for the next nine months. Today they both gave informal lunch talks, presenting some of their research on galaxy evolution. Ann noted her result that the differences between the disk-dominated and bulge-dominated populations has to be explained as a difference in bulges, not disks. Dennis showed that there are clear relationships between the orbit and star-formation history of the Large and Small Magellanic Clouds.


ultraviolet images of star-forming galaxies

I started a huge task (built on Blanton's "gmosaic_make" code that combines GALEX AIS images automatically) to create near-ultraviolet images of galaxies in the SDSS main sample whose optical colors suggest star-formation within the last Gyr. A quick inspection suggests that the K+A galaxies (those that are deficient in emission-line luminosity) also have lower ultraviolet, but I am not close to showing that post-starburst galaxies can be identified ab initio with GALEX imaging. That's the hope.



Spent the day at Rutgers yakking about CDM and its discontents.



I started (but only just started) looking at the GALEX observations in the SDSS survey area. I also prepared a talk for tomorrow at Rutgers.


astrometry vs wavelength

I worked most of the day preparing data for our test of the wavelength (bandpass) sensitivity of the efficacy of our astrometry engine.