spectrographic throughput

Optimistically, we (Blanton, Bolton, Burles, Masjedi, myself) decided that the PRIMUS prism spectroscopy of redshift unity galaxies will start to measure good redshifts as soon as we understand the throughput well. The throughput is not trivial to measure, and it depends on airmass, photometricity, and other factors. But we spent all day working on it and are very close to having some reasonable estimates. What I don't understand is what kind of data we need to take each PRIMUS night to ensure that, after the fact, we will be able to deteremine the throughput accurately enough for good redshift determination.


stellar radial velocities, spectral calibration

I spent a good fraction of today discussing what could be done with a large sample of stellar radial velocities, heterogeneously selected (as are the stellar spectroscopic targets of the Sloan Digital Sky Survey) with Willman, Zolotov, and Hans-Walter Rix (MPIA, Heidelberg). There are lots of opportunities to discover Milky Way substructure in this sample.

We have decided, optimistically, that we will be getting redshifts out of PRIMUS with high efficiency as soon as the calibration information—wavelength solutions, throughputs, atmospheric absorptions, and point-spread functions—is all known. We worked on all those things today.


prism spectroscopy

Burles, Bolton, Masjedi, and I worked on calibration and understanding of the low-resolution prisim spectroscopy we have of very faint galaxies as part of the PRIMUS project. The spectra are so low in resolution that both the point-spread function in the spectral direction and the sensitivity variation across pixels matter for the extraction of spectra and determination of redshifts.


Milky Way substructure

Willman told us in group meeting about all the newly discovered Milky Way substructure, including four new ultra-faint dwarfs. In all, about seven dwarfs have been discovered in the seventh of the sky covered by SDSS imaging so far, so we expect many tens of new dwarfs once we have good imaging all-sky. Substructure problem (ie, the problem that the simulations of Milky-Way-like galaxies put many more dwarfs in the halo than we have observed): be gone! After group meeting, Willman, Zolotov, and I discussed possible projects to find yet more substructure in the near term.



Finished the revision of the post-starburst galaxy manuscript, and resubmitted it, and posted it to astro-ph. It should come out on Monday morning on astro-ph. Thanks everyone for helpful comments and work. If you want an "advance copy" just let me know.

[Added later:] Amazingly, our scientific editor at the ApJ sent us the acceptance for this paper within five hours of the re-submission. Let's hear it for the efficient and excellent ApJ!



For a few minutes, Zolotov and I thought we had discovered something big in the SDSS stellar distribution. Then we realized that it was just the Sagittarius stream!


infall regions

Quintero saved me today after I posted yesterday about infall regions and then realized, late at night, that I had used the wrong data subsample. Luckily the right one—obtained for me by Quintero minutes before he had to jet for his flight to Germany—showed the same effect! Whew! The rest of the day was spent working on text.


infall regions of clusters, star-formation histories

Masjedi gave me great comments on the recently re-completed post-starburst environments paper. He noted that some of the discussion about infall regions was not quite right in tone, and in fact we do have a marginally significant detection of the infall of galaxies into clusters. I am incorporating his comments now.

Bhardwaj is very close to having a full-up star-formation history fitting code, specialized for fitting old stellar populations with small amounts of recent star formation. This will be used to investigate UV-excess and mid-IR-excess early-type galaxies, and a host of other fun things.


tidal stream, merging

It appears that Zolotov does confirm the Grillmair et al tidal stream candidate—we can't say if it is a stream, but a linear feature does appear to be there.

Michael Brown (Princeton) spoke about dry merger rates and the fading of red galaxy populations from his analysis of the NOAO Deep Wide Field Survey and the Spitzer IRAC Shallow Survey. He finds that dry merger rates must be fairly low since redshift of unity (consistent with Masjedi's work), but that significant stellar mass is being added to the red sequence over the same time period. His results have implications for the small-scale correlation function (a la Masjedi) and the abundance of post-starburst galaxies (a la Quintero) and the evolution of these with cosmic time.



I spent the day debugging code, one of my most favorite activities. I helped Bhardwaj debug the code she is writing to update and improve upon the kind of mean spectral analysis that Eisenstein and I did back when we had fewer data and less good models. I helped Quintero debug his code to compare the environmental dependences of galaxies in the models and in the real Universe. I helped Zolotov debug her code to re-find, in a simple way, the known Milky Way tidal streams.



Arrgh! I spent a significant fraction of today dealing with a problem caused by the fact that few consider standards—agreed-upon conventions for formatting data, for example—important. In this case, some of Willman's images, taken at some typical observatory, had astrometric information that conformed to no existing WCS standard or even convention! If you aren't going to obey the standards, then at least obey the conventions. One of the big goals of astrometry.net is to get all image WCS onto one of the standard formats, but that means that astrometry.net is going to have to know quite a few of the non-standard, idiosyncratic ways people have chosen—independently—to put the WCS into an image. Until astrometry.net standardizes everything, there will be no grid or interoperability among heterogeneous data sets.


astrometry tweak

I spent today on issues related to getting our tweak code up and working on the WWW. The tweak code takes an image with reasonable astrometric WCS (good to a few arcmin), and replaces it with more accurate WCS (good to better than an arcsec), computed at user-set polynomial order, and puts that more accurate WCS into the image header in a standards-compliant way. This will not be a unique capability of astrometry.net, but it is essential to its future, unique capabilities.


low-luminosity galaxies

Blanton showed us some of his low-luminosity galaxy results in group meeting today. I was impressed that he can show that lower luminosity galaxies have similar baryon-to-dark-matter ratios to those of higher luminosity galaxies (though they have much higher mass-to-light ratios), showing that stellar winds and supernovae do not drive out gas preferentially in smaller galaxies. He also has reasonable evidence that the mass-velocity relation (the extension of the Tully-Fisher relation) at low masses is consistent with what one would expect if there is a CDM-predicted abundance of dark halos, occupied by galaxies in a more-or-less monotonic way by luminosity.



Spent all of my research time in the last two days getting our post-starburst stuff up to DR4; I just finished now. So all I have to do is clean up a few things and resubmit.


tidal streams

Adi Zolotov and I spoke at some length about possibly repeating, in a hands-off and quantitative way, the discovery of a putative tidal stream by Grillmair et al. We ought to recover that stream, and maybe others, and we ought to be able to put strong limits on what else is out there.


getting up-to-date

Masjedi convinced me that since the post-starburst galaxy environments paper contains some tantalizingly marginal results, we should update its data to the most recent, huge SDSS footprint (for the first time ever working on the Sloan Digital Sky Survey, I feel numbers limited!). So I obeyed Masjedi's direct order and started work on that. In principle, it just involves swapping one file in for another, but we all know that it never works in practice (and it didn't today).


mass accretion

At group meeting, Masjedi showed an amazing array of results, including that: the growth of luminous red galaxies by merging and accretion is limited to a few percent per Gyr at redshift of one third; dry mergers dominate the accretion rate; the transition from two-halo to one-halo term in the galaxy correlation function is clearly visible in the LRG-normal-galaxy cross-correlation; and some fraction of galaxies near LRGs seem to have tidally triggered star formation.

I didn't do much today except advise the troops.



Ah the life of a professional scientist: An entire weekend of response-to-referee.


multi-wavelength galaxies, stellar densities

David Schiminovich (Columbia) and Ben Johnson (Columbia) came down for the morning and lunch ostensibly to discuss their big Spitzer/IRS program on SDSS galaxies, but, as always we spent the entire time talking about galaxy evolution (and philosophy). Interestingly, I think they are the only people in the world doing a large infrared spectroscopy program on a wide range of optically selected normal galaxies—most large Spitzer programs are aimed at IR-luminous sources like ULIRGs and AGN. They bought my lunch, which was not commensurate with the lack of help I gave them on their IRS data—thanks!

I started to take a look at an old paper by McIntosh et al that might have something to say about my ideas about using central stellar densities to put hard constraints on galaxy evolution scenarios.


post-starburst galaxies

I am still working on the response to referee for—and revision of—our paper on post-starburst galaxy environments, that rules out external tides and galaxy-IGM interactions as the causes of the majority of these events.