2005-06-30

tidal triggers and infall regions

I wrote more text, including mentions of the fact that we can rule out tidal triggers for the (majority of the) K+A galaxies, and that we see no excess of K+As in cluster infall regions. This latter is in direct contradiction to many off-hand claims (based on data-free arguments) in the literature. I hope I can trace some of those down.

2005-06-29

environments, color pictures, mergers

Wrote text in the post-starburst environment paper.

Helped Burles and Tongyan with color pictures of gravitational lenses.

Worked out (yes, it is obvious) that if the merger rate onto LRGs as a function of secondary magnitude is proportional to the luminosity function, then the mass-weighted merger rate will be dominated by galaxies around L-star. If it is going to diverge to the faint end, then the LRG-galaxy cross correlation will be a very strong function of the secondary galaxy magnitude, with dwarfs having much higher amplitude cross-correlation than giants. Doesn't seem likely; in fact I think we know this not to be true. The assertion in Murali et al 2002 that the mass build-up in galaxies is dominated by smooth accretion rather than merger events (is this the work upon which Romeel's assertion is based?) is not very robust, as it involves a huge extrapolation below the mass resolution of the simulation.

2005-06-28

feedback, merger rate "spectrum"

I gave feedback on recent progress on Quintero's clustocentric project, and Sam and Morad's work towards the first milestone on astrometry.net.

I put two more figures into the post-starburst environments paper, but unless I write some text for that paper tonight, I have failed today.

Morad has a great plan for the next project on the merger rate: determination of the mass accretion rate onto LRGs as a function of secondary mass. He will find out what galaxy masses dominate the mass accretion rate. One question: if the accretion onto galaxies is dominated by steady rather than impulsive (merger) events (as Romeel instructs us), does that mean that the mass spectrum of accretion events ought to "diverge" towards the low-mass end?

2005-06-24

merger rate

On day five of The Fabulous, a bit of heat broke out about the merger rate, with the van Dokkums and the Conselices arguing for high rates, and the Pattons and Hoggs for low. Basically, you get low rates if you assume all close pairs merge on a dynamical time (the maximal possible assumption), and you get high rates if you assume all high-asymmetry or faintly-tidal-featured galaxy had a merger in the last dynamical time. Which is more believable? Well, since the close pair rate is an absolute upper limit, I have to assume that either tidal features last for a long time, or they are raised by minor mergers. I don't think there are any other options for resolving this discrepancy.

In related news, it came up at coffee that although the galaxy-galaxy merger rate may or may not be expected to be low, the galaxy group-group merger rate has to be high, because halo mergers are frequent. Now that Berlind has a catalog, lets do it.

2005-06-23

very high redshift

On day four of The Fabulous, many people showed believable galaxy luminosity functions for redshift 5, 6, 7, and maybe 8, or 10! Mannuci called redshift 4.5 "low redshift" without any hint of irony.

2005-06-22

kinematics, chemical abundances

On day three of The Fabulous, Flores showed amazing IFU spectra that clearly show a cold, rotating disk and a hot bulge in ordinary, moderate redshift galaxies. He could potentially do real bulge-disk decomposition—ie, with no assumption about radial profiles of either component—although the experiment would certainly not be trivial.

Pettini and others discussed chemical abundances. One of Pettini's punchlines (old news, now) is that essentially all gravitationally bound systems with lots of stars have solar abundances, and systems found any other way (eg, by absorption) have abundances all over the hoo. Many were unsurprised; after all it doesn't take long to get to solar. But isn't it surprising that they never get above solar? (BTW, Pettini also showed that some metallicity calibrations are suspect, and many high metallicity results will come down.)

All this is related to the G-dwarf problem. Yesterday, Crampton showed that the evolution he sees in the metallicity-mass relation is consistent with the global star-formation rate density results. If so, then the Milky Way must be a strange outlier (because by Crampton's standards it would show a huge discrepancy between its stellar and metallicity-based star-formation history determinations).

[Disclosure: this post was posted two days late]

2005-06-21

redshift 5, accretion

On day two of The Fabulous, Ando showed an absolutely incredible mean Subaru spectrum of about 10 redshift 5 (!) galaxies. It shows strong, very well-detected interstellar absorption lines, so presumably the ISM is metal-enriched. Good question: Is the enrichment one infers consistent with the hypothesis that earlier generations of stars in these objects were responsible for re-ionizing the Universe at some earlier redshift?

Romeel Davé gave a very nice overview of the status of simulations/theory in galaxy evolution, including not just the successes, but a thoughtful list of the big, generic conflicts/issues. He also emphasized the importance of accretion, noting that in the simulations, the bulk of galaxy growth is through slow, steady (not major-merger) accretion, and that such accretion is substantial. He distinguished two kinds of accretion, a "hot" mode in which accreted gas hits a shock at the virial radius and is heated to near the virial temperature as it falls in, and a "cold" mode in which no shock occurs. The latter is more common in lower-mass halos and at earlier times, and it can lead to very efficient building of stellar (or black hole) mass. Romeel suggested that the dichotomy of these modes might be related to the "bimodality" in galaxy spectral energy distributions.

2005-06-20

metallicities, old galaxies

In the Fabulous Destiny of Galaxies, day one, two speakers (Devriendt and Ellison) mentioned the issue that detailed chemical abundances are some kind of integral of the star-formation history (a pretty complicated one if you think about it). This brought up two ideas. The first is that the luminosity is also some kind of integral; it would be interesting to ask about what aspects of star-formation history have very different impacts on the two different integrals. There are really many different integrals, because you can look at any element, and you can measure the luminosity in any band.

Two other speakers (Yi and Fontana, I think) mentioned the fact that semi-analytic models (and, presumably, simulations) are unable to make massive galaxies as old and dead, on average, as they are observed; indeed all models produce a significant population of blue, very massive galaxies (which are never observed in the local Universe).

2005-06-14

feedback, correlation function

I gave feedback on results from Quintero, and Burles's student Tongyan. For Tongyan I made this picture:

Morad made the figure below, among others, with minor suggestions (and bitmapping services) from me, for Eisenstein's talk at the upcoming SDSS meeting.

The correlation function of the LRGs is incredibly close to a r−2 power law over four orders of magnitude in distance, eight in correlation amplitude!

2005-06-13

paragraphs

I wrote two (2) paragraphs of the K+A environments paper. D'oh! With my work on the weekend it now has a couple of pages of content.

2005-06-10

outlines, information

I outlined the K+A environments paper.

I started to outline my talk for Marseille, but then got stuck on the question of whether or not to go postal on information and statistics. The issues are things like:

  • Look at concentration as a function of environment or look at environment as a function of concentration? Since concentration is measured at much higher signal-to-noise than (our measures of) environment, the latter contains much more information, and is more constraining on models, even if you prefer to think about it the other way.
  • Fit models to the color–magnitude diagram or fit models to the age–mass diagram? Since the ages and masses are derived from the colors and magnitudes, and because there are many uncontrolled parameters in going from color and magnitude to age and mass, it is almost always much more constraining to fit models to color and magnitude, even if it is age and mass that you really care about. Besides which, the age–mass relation contains much less information, in the sense of, say, the Cramer–Rao bound.
These points are simultaneously trivial, important, oft-ignored, and audience turn-offs.

2005-06-08

post-starburst hypotheses

Today I worked out what hypotheses we are testing with the K+A environment research.

(1) Post-starburst galaxies lie in the same range of environments as "all" or "ordinary" galaxies.
(2) Post-starburst galaxies lie in the same range of environments as bulge-dominated or "early-type" galaxies.
These hypotheses were both ruled out by Quintero et al and by Blake et al; in fact hypothesis (1) was very unlikely from the start, since no spectrally or morphologically selected subsample has the same environment distribution as "all" the galaxies ("all" doesn't really have a meaning here).

Indeed, both Quintero et al and Blake et al concluded that the mean or typical environments of K+As is the same as those of disk-dominated or "spiral" or star-forming galaxies. We already know, from work like Blanton et al, that star formation is the galaxy property most tied to environment. This leads to two new hypotheses

(3-weak) Post-starburst galaxies lie in the same range of environments as disk-dominated or "spiral" galaxies.
(3-strong) Post-starburst galaxies, at every measure of A-star excess (A/K), lie in the same range of environments as the subsample of disk-dominated or "spiral" galaxies that has the same value of A/K.

Our results confirm hypothesis (3-strong): Of all the star-formation-rate indicators, it is A-star excess that predicts the environment distribution, not H-alpha (since K+As have none by definition). This fits in with the idea that A/K evolves on timescales that are long like dynamical times.

The only statistically significant deviations we find from (3-strong) are at very small physical scales, where the dynamical time is likely shorter than the lifetime of an A star.

2005-06-07

nothing

Today I thought about my review talk for Marseille, I thought about my K+A environments project, I thought about... Okay, I admit it, I did no research today. Dur.

Must... do... something... before... sleep!

2005-06-06

K+A environments

Worked on re-factoring the K+A environment code and wrote down everything about the K+A environment results that is unique and new. Somewhat concerned about the number of LPUs we have.

2005-06-03

photometricity, outliers

Oddly, I am responsible for photometricity monitoring on the photoop team. Today I fixed bugs in ancient 10-micron cloud-camera analysis software (in the ircam subdirectory of the photoop repository at Princeton), and started it running on all the cloud-camera data ever taken at Apache Point Observatory in preparation for the next round of SDSS data processing at Princeton.

I also wasted more time thinking about outlier rejection, and implemented a Markov Chain Monte-Carlo method. D'oh!

2005-06-02

outlier rejection

I worked on a new idea for totally principled outlier rejection in the determination of weighted means. The idea is to project over all possible rejection permutations and all possible rejection penalties (ie, likelihoods or probabilities that each of the input data is not rejectable). This is in preparation for image combination, but is so far off the critical path, it was a complete waste of time.

2005-06-01

galaxocentric distances, ACS thumbnails

Morad and I fixed bugs in the galaxocentric distances calculation (we were being too clever) and discussed the result. It appears that tidal triggering for K+A events is almost certainly ruled out. There is one more test to perform, which is to simply look at the overdensities on a 1 Mpc scale, but I don't expect it to give different results than the nearest-neighbor distances.

I cleaned up some of my ACS thumbnail cutout code (hogg_acs_cutout in astrometry/pro/acs) and shipped it off to Lexi Moustakas for testing and experimentation.