[Just back from travel, hence intermittent posting. The following was composed on an airplane.]
I spent this morning (Thursday) in San Francisco (thank you,
delayed flight) working with Roweis on what we call the verify
step in astrometry.net. This is the step where we decide, for a given
hypothesis about an image's pointing, rotation, or scale, whether that
hypothesis is correct (or very close to correct). Roweis and I are
proposing that we base this decision on an approximate but
conservative estimate of the false-positive probability; that is, we
consider a hypothesis correct if the match we observe between the
image stars and the catalog stars is extremely unlikely to have
occurred by chance. One interesting aspect of this is that we have to
track all hypotheses we have tested so far: Your false positive rate
goes up the more you look.
Philosophically, the reason that astrometry.net's internals
work is that the verify step is very simple and fast. Our quad
matching
system is very unreliable; it generates thousands of
hypotheses, only one of which is correct. But because the verify step
is fast, we don't mind having many hypotheses to check. The least
flattering description of our system is that it does brute-force
search of every conceivable hypothesis (of which there are many
billions), but with the quad system ordering the hypotheses so
that we are very likely to hit the correct one in the first few
thousand!
I spent yesterday (Wednesday) battling it out with Santa Cruzians about the red sequence of early-type galaxies and constraints on any processes of galaxy–galaxy merging that can cause it to evolve. That was lively! Also Croton (Berkeley) and Wechsler (Stanford) came to UCSC to join the battle.
It is hard to summarize the discussion here, but points that came
up include: If you want to look at the stellar populations of a galaxy
that is post-merger, you have to consider that some of the stars may
have been thrown to large radius and are now part of the intragroup
stellar population. In thinking about the evolution of the
tilt
of the red sequence, you have to correctly treat the
differential evolution that comes from blue galaxies fading more into
the L-star part of the sequence, percentage-wise, than into the
high-mass part. There might be massive, star-forming progenitors of
the massive red galaxies hiding among ULIRGs and luminous AGN. The
observed evolution of the total mass on the red sequence does not
require a lot of evolution at the most massive end, though that
is in large part because the signal-to-noise of the observations is
limited. And etc. Interestingly, I think we all agreed that
merging ought to make the massive end of the red sequence become
broader
(in some metric) with cosmic time.
Bummer I missed this session, though I enjoyed your talk.
ReplyDeleteFirst, a question. Garth and I came away with the impression that Erik Bell got a similar answer to you with regard to the number of mergers among massive galaxies. Instead, according to him, a huge fraction of z~0.7 galaxies will merge. Apparently this all happens between z~0.7 and z~0.3? Do you have any other thoughts?
Second, I wonder how much ICL/IGL is made as a function of halo mass. Clusters have large amount of light, but they are big. It seems like that the theorists are never going to tell us an answer and that measuring this could make a big impact. So, is there enough uncertainty in the ICL computation to explain away your result? Could one sweep under the ICL all of your missing second galaxies?
In answer to your first question, a lot of the difference between how Bell's paper reads and how I describe it comes from the fact that Bell and I have measured different rates: I measure the rate at which LRGs will merge, so I divide the density of LRG–LRG pairs by the density of LRGs. Bell measures the rate at which LRGs have merged, so he divides the density of sub-LRG–sub-LRG pairs by the density of LRGs. Because the luminosity function is steep, Bell gets a much higher rate. But the evolution in the observable is not as strong.
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