Is the snail two snails?

Friday mornings in NYC usually start with a free-form meeting on the 11th floor of Flatiron. Today Spergel, Johnston, Gandhi, and Price-Whelan were all at the table. We began by discussing some of the accomplishments that have set the tone and agenda of the data-group and dynamics-group activities at Flatiron. Then we started to discuss what I call The Snail: The phase spiral found by Antoja in the ESA Gaia DR2 data. As my loyal reader knows, we are trying to use it to infer the dynamical properties of the Milky Way disk. And we would also like to use it to infer things about events in the recent past of the Milky Way. We discussed the possibility (suggested by the observations) that The Snail is not just one event but really two. It looks different when you look at stars with different angular momenta (different guiding centers, and hence different histories in their orbits around the Milky Way). In general the question is: Do Snails created in simulated galaxies look anything like the Snail we have?

WISE parallaxes, binary AGN accretion

The WISE mission took two passes over the sky. So it is nearly pessimal for parallax determination; parallaxes and proper motions are all mixed up. That said, at each pass there are multiple epochs, so maybe there is a glimmer of hope. Today, in a heroic move, Lang coded up simultaneous parallax and proper motion determination in the un-coadded WISE individual frames, a la this old paper. As expected, parallaxes are badly determined. However, we can use MCMC to explore the full degeneracy between parallax and proper motion conditioned on the WISE data, combine external information, get ready for NEOWISE (or whatever comes next), and apply any prior knowledge. I don't know if it counts as research to just cheer while someone else codes?

At lunch, Brian Farris (NYU) gave a nice talk about simulations of accretion onto merging black holes. He proposes that we might be able to see late-stage (0.1 pc ish) merging black-holes by looking for periodicities in the accretion luminosity. Oddly, the periodicity is not at the orbital frequency but instead at some lower frequency that has to do with how the disk reacts to the torques from the orbiting pair of holes.

KIPAC@10, day 2

Today was the second day (I missed the first) of the KIPAC@10 meeting at KIPAC at SLAC. There was a whirlwind of talks on compact objects and galaxy evolution, too many to summarize, but some highlights for me were the following:

Steiner (UW) showed neutron-star mass and radius measurements and discussed their implications for the properties of matter at extreme density. He showed some very noisy likelihood functions (ish) in mass–radius space, one per measured neutron star (and there are 8-ish with measurements) and tried to draw a curve through them. I have opinions about how to do that and he seems to be doing it right; each time we tried to discuss this over coffee something interrupted us.

Perna (Colorado) talked about magnetars; I hadn't appreciated how extremely short-lived these stars must be; their lifetimes are measured in kyr, which is not a unit you see every day. Romani (Stanford) made a pitch that Fermi-discovered gamma-ray pulsars are the bees knees. He didn't show folded light-curves but apparently there are now hundreds where you can see the periodicity in the (sparse) Fermi data. Tomsick (Berkeley) showed some outrageously awesome NuSTAR data, making me want to hear much more about that mission. It's PI is my old friend from graduate school, Fiona Harrison (Caltech), to drop a name.

Cordes (Cornell) talked about pulsar timing and gravitational radiation, a subject on which I have opinions (from a precision measurement perspective). He, like is common in that business, concentrated on the stochastic gravitational wave background; I would like to hear or think more about coherent source detection. It is usually easier! Along those lines, at one point Blandford (KIPAC) asked Aarons (Berkeley) if physical models of pulsar emission were likely to help in measurements of pulsar timing. Aarons didn't commit either way, but I think the answer has to be yes. Indeed, I have suggested previously that modeling the emission almost has to improve the measurements.

Stark (Arizona) showed very nice new data on galaxies at extremely high redshifts. He noted that almost every result at redshifts beyond six depends entirely on photometric redshifts. That's true, but is it a concern? I guess it is because there could be interloping lower-redshift objects (or stars) having a big effect on the conclusions. Kriek (Berkeley) and Lu (KIPAC), in separate talks, showed that it is difficult to explain the evolution of galaxies in sizes and stellar populations with simple models of star formation and merging. Also, Kriek called into question the star-formation-rate estimates people have been using, which is interesting; she finds a factor-of-two-ish range in the mistakes that could be being made, and this is the same order of magnitude as the amplitude of the variation in specific star-formation rate with galaxy mass. She didn't claim that there is an error there.

In the discussions at lunch, Stuart Lynn (Adler) pitched an idea from David Harris (NAS) that we start a journal of short contributions. Marshall was all over that; it might get launched tomorrow in the unconference session.

Dr Jiang

My PhD student Tao Jiang graduated today. His thesis involved the cross-correlations of various galaxy populations as a function of color and luminosity. He was able to show that the red-galaxy population has been growing since redshift unity mainly from merger activity, and that the growth is dominated by red-red mergers (that is, dry mergers). He also showed that the rate of blue-galaxy mergers is consistent with the rate at which blue galaxies transform into red galaxies (pass into the red population), which is pretty striking. Congratulations, Dr Jiang!

galaxy sources and sinks

In a low-research day, Jiang and I discussed the idea of using continuity to understand galaxy evolution. That is, changes in the number density of any population of galaxies should be quantitatively explained by sources of galaxies in that population, sinks, and evolution within the population. We can measure each of these terms for some galaxy populations, so there is a rich set of projects here.

students

Not much got done in the last two days (I didn't have to post yesterday because I posted on the previous Sunday; see rules). But I did get a few minutes to talk with Bovy about his centroiding tests; he confirmed that Lupton's centroiding trick in SDSS Photo performs surprisingly well relative to the best that can possibly be done with a compact point-spread function.

I spoke with Jiang about his cross-correlations. After a conversation with Blanton we decided to do every small–big cross-correlation corresponding to each of the big–small cross-correlations we are doing. This permits us to check our results (there are symmetry relations), and it also permits us to compute a death rate analogous to the accretion rates we are currently measuring. Think big fish eating small fish, and small fish getting eaten!

George Lewis (NYU) had a candidacy exam about understanding QCD backgrounds for Higgs events at LHC ATLAS. He and Kyle Cranmer (NYU) are looking at a set of data-driven methods to constrain QCD events, since precise QCD calculations are impossible for many of the relevant interactions. Of course I am a huge supporter of data-driven data analysis at LHC; I am certain this is a necessary part of any precise measurements with that incredibly complicated machine.

text, tables, figures

I got up early and worked on the text of Bovy's new piece on Oort-like methods for understanding the Milky Way disk, then the tables and figures of Jiang's piece on the small-scale clustering and merger rate of galaxies of a range of colors and luminosities. I don't have much to say about these activities, since they are entirely read-and-mark-up, but it was very satisfying to sink a few hours into my students' (excellent) work.

obituaries done, merger rates measured

I finished my two obituaries for Sam Roweis, one for our high school and one for the American Astronomical Society. You can read the submitted version of the AAS obituary here (PDF). I also spent some time writing in my much longer reminiscence, which I am hoping will be a document that captures what I learned from Roweis in a professional and personal way.

I also spent a long time talking to Tao Jiang, the student who is following up Masjedi's work on merger rates, but now in the full SDSS Main Sample, and thinking about what to do with SDSS-III. I am pretty excited about our results which have very high signal-to-noise.

chemical abundances

With Zolotov I discussed her results relating stellar chemical abundances to stellar origins in simulations of Milky-Way-like galaxies.

galaxy clustering

Tao Jiang (NYU) brought me back up to date on his project to measure very small-scale galax–galaxy cross-correlation functions in the SDSS Main sample (that is, normal, low-redshift galaxies). His goal is to measure the merger rate as a function of mass, color, and mass ratio.

In the rest of my research time today I worked on some crazy celestial mechanics ideas; conversations with Christian Schwab (Heidelberg Sternwarte) got me thinking about how one explores the space of possible model fits to exoplanet radial velocity data.

no more streams, Cox

T. J. Cox (Harvard) gave a nice seminar at the MPIA today about merging disk galaxies in realistic simulations. He does a nice job of creating a huge diversity of spheroidal galaxies, with gas content and/or dissipation one of the dominant parameters. He (indirectly) confirms my methodologies for constraining the galaxy–galaxy merger rate and growth rate of spheroidal galaxies.

Today Lang and I ruled out the streams we have been finding in 5D this week. There is structure in the data that is almost certainly systematic, not real.

triaxial Galaxy?

At morning coffee at OCIW, David Law (UCLA) told us that he has evidence—from the morphology of the Sagittarius stream, mainly—that the Milky Way halo could be triaxial. This is pretty intriguing, because it is the generic prediction for dark-matter halos, as I understand is (substructure notwithstanding). He reminded me of the paradox that the Sagittarius stream must—in any model—be very old, which goes against the idea that galaxies are generically building up their mass through mergers.

stars accreted by the Galaxy

At lunch, Young Wook Lee (Yonsei) gave a nice talk on the blue horizontal branch in Milky Way globular clusters, showing that its morphology and population is a very strong function of metallicity and, in particular, helium abundance. It appears that many of the Milky Way's most dense globular clusters have multiple populations with different helium abundances. This is hard to explain, but Lee hypothesizes that these may not be what we traditionally think of as globular clusters but rather the remnants of disrupted substructures accreted onto the Milky Way long ago.

Zolotov and I spent the afternoon planning her first paper and her dissertation, in preparation for her oral exam. Her first paper will focus on the kinematics of stars with different origins (formed in situ, accreted recently, accreted long ago) in a simulated Milky-Way-like galaxy.

respond to referee

I finally got the response to referee completed on the Masjedi et al paper on the mass growth of LRGs through merging (major and minor). This task took ages, despite a very straightforward and constructive referee report. I celebrated.

merging

Dan McIntosh (UMass) gave a nice group meeting talk on galaxy mergers in SDSS as a function of mass, mass ratio, and environment. During the talk I think we all realized that measures of the merger rate are in some sense more precise measures of galaxy evolution than measures of the differences between different redshifts, but they are not necessarily more accurate because they involve substantial uncertainties in their interpretation.

integrating correlation functions

I re-wrote the part of Masjedi's recent paper in which we describe how we integrated the correlation functions, because it turns out that we didn't describe it quite correctly. We did the right thing in the presence of possible software issues at very small scales; but certainly not what you would do if you weren't worried about the software.

build-up of mass through mergers

I read a few papers today on the build-up of galaxy masses through mergers. The (very constructive) referee on our recent Masjedi et al paper pointed us at some new papers on the subject that find very small amounts of evolution since redshifts around unity. Given cosmic variance and sample sizes, plus uncertainties in K and evolution corrections, nothing is nearly as precise as our direct measure of the accretion rate, so they only confirm our result at the order-of-magnitude level.

another referee report

The gods are smiling, because I got two friendly, constructive referee reports in the same week. When's the last time that happened? I spent time this morning working on the report on Masjedi et al; it is only 07:30 and I have already put in some solid research today!

submitted!

I finished the Masjedi, Hogg, and Blanton paper on the accretion rate for luminous red galaxies and submitted it to the Astrophysical Journal. It has a nice punchline: The accretion rate, even when integrating over a factor of 50 in merger mass ratios, is less than a few percent per Gyr, substantially lower than that predicted in CDM simulations, and much lower than many morphological measures of the merger or accretion rate. It will appear tomorrow as arXiv:0708.3240.

galaxy merging

With funding proposals and the beginning of term looming, I didn't get much time for research yesterday and today, but what I got was spent working through the literature on galaxy merging for the LRG accretion rate paper, which is so close to being done!