is it surprising that there are high-redshift supermassive black holes?

A nice talk at MPIA by Hanna Ũbler (MPE) about very high-redshift (redshifts 8 to 14 even) galaxies and their black-hole contents started some nice discussions in the audience and afterwards about the formation of black holes. Because of the Eddington limit (which is a limit on luminosity), black-hole growth is probably limited. The limit is on luminosity, not mass accretion rate, and the relationship between these is the radiative efficiency. As the efficiency goes down, the mass accretion rate of an Eddington accretor goes up. So the question: When can you first form a super-massive (10 million solar masses, say) black hole is a question simultaneously about seed black holes and about radiative efficiency. Anyway, this is all unfortunate, because if the efficiency couldn't get very low, then the black holes we find with NASA JWST would already be putting very strong pressure on fundamental physics in the early Universe.

how did the Solar System form?

I saw a very nice talk today by Philippine Griveaud (MPIA) about how the Solar System formed. The idea is that the giant planets formed in an accretion disk. Their formation opened gaps and caused migration (first Type I and then Type II, if you must know :). That migration pulled them into a resonant chain. That is, if the giant planets formed the way we think they formed, they must have been in a resonant chain. But they aren't in such a chain now; what gives?

The idea is that when the gas is expended (or blown out by winds), the remaining planetestimals (think: asteroids, comets, Kuiper Belt objects) interact with the planets such that they get moved from orbit to orbit and eventually ejected. These dynamical interactions break the resonant chain, migrate the giant planets to their current locations, and scatter rocks and ice balls into the interstellar regions.

It was a great talk, but also led to a lot of interesting questions, such as: How does this all fit in with the formation of the rocky planets? And how does this square with our observations (growing rapidly, apparently) of interstellar asteroids? Oh and: How does all this connect to observations of debris disks, which I now (officially) love.

extreme infrared excesses

Gaby Contardo (SISSA) showed up in Heidelberg today to make progress on our project on infrared excesses in normal, non-young FGK stars. Because we are using NASA WISE data (along with ESA Gaia and NASA 2MASS), we are only sensitive to bright, hot infrared excesses, much hotter and brighter than typical debris disks around old stars. We have some candidates, which range in temperature from 300 to 1500 K and are reprocessing maybe one percent or a fraction of a percent of the stellar light. (Warning: I haven't calculated this; this is just a guesstimate based on looking at plots.) What are those things? Today we figured out that they can't be warm substellar companions, so they have to be dust (I guess??).

a well-posed problem in gastrophysics

Magda Siwek (Harvard) gave an execellent NYU Astrophysics Seminar today, about evolution of binary systems when the binary is accreting from a circumbinary disk. She sets a few (just a few) disk parameters, and then sets the mass ratio and eccentricity of the binary, and seeks steady-state (low disk-mass or low accretion-rate) solutions. By ignoring electromagnetic fields and various bits of microphysics, she can create a setup that is completely scale-free, so it applies (approximately) from all scales from exoplanets to super-massive black holes. That's brilliant. She finds that the eccentricities are in general driven to non-zero steady-state values, which depend (strongly) on mass ratio and (maybe weakly) on disk parameters. That's a nice problem, and observationally relevant to projects we are doing right now.

accretion and mathematical physics

In the CCPP brown-bag today, Andrei Gruzinov (NYU) went through the full mathematical-physics argument of Bondi (from the 1950s) that leads to the Bondi formula for accretion from a stationary, thermal gas onto a point mass. He also talked about a generalization of the Bondi argument that he developed this year (permitting the gas to be moving relative to the point mass) and also a bevy of reasons, both theoretical and observational, that the Bondi solution never actually applies ever in practice! Haha, but beautiful stuff.

exciting stars

Stars and Exoplanets Meeting at Flatiron was a delight today. Lachlan Lancaster (Princeton) showed his results on a really interesting object he found in the ESA Gaia data. He was inspired by the idea that star clusters might have central black holes, which might retain a very dense, very luminous nuclear star cluster even after the cluster disrupts. But his search of the Gaia data was so simple: Look for things that are apparently bright but low in parallax (large in distance). Duh! And what he found is a very bright “star” that is variable, shows emission lines, and is above the top of the H–R diagram! The ideas from the room ranged from extremely young star to microquasar to technosignatures (who suggested that?). And the thing is incredibly variable.

But there was lots more! I won't do everything, but I will say that Thankful Cromartie (Virginia) showed data from pulsar monitoring (as part of a pulsar-timing project for gravitational waves). She showed that she can very clearly see the Shapiro time delay in the pulses when they pass by the neutron star that is in orbit around the pulsar. This lets them measure the mass of the neutron star accurately. It is very massive! i think it must be one of the most massive neutron stars known, which, in turn, will put pressure on the equations of state. Beautiful results from beautiful data.

actions useful? and stars meeting

In the morning I met with Gus Beane (Flatiron) to discuss his use and understanding of actions in empirical work on the Milky Way, following up on the blow-up of last week. We discussed the point that small issues with the Galactocentric coordinate system could totally mess any action calculation, even if the actions make sense, and the point that there are many possible galaxies we might live in for which the actions don't even make sense. We vowed to move the conversation / argument going on at Flatiron towards the question of what we are trying to achieve with these calculations. Are they just orbit labels? Or are they quasi-invariants? Or are we using them to match up stars that are far apart?

Stars Meeting was a whirlwind of interesting things! Kim Bott (UW) told us about polarimetry for exoplanet discovery and characterization. Evan Bauer (UCSB) told us about accretion signatures on white dwarfs and how they have probably been mis-interpreted (but in a way that makes accretion more important!). And Suroor Gandhi (NYU) told us about relationships between ages, abundances, and actions in the Milky Way that suggest that there are interesting relationships at all ages, and at all abundances. Hard to summarize, but there are lots of things to think about in there.

chemistry in protoplanetary disks

My research highlight today was a great talk by Ilse Cleeves (Harvard) about ALMA observations of the dust and molecular gas in proto-planetary disks. She showed that you can see chemical gradients in the disks, including rings and lines of formation of molecules. Much of what's visible is on the outer surfaces of the disk, which is illuminated by the young, accreting star, because the interior parts of the disk are optically thick. Because the chemical models are heavy and imperfect, I proposed looking at latent-variable models to describe the observed molecular abundances; maybe there are interesting features to be found even without relying on physical or chemical models?

The most remarkable thing she showed is time-domain chemical results: She can see chemical changes in real time as the disk responds to (presumably) stellar flares on day-ish time-scales! We discussed methods for distinguishing different kinds of events that might be triggering the chemical changes. Expect denser time sampling in the future; it's ALMA proposal-writing season!

Technically, my favorite part of Cleeves's work is that she does all of her model-fitting and hypothesis comparing in the visibilities. That is, she doesn't make an image from the interferometric data and then model it: She takes her models to the Fourier domain and compares to the raw data. That's classy.

BHs in GCs, and a new job

In Stars group meeting, Ruth Angus (Columbia) showed her catalog of rotation periods in the Kepler and K2 fields. She has a huge number! We discussed visualizations of these that would be convincing and also possibly create new scientific leads.

Also in Stars group meeting, Arash Bahramian (MSU) spoke about black holes in globular clusters. He discussed how they use simultaneous radio and X-ray observations to separate the BHs from neutron stars: Radio reveals jet energy and X-ray reveals accretion energy, which (empirically) are different for BHs and NSs. However, in terms of making a data-driven model, the only situations in which you are confident that something is a NS is when you see X-ray bursts (because: surface effect) and the only situations in which you are confident that something is a BH is when you can see a dynamical mass substantially greater than 1.4 Solar masses (because: equation of state). He highlighted some oddities around the cluster Terzan 5, which is the globular cluster with the largest number of X-ray sources, and also an extremely high density and inferred stellar collision rate. This was followed by much discussion of relationships between collision rate and other cluster properties, and also some discussion of some individual x-ray sources.

[In non-research news: Today I became an employee of the Flatiron Institute, as a new group leader within the CCA! Prior to today I was only in a consulting role.]

Dr Geoff Ryan

Today Geoff Ryan (NYU) defended his PhD. I wrote a few things about his work here last week and he did not disappoint in the defense. The key idea I take from his work is: In an axisymmetric system (axisymmetric matter distribution and axisymmetric force law), material will not accrete without viscosity; it will settle into an incredibly long-lived disk (like Saturn's rings!). This problem has been solved by adding viscosity (artificially, but we do expect effective sub-grid viscosity from turbulence and magnetic fields), but less has been done about non-axisymmetry. Ryan shows that in the case of a binary system (this generates the non-axisymmetry), accretion can be driven without any viscosity. That's important and deep. He also talked about numerics, and also about GRB afterglows. It was a great event and we will be sad to see him go.

accretion onto binary black holes

I talked to Ana Bonaca (Harvard) and Lauren Anderson (Flatiron) about their projects in the morning. With Bonaca I discussed the computation of numerically stable derivatives with respect to parameters. This is not a trivial problem when the model (of which you are taking derivatives) is itself a simulation or computation. With Anderson we edited and prioritized the to-do list to finish writing the first draft of her paper.

At lunch time, Geoff Ryan (NYU) gave the CCPP brown-bag talk, about accretion modes for binary black holes. Because the black holes orbit in a cavity in the circum-binary accretion disk, and then are fed by a stream (from the inner edge of the cavity), there is an unavoidable creation of shocks, either in transient activity or in steady state. He analyzed the steady-state solution, and finds that the shocks drive accretion. It is a beautiful model for accretion that does not depend in any way on any kind of artificial or sub-grid viscosity.

carbon stars, regulation of star formation, and so much more

Rix called me to discuss the problem that when we compare the chemical abundances in pairs of stars, we get stars that are more identical than we expect, given our noise model for chemical abundances. That is, we see things with chi-squared (far) less than the number of elements. This means (I think) that our noise estimation is overly conservative: There are (at least some) stars that we are observing at very good precision. Further evidence for my view is that there are more such (very close) pairs within open clusters than across open clusters (or in the field).

In stars group meeting, Jill Knapp (Princeton) spoke about Carbon stars (stars with more carbon than oxygen, and I really mean more in counts of atoms). She discussed dredge-up and accretion origins for these, and how we might distinguish these. She has some results on the abundance of Carbon stars as a function of expected (from stellar models) surface-convection properties, which suggest accretion origins. But it is early days.

Chang-Goo Kim (Princeton) told us about simulations that are designed to understand the regulation of star formation in galaxy disks (kpc scales). He pointed out the importance of gravity in setting the star-formation rate; these arguments are always reminiscent (to me) of the Eddington argument. His simulations include supernovae feedback in the form of mechanical and radiation energy, and magnetic turbulence and cosmic ray pressure. He emphasized that conclusions about feedback-regulated star formation depend strongly on assumptions about spatial correlations and locations (think escape over time) of the supernovae relative to the dense molecular cloud in which the star formation occurs. Fundamentally the thing that sets the star-formation rate is the pressure, which can be hydrostatic or turbulent or both.

Semyeong Oh (Princeton) and I led a discussion on the lowest-hanging fruit for projects that exploit her comoving star (and group) catalog from TGAS. Some of the lowest-hanging include investigations of the locations of the pairs in phase space, to look at heating, age, and formation mechanisms.

stars at low resolution, pair coding, accretion

Today Kathryn Johnston (Columbia) convened the first of (we hope) many Local-Group group meetings up at Columbia, with people present from NYU, CCA, Columbia, and Princeton. Johnston reported on things she learned at the recent meeting in Paris. Of particular interest to her—and everyone, apparently—was work by Yuan-Sen Ting (Harvard), building on experiments by Anna Y. Q. Ho (Caltech), showing that it is possible to get detailed stellar abundances, even without huge covariances, out of low-resolution spectra. The point is that as resolution decreases, the information per line gets worse, but you also (usually) get more spectral coverage, and this (mostly) compensates. This could have a huge impact on the future of stellar astrophysics.

I spent time today pair-coding (with Adrian Price-Whelan, Princeton) the analytic marginalized likelihood that Semyeong Oh (Princeton) and Price-Whelan and I have been working on. We found a couple bugs and by the end of the screen-sharing video call (yes, that's the way we do it), we had a marginalized likelihood ratio that seems to be delivering very good answers, and fast! Very excited.

The research day ended with a great astrophysics seminar at NYU by Zoltan Haiman (Columbia, NYU) about fast growth of black holes in the early Universe. He has found a spherically symmetric, steady-state, achievable accretion process that is (much) faster than Eddington, using the same assumptions (essentially). I need to think about it and understand it better. The Eddington limit is one of the most secure, robust, and well-tested arguments in all of astrophysics!