Oxford astro-ph Coffee

Boylan-Kolchin et al identify a new problem with CDM at sub-galactic scales: the Aquarius simulated MW galaxy halos have subhalos that are about 5 times more massive than the actual dwarf satellites we see. Are we underestimating the MW satellites' masses somehow? Or is their something wrong with the simulations? Or both? Anyway, as Phil B said: add it to the list of things to investigate about CDM :-)

The observational counterpart? http://arxiv.org/abs/1111.2611

Interesting: Strigari & Wechsler prefer to state the problem as the sims predicting 25-75 times as many subhalos at the Fornax mass scale as are observed in the MW system - and in the paper you posted they look at thousands of MW analogs in the SDSS survey and find that the MW is not atypical. This strengthens MBK's conclusion, that there is a problem with CDM - although note that S&W put the emphasis on galaxy formation not being well understood at this mass scale. They imagine that there really are all those dark Fornaxes out there! Pretty cool - now, if we could just see them somehow...

There are a number of `screening' mechanisms that are designed to suppress the effects of modified gravity below cluster scales. What are the characteristic radii at which this switch-off occurs in the different mechanisms?

Interpolation schemes for modelling the PSF, including some fun stuff like Gaussian Processes.

Now we need to get our paper out ASAP and start doing comparisons! :-) Re: "optimal" interpolation: I think they do a nice job of showing (and agreeing with us ;-) that you do best by having a very flexible, multi-scale model for the underlying maps. We also incorporate information about the statistics of the atmospheric PSF pattern in our prior; Kriging is more local, in that the covariance function is estimated only from the data in hand. Which makes me wonder what happens when the data sparseness goes up...

CosmoMC gets upgraded by the Cavendish inference team (Hobson, Feroz, and now Graff) - they approximate complex likelihood functions with neural networks, which are then much much faster to evaluate. Could be a real time-saver

Kelly and Kirshner look at the SDSS images and spectra of the host galaxies of more than 500 nearby supernovae. Seems like an interesting complement to the SNLS host studies, I'd be interested to hear from Mark where this one fits. They can resolve the galaxies very well, so can make statements like "the SN Ic-BL and SN IIb explode in exceptionally blue locations" :-)

This looks like it might be interesting - new optical spectra and Spitzer IR data for 4 galaxies at z=2 show that the dust in these systems is rather different than in the local universe. The high magnification provided gravitational lenses arranged in front of them helped a lot!

Well, alright it just looks like SMC dust instead of MW dust but still, cool that we can measure it in galaxies at z=2!

Suyu et al combine strong gravitational lensing and stellar kinematics data for a spiral galaxy to measure the mass of both the disk and the dark matter halo. The constraints are very strong - they find an oblate, flattened halo, and get a disk stellar mass with small uncertainty (0.1dex); when they compare this with the stellar mass from the disk colours and K-band magnitude they find that stellar population models with Chabrier IMF work, and Salpeter does not - the opposite to the case of massive elliptical galaxies.

Andy Lawrence suggests that the unaccounted for UV flux in AGN spectra could be coming from "clouds" of gas orbiting the black hole at ~10 Schwarzschild radii, and reflecting light as UV line emission which all gets blurred together to make a broad continuum due to the insane speeds involved.

I saw Andy yesterday at the Edinburgh sims workshop - he agreed that his clouds were probably shreds, but all that really matters is the filling factor and ionisation state, and the code he used to play around with it all is called "cloudy"... I told him we found the model pretty plausible (stuff comes off accretion disks and stays in orbit, absorbing and emitting, fine), his hope is that someone makes a more detailed model, checks his results and does some inference. That'll be Lance then...

Axions are ultra-light particles that arise in string theory, and could make up some part of the dark matter in the universe. If so, what are the prospects for detecting their effects with galaxy surveys/CMB/weak lensing with future experiments? Come and ask the authors.

Pontzen and I were sitting there reckoning axions look a lot like hot dark matter - but I guess there must be some key difference at large angular scales to enable Dodi et al to distinguish between massive neutrinos and axions in a Planck+Euclid combo. Pretty cool :-)

Hada et al reckon they can position the black hole to +/- 20 microarcsec, and resolve jet emission 100 Schwarzschild radii away!

I've not read the whole thing (it's 44 pages!) but Yu Lu is, IMO, doing the Right Thing in this field - he takes a Semi Analytic Model of galaxy formation, and *actually fits* all the parameters to the data (in this case, the observed K-band luminosity function). Some parameters are well-constrained (implying we may have learnt something about galaxies), while others show strong degeneracies, indicating what new physics needs to be included. Seems like the models in most of the parameter volume fail to predict some other datasets, giving more clues on how to improve the model. The key thing is that by doing the inference properly, Lu has elevated SAM study to a quantified learning process.

There's been quite a bit of discussion about this issue this summer, it came up at the Bologna dark matter meeting as well as the Aosta strong lensing workshop. Basically, in strong lens systems we infer *more* subhalos/satellites than CDM predicts for massive lens galaxies (the satellites cause "millilensing", where the quasar/radio source image fluxes are affected by the small amounts of additional magnification and demagnification). One suggested resolution to this problem is to include all the subhalos along the line of sight - Metcalf (2008) claimed this was the answer, and now Dan Dan Xu has tested this claim using the Millenium Simulation, and various assumptions for the satellite subhalo density profile.

A new method to determine the luminosity-distance/redshift relation from gravitational waves, without the need to find an EM counterpart? I would love someone with more NS knowledge to explain the details!

"An expanding fluid leaves its equilibrium state; the energy density decreases and the pressure also decreases. In the absence of bulk viscosity, the fluid relaxes instantaneously and pressure and density are related by the equation of state. Bulk viscosity dampens this behavior by introducing a finite relaxation timescale, hence producing a shift between the equation of state pressure and the true pressure. We note that for a large enough ζ, the effective pressure becomes negative and could mimic a dark energy behavior." In particular, authors Gagnon & Lesgourges predict effective neutrino number around 3.04, w0 around -0.9, and wa around 0.1. Something to shoot for!

Interesting Nature letter suggested by Pat Roche

Daniel Mortlock talked me through this when I visited ICL recently, its a nice result, especially the deep GP trough. The survey paper (that preceded this one) was interesting too, for its treatment of detection probablity and completeness, worth a look for anyone interesting in finding (and not finding) things.

Interesting letter discussing re-ionizing the universe through accretion shocks around massive halos in the early universe.

Paper discussing how lensing can bias your reconstruction of reionisation field.

One of the interesting things about this paper, is that they have a modular code, StringFast which they release here: http://www.astro.ubc.ca/people/scott/stringfast.html

Nice to see our (Morganson et al, ref 17) lensing limit on the string tension (from not finding any sets of split pairs of faint blue galaxy images in the HST archive) is still competitive - its equal to the CMB limit they take as their best case scenario :-) Mind you, we could only rule out long straight strings.We could probably put some limit on their wiggliness parametr too I suppose - but its nice that the CMB power doesn't care about wiggliness or anything.