Back in March we had yet another apparently insurmountable challenge to the standard model of cosmology, and this paper from May claims that the problems have once again been surmounted after all.
To recap, last time a paper by Asencio et al. claimed that the El Gordo galaxy cluster is just too damn fat (i.e. massive) to exist. Or rather, according to the standard model of cosmology it shouldn't be possible to assemble such a gigantic behemoth in so short a time, and the collision velocity of its merging sub-clusters is too dang high. The authors looked at a huge simulation suite and found that no such objects should be formed at all, which is a pretty damning result if you take it at face value.
While the claims were made with a somewhat... robust level of conviction, the basic idea seemed reasonable enough to me. My main concern was whether the mass might somehow have been overestimated, since the frequency of such objects is very strongly mass-dependent. There's also the matter of the small area of the survey in which it was found. This means we have no clue whether El Gordo is a hideous bloated freak we can cheerfully ignore, or a representative of a much more interesting, widespread problem that we ought to confront.
Like the Bullet Cluster before it, this paper by Kim et al. circumvents the problem by saying that Asencio et al. were looking for the wrong object. They use new Hubble data to get gravitational lensing estimates of the mass, combined with simulations to figure out the most probable collision velocity whilst accounting for "radio relics" that were previously ignored. The bottom line is they say there's no serious conflict with the standard model after all.
So who's right ?
Difficult to say. I'm not expert in the lensing techniques they consider, so this was a tough read for me. Fortunately most of the rest is easier, and there are some interesting and very stark contradictions with the Asencio paper.
First, their new measurements decrease the mass by a modest but significant 20% or so compared to previous estimates (50% compared to the value Asencio used). This, they say, is because the previous results had to extrapolate out to the full size of the cluster, whereas their own data covers a larger area so this isn't necessary. Well, maybe, but their figure shows the mass of the cluster continues to increase out to their observed radius limit, and shows no signs of reaching a plateau, so I'm not sure why they're so confident about their new value.
And anyway this decrease isn't enough to bring it into compatibility with the masses Asencio found in their simulation. Here they appear to be in almost direct disagreement : Kim say the chance of such a cluster existing at such a distance is about 10%, whereas Asencio said it was close to zero. Even given the relatively small initial survey volume, Kim say this isn't surprising that such a monster was found, owing to the observational uncertainties on both the cluster properties and the uncertainties in the cosmological parameter values. But why there's such a stark difference in these claims is very unclear.
What about the collision velocity ? Here it gets worse. Whereas Asencio searched for clusters colliding at an enormous speed of > 2,500 km/s, Kim say the true velocity is likely to be closer to 450 km/s, which would certainly pose no difficulty whatever for the standard model. But their conclusion here is frustratingly brief. They used some simple models to find the general parameters, then ran a hydrodynamic code (i.e. something very sophisticated) to verify it. But do we get to see this fancy simulation ? Heck no. And their description of what they did is frankly confusing, apparently deliberately excluding cases they consider unphysical and then coming up with a velocity they had already pre-excluded !
What I think they're trying to say, which may offer a way out of this mess, is that previous authors began with high infall velocities and/or started with the two cluster components too close together. Kim et al. seem to be saying that actually the two subclusters started off both further away and less massive, so their initial infall velocity was much smaller. Presumably, as they approach each other, they accumulate other background galaxies and grow in mass, eventually reaching a higher velocity for the collision itself (which therefore poses no challenge to physics : more mass => higher velocity). Hence Asencio et al.'s statistics are all correct, but they were looking for the wrong sort of progenitor objects. El Gordo's parents weren't necessarily all that big or fast when they first started their doomed embrace - their romance started gradually, only reaching a frenzy at the final climax.
Ahem. Anyway, my impression is that this would have been considerably better as two papers. Most of this one is about the lensing measurements, with the simulation stuff being too tacked on and confusing. It would have been nice to have a much more rigorous examination of the Asencio result, e.g. how rare is El Gordo itself (rather than its parents) according to their simulations ?
My guess is that something like this analysis of Kim et al. will win out eventually : El Gordo will turn out to be an interesting beast, but not the CDM-slaying monster it's purported to be. But for now, Kim's result is just too unclear to be all that compelling. I suspect I'm missing something.
Head-to-Toe Measurement of El Gordo
We present an improved weak-lensing (WL) study of the high −z (z=0.87) merging galaxy cluster ACT-CL J0102-4915 (El Gordo) based on new wide-field Hubble Space Telescope (HST) imaging data. The new imaging data cover the 3.5 × 3.5 Mpc region centered on the cluster and enable us to detect WL signals beyond the virial radius, which was not possible in previous studies. Our updated mass is a more direct measurement since we are not extrapolating to R200 as in all previous studies. The new mass is compatible with the current ΛCDM cosmology.
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