Archive for Extreme Clusters

MADCOWS and Extreme Galaxy Clusters

Posted in The Universe and Stuff, Uncategorized with tags , , , on November 4, 2015 by telescoper

I thought I’d do a quick post just to have an excuse to post this very pretty picture I found in a press release from  JPL:

extreme cluster

This is a distant galaxy cluster found in the “Massive And Distance Clusters Of Wise Survey“, which is known by its acronym “MADCOWS”. Ho Ho Ho. If the previous link is inaccessible, because you don’t have a subscription, then don’t worry: the paper concerned is available for free on the arXiv. If the previous link isn’t inaccessible, because you do have a subscription, then do worry because you’re wasting your money…

Anyway the abstract of the paper, by Gonzalez et al., reads:

We present confirmation of the cluster MOO J1142+1527, a massive galaxy cluster discovered as part of the Massive and Distant Clusters of WISE Survey. The cluster is confirmed to lie at z = 1.19, and using the Combined Array for Research in Millimeter-wave Astronomy we robustly detect the Sunyaev–Zel’dovich (SZ) decrement at 13.2σ. The SZ data imply a mass of M200m = (1.1 ± 0.2) × 1015M, making MOO J1142+1527 the most massive galaxy cluster known at z > 1.15 and the second most massive cluster known at z > 1. For a standard ΛCDM cosmology it is further expected to be one of the ~5 most massive clusters expected to exist at z ≥ 1.19 over the entire sky. Our ongoing Spitzer program targeting ~1750 additional candidate clusters will identify comparably rich galaxy clusters over the full extragalactic sky.

I added the link to WISE, by the way.

This cluster is obviously an impressive object, and galaxy clusters are always “extreme” in the sense that they are defined to be particularly large concentrations of mass, but this one is actually in line with theoretical expectations for such objects. The following graph shows the spread of extreme cluster masses expected as a function of redshift:

If you mentally plot the mass and redshift of this beastie on the diagram you’ll see that it’s well within the comfort zone. As extreme objects go, this one is quite normal!

Article of the Day!

Posted in The Universe and Stuff with tags , , , , , , on July 31, 2013 by telescoper

Back in the office today, the heatwave having given way to grey drizzle and cool breezes (at least for the time being). I’ve got stacks of paperwork to catch up on, but fortunately I’ve got time to post a quick congratulatory message to Ian Harrison, who is author of today’s NASA ADS Article of the Day! Ian is a PhD student in the School of Physics & Astronomy at Cardiff University and was supervised by me until I abandoned ship to come here to Sussex earlier this year; he’s got a postdoctoral research position lined up in the Midlands (Manchester) when he finishes his thesis. The other author, Shaun Hotchkiss, is coming to Sussex as a postdoctoral researcher in October.

Anyway, the paper is a nice one, called A consistent approach to falsifying ΛCDM with rare galaxy clusters. Here’s the abstract:

We consider methods with which to answer the question “is any observed galaxy cluster too unusual for ΛCDM?” After emphasising that many previous attempts to answer this question will overestimate the confidence level at which ΛCDM can be ruled out, we outline a consistent approach to these rare clusters, which allows the question to be answered. We define three statistical measures, each of which are sensitive to changes in cluster populations arising from different modifications to the cosmological model. We also use these properties to define the “equivalent mass at redshift zero” for a cluster — the mass of an equally unusual cluster today. This quantity is independent of the observational survey in which the cluster was found, which makes it an ideal proxy for ranking the relative unusualness of clusters detected by different surveys. These methods are then used on a comprehensive sample of observed galaxy clusters and we confirm that all are less than 2σ deviations from the ΛCDM expectation. Whereas we have only applied our method to galaxy clusters, it is applicable to any isolated, collapsed, halo. As motivation for future surveys, we also calculate where in the mass redshift plane the rarest halo is most likely to be found, giving information as to which objects might be the most fruitful in the search for new physics.

In case you’re wondering, the rather Popperian nature of the title is not the reason why I’m not among the authors. I’m just not the sort of supervisor who feels he should always be an author of papers done by his research students even when they had the idea and did all the work themselves. From what I’ve heard talking to others, we’re a dying breed!