CDMS-2's 1-sigma dark matter result.

After much speculation the CDMS-2 "dark matter detection result" announced yesterday turns out to be 2 events with an expected background rate of 0.8 events in the 2007-2008 time frame of the experiment (SciAm news piece).

Exactly two events would happen by chance ~14% of the time (>=2 events would happen 20% of the time), so we're basically talking about a ~1 sigma result. Meh. Hardly impressive. It'd take 10 events under a 0.8 count background rate to get me excited.

BBC discusses costs of ESA Cosmic Vision contenders

An article on the BBC by Jonathan Amos discusses the contenders for the two slots in ESA's Cosmic Visions program for 2017-2018 launches. Interestingly the article's focus is on the costs (see graph taken from article), but it also has a nice description of each mission (many of which I hadn't heard of).

The missions, and my shorter summary of their nature and aims, are:

• SPICA: Joint ESA/JAXA infrared space telescope (5 to 210 micron wavelength range) with a 3.5m primary mirror.
  
• Euclid: Map mass distributions using baryonic acoustic oscillations and weak lensing.
• PLATO: A planet hunter with a particular emphasis on finding Earth-like and super-earth terrestrial planets using milli-magnitude accuracy photometry.
• Solar Orbiter: Study the Sun and Solar wind from a relatively close-in orbit (as close as 48 Solar radii, it claims).
• Marco Polo: Joint ESA/JAXA sample return mission from a near-earth asteroid. Note the high cost!
• Cross-Scale: Study MHD plasma properties in the terrestrial magnetosphere and bow shock. 7 ESA spacecraft forming 2 nested tetrahedra with a shard corner. (International collaboration will produce the optimum fleet of 12 spacecraft in 3 nested tetrahedra.) High cost!
  

Interesting Astrophysics: Apr 06 to Apr 10

It has been a relatively quiet week in terms of papers and preprints that I consider interesting enough to make a note of. Perhaps the most interesting is Martinez and Trimble's "Cosmologists in the dark", largely as it is a historical review rather than a technical paper.

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Galaxies and Starbursts

An ultraviolet study of nearby luminous infrared galaxies: star formation histories and the role of AGN
Sugata Kaviraj, 2009, MNRAS, 394, 1167
Full Text: HTML, PDF (Size: 13559K)

Over half of the far-infrared background light comes from galaxies at z > 1.2
Mark J. Devlin, et al, arXiv:0904.1201 [pdf]

Comments: Accepted to Nature. Maps available at this http URL

Journal-ref: Nature, vol. 458, 737-739 (2009)

BLAST: A Far-Infrared Measurement of the History of Star Formation
Enzo Pascale, et al, arXiv:0904.1206 [ps, pdf, other]

Comments: Submitted to ApJ. Maps available at this http URL

X-ray nuclear activity in nearby galaxies
Wei Ming Zhang, Roberto Soria, Shuang Nan Zhang, Douglas A. Swartz, JiFeng Liu, arXiv:0904.1091 [ps, pdf, other]
Comments: 27 pages, 353 kB, accepted by ApJ

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Cosmology

Cosmologists in the dark
Vicent J. Martinez, Virginia Trimble, arXiv:0904.1126 [ps, pdf, other]
Comments: 10 pages, ASP style (asp2006.sty), invited talk, to be published in the proceedings of the conference "Cosmology across Cultures" (held at Granada, Spain, on 2008, September 8th to 12th), J. A. Belmonte, F. Prada, J. A. Rubino Martin, & A. Alberdi, Eds., ASP, S. Francisco. Comments are welcome

A nice discussion of the history of the last century of cosmology that maintains a fair balance in its critiques. It certainly does a nice job of discussing the problems with Steady State and Quasi Steady State models. The title, which you might think of as a dig at cosmologists, is actually in reference to the quote from T.S. Elliot's "East Coker" (which I'm sure you know, or should know anyway).

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Black Holes and AGN

The Contribution of Active Galactic Nuclei to the Microjansky Radio Population
D.R. Ballantyne, arXiv:0904.0996 [ps, pdf, other]
Comments: 30 pages, 11 figures, accepted by the ApJ

Active Galactic Nuclei with Starbursts: Sources for Ultra High Energy Cosmic Rays
P.L. Biermann, J.K. Becker, L. Caramete, L. Gergely, I.C. Maris, A. Meli, V. de Souza, T. Stanev, arXiv:0904.1507 [ps, pdf, other]
Comments: 4 pages, 1 figure, proceedings of "High-Energy Gamma-rays and Neutrinos from Extra-Galactic Sources", Heidelberg

Quasar Feedback: More Bang for Your Buck
Philip F. Hopkins, Martin Elvis, arXiv:0904.0649 [ps, pdf, other]
Comments: 7 pages, 2 figures, submitted to MNRAS

Meh. I must be feeling extra cynical today as I really don't think this says anything new other than collecting old results together and packaging them together with a cartoon picture.

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Numerical Astrophysics and Hydrodynamics

The turbulent destruction of clouds – I. A k–ε treatment of turbulence in 2D models of adiabatic shock–cloud interactions
J. M. Pittard, S. A. E. G. Falle, T. W. Hartquist, J. E. Dyson, 2009, MNRAS, 394, 1351
Full Text: HTML, PDF (Size: 52479K)

Numerical approaches to star formation and SuperNovae energy feedback in simulations of galaxy clusters
Martina Giovalli, arXiv:0904.1399 [pdf, other]
Comments: Ph.D. Thesis, 203 pages

Note to self: Might be good. Try to find the time to read this carefully.

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Stars, Supernovae and Planets

The Core-collapse rate from the Supernova Legacy Survey
G. Bazin, et al, arXiv:0904.1066 [ps, pdf, other]
Comments: accepted Astronomy and Astrophysics

On the Mass and Metallicity Distributions of the Parent AGB Stars of O-rich Presolar Stardust Grains
Larry R. Nittler, arXiv:0904.1388 [ps, pdf, other]
Comments: 8 pages, 5 figures; accepted for publication in the Publications of the Astronomical Society of Australia

I found this part of the abstract particularly interesting: "3) The Sun appears to have a moderately low metallicity for its age and/or unusual 17O/16O and 18O/16O ratios for its metallicity. 4) The Solar 17O/18O ratio, while unusual relative to present-day molecular clouds and protostars, was not atypical for the presolar disk and does not require self-pollution of the protosolar molecular cloud by supernova ejecta."

Press releases from last week's AAS meeting in Long Beach, CA.

A variety of astronomy highlights and press releases from last week's AAS meeting in Long Beach, CA made it into the popular science media, in particular Space.com, but also the BBC and the NYT (unless otherwise noted, the links are to a Space.com article). I chosen to cover four stories from the AAS that made it into the popular science media.

Unfortunately the meeting website and the AAS don't seem to link to or archive the these press releases. In some cases all that is available is the pop science write-up, which may or may not accurately reflect what the scientists involved wanted to present. It used to be the case that press releases only happened on the publication of a full peer reviewed Journal article, but this is the AAS where you typically announce work-in-progress and so full papers may still be in the works.

(1.) "Mystery Roar from Faraway Space" by Andrea Thompson discusses an excess component in the Extragalactic Radio background (in the 3 to 90 GHz band) discovered in data from the ARCADE 2 balloon experiment. Emission in this band is a combination of a signal due to the Cosmic Microwave Background (the remainder radiation from the Big Bang), a foreground signal from our own Galaxy, and the integrated emission from extra-galactic sources (in particular galaxies). The claim is that after accounting for known sources and the expected contribution from galaxies there is a residual component to the radio background with a spectral index of -2.56 +/- 0.04 (the actual submitted paper is Seiffert et al, arXiv:0901.0559).

The actual paper spends quite a bit of time trying to demonstrate that they have good "evidence for isotropic radio emission... beyond what can be explained Galactic emission and the unresolved emission from the known population of discrete sources" [i.e. galaxies], but the paper doesn't really speculate as to what the source of this component is. The argument that this excess can not be due to galaxies is a pretty good one. Astronomers have a good idea of how much radio emission to expect from all the galaxies in the Universe because galaxies obey the relatively tight radio/FIR correlation (the massive stars whose explosions ultimate create the cosmic ray electrons responsible for galactic radio emission also heat dust to IR-emitting temperatures), and if galaxies were responsible for the excess radio emission then we'd expect a higher IR background than we see.

Thompson's write up (nicely) doesn't go beyond the submitted paper: 'For now, the origin of the signal remains a mystery. "We really don't know what it is," said team member Michael Seiffert of NASA's Jet Propulsion Laboratory in Pasadena, Calif.'

This story is also covered by Dennis Overbye at the NYT on January 7th ("Theory Ties Radio Signal to Universe's First Stars"). This article goes rather further than the actual submitted paper, spending quite a bit of time discussing Alan Kogut's (NASA/GSFC) admittedly "wildly speculative" suggestion that the excess emission originates from black holes formed by the collapse of the first stars to form in the Universe (Population III stars).

(2.) "Explosions Starve Black Holes" by Andrea Thompson discusses results presented by Daniel Wang (Hi Daniel), but I'm confused as what new result its reporting, or whether its theory or observational data despite the subject matter (galaxies, X-rays, supernova-heated hot gas) being very close to stuff I do. I could not find an actual press release on the UMass or JPL web sites, nor a paper on arXiv. The the idea that supernova heating in prevents/explains the lack of accumulation of large masses of cool/warm interstellar medium (by cool or warm I mean gas with temperatures less a few million degrees) from recycled gas from evolved stars in old stellar populations (ellipticals, bulges) is not new, dating back to the 1960's. That AGN will accrete less in a hot low density ISM is also
self-evident. Nevertheless, despite my lack of understanding regarding the news item I've included it in this list because I know Daniel and hot gas in galaxies also deserves more media attention.

(3.) "Surprise! Milky Way Much More Massive" by Andrea Thompson discusses new Very Long Baseline Array (VLBA) radio observations of Masers that indicate that the rotational velocity (and hence mass) of our Milky Way galaxy is higher than previously thought (by approximately 15% above the old standard of rotation speed 220 km/s, and ~50% higher in mass). This is also covered by Kenneth Chang at the NYT ("Data Uncover Bigger Galaxy in Cosmos, and It’s Ours" ), at the BBC ("Milky Way 'bigger than thought'" ), and NRAO's full press release ("Milky Way a Swifter Spinner, More Massive, New Measurements Show") is here. The image shown at the start of this post is taken from the NRAO press release, and shows an artist's impression of the Milky Way (as viewed from above) along with our location (red circle) and the location of the maser sites (green, blue circle?).

The basic method is to exploit the amazing spatial resolution given by very long-baseline radio interferometry to measure trigonometric parallaxes (and hence the distance) to methanol masers (simpler wikipedia article on masers here) in massive star forming regions in our galaxy. This is a direct distance measurement, and hence doesn't rely on multiple error-prone steps in the Cosmological Distance Ladder. From the doppler shifts of the radio emission one also has the line-of-sight velocity of the source with respect to us. Thus you have size and velocity measurements, although how you go beyond that to get a rotation model for the Galaxy isn't immediately obvious to me (remedial reading of Galactic Dynamics or Galactic Astronomy is hereby self-assigned).

Unfortunately this is another result that doesn't appear (AFAIK) to be associated with a publically available recent paper (no obvious arXiv preprint, no published paper that I can find, no paper mentioned in the NRAO press release). Papers on distances to individual star forming by members of this team are on arXiv, but don't mention changing the previously accepted value of the Milky Way's rotational velocity (Brunthaler et al, arXiv:0811.0713; Rygl et al, arXiv:0812.0905 ).

However, I did remember this issue was mentioned in Shattow and Loeb's paper on the LMC 's orbit (2008, MNRAS, 392, L21 ), and sure enough they say: "Also, within the past five years, the circular velocity of the MW and the distance between the Sun and the Galactic Centre have been updated by Reid & Brunthaler (2004; hereafter RB04) and Gillensen, Genzel & Eisenhouer (private communication; hereafter GGE08), respectively. These increased the likely circular velocity of the MW from the IAU standard of Vcirc = 220 to 251 ± 15 km s−1. A value of 220 km s−1 now corresponds to a reduction of the best-fitting value by two standard deviations (and equivalent to moving the Sun a total of 1 kpc closer to the centre of the Galaxy). Uemura et al. (2000) used parallax measurements from Hipparcos and SKYMAP to obtain a similar value of Vcirc = 255 ± 8 km s−1."

Here are the links to the Reid and Brunthaler (2004) and Uemura et al (2000) papers:
1. "The Proper Motion of Sagittarius A*. II. The Mass of Sagittarius A*," Reid and Brunthaler, 2004, ApJ, 616, 872. (Note that this paper gives theta_0 = 236 +/- 15 km/s, with the error dominated by the 0.5 kpc uncertainty in R_0.)
2. "Galactic Rotation Derived from OB Stars Using Hipparcos Proper Motions and Radial Velocities," Uemura et al, 2000, PASJ, 52, 143. They obtained v_theta = 255 +/- 8 km/s.

So although this result might actually be a few years old and not "hot of the presses" new, it certainly was news to me and is a nice example of how we continue to reassess the nature of our own Galaxy.

(4.) "Black Holes Preceded Galaxies, Discovery Suggests" by Andrea Thompson. This is a story that got a lot of press and blog attention, much of which typically fails to include the important qualifier "suggests" that Thompson includes in her title (good for you Andrea!). The BBC article is another offender ("Black holes 'preceded galaxies'" ). Interestingly this story was not touched by the NYT. The core scientific result is an impressive technical achievement and is scientifically interesting and important. Unfortunately the press releases go beyond what the data is saying.

One of the most striking differences between our current view of the supermassive black holes (SMBHs) that power AGN and the galaxies within which they reside, and what we knew say 15 years ago when I was in grad school is that we now that galaxy growth and black hole growth are linked some how. In the local z=0 (here and now) Universe, 13.7 billion years after the Big Bang, those galaxies that have SMBHs (not all do, many lower mass galaxies appear not to have SMBHs at all) have SMBHs that weigh typically about 0.1% of the galaxy mass (there are of course arguments over whether the fundamental relationship is BH mass to galaxy stellar (baryonic) mass or to just the mass of the galaxies Bulge stellar population, or the mass of the entire halo including the Dark Matter, but you get the picture).

This strongly suggests that galaxy growth (star formation from gas) and black hole growth (accretion of gas) are somehow linked and possibly regulate one another. This was a shocking result. Some astronomers now speculate that AGN control or limit star formation on galactic scales, other speculate that star formation regulates/limits accretion onto SMBHs, and other suggest that the size of the galaxy's dark matter halo ultimately controls both star formation and the accretion onto the SMBH. All very different and much more complex and almost ecological compared to the view back in the early 1990's where theories of galaxy formation and AGN growth were essentially separate, their evolution disconnected from one another.

The Carilli et al press release is basically about measuring the ratio of the mass of the SMBH to the galaxy in four high redshift AGN. Do these much younger galaxies, only a billion or two years old, have the same ratio as the older galaxies now do? That is, were the BH and stellar growth rates constrained by whatever process in the same way over the entire history of the Universe? This is a fascinating question, and the answer appears to be no. In Carilli et al's four high-z AGN the SMBH appears to be proportionally much more massive than in local galaxies. Or alternatively the galaxies (mainly stars) are less massive.

Whichever way you put it, the black holes in these particular galaxies in early Universe appear to have grown faster relative to the galaxy than the long term average we now see imprinted in galaxies at z=0 . Qualitatively this result is not entirely unexpected (read the nice introduction to Haiman, 2006, New Astronomy Reviews, 50, 672 ) but actually managing to get quantitative measurements is a great piece of science nonetheless. Good job (so far) guys!

Its at this point things start to go wrong, IMHO. These galaxies they're observing are young, but not primeval - and they still have most of their mass in stars. The period is still ~1.5 billion years after the Big Bang and about a billion years after the first stars and galaxies began to form.

Yet the result is extrapolated to much earlier times and to discuss what came first: Black Holes or stars? “Black holes came first and somehow—we don’t know how—grew the galaxy around them,” Carilli says [sciam.com ]. "The implication [emphasis mine] is that the black holes started growing first." Fabian Walter is quoted as saying [BBC article ]. But the data itself doesn't tell you what happed. Its at least equally plausible that the first stars came before the black holes, but then the black hole growth rate briefly surged.

The data says that the SMBHs are proportionally more massive with respect to the galaxy mass at high redshift than at low redshift. A reasonable extrapolation of this is to say that the black holes grew proportionally faster than the galaxies, averaged over the entire 1.5 billion year period after the Big Bang. But its a big leap to say the Black Holes formed first. If the stellar/galactic and Black Hole growth rates are no longer inextricably coupled then we have less, not more, of a handle on the age!

Yet from reading the popular press Joe Public would get the incorrect impression that science has solved the problem and worked out that Black Holes came before galaxies. I only highlight this particular case because this seems me to be a typical problem with popularizing science and with press releases: The qualifying statements get stripped out, and the possible-but-speculative implications of a result end up getting higher billing than the real science itself.

As a side note, the SciAm article on this has an update mentioning that even the basic science result may not be robust but instead be a statistical artifact. No doubt this work will spur other groups to perform similar studies, so time will tell whether the results of relatively larger SMBH to galaxy mass in the early Universe holds true. Nevertheless, very interesting work despite the annoying PR spin.

Dark Matter Annihilation?

The physicists are getting all excited by the hints from PAMELA and ATIC of an excess in the electron cosmic ray spectrum at around 620 GeV - could this be a signal of dark matter (WIMPs) interacting with itself and annihilating?

Dennis Overbye has a nice article in yesterdays NYT suitable for a general readership ("A Whisper, Perhaps, From the Universe's Dark Side"). Yousaf Butt wrote the Nature "News and Views" article that serves as a good introduction or perspective on the Chang et al (2008, Nature, 456, 362, "An excess of cosmic ray electrons at energies of 300–800 GeV" article that has kick started the media interest.

[Image is Figure 2 from the Chang et al paper, accessible with caption here as long as you have a Nature subscription]