Finding Hidden Black Holes

The super-massive black holes thought to lurk in nearby galaxies present us with a problem. They should suck in surrounding gas and dust to produce x-rays, and it has been the assumption that black holes hidden by such materials, also known as ‘Compton-thick objects,’ are responsible for much of the overall x-ray background. Yet an x-ray census using data from Integral, ESA’s orbiting International Gamma Ray Astrophysics Laboratory, showed that a mere 15 percent of black hole galaxies detected were of the hidden Compton-thick variety.

And later work at NASA (GSFC) and the Integral Science Data Centre (Geneva), using two years of Integral data, shows an even smaller fraction. So where do the x-rays come from? “Naturally, it is difficult to find something we know is hiding well and which has eluded detection so far,” says Volker Beckmann (NASA GSFC, and lead author of an upcoming paper on the subject). “Integral is a telescope that should see nearby hidden black holes, but we have come up short.”

A hidden black hole

Image: This artist’s impression shows the thick dust torus that astronomers believe surrounds many supermassive black holes and their accretion discs. When the torus is seen edge-on as in this case, much of the light emitted by the accretion disc is blocked, creating a “hidden” black hole. Credit: ESA / V. Beckmann (NASA-GSFC).

The implication is that if hidden black holes really are the source of most of the x-ray background radiation, they must be located in the distant universe. That could be the outcome if the super-massive black holes near us have all had time to consume the gas and dust that once surrounded them, leaving them with little material to feast on. After all, the x-rays used in these observations are produced by the heating of such material as it falls into the black hole, and a lack of gas and dust would make the black hole much harder to detect.

Larger surveys are ahead, hoping to track the evolution of black holes going back into earlier epochs. Such surveys (one is planned using data from the Swift spacecraft) may show whether this revised theory is viable, or whether nearby black holes are simply hidden much more deeply than previously believed.

You can find the Integral survey results in Bassani et al., “Integral IBIS Extragalactic survey: Active Galactic Nuclei Selected at 20-100 keV,” The Astrophysical Journal 636 (10 January 2006), pp. L65-L68. Also see Beckmann et al., “The Hard X-ray 20-40keV AGN Luminosity Function,” scheduled for The Astrophysical Journal (pre-print available here).

Extraterrestrial Inflows and Ice Ages

40,000 tons of extraterrestrial matter are believed to hit the Earth every year. This from the current issue of Science, where researchers from New York (Columbia University) and Bremerhaven (Alfred-Wegener-Institut) present a study of helium isotopes found in Antarctic ice cores. Over the last 30,000 years, the scientists believe, the amount of 3He, a rare isotope found in cosmic dust, exceeds that found in terrestrial dust in ice by a factor of 5000. We have, the investigation indicates, been subject to a constant rain of cosmic dust particles over this period.

Which is interesting in its own right, but becomes more pointed when you look at the measurements of the helium isotope 4He, which is much more common on Earth. Indications point to a change of origins in terrestrial dust between the last Ice Age and the current interglacial warm period.

Says Gisela Winckler (Lamont-Doherty Earth Observatory, Columbia University):

“The terrestrial dust coming down on Antarctica during the Ice Age obviously is not the same as that during warm periods. This may be due to the mineral dust originating from different regional sources or to changes in weathering, the process responsible for production of dust.”

And from the paper itself:

We suggest that different dust sources, exposed continental shelves, or freshly generated glaciogenic material may have influenced the glacial dust deposition on the Antarctic ice sheet.

The Antarctic evidence suggests that changes in terrestrial dust between the last Ice Age and today are significant in terms of climate change, whereas extraterrestrial matter inflows seem constant during the same period. As the scientists put it in their abstract: “This finding excludes 3He as a pacemaker of late Pleistocene glacial cycles. Rather, it supports 3He as a constant flux parameter in paleoclimatic studies.”

The paper is Winckler and Fischer, “30,000 Years of Cosmic Dust in Antarctic Ice,” Science 313, p. 491 (July 28, 2006), with abstract available here. Thanks to Anthony Kendall, author of the fine Anthonares site, for his help in preparing this story.

A Fine Drizzle on Titan

The current issue of Nature features a look at Huygens data with a big payoff: rain is falling on Titan, continues to fall as we speak, and will probably keep falling for a long time. Says Christopher McKay (NASA Ames), a co-author of the paper, “The rain on Titan is just a slight drizzle, but it rains all the time, day in, day out. It makes the ground wet and muddy with liquid methane. This is why the Huygens probe landed with a splat. It landed in methane mud.” All this from low, methane-nitrogen clouds that are barely visible but appear to be widespread, affecting weather globally. A Nature feature on this work, from which the McKay quote is drawn, can be found here.

Methane rains are what you get at temperatures of minus 179 degrees Celsius, as are the river-like features also found by the probe as it descended on January 14, 2005. The latter surely derive from the ceaseless rains as well. McKay says the rain equals roughly two inches a year, about as much as Death Valley gets on Earth, but on Titan the rain falls all year round. All this at the same time that Cassini’s radar images have shown lakes in Titan’s northern hemisphere. We’re talking bodies of water as large as North America’s Great Lakes.

Interestingly, drizzle alone can’t account for the geological features thus far observed on Titan, indicating more substantial rainstorms at other times. And another letter to the same issue of Nature by Ricardo Hueso and Agustín Sánchez-Lavega (University of the Basque Country, Bilbao, Spain) discusses their simulations of Titan’s methane clouds, which found a substantial likelihood of bouts of heavy rainfall during what they depict as severe convective storms.

Huygens landing site

Image: This image provides a comparison between the Huygens landing site on Titan as viewed by the Cassini Imaging Science Subsystem (ISS) and the NACO/SDI instrument mounted on the 8-metre Yepun telescope of the VLT (Very Large Telescope) station, in Chile. Credit: NASA/JPL/Cassini-ISS/Space Science Institute and ESO/NACO-SDI/VLT.

Be aware as well of an extensive series of papers on ground-based observations during the Huygens descent and landing including Witasse, Lebreton et al., “Overview of the coordinated ground-based observations of Titan during the Huygens mission,” in the Journal of Geophysical Research (July 27, 2006). The Titan landing was one of the largest ground-based observational campaigns ever undertaken in support of a space mission.

From an ESA news release on the effort:

The radio experiments worked beyond expectations and even proved to be a ‘safety net’ when the reception of Huygens’ second communications channel failed during the descent. The data from several of Huygens’ six experiments was lost, including that required for the Huygens radio experiment to track the winds during the whole descent. The Doppler-tracking data from the Green Bank Telescope (West Virginia, America) and from Parkes (Australia) provided real-time information about the probe’s drift in the winds. The processing of the VLBI data set is not yet completed but initial results look very promising.

The Very Long Baseline Interferometry (VLBI) observations, which included the use of 17 telescopes, should provide a second window on the Huygens’ landing. We already have an exact fix on Huygens touchdown (10.33 degrees south and 192.32 degrees west) from a combination of Cassini and Huygens data. VLBI data will provide an independent reconstruction of the descent sequence.

Zoom in on COSMOS

We are entering a great era when it comes to research tools for the study of deep space. But as new technologies create datasets we’re able to distribute globally, we need to consolidate our gains to make them available to broader audiences. That’s why the creation of a Web-based utility called COSMOS SkyWalker is such heartening news. Using it, huge and minutely detailed images from sources like the Hubble Space Telescope’s Advanced Camera for Surveys can be managed for presentations and study over the Internet.

The problem is no small one. Compare the Hubble Ultra Deep Field (UDF), which contains some 10,000 galaxies, to the Cosmological Evolution Survey (COSMOS), housing no less than 2 million. The image areas on these surveys are contiguous and made up of an extraordinary number of data pixels, some 1010 pixels for COSMOS. That kind of scale makes it all but impossible to show both size and detail at the same time. Shipping the complete COSMOS ACS image over the Internet, even in compressed JPEG format, is not feasible, nor are average PCs up to the challenge of displaying such imagery.

SkyWalker can be used to browse large images and view any part of them on the screen. Because it works with HTML and JavaScript only (integrated in Web browsers like Firefox and Internet Explorer), it is usable without specialized software. The key to SkyWalker is that images can be browsed without downloading them in their entirety, allowing the user to pan around in the image, moving a pointer and tapping several zoom levels for study and presentation.

Yes, there are dedicated scientific data viewers, but SkyWalker fills a notable gap, being useful for quick access to ACS imagery and especially handy for those of us who occasionally present live material to non-scientific audiences. The COSMOS ACS mosaic is now available on the SkyWalker site, but other datasets are being added. For background on how the software works, check Jahnke, Sanchez and Koekemoer, “Seeing the Sky Through Hubble’s Eye: The COSMOS SkyWalker,” avalable here.

55 Cancri: Modeling a Terrestrial World

For Centauri Dreams, the most exciting part of the exoplanet hunt is the refinement of our models. We know, for example, of numerous planetary systems dominated by gas giants. Now we’re trying to figure out which of these may contain smaller, rocky worlds, and that means learning more about solar system dynamics. A step in the right direction emerges from a June paper that analyzes what happens to moon-sized protoplanets as they evolve in systems with gas giants.

Based on computer simulations, the work assumes a giant planet the size of Jupiter and manipulates the position and mass of the protoplanets in these settings over time, testing four systems with known planets: 55 Cancri, HD 38529, HD 37124 and HD 74156. The most interesting result is the ready formation of terrestrial worlds around 55 Cancri, often with orbits in the habitable zone. HD 38529 also produced a rocky world, one about the size of Mars, and showed conditions favorable to an asteroid belt as well. No further planets evolved around HD 37124 and HD 74156.

“It’s exciting that our models show a habitable planet, a planet with mass, temperature and water content similar to Earth’s, could have formed in one of the first extrasolar multi-planet systems detected,” said Rory Barnes, a postdoctoral researcher at the University of Arizona who is a co-author of the study.

Exciting indeed. 55 Cancri, a G-class star in Cancer 41 light years from Earth, is already thought to be orbited by as many as four planets. Learning more about such systems will help us refine the target list for planet hunting space missions like New Worlds Imager. Remarkably, we may be no more than a decade or so away from being able to see such planets, and transit methods may snare a terrestrial world sooner still.

The paper is Raymond, Barnes and Kaib, “Predicting Planets in Known Extrasolar Planetary Systems. III. Forming Terrestrial Planets,” Astrophysical Journal 644, pp. 1223-1231, with abstract here.