Tuesday, August 28, 2007

The harvestmen of deep time

[image source: Gonzalo Giribet / NY times]

Carl Zimmer has produced yet another fascinating article in the science section of the NYT, featuring Dr Gonzalo Giribet and his group's research on the evolution of mite harvestmen (a relative of the daddy longlegs).

In short, as any species of mite harvestmen has a small physical range (apparently of order 50 miles or so) and hence do not spread or disperse much on their own, they provide a great way of tracing continental motion on time scales of hundreds of millions of years.

Monday, August 20, 2007

FUSE observations of O VI in Superwinds

Following up from the previous obituary for the Far Ultraviolet Spectroscopic Explorer (FUSE) I thought I'd start to discuss one way in which FUSE advanced the scientific topic I study most: starburst driven superwinds.

FUSE observations of starburst galaxies and superwinds yielded many important results, although they generated as many questions as they answered. In particular the 1032 and 1038 Angstrom doublet of the O VI ion (five times ionized oxygen, O^5+). This coronal phase gas could be used to measure the velocity and amount of material at a temperature of about 300,000 K (3e5 K), gas far hotter than the 10,000 K gas seen in Hubble observations of superwinds (this latter gas phase is technically termed warm ionized gas to differentiate it from the hotter ionized phases detected with O VI or X-rays).

Although 3e5 K is still much cooler than the temperature of the X-ray emitting gas in superwinds (which has temperatures of several times 1e6 to several times 1e7 K)
, this is still the hottest gas for which the velocity in a superwind has been measured.

The standard theoretical model for superwinds predicts that the hotter the gas phase the higher its outflow velocity. This is because it is the gas thermal pressure and ram pressure-driven expansion of the hottest material (the merged supernova and stellar wind ejecta) that sweeps up and accelerates the cooler ambient gas clouds that are seen in optical observations of superwinds. As this acceleration process is imperfect the cooler material is never accelerated to the same velocity as the hot gas.

In other theoretical models for superwinds, for example radiation-pressure driven winds or cosmic-ray driven winds, this specific variation of outflow velocity with temperature is not expected. Thus measurements of the outflow velocity of different gas phases at different temperatures can be used as a test of our theories of how superwinds are created and how they work physically.

Initial observations with FUSE of the starbursting dwarf galaxy NGC 1705 (Heckman et al, 2001, ApJ, 554, 1021) suggested that this O VI-absorbing gas was flowing outward faster than the warm and neutral ionized gaseous media in line with expectations from the standard superwind model [and that the amount of O VI absorbing material was inconsistent with thermal condiction in the shell of a superbubble, a slightly different model for what was is happening in NGC 1705].

The image shown above is an optical image of NGC 1705 taken from the SINGG survey of Meurer et al 2006, ApJS, 165, 307. The red in this image is H-alpha emission, light from warm ionized hydrogen with a temperature of 8000 - 10000 Kelvin. The filaments, shell and arcs of ionized hydrogen cover a region about 1 kpc (about 3000 light years) in diameter.

Since those early FUSE results on NGC 1705 were published in 2001 many more FUSE observations of a variety of different starbursting galaxies were taken, most of them analyzed by colleagues here at Johns Hopkins such as Charles Hoopes and John Grimes. In a future post I'll summarize what the latest thinking is on O VI in starbursts and the issue of phase-dependent velocities in superwinds.

FUSE is dead, long live FUSE!

The Far Ultraviolet Spectroscopic Explorer, a UV space telescope launched by NASA in June 1999 with a nominal lifetime of 3 years, ceased working for the last time on July 12th 2007.

FUSE Mission Status--Aug. 17, 2007
Dear Colleagues,

As reported previously in the FUSE newsletter, the last operational reaction wheel on FUSE stopped temporarily in early May 2007. It was restarted and science operations resumed on June 12. However, on July 12 the wheel stopped again. This time the stoppage was very abrupt indicating a large braking force. Attempts to restart any of the wheels over the last four weeks have been unsuccessful. Although the instrument remains in excellent condition, the FUSE satellite is currently incapable of the fine pointing control required to continue its science mission, and there is no real prospect for recovering this capability.

Regrettably, we have concluded that the scientific mission of the Far Ultraviolet Spectroscopic Explorer is no longer viable. The NASA Science Mission Directorate has accepted our recommendation to terminate the mission. The FUSE Project has started closeout activities and will complete the final CalFUSE 3.2 reprocessing of the entire science mission data set in mid 2008. The FUSE archive at MAST will be an ongoing legacy of the mission, and an important resource for years to come. Future editions of the FUSE Newsletter will provide details of our plans for the FUSE mission archive at
MAST. Also, watch the FUSE web page for updates.

The FUSE mission has been a fantastic success by any measure. 678 science programs (GI, PI team, and discretionary time) have obtained 67 Msec of observing time, over 5100 observations of about 2800 unique targets. There are over 430 peer-reviewed papers based on FUSE data and the number continues to grow. The story is not quite over, though. Twenty five of the 68 programs selected for Cycle 8 obtained data this Spring and Summer before the reaction wheel stopped for the last time. These data have been archived recently and should lead to further exciting results in the near future. Utilization of the FUSE archive will continue the flow of new results.

The Astrophysics Division intends to place special emphasis on FUSE archival research in the 2008 Astrophysics Data Program that will be part of the 2008 ROSES proposal solicitation.

The success of FUSE is a result of the combined efforts of the scientists and engineers who built and operated it plus the scientists who proposed, analyzed, and interpreted the observations. FUSE's legacy is a testament to the creativity, ingenuity, and hard work of all of you. We acknowledge your efforts and enthusiasm with gratitude.

George Sonneborn Warren Moos
Project Scientist Principal Investigator
NASA/GSFC Johns Hopkins University
Over the years scientists and engineers had worked wonders to overcome many hardware failures and software glitches to keep FUSE working beyond its normal lifetime, but this final problem with the reaction wheel appears fatal. FUSE will live on as scientists continue to analyze and re-analyze the data archive accumulated over its many years of successful observing.

Unlike the Hubble Space Telescope, which is primarily an imager, FUSE consisted of a set of four co-aligned UV-sensitive spectrometers (see the FUSE User Guide more a technical description of the instrument) and thus can not create pretty images of the sort that have made Hubble famous. FUSE has been very successful scientifically, but without images it is hard to convey this, and why it had unique capabilities that Hubble (or its eventual replacement JWST) can not match, to the public.

Friday, August 17, 2007

Mira's tail, 13 light years long and my favorite color: ultra-violet

The image above may look like a comet, but in fact the stream you see in this image taken by NASA ultra-violet space telescope GALEX is 13 light years long, and is comprised of gas thrown off the red giant star Mira over the last 30 000 years or so as it moves through space at a speed of about 130 km/s.

For more information read the press releases (here, and here with a really nice animation), which do a good job of explaining what is physically happening and how this discovery happened. Some of the images are quite impressive, even from a purely aesthetic point of view.

A BBC story is somewhat less informative, but it does includes more quotes from one of the primary scientists involved in this discovery< Mark Siebert (who just happened to be a grad student in the Astro Dept here at Johns Hopkins before moving out west. Hi Mark!).

Mira is a very well studied star and the archetype of a specific class of variable star (Mira variables), but this huge trail of cast-off gas was only discovered in GALEX images taken last year. This kind of discovery is yet another example that illustrates why having telescopes operating at wavelengths other than the optical is vital for advancing our understanding of the Universe. Hubble or ground-based telescopes like Keck and the VLT, with their primarily optical detectors and tiny fields of view, could never have made this discovery.

Almost as impressive as the scientific discovery itself is the effort JPL and GALEX PR team put into producing the press releases. You can see 6 different spins of the story one after the other. The work on the animations are also first class.

Friday, August 10, 2007

New Homo Erectus and Homo Habilis fossil finds

To get beyond the somewhat distorted press accounts regarding the Spoor et al letter in Nature on "Implications of new early Homo fossils from Ileret, east of Lake Turkana, Kenya" you should read this post at John Hawk's anthropology web log.

As with much press coverage of scientific issues something that has been known about for many decades: that multiple species of Homo coexisted at the same time, and that the evolutionary tree of genus Homo is quite bushy and complicated, and not a simple linear ladder of "progress"; is being presented as a new discovery, and furthermore the actual issues discussed in the Spoor letter aren't covered in the press coverage.

The image is a to-scale superposition of the skulls of the young adult (or late subadult) Homo Erectus (KNM-ER 42700, one of the subjects of the Spoor et al letter, cranial volume about 700 ml) on top of the skull of the largest known Homo Erectus skull (OH 9). A cool image that illustrates the diversity within Homo Erectus.

Just FYI, 90% of modern humans have cranial volumes in the range 1040 to 1595 ml [talk.origins FAQ], with volumes less than 1000 ml being extremely uncommon.