Professor Hai Fu led an international team of astronomers in the discovery of a rare encounter between two massive and gas-rich galaxies in a survey from the Herschel telescope. The event took place when the Universe was only about three billion years old and involved two galaxies forming stars with exceptional efficiency whilst in the process of merging. This galactic collision would go on to form a very massive elliptical galaxy with hardly any star formation activity. The discovery suggests a viable mechanism for the origin of the puzzling 'red and dead' galaxies that are seen in the young Universe.
Each of the two galaxies has a stellar mass of about 100 billion solar masses, and they each contain roughly the same amount of gas. The astronomers estimated that the merging process would take at most 200 million years to complete, resulting in a massive elliptical galaxy of about 400 billion solar masses.
"We see these two galaxies forming stars at a phenomenal rate – about 2000 stars like the Sun per year – and the conversion of gas into stars is more efficient than in normal galaxies by an order of magnitude," says Fu.
Such a high star-formation rate, spurred by the merger process itself, is not sustainable and would not take long to exhaust the gas reservoir of both progenitor galaxies and quench star formation. The astronomers believe that such a merger must have produced a descendant galaxy with passive star-forming activity and a stellar population of old and red stars.
Ref: Hai Fu, et al., "The rapid assembly of an elliptical galaxy of 400 billion solar masses at a redshift of 2.3", 2013, Nature. DOI: 10.1038/nature12184
The phrase "harvest moon" recently took on a new meaning as a University of Iowa astrophysicist and his UI colleagues used the moon in an attempt to harvest evidence of elusive cosmic particles called ultra high energy (UHE) neutrinos in the most sensitive such radio search ever attempted.
The results of the project -- conducted by professor Robert Mutel, associate professor Kenneth Gayley and National Research Council post-doctoral fellow Theodore Jaeger, all of the UI Department of Physics and Astronomy -- have just been published in the December 2010 issue of the journal Astroparticle Physics. What the researchers hoped to detect was the tell-tale signatures of neutrinos, elementary particles of neutral charge, in order to learn more about the fundamental building blocks of matter.
Although they didn't detect neutrino-generated pulses, they did succeed in setting new, lower limits on the cosmic UHE neutrino flux. These limits provide important constraints on models of neutrino generation from a variety of cosmic neutrino generation models. In particular, the lower limit eliminates some Z-burst models which had predicted UHE neutrinos originating from the halo of the Milky Way galaxy. In addition, the pulse detection scheme and a new analysis of the moon as a neutrino target developed by the UI team will guide future moon-based radio searches with the next generation of radio telescope arrays.
Ref: Jaeger, T., Mutel, R., Gayley, K. 2010, Project RESUN, a Radio EVLA Search for UHE Neutrinos, Astroparticle Physics, 34, 293.
Philip Kaaret, professor of physics and astronomy, and his colleagues have found good evidence for the existence of two medium-sized black holes close to the center of a nearby starburst galaxy, M82, located 12 million light years from Earth. Because they avoided falling into the galactic center, the black holes may help scientists understand the seeds that give rise to supermassive black holes in other galaxies, including our own Milky Way galaxy. Professor Kaaret said that one current theory of supermassive black hole formation suggests chain reaction collisions of stars within compact star clusters can create extremely massive stars, which, in turn, collapse to form intermediate-mass black holes. The star clusters migrate to the centers of galaxies, where intermediate-mass black holes merge into supermassive black holes. Clusters that are insufficiently massive or too distant from the galactic center would survive, as would any black holes they contain.
"Finding two medium-sized black holes in one galaxy that's similar to small, star-forming galaxies that predominated when the universe was young suggests that this process may have been important in forming large galaxies like our own Milky Way," Kaaret said. His colleagues include Hua Feng of Tsinghua University in China and a former UI postdoctoral student, lead author of two papers on the subject recently published in The Astrophysical Journal.
Ref: Kaaret, P., Feng, H., and Wong, D., Tao, L. 2010, Direct Detection of an Ultraluminous Ultraviolet Source, ApJL, 714, L167.
Graduate student Bill Peterson, along with Professor Robert Mutel and colleagues from NRAO and ETH Zurich, have made the first images of a coronal loop structure on a star other than the Sun. The close binary Algol system contains a radio-bright KIV subgiant star in a very close (0.062 astronomical units) and rapid (2.86 day) orbit with a main sequence B8 star. Because the rotation periods of the two stars are tidally locked to the orbital period, the rapid rotation drives a magnetic dynamo. A large body of evidence points to the existence of an extended, complex coronal magnetosphere originating at the cooler K subgiant. The detailed morphology of the subgiant's corona and its possible interaction with its companion are unknown, though theory predicts that the coronal plasma should be confined in a magnetic loop structure, as seen on the Sun. In the January 14, 2010 issue of Nature we report multi-epoch radio imaging of the Algol system, in which we see a large, persistent coronal loop approximately one subgiant diameter in height, whose base is straddling the subgiant and whose apex is oriented towards the B8 star. This suggests that a persistent asymmetric magnetic field structure is aligned between the two stars. The loop is larger than anticipated theoretically, but the size may be the result of a magnetic interaction between the two stars.