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Galactic Russian Dolls Stop Star formation

Astronomers have long puzzled over why some elliptical galaxies stop forming many new stars, despite having the materials to do so. Now, an observation using NASA’s Chandra X-ray Observatory by a team of astronomers including assistant professor Mateusz Ruszkowski has an answer.

X-ray observations of galaxies show that many galaxies are surrounded by halos of hot gas. “For decades astronomers were puzzled by the presence of the warm gas around these objects. The gas was expected to cool down and form a lot of stars” said Prof. Ruszkowski in an interview with the UofM News Service.

M 84 x-ray image with circles indicating the location of bubbles The X-ray observation of M84, a giant elliptical galaxy in the Virgo cluster around 55 million light years away, show how the supermassive black hole at the center of the galaxy may be heating the gas. The image shows that the black hole has regular, repeated outbursts, which heats the gas in the halo. “Now, we see clear and direct evidence that the heating mechanism of black holes is persistent, producing enough heat to significantly suppress star formation. These plasma bubbles are caused by bursts of energy that happen one after another rather than occasionally, and the direct evidence for such periodic behavior is difficult to find.”

The image to the right is an x-ray image of M84, with red lines showing the location of the bubbles. Some of the bubbles are inside others, like a Russian matryoshka doll. In the X-ray image, the topmost bubble appears to be in the process of popping, releasing new superheated gas into the halo and the space between galaxies.

The team also produced a numerical simulation of the waves produced as the bubbles expand. The simulation shows that multiple outbursts can lead to the nested bubbles in the observation. Click the image at right to see the animation of the simulation.

The repeated outbursts pump energy into the gas and dust on the galaxy and between it and other galaxies. This prevents the gas from cooling enough to form new stars. The lead author of the paper in the astrophysical journal, Alexis Finoguenov, of UMBC and the Max-Planck Institute for Extraterrestrial Physics in Germany, compares the actions of the black hole to a human heart. “Just like our hearts periodically pump our circulatory systems to keep us alive, black holes give galaxies a vital warm component. They are a careful creation of nature, allowing a galaxy to maintain a fragile equilibrium.”

The paper “In-Depth Chandra Study of the AGN Feedback in Virgo Elliptical Galaxy M84” appears in The Astrophysical Journal, 686:911–917, 2008 October 20.
dditional material for this articles comes from http://www.ns.umich.edu/htdocs/releases/story.php?id=6837 and
http://chandra.harvard.edu/photo/2008/m84/

Related stories:: UM astronomers help reveal origin of black hole jet.; New Movies Help Astronomers Understand Active Galactic Nuclei; Black Holes Light Up the Universe

Two Michigan Astronomers Selected to Help Guide New X-ray Observatory

Michigan Astronomy professors Joel Bregman and Jon Miller have been selected to serve on the Science Definition Team for the International X-ray Observatory (Formerly Constellation-X).

IXO will is a joint venture between the United States (NASA), Europe (ESA), and Japan (JAXA). It will study some of the most compelling ideas and problems in the universe, including the evolution of large-scale structure, the nature of space and time close to black holes, ultra-dense states of matter, feedback cycles, and the first quasars. Michigan is the only US institution to have two scientists on the team.

Visit the IXO website at http://ixo.gsfc.nasa.gov/

Installation of Dennison Mural Begins

Installation of the astronomy themed window mural began on September 26 with a line drawing of a radio telescope on the eastern-most windows (Click the images for a bigger version).

Over the next several weeks, Prof. Jim Cogswell will add several more images, ranging from equations to a gamma-ray image of the Milky Way. The images were provided by faculty in the astronomy department, based on their current projects and research.

The window mural is the first of many projects planned for the Winter ’09 Theme Semester and International Year of Astronomy.

Highlights of the department’s plans for the Theme Semester and IYA are available on our Events page: http://astro.lsa.umich.edu/about/events.php
The Theme Semester website is at http://www.lsa.umich.edu/universe/
Visit the International Year of Astronomy page at http://www.astronomy2009.org/

Astronomers Rediscover Young Supernova Remnant

One type of supernova occurs when a massive star dies: its outer layers collapse then bounce off the core causing a massive explosion. Normally, a supernova should occur roughly every 50 years in our galaxy. The last time a supernova was observed in our galaxy was Kepler’s supernova in 1604, and there are only a few dozen supernova remnants (SNRs) known to have occurred during all of human history. Astronomers have long believed the “missing” supernovas occurred in dusty regions that block the visible light.

When massive stars collapse, the outer layers expand away, forming the SNR. A typical SNR expands away from the center point in roughly spherical shells, so they have a roughly spherical shape. The core of the star is usually left behind, as a central compact object (CCR).

When astronomers first pointed their radio telescopes at G350.1-0.3 in the early 70s, the light coming from it indicated it might be a SNR. But radio observations in the mid-80s showed an irregular shape that didn’t really look like a SNR (the white lines in the image at left). Many astronomers thought it was more likely a background galaxy, so it was downgraded to a SNR candidate and was even taken off many lists of SNRs. G350.1-0.3 was mostly forgotten.

Then in 2005, new data were published, which indicated that G350.1-0.3 had to be between 15 and 34.9 thousand light years away. Astronomically speaking, that’s practically on our back doorstep, and definitely in our own galaxy. There was no way this could be a background galaxy.

A team of astronomers, which included Jon Miller of UofM, led by Bryan Gaensler and Anant Tanna of the University of Sydney used the European Space Agency’s XMM-Newton X-ray observatory to look at G350.1-0.3, and reviewed some earlier radio observations. Their observations lead them to conclude that G350.1-0.3 is in fact a SNR.

The light coming from G350.1-0.3 indicates that it is exactly the kind of material you expect from a supernova caused by the collapse of a massive star. They found it is a mere 15 thousand light years away and is roughly 900 years old. The strange shape comes from the surrounding material. G350.1-0.3 is in a dense, dusty area of the galaxy, so the material did not expand evenly. It is so dusty in fact, that according to Gaensler “Even if you'd been looking straight at it when it exploded, it would've been invisible to the naked eye.”

Additionally, there is an object very close to G350.1-0.3, the round blue object on the right side of the image, called XMMU J172054.5-372652 that appears to be a neutron star, a common type of CCR. It may seem odd that the CCO is not actually at the center of G350.1-0.3, but there are two possible explanations for this. In other supernovas, the CCO has been “kicked out” of the central region by the explosion, so that after a few thousand years it is no longer anywhere near the center. XMMU J172054.5-372652 is rather far from G350.1-0.3 for this to be the case, but it isn’t impossible. The other possible explanation is that XMMU J172054.5-372652 is actually at the center of the SNR, and the region is so dusty that another component of the SNR is actually still hidden from our view.

G350.1-0.3 is the most recent of several young SNRs discovered in recent years in our galaxy. So far, all the newly discovered young SNRs have been in dusty regions, and were only discovered after observations with x-ray and gamma ray telescopes.

This research was published as “The (Re-)Discovery of G350.1−0.3: A Young, Luminous Supernova Remnant and Its Neutron Star”, B. M. Gaensler, A. Tanna, P. O. Slane, C. L. Brogan, J. D. Gelfand, N. M. McClure-Griffiths, F. Camilo, C.-Y. Ng, and J. M. Miller; The Astrophysical Journal Letters, 680:L37–L40, 2008 June 10
Additional information and quotes for this article were taken from the ESA news article “Detective astronomers unearth hidden celestial gem” at http://www.esa.int/esaCP/SEM1OPUG3HF_index_0.html
The radio and x-ray composit image came from http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=42879

Related stories:: Neutron star observations provide groundbreaking test of relativity.

Imaging Stars with CHARA

The Center for High Angular Resolution Astronomy (CHARA) is a six telescope optical/infrared array on Mount Wilson in California. Astronomers the University of Michigan designed and built the infrared combiner used to image the surfaces of stars and close binaries. John Monnier gave an invited talk at the summer 2008 meeting of the American Astronomical Society (AAS) on “Imaging with the CHARA Interferometer.”

Professor Monnier began his talk with the Hubble Deep Field image, to illustrate how difficult it is to study stars. In this image, galaxies billions of light years distant are resolved well enough to distinguish their types and basic characteristics. However, the one star in the image, identifiable by the diffraction spikes, is a single point of light. That star is within our own galaxy, a few thousand light years distant at most. In order to resolve even the closest stars, we need 10 times better resolution than the resolution we need to identify most distant galaxies in the universe.

The CHARA interferometer has a resolution of 0.3 – 1 milli-arc-second. That’s roughly the same as being able to distinguish a human hair at a distance of 10 football fields, and is small enough to resolve large surface features on nearby large stars. The first star imaged by CHARA was Altair, and the results were published in Science in May 2007.

According to the Von Zeipel model, massive stars that rotate rapidly should exhibit gravity darkening. The equatorial region of a rapidly rotating star will bulge out, allowing it to cool, which causes it to become somewhat dimmer. This effect is known as gravity darkening. Since the equator is dimmer than the pole, the angle of the star with respect to us can affect how bright the star looks. For example if we are looking at the star’s pole, it will look brighter than it should, which will cause astronomers to underestimate its distance from us, and overestimate its mass.

CHARA images of Altair show it is longer on one axis than the other and exhibits equatorial darkening, clearly indicating it is a rapid rotator. However, the amount of darkening seen in the observations is greater than that predicted by the Von Zeipel model. This is probably because the model assumes the stars rotate like a solid body, like the Earth. The observations fit better with models that assume differential rotation, like the Sun, where the equator actually rotates faster than the poles.
CHARA has also imaged Vega, Achenar, Regulus, and Aldeberan, and shown all of them are rapid rotators that exhibit gravity darkening. Papers on these stars are forthcoming.

These models could eventually be used to measure the mass of individual stars. The temperature gradation shows the star’s inclination and shape. The shape can be used to determine the centrifugal force, which is determined by gravity. Since gravity depends only on the mass, knowing the force of gravity leads to the star’s mass. Observations to test this are being planned.

In addition to imaging individual stars, CHARA holds great promise for imaging tight binary systems. A team headed by graduate student Ming Zhao recently imaged beta-Lyrae, the tightest binary system ever resolved. Their image partially resolves the accretion disk between the two stars. CHARA can resolve features as small as 150 micro-arc-seconds for close binary stars.

More information and images will be coming out soon, in the paper “First Resolved Images of the Eclipsing and interacting binary \beta Lyrae” by M. Zhao, D. Gies, J. D. Monnier, N. Thureau, E. Pedretti, F. Baron, A. Merand, T. ten Brummelaar, H. McAlister, S. T. Ridgway, N. Turner, J. Sturmann, and L. Sturmann, which is currently undergoing review.

Related stories:: Astronomers Capture First Image of the Surface of a Sun-Like Star

Student Image Included In Podcast

The image of a young star system taken by a team of astronomers that included graduate student John Tobin was recently included in a Spitzer Space telescope HD video podcast on protostellar jets. The podcast tells about what we can learn from infra-red images of very young star systems. The image was used as an example of a jet that can only be seen by observing in the infra red.

Download and view the podcast at http://www.spitzer.caltech.edu/features/hd/files/HUHD_019_ProtostellarJets.m4v. You can view other Hidden Universe High Definition vodcasts or subscribe to he series by going to http://www.spitzer.caltech.edu/features/hd/index.shtml.

The original image press release and full credits are available at http://www.spitzer.caltech.edu/Media/releases/ssc2007-19/ssc2007-19b.shtml
Also, check out the original article about thisunder Related Stories below.

Related stories:: A Protostellar Envelope in a Young Stellar System

UM astronomers help reveal origin of black hole jet

Blazars are some of the most energetic objects in the universe. They are a type of active galaxy similar to a quasar, powered by massive black holes at their core, and fueled by the stars and gas near the center of the galaxy. The exact mechanism for their exceptionally bright jets was uncertain until recently.

When the stars and gas get too close to a black hole, they get caught in its gravity. The gravity of the black hole is so great it rips the material apart, eventually turning it into plasma. The material has too much inertia to fall straight into the black hole. Instead it orbits several times before falling in, forming something astronomers call an accretion disk. The orbiting plasma generates a magnetic field, which is twisted into a corkscrew pattern by the rotation of the accretion disk and black hole. Some of the charged particles get caught in the magnetic field before they get too close to the black hole and are flung out at near light speed along the twisted magnetic field. The image at left shows an artist's concept of a black hole with an accretion disk and jets.

The theoretical models for blazars predict that the supermassive black hole at the center of the galaxy should have an incredibly strong magnetic field and a huge accretion disk. The particles should emit light as they travel along the magnetic field, appearing as a jet from the center of the galaxy. Astronomers should be able to observe the change in polarization of the light from the jets – the same property of light that allows polarizing sunglasses to cut down on glare from horizontal surfaces. Additionally, the particles should brighten drastically when they hit a shock wave, creating a temporary brightening in one location along the jet. An animationa is available at http://www.bu.edu/blazars/bllac_files/agn_nature_cam3_360sqpix.mov

An international team of astronomers headed by Alan Marscher of Boston University observed BL Lacertae, a blazar about 950 million light years from Earth. The team included University of Michigan radio astronomers Margo and Hugh Aller, as well as visible, x-ray and gamma ray observers. The results are amazing.

"This is the first observational evidence that really fits with the picture that the theoreticians have had," said Margo Aller, a University of Michigan radio astronomer. According to Alan Marscher "We have gotten the clearest look yet at the innermost portion of the jet, where the particles actually are accelerated"

Other observations have been unclear about what was happening. "What's really been a mystery was that we could see there were these really high-energy particles, but we didn't know how they were created, how they were accelerated. It turns out that the model matches the data. We can actually see the particles gaining velocity as they are accelerated along this magnetic field,” said Hugh Aller.
According to Margo Aller, "The reason we have this evidence is a very fine sampling of a large number of instruments, including the Michigan radio telescope."

The Michigan Radio Telescope is located at the Peach Mountain Observatory in Dexter. It has been in operation since 1958. It is open to the public on the third Sunday of September every year, from 2 - 4:30 in the afternoon.

More on this topic is available from
http://www.ns.umich.edu/htdocs/releases/story.php?id=6499
http://www.reuters.com/article/scienceNews/idUSN2338757920080424
http://www.space.com/scienceastronomy/080428-mm-black-hole-blazar.html
Images and animations are from
http://www.bu.edu/blazars/BLLac.html
The letter "The inner jet of an active galactic nucleus as revealed by a radio-to-big gamma-ray outburst" appeared in the 24 April 2008 Nature, vol 452 pp 966-9 available at
http://www.nature.com/nature/journal/v452/n7190/full/nature06895.html

Related stories:: New Movies Help Astronomers Understand Active Galactic Nuclei; Triple Quasar Systems Common in the Early Universe; Black Holes Light Up the Universe

New Movies Help Astronomers Understand Active Galactic Nuclei

University of Michigan radio astronomers Hugh and Mago Aller are members of the MOJAVE (Monitoring Of Jets in Active galactic nuclei with VLBA Experiments) program. MOJAVE recently released movies of 100 jets from active galactic nuclei.

Active galaxies have unusually bright centers (galactic nuclei). Quasars are the most well known and brightest type of active galaxy. Active galactic nuclei are typically much brighter than average at shorter wavelengths, and usually have a radio lobe directed perpendicular to the plane of the galaxy. (The image at right is a multi-wavelength composite of the Centaurs A galaxy. The dusty disk runs lower left to upper right, and the green colored radio jet runs upper left to lower right. Click the image to go to the Chandra page to see the individual images that went into the composite.)

Active galactic nuclei are driven by a black hole with an accretion disk. Material will orbit a black hole several times before falling in. The orbiting material tends to form into a disk around the black hole, called an accretion disk. Before falling in, some of the material can be accelerated enough to get kicked up out of the accretion disk. This material emits radio waves, and forms the radio jets. Understanding these jets helps astronomers understand the black holes and accretion disks that power the active galactic nuclei.

There have been some surprises in the observations. For example, some components of the jets appear to be moving faster than the speed of light. Other jets appear to twist around erratically, change direction, or show sudden changes in their magnetic field. A few galaxies even appear to have only one radio lobe instead of two. Detailed observations are needed to understand what is really happening in all these cases.

The MOJAVE project regularly images jets from active galaxies. It began in 2002, and is a successor to an earlier VLBA (Very Long Baseline Array) that ran from 1994 to 2002. In January ’08 it released data from over 100 objects as time-lapse movies.

In particular, astronomers hope to combine these data with observations from NASA’s GLAST satellite, which is expected to launch in May ’08.

More on the MOJAVE program, and the movies, are available at http://www.physics.purdue.edu/MOJAVE/
The press release about the movies is at http://www.nrao.edu/pr/2008/mojave