Universe Today

Image credit: ESO

An international team of astronomers have used the European Southern Observatory’s Very Large Telescope (VLT) to look deep into space and see galaxies located 12.6 billion light-years away – these galaxies are being seen when the Universe was only 10% of its current age. Few galaxies this old have been found, and this new collection has helped the astronomers conclude that they are a part of a cosmic Dark Age, when luminous galaxies were rarer – there were many more only 500 million years later.

Using the ESO Very Large Telescope (VLT), two astronomers from Germany and the UK [2] have discovered some of the most distant galaxies ever seen. They are located about 12,600 million light-years away.

It has taken the light now recorded by the VLT about nine-tenths of the age of the Universe to traverse this huge distance. We therefore observe those galaxies as they were at a time when the Universe was very young, less than about 10% of its present age. At this time, the Universe was emerging from a long period known as the “Dark Ages”, entering the luminous “Cosmic Renaissance” epoch.

Unlike previous studies which resulted in the discovery of a few, widely dispersed galaxies at this early epoch, the present study found at least six remote citizens within a small sky area, less than five per cent the size of the full moon! This allowed understanding the evolution of these galaxies and how they affect the state of the Universe in its youth.

In particular, the astronomers conclude on the basis of their unique data that there were considerably fewer luminous galaxies in the Universe at this early stage than 500 million years later.

There must therefore be many less luminous galaxies in the region of space that they studied, too faint to be detected in this study. It must be those still unidentified galaxies that emit the majority of the energetic photons needed to ionise the hydrogen in the Universe at that particularly epoch.

From the Big Bang to the Cosmic Renaissance
Nowadays, the Universe is pervaded by energetic ultraviolet radiation, produced by quasars and hot stars. The short-wavelength photons liberate electrons from the hydrogen atoms that make up the diffuse intergalactic medium and the latter is therefore almost completely ionised. There was, however, an early epoch in the history of the Universe when this was not so.

The Universe emanated from a hot and extremely dense initial state, the so-called Big Bang. Astronomers now believe that it took place about 13,700 million years ago.

During the first few minutes, enormous quantities of protons, neutrons and electrons were produced. The Universe was so hot that protons and electrons were floating freely: the Universe was fully ionised.

After some 100,000 years, the Universe had cooled down to a few thousand degrees and the nuclei and electrons now combined to form atoms. Cosmologists refer to this moment as the “recombination epoch”. The microwave background radiation we now observe from all directions depicts the state of great uniformity in the Universe at that distant epoch.

However, this was also the time when the Universe plunged into darkness. On one side, the relic radiation from the primordial fireball had been stretched by the cosmic expansion towards longer wavelengths and was therefore no more able to ionise the hydrogen. On the contrary, it was trapped by the hydrogen atoms just formed. On the other side, no stars nor quasars had yet been formed which could illuminate the vast space. This sombre era is therefore quite reasonably dubbed the “Dark Ages”. Observations have not yet been able to penetrate into this remote age – our knowledge is still rudimentary and is all based on theoretical calculations.

A few hundred million years later, or at least so astronomers believe, some very first massive objects had formed out of the huge clouds of gas that had moved together. The first generation of stars and, somewhat later, the first galaxies and quasars, produced intensive ultraviolet radiation. That radiation could not travel very far, however, as it would be immediately absorbed by the hydrogen atoms which were again ionised in this process.

The intergalactic gas thus again became ionised in steadily growing spheres around the ionising sources. At some moment, these spheres had become so big that they overlapped completely: the fog over the Universe had lifted !

This was the end of the Dark Ages and, with a term again taken over from human history, is sometimes referred as the “Cosmic Renaissance”. Describing the most significant feature of this period, astronomers also call it the “epoch of reionisation”.

Finding the Most Distant Galaxies with the VLT
To cast some light on the state of the Universe at the end of the Dark Ages, it is necessary to discover and study extremely distant (i.e. high-redshift [2]) galaxies. Various observational methods may be used – for instance, distant galaxies have been found by means of narrow-band imaging (e.g., ESO PR 12/03), by use of images that have been gravitationally enhanced by massive clusters, and also serendipitously.

Matthew Lehnert from the MPE in Garching, Germany, and Malcolm Bremer from the University of Bristol, UK, used a special technique that takes advantage of the change of the observed colours of a distant galaxy that is caused by absorption in the intervening intergalactic medium. Galaxies at redshifts of 4.8 to 5.8 [2] can be found by looking for galaxies which appear comparatively bright in red optical light and which are faint or undetected in the green light. Such “breaks” in the light distribution of individual galaxies provide strong evidence that the galaxy might be located at high redshift and that its light started on its long journey towards us, only some 1000 million years after the Big Bang.

For this, they first used the FORS2 multi-mode instrument on the 8.2-m VLT YEPUN telescope to take extremely “deep” pictures through three optical filters (green, red and very-red) of a small area of sky (40 square arcmin, or approx. 5 percent the size of the full moon). These images revealed about 20 galaxies with large breaks between the green and red filters, suggesting that they were located at high redshift. Spectra of these galaxies were then obtained with the same instrument, in order to measure their true redshifts.

“The key to the success of these observations was the use of the great new red-enhanced detector available on FORS2”, says Malcolm Bremer.

The spectra indicated that six galaxies are located at distances corresponding to redshifts between 4.8 and 5.8; other galaxies were closer. Surprisingly, and to the delight of the astronomers, one emission line was seen in another faint galaxy that was observed by chance (it happened to be located in one of the FORS2 slitlets) that may possibly be located even further away, at a redshift of 6.6. If this would be confirmed by subsequent more detailed observations, that galaxy would be a contender for the gold medal as the most distant one known!

The Earliest Known Galaxies
The spectra revealed that these galaxies are actively forming stars and are probably no older than 100 million years, perhaps even younger. However, their numbers and observed brightness suggest that luminous galaxies at these redshifts are fewer and less luminous than similarly selected galaxies nearer to us.

“Our findings show that the combined ultraviolet light from the discovered galaxies is insufficient to fully ionise the surrounding gas”, explains Malcom Bremer. “This leads us to the conclusion that there must be many more smaller and less luminous galaxies in the region of space that we studied, too faint to be detected in this way. It must be these still unseen galaxies that emit the majority of the energetic photons necessary to ionise the hydrogen in the Universe.”

“The next step will be to use the VLT to find more and fainter galaxies at even higher redshifts”, adds Matthew Lehnert. “With a larger sample of such distant objects, we can then obtain insight into their nature and the variation of their density in the sky.”

A British Premiere
The observations presented here are among the first major discoveries by British scientists since the UK became a member of ESO in July 2002. Richard Wade from the Particle Physics and Astronomy Research Council (PPARC), which funds the UK subscription to ESO, is very pleased: “In joining the European Southern Observatory, UK astronomers have been granted access to world-leading facilities, such as the VLT. These exciting new results, of which I am sure there will be many more to come, illustrate how UK astronomers are contributing with cutting-edge discoveries.”

More information
The results described in this Press Release are about to appear in the research journal Astrophysical Journal (” Luminous Lyman Break Galaxies at z>5 and the Source of Reionization” by M. D. Lehnert and M. Bremer). It is available electronically as astro-ph/0212431.

Notes
[1]: This is a coordinated ESO/PPARC Press Release. The PPARC version of the release can be found here.

[2]: This work was carried out by Malcolm Bremer (University of Bristol, The United Kingdom) and Matthew Lehnert (Max-Planck-Institut f?r Extraterrestrische Physik, Garching, Germany).

[3]: The measured redshifts of the galaxies in the Bremer Deep Field are z = 4.8-5.8, with one unexpected (and surprising) redshift of 6.6. In astronomy, the redshift denotes the fraction by which the lines in the spectrum of an object are shifted towards longer wavelengths. The observed redshift of a remote galaxy provides an estimate of its distance. The distances indicated in the present text are based on an age of the Universe of 13.7 billion years. At the indicated redshift, the Lyman-alpha line of atomic hydrogen (rest wavelength 121.6 nm) is observed at 680 to 920 nm, i.e. in the red spectral region.

Original Source: ESO News Release

Image credit: ESA

The European Space Agency’s Ulysses spacecraft has confirmed that the Sun’s 11-year cycle that causes it to switch magnetic poles allows interstellar dust to enter our Solar System in greater quantities. The Sun normally puts a protective magnetic bubble around the solar system to push dust around us, but during this pole-switch, the bubble disappears for a little while. Astronomers believe this will increase the amount of material that falls on the Earth to 40,000 tonnes of dust a day – it won’t really cause a problem; however, we may be able to see some more faint falling stars.

Astronomers once thought they understood how the Sun worked. A large ball of gas, generating energy by nuclear fusion, it also created a magnetic field enclosing Earth and the other planets in a gigantic magnetic bubble.

This bubble protected us from the dusty cosmic debris that shoots through space beyond the Solar System. Thanks to ESA’s solar polewatcher Ulysses, that picture is changing…

11-year switch
Ulysses has revealed a complexity to the Sun’s magnetic field that astronomers had never imagined. The Sun’s magnetic field consists of a north pole, where the field flows out of the Sun and a south pole, where the field reenters. Usually, these line up, more-or-less, with the rotation axis of the Sun. Every 11 years the Sun reaches a peak of activity that triggers the magnetic poles to exchange places. The reversal was thought to be a rapid process but, thanks to Ulysses, astronomers now know it is gradual and could take as much as seven years to complete.

During this slow-motion reversal, the line connecting the poles – known as the magnetic axis – comes close to the Sun’s equator and is swept through space like the beam of a light house. Eventually it passes through this region and lines up with the opposite pole.

Imagine if this happened on Earth! Compasses would become useless, given that they rely on the fact that Earth’s magnetic axis is roughly coincident with its rotation axis, which passes through the North and South geographic Pole. Although it seems surprising, magnetic pole reversals have happened on Earth also. The last time was about 740 000 years ago. After studying magnetic rocks, scientists conclude that field reversals on Earth take place once every 5000 to 50 million years (but are impossible to predict). Reversals on the Sun, however, are almost as regular as clockwork – every 11 years, with its magnetic axis changing position for most of that time.

More shooting stars
Earth’s magnetic field is more stable because it arises in the metal-dominated regions in the deep interior of the planet. The Sun’s field, however, comes from a high-temperature, electrified gas called plasma so it is a much more volatile thing. Loops of the magnetic field can burst through the surface of the Sun and when they do, they create the dark patches known as sunspots.

Astronomers are still studying the precise reasons behind the Sun’s 11-year magnetic flips. However, using Ulysses, they have now shown that, when the Sun’s magnetic axis points near its equator, it allows much more cosmic dust to enter the Solar System than normal. What does that mean for us?

If there is more dust in the Solar System, more of it will fall on Earth also. Scientists estimate that in the coming years, about 40 000 tonnes of dust could fall on Earth every day. However, most of it will be so small that it will burn up in the atmosphere before reaching the ground. This will certainly increase the number of faint shooting stars during the next 11 years, but fortunately the Earth will not become a dustier place!

Original Source: ESA News Release

Image credit: NSF

Astronomers have used the accuracy of the National Science Foundation’s Very Long Baseline Array (VLBA) to pinpoint the distance to a pulsar. The object, called PSR B0656+14, was previously thought to be up to 2,500 light-years away but it was at the same location in the sky as a supernova remnant which is only 1,000 light years away. This was thought to be a coincidence, but the new measurement from the VLBA pegs the pulsar at 950 light years away; the same distance as the remnant – they were both created by the same supernova blast.

Location, location, and location. The old real-estate adage about what’s really important proved applicable to astrophysics as astronomers used the sharp radio “vision” of the National Science Foundation’s Very Long Baseline Array (VLBA) to pinpoint the distance to a pulsar. Their accurate distance measurement then resolved a dispute over the pulsar’s birthplace, allowed the astronomers to determine the size of its neutron star and possibly solve a mystery about cosmic rays.

“Getting an accurate distance to this pulsar gave us a real bonanza,” said Walter Brisken, of the National Radio Astronomy Observatory (NRAO) in Socorro, NM.

The pulsar, called PSR B0656+14, is in the constellation Gemini, and appears to be near the center of a circular supernova remnant that straddles Gemini and its neighboring constellation, Monoceros, and is thus called the Monogem Ring. Since pulsars are superdense, spinning neutron stars left over when a massive star explodes as a supernova, it was logical to assume that the Monogem Ring, the shell of debris from a supernova explosion, was the remnant of the blast that created the pulsar.

However, astronomers using indirect methods of determining the distance to the pulsar had concluded that it was nearly 2500 light-years from Earth. On the other hand, the supernova remnant was determined to be only about 1000 light-years from Earth. It seemed unlikely that the two were related, but instead appeared nearby in the sky purely by a chance juxtaposition.

Brisken and his colleagues used the VLBA to make precise measurements of the sky position of PSR B0656+14 from 2000 to 2002. They were able to detect the slight offset in the object’s apparent position when viewed from opposite sides of Earth’s orbit around the Sun. This effect, called parallax, provides a direct measurement of distance.

“Our measurements showed that the pulsar is about 950 light-years from Earth, essentially the same distance as the supernova remnant,” said Steve Thorsett, of the University of California, Santa Cruz. “That means that the two almost certainly were created by the same supernova blast,” he added.

With that problem solved. the astronomers then turned to studying the pulsar’s neutron star itself. Using a variety of data from different telescopes and armed with the new distance measurement, they determined that the neutron star is between 16 and 25 miles in diameter. In such a small size, it packs a mass roughly equal to that of the Sun.

The next result of learning the pulsar’s actual distance was to provide a possible answer to a longstanding question about cosmic rays. Cosmic rays are subatomic particles or atomic nuclei accelerated to nearly the speed of light. Shock waves in supernova remnants are thought to be responsible for accelerating many of these particles.

Scientists can measure the energy of cosmic rays, and had noted an excess of such rays in a specific energy range. Some researchers had suggested that the excess could come from a single supernova remnant about 1000 light-years away whose supernova explosion was about 100,000 years ago. The principal difficulty with this suggestion was that there was no accepted candidate for such a source.

“Our measurement now puts PSR B0656+14 and the Monogem Ring at exactly the right place and at exactly the right age to be the source of this excess of cosmic rays,” Brisken said.

With the ability of the VLBA, one of the telescopes of the NRAO, to make extremely precise position measurements, the astronomers expect to improve the accuracy of their distance determination even more.

“This pulsar is becoming a fascinating laboratory for studying astrophysics and nuclear physics,” Thorsett said.

In addition to Brisken and Thorsett, the team of astronomers includes Aaron Golden of the National University of Ireland, Robert Benjamin of the University of Wisconsin, and Miller Goss of NRAO. The scientists are reporting their results in papers appearing in the Astrophysical Journal Letters in August.

The VLBA is a continent-wide system of ten radio- telescope antennas, ranging from Hawaii in the west to the U.S. Virgin Islands in the east, providing the greatest resolving power, or ability to see fine detail, in astronomy. Dedicated in 1993, the VLBA is operated from the NRAO’s Array Operations Center in Socorro, New Mexico.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Original Source: NRAO News Release