Universe Today

Image credit: SDSS

Gravitational lensing happens when the light from a distant object, such as a quasar, is distorted by the gravity of a closer object. Astronomers have discovered just such a lens, where the distortions are so great, they have to be caused by a significant amount of dark matter – the visible material alone couldn’t be responsible. Dark matter is predicted by its gravitational influence on galaxies and stars in the Universe, but so far, astronomers aren’t really sure what it is; whether it’s just regular matter which is too cold to be seen from Earth, or some kind of exotic particle.

Sloan Digital Sky Survey scientists have discovered a gravitationally lensed quasar with the largest separation ever recorded, and, contrary to expectations, found that four of the most distant, most luminous quasars known are not gravitationally lensed.

Albert Einstein’s Theory of General Relativity predicts that the gravitational pull of a massive body can act as a lens, bending and distorting the light of a distant object. A massive structure somewhere between a distant quasar and Earth can “lens” the light of a quasar, making the image substantially brighter and producing several images of one object.

In a paper published in the December 18/25 edition of NATURE magazine, a Sloan Digital Sky Survey (SDSS) team led by University of Tokyo graduate students Naohisa Inada and Masamune Oguri report that four quasars in close proximity are, in fact, the light from one quasar split into four images by gravitational lensing.

More than 80 gravitationally lensed quasars have been discovered since the first example was found in 1979. A dozen of the cataloged lensed quasars are SDSS discoveries, of which half are the result of the work of Inada and his team.

But what makes this latest finding so dramatic is that the separation between the four images is twice as large as that of any previously known gravitationally lensed quasar. Until the discovery of this quadruple lens quasar, the largest separation known in a gravitationally lensed quasar was 7 arcseconds. The quasar found by the SDSS team lies in the constellation Leo Minor; it consists of four images separated by 14.62 arcseconds.

In order to produce such a large separation, the concentration of matter giving rise to the lensing has to be particularly high. There is a cluster of galaxies in the foreground of this gravitational lens; the dark matter associated with the cluster must be responsible for the unprecedented large separation.

“Additional observations obtained at the Subaru 8.2 meter telescope and Keck telescope confirmed that this system is indeed a gravitational lens,” explains Inada. “Quasars split this much by gravitational lensing are predicted to be very rare, and thus can only be discovered in very large surveys like the SDSS.”

Oguri added: “Discovering one such wide gravitational lens out of over 30,000 SDSS quasars surveyed to date is perfectly consistent with theoretical expectations of models in which the universe is dominated by cold dark matter. This offers additional strong evidence for such models.” (Cold dark matter, unlike hot dark matter, forms tight clumps, the kind that causes this kind of gravitational lens.)

“The gravitational lens we have discovered will provide an ideal laboratory to explore the relation between visible objects and invisible dark matter in the universe,” Oguri explained.

In a second paper to be published in the Astronomical Journal in March 2004, a team led by Gordon Richards of Princeton University used the high resolution of the Hubble Space Telescope to examine four of the most distant known quasars discovered by SDSS for signs of gravitational lensing.

Looking to great distances in astronomy is looking back in time. These quasars are seen at a time when the universe was less than 10percent of its present age. These quasars are tremendously luminous, and are thought to be powered by enormous black holes with masses several billion times that of the Sun. The researchers said it is a real mystery how such massive black holes could have formed so early in the universe. Yet if these objects are gravitationally lensed, SDSS researchers would infer substantially smaller luminosities and therefore black hole masses, making it easier to explain their formation.

“The more distant a quasar, the more likely a galaxy lies between it and the viewer. This is why we expected the most distant quasars to be lensed,” explained SDSS researcher Xiaohui Fan of the University of Arizona. However, contrary to expectations, none of the four shows any sign of multiple images that is the hallmark of lensing.

“Only a small fraction of quasars are gravitationally lensed. However, quasars this bright are very rare in the distant universe. Since lensing causes quasars to appear brighter and therefore easier to detect, we expected that our distant quasars were the ones most likely to be lensed,” suggested team member Zoltan Haiman of Columbia University.

“The fact that these quasars are not lensed says that astronomers have to take seriously the idea that quasars a few billion times the mass of the Sun formed less than a billion years after the Big Bang”, said Richards. “We’re now looking for more examples of high-redshift quasars in the SDSS to give theorists even more supermassive black holes to explain.”

Original Source: SDSS News Release

Image credit: ESO

The European Southern Observatory has released three new images of distant spiral galaxies, which were taken while astronomers were searching for quasars. NGC 613 is a beautiful barred spiral galaxy in the southern constellation of Sculptor; NGC 1792 is a starburst spiral galaxy located in the southern constellation of Columba; and NGC 3627 is also known as Messier 66 and located in the constellation Leo.

Not so long ago, the real nature of the “spiral nebulae”, spiral-shaped objects observed in the sky through telescopes, was still unknown. This long-standing issue was finally settled in 1924 when the famous American astronomer Edwin Hubble provided conclusive evidence that they are located outside our own galaxy and are in fact “island universes” of their own.

Nowadays, we know that the Milky Way is just one of billions of galaxies in the Universe. They come in vastly different shapes – spiral, elliptical, irregular – and many of them are simply beautiful, especially the spiral ones.

Astronomers Mark Neeser from the Universit?ts-Sternwarte M?nchen (Germany) and Peter Barthel from the Kapteyn Institute in Groningen (The Netherlands) were clearly not insensitive to this when they obtained images of three beautiful spiral galaxies with ESO’s Very Large Telescope (VLT). They did this in twilight during the early morning when they had to stop their normal observing programme, searching for very distant and faint quasars.

The resulting colour images (ESO PR Photos 33a-c/03) were produced by combining several CCD images in three different wavebands from the FORS multi-mode instruments.

The three galaxies are known as NGC 613, NGC 1792 and NGC 3627. They are characterized by strong far-infrared, as well as radio emission, indicative of substantial ongoing star-formation activity. Indeed, these images all display prominent dust as well as features related to young stars, clear signs of intensive star-formation.

NGC 613
NGC 613 is a beautiful barred spiral galaxy in the southern constellation Sculptor. This galaxy is inclined by 32 degrees and, contrary to most barred spirals, has many arms that give it a tentacular appearance.

Prominent dust lanes are visible along the large-scale bar. Extensive star-formation occurs in this area, at the ends of the bar, and also in the nuclear regions of the galaxy. The gas at the centre, as well as the radio properties are indicative of the presence of a massive black hole in the centre of NGC 613.

NGC 1792
NGC 1792 is located in the southern constellation Columba (The Dove) – almost on the border with the constellation Caelum (The Graving Tool) – and is a so-called starburst spiral galaxy. Its optical appearance is quite chaotic, due to the patchy distribution of dust throughout the disc of this galaxy. It is very rich in neutral hydrogen gas – fuel for the formation of new stars – and is indeed rapidly forming such stars. The galaxy is characterized by unusually luminous far-infrared radiation; this is due to dust heated by young stars.

M 66 (NGC 3627)
The third galaxy is NGC 3627, also known as Messier 66, i.e. it is the 66th object in the famous catalogue of nebulae by French astronomer Charles Messier (1730 – 1817). It is located in the constellation Leo (The Lion).

NGC 3627 is a beautiful spiral with a well-developed central bulge. It also displays large-scale dust lanes. Many regions of warm hydrogen gas are seen throughout the disc of this galaxy. The latter regions are being ionised by radiation from clusters of newborn stars. Very active star-formation is most likely also occurring in the nuclear regions of NGC 3627.

The galaxy forms, together with its neighbours M 65 and NGC 3628, the so-called “Leo Triplet”; they are located at a distance of about 35 million light-years. M 66 is the largest of the three. Its spiral arms appear distorted and displaced above the main plane of the galaxy. The asymmetric appearance is most likely due to gravitational interaction with its neighbours.

Original Source: ESO News Release

Image credit: ESO

Although the Universe is currently a beige colour overall, it used to be more blue, according to astronomers with the European Southern Observatory. This was caused by the predominantly hot, young blue stars in the most distant galaxies – astronomers are seeing them when the Universe was only 2.5 billion years old. The astronomers worked out the distance and colour to 300 galaxies which were contained within the Hubble Deep Sky survey, which took a deep look at a region of sky in the southern constellation of Tuscanae.

An international team of astronomers [1] has determined the colour of the Universe when it was very young. While the Universe is now kind of beige, it was much bluer in the distant past, at a time when it was only 2,500 million years old.

This is the outcome of an extensive and thorough analysis of more than 300 galaxies seen within a small southern sky area, the so-called Hubble Deep Field South. The main goal of this advanced study was to understand how the stellar content of the Universe was assembled and has changed over time.

Dutch astronomer Marijn Franx, a team member from the Leiden Observatory (The Netherlands), explains: “The blue colour of the early Universe is caused by the predominantly blue light from young stars in the galaxies. The redder colour of the Universe today is caused by the relatively larger number of older, redder stars.”

The team leader, Gregory Rudnick from the Max-Planck Institut f?r Astrophysics (Garching, Germany) adds: “Since the total amount of light in the Universe in the past was about the same as today and a young blue star emits much more light than an old red star, there must have been significantly fewer stars in the young Universe than there is now. Our new findings imply that the majority of stars in the Universe were formed comparatively late, not so long before our Sun was born, at a moment when the Universe was around 7,000 million years old.”

These new results are based on unique data collected during more than 100 hours of observations with the ISAAC multi-mode instrument at ESO’s Very Large Telescope (VLT), as part of a major research project, the Faint InfraRed Extragalactic Survey (FIRES). The distances to the galaxies were estimated from their brightness in different optical near-infrared wavelength bands.

Observing the early Universe
It is now well known that the Sun was formed some 4.5 billion years ago. But when did most of the other stars in our home Galaxy form? And what about stars in other galaxies? These are some of the key questions in present-day astronomy, but they can only be answered by means of observations with the world’s largest telescopes.

One way to address these issues is to observe the very young Universe directly – by looking back in time. For this, astronomers take advantage of the fact that light emitted by very distant galaxies travels a long time before reaching us. Thus, when astronomers look at such remote objects, they see them as they appeared long ago.

Those remote galaxies are extremely faint, however, and these observations are therefore technically difficult. Another complication is that, due to the expansion of the Universe, light from those galaxies is shifted towards longer wavelengths [2], out of the optical wavelength range and into the infrared region.

In order to study those early galaxies in some detail, astronomers must therefore use the largest ground-based telescopes, collecting their faint light during very long exposures. In addition they must use infrared-sensitive detectors.

Telescopes as giant eyes
The “Hubble Deep Field South (HDF-S)” is a very small portion of the sky in the southern constellation Tucanae (“the Toucan”). It was selected for very detailed studies with the Hubble Space Telescope (HST) and other powerful telescopes. Optical images of this field obtained by the HST represent a total exposure time of 140 hours. Many ground-based telescopes have also obtained images and spectra of objects in this sky area, in particular the ESO telescopes in Chile.

A sky area of 2.5 x 2.5 arcmin2 in the direction of HDF-S was observed in the context of a thorough study (the Faint InfraRed Extragalactic Survey; FIRES, see ESO PR 23/02). It is slightly larger than the field covered by the WFPC2 camera on the HST, but still 100 times smaller than the area subtended by the full moon.

Whenever this field was visible from the ESO Paranal Observatory and the atmospheric conditions were optimal, ESO astronomers pointed the 8.2-m VLT ANTU telescope in this direction, taking near-infrared images with the ISAAC multi-mode instrument. Altogether, the field was observed for more than 100 hours and the resulting images (see ESO PR 23/02), are the deepest ground-based views in the near-infrared Js- and H-bands. The Ks-band image is the deepest ever obtained of any sky field in this spectral band, whether from the ground or from space.

These unique data provide an exceptional view and have now allowed unprecedented studies of the galaxy population in the young Universe. Indeed, because of the exceptional seeing conditions at Paranal, the data obtained with the VLT have an excellent image sharpness (a “seeing” of 0.48 arcsec) and can be combined with the HST optical data with almost no loss of quality.

A bluer colour
The astronomers were able to detect unambiguously about 300 galaxies on these images. For each of them, they measured the distance by determining the redshift [2]. This was done by means of a newly improved method that is based on the comparison of the brightness of each object in all the individual spectral bands with that of a set of nearby galaxies.

In this way, galaxies were found in the field with redshifts as high as z = 3.2, corresponding to distances around 11,500 million light-years. In other words, the astronomers were seeing the light of these very remote galaxies as they were when the Universe was only about 2.2 billion year old.

The astronomers next determined the amount of light emitted by each galaxy in such a way that the effects of the redshift were “removed”. That is, they measured the amount of light at different wavelengths (colours) as it would have been recorded by an observer near that galaxy. This, of course, only refers to the light from stars that are not heavily obscured by dust.

Summing up the light emitted at different wavelengths by all galaxies at a given cosmic epoch, the astronomers could then also determine the average colour of the Universe (the “cosmic colour”) at that epoch. Moreover, they were able to measure how that colour has changed, as the Universe became older.

They conclude that the cosmic colour is getting redder with time. In particular, it was much bluer in the past; now, at the age of nearly 14,000 million years, the Universe has a kind of beige colour.

When did stars form ?
The change of the cosmic colour with time may be interesting in itself, but it is also an essential tool for determining how rapidly stars were assembled in the Universe.

Indeed, while the star-formation in individual galaxies may have complicated histories, sometimes accelerating into true “star-bursts”, the new observations – now based on many galaxies – show that the “average history” of star-formation in the Universe is much simpler. This is evident by the observed, smooth change of the cosmic colour as the Universe became older.

Using the cosmic colour the astronomers were also able to determine how the mean age of relatively unobscured stars in the Universe changed with time. Since the Universe was much bluer in the past than it is now, they concluded that the Universe is not producing as many blue (high mass, short-lived) stars now as it was earlier, while at the same time the red (low mass, long-lived) stars from earlier generations of star formation are still present. Blue, massive stars die more quickly than red, low-mass stars, and therefore as the age of a group of stars increases, the blue short-lived stars die and the average colour of the group becomes redder. So did the Universe as a whole.

This behaviour bears some resemblance with the ageing trend in modern Western countries where less babies are born than in the past and people live longer than in the past, with the total effect that the mean age of the population is rising.

The astronomers determined how many stars had already formed when the Universe was only about 3,000 million years old. Young stars (of blue colour) emit more light than older (redder) stars. However, since there was just about as much light in the young Universe as there is today – although the galaxies are now much redder – this implies that there were fewer stars in the early Universe than today. The present study inidcates that there were ten times fewer stars at that early time than there is now.

Finally, the astronomers found that roughly half of the stars in the observed galaxies have been formed after the time when the Universe was about half as old (7,000 million years after the Big Bang) as it is today (14,000 million years).

Although this result was derived from a study of a very small sky field, and therefore may not be completely representative of the Universe as a whole, the present result has been shown to hold in other sky fields.

Original Source: ESO News Release