* Astronomy

Members Login
Post Info
TOPIC: Distant Galaxies


L

Posts: 131433
Date:
Ultra-Compact Dwarf Galaxies
Permalink  
 


Multi-colour Imaging of Ultra-compact Objects in the Fornax Cluster
Authors: A. M. Karick, M. D. Gregg, M. J. Drinkwater, M. Hilker, P. Firth

Ultra-compact dwarf galaxies (UCDs) are a new type of galaxy we have discovered in the central region of the Fornax and Virgo clusters. Unresolved in ground-based imaging, UCDs have spectra typical of old stellar systems.
Ninety-two have been found in Fornax, making them the most numerous galaxy type in the cluster. Here we present multicolour (u'g'r'i'z') imaging of the central region of the Fornax Cluster using the CTIO 4m Mosaic Telescope.
The colour-magnitude relation for bright UCDs is qualitatively consistent with UCDs being the stripped nuclei of dE,Ns. However at faint magnitudes, GCs and UCDs cannot be distinguished by colour alone. High resolution spectroscopy to measure their internal velocity dispersions and metallicities, is needed to distinguish between GCs and UCDs.

Read more (PDF)

__________________


L

Posts: 131433
Date:
RE: Distant Galaxies
Permalink  
 


A team of astronomers led by Nikhil Padmanabhan and David Schlegel has published the largest three-dimensional map of the universe ever constructed, a wedge-shaped slice of the cosmos that spans a tenth of the northern sky, encompasses 600,000 uniquely luminous red galaxies, and extends 5.6 billion light-years deep into space, equivalent to 40 percent of the way back in time to the Big Bang.

Read more

__________________


L

Posts: 131433
Date:
Distant Elements
Permalink  
 


Deep observations of two X-ray bright clusters of galaxies with ESA's XMM-Newton satellite allowed a group of international astronomers to measure their chemical composition with an unprecedented accuracy. Knowing the chemical composition of galaxy clusters is of crucial importance to understanding the origin of chemical elements in the Universe.

Clusters, or conglomerates, of galaxies are the largest objects in the Universe. By looking at them through optical telescopes it is possible to see hundreds or even thousands of galaxies occupying a volume a few million light years across. However, such telescopes only reveal the tip of the iceberg. In fact most of the atoms in galaxy clusters are in the form of hot gas emitting X-ray radiation, with the mass of the hot gas five times larger than the mass in the cluster's galaxies themselves.
Most of the chemical elements produced in the stars of galaxy clusters - expelled into the surrounding space by supernova explosions and by stellar winds - become part of the hot X-ray emitting gas. Astronomers divide supernovae into two basic types: 'core collapse' and 'Type Ia' supernovae. The 'core collapse' supernovae originate when a star at the end of its life collapses into a neutron star or a black hole. These supernovae produce lots of oxygen, neon and magnesium. The Type Ia supernovae explode when a white dwarf star consuming matter from a companion star becomes too massive and completely disintegrates. This type produces lots of iron and nickel.


These X-ray images of the clusters of galaxies ‘Sersic 159-03’(right) and ‘2A 0335+096’ (left) were taken by the European Photon Imaging Camera (EPIC) on-board ESA’s XMM-Newton, in November 2002 and August 2003 respectively.
Credit: ESA and the XMM-Newton EPIC consortium


Respectively in November 2002 and August 2003, and for one and a half day each time, XMM-Newton's made deep observations of the two galaxy clusters called 'Sersic 159-03' and '2A 0335+096'. Thanks to these data the astronomers could determine the abundances of nine chemical elements in the clusters 'plasma' – a gas containing charged particles such as ions and electrons.
These elements include oxygen, iron, neon, magnesium, silicon, argon, calcium, nickel, and - detected for the first time ever in a galaxy cluster - chromium.

"Comparing the abundances of the detected elements to the yields of supernovae calculated theoretically, we found that about 30 percent of the supernovae in these clusters were exploding white dwarfs ('Type Ia') and the rest were collapsing stars at the end of their lives ('core collapse'). This number is in between the value found for our own Galaxy (where Type Ia supernovae represent about 13 percent of the supernovae 'population') and the current frequency of supernovae events as determined by the Lick Observatory Supernova Search project (according to which about 42 percent of all observed supernovae are Type Ia)" - Norbert Werner, from the SRON Netherlands Institute for Space Research (Utrecht, Netherlands) and one of the lead authors of these results.

The astronomers also found that all supernova models predict much less calcium than what is observed in clusters and that the observed nickel abundance cannot be reproduced by these models. These discrepancies indicate that that the details of supernova enrichment is not yet clearly understood. Since clusters of galaxies are believed to be fair samples of the Universe, their X-ray spectroscopy can help to improve the supernova models.

The spatial distribution of elements across a cluster also holds information about the history of clusters themselves. The distribution of elements in 2A 0335+096 indicates an ongoing merger. The distribution of oxygen and iron across Sersic 159-03 indicates that while most of the enrichment by the core collapse supernovae happened long time ago, Type Ia supernovae still continue to enrich the hot gas by heavy elements especially in the core of the cluster.

Source

__________________


L

Posts: 131433
Date:
RX J1416.4+2315
Permalink  
 


Taking advantage of the high sensitivity of ESA's XMM-Newton and the sharp vision of NASA's Chandra X-Ray space observatories, astronomers have studied the behaviour of massive fossil galaxy clusters, trying to find out how they find the time to form…

Many galaxies reside in galaxy groups, where they experience close encounters with their neighbours and interact gravitationally with the dark matter - mass which permeates the whole intergalactic space but is not directly visible because it doesn’t emit radiation.
These interactions cause large galaxies to spiral slowly towards the centre of the group, where they can merge to form a single giant central galaxy, which progressively swallows all its neighbours.
If this process runs to completion, and no new galaxies fall into the group, then the result is an object dubbed a 'fossil group', in which almost all the stars are collected into a single giant galaxy, which sits at the centre of a group-sized dark matter halo. The presence of this halo can be inferred from the presence of extensive hot gas, which fills the gravitational potential wells of many groups and emits X-rays.
A group of international astronomers studied in detail the physical features of the most massive and hot known fossil group, with the main aim to solve a puzzle and understand the formation of massive fossils. In fact, according to simple theoretical models, they simply could not have formed in the time available to them!


Expand (130kb, 854 x 794)

The fossil group investigated, called 'RX J1416.4+2315', is dominated by a single elliptical galaxy located one and a half thousand million light years away from us, and it is 500 thousand million times more luminous than the Sun.
The XMM-Newton and Chandra X-ray observations, combined with optical and infrared analyses, revealed that group sits within a hot gas halo extending over three million light years and heated to a temperature of 50 million degrees, mainly due to shock heating as a result of gravitational collapse.

Such a high temperature, about as twice as the previously estimated values, is usually characteristic of galaxy clusters. Another interesting feature of the whole cluster system is its large mass, reaching over 300 trillion solar masses. Only about two percent of it in the form of stars in galaxies, and 15 percent in the form of hot gas emitting X-rays. The major contributor to the mass of the system is the invisible dark matter, which gravitationally binds the other components.
According to calculations, a fossil cluster as massive as RX J1416.4+2315 would have not had the time to form during the whole age of the universe. The key process in the formation of such fossil groups is the process known as 'dynamical friction', whereby a large galaxy loses its orbital energy to the surrounding dark matter. This process is less effective when galaxies are moving more quickly, which they do in massive 'clusters' of galaxies.
This, in principle, sets an upper limit to the size and mass of fossil groups. The exact limits are, however, still unknown since the geometry and mass distribution of groups may differ from that assumed in simple theoretical models.

"Simple models to describe the dynamical friction assume that the merging galaxies move along circular orbits around the centre of the cluster mass. Instead, if we assume that galaxies fall towards the centre of the developing cluster in an asymmetric way, such as along a filament, the dynamic friction and so the cluster formation process may occur in a shorter time scale. Such a hypothesis is supported by the highly elongated X-ray emission we observed in RX J1416.4+2315, to sustain the idea of a collapse along a dominant filament" - Habib Khosroshahi, University of Birmingham (UK), first author of the results.

The optical brightness of the central dominant galaxy in this fossil is similar to that of brightest galaxies in large clusters (called 'BCGs'). According to the astronomers, this implies that such galaxies could have originated in fossil groups around which the cluster builds up later. This offers an alternative mechanism for the formation of BCGs compared to the existing scenarios in which BCGs form within clusters during or after the cluster collapse.

"The study of massive fossil groups such as RX J1416.4+2315 is important to test our understanding of the formation of structure in the universe. Cosmological simulations are underway which attempt to reproduce the properties we observe, in order to understand how these extreme systems develop" - Habib Khosroshahi.


__________________


L

Posts: 131433
Date:
RE: Distant Galaxies
Permalink  
 


The evolution of galaxies from primeval irregulars to present-day ellipticals

Ultra-high resolution super computer simulations have succeeded in revealing the future of the earliest and most distant galaxies known. The Subaru telescope and other recent advances in technology have led to the discovery of a large number of galaxies that existed when the universe was less than a quarter of its current age.


Expand (263kb, 900 x 655)

Dr. Masao Mori (University of California, Los Angeles and Senshu University) and Dr. Masayuki Umemura (University of Tsukuba) calculated in detail how the life-cycle of stars and mergers of these early galaxies lead to the formation of the galaxies in the universe today. This research was published in the March 30, 2006, edition of Nature.

Source

__________________


L

Posts: 131433
Date:
Abell 1689
Permalink  
 


A Gemini/HST survey reveals building-block process in evolution of massive galaxy clusters

A study of the Universe’s most massive galaxy clusters has shown that mergers play a vital role in their evolution.

Astronomers at Oxford University and the Gemini Observatory used a combination of data from the twin Gemini Telescopes, located in Hawaii and Chile, and the Hubble Space Telescope (HST) to study populations of stars in the Universe’s most massive galaxy clusters over a range of epochs – the earliest being half the age of the Universe. The HST images were used to map the light distribution of the galaxies in the cluster. Data from the Gemini Multi-Object Spectrograph allowed the team to analyse the light from galaxies to determine their masses, ages and chemical compositions.

"We still don’t have a clear picture of how galaxies develop over the history of the Universe. The strength of this study is that we are able to look at galaxy clusters over a range of epochs" - Dr Jordi Barr of Oxford University, who is presenting some of the first results of the Gemini/HST Galaxy Cluster Project at the RAS National Astronomy Meeting on 5th April.

Galaxy clusters contain the most massive galaxies in the Universe. Until recently, astronomers believed that all galaxies in the centres of clusters formed rapidly and then aged without any further changes to their structure in a process known as “Passive Evolution”. Results from the Gemini/HST Galaxy Cluster Project now show that this cannot be the case.

"When we’re looking at the most distant galaxy clusters, we are looking back in time to clusters that are in early stages of their formation. The young galaxies in distant clusters appear to be very different from those in the mature clusters that we see in the local Universe. We found the earliest galaxy clusters have a huge variation in the abundances of elements such as oxygen and magnesium, whereas the chemistry of galaxies in the sample of closer clusters appears to be much more homogenous. This difference in chemistry proves that the clusters must actively change over time. If the galaxies in the old clusters have acquired a complete ‘set’ of elements, it’s most likely that they have formed from the mergers of several young galaxies" - Dr Jordi Barr.


Hubble Space Telescope image of Abell 1689, one of the galaxy clusters used in the sample for the study.
Image credit: NASA


The group found that the star-formation in galaxies is dependent on mass and that in lower mass galaxies star-formation continues for longer. The most massive galaxies in clusters appear to have formed all their stars by the time the universe is just over a billion years old, whereas the lower mass galaxies finish forming their stars some 4 billion years later.

"We see the effects of star-formation in low mass galaxies but are unsure about why it’s happening. It’s possible that star-formation can be shut down very rapidly in dense environments and that the lower mass galaxies are recent arrivals that are forming stars over a longer period outside the cluster, then falling in. But we are still speculating..." - Dr Jordi Barr.

The group’s observations of merging galaxy clusters showed that a large proportion of the galaxies in those clusters have undergone recent bursts of star formation. This indicates that star formation may be triggered if galaxies are thrown, during the course of a merger, into contact with the gaseous medium pervading the cluster.

Future observations are planned at X-ray wavelengths to study the interactions between galaxies and the distribution and temperature of the surrounding gas.

Read more

__________________


L

Posts: 131433
Date:
Abell 2218
Permalink  
 


Title: Spitzer Massive Lensing Cluster Survey
Authors: E. Egami, G. H. Rieke, J. R. Rigby, C. Papovich, J.-P. Kneib, G. P. Smith, E. Le Floc'h, K. A. Misselt, P. G. Perez-Gonzalez, J.-S. Huang, H. Dole, D. T. Frayer

Astronomers are currently undertaking a Spitzer GTO program to image ~30 massive lensing clusters at moderate redshift with both IRAC and MIPS. By taking advantage of the gravitatinoal lensing power of these clusters, they will study the population of faint galaxies that are below the nominal Spitzer detection limits.
Here, they present a few examples of their science programs.


Spitzer images of Abell 2218:
Left – IRAC 4.5 µm image. Galaxies with spectroscopic redshifts are marked with circles.
Right – MIPS 24 µm image. The spectroscopic redshifts are listed. The triply lensed images of the z ~ 2.5 submillimeter galaxy are marked with circles. In both images, the position of the cD galaxy is marked with a cross.


Read more (PDF)

__________________


L

Posts: 131433
Date:
ISCS J143809+341419
Permalink  
 


Astronomers using the Spitzer Space Telescope have conducted a cosmic safari to seek out a rare galactic species. Their specimens -- clusters of galaxies in the very distant universe -- are few and far between, and have hardly ever been detected beyond a distance of 7 billion light-years from Earth.

To find the clusters, the team carefully sifted through Spitzer infrared pictures and ground-based catalogues; estimated rough distances based on the cluster galaxies' colours; and verified suspicions using a spectrograph instrument at the W.M. Keck Observatory in Hawaii.
Ultimately, the expedition resulted in quite a galactic catch -- the most distant galaxy cluster ever seen, located 9 billion light-years away. This means the cluster lived in an era when the universe was a mere 4.5 billion years old. The universe is believed to be 13.7 billion years old.

"Detecting a galaxy cluster 9 billion light-years away is very exciting. It's really amazing that Spitzer's 85-centimeter telescope can see 9 billion years back in time" - Dr. Peter Eisenhardt of NASA's Jet Propulsion Laboratory, the study's lead investigator.


Expand (316kb, 800x900)
ISCS J143809+341419
Position (J2000): RA: 14:38:09 Dec: +34:14:19
Some of the oldest galaxy clusters in the universe pose for Spitzer's Infrared Array Camera. The green blobs are Milky Way stars along the line of sight.
Credit: NASA/JPL-Caltech/S.A. Stanford (UC Davis/LLNL)


Using the same methods, the astronomers also found three other clusters living between 7 and 9 billion light-years away.

"Spitzer is an excellent instrument for detecting very distant galaxy clusters because they stand out so brightly in the infrared. You can think of these distant galaxy cluster surveys as a game of 'Where's Waldo?' With an optical telescope you can spot 'Waldo,' or the distant galaxy clusters, by carefully searching for them amongst a sea of faint galaxies. But in the Spitzer data, it's as though Waldo is wearing a bright neon hat and can be easily picked out of the crowd" - Dr. Mark Brodwin, co-investigator, also of JPL.

Galaxy clusters are the largest gravitationally bound structures in the universe. A typical cluster can contain thousands of galaxies and trillions of stars. Because of their huge size and mass, they are relatively rare. For example, if Earth were to represent the entire universe, then countries would be the equivalent of galaxies, and continents would be the galaxy clusters.
Galaxy clusters grow like snowballs, picking up new galaxies from gravitational interactions over billions of years. For this reason, team members say these behemoths should be even rarer in the very distant universe.

"The ultimate goal of this research is to find out when the galaxies in this and other distant clusters formed" - Dr. Adam Stanford, University of California at Davis, co-investigator.

Stanford is the lead author of a paper on the most distant galaxy cluster's discovery, which was published in the December 2005 issue of Astrophysical Journal Letters.
This is the second time Eisenhardt and Stanford have broken the record for capturing the most distant galaxy cluster. Both say they accidentally broke the record in 1997 when they detected a cluster located 8.7 billion light-years away. The discovery was made by a deep survey of a 0.03-degree patch of sky, or an area significantly smaller than a pea held out at arms length, for 30 nights at the Kitt Peak National Observatory in Arizona.

"We were lucky in 1997 because we weren't looking for galaxy clusters and found the most distant one ever detected in a very small patch of sky. Because galaxy clusters are so massive and rare, you typically need to deeply survey a large area of sky to find them" - Dr. Adam Stanford.

"With Spitzer's great infrared sensitivity we surveyed more deeply in 90 seconds than we could in hours of exposure in the 1997 observations, and we used this advantage to survey a region 300 times larger" - Dr. Peter Eisenhardt.

The 9 billion-year-old cluster is just one of 25 the team captured on their Spitzer safari. They are currently preparing for more observations this spring at the W.M. Keck Observatory to confirm the distance of additional galaxy clusters from their sample. According to Eisenhardt, some of the clusters awaiting confirmation may be even more distant than the current record holder.

Read more

__________________


L

Posts: 131433
Date:
GIRAFFE
Permalink  
 


GIRAFFE at VLT reveals the turbulent life of distant galaxies

Studying several tens of distant galaxies, an international team of astronomers found that galaxies had the same amount of dark matter relative to stars 6 billion years ago as they have now. If confirmed, this suggests a much closer interplay between dark and normal matter than previously believed.
The scientists also found that as many as 4 out of 10 galaxies are out of balance. These results shed a new light on how galaxies form and evolve since the Universe was only half its current age.

"This may imply that collisions and merging are important in the formation and evolution of galaxies" - François Hammer, Paris Observatory, France, and one of the leaders of the team.

The scientists were interested in finding out how galaxies that are far away - thus seen as they were when the Universe was younger - evolved into the ones nearby. In particular, they wanted to study the importance of dark matter in galaxies.

"Dark matter, which composes about 25% of the Universe, is a simple word to describe something we really don't understand. From looking at how galaxy rotates, we know that dark matter must be present, as otherwise these gigantic structures would just dissolve" - Hector Flores, co-leader.

In nearby galaxies, and in our own Milky Way for that matter, astronomers have found that there exist a relation between the amount of dark matter and ordinary stars: for every kilogram of material within a star there is roughly 30 kilograms of dark matter. But does this relation between dark and ordinary matter still hold in the Universe's past?
This required measuring the velocity in different parts of distant galaxies, a rather tricky experiment: previous measurements were indeed unable to probe these galaxies in sufficient details, since they had to select a single slit, i.e. a single direction, across the galaxy.
Things changed with the availability of the multi-object GIRAFFE spectrograph, now installed on the 8.2-m Kueyen Unit Telescope of ESO's Very Large Telescope (VLT) at the Paranal Observatory (Chile).

In one mode, known as "3-D spectroscopy" or "integrated field", this instrument can obtain simultaneous spectra of smaller areas of extended objects like galaxies or nebulae. For this, 15 deployable fibre bundles, the so-called Integral Field Units (IFUs) , cf. ESO PR 01/02 , are used to make meticulous measurements of distant galaxies. Each IFU is a microscopic, state-of-the-art two-dimensional lens array with an aperture of 3 x 2 arcsec2 on the sky. It is like an insect's eye, with twenty micro-lenses coupled with optical fibres leading the light recorded at each point in the field to the entry slit of the spectrograph.

"GIRAFFE on ESO's VLT is the only instrument in the world that is able to analyse simultaneously the light coming from 15 galaxies covering a field of view almost as large as the full moon. Every galaxy observed in this mode is split into continuous smaller areas where spectra are obtained at the same time" - Mathieu Puech, lead author of one the papers presenting the results


Expand (351kb, 1106 x 800 )
Three examples of results obtained with GIRAFFE on distant galaxies. In the first column, images of galaxies as obtained with the Hubble Space Telescope are shown. The second column is the velocity field as deduced from GIRAFFE observations: the reddish parts show material moving away from us with respect to the mean velocity of the galaxy, while the blue parts are moving towards us. The scale in km/s is shown on the right. The last column is a map of electron density per cubic centimetre. The first object corresponds to a spiral galaxy forming star at a frantic rate of 100 solar masses per year. The electron density map allows the astronomers to localise the region of star formation as the black region on the left. The second object is a galaxy which is clearly "out of balance" and therefore shows a very perturbed velocity field. The third object appears to show an outflow - matter being ejected perpendicular to the plane of the galaxy.

The astronomers used GIRAFFE to measure the velocity fields of several tens of distant galaxies, leading to the surprising discovery that as much as 40% of distant galaxies were "out of balance" - their internal motions were very disturbed - a possible sign that they are still showing the aftermath of collisions between galaxies.
When they limited themselves to only those galaxies that have apparently reached their equilibrium, the scientists found that the relation between the dark matter and the stellar content did not appear to have evolved during the last 6 billions years.

Thanks to its exquisite spectral resolution, GIRAFFE also allows for the first time to study the distribution of gas as a function of its density in such distant galaxies. The most spectacular results reveal a possible outflow of gas and energy driven by the intense star-formation within the galaxy and a giant region of very hot gas (HII region) in a galaxy in equilibrium that produces many stars.



Expand (199kb, 800 x 958)
Relation between the total mass of the halo of distant galaxies (thus including dark matter) and the mass contained in stars. Red triangles, green squares and blue dots represent complex kinematics, perturbed rotations and rotating discs, respectively. Full and dotted lines represent the relation valid for galaxies in the local Universe. This plot shows that all the scatter of the relation is caused by interloper galaxies with kinematics classified either as complex or perturbed. Considering only rotating discs, i.e. galaxies that may have reached equilibrium, the relationship at moderate redshift is similar (slope, zero point and scatter) to that in the local universe.

"Such a technique can be expanded to obtain maps of many physical and chemical characteristics of distant galaxies, enabling us to study in detail how they assembled their mass during their entire life. In many respects, GIRAFFE and its multi-integral field mode gives us a first flavour of what will be achieved with future extremely large telescopes" - François Hammer.


The GIRAFFE instrument allows obtaining high-quality spectra of a large variety of celestial objects, from individual stars in the Milky Way and other nearby galaxies, to very distant galaxies. It functions by means of multiple optical fibres that guide the light from the telescope's focal plane into the entry slit of the spectrograph. Here the light is dispersed into its different colours. GIRAFFE and these fibres are an integral part of the advanced Fibre Large Array Multi-Element Spectrograph (FLAMES) facility which also includes the OzPoz positioner and an optical field corrector. It is the outcome of a collaboration between ESO, Observatoire de Paris-Meudon, Observatoire de Genève-Lausanne and the Anglo Australian Observatory (AAO) . The principle of this instrument involves the positioning in the telescope's focal plane of a large number of optical fibres. This is done in such a way that each of them guides the light from one particular celestial object towards the spectrograph that records the spectra of all these objects simultaneously. The size of the available field-of-view is no less than about 25 arcmin across, i.e. almost as large as the full moon. The individual fibres are moved and positioned "on the objects" in the field by means of the OzPoz positioner.

Source ESO

__________________


L

Posts: 131433
Date:
Lyman-break technique
Permalink  
 


A team of astronomers from France, the USA, Japan, and Korea, led by Denis Burgarella has recently discovered new galaxies in the Early Universe. They have been detected for the first time both in the near-UV and in the far-infrared wavelengths. Their findings will be reported in a coming issue of Astronomy & Astrophysics. This discovery is a new step in understanding how galaxies evolve.

The astronomer Denis Burgarella (Observatoire Astronomique Marseille Provence, Laboratoire d’Astrophysique de Marseille, France) and his colleagues from France, the USA, Japan, and Korea, have recently announced their discovery of new galaxies in the Early Universe both for the first time in the near-UV and in the far-infrared wavelengths. This discovery leads to the first thorough investigation of early galaxies. Figure 1 shows some of these new galaxies. The discovery will be reported in a coming issue of Astronomy & Astrophysics.

The knowledge of early galaxies has made major progress in the past ten years. From the end of 1995, astronomers have been using a new technique, known as the “Lyman-break technique”. This technique allows very distant galaxies to be detected. They are seen as they were when the Universe was much younger, thus providing clues to how galaxies formed and evolved. The Lyman-break technique has moved the frontier of distant galaxy surveys further up to redshift z=6-7 (that is about 5% of the present age of the Universe). In astronomy, the redshift denotes the shift of a light wave from a galaxy moving away from the Earth. The light wave is shifted toward longer wavelengths, that is, toward the red end of the spectrum. The higher the redshift of a galaxy is, the farther it is from us.

The Lyman-break technique is based on the characteristic “disappearance” of distant galaxies observed in the far-UV wavelengths. As light from a distant galaxy is almost fully absorbed by hydrogen at 0.912 nm (due to the absorption lines of hydrogen, discovered by the physicist Theodore Lyman), the galaxy “disappears” in the far-ultraviolet filter. Figure 2 illustrates the “disappearance” of the galaxy in the far-UV filter. The Lyman discontinuity should theoretically occur at 0.912 nm. Photons at shorter wavelengths are absorbed by hydrogen around stars or within the observed galaxies.
For high-redshift galaxies, the Lyman discontinuity is redshifted so that it occurs at a longer wavelength and can be observed from the Earth. From ground-based observations, astronomers can currently detect galaxies with a redshift range of z~3 to z~6. However, once detected, it is still very difficult to obtain additional information on these galaxies because they are very faint.

Source

__________________
«First  <  117 18 19 20  >  Last»  | Page of 20  sorted by
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard