"GALEX - Galaxy Evolution Explorer (II)" 
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|  GALEX has a 50-centimeter-diameter telescope |
| [30.4.2003] Die Bahn des Galaxy Evolution Explorers der NASA (in 690-km Höhe) ist um 29° gegen den Äquator geneigt. Der kleine Satellit hat ein nur 50 cm großes Teleskop, aber das genügt: Mit seinen Detektoren für das nahe und ferne UV-Licht wird er sich besonders hellen Himmelsobjekten zuwenden, jungen Sternen nämlich - dies aber in Sternbildungsregionen in Millionen Galaxien. Mehrere Himmelsdurchmusterungen sind geplant, darunter einige, die es so noch nie gegeben hat. "Diese Mission wird uns die erste umfassende Karte eines Universums aus Galaxien liefern, die noch 'im Bau' sind," sagt GALEX Principal Investigator Christopher Martin vom Caltech: Die Entfernungen und Sternbildungsraten von gut einer Million Galaxien sollen bestimmt werden, um auf diese Weise die Geschichte der Sternentstehung und auch der Bildung der schweren chemischen Elemente besser zu verstehen. Insbesondere sollen die Daten von GALEX mit vielen bestehenden Himmelsdurchmusterungen (etwa im Infraroten durch den Satelliten IRAS) kombiniert werden - und wenn der Satellit auf ein besonders faszinierendes Einzelobjekt stößt, dann kann Hubble es genauer unter die Lupe nehmen. | |
Galaxy Evolution Explorer
Field of View
The Galaxy Evolution Explorer specializes in surveying galaxies in ultraviolet light. Its telescope, 50 centimeters in diameter, has a field of view that is much wider than most ground-based and space-based telescopes. This field of view, nearly three times the diameter of the Moon, allowed the Galaxy Evolution Explorer to discover seemingly newborn galaxies in our local universe. Image credit: NASA/JPL-Caltech/NOAO |
GALEX sieht seltene, extrem UV-helle Galaxien
Die große ultraviolette Himmelsdurchmusterung mit dem kleinen NASA-Satelliten GALEX hat eine bislang übersehene Galaxienklasse aufgespürt, deren Rolle in der Evolution des Kosmos zwar noch nicht ganz klar ist, die aber nun Gegenstand intensiver Nachbeobachtungen werden dürfte. Diese drei dutzend Galaxien - nur etwa jede 3000. in der weiteren Umgebung der Milchstraße gehört dazu - haben eine zehn-mal höhere ultraviolette Leuchtkraft als die Milchstraße, sind aber zugleich mit rund 10'000 Lichtjahren Durchmesser deutlich kompakter. In 2 bis 4 Lichtjahren Entfernung sollten die »blue blobs« aber für größere Teleskope noch aufzulösen und im Detail zu untersuchen sein - was ausgesprochend lohnend sein sollte: Nach allen bisher vorliegenden Daten entsprechen sie praktisch in jedem Detail den allerersten großen Galaxien des Universums, die vor rund 10 Mrd. Jahren aufleuchteten (und natürlich viel schwerer zu beobachten sind).
Leuchtkraft im kurzwelligen Licht, Durchmesser, Massen etc. stimmen mit den Urgalaxien überein, allerdings fehlt noch der endgültige Beweis, daß es sich bei den GALEX-Galaxien wirklich um junge Sternsysteme handelt, die erst vor 100 Mio. bis 1 Mrd. Jahren entstanden sind: Man muß sie sich auch bei langen Wellen ganz genau anschauen um feststellen zu können, ob es da vielleicht noch viel ältere Sternpopulationen gibt, die der UV-Glanz völlig überstrahlt. Auf jeden Fall aber erleben die GALEX-Galaxien in der kosmischen »Gegenwart« außergewöhnlich heftige Episoden von Sternentstehung, denn dies ist die offensichtliche Erklärung ihrer enormen UV-Leuchtkraft. Im heute populären Bild der hierarchischen Galaxienentstehung waren es solcherlei Galaxien, aus denen später durch Verschmelzungen die heutigen Milchstraßen entstanden. Und auch wenn die GALEX-Funde Raritäten sind: Im Prinzip könnten diese Prozesse auch heute noch ablaufen.
GALEX survey turns up strange new class of nearby UV-bright galaxies, resemble those 10 Gyr ago.
The Galaxy Evolution Explorer was launched on April 28, 2003.
Its mission is to study the shape, brightness, size and distance of galaxies across 10 billion years of cosmic history. The Explorer's 50-centimeter-diameter telescope sweeps the skies in search of ultraviolet-light sources.
Ultraviolet is light from the higher end of the electromagnetic spectrum, just above visible light in frequency, but below X-rays and gamma rays. While a small amount of ultraviolet penetrates Earth's atmosphere, causing sunburn, the Galaxy Evolution Explorer observes those ultraviolet frequencies that can only be seen from space.
The Galaxy Evolution Explorer mission is led by the California Institute of Technology, Pasadena, Calif., which is also responsible for science operations and data analysis. NASA's Jet Propulsion Laboratory, Pasadena, Calif., a division of Caltech, manages the mission and built the science instrument. The mission was developed under NASA's Explorers Program, managed by the Goddard Space Flight Center, Greenbelt, Md. South Korea and France are the international partners in the mission.
The telescope surveyed thousands of galaxies before finding three-dozen of these newborns.
It's the first major discovery of the Galaxy Evolution Explorer satellite: a new class of rare by nearby galaxies with a high UV flux. In all aspects they resemble modern large galaxies (like ours) as they looked like 10 billion years ago - but the new galaxies are relatively close to us, ranging from two to four billion light-years away; they may be as young as 100 million to one billion years old. The discovery suggests our aging universe is still alive with youth.
It also offers astronomers their first, close-up glimpse at what our galaxy probably looked like when it was in its infancy. Now we can study the ancestors to galaxies much like our Milky Way in much more detail than ever before: It's like finding a living fossil in your own backyard. The new discoveries are called ultraviolet luminous galaxies: They were discovered after the Galaxy Evolution Explorer scanned a large portion of the sky with its highly sensitive ultraviolet light detectors. Since young stars pack most of their light into ultraviolet wavelengths, young galaxies appear to the spacecraft like diamonds in a field of stones.
Astronomers mined for these rare gems before, but missed them; since they weren't able examine a large enough slice of the sky. GALEX surveyed thousands of galaxies before finding these few dozen ultraviolet-bright ones. They are about 10 times as bright in ultraviolet wavelengths as the Milky Way, indicating that they are teeming with violent star-forming regions and exploding supernova, which are characteristics of youth. K1.1
Aging Universe may Still be Spawning Massive Galaxies
NASA's Galaxy Evolution Explorer has spotted what appear to be massive "baby" galaxies in our corner of the universe. Previously, astronomers thought the universe's birth rate had dramatically declined and only small galaxies were forming.
"We knew there were really massive young galaxies eons ago, but we thought they had all matured into older ones more like our Milky Way. If these galaxies are indeed newly formed, then this implies parts of the universe are still hotbeds of galaxy birth," said Chris Martin. He is principal investigator for the Galaxy Evolution Explorer, and co-author of the study.
Martin and colleagues, led by Dr. Tim Heckman of Johns Hopkins University, Baltimore, Md., unearthed three-dozen bright, compact galaxies that greatly resemble the youthful galaxies of more than 10 billions years ago. These new galaxies are relatively close to us, ranging from two to four billion light-years away. They may be as young as 100 million to one billion years old. The Milky Way is approximately 10 billion years old.
This artist's conception illustrates the decline in our universe's "birth-rate" over time. When the
universe was young, massive galaxies were forming regularly, like
baby bees in a bustling hive. In time, the universe bore fewer and
fewer "offspring," and newborn galaxies (white circles) matured into
older ones more like our own Milky Way (spirals).
Image credit: NASA/JPL-Caltech/NOAO |
"Now we can study the ancestors to galaxies much like our Milky Way in much more detail than ever before," Heckman said. "It's like finding a living fossil in your own backyard. We thought this type of galaxy had gone extinct, but in fact newborn galaxies are alive and well in the universe," he added.
The new discoveries are of a type called ultraviolet luminous galaxies. They were discovered after the Galaxy Evolution Explorer scanned a large portion of the sky with its highly sensitive ultraviolet light detectors. Since young stars pack most of their light into ultraviolet wavelengths, young galaxies appear to the spacecraft like diamonds in a field of stones. Astronomers mined for these rare gems before, but missed them because they weren't able to examine a large enough slice of the sky.
"The Galaxy Evolution Explorer surveyed thousands of galaxies before finding these few dozen ultraviolet-bright ones," said Dr. Michael Rich, a co-author of the study.
The newfound galaxies are about 10 times as bright in ultraviolet wavelengths as the Milky Way. This indicates they are teeming with violent star-forming regions and exploding supernova, which are characteristics of youth.
When our universe was young, massive galaxies were regularly bursting into existence. Over time, the universe bore fewer and fewer galactic progeny, and its newborn galaxies grew up into ones that look like our own. Until now, astronomers thought they had seen the last of these giant babies.
The results will be published in an upcoming special issue of Astrophysical Journal Letters, along with several other papers describing new results from the Galaxy Evolution Explorer.
K1.2
Baby Galaxies in the Adult Universe
Image credit: NASA/JPL-Caltech/NOAO
Baby Galaxies in the Adult UniverseSix of the three-dozen "ultraviolet luminous galaxies" "ultraviolet luminous galaxies" Clockwise beginning from the upper left, they are called: GALEX J232539.24+004507.1, GALEX J231812.98-004126.1, GALEX J015028.39+130858.5, GALEX J021348.52+125951.3, GALEX J143417.15+020742.5, GALEX J020354.02-092452.5. |
The discovery not only suggests that our universe may still be alive with youth, but also offers astronomers their first close-up look at what appear to be baby galaxies. Prior to the new result, astronomers had to peer about 11 billion light-years into the distant universe to see newborn galaxies. The newfound galaxies are only about 2 to 4 billion light-years away.
Nearby Newborns
This image shows six of the three-dozen "ultraviolet luminous galaxies" spotted in our corner of the universe by NASA's Galaxy Evolution Explorer. These massive galaxies greatly resemble newborn galaxies that were common in the early universe. The discovery came as a surprise, because astronomers had thought that the universe's "birth-rate" had declined, and that massive galaxies were no longer forming.
The galaxies, located in the center of each panel, were discovered after the Galaxy Evolution Explorer scanned a large portion of the sky with its highly sensitive ultraviolet-light detectors. Because young stars pack most of their light into ultraviolet wavelengths, young galaxies appear to the Galaxy Evolution Explorer like diamonds in a field of stones. Astronomers mined for these rare "gems" before, but missed them because they weren't able to examine a large enough slice of the sky. The Galaxy Evolution Explorer surveyed thousands of nearby galaxies before finding three-dozen newborns.
While still relatively close in astronomical terms, these galaxies are far enough away to appear small to the Galaxy Evolution Explorer.
K1.3
Lya-Emitting Galaxies at 0.2K2
M81: A Galaxy in Different Lights
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Image credit: NASA/JPL-Caltech/NOAO
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While visible-light images of galaxies reveal the distribution of stars, ultraviolet-light images highlight the most active, young stars. The ultraviolet image of Messier 81 shows that the galaxy's spiral arms are dotted with pockets of violent star-forming activity.
NASA's Galaxy Evolution Explorer is sweeping the entire sky in ultraviolet wavelengths, mapping galaxies and their star-formation rates across 10 billion years of cosmic time.
The visible-light image is from the National Optical Astronomy Observatory. The ultraviolet-light image was taken by the Galaxy Evolution Explorer.
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A Galaxy in Different Lights | |
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Authors: L. Bianchi, D.A. Thilker, D. Burgarella et al. |
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Journal-ref: ApJ 619 (2005) L71-L74 [  ] |
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Title: Recent star formation in nearby galaxies from GALEX imaging: M101 and M51 |
Image credit: NASA/JPL
Fig. — M106 (GALEX five years in space)
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M106's extended arms are the blue filaments that curve around the edge of the galaxy, creating its outer disk. Tints of blue in the galaxy's arms reveal hot, young, massive stars. Meanwhile, traces of gold toward the center reveal an older stellar population and the presence of obscuring dust.
From 24 million-light years away, neighboring galaxy NGC 4248 can be seen sitting just right of M106. The irregular galaxy looks like a yellow smudge, with a bluish-white bar in the center. The galaxy's outer golden glow indicates a population of older stars, while the blue central region shows a younger stellar demographic.
Dwarf galaxy UGC 7365 also emerges at the bottom center of the frame, as a faint yellow smudge directly below M106. This galaxy is not forming any new stars, and looks much smaller than M106 despite being closer to Earth, at 14 million light-years away.
M106, also known as NGC 4258, is located in the constellation Canes Venatici. This image is a two-color composite, with far-ultraviolet light as blue, and near-ultraviolet light as red.
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Abell clusters
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How to Spot a Young Galaxy
This animation demonstrates that young galaxies are best viewed in ultraviolet light. It consists of artist conceptions of a typical mature galaxy like our own Milky Way (left) and a typical young galaxy (right). The movie begins by showing the pair in visible light, then transitions into ultraviolet views. While young and old galaxies look equally bright when viewed in visible wavelengths, young galaxies are glaringly bright compared to older galaxies in ultraviolet wavelengths. In the final ultraviolet view, the brightness levels have been dimmed to highlight features of the young galaxy. Young galaxies light up in the ultraviolet because they are filled with hot, newborn stars – objects that pack most of their light into ultraviolet wavelengths. Older galaxies have less star-forming activity and thus give off less ultraviolet light. Both young and old stars radiate visible light, so young and old galaxies look similar when viewed in this wavelength. NASA's Galaxy Evolution Explorer, with its highly sensitive ultraviolet detectors, spotted what appear to be three-dozen massive young galaxies in our corner of the universe. The findings came as a surprise, because astronomers had thought that the universe's "birth-rate" had declined, and that massive galaxies were no longer forming. The artist concepts of the mature galaxy are based on images of the nearby galaxy called Messier 81. The artist concepts of the young galaxy are based on images of the Bearclaw Galaxy. Image credit: NASA/JPL-Caltech |
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Bulges and Black Holes in the Local Universe
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Authors: G. Kauffmann, T.M. Heckman, T. Budavari, S. Charlot, C.G. Hoopes, D.C. Martin, M. Seibert, T.A. Barlow, L. Bianchi, T. Conrow, Jose Donas, Karl Forster, P.G. Friedman, Young-Wook Lee, B.F. Madore, Bruno Milliard, P.F. Morrissey, S.G. Neff, R.M. Rich, D. Schiminovich, Todd Small, A.S. Szalay, T.K. Wyder, S.K. Yi |
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Journal-ref: ApJS 173 (2007) 357 [astro-ph/0609436 ] |
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Title: Ongoing Formation of Bulges and Black Holes in the Local Universe: New Insights from GALEX |
| Abstract:
We analyze a volume-limited sample of massive bulge-dominated galaxies with
data from both the Sloan Digital Sky Survey and the Galaxy Evolution Explorer (GALEX) satellite.
The galaxies have central velocity dispersions greater than 100 km/s and stellar surface mass densities that lie above the value where galaxies transition from actively star forming to passive systems. The sample is limited to redshifts 0.03<z<0.07. At these distances, the SDSS spectra sample the light from the bulge-dominated central regions of the galaxies. The GALEX NUV data provide high sensitivity to low rates of global star formation in these systems. Our sample of bulge-dominated galaxies exhibits a much larger dispersion in NUV-r colour than in optical g-r colour. Nearly all of the galaxies with bluer NUV-r colours are AGN. Both GALEX images and SDSS colour profiles demonstrate that the excess UV light is associated with an extended disk. We find that galaxies with red outer regions almost never have a young bulge or a strong AGN. Galaxies with blue outer regions have bulges and black holes that span a wide range in age and accretion rate. Galaxies with young bulges and strongly accreting black holes almost always have blue outer disks. Our suggested scenario is one in which the source of gas that builds the bulge and black hole is a low mass reservoir of cold gas in the disk.The presence of this gas is a necessary, but not sufficient condition for bulge and black hole growth. Some mechanism must transport this gas inwards in a time variable way. As the gas in the disk is converted into stars, the galaxies will turn red, but further gas infall can bring them back into the blue NUV-r sequence. 1. Introduction
The formation and evolutionary history of massive bulge-dominated galaxies has been a
subject of considerable controversy over the past decade. On the one hand, the colours and
detailed spectral energy distributions of these galaxies indicate that their stellar populations
are predominantly old and metal-rich (see Renzini (2006) for a recent review). The dispersion
in spectral properties between different systems is small, indicating that the stars in different
galaxies were formed at roughly similar epochs and less than a few gigayears after the Big Bang.
On the other hand, a theoretical paradigm for the assembly of structure in the Universe has emerged in recent years, which at first sight has appeared to contradict this observational data. According to this paradigm, the largest structures in the Universe form hierarchically through the merging of small virialized condensations (halos) to form larger and larger systems. This means that the dark matter halos that host massive bulge-dominated galaxies are predicted to have assembled relatively recently. In addition, a significant fraction of these galaxies are expected to reside at the centers of massive halos, where gas is expected to reach high enough densities to be able to cool through radiative processes. The predicted cooling rates in these halos are high enough to produce a population of young stars that should be easily detectable in the spectra of their central galaxies. | |
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Tidal Disruption of a Star by a SmBH
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Authors: S. Gezari, D. C. Martin, B. Milliard, S. Basa, J. P. Halpern, K. Forster, P. G. Friedman, P. Morrissey, S. G. Neff, D. Schiminovich, M. Seibert, T. Small, T. K. Wyder |
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Journal-ref: ApJ 653 (2006) L25 [astro-ph/0612069 ] |
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Title: Ultraviolet Detection of the Tidal Disruption of a Star by a Supermassive Black Hole |
| Abstract:
A supermassive black hole in the nucleus of a galaxy will be revealed when a
star passes close enough to be torn apart by tidal forces and a flare of
radiation is emitted by the stream of stellar debris that plunges into the
black hole. Since common active galactic nuclei have accreting black holes that
can also produce flares, a convincing demonstration that a stellar tidal
disruption has occurred generally begins with a ``normal'' galaxy that has no
evidence of prior nuclear activity.
Here we report a luminous UV flare from an elliptical galaxy at z = 0.37 in the Groth field of the GALEX Deep Imaging Survey that has no evidence of a Seyfert nucleus from optical spectroscopy and X-ray imaging obtained during the flare. Multiwavelength data collected at the time of the event, and for 2 years following, allow us to constrain, for the first time, the spectral energy distribution of a candidate tidal disruption flare from optical through X-rays. The luminosity and temperature of the radiation and the decay curve of the flare are in excellent agreement with theoretical predictions for the tidal disruption of a star, and provide the strongest empirical evidence for a stellar disruption event to date. | |
K7
UV Star Formation Rates in the Local Universe
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Authors: S. Salim, R.M. Rich, S. Charlot, J. Brinchmann, B.D. Johnson, David Schiminovich, Mark Seibert, Ryan Mallery, Timothy M. Heckman, Karl Forster, Peter G. Friedman, D. Christopher Martin, Patrick Morrissey, Susan G. Neff, Todd Small, Ted K. Wyder, Luciana Bianchi, Jose Donas, Young-Wook Lee, Barry F. Madore, Bruno Milliard, Alex S. Szalay, Barry Y. Welsh, Sukyoung K. Yi |
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Journal-ref: ApJ 173 (2007) 267 [0704.3611 ] |
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Title: UV Star Formation Rates in the Local Universe |
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We measure star formation rates of ~50,000 optically-selected galaxies in the
local universe (z~0.1), spanning a range from gas-rich dwarfs to massive
ellipticals. We obtain dust-corrected SFRs by fitting the GALEX (UV) and SDSS
(optical) photometry to a library of population synthesis models that include
dust attenuation. For star-forming galaxies, our UV-based SFRs compare
remarkably well with those derived from SDSS H alpha. Deviations from perfect
agreement between these two methods are due to differences in the dust
attenuation estimates. In contrast to H alpha, UV provides reliable SFRs for
galaxies with weak or no H alpha emission, and where H alpha is contaminated
with an emission from an AGN. We use full-SED SFRs to calibrate a simple
prescription that uses GALEX UV magnitudes to produce good SFRs for normal
star-forming galaxies. The specific SFR is considered as a function of stellar
mass for (1) star-forming galaxies with no AGN, (2) those hosting an AGN, and
for (3) galaxies without H alpha emission. We find that the three have distinct
star formation histories, with AGN lying intermediate between the star-forming
and the quiescent galaxies. Normal star forming galaxies (without an AGN) lie
on a relatively narrow linear sequence. Remarkably, galaxies hosting a strong
AGN appear to represent the massive continuation of this sequence. Weak AGN,
while also massive, have lower SFR, sometimes extending to the realm of
quiescent galaxies. We propose an evolutionary sequence for massive galaxies
that smoothly connects normal star-forming galaxies to quiescent (red sequence)
galaxies via strong and weak AGN. We confirm that some galaxies with no H alpha
emission show signs of SF in the UV. We derive a UV-based cosmic SFR density at
z=0.1 with smaller total error than previous measurements.
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| Literatur zu "GALEX" | |||
| L. Bianchi, D.A. Thilker, D. Burgarella et al. et al. | 2005 | ApJ 619, L71 |
"Recent star formation in nearby galaxies from GALEX imaging: M101 and M51"
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| T.M. Heckman, C.G. Hoopes, M. Seibert, C. Martin, et al. | 2005 | ApJ 619, L35-8 |
"The Properties of Ultraviolet-Luminous Galaxies at the Current Epoch"
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| S. Gezari, D. C. Martin, B. Milliard, et al. | 2006 | ApJ 653, L25 |
"Ultraviolet Detection of the Tidal Disruption of a Star by a Supermassive Black Hole"
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| C.H. Ree, Y-W Lee, S.K. Yi, et al. | 2007 | ApJ 173, 607 |
"Look-back Time Evolution of Far-UV Flux from the Brightest Cluster Elliptical Galaxies at z < 0.2"
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| G. Kauffmann, T.M. Heckman, T. Budavari, et al. | 2007 | ApJ 173, 357 |
"Ongoing Formation of Bulges and Black Holes in the Local Universe: New Insights from GALEX"
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| S. Salim, R.M. Rich, S. Charlot, et al. | 2007 | ApJ 173, 267 |
"UV Star Formation Rates in the Local Universe"
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| Deharveng, J-M.; Small, T.; Barlow, T.A.; et al. | 2008 | ApJ 680, 1072 |
"Lya-Emitting Galaxies at 0.2 |
 | H. Heintzmann | ( Eintrag vom 20.5.2008) |
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