Galaxy Evolution Explorer
GALEX sieht seltene, extrem UV-helle Galaxien
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
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 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
[December 21, 2004]
GALEX deep survey — UVLGs — ultraviolet-luminous galaxies|
||Authors: T.M. Heckman, C.G. Hoopes, M. Seibert, C. Martin, S. Salim,
R.M. Rich, G. Kauffmann, S. Charlot, T.A. Barlow, L. Bianchi, Yong-Ik Byun,
J. Donas, K. Forster, P.N. Jelinsky, Y-W Lee, B.F. Madore, R.F. Malina, B. Milliard,
P.F. Morrissey, S.G. Neff, D. Schiminovich, O.H.W. Siegmund, T. Small, A.S. Szalay,
B.Y. Welsh, T.K. Wyder|
||Journal-ref: ApJ 619 (2005) L35-L38 [astro-ph/0412577 ]|
||Title: The Properties of Ultraviolet-Luminous Galaxies at the Current Epoch|
We have used the first matched set of GALEX and SDSS data to investigate the
properties of a sample of 74 nearby galaxies with far-ultraviolet luminosities
chosen to overlap the luminosity range of typical high-z Lyman Break Galaxies (LBGs).
GALEX deep surveys have shown that ultraviolet-luminous galaxies
(UVLGs) similar to these are the fastest evolving component of the UV galaxy population.
Model fits to the combined GALEX and SDSS photometry yield typical
FUV extinctions similar to LBGs. The implied star formation rates are SFR ~ 3
to 30 solar mass per year. This overlaps the range of SFRs for LBGs.
We find a strong inverse correlation between galaxy mass and far-ultraviolet surface
brightness, and on this basis divide the sample into ``large'' and ``compact''
UVLGs. The compact UVLGs have half-light radii of a few kpc or less (similar to
LBGs). They are relatively low mass galaxies (~10 billion solar masses) with
typical velocity dispersions of 60 to 150 km/s. They span a range in
metallicity from 0.3 to 1 times solar, have blue optical-UV colors, and are
forming stars at a rate sufficient to build the present galaxy in ~a Gigayear.
In all these respects they appear similar to the LBG population.
These ``living fossils'' may therefore provide an opportunity for detailed investigation of
the physical processes occurring in typical star forming galaxies in the early universe.
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.
The recent 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.
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
Previously, astronomers thought that the universe had ceased to give rise to massive, young galaxies, but
findings from NASA's Galaxy Evolution Explorer suggest that may not be the case.
Surveying thousands of nearby galaxies with its highly sensitive
ultraviolet eyes, the telescope spotted three dozen that greatly
resemble youthful galaxies from billions of years ago. In this
illustration, those galaxies are represented as white circles on the right, or "today" side of the timeline.
Image credit: NASA/JPL-Caltech/NOAO
Baby Galaxies in the Adult Universe
Six 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.
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 0.2
| — |
||Authors: Deharveng, J-M.; Small, T.; Barlow, T.A.; et al.|
||Journal-ref: ApJ 680 (2008) 1072 [0803.1924 ]|
||Title: Lya-Emitting Galaxies at 0.2|
We have used the GALEX (Galaxy Evolution Explorer) spectroscopic survey mode, with a resolution of ~8 Å in
the far-ultraviolet (FUV; 1350-1750 Å) and ~20 Å in the near-ultraviolet (NUV; 1950-2750 Å) for a systematic
search of Lya-emitting galaxies at low redshift.
Our aim is to fill a gap between high-redshift surveys and a small set of objects studied in detail
in the nearby universe.
A blind search of 7018 spectra extracted in five deep
exposures (5.65 deg2) has resulted in 96 Lya-emitting
galaxy candidates in the FUV domain after accounting for broad-line
AGNs. The Lya equivalent widths (EWs) are consistent with stellar
population model predictions and show no trends as a function of UV
color or UV luminosity, with the exception of a possible decrease in the
most luminous objects that may be due to small-number statistics. The
objects' distribution in EW is similar to that at z~3, but their
fraction among star-forming galaxies is smaller.
Avoiding uncertain candidates, a subsample of 66 objects in the range 0.2<z<0.35 has
been used to build a Lya luminosity function (LF). The
incompleteness due to objects with significant Lya emission but a
UV continuum too low for spectral extraction has been evaluated. A
comparison with Ha LFs in the same redshift domain is consistent
with an average Lya/Ha of ~1
in about 15% of the star-forming galaxies. A comparison with high-redshift
implies an increase of the Lya luminosity density by a factor of
about 16 from z~0.3 to z~3. By comparison with the factor of 5 increase
in the UV luminosity density in the same redshift range, this suggests
an increase of the average Lya escape fraction with redshift.
Lya emission remains the only mean for identifying galaxies when the
continuum becomes too faint to be detected, and, following Cowie & Hu (1998) and Hu, Cowie, & McMahon
(1998), increasingly deeper and larger surveys have come into a widespread use for detecting
galaxies at high redshifts. Beyond the redshift of ~ 6, the increasingly neutral IGM is not
a complete obstacle to the visibility of Lya emission (e.g.) (Haiman 2002)
and the density evolution of Lya emitters may even help to trace the
history of the cosmic re-ionization.
Although nearby galaxies have played a key role for understanding the factors affecting
Lya escape, their observations, using space-borne UV spectrographs in
pointing mode, were directed to specific and known objects. As a consequence, there is not yet a systematic
survey for redshifts smaller than those reachable from the ground.
We use here the GALEX spectroscopic survey mode for the first systematic search of
Lya emitting galaxies at low redshift. Goals are to understand
• (i) whether the Lya escape is related to specific
properties of galaxies,
• (ii) whether the Lya emission evolves from current
epoch to high z as the cosmic star formation rate traced by Balmer lines or the UV continuum of galaxies.
If so, the average relationship between the massive stellar content of the galaxies and the
Lya emission would be constant over time;
the Lya emission might be used as a tracer of star formation, with an
empirical calibration encapsulating the average effects of resonant scattering. If not, there
would be an evidence for cosmic evolution of the physical processes, especially galactic winds,
expected to play a central role in the Lya escape from galaxies.
Cowie, L. L., & Hu, E. M. 1998, AJ 115, 1319
Hu, E. M., Cowie, L. L., & McMahon, R. G. 1998, ApJ 502, 99
K2 M81: A Galaxy in Different Lights
M81 (NGC 3031) with SN 1993J
A Galaxy in Different Lights (ii)
This side-by-side comparison shows the nearby galaxy Messier 81, which is similar to our own Milky Way,
in both visible (left) and ultraviolet light (right).
Image credit: NASA/JPL-Caltech/NOAO
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.
A Galaxy in Different Lights
M51 as observed by SDSS, Spitzer, GALEX and HST
GALEX image of Messier 101
| — |
||Authors: L. Bianchi, D.A. Thilker, D. Burgarella et al.|
||Journal-ref: ApJ 619 (2005) L71-L74 [ ]|
||Title: Recent star formation in nearby galaxies from GALEX imaging: M101 and M51|
Galaxy Evolution Explorer Celebrates Five Years in Space
[April 28, 2008] A Cosmic Embrace
Since its launch five years ago, the Galaxy Evolution Explorer has photographed hundreds
of millions of galaxies in ultraviolet light. M106 is one of those galaxies, and from 22 million light
years away, it strikes a pose in blue and gold for this new commemorative portrait.
From 22 million light-years away, galaxy M106 extends two ultraviolet-bright spiral arms in this image
from NASA's Galaxy Evolution Explorer.
Image credit: NASA/JPL
Fig. — M106 (GALEX five years in space)
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
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.
K3 Abell clusters
| — |
||Authors: C.H. Ree, Y-W Lee, S.K. Yi, Suk-Jin Yoon, R.M. Rich, J-M Deharveng,
Young-Jong Sohn, Sugata Kaviraj, Jonghwan Rhee, Yun-Kyeong Sheen, Kevin Schawinski, Soo-Chang Rey,
Alessandro Boselli, Jaehyon Rhee, Jose Donas, Mark Seibert, Ted K. Wyder, Tom A. Barlow, Luciana Bianchi,
Karl Forster, Peter G. Friedman, Timothy M. Heckman, B.F. Madore, D.C. Martin, Bruno Milliard,
Patrick Morrissey, S.G. Neff, David Schiminovich, Todd Small, A.S. Szalay, Barry Y. Welsh|
||Journal-ref: ApJ 173 (2007) 607 [astro-ph/0703503 ]|
||Title: The Look-back Time Evolution of Far-Ultraviolet Flux from
the Brightest Cluster Elliptical Galaxies at z < 0.2|
We present the GALEX UV photometry of the elliptical galaxies in
Abell clusters at moderate redshifts (z < 0.2) for the study of the
look-back time evolution of the UV upturn phenomenon.
The brightest elliptical galaxies (M_r < -22) in 12 remote clusters are compared with the nearby
giant elliptical galaxies of comparable optical luminosity in the Fornax and
Virgo clusters. The sample galaxies presented here appear to be quiescent
without signs of massive star formation or strong nuclear activity, and show
smooth, extended profiles in their UV images indicating that the far-UV (FUV)
light is mostly produced by hot stars in the underlying old stellar
population. Compared to their counterparts in nearby clusters, the FUV flux of
cluster giant elliptical galaxies at moderate redshifts fades rapidly with ~ 2
Gyrs of look-back time, and the observed pace in FUV - V color evolution
agrees reasonably well with the prediction from the population synthesis
models where the dominant FUV source is hot horizontal-branch stars and their
progeny. A similar amount of color spread (~ 1 mag) in FUV - V exists among
the brightest cluster elliptical galaxies at z ~ 0.1, as observed among the
nearby giant elliptical galaxies of comparable optical luminosity.
Tab. 1.— NASA/IPAC Extragalactic Database (http://nedwww.ipac.caltech.edu)
The ultraviolet (UV) upturn phenomenon of early-type galaxies is the rising flux with decreasing wavelength
from 2500°A to the Lyman limit. Since its first detection (Code 1969), many UV space facilities have targeted
nearby elliptical galaxies and spiral bulges in order to investigate the spectral and photometric characteristics
of the UV upturn in early-type galaxies and its connection to the physical properties.
Although it is now well established that the far-UV (FUV) flux of nearby early-type
galaxies originates from a minority population of old hot helium-burning horizontal-branch (HB) stars, the
remaining issue on their metallicities has an outstanding implication on the galaxy evolution.
K4 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
K5 Bulges and Black Holes in the Local Universe
| — |
||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|
||Journal-ref: ApJS 173 (2007) 357 [astro-ph/0609436 ]|
||Title: Ongoing Formation of Bulges and Black Holes in the Local Universe: New Insights from GALEX|
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.
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.
K6 Tidal Disruption of a Star by a SmBH
| — |
||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|
||Journal-ref: ApJ 653 (2006) L25 [astro-ph/0612069 ]|
||Title: Ultraviolet Detection of the Tidal Disruption of a Star by a Supermassive Black Hole|
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
| — |
||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|
||Journal-ref: ApJ 173 (2007) 267 [0704.3611 ]|
||Title: UV Star Formation Rates in the Local Universe|
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.
|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"
|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"
|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"
|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"
|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"
|S. Salim, R.M. Rich, S. Charlot, et al.||2007||ApJ 173, 267||
"UV Star Formation Rates in the Local Universe"
|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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