Artikel zu "M33 (NGC 598)"
M33 (main) XMM-Newton survey of the Local Group galaxy M33 K1.1 M33 X-8: Ultraluminous X-ray Source in M33 (XMM-Newton) K1.2 M33 X-7 (Chandra) K1.3 M33 X-7 — A 15.65 solar mass black hole K1.4 M33 X-7 — Spin Parameter a* = 0.77 Sternleiche bricht Masse- Rekord K2 M33 in radio and optical (D = 800 kpc) K3 Radio distance (D = 730 kpc) K4	The black-hole binary V4641 Sagittarii Literatur

M33 (NGC 598) - Triangulum Galaxy

K1.1 M33 X-8: Ultraluminous X-ray Source in M33 (XMM-Newton)

M33 X-8 — M_BH ~ 12 M — L_{0.5-10 keV} = 2·10³⁹ erg s^-1
	Authors: L. Foschini, J. Rodriguez, Y. Fuchs, L. C. Ho, M. Dadina, G. Di Cocco, T. J.-L. Courvoisier, G. Malaguti
	Journal-ref: A&A 416 (2004) 529 [astro-ph/0312118 ]
	Abstract: We present observations with XMM-Newton of M33 X-8, the ultraluminous X-ray source L_{0.5-10 keV} = 2·10³⁹ erg s^-1 closest to the centre of the galaxy. The best-fit model is similar to the typical model of Galactic black holes in very high state. Comparison with previous observations indicates that the source is still in a very high state after about 20 years of observations. No state transition has been observed even during the present set of XMM-Newton observations. We estimate the lower limit of the mass of the black hole > 6 M, but with proper parameters taking into account different effects, the best estimate becomes 12 M. Our analysis favours the hypothesis that M33 X-8 is a stellar mass black hole candidate, in agreement with the findings of other authors. In addition, we propose a different model where the high luminosity of the source is likely to be due to orientation effects of the accretion disc and anisotropies in the Comptonized emission.

M33 (NGC 598) is one of the nearest spiral galaxies (d = 840 kpc). Classified as SA(s)cd, it has an inclination angle of 55°.
About 20 years ago, with the early observations of nearby spiral galaxies by the Einstein satellite, a new class of intermediate luminosity (L_X = 10³⁹-10⁴⁰ erg s^-1) X-ray sources was discovered (cf Fabbiano 1989). These sources, later defined as ultraluminous X-ray sources (ULX, Makishima et al. 2000), were immediately intriguing, since one of the proposed model is that they could be intermediate mass black holes (10²-10⁴M) accreting at sub-Eddington rates, the missing link between stellar mass X-ray binaries and active galactic nuclei (see Miller & Colbert 2003 for a review on intermediate mass black holes and their relationship with ULX). However, there are also other explanations available, which do not require a new class of object. According to these models, the ULX are stellar mass X-ray binaries, but either with truly super-Eddington accretion rate, or with sub-Eddington rate, but with some type of collimated emission, either simply anisotropic or relativistic to increase the observed luminosity.
The threshold to define an ULX is now generally set to L_X = 10³⁹ erg s^-1, without any reference to the physical mechanism responsible for this value.
Surveys of ULX (e.g. with ROSAT Colbert & Ptak 2002, with XMM—Newton Foschini et al. 2002, with Chandra Colbert et al. 2003) can give gross information about these sources, their statistical properties, their relationships with the host galaxy. However, to improve the understanding of these sources, a detailed study of nearby ULX with high signal—to—noise data are needed.
Since the first observations with the {Einstein} satellite (Long et al. 1981), it was clear that the central source (M33 X-8) had particular features (luminosity in the 0.2-4 keV energy range of about 10³⁹ erg s^-1, soft spectrum, excess of absorption along the line of sight) suggesting that the source is somewhat different from an active galactic nucleus (Trinchieri et al. 1988). The authors suggested the possibility that M33 contains a new type of X-ray binary system.
Later on, ASCA (Takano et al. 1994) observations extended up to 7 keV and strengthened the early results of Trinchieri et al. (1988). The best-fit model was composed of a multicolour disc (MCD) plus a power law at high energies, consistent with that of Galactic black holes in their high state. However, Schulman & Bregman (1995), based on {ROSAT} observations, conclude that the probability of such an unusual X-ray binary close to the centre of M33 is very small.
Another point which makes M33 X-8 an unusual source is the steadiness of its flux, except for a modulation of ~ 20% with a period of 106 days (Dubus et al. 1997). This discovery strengthened the hypothesis of a binary system, since the modulations can be due to the precession motion of the accretion disc (cf. Maloney et al. 1996). It is important to add that there is a lack of information at wavelengths other than X-rays for the source, since the source is located in a crowded region, so that it is difficult to find the right counterpart or the companion star.
The recent increase of interest for ultraluminous X-ray source phenomenon gave new light to the study of M33 X-8. Indeed, since the spatial resolution of Einstein, ROSAT, and ASCA were not sufficient to rule out the possibility of a small offset of the source from the optical centre, Makishima et al. (2000) suggested that X-8 could be an ULX. However, {Chandra} observations put the tightest constraints on the position of X-8 (Dubus & Rutledge 2002). The authors found a possible counterparts at radio wavelengths: it was identified with the point source n. 102 discovered by (Gordon et al. 1999) with the VLA at 20 and 6 cm. In the near-IR, M33 X—8 is at the 2MASS position of the nucleus (2MASS J01335089+3039365) within 0.6'', which corresponds to about 2.3 pc at the distance of 795 kpc (Dubus & Rutledge 2002).
The hypothesis of an active galactic nucleus (AGN) in the centre of M33 is inconsistent with the upper limits on the central black hole mass obtained from the velocity dispersion measurements of the nuclear region: Kormendy & McClure (1993) gave an upper limit of 5·10⁴M, by using the Canada-France-Hawaii Telescope. Recently, Gebhardt et al. (2001) set, with the {Hubble Space Telescope (HST)}, an upper limit to only 1500 M.
Moreover the 106 days periodicity is not consistent with the AGN hypothesis. The possibility that M33 X-8 is an ULX is the best explanation, as already suggested by Makishima et al. (2000), although the source is very close to the centre of M33.

K1.2 M33 X-7 (Chandra)

M33 X-7 — 0.31 s pulse period — 3.45 d orbital period — L_X » 10³⁸ erg s^-1
	Authors: W. Pietsch, F. Haberl, M. Sasaki, T.J. Gaetz, P.P. Plucinsky, P. Ghavamian, K.S. Long, T.G. Pannuti
	Journal-ref: ApJ 646 (2006) 420–428 [astro-ph/0603698 ]
	Title: M33 X-7: ChASeM 33 reveals the first eclipsing black hole X-ray binary
Abstract: The first observations conducted as part of the Chandra ACIS survey of M 33 (ChASeM33) sampled the eclipsing X-ray binary M33 X-7 over a large part of the 3.45 d orbital period and have resolved eclipse ingress and egress for the first time. The occurrence of the X-ray eclipse allows us to determine an improved ephemeris of mid-eclipse and binary period as HJD (2453639.119 ± 0.005) ± N x (3.453014 ± 0.000020) and constrain the eclipse half angle to (26.5 ± 1.1) degree. There are indications for a shortening of the orbital period. The X-ray spectrum is best described by a disk blackbody spectrum typical for black hole X-ray binaries in the Galaxy. We find a flat power density spectrum and no significant regular pulsations were found in the frequency range of 10^-4 to 0.15 Hz. HST WFPC2 images resolve the optical counterpart, which can be identified as an O6III star with the help of extinction and colour corrections derived from the X-ray absorption. Based on the optical light curve, the mass of the compact object in the system most likely exceeds 9 M. This mass, the shape of the X-ray spectrum and the short term X-ray time variability identify M33 X-7 as the first eclipsing black hole high mass X-ray binary.
Introduction: The orbital period, pulse period and observed X-ray luminosity are remarkably similar to those of the Small Magellanic Cloud neutron star XRB SMC X-1 (Liu et al. 2000). X-7 was the first and only identified eclipsing accreting binary system with an X-ray source in an external galaxy other than the Magellanic Clouds before the detection of similar behavior based on XMM-Newton and Chandra data of the NGC 253 X-ray source RX J004717.4-251811. References Pietsch, W., Mochejska, B.J., Misanovic, Z., Haberl, F., Ehle, M. & Trinchieri, G. A&A 413, 879–887 (2004). The eclipsing massive X-ray binary M 33 X-7: New X-ray observations and optical identification

K1.3 M33 X-7 — A 15.65 solar mass black hole

Zum Thema
IC 10 X-1, a Black Hole (M_BH ~ 26 M)	List: BHs

M33 X-7 — M_BH ~ 15.6 M — M_comp ~ 70 M

Authors: J.A. Orosz, J.E. McClintock, R. Narayan, C.D. Bailyn, J.D. Hartman, L. Mracri, J. Liu, Wolfgang Pietsch, R.A. Remillard, A. Shporer, T. Mazeh

Journal-ref: Nature 449 (2007) 872-875 [0710.3165

]

Title: A 15.65 solar mass black hole in an eclipsing binary in the nearby spiral galaxy Messier 33

X-ray source   Optical/IR  BH mass     Comp. mass 
               counterpart   (M)         (M)
GRS 1915+105   V1487 Aql   14.0 ± 4.4    0.81
GS 2023+338    V404 Cyg    12 ± 2        0.6
A0620-00       V616 Mon    10 ± 5        0.6
GS 2000+25     QZ Vul      10 ± 4        0.5
XTE J1550-564  V381 Nor    9.6 ± 1       ...
4U 1543-47     IL Lup      9.4 ± 1       2.5
Cyg X-1        HDE 226868  > 4.8         > 11.7
LMC X-1        ...         8 - 20        ...

Table 1. Dynamical measurements of massive stellar BHs.

Image credit: Orosz et al. (2007)

FIG. 4.— Schematic diagram of M33 X-7. The companion star and the accretion disk surrounding the black hole are shown to scale as seen projected onto the plane of the sky at three orbital phases. The colours on the star represent temperatures (not intensities), with cooler temperatures shown by darker colours as denoted on the bar. The distance between the Sun and Mercury is indicated and the figure is scaled in solar radii.

Abstract: Stellar-mass black holes are discovered in X-ray emitting binary systems, where their mass can be determined from the dynamics of their companion stars.
Models of stellar evolution have difficulty producing black holes in close binaries with masses >10 M, which is consistent with the fact that the most massive stellar black holes known so all have masses within 1 s of 10 M.
Here we report a mass of 15.65 ± 1.45 M for the black hole in the recently discovered system M33 X-7, which is located in the nearby galaxy Messier 33 (M33) and is the only known black hole that is in an eclipsing binary.
In order to produce such a massive black hole, the progenitor star must have retained much of its outer envelope until after helium fusion in the core was completed.
On the other hand, in order for the black hole to be in its present 3.45 day orbit about its 70.0 ± 6.9 M companion, there must have been a ``common envelope'' phase of evolution in which a significant amount of mass was lost from the system.
We find the common envelope phase could not have occured in M33 X-7 unless the amount of mass lost from the progenitor during its evolution was an order of magnitude less than what is usually assumed in evolutionary models of massive stars.

Optical imaging and spectroscopic observations of M33 X-7 were obtained in service mode with the 8.2m Gemini North Telescope between 2006 August 18 and November 16.
The temperature of the companion star was determined by comparing its averaged spectrum to a collection of synthetic spectra. A good match to the observed spectrum is provided by the model with T_eff ~ 35000 K, which corresponds to a spectral type of O7III to O8III.
For the radius of the O-star we find a radius of R_comp = 19.6 ± 0.9 R and
a luminosity of log(L_comp/L) = 5.72 ± 0.07.
M33 X-7 is a key system in the study of high mass stars, high mass X-ray binaries, and high mass black holes. A ~ 16M black hole paired with a ~ 70M secondary with a separation of only ~ 42R is very difficult to explain using stellar evolutionary models.
Since the radius of the black hole progenitor would have been much larger than the current orbital separation, the two stars must have been brought closer together via some kind of “common envelope” phase which results in a significant amount of mass lost from the progenitor, and very little mass gained by the secondary.
On the other hand, in order for the core mass to remain large enough to produce a ~ 16M black hole, the outer envelope of the progenitor needs to be intact until core He burning is completed. Hence we require that the common envelope phase begins only after core He burning in the progenitor is complete (case C mass transfer).
The determination of an accurate mass for M33 X-7 – located at a distance of more than 16 times that of any other confirmed stellar black hole – marks a major advance in our capability to study black holes in Local Group galaxies beyond the MilkyWay.

References
Brown, G.E. Heger, A. Langer, N. Lee, C.-H. Wellstein, S., & Bethe, H.A. 
Charles, P. A. & Coe, M. J. Compact Stellar X-ray Sources   
Greiner, J., Cuby, J.G., & McCaughrean Nature 414, 522–525 (2001). 
Neil, E. T., Bailyn, C. D., & Cobb, B. E. ApJ 657, 409-14 (2007)
Orosz, J. A. IAU Symp. 212 365–371 (2003). 
Pietsch, W. et al. M33 X-7: ApJ 646, 420–428 (2006). 
Remillard, R.A. & McClintock, J.E.  ARAA 44, 49–92 (2006). 
Tauris, T.M. & van den Heuvel, E.P.J. Compact Stellar X-ray Sources

Sternleiche bricht Masse- Rekord

[18. Oktober 2007] Schwarzes Loch M33 X-7
US-Astronomen haben das bisher größte stellare Schwarze Loch im Weltall entdeckt. Das beim Kollaps eines Sterns entstandene Objekt wiegt fast 16 Mal soviel wie die Sonne. Nach bisherigen Theorien über die Entstehung Schwarzer Löcher dürfte es eigentlich gar nicht existieren.

Image credit:

FIG. — Rechts in der Illustration die Akkretionscheibe von M33 X-7 zu sehen, links der noch massereichere Begleitstern.

US-Astronomen haben ein wahres Ungetüm im All aufgespürt: Das jetzt entdeckte etwa 800 kpc von der Erde entfernte stellare Schwarze Loch hat die 15,7-fache Masse unserer Sonne, berichten Jerome Orosz und seine Kollegen. Damit handele es sich um das schwerste Schwarze Loch seiner Art, schreiben die Forscher im Magazin "Nature".
Stellare Schwarze Löcher sind Sternenleichen und wesentlich kleiner als sogenannte supermassive Schwarze Löcher. Sie entstehen, wenn ein Stern seinen Brennstoffvorrat verbraucht hat und unter dem eigenen Gewicht innerhalb von Sekunden kollabiert. Dabei blitzt der Stern noch einmal kurz als Supernova auf. "Übrig bleibt eine extrem dichte und massereiche Sternleiche, deren Anziehungskraft nicht einmal Licht zu entkommen vermag: ein Schwarzes Loch", erklärten die Wissenschaftler.
Eine Partnersonne ziehe alle dreieinhalb Tage an dem Objekt mit der Bezeichnung M33 X-7 vorüber und schirme die Röntgenstrahlung ab, die das Massemonster umgebe. Dieser Stern war es auch, der die Vermessung des Schwarzen Lochs erlaubte.
Aus der Dauer der Abschirmung und der Geschwindigkeit des Begleitsterns "konnten wir sehr exakt die Massen der beiden Komponenten des Doppelsternsystems ableiten", sagte Wolfgang Pietsch, der an der Entdeckung beteiligt war. Der Begleitstern selbst habe die 70-fache Masse unserer Sonne.
Die Entdeckung ist auch deshalb bemerkenswert, weil Astrophysiker Schwierigkeiten haben, mit ihren bisherigen Modellen die Entstehung von stellaren Schwarzen Löchern schwerer als zehn Sonnenmassen zu erklären. Es stellten sich nun eine Menge neuer Fragen, sagte Orosz.

K1.4 M33 X-7 — Spin Parameter a* = 0.77

M33 X-7 — D = 840 ± 20 kpc — i_orb = 74.6° ± 1.0° — a* = 0.77 — M_BH = 15.6 ± 1.45 M
	Authors: J. Liu, J. McClintock, R. Narayan, S. Davis, J. Orosz
	Journal-ref: ApJ (2008) L [0803.1834 ]
	Title: Precise Measurement of the Spin Parameter of the Stellar-Mass Black Hole M33 X-7
Abstract: In prior work, Chandra and Gemini-North observations of the eclipsing X-ray binary M33 X-7 have yielded measurements of the mass of its black hole primary and the system's orbital inclination angle of unprecedented accuracy. Likewise, the distance to the binary is known to a few percent. In an analysis based on these precise results, fifteen Chandra and XMM-Newton X-ray spectra, and our fully relativistic accretion disk model, we find that the dimensionless spin parameter of the black hole primary is a* = 0.77 ± 0.05. The error includes all sources of observational uncertaity. Four Chandra spectra of the highest quality, which were obtained over a span of several years, all lead to the same estimate of spin to within statistical errors (2%), and this estimate is confirmed by 11 spectra of lower quality. There are two remaining uncertainties: • (1) the validity of the relativistic model used to analyze the observations, which is being addressed in ongoing theoretical work; and • (2) our assumption that the black hole spin is approximately aligned with the angular momentum vector of the binary, which can be addressed by a future X-ray polarimetry mission. 1. Introduction M33 X-7 is the first stellar-mass black hole to be discovered that is eclipsed by its companion (Pietsch et al. 2006). The X-ray eclipse and the precisely known distance of this system, D = 840 ± 20 kpc, underpin the most accurate dynamical model that has been achieved for any of the 21 known black hole binaries (Orosz et al. 2007). The two dynamical parameters of interest in this Letter are the black hole mass M_BH = 15.6 ± 1.45 M and the orbital inclination angle i_orb = 74.6° ± 1.0° (Orosz et al. 2007). References Orosz, J.A., McClintock, J.E., Narayan, R. et al. 2007, Nature 449, 872 Pietsch, W. et al. M33 X-7: ApJ 646, 420–428 (2006). Remillard, R.A. & McClintock, J.E. ARAA 44, 49–92 (2006).

K2 M33 in radio and optical

Parallax distance to M33 — D = 800 kpc
	Authors: A. L. Argon, L. J. Greenhill, J. M. Moran, M. J. Reid, K. M. Menten, M. Inoue
	Journal-ref: ApJ 615 (2004) 702 [astro-ph/0407486 ]
	The IC133 Water Vapor Maser in the Galaxy M33: A Geometric Distance We report on the results of a 14 year long VLBI study of proper motions in the IC133 H2O maser source in the galaxy M33. The method of Ordered Motion Parallax was used to model the 3-dimensional structure and dynamics of IC133 and obtain a distance estimate, 800 ± 180 kpc. Our technique for determining the distance to M33 is independent of calibrations common to other distance indicators, such as Cepheid Period-Luminosity relations, and therefore provides an important check for previous distance determinations.

Erstmals Eigenbewegung einer Galaxie gemessen!
Es war eine der großen Hürden der beobachtenden Astronomie, und nach fast 100 Jahren ist sie genommen:
Zum ersten Mal konnte die Bewegung einer anderen Galaxie in der Himmelsebene direkt gemessen werden. Das Kunststück gelang mit der »Triangel-Galaxie« Messier 33:
Mit dem Very Long Baseline Array aus Radioteleskopen konnte die Eigenbewegung mehrerer Maserquellen gegenüber ferneren Radioquellen verfolgt werden. Die Winkelgeschwindigkeit ist unglaublich gering:
30 Mikrobogensekunden pro Jahr, was einem Monddurchmesser in 60 Mio. Jahren. Zusammen mit der viel einfacher zu messenden Radialgeschwindigkeit kennt man nun die komplette räumliche Bewegung von M 33 gegenüber der Milchstraße.

Image courtesy of T. Rector (NRAO/AUI/NSF and NOAO/AURA/NSF), D. Thilker (NRAO/AUI/NSF) and R. Braun (ASTRON).

Radio/Optical Image of M33
This image of the Triangulum Galaxy was created by combining optical data from the 0.9-meter telescope on Kitt Peak in Arizona with radio data from the Very Large Array (VLA) telescope in New Mexico and the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands.

Also known as M33, the Triangulum Galaxy is part of the Local Group of galaxies, which includes the Andromeda Galaxy (M31) and our galaxy, the Milky Way. M33 is over thirty thousand light years across, and more than two million light years away.

The optical data in this image show the many stars within the galaxy as well as reddish star forming regions that are filled with hot Hydrogen gas.

The radio data reveal the cool Hydrogen gas within the galaxy, gas which cannot be seen with an optical telescope. Combined together, the radio and optical give a more comprehensive view of star formation in this galaxy.

The color image was generated by combining images taken with the 0.9-meter telescope through four filters (B, V, I and Hydrogen-alpha) and 21cm neutral Hydrogen data taken with the VLA and WSRT (shown in blue-violet).

The image is one square degree in field of view, roughly five times the size of the Moon.

The VLBA is a system of ten radio-telescope antennas, each with a dish 25 meters (82 feet) in diameter and weighing 240 tons. From Mauna Kea on the Big Island of Hawaii to St. Croix in the U.S. Virgin Islands, the VLBA spans more than 5,000 miles, providing astronomers with the sharpest vision of any telescope on Earth or in orbit. Dedicated in 1993, the VLBA has an ability to see fine detail equivalent to being able to stand in New York and read a newspaper in Los Angeles.

The VLBA's scientific achievements include

making the most accurate distance measurement ever made of an object beyond the Milky Way Galaxy;
the first mapping of the magnetic field of a star other than the Sun;
movies of motions in powerful cosmic jets and of distant supernova explosions;
the first measurement of the propagation speed of gravity; and
long-term measurements that have improved
the reference frame used to map the Universe and
detect tectonic motions of Earth's continents.

K3 Radio distance — Kosmische Choreografie

Parallax distance to M33 — D = 730 kpc
	Authors: A. Brunthaler, M.J. Reid, H. Falcke, L.J. Greenhill, C. Henkel
	Science 307 (2005) 1440 [astro-ph/0503058 ]
	The Geometric Distance and Proper Motion of the Triangulum Galaxy (M33) We measured the angular rotation and proper motion of the Triangulum Galaxy (M33) with the Very Long Baseline Array by observing two H2O masers on opposite sides of the galaxy. By comparing the angular rotation rate with the inclination and rotation speed, we obtained a distance of 730 ± 168 kiloparsecs. This distance is consistent with the most recent Cepheid distance measurement. M33 is moving with a velocity of 190 ± 59 km/s relative to the Milky Way. These measurements promise a new method to determine dynamical models for the Local Group and the mass and dark matter halos of M31, M33 and the Milky Way.

Radioastronomen messen erstmals den Tanz einer benachbarten Galaxie und bestimmen präzise deren Entfernung
Mit einer Armada von Radioteleskopen haben sich Astronomen einen 80 Jahre lang gehegten Traum erfüllt und erstmals die Bewegung einer benachbarten Galaxie am Himmel direkt nachgewiesen. Die Wissenschaftler hoffen, mit diesen Messungen das zukünftige Schicksal unseres eigenen Sternsystems, der Milchstraße, vorherzusagen. Die Forscher bestimmten außerdem die Entfernung der Galaxie M33 sehr präzise zu 2,4 Millionen Lichtjahren.

Bild: Travis Rector (NRAO/AUI/NSF und NOAO/AURA/NSF), David Thilker (NRAO/AUI/NSF), und Robert Braun (ASTRON)

Abb. 1: Galaxie M33 im Sternbild Dreieck. Die Positionen, in denen Wassermaser-Aktivität nachgewiesen wurde, sind markiert.

Galaxien bestehen aus Milliarden von Sternen sowie Staub- und Gaswolken und formen oftmals große Ansammlungen. Unsere Milchstraße gehört zu einem kleineren Galaxienhaufen, der Lokalen Gruppe. Unter dem Einfluss der Gravitation umkreisen sich die Mitglieder dieser Galaxienfamilie und führen dabei einen galaktischen Tanz auf, der mehrere Milliarden Jahre dauert.
Wegen der riesigen Abstände zwischen den Galaxien erscheinen deren Bewegungen sehr langsam, gleichsam wie in extremer Zeitlupe. Daher sind ferne Milchstraßensysteme für den Beobachter am Himmel üblicherweise statische Objekte. In den 1920er-Jahren hatte der niederländische Astronom Adriaan van Maanen jedoch verkündet, die Drehungen und Bewegungen von so genannten Spiralnebeln - wie Galaxien zu der Zeit genannt wurden - gemessen zu haben.
Der amerikanische Forscher Edwin Hubble konnte jedoch kurz darauf diese Behauptung im Rahmen einer berühmten Debatte über die Größe des Universums widerlegen. Er zeigte, dass die "Spiralnebel" eigenständige Galaxien sind - und viel zu weit von uns entfernt, um deren Dynamik mit den damals verfügbaren astronomischen Instrumenten aufzuspüren.
Genau das ist jetzt einem internationalen Team mit präzisen Radiobeobachtungen gelungen. Die Forscher haben die Bewegung von Wasserdampfwolken in der nahe gelegenen Galaxie M33 über einen Zeitraum von drei Jahren verfolgt. Der Wasserdampf verhält sich wie ein natürlicher Laser, der aber Radiowellen aussendet. Das Ergebnis der Messungen: Die Galaxie "tanzt" 100-mal langsamer als von van Maanen behauptet. "Mehr als 80 Jahre später ist damit der Traum des niederländischen Astronomen Realität geworden - allerdings anders, als er sich das vorgestellt hat", sagt Andreas Brunthaler.

Bild: Bill Saxton, NRAO/AUI/NSF

Abb. 2: Dreidimensionale Abbildung der Galaxien in der Lokalen Gruppe sowie der gemessene Geschwindigkeitsvektor von M33. Der Geschwindigkeitsvektor der Andromedagalaxie (M31) zeigt nur die bekannte Bewegung auf die Milchstraße an.

Die Messungen zeigen, dass die Wasserdampf-Regionen zusammen mit der Galaxie pro Jahr nur um etwa 30 Mikrobogensekunden am Himmel wandern. Die Messgenauigkeit betrug 5 Mikrobogensekunden jährlich. Zum Vergleich: Aus einer Distanz von 500 Kilometern ließe sich damit noch eine Verschiebung von 0,01 Millimeter pro Jahr entdecken. "Mit der von uns erreichten Präzision könnten wir von Bonn aus sehen, wenn sich in Berlin etwas um Haaresbreite bewegt", sagt Heino Falcke, der die Arbeit in Bonn betreute. Für ihre Beobachtung schalteten die Astronomen mit Hilfe der interkontinentalen Radiointerferometrie ("Very Long Baseline Interferometry", VLBI) tausende Kilometer voneinander entfernte Radioteleskope zu einem Riesenteleskop zusammen.
Die Ergebnisse zeigen, dass sich M33 mit 190 Kilometer pro Sekunde um unsere Milchstraße und in Richtung unserer Schwestergalaxie, dem Andromedanebel, bewegt. Auch wenn M33 auf dieses System zurast, wird sie es knapp verfehlen. Die Forscher hoffen, dass sich mit diesen Resultaten sowohl die Entstehungsgeschichte der Milchstraße als auch deren zukünftige Entwicklung besser verstehen lassen. So wäre es nach derzeitigem Kenntnisstand durchaus möglich, dass die Milchstraße in einigen Milliarden Jahren mit der Andromedagalaxie kollidieren und verschmelzen wird.
Dank ihrer Beobachtungstechnik haben die Wissenschaftler aus den gemessenen Daten aber auch die Entfernung der Galaxie M33 auf rein geometrischem Weg direkt bestimmt und das Universum in unserer Nachbarschaft neu vermessen. Demnach ist M33 etwa 2,4 Millionen Lichtjahre von der Erde entfernt. Genaue Entfernungsbestimmungen stellen grundsätzlich ein großes Problem in der Astronomie dar. Weil nicht einfach ein Maßband durch das Weltall gelegt werden kann, müssen die Forscher komplizierte Methoden benutzen, die jedoch häufig mit unbekannten Fehlern behaftet sind. Daher ist es wichtig, direkte geometrische Entfernungen zu ermitteln. Diese Messungen dienen dazu, die extragalaktische Entfernungsskala neu zu eichen - und mit jedem weiteren Jahr des Experiments werden die Resultate genauer.
Exakte Entfernungen und Bewegungen werden in der Astronomie außerdem dazu genutzt, die Masse von Objekten abzuschätzen. Frühere Beobachtungen haben gezeigt, dass der größte Teil des Universums in einer mysteriösen dunklen Materie steckt. Die Astronomen erwarten, mit weiteren Beobachtungen von Galaxienbewegungen unsere Milchstraße und ihre Nachbargalaxien genau zu "wiegen". Das wird zeigen, wie viel dunkle Materie es im lokalen Universum gibt.

K4

Literatur zu "M33"
L. Foschini, J. Rodriguez, Y. Fuchs, et al.	2004	A&A 416, 529	"XMM—Newton Observations of the Ultraluminous X-ray Source in M33"
A. L. Argon, L. J. Greenhill, J. M. Moran, et al.	2004	ApJ 615, 702	"The IC133 Water Vapor Maser in the Galaxy M33: A Geometric Distance"
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H. Heintzmann

( Eintrag vom 12.3.2008)

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