Wissenschaftliche Ergebnisse / Galaxien und agn 3 Galaxien und agn / Galaxies and agn

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Wissenschaftliche Ergebnisse / Galaxien und AGN

2.3 Galaxien und AGn / Galaxies and AGN


Nearby normal, starburst, and active galaxies provide the perfect laboratories for detailed investigation of the processes important for galaxy evolution. With the scientific expertise and instrument development capability spanning nearly the entire spectrum of wavelengths – from gamma-rays, x-rays, and near-IR through millimetre, scientists at MPE are in the unique position of being able to study these systems over a wide range of size scales, morphological type, and activity. This was another fascinating year for extragalactic research at MPE, with the Galactic Centre again taking centre stage. We were witness to spectacular flaring events at both infrared and x-ray wavelengths. In October of 2002, we observed with XMM-Newton the brightest X-ray flare reported so far from Sgr A* with a duration shorter than one hour. Then, on the 9th of May, during routine observations of the Galactic Centre star cluster at 1.7 microns with the CONICA/NAOS adaptive optics imager/near infrared camera at the ESO VLT, we witnessed a powerful IR flare at the location of the black hole (Sgr A*) itself. This was the first time that such an event has been observed in the infrared. In this chapter, we present these and the many other highlights from the exciting extragalactic research that we have undertaken in 2003 at MPE.

2.3.1 Das Galaktische Zentrum / The Galactic Center

/ Near-infrared flares from the black hole

As the nearest galactic nucleus, the centre of the Milky Way is a unique laboratory for study of the physical processes that operate in the vicinity of a supermassive black hole. Near-infrared high-resolution observations of the galactic centre (GC) became possible since the beginning of the 1990s. Since then, the GC stellar cluster was regularly monitored by high-resolution NIR imaging. However, in spite of all efforts, we were not able to unambiguously detect a NIR counterpart of Sgr A*, the putative black hole, before 2003. On the 9th of May, during routine observations of the GC star cluster at 1.7 microns with the CONICA/NAOS adaptive optics imager/near infrared camera at the ESO VLT, we witnessed a powerful flare at the location of the black hole (Fig. 1 & 2). Within a few minutes, the flux of a faint source increased by a factor of 5-6 and fainted again after about 30 min. The flare was found to have happened within a few milli-arcseconds of the position of Sgr A*. The short rise-and-decay times told us that the source of the flare was located within less than 10 Schwarzschild radii of the black hole.

Abb. 1: _2.3_ott_gc1.eps

Fig 1: Detection of NIR emission from Sgr A*. The images show raw AO images (60 s total exposure time) of an area 1''x1'' around Sgr A*, observed on May 9, 2003 UT. The left image was taken at the beginning of the observations; the right image about 40 min later. The flaring source is easily detected in the right image. Its position is offset -1.4±3.0 mas in R.A. and -0.2±3.0 in Dec. from the dynamical position of Sgr A* as it was determined from the orbit of the star S2. The star S2 is marked by a cross, the position of Sgr A* is indicated by a white circle.

During subsequent observations in 2003, we observed more flares from Sgr A* as well as quiescent emission from a source at this location. With hindsight, we could also detect a flaring source in older, longer wavelength data from 2002. Up until now, we have observed four flares in the H, K and L-bands (1.7, 2.2 and 3.8 microns). The flares were observed at four different epochs within a few milli-arcseconds of the location of Sgr A*, which makes it highly probable that they are indeed associated with matter in the immediate environment of the black hole, which is also reflected in the very short rise-and-fall time scales of the light curves.

The quiescent and flaring NIR emission from Sgr A* fills an important gap in our knowledge of the spectrum of this source and will allow us to constrain the existing models of how the radiation is produced (Fig. 3). While the quiescent emission appears to be largely consistent with an origin in the high-energy tail of a synchrotron spectrum, the mechanism of the NIR flares is uncertain. Although the NIR flares were observed at different epochs, they might hint at a blue colour of the flares, which would be a challenge to current theories. Simultaneous, multi-wavelength NIR and X-ray observations of the GC are planned for 2004. The chances are high that these observations will provide the required data to constrain the models and to establish (or exclude) a relation between the X-ray and NIR variability.

Abb. 2: _2.3_ott_gc2.jpg

Fig 2: Light curves of the Sgr A* NIR flares in 2002 and 2003, observed with NACO/VLT. The L'-band flare on August 30, 2002, was only partially covered by observations. Gaps in the time series of the H-band flare on May 10, 2003, and of the KS-band flare on June 15, 2003, are due to sky observations and instrument failure, respectively. For comparison, the steady emission of the star S1 near Sgr A* is shown in all plots (light grey data points). Arrows in the plots of the two KS-band flares indicate substructure peaks of the flares. Both KS-band flares show very similar quasi-periodicity, although the second flare was observed more than 24 h after the first one and must thus have been an unrelated event. The upper right panel shows the normalised power spectrum of the two KS-band flares. Both of them show a significant peak at a frequency corresponding to time scales of 16.8±2.0 min. In both cases, the power spectrum of S1 does not show such a frequency.

Abb. 3: _2.3_ott_gc3.eps

Bildunterschrift kürzer!!!

Fig 3: Spectral energy distribution of the emission from Sgr A* showing the extinction and absorption corrected luminosities. All error bars are ±1 σ and include statistical and systematic errors. Black triangles denote the radio spectrum of Sgr A*. Open grey circles mark various infrared upper limits from the literature. The three X-ray data ranges are (from bottom to top) the quiescent state as determined with Chandra (black; Baganoff et al. 2003), the autumn 2000 Chandra flare (red; Baganoff et al. 2000), and the autumn 2002 flare observed by XMM (light blue; Porquet et al. 2003). Open red squares with crosses mark the de-reddened peak emission (minus quiescent emission) of the four NIR flares. Open blue circles mark the de-reddened H, KS, and L' luminosities of the quiescent state, derived from the local background subtracted flux density of the point source at the position at Sgr A*. This eliminates the contribution from extended, light due to the stellar cusp around Sgr A*.

/ A spin measurement of the black hole?

The two K-band flares observed on the 15th and 16th of June 2003 are the flares that were completely covered by observations. Although they happened more than 24 hours apart and thus appear to be unrelated events, they both show a striking quasi-periodicity of the flare with a period of about 17 min. Of all possible periodic processes near a black hole (acoustic modes of a thin disk, Lense-Thirring precession, precession of orbital nodes, orbital motion), the period of matter circling the black hole near the last stable orbit is the shortest one. The observed period of 17 min is so short, however, that the only reasonable explanation is that the oscillations are produced by Doppler boosting of hot gas near the last stable orbit of a spinning (Kerr) black hole. The spin of the black hole will allow for a last stable orbit closer to the event horizon and thus with a shorter orbital frequency. From the observed 17 min period we estimate that the supermassive black hole Sgr A* has a spin that is half as big as the maximum possible spin of such an object.

Additional observations of flares and their quasi-periodicity will be needed in order to confirm this result. Should the quasi-periodicity indeed be an intrinsic feature of the flares then this will mean that the era of black hole physics has begun with the properties of black holes accessible to direct measurements!

Stellar Population and Dynamics of the Galactic Center Star Cluster

We observed the crowded central parsec in spring, 2003, with the new integral field spectrometer for the VLT, SPIFFI, obtaining the deepest yet near-infrared, high angular resolution, integral field spectroscopy of the region. SPIFFI provides simultaneous spectra of all stars in the field of view, permitting spectroscopic classification of unprecedented numbers of bright blue and red stars (Fig. 4). Combining proper motions and radial velocities for the blue stars reveals their surprising dynamics: they populate two rotating stellar discs that are at large angles to each other, and that rotate in the opposite sense to the rest of the Galaxy. The stars in these two discs have very similar stellar content and appear to have formed coevally about 5 Myr ago in a metal-rich starburst that lasted for several Myr. How did these massive stars come to exist so near to the central black hole? They are too young to have formed further away and migrated in, but strong tidal forces would prevent star formation by the usual mechanism of molecular cloud collapse. The presence of two coeval stellar discs suggests an origin in a sudden dissipative event, such as the collision of two infalling clouds that created debris gas discs that then formed the stars.

The SPIFFI spectroscopy, together with other spectroscopy and proper motion measurements, also allowed a new geometric distance measurement to the Galactic Centre of 7.94 0.42 kpc, which confirms and improves previous primary distance measurements that are critical rungs in the extragalactic distance ladder (Fig. 5).

Abb. 4: _2.3_ott_gc4.gif

Fig. 4: Selected SPIFFI spectra superposed on a NACO H/K/L’ colour composite image of the central region. The spectra display the wide range of stellar types found in the cluster: late-type main sequence O stars (the star S2 near Sgr A*; circle in image), luminous blue variables (IRS16SW, lower right), early WN (middle left) and WC (top right) Wolf-Rayet stars, red supergiants (the brightest star IRS7 at the top/middle of the image), bright asymptotic giant branch stars (IRS9, lower left) and normal red giants (top left). Note that in the case of the dusty WC5/6 star IRS3 (top right) we first subtracted a strong featureless power-law to emphasize the characteristic carbon features.

Abb. 5: _2.3_ott_gc5.gif

Fig 5: Geometric determination of the distance from the Sun to the Galactic Centre from a precision measurement of the star S2 that orbits the super massive black hole. The star's radial velocity is measured from its Br-gamma Doppler shift (from SPIFFI, INSPECT/Keck--Ghee et al. 2003, and NACO spectra), while its proper motion is measured from SHARP/NTT and NACO images. The orbital solution ties together the angular and absolute velocities to yield the distance to the S2/Sgr A* binary system.

XMM-Newton observations of a spectacular X-ray flare from Sgr A*

We have observed on October 3, 2002 with XMM-Newton (during an exposure of about four hours), the brightest X-ray flare reported so far from Sgr A* with a duration shorter than one hour (~2.7 ski; Fig. 6). The light curve is almost symmetrical with respect to the peak flare, and no significant difference between the soft and hard X-ray range is detected. The overall flare spectrum is well represented by an absorbed power-law with a soft photon spectral index of Γ=2.50.3, and a peak 2-10 keV luminosity of about 3.6x1035 erg/s, i.e. a factor 160 higher than the Sgr A* quiescent value. No significant spectral change during the flare is observed. This X-ray flare is very different from other bright flares reported so far: it is by far much brighter and softer. The present accurate determination of the flare characteristics challenges the current interpretation of the physical processes occurring inside the very close environment of Sgr A* by providing very strong constraints for the theoretical flare models.


Abb. 6: _2.3_Porquet_gc.jpg

Fig. 6: A three-colour image of our Galactic Centre observed in X-rays by XMM-Newton (red: 0.5 to 2 keV, green: 2 to 5 keV, blue 5 to 10 keV). At left: the long exposure (12 hours) in February 26, 2002 shows the quiescent phase of Sgr A*. At right: The same region during the shorter observation (4.5 hours) in October 3, 2002 showing Sgr A* during its X-ray flare.

2.3.2 Nahe normale Galaxien / Nearby Normal Galaxies

Die Magellanschen Wolken / The Magellanic Clouds

Photo-dissociated regions (PDRs) are those components of the Interstellar Medium (ISM) where physics and chemistry are mainly driven by the Far UV photons (i.e. interface between the HII region and molecular cloud). Their role in star formation and thus in galaxy evolution is very important, because they could be the dominant component of the atomic neutral ISM in galaxies. Here, we present ISOCAM spectro-imaging data between 5 and 18 m (/ ~ 50), of the HII complex N4 in the Large Magellanic Cloud (LMC). Together with colleagues in France and Chile, we have imaged the single Infrared Bands (IBs) and the ionised gas lines. We then subtracted the fitted features to produce a data cube representing the "pure" MIR continuum. Fig. 7 shows the 7.7 m Infrared Band (IB) map with the SIV and the pure 15 m continuum contours overlaid. PAH emission arises in a shell, and it is brightest in the north front with two unresolved peaks. All maps in the other IBs show a very similar morphology suggesting that IB carriers have the same origin, and are excited by the same mechanism. The ionised gas arises mainly in the dust cavity close to the location of the two purported exciting stars. The figure also shows spectra of four typical components of an HII complex: the eastern CO peak (outside the ionised gas), the dust bright front of the nebula, the peak of the ionised gas and the peak in the 15 m pure continuum emission. On the molecular cloud, both IBs and the continuum are very weak, which is typical of the quiescent region. IBs are very strong in the dense north front, which is typical of the PDR, and they disappear where the ionised gas reaches its maximum, most likely because they are destroyed by the intense radiation field. The IB emission ratio is quite constant throughout the dust shell, suggesting that here their carriers (PAHs) are all in the same ionisation state and/or hydrogenation. No significant differences from the typical dust feature ratio found in normal metallicity environments have been found. Finally, the "pure" continuum 15 m map peaks on a point-like source, corresponding to a bright spot in the dust shell. NIR data reveal that there is a red star, which is probably deeply embedded, as is also suggested by the deep silicate absorption in the corresponding spectrum.

Abb. 7: _2.3_contursi_LMC.ps

Fig. 7: Emission in the 7.7 m PAH feature of the HII complex N4 in the LMC. White contours: emission in the SIV (15.6 m) line; black contours: emission in the pure continuum at 15 m. Also shown are MIR ISOCAM spectra of 4 representative HII-PDR-molecular cloud components of the HII complex.

During the 1990s an increasing number of hard X-ray transients  often pulsars  were discovered in the Small Magellanic Cloud (SMC) by ASCA, ROSAT and RXTE. Many of them were optically identified with Be stars forming a binary system with a neutron star, which powers the X-ray emission via accretion of matter. XMM-Newton observations of the SMC continue to reveal new Be/X-ray binary pulsars. After RX J0101.3-7211 (neutron star spin period 452.2 s), XMMU J005605.2-722200 (140.1 s) and RX J0057.8-7207 (152.34 s), we discovered pulsations with a period of 263.6 s in EPIC data of RX J0047.3-7312. Together with measurements from previous satellites this has enabled us to follow the spin period history over many years, and led to the discovery of large spin period changes, in particular for some of the long period pulsars (Fig. 8). The number of high mass X-ray binaries and candidates in the SMC comprises now 65 objects of which at least 37 showing X-ray pulsations. This is the highest number of such systems known from any galaxy, including the Milky Way, which contains about a factor of 100 more mass than the SMC.

Abb. 8: _2.3_pietsch_XG1.ps

Fig 8: Spin history of three long period binary pulsars in the SMC. The last two points are XMM-Newton measurements.

/ Suche nach Dunkler Materie im Rahmen des WeCAPP Projektes

Messungen der Rotationsgeschwindigkeiten bei Spiralgalaxien legen auch auf diesen Skalen die Existenz von Dunkler Materie nahe. Neben hochenergetischen Teilchen, die von verschiedenen Theorien vorhergesagt werden, kommt auch baryonische Materie als Kandidat für den Hauptbestandteil an Materie dieser Galaxien in Frage. Bei diesen, unter dem Namen MACHOs (MAssive Compact Halo Objects) zusamengefaten Objekten handelt es sich um Objekte, die, weil sie zu schwach oder gar nicht leuchten, bisher in den Himmelsdurchmusterungen unentdeckt blieben. Unter diese Gruppe fallen Braune Zwerge, frühzeitliche Schwarze Löcher, aber auch Überreste einer frühen Generation von Sternen, die sich zu Weien Zwergen, Neutronensternen oder Schwarzen Löchern entwickelt haben.

Die direkteste Methode für den Nachweis dieser dunklen Halo-Objekte bietet der sogenannte „Gravitationslinseneffekt“, eine von der Allgemeinen Relativitätstheorie vorhergesagte Eigenschaft von Materie, Licht abzulenken und zu verstärken. Im Jahre 1993 konnte erstmals das Aufhellen eines Sterns in der Grossen Magellanschen Wolke (LMC) durch den Gravitationslinseneffekt eines Objekts, das den Sehstrahl kreuzte, nachgewiesen werden. Diese Mikrolinsen (mikro deshalb, da ihre Lichtablenkung zwar vorhanden, aber zu klein ist, um sie nachzuweisen), verursachen einen charakteristischen Helligkeitsanstieg, der eindeutig auf die Raumkrümmung schliessen lässt. Findet man ausreichend viele dieser sehr seltenen (Wahrscheinlichkeit ca. 10-6) Ereignisse, können schliesslich Aussagen über die Art und Konzentration der kompakten Dunklen Materie gemacht werden.

An der Universitäts-Sternwarte München wurde 1997 mit dem Wendelstein Calar Alto Pixellensing Projekt (WeCAPP) ein Gravitationslinsenexperiment zur Suche nach MACHOs zwischen der Andromeda-Galaxie und der Milchstrasse gestartet. Die starke Neigung (77) der Andromeda-Galaxie (M 31) und ihre Entfernung (d=770 kpc) ermöglicht die gleichzeitige Beobachtung von unterschiedlich dichten Bereichen des M 31 Halos. Im Gegensatz zu klassischen Mikrolinsen-Experimenten werden nicht einzelne Sterne vermessen, sondern Veränderungen der Flächenhelligkeit der Galaxie untersucht, hervorgerufen durch die Aufhellung eines einzelnen Sterns. Mit Hilfe der Methode des “Difference Imaging” gelang es, auch diese sehr sternreichen Gebiete mit Hunderten von Sternen in einem Pixel nach Veränderlichen Quellen zu untersuchen.

Abb. 9: Echtfarbenbild des WeCAPP Feldes in M 31. Das Bild hat eine Kantenlänge von etwa 17’ und wurde aus optischen Aufnahmen in den Filtern I, R und V zusammengesetzt.

Fig. 9: _2.3_fliri1.jpg

Das Wendelstein Calar Alto Pixellensing Projekt beobachtet seit 1999 einen ca. 17’x17’ grossen Bereich des Zentrums von M 31 (Abb.9) parallel mit dem 1.23 m Teleskop am Calar Alto und dem institutseigenen 0.8 m Teleskop am Wendelstein in den beiden optischen Filtern R und I. Die hierbei gewonnenen Daten stellen die, was die zeitliche Überdeckung betrifft, umfangreichste und vollständigste Datenbasis des Bulges von M 31 dar. Die Reduktion der Calar Alto Daten der Beobachtungskampagne 2000/2001 erbrachte den Nachweis zweier hochverstärkter Mikrolinsen-Ereignisse mit sehr gutem Signal-zu-Rausch Verhältnis. Beide Ereignisse, WeCAPP-GL1 und WeCAPP-GL2, zeigen achromatische Lichtkurven, die der durch die Theorie vorhergesagten charakteristischen Form entsprechen (Abb. 10).

GL1, die bestgesampelte Gravitationslinse in M 31, wurde ebenfalls von dem französischen Konkurrenzprojekt AGAPE nachgewiesen, allerdings mit deutlich schlechterer Zeitüberdeckung. Die Kombination beider Datensätze erlaubte eine Abschätzung der Masse des gelinsten Objekts. Für eine Linse im Dunklen Halo von M 31 ergibt sich eine Masse von 0.08 M (GL2: 0.02 M), Linsen mit einer Sonnenmasse sind um einen Faktor zwei unwahrscheinlicher.

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