Appreciate the challenges to observing the centre of our Galaxy. Know the observing tools which do penetrate to the Galactic centre. Learn of the objects currently observed at the Galactic centre. Summary This Activity should enable you to: AAT 028 Absorbing material blocking visual observation of the Galactic centre
Recent history The story of the study of the central regions of our Milky Way Galaxy parallels: the progress in observation at wavelengths other than visual - infrared radio X-ray gamma ray the progress in telescopes using these wavelengths - higher resolutions and orbiting telescopes. the discovery of phenomena associated with the central regions of other galaxies black holes jets Lets briefly see some examples of these, in order to see what features we might expect to find at the centre of our galaxy.
High resolution of galactic centres Even in visible light, the Hubble Space Telescope resolved a bright split source at the centre of the M31 galaxy in Andromeda. 180,000 light years NOAO Ground view of M31 core 2,000 light years 40 light years HST view of M31 nucleus
HST Black Hole images A black hole itself, by definition, cannot be imaged. Radiation, emitted by gas and dust orbiting a massive object at high speed, is detectable. Estimation of the rotation speed and the orbit radius leads (by Keplers 3rd Law) to the central mass and an upper limit to its diameter. The HST has imaged several objects meeting black hole* criteria; this dramatic image is from an object at the centre of galaxy NGC4261. *Click here to find out about black holesClick here
Model: Material orbiting a central object - with higher velocities toward centre slit HST Imaging Spectrograph UKS 024 The Virgo cluster M84 HST M84 nucleus slit Finding: Orbital speeds of 400 km/sec within 26 light years of the central object. HST Spectrograph black hole evidence The HST imaged a spectrum of the core of the M84 galaxy (May 1997). Doppler shift of central line
M84 central object mass Compute the mass using the form of Keplers 3 rd law*: In the above units, d=1,641,000 AU and P=122,523 years. Assuming m can be neglected compared with M, d 3 /P 2 gives M=294 million solar masses! The escape velocity for such a massive object, of upper size limit set by HST resolution, is greater than c, the velocity of light. By definition, a black hole is inferred. Where mass m (solar masses) orbits mass M at a distance d (astronomical units) in a period of P (years). d 3 /P 2 = M + m Use the M84 finding of orbital speeds of 400 km/sec within 26 light years of the central object. *Click here to revise Keplers 3 rd LawClick here
Jets from Galactic Nuclei Some galaxies show jets of material emitted in opposite directions from their nuclei. This galaxy's nucleus is the small point in the centre of the image. These jets impact material surrounding the galaxy, giving rise to the giant "lobes" of radio emission seen in this image. The energy required to produce these jets is believed to be due to the influence of a black hole millions of times more massive than the Sun. Cygnus A radio source VLA
Back to our own Galaxy With our appetite whetted by what may lurk at our Galaxys centre, what are the observational difficulties involved? The previous Activity showed the difficulty in identifying spiral arms in our own Galaxy, though they are clearly evident in external galaxies. The difficulty is worse when our target is the very nucleus of our Galaxy - some 8Kpc away across the densest regions of absorbing gas and dust in the Galactic plane. Visible light is reduced by 28 magnitudes. As with infrared night vision and for observations through fog or dust on Earth, other wavelengths are needed for astronomy of the Galactic centre.
Radiation reaching us from the centre Astronomy now utilizes a wide range of the electromagnetic spectrum. < 0.6nm X-rays detected from central region by orbiting Einstein X-Ray Observatory 21 cm radio detects H I regions >100pc from centre 511 keV gamma rays detected from Galactic nucleus 2.2 m infrared enables detection of central old Population I K and M giant stars (temperature ~4000 o K)
Instruments A selection of ground based and satellite telescopes Australia Telescope NRAO 12m millimetre telescope at Kitt Peak COBE Satellite Einstein X-Ray satellite Parkes 64m Very Large Array
Abbreviations From this point on, certain terminology (eg centre) will save repetition of full terms such as the Galactic centre. Bulge - central ~3kpc diameter region of the Galaxy. Central region - central ~300pc diameter region. Centre - ~20pc diameter centre of Galaxy. Nucleus - ~3 parsec diameter core of Galaxy. The region of the electromagnetic spectrum used for an observation will appear simply as 21cm, 2.2 m, <0.6nm etc
Radio (0.4 GHz * ) Atomic hydrogen Radio (2.7 GHz * ) Molecular hydrogen Infrared Near Infrared Visual X-Ray Gamma Ray Location key Multi wave- length images The website nicely presents multi- wavelength panoramic views along the plane of the Milky Way, of which just 60 o either side of the centre (l=0 o ) are reproduced here. The website includes references to authors, observations and background material. * Click here to be reminded about GHz Click here
Highlights Radio (0.4 GHz*) Atomic Hydrogen Radio (2.7 GHz) Molecular H 2 Infrared Near Infrared Visual X-Ray Gamma Ray Location key Note the hopelessness of visual observations of the Galactic centre. Note the high central intensities in: near infrared 0.4GHz radio gamma ray Note the quiet centre for atomic hydrogen.
The Galactic Bulge IRAS 12 m shows strong sources from asymptotic giant branch (AGB) stars in the H-R diagram*. Apart from the very centre, the rotation curve* shows that bulge stars rotate with similar periods (like a solid body) with higher velocities for larger orbits about the centre. The bulge, about 3kpc diameter, comprises heavy element enriched stars, especially type M giants, Pop I K giants, and a few metal rich RR Lyrae stars *. As we journey to the centre of the Galaxy, we take a quick glance at its central bulge. * Click here to revise H-R diagrams Click here * Click here to find out about RR Lyrae starsClick here * we met rotation curves in the Activity on Galactic Rotation The derived bulge mass is some 10 billion solar masses.
Radius (pc) Enclosed solar masses 21cm The inner Galactic Bulge The high central stellar density affects the velocity curve. Mechanical energy exchange from close stellar encounters should lead to a close-to-flat rotation curve, with enclosed mass proportional to orbit radius. This is confirmed, from various indicators, down to r = 2pc. Closer in, velocities increase significantly, suggesting ~4x10 6 solar masses in the inner 0.5pc. 2.2 m from old Pop I K and M giants indicates a high central stellar density.
Radio Map of Central Region Continuous radio emission from the Galactic central region shows a string of radio sources in the galactic plane. Centre of Galaxy o latitude Sagittarius B Sagittarius A The strongest source is Sagittarius A (Sgr A), followed, like the split source in M31, by nearby Sgr B. This region is about 270x90 parsecs. longitude0 o 00 0 o 30 1 o o 30Galactic Galactic equator B1 B2 C
Sources of energy As we introduce each type of source detected in the Galactic centre region, we will consider what it might consist of - from the point of view of energy production or mass involved (in solar units). For example, some of the sources in the last frame show characteristics of H II regions. The O and B stars necessary to keep these regions ionized and emitting radiation, is estimated to be equivalent to about five million Suns (close to that from velocity measures). To a first approximation this is about 7 times the density of stars in the solar neighbourhood. … Night skies would be rather bright!
Clues to magnetic fields Unusual filamentary features appear near Sgr A. 20cm radiation produced by synchrotron radiation* reveals filaments which stretch for 20pc, at right angles to the galactic plane, and then make an almost right- angle turn. Sgr A simulated image From the strength and polarization of the radiation, magnetic fields would be two to four orders of magnitude weaker than the Earths magnetic field. * Click here to find out about synchrotron radiation Click here
High resolution radio information We now turn to high resolution radio mapping of the Galactic centre, for which the Very Large Array (VLA) - introduced in the next frame - has been at the forefront.
The Very Large Array (VLA) A high resolution radio interferometer. Near Socorro, New Mexico, the VLA consists of 27 antennas arranged in a huge Y pattern up to 36km across. Each antenna is 25 meters in diameter. They are combined electronically to give the resolution of an antenna 36km across, with the sensitivity of a dish 130 meters in diameter. At its highest frequency, 43GHz, its resolution is 0.04 arc. NRAO Photo by Dave Finley Internet:
RA Dec Sgr A East Sgr A West Sgr A* The VLA resolves Sagittarius A This image, from 6cm and 20cm radiation, resolves detail down to ~2 arc. It shows the following components: Sgr East: A non-thermal shell-like structure, usually interpreted as a supernova remnant. Sgr West: A spiral shaped thermal source, like an H II region. Within Sgr West is a non- thermal point source <0.1 diameter, given the name Sgr A* - -pronounced Sadge-A-Star
Sagittarius A West Here we show the various named regions of this complex. RA 17 h 42 m 31 s 29 s 27 s 25 s 1 parsec Northern arm Western arc Dust and gas disk Galactic equator Background Sgr A* far infrared m 2cm microwave Diagram indicative only Eastern arm Bar -29 o o o 58 N E Dec Doppler shifts, from Ne II infrared emission at 12.8 m, reveal high velocities in the bar region.
The various arms of the mini- spiral pattern are as labelled. The Sgr A* radio luminosity is ~2x10 27 W from within a diameter of less than 20AU. The next frame gives a mass estimate from the higher velocities within the bar region. The general nature of their radial velocity (recession, approach) is indicated (up to ~130km/sec). Sgr A* 1 parsec The Sgr A West mini-spiral and Sgr A* Rotation velocities increase toward the site of Sgr A*. Bar Eastern arm Northern arm Western arc
Mass estimate within Sgr A* A gas cloud r=0.3pc from the centre has a measured velocity of v=260km/sec. If this is orbiting a central mass, calculate that mass. Use either M=v 2 r/G in standard units and work through to a result in solar masses, or Keplers law, r 3 /P 2 =M, which gives M in solar masses if we first calculate distance r in AU and period P in years. In this case r=0.3 parsecs or ~61679 AU and P=7089 years, leading to M = 4.7 million solar masses! Could Sgr A* be a massive black hole? The Schwarzschild radius (within which light cannot escape) is R s =2GM/c 2 = 0.09AU This is well below the current resolution limit.
X-ray emission Time variable X-rays have been detected from the region of Sgr A West including Sgr A*. One X-ray mechanism involves accretion disks around dense stars - white dwarfs, neutron stars or black holes - another hint to the nature of Sgr A*. X-ray images of the Galactic nuclear region The speed of light limits the diameter d of an object from which time fluctuations t of radiation are observed: d
Gamma rays Gamma rays at 511 keV have been observed from a source less than 0.3pc diameter almost coincident with the Galactic centre. 511keV, the rest mass energy of an electron, is a signature of electron-positron annihilation. Since it is believed black holes can produce positrons in the space around them, this seems to support a black hole as a candidate for Sgr A*. However the enormous 511keV luminosity of about 5x10 4 times the solar luminosity implies a smaller black hole (~500 solar masses) than that envisioned for the Galactic centre.
VLA =90cm - central region, wide field Compare with the earlier Radio Map. Note the shell-like structure of super- novae remnants (SNRs). Note the fine threads at high angles to the Galactic plane and extending for tens of parsecs. NRAO Image: Kassim, LaRosa, Lazio & Hyman 1999
Supernovae activity Even higher energy 1.8MeV gamma rays have been detected. The 1.8MeV line is produced by the decay of 26 Al to 26 Mg. 26 Al has a half-life of 716,000 years and is only produced in small amounts in supernovae & novae explosions and possibly Wolf-Rayet * stars. The detected presence of ~5 solar masses of 26 Al suggests that a large number of supernovae have occurred in the Galactic centre over the last million years. It certainly appears to be an active environment! * Click here to find out about Wolf-Rayet stars Click here
The importance of Galactic centre studies Other galaxies also appear to have black holes at their centres. Some are relatively quiet while others have extremely active nuclei. Back to our own Galaxy, the rotational dynamics, mass distribution and energy processes of the overall Galaxy may lead to the production of high mass density (including black hole(s) at the centre, or if supermassive black holes were produced at the galaxys embrionic stage they may in some way power other features of the Galaxy - even spiral arms.
WavelengthTelescopeRegionSource radio nucleusSgr A and a string of sources; H II and SNR characteristics IR/RadioNucleusmetal-rich giants, low mass dwarfs mDust at Sgr A heated by OB stars McentreDust heated by Pop I and O stars 12.8 mnucleusNe II emission; Doppler shift 200km/sec within 1.5pc of centre 12 mIRASBulgeAGB stars 2.2 mcentrePop I K giants <0.6 nm X-rayEinstein<100pcweak sources in weaker halo ~10 -3 nm raynucleus<0.3pc size, at or near nucleus Summary i) Some of the wavelengths and sources weve visited.
Summary ii) All the observations point to massive objects within a very small radius of the Galactic centre. They may take the form of: a) a massive black hole of ~4x10 6 solar masses, b) a very dense star cluster of ~10 6 solar masses within 2pc of the centre. Additional support for the black hole scenario comes from similar evidence in other galaxies.
Hubble Space Telescope images indexed by subject: ESO (European Southern Observatory) VLT images: NRAO VLA 90cm radio image of Galactic centre region NRAO VLA site images VLA: Cygnus A COBE and Einstein satellite pictures: Image Credits VLA
The origin of the Milky Way is the subject of the next Activities. Hit the Esc key (escape) to return to the Index Page
Thus: f=c/ or =c/f c=3x10 5 km/sec What wavelengths would 2.7Ghz and 0.4 GHz be? Frequencies and wavelength Explanation Since the days of tinkering with valves, radio astronomers often refer to frequencies (f) rather than wavelengths ( ). The frequency of passing wavecrests = speed of wave / wavelength 2.7GHz: = 3x10 5 /(2.7x10 9 ) = 1.11x10 -4 km = 11.1 cm background The previous frame showed radio observations expressed in GHz. (GigaHertz or 10 9 cycles per second) What frequency is the 21cm hydrogen line? f = 3x10 5 /(.21x10 -3 ) = 1.428x10 9 = ~1.4 GHz 0.4GHz: = 3x10 5 /(0.4x10 9 ) = 7.5 x10 -4 km = 75 cm Click hereClick here to return to the Activity
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Introduction to Synchrotron Radiation background The following section is a brief introduction to thermal and non-thermal processes, and in particular, synchrotron radiation.
Thermal Radiation Conventionally, thermal radiation refers to black body radiation at a given temperature. background In astrophysics the term thermal includes absorption, emission and scattering processes arising from any interactions between electrons and atoms or molecules in a hot medium - including: excitation/de-excitation 1 within the atom ionization/recombination 2 to/from free electrons free-free processes 3 between electrons, photons and ions
Electrons ( mass m, charge q ) spiral, in magnetic field B, at angular frequency =qB/( mc) c = speed of light v = electron velocity = Non-thermal radiation Involving physical processes not dependent on temperature. (Including the MASER process, not covered here.) background Non-thermal processes include synchrotron radiation from electrons, moving at near light speeds, and spiralling along magnetic flux lines. The radiation is polarized and the process relativistic. Click hereClick here to return to the Activity
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About Wolf-Rayet Stars Although we feel that we know a lot about stellar evolution, (even if only through indirect evidence), there are still some fascinating stellar objects which are hard to explain. Wolf-Rayet stars are very hot (T~30,000 K), massive (perhaps 10 to 40 M ) stars which are often found in binary systems (which we use to estimate their mass), are losing mass at very high rates, and exhibit strong, wide emission lines of nitrogen, oxygen and carbon and weak or nonexistent hydrogen lines. It is believed that the high rate of mass loss in these (probably) post-main-sequence stars has stripped them of most of their hydrogen envelopes, exposing nuclear processed material in inner layers near their cores. background
If indeed these stars turn out to be typically in binary systems, they may turn out to be the more massive and faster evolving partners. Theoretical models suggest that a Wolf-Rayet star in a binary system is just past its red supergiant stage, where much of its envelope has swollen up and spilled over onto its companion, and just before it undergoes a supernova explosion! However without more evidence, we cant be sure exactly what these intriguing stars are. Back to the Activity background
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Giants Luminosity L/L Absolute Magnitude Temperature (K) The HORIZONTAL BRANCH Instability Strip …that also happen to fall within the Instability Strip, the region of the H-R diagram which contains variable stars. RR Lyrae Variables are stars on a region of the H-R diagram called the helium burning Horizontal Branch RR Lyrae Stars background
The great thing about RR Lyrae Variables is that they are bright and all at about the same absolute magnitude This is because RR Lyrae Variables all have about the same mass and are all at the same phase in their evolution. As long as we can find such stars their brightness immediately tells us their distance, and therefore the distance to the cluster they are in. Back to the Activity background
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H-R Diagrams In 1905 the Danish astronomer Ejnar Hertzsprung noticed that a graph of the absolute magnitudes of stars versus their colour showed a few very regular groupings. A bit later on, Henry Russell in America noticed the same thing, although he used spectral type rather than colour. blue yellow red Absolute magnitude -10 bright +15 faint Spectral type O5 M8 Thats why the diagrams you are about to study are called Hertzsprung-Russell Diagrams (H-R for short). background
Temperature versus Type Later on, when the link between spectral type and temperature was realised, H-R diagrams began to appear with temperature along the horizontal axis instead. Absolute magnitude -10 bright +15 faint Spectral type O5 M8 Boring! Why have you suddenly gone all historical? temperature 2500 Because we have to explain why temperature goes down along the horizontal axis of an H-R diagram: a long time ago, astronomers listed stars by colour, from blue (hot) to red (cool). Ahhhh. blue Colour red
O B A F G K M lowluminosityhigh White dwarfs Red dwarfs Main sequence Super- giants Giants H-R diagrams and spectral classes Well use this version of an H-R diagram to show how spectral classes appear in that format. background
Looking for patterns hightemperaturelow lowluminosityhigh Huge, cool stars appear in the top right, and small, hot stars tend to gather in the bottom left. But the rest of the stars lie somewhere along the main sequence. T increasing L increasing Mass increasing Back to the Activity background
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M m radius d period P G and 4 2 are constants M is the mass of the Sun Johannes Keplers third law for planets: There is a fixed relationship between the cube of the radius (d) of a planets orbit and the square of its period (P) of orbit. Back to the Activity In other situations where objects are in orbit the law still applies, but if the mass m is not tiny compared to M then the formula becomes d 3 /P 2 = M + m background
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Black Holes When a star more massive than 8 M reaches the end of its life, the stars gravity is so strong that it collapses into an object of zero radius and infinite density - a black hole. The gravitational field of a black hole is so strong that even light cannot escape. For this reason, black holes are not directly observable. Back to the Activity background