ASTR112 The Galaxy Lecture 7 Prof. John Hearnshaw 11. The galactic nucleus and central bulge 11.1 Infrared observations (cont.) 11.2 Radio observations

ASTR112 The GalaxyLecture 7

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11. The galactic nucleus and central bulge 11.1 Infrared observations (cont.) 11.2 Radio observations 11.3 The nature of the galactic nucleus


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awIn the 1980s the IRAS infrared astronomy satellite surveyed the Milky Way in infrared. This image shows thermal emission from warm dust at mid to far IR wavelengths.Note: zodiacal light, Magellanic Clouds


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IRAS false-colour image of the central Milky Way, showing that dust is confined to a very thin layer


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In the early 1990s the COBEsatellite did an all sky IRsurvey at near and far IRwavelengths. One of COBE’sinstruments was DIRBE,the Diffuse InfraRedBackground Experiment.


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COBE/DIRBE near IR view of the Galaxy showingthe distribution of red stars in the disk and bulge.


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A near IR view of the whole Milky Way showing the distribution of cool stars, including the concentration in the galactic centre.

A far IR view of the Milky Wayshowing the dust distribution.

Both images were from theCOBE satellite, 1995.


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aw2MASS – the 2 micron all-sky survey. This wide-fieldimage shows red stars and warm dust in the central bulge of the Milky Way. Stars are shown blue, the cooler dust isred in this false colour image at several near IR wavelengths.


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Types of radio emission in astronomy

Four different types of radio source:• Synchrotron radiation (high energy electrons spiralling in a magnetic field)• 21-cm radiation from HI (neutral H atoms)• Molecular emission lines (from dense molecular clouds)• HII sources, emitting both (a) a free-free continuous spectrum and (b) H emission lines


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Synchrotron sources are often described as non-thermal radio emission, as the high electron energies result from some process other than hightemperature.

HII regions are thermal (i.e. hot) sources, and the free-free radiation is also known by the name ‘thermal Bremsstrahlung’ which results from therandom motion of electrons in a hot ionized gas.


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The different continuous spectra from HII (thermal)and supernova remnant (non-thermal, synchrotron)radio sources


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All four types of radio source are observedfrom the central region (nucleus) of the Galaxy. There is no extinction at all by dust.

• Sgr A (or Sgr A*) is a synchrotron source at very centre of Galaxy• Sgr A East (a non-thermal supernova remnant)• Sgr A West (a thermal source – HII region)• Sgr B (about 0.7º east of Sgr A on galactic equator) and other sources nearby are thermal HII regions


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21-cm radiation from galactic centre

Radio telescopes can measure the mass of H(from intensity of emission) and its velocityin line-of-sight, VR (from Doppler effect)

The observations show a 3 kpc expanding androtating arm (VR ~ 50 km/s) of HI gas extending ~ 3 kpc from centre.


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21-cm intensity plot near the galactic centre.

Vertical axis: radial velocityHorizontal axis: gal. latitude b


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awA false colour image showing 21-cm profiles near the galactic centre. The vertical axis is latitude, b. Thehorizontal axis is radial velocity. Note the 3 kpc arm tothe left of the main peak.


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Within 2 kpc of the centre there is a rapidly rotating HI disk of gas, titlted at about 40º to the galactic plane.• The disk’s rotational velocity is ~360 km/s• Mass ~107 M⊙ of HI, and ~109 to 1010 M⊙

of molecular hydrogen, H2 • Thickness of inner disk ~100 pc• Molecular emission lines from various molecules especially OH, H2O, NH3, CO and H2CO are also observed from this disk.


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The Galaxy at different wavelengths

Radio 73.5 cm

Radio HI 21 cm

2.6 mm H2

Mid and far IR (dust)

Near IR (stars)

Optical (nearby starsand dark clouds)X-raysGamma rays


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Nature of the galactic nucleus

• Innermost 1.5 pc contains a very small source (Sgr A*)

• Even around this region there exists gas with circular velocity of ~200 km/s indicating a small but massive central object, M ~ 106 M⊙ . This is probably a massive black hole.

• Size of central region ~140 pc


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The very centre of the Galaxy, the nucleus,is seen in this near IRimage from ESO(European SouthernObservatory). A very crowded field of red stars is seen. The arrows show the presumed location of a supermassive black hole.Stars and dust orbit this massive object.


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IR sources in the galactic nucleus. This is a mid-IR image of the nucleus taken with the Keck II telescope. The suspected black hole is in centre of image. Swirling warm dust is spiralling into the hole fromthe Nothern Arm. Dust enshrouded supergiant stars are the IR sources


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Far infrared and 2-cm radiothermal emission from the region of the galactic centre.


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Radio contour imageof the galactic centre,showing Sgr A andSgr B sources.


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Radio image of the galactic nucleus at 1-m wavelengthshowing Sgr A and Sgr B and various other sources.Image size ~ 4 º × 4º. VLA radio image, New Mexico


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The nucleus of the Milky Way

The nucleus contains dense molecular clouds, supernovaremnants (SNR), mysterious filaments of gas, and a massive black hole (MBH ~ 2.6 × 106 M⊙).Within Sgr A is Sgr A*, the presumed black hole site. Sgr A* is also a recently discovered X-ray source, whichis characteristic emission from an accretion disk thatsurrounds a supermassive black hole.


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End of lecture 7End of lecture 7G

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Documents

ASTR112 The Galaxy Lecture 7 Prof. John Hearnshaw 11. The galactic nucleus and central bulge 11.1 Infrared observations (cont.) 11.2 Radio observations