11. The galactic nucleus and central bulge 11.1 Infrared observations (cont.)

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

2. In 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

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

4. In the early 1990s the COBE satellite did an all sky IR survey at near and far IR wavelengths. One of COBE’s instruments was DIRBE, the Diffuse InfraRed Background Experiment.

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

6. 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 Way showing the dust distribution. Both images were from the COBE satellite, 1995.

7. 2MASS – the 2 micron all-sky survey. This wide-field image shows red stars and warm dust in the central bulge of the Milky Way. Stars are shown blue, the cooler dust is red in this false colour image at several near IR wavelengths.

8. 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

9. Synchrotron sources are often described as non-thermal radio emission, as the high electron energies result from some process other than high temperature. HII regions are thermal (i.e. hot) sources, and the free-free radiation is also known by the name ‘thermal Bremsstrahlung’ which results from the random motion of electrons in a hot ionized gas.

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

11. All four types of radio source are observed • from 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

12. 21-cm radiation from galactic centre Radio telescopes can measure the mass of H (from intensity of emission) and its velocity in line-of-sight, VR (from Doppler effect) The observations show a 3 kpc expanding and rotating arm (VR ~ 50 km/s) of HI gas extending ~ 3 kpc from centre.

13. 21-cm intensity plot near the galactic centre. Vertical axis: radial velocity Horizontal axis: gal. latitude b

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

15. 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 ~107M⊙ of HI, and ~109 to 1010M⊙ • 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.

16. Radio 73.5 cm Radio HI 21 cm 2.6 mm H2 Mid and far IR (dust) Near IR (stars) Optical (nearby stars and dark clouds) X-rays Gamma rays The Galaxy at different wavelengths

17. 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 ~ 106M⊙ . This is probably a • massive black hole. • Size of central region ~140 pc

18. The very centre of the Galaxy, the nucleus, is seen in this near IR image from ESO (European Southern Observatory). 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.

19. 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 from the Nothern Arm. Dust enshrouded supergiant stars are the IR sources

20. Far infrared and 2-cm radio thermal emission from the region of the galactic centre.

21. Radio contour image of the galactic centre, showing Sgr A and Sgr B sources.

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

23. The nucleus of the Milky Way The nucleus contains dense molecular clouds, supernova remnants (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, which is characteristic emission from an accretion disk that surrounds a supermassive black hole.

24. End of lecture 7 Galactic nucleus in the near IR (ESO)