The Galactic Centre: a hidden treasure How can we study the Galactic centre? How can we study the Galactic centre? What is do we find there? What is do we find there? What does it tell us about our Galaxy, and galaxies in general? What does it tell us about our Galaxy, and galaxies in general?

Galactic Bulges Classic view of spiral galaxies Classic view of spiral galaxies young stars in spiral arms old stars in halo and bulge therefore, expect Galactic centre to be a quiet collection of old stars (like a globular cluster)? therefore, expect Galactic centre to be a quiet collection of old stars (like a globular cluster)?

Observing the Milky Way 75 cm 21 cm 11 cm 2.6 mm 60 μm 8 μm 2 μm optical X-ray γ-ray

The Galactic Centre: Radio central 4° square central 4° square numerous supernova remnants numerous supernova remnants so, not a quiet region of old stars! filamentary structures suggesting complicated magnetic fields filamentary structures suggesting complicated magnetic fields bright central source, Sgr A bright central source, Sgr A

Zooming In The Very Large Array, New Mexico The Very Large Array, New Mexico 27 telescopes in Mercedes star movable on rails data processed to mimic one large telescope resolution can be better than HST!

Zooming In NRAO / AUI / NSF 20 cm 6 cm 3.6 cm1.2 cm Sgr A*

The Galactic Centre: X-rays X-ray images pick up hot gas and “astrophysical par- ticle accelerators” such as pulsars and X-ray binaries radio image for comparison images from Chandra website

The Galactic Centre: X-rays radio image for comparison

The Galactic Centre: infra-red 4.5° MSX mid-infrared view (4.2 – 26 microns) showing mostly warm dust

The Galactic Centre: infra-red MPG/ESO near infrared (2 microns) shows stars

Mysteries of the Galactic Centre A region of intense star formation surrounding a point source of radio and X-ray emission A region of intense star formation surrounding a point source of radio and X-ray emission Where does the gas come from? What is the central point source?

Bringing in the Gas Evidence from star counts and extinction studies shows Milky Way is a barred spiral with a ring structure, like M95 Evidence from star counts and extinction studies shows Milky Way is a barred spiral with a ring structure, like M95 This allows gas from the disc to be drawn in towards the centre This allows gas from the disc to be drawn in towards the centre Image by Jean-Charles Cuillandre and Giovanni Anselme, CFHT

Sgr A* Stars in infra-red images are seen to move around Sgr A* Stars in infra-red images are seen to move around Sgr A* plot orbits plot orbits determine mass of Sgr A* determine mass of Sgr A* result: approximately solar masses! result: approximately 4 million solar masses!

The Milky Way’s dark heart Is Sgr A* a supermassive black hole? Is Sgr A* a supermassive black hole?  YES! How do we know? How do we know?  Basically, it’s too small to be anything else orbit of star S2 orbit of star S2 rapid flares in X-ray and near infra-red rapid flares in X-ray and near infra-red

The black hole mystery The Milky Way is not alone! The Milky Way is not alone! most large galaxies have central supermassive black holes most large galaxies have central supermassive black holes the black hole mass is closely correlated with the mass of the galaxy’s bulge the black hole mass is closely correlated with the mass of the galaxy’s bulge we don’t know why (yet) we don’t know why (yet) note that we believe these central black holes are the power sources of active galactic nuclei note that we believe these central black holes are the power sources of active galactic nuclei

Conclusions The centre of the Milky Way galaxy is an exciting place (if not an ideal holiday spot!) The centre of the Milky Way galaxy is an exciting place (if not an ideal holiday spot!) it is a site of active star formation with numerous young, massive stars and recent supernovae it is a site of active star formation with numerous young, massive stars and recent supernovae at its heart is a 3-million-solar-mass black hole at its heart is a 3-million-solar-mass black hole further studies may give new insights on galaxy formation further studies may give new insights on galaxy formation