The Galactic Centre:The Galactic Centre:a hidden treasurea 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 What does it tell us about our Galaxy, and galaxies in

general?general?

Galactic BulgesGalactic Bulges

Classic view of spiral Classic view of spiral galaxiesgalaxies young stars in spiral young stars in spiral

armsarms old stars in halo and old stars in halo and

bulgebulge therefore, expect Galactic therefore, expect Galactic

centre to be a quiet centre to be a quiet collection of old stars collection of old stars (like a globular cluster)? (like a globular cluster)?

Observing the Milky WayObserving the Milky Way

75 cm

21 cm

11 cm

2.6 mm

60 μm

8 μm

2 μm

optical

X-ray

γ-ray

http://adc.gsfc.nasa.gov/mw/milkyway.html

The Galactic Centre: RadioThe Galactic Centre: Radio

central 4central 4° square° square numerous supernova numerous supernova

remnantsremnants so, not a quiet region of old so, not a quiet region of old

stars!stars!

filamentary structures filamentary structures suggesting complicated suggesting complicated magnetic fieldsmagnetic fields

bright central source, bright central source, Sgr ASgr A

Zooming InZooming In

The Very Large The Very Large Array, New Array, New MexicoMexico 27 telescopes in 27 telescopes in

Mercedes starMercedes star movable on railsmovable on rails data processed to data processed to

mimic one large mimic one large telescopetelescope

resolution can be resolution can be better than HST!better than HST!

Zooming InZooming In

NRAO / AUI / NSF

20 cm6 cm

3.6 cm1.2 cm

Sgr A*

The Galactic Centre: X-raysThe 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 comparisonimages from

Chandra website

The Galactic Centre: X-raysThe Galactic Centre: X-rays

radio image for comparison

The Galactic Centre: infra-redThe Galactic Centre: infra-red

4.5°

MSX

mid-infrared view (4.2 – 26 microns) showing mostly warm dust

The Galactic Centre: infra-redThe Galactic Centre: infra-red

MPG/ESO

near infrared (2 microns) shows stars

Mysteries of the Galactic CentreMysteries of the Galactic Centre

A region of intense star A region of intense star formation surrounding a formation surrounding a point source of radio and point source of radio and X-ray emissionX-ray emission Where does the gas come Where does the gas come

from?from? What is the central point What is the central point

source?source?

Bringing in the GasBringing in the Gas

Evidence from star Evidence from star counts and extinction counts and extinction studies shows Milky studies shows Milky Way is a barred Way is a barred spiral with a ring spiral with a ring structure, like M95structure, like M95

This allows gas from This allows gas from the disc to be drawn the disc to be drawn in towards the centrein towards the centre

Image by Jean-Charles Cuillandre and Giovanni Anselme, CFHT

Sgr A*Sgr A*

Stars in infra-red images Stars in infra-red images are seen to move are seen to move around Sgr A*around Sgr A* plot orbitsplot orbits determine mass of Sgr A*determine mass of Sgr A* result: approximately result: approximately

4 million solar masses! solar masses!

The Milky Way’s dark heartThe Milky Way’s dark heart

Is Sgr A* a supermassive Is Sgr A* a supermassive black hole?black hole?

YES!YES! How do we know?How do we know? Basically, it’s too small to Basically, it’s too small to

be anything elsebe anything else orbit of star S2orbit of star S2 rapid flares in X-ray and rapid flares in X-ray and

near infra-rednear infra-red

The black hole mysteryThe black hole mystery

The Milky Way is not alone!The Milky Way is not alone! most large galaxies have central most large galaxies have central

supermassive black holessupermassive black holes the black hole mass is closely the black hole mass is closely

correlated with the mass of the correlated with the mass of the galaxy’s bulgegalaxy’s bulge

we don’t know why (yet)we don’t know why (yet) note that we believe these central note that we believe these central

black holes are the black holes are the power sources of power sources of active galactic nucleiactive galactic nuclei

ConclusionsConclusions

The centre of the Milky Way galaxy is an The centre of the Milky Way galaxy is an exciting place (if not an ideal holiday spot!)exciting place (if not an ideal holiday spot!) it is a site of active star formation with numerous it is a site of active star formation with numerous

young, massive stars and recent supernovaeyoung, massive stars and recent supernovae at its heart is a 3-million-solar-mass black holeat its heart is a 3-million-solar-mass black hole further studies may give new insights on galaxy further studies may give new insights on galaxy

formationformation