Supermassive Black Holes at the Centers of

GalaxiesSingles and Pairs

• using X-rays to study black holes• disruption of stars by massive

black holes• pairs of massive black holes

Stefanie Komossa Max-Planck-Institut für extraterr. Physik, Garching

BRIGFOS, Wyboston, 30.3.-2.4.2006

Introduction:

• how frequent are supermassive black holes, do they exist in all galaxies ?

‚hidden‘ and ‚dormant‘ black holes difficult to find

• when did most supermassive black holes form ?

before, simultaneous with, after the galaxies ? • how do most supermassive black holes grow ... ...up to billion times the mass of our sun ?

Important questions in the context of galaxy & quasar formation and evolution:

X-ray astronomy

X-rays are emitted from matter in the immediate vicinity of a black hole

X-ray astronomy

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X-ray source

hand film in camera

X-ray satellite

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growth of supermassive black holes

(1) accretion ofsurroundin

gmatter

by (3) black hole – black hole merging

(2) capture/ disruption of stars

until recently, (2) & (3) remained elusive, observationally

discovery of an enormeously powerful outburst of X-ray radiation from several galaxies thousands of times brigther in X-rays than all the billions of stars of these galaxies taken together

flares from the centers of galaxies

X-rays: blue, visible light: red

X-ray satellite

galaxy

• otherwise, these galaxies are „normal“, inconspicuous, non-active galaxies

• X-rays dropped by factors up to 6000, years after the peak

flares from the centers of galaxies

huge energy output, in short time, right from the centers of otherwise „inactive“ galaxies

flares: interpretation

long sought disruption of a star by a super- massive black hole at the center of each flaring galaxy

• star is ripped apart once the BH‘s tidal forces exceed the selfgravity of the star

• for very massive BHs, solar-type stars are swallowed whole

• infalling compact stars of high density (white dwarfs, neutron stars) will survive Nils´ talk

flares from the centers of galaxies

• masses of the black holes in the flaring galaxies: M ~ 10-100 million solar masses

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diustidal radius

what about a stellar disruption flare in our own Galactic Center ?

• expected ~ once every 10.000 yrs• the X-ray luminosity of the Galactic

Center would increase 100 billion times !• the Galactic Center would be the

brightest object in the X-ray sky apart from the sun, if we had an unobscured view on it !

• detectors aboard X-ray observatories like Chandra would be destroyed if they looked at such a bright source !

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future searches for stellar disruption flares

• most efficient: future X-ray all-sky surveys• applications: (1) detection of massive black holes out to

large distances (2) flare illuminates surrounding

circumnuclear material which otherwise lingers in darkness study its properties

(3) accretion of stellar debris probe the realm of strong gravity

(4) signatures from captured compact stars

growth of supermassive black holes

(1) accretion ofsurroundin

gmatter

by

(3) black hole – black hole merging

(2) capture/ disruption of stars

• massive galaxy, ~400 million lightyears away

• peculiar shape aftermath of galaxy collision

• center heavily veiled by gas & dust

• need hi-energy X-rays to peer into its center

pairs of black holes: the galaxy NGC 6240

X-ray image red: low energy blue: high energy

pairs of black holes• X-ray spectra of the nuclei:

Southern nucleus, Northern nucleus looks similar

black hole mergers

: pair of black holes at the center of this galaxyfirst found in ´03, several more similar galaxies known now

• what is the fate of these two black holes ?

• exciting future prospect: detection of gravitational waves from the merging black holes

Nils´ talk

Summary

• presence and properties of black holes are intimately linked to the formation and evolution of galaxies in the universe;

two processes long predicted by theory recently obs.: • disruption of stars by massive black holes at

centers of sev. galaxies; appear as spectacular X-ray flares in the sky; provide completely independent route to find and study black holes and their environment

• active pair of black holes, merging; in nearby galaxy many more out there

• ongoing & future search for more such events: detect black hole(masses) out to large distances, study black hole environment, probe the realm of strong gravity

• these observations help us understand how galaxies form and evolve, and how black holes grow