The Black Hole in M 31 M 31 the Andromeda Galaxy

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The Black Hole in M 31M 31 the Andromeda Galaxy
Michael Hilz
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Introduction

• - M 31 is a member of our Local Group
• together with our Galaxy and companions
• - M 31 will merge with the Milky Way in 10
  Billion years ? perhaps Elliptical Galaxy
• - M 31 is two times larger than the Milky Way
• - M 31 is the second galaxy with good evidence
  for a black hole as the supermassive central dark
  object
• ? stellar dynamics

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Observations until 2005

• - Asymmetric nucleus, i.e. offset of the
  brightest point from bulge center and velocity
  dispersion peak
• - Hubble Space Telescope imaging shows
• double nucleus with two bright intensity peaks
  in a spectrum including Ca II lines (red part of
  visible spectrum)
• (bright nucleus P 1, faint nucleus P 2)
• - Rotation curve is approximately symmetric
  about the faint nucleus P2 !

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What is this double peak?

• 1) Almost completed galaxy merger ?
• implausible, because two cluster orbiting
  around each other would merge in less than 108
  years by dynamical friction ltlt Hubble time
  1010 yr
• Both nuclei are part of the same eccentric disk
  of stars (see Tremaine 1995)

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• 2) Both nuclei are part of the same eccentric
  disk of stars
• ? requirement for an almost Keplerian disk is a
  black hole in P 2
• ? brighter nucleus P 1 is farther from the black
  hole and results from the lingering of stars near
  apocenter
• ? because the stars linger at P 1 the velocity V
  and velocity dispersion s should be small
• ? at P 2 on the anti P 1 side of the black hole
  V and s should be very high

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additionally, the theoretical models made with
older observations fit excellently the new
measurements with higher resolution ? success of
the eccentric disk model
P 1 P3
P 2
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• Problem
• first models were without self-gravity of the
  stars
• When the disk has 10 of the black hole mass
  self-gravity becomes important
• there are many different models which include
  self-gravity and all of them dont really fit the
  observations!
• Estimates of the black hole mass are very
  uncertain - within a range over 3-10 107 Msun

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Discovery of a third nucleus

• With new observations from STIS spectroscopy
  (HST) a third nucleus was detected in M 31 (see
  Bender et al. 2005).
• The new spectrum includes more wavelengths
• the calcium triplet, 8498 ?, 8542 ?, 8662 ?
• additionally 2700 5200 ?

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• appearance of a new strong intensity peak in the
  blue regime of the fainter nucleus P 2
• much stronger than P 1
• identification as a new blue nucleus P 3

P 2 P 1
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What is P 3 ?

• greatest brightness difference between P1 and
  P2/P3 at 3800 3950 ?
• P3 less prominent at 3600 3750 ?
• ? strong Balmer break in the spectrum
• totally different stellar population in
  P3 with Balmer lines and Balmer break
• ? best match of the observed spectra by giant
  and dwarf stars of spectral type A

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• Using an A0 dwarf star and an A0 giant star along
  with a fit of a 200 Million years old starburst
  population as a template one gets an average
  velocity dispersion of
• s 977 106 km/s
• This is the highest velocity dispersion of
  any known galaxy !

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• For comparison
• - center of the Milky Way s 500 800 km/s
• red stars in P2 s 250 km/s
• - blue stars in P3 s 1000 km/s
• ? supermassive black hole in P3 !

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• Fits to the measured spectrum
• 1) Best fit by a 200 Myr starburst population
  with 200 stars between A5 B5
• Problem
• - very hard to form stars so close to a BH
• the complex process is not fully unterstood but
  under extreme conditions it seems to be possible
• formation due to star collisions is impossible

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• 2) Fit of White Dwarf Stars
• - fit to the line widths
• is less good but possible
• - the fit to the Balmer
• break with a white dwarf
• template is impossible,
• even if we choose
• different parameters for
• temperature and gravity

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? P 3 consists of A 5 B 5 stars !
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Light distribution

• important for dynamical analysis
• fit of P 3 to Sersic models with
• ? P 3 is a eccentric disk with inclination i
  55 2

• I(r) I0exp-(r/r0)1/n

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Position angel
Red spectrum
Blue spectrum
How you rotate the slit of the observation
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Mass of the central dark object

• new estimate with the eccentric blue disc
• Only free parameter is inclination which is
  between 55 lt i lt58
• ?best estimate MBH 1.4-0,30,9 108 Msun
• Problem
• difference to the MBH sbulge relation
• also detected in our Galaxy
• ? scattering of MBH sbulge relation for low
  mass galaxies

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What is the central dark object ?

• Brown dwarfs
• impossible because they collide very often and
  get converted back to gas
• Intermediate White dwarfs
• when they collide they make Supernovae Ia, but
  we observe too few events
• 3) Heavy remnants like neutron stars
• - this is a possible alternative if one
  considers the evaporation times

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• - however, problems with the stellar evolution
  in former times
• ? too many stars in an area of about 0.113 pc
• ? contradiction to our knowledge of star and
  star cluster evolution

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Conclusions

• M 31 the third (?) galaxy with a Black Hole as
  the central dark object
• MBH 1.4-0,30,9 108 Msun
• a new hint that all dynamically detected dark
  objects are supermassive black holes
• ? but with only three cases we cannot be
  sure