Transcript dark matter

Our Galaxy, the Milky Way
Ken Freeman,
Research School of Astronomy & Astrophysics, ANU
& UWA
Public lecture August 12, 2010
The Milky Way on a winter’s night in the south
NGC 891
Keeler (1899)
The Kapteyn Universe 1922
Kapteyn’s Universe
Shapley and the distance to the Galactic center
12 kpc
Lindblad interpreted
the pattern of stellar
motions as due to
Galactic rotation
Oort worked out the
theory of Galactic
rotation
Need to tell you about dark matter now. There is more
to the Galaxy than what we can see in this image. In fact,
this is only a tiny fraction of it
30 kpc
NGC 2997 - a typical disk-like spiral galaxy - flat and rotating,
but not rotating as a rigid body
halo
The Westerbork telescope
Rotation curve of a spiral galaxy : gravity provides the
acceleration V2/R needed for the stars and gas to go around in
circular motion. The gravitational field of
the stars and gas alone is not consistent with the flat rotation
curve. A dark halo is needed
Albert Bosma’s (1978)
thesis from the
Kapteyn Institute
helped to make the case
for the dark halos of
galaxies
30 kpc
Rotation of the Galaxy
Merrifield (1992)
M31
M31 and the Milky Way are now
approaching at 118 km s -1. Their
separation is about 750 kpc
To acquire this velocity of approach
in the life of the universe means that
the total mass of the Milky Way
is at least 120 x 10 10 M.
The stellar mass is about
6 x 1010 M, so the ratio
of dark to stellar mass is ~ 20
The dark halo extends out to
at least 120 kpc, far beyond
the disk's radius of ~ 20 kpc
(Kahn & Woltjer 1959)
118 km s -1
Milky Way
The ratio of dark mass to stellar mass is
typically about 20: 1
The dark halos extend 5 to 10 times further out
than the disks of these galaxies
Dark matter dominates the mass budget of the universe. It is very important
for galaxy formation. The Big Bang was 13.7 Gyr ago. We see that galaxies
are already forming 0.5 Gyr after the Big Bang. Without dark matter, this
could not happen - it would take galaxies much longer to form
MOVIE
Start by showing a numerical simulation of galaxy formation.
The simulation summarizes our current view of how a disk galaxy
like the Milky Way came together from dark matter and baryons,
through the merging of smaller objects in the cosmological
hierarchy.
• much dynamical and chemical evolution
• halo formation starts at high z
• dissipative formation of the disk
Simulation of
galaxy formation
• cool gas
• warm gas
• hot gas
QuickTime™ and a
Microsoft Video 1 decompressor
are needed to see this picture.
Movie synopsis
• z ~ 13 : star formation begins - drives gas out of the
protogalactic dark matter mini-halos. Surviving stars will
become part of the stellar halo - the oldest stars in the Galaxy
• z ~ 3 : galaxy is partly assembled - surrounded
by hot gas which is cooling out to form the disk
• z ~ 2 : large lumps are falling in - now have a
well defined rotating disk galaxy.
You saw the evolution of the baryons. There is about 10 x
more dark matter in a dark halo, underlying what you saw:
The dark halo was built up from mergers of smaller sub-halos
Saw spiral structure developing in the gas
Merging of galaxies is still going on now
Merging of galaxies is still
happening now
Merging stimulates star formation and disrupts the galaxies. This is NGC 4038/ 9 :
two large merging spirals. The end product of the merger is often an elliptical galaxy.
(In a few Gyr, the Milky Way will probably merge with M31)
NGC 5907: debris of small galaxy accreted by a larger galaxy
Our Galaxy has a similar structure from the disrupting Sgr dwarf
APOD
You could see spiral structure coming and going in the movie
Our Galaxy has spiral structure, though it is difficult to map
this
structure from inside the Galaxy. Look first at the spiral
structure of a nearby spiral M83
The nearby spiral galaxy M83 in blue light (L) and in the NIR (R)
The blue image shows young star-forming regions and is affected by dust
obscuration. The NIR image shows mainly the old stars and is unaffected
by dust. Note how clearly the central bar can be seen in the NIR image
Sun
Artist’s sketch of the spiral structure of our Galaxy,
based on optical observations of young stars and radio
observations of hydrogen and CO.
APOD
For a long time, spiral structure was thought to be a steady wave,
propagating around the Galaxy.
Now we think it comes and goes, as in the movie - needs gas to maintain
the spiral structure and star formation. Without replenishment of the gas,
the star formation and spiral structure should go out in a few Gyr. Some
gas comes from dying stars, but not enough. New gas may come from the
infall of small galaxies and gas clouds - one of the big present puzzles.
The Magellanic
Stream in
neutral hydrogen
(APOD, 2010)
Most spirals (including our Galaxy) have a second thicker disk
component . In some galaxies, it is easily seen
The thin disk
The thick disk
NGC 4762 - a disk galaxy with a bright thick disk (Tsikoudi 1980)
Our Galaxy has a significant thick disk
It was discovered via star counts at the South
Galactic Pole
• it is about three times thicker than the
thin disk
thin disk
thick disk
The discovery of the thick disk of our Galaxy:
star counts towards the South Galactic Pole
Gilmore & Reid 1983
The thick disk of our Galaxy:
• its mass is about 10% of the mass of the thin disk
• its stars are old (> 12 Gyr) and have less metals
than the thin disk
• its stars are enriched in alpha-elements (Mg, Si, Ca)
so its star formation was rapid, ~ 1 Gyr
Thick disk chemical element ratios
The ratio /Fe is about 2.5 times enhanced in the thick disk
-1.0
-0.5
0.0
Mg enrichment in the thick disk: thick disk appears
chemically distinct from the thin disk
Fuhrmann 2008
0.5
-enrichment relative to Fe means rapid chemical evolution
Element-building occurs mainly via supernova explosions: exploding stars
return chemically enriched gas to the interstellar gas - then new stars form.
The alpha elements (Mg, Ca, Si, Ti) come mainly from massive supernovae:
stars with masses > 10 M which explode within a few million years
Fe comes mainly from low mass supernovae which take 1-2 Gyr to explode
So a high alpha/Fe ratio means that the star formation and chemical
evolution ended before the low mass supernovae had time to explode and
enrich the interstellar gas.
The thick disk stars formed mostly more than 12 Gyr ago.
Then there was a pause in star formation, until the thin disk stars
started to form about 10 Gyr ago. Thin disk star formation has
continued at a more-or-less constant rate up to the present time.
How do thick disks form ?
• large energetic star-bursts driven by early gas-rich mergers
(Brook et al 2004). Clump cluster galaxies at high redshift
may be such events.
• accretion debris (Abadi et al 2003, Walker et al 1996)
• early thin disk, heated by accretion events: Thin disk formation
begins early, at z = 2 to 3. Partly disrupted during merger
epoch which heats it into thick disk observed now, The rest
of the gas then gradually settles to form the present thin disk as
merger activity dies down.
Clump cluster galaxy at z = 1.6: these are common
at high redshift as galaxies are assembling
(Bournand et al 2008)
Heating of the early thin disk by accretion of a small satellite galaxy
Chemical element abundance ratios in small galaxies
Abundance ratios
reflect different
star formation
histories
LMC
Sgr
Fornax
Sculptor
Pompeia, Hill et al. 2008
Sbordone et al. 2007
Letarte PhD 2007
Hill et al. 2008 in prep
+ Geisler et al. 2005
Carina Koch et al. 2008
+ Shetrone et al. 2003
Milky-Way Venn et al. 2004
Venn 2008
We can use the chemical element abundance patterns to probe the formation
of the Galactic thick disk.
Small galaxies have distinctive and different abundance patterns:
if the thick disk was built up partly by accretion of small galaxies, we will be
able to recognise the imprint of these accreted small galaxies in the
abundance patterns of the thick disk.
This is called chemical tagging. It needs a huge number of stellar spectra.
This kind of data does not exist yet: it is one of the goals for the HERMES
survey.
With the new HERMES instrument on the AAT, we will measure the
chemical abundances of many elements for a million stars, mostly in the thin
and thick disks
High resolution spectroscopy
redder
bluer
High resolution spectrum
17A window on the solar spectrum revealing lines of Fe, Cr, Ti,
V, Co, Mg, Mn, Nd, Cu, Ce, Sc, Gd, Zr, Dy
HERMES is a new high resolution multiobject spectrometer on the AAT
spectral resolution 28,000
400 optical fibres over  square degrees
4 VPH gratings ~ 1000 Å
First light ~2012 on AAT
Will get spectra for a million stars
The four wavelength
bands are chosen to detect
lines of elements needed
for chemical tagging
The central regions of our Galaxy are dominated by the bar/bulge
The boxy appearance of the bulge is typical of
galactic bars seen edge-on. Where do these
bars come from: they are very common ? About
2/3 of spiral galaxies show some kind of central
bar structure in the infra-red.
The bars come naturally
from instabilities of the disk.
A rotating disk is often
unstable to forming a flat
bar structure at its center.
This flat bar in turn is often
unstable to vertical buckling
which generates the boxy
appearance.
Shen, 2010
QuickTime™ and a
FLIC ŽØª° decompressor
are needed to see this picture.
If this is right, then the stars in the bulge were once part of the inner disk
We are doing a survey of about 30,000 stars
in the bulge and the adjacent disk, to
measure the chemical properties of stars (Fe,
Mg, Ca, Ti, Al) in the bulge and adjacent
disk: are they similar, as we would expect if
the bar/bulge grew out of the disk ? We use
the AAOmega fiber spectrometer on the
AAT, to acquire medium-resolution spectra
of about 350 stars at a time.
Melissa Ness
At the center of the bulge lies a supermassive
black hole. Central BHs are common in
galaxies: their mass is usually a small
fraction of the bulge mass.
The Galactic bulge:
mass = 2.1010 M
The central black hole:
mass = 4.106 M
The black hole itself is invisible, but we can measure its gravitational effect
on nearby stars. Turbulence in the earth’s atmosphere blurs out any details
smaller than about 0.5 arcsec (0.25 mm at 100 m). To see stars close
enough to the black hole (0.1 arcsec), adaptive optics are needed to correct
for the effects of the atmospheric turbulence.
natural seeing image
(0.26 arcsec)
AO image
(0.060 arcsec)
Gemini Observatory
The Galactic black hole
is located at the
The nearby young stars are
moving very fast in the intense
gravitational field of the black
hole
Star S0-2 has made a
complete orbit in 15 years !
(0".1 ~ 1000 AU)
A. Ghez, UCLA
Mass of the black hole
Black holes in
other galaxies
Mass of the BH
 brightness of the
bulge, but with fairly
large scatter
Brightness of the bulge
Gültekin et al 2009
Mass of the black hole
Central black holes in
many galaxies,
including the MW
Mass of the BH
  4 with smaller
scatter
MW
Random velocity of stars in the bulge, away from the BH
The formation of the
black hole and the
properties of the
surrounding bulge
are closely related:
maybe they formed
together ?
The End