Dwarf galaxies

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Dwarf galaxies Galaxias enanas
Michael Hilker (ESO/Garching/Germany)
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What is a galaxy?
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The word galaxía is Greek and means Milky
Way Originally, this word describes the
diffuse, white band of light that stretches
across the night sky. In the Greek mythology
it was explained by a river of milk that flows
out of Heras breast. Also the Romans used the
word Milky Way Via Lactea
Todays definition of a galaxy A galaxy is a
huge agglomeration of billions of stars, gas
and dust, which all is hold together by the
forces of gravity. In a typical galaxy like our
Milky Way one can find more than 10 billion
(gt10.000.000.000!!!) stars distributed over an
area with a diameter of about 100.000 light years.
That means the light that was emitted from a star
located at one side of the Milky Way needs
100.000 years to get to the opposite side of our
Galaxy.
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We are living in a galaxy the Milky Way!
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Our Galaxy the Milky Way
Glance at the Milky Way through a telescope
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Stars within 12 light years around the Sun
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Stars within 250 light years around the Sun
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Optical view of the Milky Way once around the
full sky
Infrared image of the Milky Way by the COBE
satellite
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NGC 4565 a distant spiral galaxy from edge-on
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M101 a distant spiral galaxy from face-on
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Objects in the neighbouring spiral arms
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Milky Way overview
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Schematic overview of the Milky Way
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Globular cluster M10
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A journey through the nearby Universe
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The variety of galaxies
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Elliptical galaxies
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Lenticular galaxies
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Disk/spiral galaxies
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Spirals with a bar
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and dwarf galaxies of various types
Ultra-compact dwarfs
Dwarf spheroidals
Blue compact dwarfs
Dwarf irregulars
Dwarf ellipticals
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Most luminous galaxy types
bright
cE UCD GC
faint
red
blue
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Why do we care about dwarf galaxies?
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• Motivation
• Dwarf galaxies are the most abundant type of
  galaxies in
• the Universe.
• Dwarf-sized galaxies are supposed to be the
  building
• blocks of larger galaxies. In the very early
  Universe they
• were the first galaxies that formed.
• The stars and gas of dwarf galaxies have low
  abundances
• of heavy elements, thus they reflect the
  chemical enrich-
• ment history of the early Universe.
• The faintest dwarf galaxies seem to contain the
  largest
• fraction of dark matter of any galaxy type,
  thus they are
• important probes of the dark matter paradigm.

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Big Question How do the largest galaxies form?
Galaxy evolution merger history (?CDM)
stellar evolution
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The pedigree of our Milky Way?
some billion years ago
Dwarf galaxies
today
Milky Way
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A dwarf galaxy pair in our neighbourhood the
Magellanic Clouds
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The Magellanic Clouds dLMC 50 kpc dSMC 60
kpc vLSR,LMC 266 km/s vLSR,SMC 149 km/s
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The Large Magellanic Cloud (LMC)
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Stars and their evolution
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Spectra of different stars
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The Magellanic Clouds as seen by radio telescopes
Density distribution of neutral hydrogen
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Distribution of gas around the Milky Way
Magellanic Clouds
Magellanic Stream
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How galaxies can loose their gas
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Density
Pressure
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Dwarf galaxies with a large gas content
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Dwarf irregular galaxies (dIrr)
IC 10

• Gas-rich galaxies with irregular optical
  appearance
• dominated by scattered regions of ionized gas.
• mass in gas is comparable to or higher than mass
• in stars.

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The dwarf irregular galaxy NGC 6822

• Continuous star formation over the
• lifetime of the Universe.
• Occurrence Low-density regions
• preferred (galaxy groups, field).

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Blue compact dwarf galaxies (BCDs)
I Zwicky 18

• Blue amorphous galaxies, dominated by
• giant HII regions.
• mass in gas similar to mass in stars.

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The blue compact dwarf galaxy NGC 1705

• Gas, stars and starburst regions
• with super star clusters are
• centrally concentrated.
• Among the most metal-poor
• galaxies, analogs to galaxies in
• the early Universe.
• Occurrence Low-density regions
• preferred.

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Back to our neighbourhood the Local Group of
galaxies
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Satellite galaxies of the Milky Way
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The Local Group
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Our neighbouring spiral galaxy Andromeda (M31)
M31 has a dozen of satellite dwarf galaxies
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Dwarf elliptical galaxies (dEs)
NGC 205, a nucleated dE

• High central stellar densities,
• often with a nuclear star
• cluster.
• May have very little gas and
• ongoing star formation.
• Enriched in chemical
• elements.
• Occurrence high-density
• regions preferred mostly
• satellites to major galaxies.

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Dwarf spheroidal galaxies (dSphs)

• Very low density of stars,
• deficient of gas.
• No ongoing star formation.
• Faint dSphs have similar
• luminosity as globular clusters,
• but are 100x more extended.

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The dwarf spheroidal galaxy Leo I

• Least massive galaxies known,
• dominated by dark matter.
• Occurrence high-density
• regions preferred mostly
• satellites to major galaxies.

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The quest for ever fainter and fainter dwarf
spheroidals
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Summary of galaxies of the Local Group

• Local Group members
• gt56 galaxies within 1 Mpc.
• 3 spiral galaxies ( 90
• luminosity).
• gt51 dwarf and satellite
• galaxies (MV 18).

spirals
dEs
dIrrs
dIrrs/dSphs
dSphs
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The closest dwarf galaxy a galaxy under
destruction
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Location of the Sagittarius dwarf galaxy
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The Sagittarius stream
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Sagittarius dwarf galaxy an artists view
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Tidal streams around the Sombrero galaxy
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The amazing case of NGC 5907
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How dwarf galaxies get transformed and disrupted
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Simulation of the transformation of a disk galaxy
into a dwarf spheroidal
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Simulation of the transformation of a disk galaxy
into a dwarf spheroidal
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The nucleus of a dwarf galaxy can survive
the disruption of its main body
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The nucleus of a dwarf galaxy can survive
the disruption of its main body
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Disruption of a dwarf elliptical in a dense
environment
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Galaxies in interaction - death and birth dwarf
galaxies
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Major galaxies can get very close to each other
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Witnessing the formation of tidal dwarf
galaxies(?)
The tadpole galaxy a merger of two major galaxies
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Tidal dwarf galaxies (TDGs)

• Galaxies made out of collisional
• debris expelled by tidal forces.
• Do not possess dark matter.
• Occurrence around interacting/
• merged galaxies.

Merging galaxies with TDG candidates
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The densest regions of the Universe the realm
of dwarf galaxies in galaxy clusters
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Nearby galaxy clusters
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Nearby superclusters of galaxies
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The Hercules galaxy cluster
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The Perseus galaxy cluster
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NGC 3311 3593 km/s NGC 3309 4075 km/s
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The galaxy population of the Virgo cluster
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Dwarf galaxies of the Virgo cluster
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The Fornax cluster
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Observations Las Campanas December 99 2.5m Du
Pont WFCCD
The Fornax Cluster
3-color composit of the central field with NGC
1399, 1404 and 1427A
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Ultra-compact dwarf galaxies (UCDs)

• Very compact, luminous,
• globular-cluster-like objects.
• Few have very low surface
• brightness envelopes.
• Old stellar populations.

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brightest GCs, isolated nuclei?
UCDs their origin is under debate
compact elliptical (cE)
?
?
?
nucleated dwarf elliptical (dE,N)
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super-cluster complex
Antennae M(V)-14 mag
blue compact dwarfs (BCDs)
NGC 1705 M(V)-16 mag
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The very distant Universe the first galaxies
are dwarfish!
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Observation of the famous Hubble Deep Field
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High redshift galaxies in the Hubble deep field
Small irregular lumps of star- forming material
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At the beginning of the Universe all galaxies are
dwarf galaxies
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• Summary
• Dwarf galaxies are the first galaxies of the
  Universe.
• Thus, they contain information on the first
  phases
• of structure formation and element enrichment
  in
• the Universe.
• In the vicinity of giant galaxies dwarf galaxies
  
• change their appearance from gas-rich
  irregulars
• to gas-poor spheroidal galaxies.
• Dwarf galaxies can be destroyed but also new
• ones can be born in galaxy interactions.
• Dwarf galaxies probe the paradigm of dark
  matter.

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En conclusion Las galaxias enanas son muy
cheveres
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Muchas gracias por su atencion!