Phil James

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Phil James
22nd August 2007 STFC PhD
Summer School, Durham
The Structure of Galaxies

• Liverpool John Moores University
• Astrophysics Research Institute

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Talk overview

• Look at the diversity of galaxy Structures seen
  in the local Universe
• Link structural properties with Content of
  galaxies (gas, stars, dust, dark matter, black
  holes) and Processes connecting these
• Identify open questions galaxies are far from
  being fully understood

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Im not going to mention this
Diagram courtesy Space Telescope Science Institute
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but we need to understand this
Baldry et al. 2004 66846 SDSS galaxies 0.004ltzlt0.0
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Red and blue sequence galaxies in the Virgo
Cluster
Image CFHT
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STRUCTURES Field galaxies
Image A. Block
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Structures Disks
NGC 2683 Image D. Matthews A. Block
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Structures Disks and Bulges
NGC 4565 Image Hugo, Gaul Black (KPNO)
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Structures Disks and Bulges
M 104 Image HST
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Structures Bars
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STRUCTURES Elliptical galaxy
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Galaxy Contents

• Gas (atomic and molecular)
• Stars
• Dust
• Black holes
• Dark matter

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Gas in galaxies

• This is a dissipative component if 2 gas
  clouds collide, they can shock and radiate
  energy, so collisions are highly inelastic
• If there is any initial angular momentum, this
  naturally leads to the formation of a disk

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Superthin galaxies
These galaxies have little or no bulge pure disk
systems.
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Gas in galaxies

• This is a dissipative component if 2 gas
  clouds collide, they can shock and radiate
  energy, so collisions are highly inelastic
• If there is any initial angular momentum, this
  naturally leads to the formation of a disk
• The gaseous disk then forms stars, once gas
  density is sufficiently large

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Measuring star formation

• Ha from gas ionized by hot young stars

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Red light spectrum of a galaxy
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Measuring star formation

• Ha from gas ionized by hot young stars
• Mid/Far-infrared emission from hot dust around
  star formation regions (IRAS, Spitzer)

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Dust in galaxies
NGC 1410 Image Bill Keel
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Spitzer IR Space Telescope
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M81 observed by Spitzer
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Measuring star formation

• Ha from gas ionized by hot young stars
• Mid/Far-infrared emission from hot dust around
  star formation regions (IRAS, Spitzer)
• UV emission from young stars (GALEX)
• Radio emission from star formation regions, or
  from supernova remnants

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Star Formation (Schmidt-Kennicutt) Law SFR a
(Gas density)1.4
Kennicutt 1998
?Starburst nuclei
?Normal disks
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Conversion of gas to stars

• Star formation law works well, with wide
  applicability (normal galaxies and starbursts)
• It is largely empirical, however no physical
  basis for power law index
• Does it apply to star formation in densest
  regions (globular clusters and nuclear clusters)
  or is there another mode of star formation for
  these?

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Globular Cluster M3
Image K Teuwen
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T. Böker et al. 2002 HST images of compact
nuclear clusters
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Some personal opinions (not all would agree)

• Gravitational collapse of gas clouds naturally
  leads to disks in undisturbed systems
• Such disks will always start forming stars when a
  critical density is reached (note that there are
  no gas-rich, quiescent galaxies)
• This star formation is continuous, at a broadly
  constant rate, in the absence of outside
  influences, and as long as the gas supply holds up

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Star formation timescaleR-luminosity dependent
extinction correction
Bulge-dominated
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Star formation timescaleR-luminosity dependent
extinction correction
Bulge disk
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Star formation timescaleR-luminosity dependent
extinction correction
Bulge-free
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UGC 8508, Im
UGC 9240, Im
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Sm
Im
Mean R profile
Mean Ha profile
Difference, Ha-R
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But many galaxies are not disks

• Q Where do elliptical galaxies and spiral galaxy
  bulges come from?
• A This seems to require the presence of stars (a
  non-dissipative component, unlike the gas), and
  something to stir them up
• Internal processes (bars, spiral arms) seem too
  weak large bulges and elliptical galaxies
  probably need outside interference

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Simulation J. Dubinski, U. Toronto
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Some real interactions and mergers
Atlas of peculiar galaxies, H. Arp
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The Antennae, NGC 4038/4039. Colour Image HST,
B. Whitmore F. Schweizer
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Tadpole galaxy, ImageHST
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Galaxy mergers results from simulations

• Colliding disc galaxies form long tidal tails and
  arms
• After a close approach, they are likely to spiral
  together and merge
• Gas becomes centrally concentrated, ? nuclear
  starburst
• Merger remnant density profiles resemble
  elliptical galaxies or bulges
• Characteristic relaxation timescales quite
  short few x 108 years
• Summary undisturbed galaxies stay as thin
  discs, collisions make bulges or ellipticals

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• Under currently-favoured hierarchical
  cosmologies, mergers are common most bright
  galaxies will have experienced at least one
  merger since their formation. Minor mergers
  with dwarf galaxies may just build bulges or
  thicken disks major mergers of two large
  galaxies can make disks directly into
  ellipticals.

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Bulge star-formation histories

• Colours, population synthesis analyses show that
  most bulges are dominated by old stars, 10 Gyr
  old
• Bulges and ellipticals have little cold gas
• Full understanding of this involves feedback
  processes
• Feedback can come from stars (stellar winds and
  supernovae)

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Galactic superwind in starburst M82
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Bulges and feedback processes (contd.)

• The last decade has shown that bulges are
  closely linked to even more energetic phenomena
  than starbursts

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M31
Image R. Gendler
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Kormendy 1988a
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Magorrian et al. 1998
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MOST or ALL galaxies with bulges have central
supermassive black holes!
Magorrian et al. 1998
(Also Ferrarese Merritt 2000, Gebhardt et al.
2000, Tremaine et al. 2002)
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All galaxies with bulges went through a quasar
phase
Quasar Images HST
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• Quasar phase early in evolution of most galaxies
• Often linked to disturbance/mergers in HST
  imaging
• Results in ejection of gas from central regions
• Leaves a gas-free bulge with no further star
  formation
• Enriched gas from bulge can enhance metallicity
  of disk possible solution of the G-dwarf
  problem (very few low-metallicity disk stars in
  our galaxy)

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Summary

• Initial collapse of gas cloud ? rotating disk
• Gaseous disk ? star formation (S-K law)
• If left alone, SF continues at const. rate
• Bulges result from mergers, after a stellar
  component has formed
• Subsequent SF history shaped by feedback
  processes
• Bulge formation linked to supermassive nuclear
  black holes

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Summary contd

• AGN feedback ? ejection of gas from
  bulges/ellipticals, transition from blue ? red
  sequence if feedback is strong enough
• Disk (re-)establishes itself around bulge, with
  gas enriched in heavy elements
• AGN becomes a quiescent BH when gas supply
  exhausted
• SF continues in disk, at rate and for a time
  dependent on gas supply

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• Plenty of open questions with this story

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• Some people who know a lot about galaxies
  would say that most of this is WRONG. They hold
  that bulges and ellipticals can form from the
  initial collapse phase of a galaxy, with no need
  for mergers Monolithic Collapse.
• (Everyone should read the paper by Eggen,
  Lynden-Bell and Sandage on the evidence for this
  from our Galaxy)

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NGC 4449, HST Image
Why are low-mass galaxies irregular, rather than
disks, given their short relaxation timescales?
(Gas fraction, DM, SN feedback?)
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If mergers are as common as hierarchical
theories imply, how do so many disks survive?
ESO 510-G13 Image HST
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How do globular clusters form? How come the
densest stellar systems form in the lowest
density environment? Do all the stars form
before the first burst of SNe? Why no
angular momentum?
M3 Image K Teuwen
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Why do ellipticals have more globular clusters
per unit mass than spirals? Why do many galaxies
have two sets of clusters, red and blue?
M3 Image K Teuwen
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Can bulges form without outside interference?

• Some authorities (e.g. Combes) claim that bulges
  can develop through processes internal to disk
  galaxies (Secular evolution)
• Principal mechanism is bar instability in disks

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NGC 1300 Image HST
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Can bulges form without outside interference?

• Some authorities (e.g. Combes) claim that bulges
  can develop through processes internal to disk
  galaxies (Secular evolution)
• Principal mechanism is bar instability in disks
• Bars efficiently funnel gas into central regions,
  fuelling star formation and potentially building
  bulges

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NGC 1365 Image VLT
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Can bulges form without outside interference?

• Some authorities (e.g. Combes) claim that bulges
  can develop through processes internal to disk
  galaxies (Secular evolution)
• Principal mechanism is bar instability in disks
• Bars efficiently funnel gas into central regions,
  fuelling star formation and potentially building
  bulges
• However, the resulting structures are flat
  lenses ongoing debate about whether bulges
  have to bulge

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• What physical mechanism causes the mass
  scaling between bulges and nuclear black holes?
  (Not even clear to me in which direction any
  causal link should act.)
• How do the black holes form at all? Are they
  linked to the dense nuclear clusters seen in the
  bulge-free galaxies?

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The 90 we have ignored so far

• What is the dark matter?
• How is it distributed around galaxies?
• What effect does it have on bars?
• Do all galaxies have DM haloes?

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NGC 3379 PN velocities Romanovsky et al. 2003
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Final conclusions/ annoying rant

• Always have a science question in mind, whatever
  you are working on.
• Always be willing to at least consider the answer
  you dont want or dont expect.