HI in Galaxies

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HI in Galaxies
Karen ONeil NRAO - GB
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Atmospheric opacity has governed our view onto
the sky
A Brief Introduction
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The most common transition of neutral
Hydrogen (HI) is at 21-cm (1420.4058 MHz)
A Brief Introduction
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The most common transition of neutral
Hydrogen (HI) is at 21-cm (1420.4058 MHz)
A Brief Introduction

• Hyperfine transition (forbidden)

• Hyperfine transition (forbidden)
• Decay half-life of 12 million years

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The most common transition of neutral
Hydrogen (HI) is at 21-cm (1420.4058 MHz)
A Brief Introduction
1950 Ewen Purcell announced the first
discovery of HI!
Ed Purcell Taffy Bowen Doc Ewen
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The most common transition of neutral
Hydrogen (HI) is at 21-cm (1420.4058 MHz)
A Brief Introduction

• Detection of HI should be virtually impossible
• 1950 Ewen Purcell announced detection of HI!

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The most common transition of neutral
Hydrogen (HI) is at 21-cm (1420.4058 MHz)
A Brief Introduction
Atmosphere is (mostly) transparent at 1420 MHz
Galaxies typically have lots of HI
21-cm HI emission is (relatively) easy to study!
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So what?
What do you with do with it?
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Tracing the structureand star formation
potential in galaxies
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Tracing the structureand star formation
potential in galaxies
The midplane of the Milky Way Galaxy near the
constellation Perseus.
Image from CGPS www.ras.ucalgary.ca/CGPS/
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Tracing the structureand star formation
potential in galaxies
GSH138-01-094 An Expanding Shell in the Milky
Way Galaxy
Image from CGPS www.ras.ucalgary.ca/CGPS/
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Tracing the structureand star formation
potential in galaxies
Neutral Hydrogen around Cass A
Image from CGPS www.ras.ucalgary.ca/CGPS/
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Tracing the structureand star formation
potential in galaxies
Atomic Hydrogen Mushroom Cloud
Image from CGPS www.ras.ucalgary.ca/CGPS/
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Tracing the structureand star formation
potential in galaxies
M101
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Tracing the structureand star formation
potential in galaxies
UGC 12695
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Tracing the structureand star formation
potential in galaxies
From the HI Rogues Gallery - http//www.nrao.edu/
astrores/HIrogues/
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Tracing the structureand star formation
potential in galaxies
From the HI Rogues Gallery - http//www.nrao.edu/
astrores/HIrogues/
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Tracing the structureand star formation
potential in galaxies
From the HI Rogues Gallery - http//www.nrao.edu/
astrores/HIrogues/
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Tracing the structureand star formation
potential in galaxies
and even discovering new galaxies!
Kilborn, et. Al 2000
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What else can you do?
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Obtaining rotation curves
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Obtaining rotation curves
If you know an objects velocity at a given
radius, you can determine the mass enclosed by
that radius.
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Obtaining rotation curves
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Obtaining rotation curves
Dark Matter
Gas
Stars
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What is Dark Matter?
Obtaining rotation curves
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An AsideDark Matter and Missing Mass
Obtaining rotation curves

• Nucleosynthesis Big Bang predictions 60-80
  of the predicted baryonic matter in
• the universe is not visible

• Assuming Newton Einstein are right,
• gravitational constraints
• An additional 90 of non-baryonic matter
• is not visible

• Result
• What we actually study is only 2-4 of the
• stuff in the Universe!!!

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What is Dark Matter?
Obtaining rotation curves
Option 1 - Baryons
Recall that baryons are the stuff of everyday
life protons and neutrons
In fact, baryons are the heavyweight particle,
each having 3 quarks, and includes p,n,?, ?,S,
O,L
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What is Dark Matter?
Obtaining rotation curves
Option 1 - Baryons
Possibilities include

• Cold molecular gas in the form of dense clumps
• T3K, d1024 cm-2,D30 A.U., M10-3Msun
• Stellar remnants, brown/low mass stars dwarfs,
• black holes

Problems

• No evidence for sufficient quantities

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What is Dark Matter?
Obtaining rotation curves
Option 2 Supersymmetric particles
Neutralinos, gravitinos, axinos

• Neutral particles (uncharged)
• Very light, but containing mass
• Made up of neutrons, gravitons, or axions
• Created in the early Universe

Problems

• Extremely difficult to detect
• Existence not yet proven

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What is Dark Matter?
Obtaining rotation curves
Option 3 New particles/ Particle Physics
Gives lots of freedom in constraints No proof
available
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What is Dark Matter?
Obtaining rotation curves
Option 4 MOND Modified Newtonian Dynamics

• Current version proposed by Milgrom, 1983
• (but originally seen back in the 1950s)
• Attempts to bypass the need for dark matter in
• galaxies galaxy clusters

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What is Dark Matter?
Obtaining rotation curves
Option 4 MOND Modified Newtonian Dynamics

• Newtonian Physics

Fma

• MOND

Fma f(a/a0) a0constant f(x) 1 if xgtgt1 f(x)
x if xltlt1
gn g f(g/a0) In low acceleration g
v(gna0) v4 GMa0
MOND changes the force of gravity only when
acceleration (density) is low.
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What is Dark Matter?
Obtaining rotation curves
Option 4 MOND Modified Newtonian Dynamics
MOND Surface-Density Prediction

• At high density (Smatter gt a0/G ) Newtonian
  Physics dominates
• At low density (Smatter lt a0/G ) MOND dominates

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Modified Newtonian Dynamics
Obtaining rotation curves
Smatter gt a0/G
Smatter lt a0/G
Sm lt a0/G
High Density Galaxies
Low Density Galaxies
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Without MOND
Obtaining rotation curves
Smatter gt a0/G
Smatter lt a0/G
Sm lt a0/G
High Density Galaxies
Low Density Galaxies
NGC 2903
DM
gas
stars
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Modified Newtonian Dynamics
Obtaining rotation curves
Smatter gt a0/G
Smatter lt a0/G
Sm lt a0/G
High Density Galaxies
Low Density Galaxies
NGC 2903
MOND
stars
gas
a01.2X10-8 cm/s2
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Modified Newtonian Dynamics
Obtaining rotation curves
MOND and Newtonian fits to high and low density
galaxies
Sm lt a0/G
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Modified Newtonian Dynamics
Obtaining rotation curves
Sm lt a0/G
MOND accurately predicts rotation curves
observed for high and low density galaxies!!!
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Modified Newtonian Dynamics
Obtaining rotation curves
Sm lt a0/G
Should we toss out Newton and embrace MOND?

• YES!
• Accurate prediction of many rotation curves
• Eliminates need for mysterious Dark Matter in
  galaxies
• Also explains surface density limits in galaxies
• formation of bars in HSB galaxies
  

• NO!
• Needless change in fundamental physics
• Does not work 100 of the time

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Modified Newtonian Dynamics
Obtaining rotation curves
Why should we continue to investigate MOND?

• At this point, we still do not know what the
  mysterious
• Dark Matter (or Dark Force) is
• Understanding when and why MOND works for
  galaxies
• can give us considerable insight into Dark
  Matter
• Trial and error technique is how many scientific
  theories
• are realized

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Using a Single Dish Telescope
One last thought..
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Using a single dish telescope

• Results shown have information on radius vs.
  HI/velocity
• This requires imaging of galaxies (more than 1
  pixel)
• GBT is only one telescope (one pixel)

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Images Pixels
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Images Pixels
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Images Pixels
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Using a single dish telescope

• Results shown have information on radius vs.
  HI/velocity
• This requires imaging of galaxies (more than 1
  pixel)
• GBT is only one telescope (one pixel)
• Can we use the GBT to study galaxy structure and
  DM
• in galaxies smaller than the beam?

YES
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Using a single dish telescope
More than one telescope (e.g. VLA)
One telescope (e.g. GBT)
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Using a single dish telescope
Determining the Distribution
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Using a single dish telescope
Determining the Dark Matter Content
W202vrot

• Measure vrotation
• Assume a radius for gas
• Determine Mdynamical
• Measure Mgas
• Determine Mdark matter

Less accurate, but it takes (far!) less
telescope time for higher sensitivity
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The End