Download - Introduction to Galaxies
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Introduction to Galaxies
Robert Minchin
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What is a galaxy?
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What is a galaxy?
How would you define a galaxy?
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Is M83 a galaxy?
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Is M31 a galaxy?
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Is M87 a galaxy?
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Is NGC 1087 a galaxy?
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Is Arp 220 a galaxy?
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Is M78 a galaxy?
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Is the LMC a galaxy?
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Is NGC 104 a galaxy?
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Is Leo T a galaxy?
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Is UGC 9792 a galaxy?
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Is NGC 5139 a galaxy?
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Is And XII a galaxy?
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Is And XII a galaxy?
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Name Magnitude Luminosity Class
Messier 87 8.6 6.3×1010 Galaxy
Messier 31 3.4 3.6×1010 Galaxy
Arp 220 13.2 2.6×1010 Galaxy
Messier 83 7.5 1.7×1010 Galaxy
NGC 1087 11.1 1.2×1010 Galaxy
LMC 0.4 1.2×109 Galaxy
NGC 104/47 Tuc 4.0 3.6×105 Glob. C.
Andromeda XII 17 105 Galaxy
NGC 5139/ω Cen 3.7 8.3×104 Glob. C. ?
Leo T 16 5×104 Galaxy
UGC 9792/Pal 5 11.8 8.5×103 Glob. C.
Messier 78 8.3 95 Refl. Neb.
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What is a galaxy?
• Small dwarf galaxies and large globular clusters have similar morphologies and similar luminosities.
• How do we decide what is a globular cluster and what is a galaxy?
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Galaxy Rotation Curves
• The surface-brightness of a disc galaxy is given by σ(r) = σ 0e-r/h
• If M/L is constant, ρ(r) = ρ0e-r/h
• Mass within the radius r is then
M(r) = 2πρ 0 ∫0
r r e-r/h dr
• Integrating by parts gives
M(r) = 2πρ 0 h2 [1 - e-r/h(1 + r/h)]
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Galaxy Rotation Curves
• Centripetal acceleration is a = v(r)2/r
• Gravitation acceleration is g = GM(r)/r2
• Equating these, v(r)2 = GM(r)/r
• Thus, v(r) = (2πρ0Gh2[1-e-r/h(1+r/h)]/r)1/2
• This is a function that rises steeply to peak near r/h = 2 and then falls off.
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The Observed Rotation Curve
• In the 1970s, interferometers such as the VLA and WSRT observed the rotation curves of spiral galaxies
• They were able to trace the rotation curves out beyond the stellar disc.
• The resulting curves were flat or slightly rising, not falling as predicted
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The Observed Rotation Curve
• Clearly there was some other component to the galactic mass
• GM(r)/r2 = v2/r, thus v = (GM(r)/r)1/2, implying that M(r) is proportional to r
• If this new mass is spherically
distributed, M(r) = 4π ∫0
r r2 ρ(r) dr
• Thus ρ(r) is proportional to 1/r2
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The Observed Rotation Curve
• This density profile is characteristic of a isothermal sphere.
• This implies that the unknown mass is only interacting gravitationally.
• Lead to the acceptance of the existence of dark matter.
• As a general result, galaxies contain more mass than is detectable.
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What is a galaxy?
• Small dwarf galaxies and large globular clusters have similar morphologies and similar luminosities.
• How do we decide what is a globular cluster and what is a galaxy?
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What is a galaxy?
• Small dwarf galaxies and large globular clusters have similar morphologies and similar luminosities.
• How do we decide what is a globular cluster and what is a galaxy?
• Dynamics!
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Measuring Dynamics
• Dynamics can be measured by looking at emission and absorption lines from stars and star-forming regions, or from the gas in the ISM.
• Dynamics for spiral and irregular galaxies are often measured using the 21-cm line of neutral hydrogen.
• For elliptical and spheroidal galaxies, which are gas poor, velocity dispersions from stellar spectroscopy can be used
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Single-dish HI profiles
• Interferometry observations give the most accurate measurements, but are expensive in terms of telescope time.
• Using optical information on inclinations, single-dish profiles can be used to determine the rotational velocity.
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What else does HI give us?
• The recessional velocity gives a measure of how distant a galaxy is.
dMpc = V/H0
• From the distance and the flux, can find the HI mass of the galaxy:
MHI = 2.356 × 105 d2 FHI
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What else does HI give us?
• The HI mass to luminosity ratio (MHI/L) tells us how gas-rich a galaxy is.– Gas-rich galaxies are often blue, late-type
galaxies with active star formation.– Some are more intriguing objects with low
SF rates. A number of these have been turned up by HI surveys.
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Let’s follow up this galaxy!
ΔV20
Speak
Velocity
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Let’s follow up this galaxy!
FHI
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Interferometry: Continuum
L-band continuumcan give informationon star formation ornuclear activity.
Here, data from theNRAO VLA Sky Survey(NVSS) shows a high level of star formation – termed a ‘Star Burst’
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Interferometry: HI Moment 0
From HI observations,a map can be made showing where the neutral hydrogen is in the galaxy.
These data from the VLA show an HI ‘tail’ – indicating that this galaxy has interacted with another recently.
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Interferometry: Moment 1
HI observations alsogive information aboutthe velocity field of thegalaxy.
These data show that the velocity field in the tail is disturbed, but is fairly normal in the rest of the galaxy.
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HI Surveys
• Can turn up interesting objects with high HI masses or high gas fractions.
• Can side-step optical selection effects.
• Give an instant measure of the recessional velocity, velocity width, and HI flux for every object detected.
• Do not find gas-poor galaxies and require optical data for inclinations, etc.
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Case Study - NGC 7332
Optical Studies:
• NGC (Dreyer 1888): 2 galaxies– NGC 7332, NGC 7339
• Karachentseva et al. 1999): 2 more– KKR 72, KKR 73
• AGES optical (Karachentsev): 3 more– J223450+240757, J223558+234825, J223631+240814
Look at your ‘Galaxy Card’
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Case Study - NGC 7332
HI Studies:
• AGES HI: 2 new galaxies– AGES J223627+234258, AGESJ223829+235135
• Also sees NGC 7339
Look at your ‘Galaxy Card’
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Case Study - NGC 7332
J223829+235135
J223627+234258
NGC 7339
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Case Study - NGC 7332
• Have 6 galaxies from optical, 2 from HI, 1 from both
BUT – HI Studies show:
• 2 of the optical galaxies not in the group– AGES J223449+240756 = J223450+240757– AGES J223631+240823 = J223631+240814
Look at your ‘Galaxy Card’
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What happened to NGC 7332?
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HI Surveys
• HIPASS (white) – 1997 - 2005• ALFALFA (red) – 2005 - present• AGES (green) – 2005 - present• AUDS (blue) – 2008 - present
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Summary
• What is a galaxy?– A galaxy contains stars within a DM halo.– May also contain gas and dust– DM halos extend well beyond optical disc
• HI gives us dynamics and a wealth of other information
• HI surveys can detect gas-rich galaxies missed by optical surveys, but miss gas-poor galaxies seen in the optical.