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Introduction to Galaxies
Robert Minchin
What is a galaxy?
What is a galaxy?
How would you define a galaxy?
Is M83 a galaxy?
Is M31 a galaxy?
Is M87 a galaxy?
Is NGC 1087 a galaxy?
Is Arp 220 a galaxy?
Is M78 a galaxy?
Is the LMC a galaxy?
Is NGC 104 a galaxy?
Is Leo T a galaxy?
Is UGC 9792 a galaxy?
Is NGC 5139 a galaxy?
Is And XII a galaxy?
Is And XII a galaxy?
Name
Messier 87
Messier 31
Arp 220
Messier 83
NGC 1087
LMC
NGC 104/47 Tuc
Andromeda XII
NGC 5139/ω Cen
Leo T
UGC 9792/Pal 5
Messier 78
Magnitude
8.6
3.4
13.2
7.5
11.1
0.4
4.0
17
3.7
16
11.8
8.3
Luminosity
6.3×1010
3.6×1010
2.6×1010
1.7×1010
1.2×1010
1.2×109
3.6×105
105
8.3×104
5×104
8.5×103
95
Class
Galaxy
Galaxy
Galaxy
Galaxy
Galaxy
Galaxy
Glob. C.
Galaxy
Glob. C. ?
Galaxy
Glob. C.
Refl. Neb.
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?
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
r
M(r) = 2πρ 0 ∫0 r e-r/h dr
• Integrating by parts gives
M(r) = 2πρ 0 h2 [1 - e-r/h(1 + r/h)]
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.
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
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
r
distributed, M(r) = 4π ∫0 r2 ρ(r) dr
• Thus ρ(r) is proportional to 1/r2
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.
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?
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!
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
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.
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
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.
Let’s follow up this galaxy!
Speak
ΔV20
Velocity
Let’s follow up this galaxy!
FHI
Interferometry: Continuum
L-band continuum
can give information
on star formation or
nuclear activity.
Here, data from the
NRAO VLA Sky Survey
(NVSS) shows a high
level of star formation –
termed a ‘Star Burst’
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.
Interferometry: Moment 1
HI observations also
give information about
the velocity field of the
galaxy.
These data show that
the velocity field in the
tail is disturbed, but is
fairly normal in the rest
of the galaxy.
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.
Case Study - NGC 7332
Look at your ‘Galaxy Card’
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
Case Study - NGC 7332
Look at your ‘Galaxy Card’
HI Studies:
• AGES HI: 2 new galaxies
– AGES J223627+234258, AGESJ223829+235135
• Also sees NGC 7339
Case Study - NGC 7332
J223829+235135
NGC 7339
J223627+234258
Case Study - NGC 7332
Look at your ‘Galaxy Card’
• 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
What happened to NGC 7332?
HI Surveys
•
•
•
•
HIPASS (white) – 1997 - 2005
ALFALFA (red) – 2005 - present
AGES (green) – 2005 - present
AUDS (blue) – 2008 - present
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 gaspoor galaxies seen in the optical.