Download It is now recognized that the vast majority of ellipticals are of

PH607 - 4
3 Ellipticals
4 Comparison
5 Multiwavelength
3 Elliptical Galaxies
When you have seen one elliptical galaxy, you have pretty much
seen them all. Here is a picture of one such system, the nearby
elliptical M32:
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An elliptical galaxy shows no spiral structure and can vary
from almost round (what Hubble called E0) to almost cigar
shaped (called E7).
This classification is based on our perspective from Earth
and not on the actual shape. So, elliptical galaxies are
designated "E#," where # refers to their apparent flattening:
o # = 10(1 - b/a)
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M89: E0
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M59 E5
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Apparently round ellipticals are E0s
The flattest ellipticals observed are E7s.
Intrinsic 3D Shapes
3-D shapes – ellipticals are predominantly triaxial ellipsoids:
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Oblate: A = B > C (a flying saucer)
Prolate: A > B = C (a cigar)
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Triaxial A > B > C
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(A,B,C are intrinsic axis radii)
Find that galaxies are mildly triaxial:
A:B:C ~ 1 : 0.95 : 0.65 (with some dispersion ~0.2)
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Triaxiality is also supported by observations of isophotal
twists in some galaxies (would not see these if oblate or
prolate)
It was once thought that the shape of ellipticals varied
from spherical to highly elongated. The Hubble
classification of elliptical galaxies ranges from E0 for those
that are most spherical, to E7, which are long and thin.
It is now recognized that the vast majority of ellipticals
are of middling thinness, and that the Hubble
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classifications are a result of the angle with which the
galaxy is observed.
(Lenticulars. In between the ellipticals and the spirals are
the S0s which have
o
o
o
very large bulges
weak disks
no spiral structure )
Surface Brightness. The R1/4 law.
Many normal bright (Mv < -17) elliptical galaxies and the
bulges of spirals have a projected luminosity distribution
that follows a de Vaucouleurs. (or R1/4) law. The surface
brightness, I, of the bulge of the galaxy (measured in
units of L pc-2) shows a radial dependence according to:
I(R) = I(Ro) exp[ - b (R/Ro) 1/4 – 1 ]
where Re is the radius of the isophot containing half the total
luminosity, and Ie is the surface brightness at Re.
This is often referred to as a r1/4 law – and the distribution is
sometimes called a de Vaucouleurs profile.
This law is a purely empirical fit with no physical basis. However,
any theory of elliptical galaxy formation must reproduce it.
In general:
I(R) = I(Ro) exp[ - b (R/Ro) 1/n – 1 ]
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Cores of Ellipticals
• Only a few E galaxies actually have flat luminosity
profiles at small radii; instead, the profiles rise inward to
the last measured point.
• Cores may exhibit unusual kinematics; for example,
about a quarter of all elliptical galaxies have cores
which appear to counter-rotate with respect to the rest
of the galaxy .
• A few nearby E galaxies have nuclear star clusters with
densities much higher than the cores they reside in;
some of these nuclei may be rotating, disk-like systems.
Distorted Shapes of Ellipticals
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Projected axial ratios range from b/a = 1 to ~0.3, but
not flatter (Schechter 1987).
Apparent ellipticity is generally a function of projected
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radius, with a wide range of profiles.
Isophotal twists are common. Because it is highly
unlikely that intrinsically twisted galaxies could be
dynamically stable, such twists are generally
interpreted as evidence for triaxiality .
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Elliptical galaxies are not exactly elliptical; isophotes
may depart significantly from perfect ellipses. –
typical deviations of a few %
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Deviations from ellipses can be classified as disky or
boxy (peanut)
Boxy galaxies tend to be more luminous, slower
rotators
Disky: normal and low luminosity ellipticals, which
have nearly isotropic random velocities but are
flattened due to rotation.
Kinematics of Ellipticals
The rotation velocities of bright E galaxies are much
too low to account for the flattenings we observe.
Fainter E galaxies, however, rotate at about the rates
implied by their shapes (Davies 1987).
E galaxies may exhibit minor-axis rotation; more generally,
the apparent rotation axis and the apparent minor axis
may be misaligned. In most galaxies these
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misalignments are modest.
Larger galaxies have fainter effective surface
brightnesses. Mathematically speaking, the radius
(Djorgovski & Davis 1987) where Re is the
effective radius, and
is the mean surface brightness
interior to Re.
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More luminous ellipticals have lower surface
brightnesses. As
, we can substitute the
previous correlation and see that
and
therefore:
.
More luminous elliptical galaxies have larger central
velocity dispersions. This is called the Faber-Jackson
relation (Faber & Jackson 1976). Analytically this is:
.
Shells & Other `Fine Structures'
• The surface brightnesses of E galaxies do not always
decline smoothly with radius. When a smooth
luminosity profile is subtracted from the actual surface
brightness, `shells' or `ripples', centered on the
galaxy, are seen.
• The fraction of field E galaxies with shell-like features is
at least 17% and possibly more than 44%.
• The colours of shells indicate that they are composed of
stars. In many cases the shells are somewhat more
blue than the galaxies they occupy.
Gas & Dust in Ellipticals
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• 25% to more than 40% of E galaxies show features due
to dust absorption.
• The dust lanes seen in E galaxies imply that the
absorbing material is distributed in rings or disks. Dust
lanes may be aligned with either the major or minor
axes, or they may be warped.
• E galaxies contain modest amounts of cool and
warm gas, although not as much as is found in S
galaxies. A few E galaxies have extended disks of
neutral hydrogen.
• X-ray observations indicate that many ellipticals contain
109 to 1010 solar masses of gas at temperatures of ~10
million K; this hot gas typically forms a pressuresupported `atmosphere' around the galaxy.
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Tidal Features
• Elliptical galaxies in rich galaxy clusters often exhibit
luminosity profiles which fall below a de Vaucouleurs
law at large radii. Such downturns are often attributed
to tidal truncation in the mean field of the cluster.
• In contrast, E galaxies with close companions often
have luminosity profiles which rise above a de
Vaucouleurs law at large radii. These features may be
plausibly blamed on tidal interactions.
• E galaxies in closely interacting systems sometimes
exhibit outer isophotes which are visibly eggshaped and/or offset with respect to the centers of their
galaxies. Again, tidal effects are strongly implicated.
• On very deep exposures, some E galaxies are seen to
have `plumes' or `tails', while others (e.g. NGC 5128)
show rather irregular luminosity distributions. Tail-like
features may be signatures of major mergers.
4 Galaxy Constituents & Colours:
Spiral galaxies contain:
o
stars (population I and II) , gas, dust
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Elliptical galaxies contain:
o
stars (population II only – (i.e. old) stars)
o
Irregular galaxies are harder to classify. They usually
contain:
o
stars (population I (young stars) – in other words
there are significant amounts of gas in the
galaxy which is being transformed into young
stars – with ages as short as a few million
years) and some population II) , star-forming
regions , gas (a higher proportion than in
spirals)
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The Large Magellanic Cloud at optical wavelengths
Ellipticals
Spirals
Irregulars
Mass M
105 - 1013
109 – 4 x 1011
108 – 3 x 1010
Absolute mag
-9 -> -23
-15 -> -21
-13 -> -18
Luminosity L
3 x 105 - 1011
108 – 2 x 1010
107 - 109
100
2 – 20
1
1 – 200
5 - 50
1 – 10
Stellar
population
II and old I
I in arms, II and
old I overall
I, some II
Presence of dust
Almost none
Yes
Yes
Total fraction %
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83
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M/L (M / L =1)
Diameter (kpc)
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E
Colour
Red
S0
Sa
Red
Sb

Sc
Sd Irr
Blue
Blue
Stellar
Old Old +
Old +
Intermediate
Population
Intermediate Intermediate +
+ Young
Young
Star Form zero
low
higher
high
Rate
HI (gas) Zero/ low
modest
high highest
low
dust
Zero/
low
Higher
highest
Lower
(less
metals)
Dynamics Bulge/halo Disk dominated, so
dom.
rotation
The Colour-Magnitude Diagram:
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Colour: Large automated imaging surveys are better at
defining a galaxy's colour rather than morphology.
it is more natural to describe a galaxy as being on the
‘red sequence’ or ‘blue sequence’ rather than being an
‘early type’ or ‘late type’.
This interpretation also has the advantage that galaxy
colours are directly related to the star formation, dust and
metal-enrichment history of the galaxy and can thus be
more readily interpreted in theoretical models
The bimodal distribution of red and blue galaxies is seen
in analysis of Sloan Digital Sky Survey data.
Three features:
the red sequence,
the green valley
the blue cloud.
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The red sequence includes most red galaxies which are
generally elliptical galaxies.
The blue cloud includes most blue galaxies which are
generally spirals.
In between the two distributions is an underpopulated
space known as the green valley which includes a number
of red spirals.
Unlike the comparable HR diagram for stars, galaxy
properties are not necessarily completely determined by
their location on the color-magnitude diagram. The
diagram also shows considerable evolution through time.
Cluster v. Field environment
Colour (U-R) versus stellar mass relations for different
environments.
Panel (a): void-like environments while
Panel (f) cluster-like environments.
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5 Multiwavelength Views
Our view of is greatly affected by the observing
wavelength – the infrared penetrates deeper than optical
radiation.
M101, a nearby Sc galaxy:
7Mpc, The Pinwheel SAB(rs)cd
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The Whirlpool: M51…below Chandra (X-ray) and ISO (mid-IR):
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The x-ray image (left) highlights the energetic central regions of
the two interacting galaxies. Much of the diffuse glow is from multimillion degree gas. Many of the point-like sources in the x-ray
image are due to black holes and neutron stars in binary star
systems.
Mid-IR light (right) is well-suited to studying star formation and
tracing dust in spiral galaxies. This image not only shows the galaxy
cores and spiral arms, but nicely illustrates the knots of star
formation occurring in the arms of M51.
M104: (SAa, 9 Mpc) Spitzer's infrared view of the starlight, piercing
through the obscuring dust, is easily seen, along with the bulge of
stars and an otherwise hidden disk of stars within the dust ring.
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NGC253 at infrared,optical and X-ray wavelengths
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M81 at optical wavelengths (left) and the 21cm
wavelength HI tracer of atomic hydrogen gas (right) which
shows the relative intensity of emission from neutral
atomic hydrogen gas. In the pseudo-colour image, red
indicates strong radio emission and blue weaker emission.
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