Γαλαξίες – 1
Ελλειπτικοί Γαλαξίες
9 Ιανουαρίου 2013
Galaxy classification
• Classification can be based on
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morphological criteria
color indices
spectroscopic parameters (based on emission or absorption lines)
the broad-band spectral distribution (galaxies with/without radio- and/or
X-ray emission)
• The morphological classification defined by Hubble is still the bestknown today, and it is based on optical observations.
Schneider 2007
Elliptical galaxies
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Elliptical galaxies have neither spiral arms nor a disk.
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Stellar orbits in elliptical galaxies are oriented randomly.
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Nearly elliptical isophotes
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~14% of galaxies are elliptical
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Ellipticity ε ≡ 1−b/a, 0 ≤ ε ≤ 0.7 (where a and b denote the semimajor and the
semiminor axes)
– Classification of ellipticals on the basis of ε:
n=10 ε
class En (e.g. E0 circular isophotes)
E0
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E6
Ellipticals span a wide range of masses and luminosities (6 orders of
magnitude)
Types of ellipticals
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Normal ellipticals. This class includes giant ellipticals (gE’s), those of intermediate luminosity
(E’s), and compact ellipticals (cE’s), covering a range in absolute magnitudes from MB ~−23 to
MB ~−15.
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S0 galaxies are often assigned to this class of early-type galaxies. (lenticular galaxies)
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Dwarf ellipticals (dE’s). These differ from the cE’s in that they have a significantly smaller surface
brightness and a lower metallicity.
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cD galaxies (central Dominant galaxy ). These are extremely luminous (up to MB ~−25) and large (up
to R ~1Mpc) galaxies that are only found near the centers of dense clusters of galaxies.
Their surface brightness is very high close to the center, they have an extended diffuse envelope,
and they have a very high M/L ratio.
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Blue compact dwarf galaxies. These “blue compact dwarfs” (BCD’s) are clearly bluer (with B−V
between 0.0 and 0.3) than the other ellipticals, and contain an appreciable amount of gas in
comparison.
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Dwarf spheroidals (dSph’s) exhibit a very low luminosity and surface brightness. They have been
observed down to MB ~−8. Due to these properties, they have thus far only been observed in the
Local Group.
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Giant ellipticals are appreciably larger than spirals
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Dwarf ellipticals are appreciably smaller than spirals
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Dwarf ellipticals outnumber giant ellipticals by about 10:1.
<SN> is the “specific frequency”, a measure for the number of globular clusters in
relation to the visual luminosity
“Normal” elliptical
•“Perfect” r1/4 fit
• Has ring of HI
• Has supermassive BH
S0 (lenticular)
NGC1275 – cD in Perseus cluster
From Carroll & Ostlie
NGC147 – dE
• a massive network of emission-line
filaments
• supermassive central BH
Blue Compact Dwarf UGC 5497
Leo I – dsph
Brightness Profile - Effective radius
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The brightness profiles of normal E’s and cD’s
follow a de Vaucouleurs profile
     r 4
or
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re effective radius: F(r < re) = 1/2 Ftotal
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Empirical law – no physical basis
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The de Vaucouleurs profile provides the best fits
for normal E’s
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For E’s with exceptionally high (or low) luminosity
the profile decreases more slowly (or rapidly) for
larger radii.
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The profile of cD’s extends much farther out and
is not properly described by a de Vaucouleurs
profile except in its innermost part.
Seeing problems
NGC4472
Brightness Profile
in ellipticals of different class
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The effective radius Re is strongly
correlated with the absolute magnitude
MB
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The dE’s and the dSph’s clearly follow a
different distribution.
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The surface brightness in normal
E’s decreases with increasing
luminosity, while it increases for
dE’s and dSph’s.
Composition of Ellipticals
 Except for the BCD’s, elliptical galaxies
appear red when observed in the optical,
which suggests an old stellar population
(but, metallicity).
 It was once believed that ellipticals contain
neither gas nor dust, but these components
have now been found, though at a much
lower mass-fraction than in spirals.
 hot gas (~ 107 K) detected by its X-ray
emission.
 Hα emission lines of warm gas (~ 104 K)
 cold gas (~100 K) in the HI (21-cm)
 CO molecular lines.
 Many of the normal ellipticals contain visible
amounts of dust, partially manifested as a
dust disk.
 The metallicity of ellipticals and S0 galaxies
increases towards the galaxy center, as
derived from color gradients. Also in S0
galaxies the bulge appears redder than the
disk.
Dust lane in Cen A
Characteristic optical spectrum (integrated) of elliptical galaxy –
shows old population
•strong absorption lines, due to metals
in the stellar atmospheres of the
low luminosity stellar population.
•few to no emission lines
A red colour can be due to high metallicity as well as old age!
Star formation histories of model ellipticals
De Lucia et al. 2005
Colour-magnitude relation
Abell 2218
The linear relation for the brighter galaxies indicates that most of the E and S0 galaxies
within the cluster were formed via the same mechanism and that this mechanism couples
the colour of the stars formed within the galaxy to the final mass of the galaxy.
http://www.astro.lu.se/Education/utb/AST314/colMag/colMag_intro.html#a2218CM
Metallicity – Magnitude Relation
On Elliptical Galaxy Formation
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Elliptical galaxies show a remarkable uniformity in their photometric and chemical
properties,
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One of the strongest constraints being the mass-metallicity relation
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The first proposed scenario of elliptical formation was the so-called monolithic
collapse scenario (e.g. Larson, 1974).
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ellipticals are assumed to have formed at high redshift as a result of a rapid collapse of a gas
cloud.
This gas is rapidly converted into stars by means of a very strong burst
Galactic wind powered by the energy injected into the ISM by SNe and
stellar winds. carries out the residual gas from the galaxies, thus inhibiting
further star formation.
the mass-metallicity relation could be easily explained in terms of metallicity sequences,
namely the more massive objects develop the wind later (due to their deeper potential wells)
and, thus, have more time to enrich their stellar generations.
This scenario has been modified to take into account the increase of [Mg/Fe]
abundance ratio in the stars as a function of galactic mass
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Mg (by type II SNe, on short timescales), Fe (by type Ia SNe, on longer timescale),
the Mg/Fe-mass relation implies that the more massive objects should have formed faster
than the less massive ones
Pipino & Matteucci (2004) implemented an infall term in the chemical evolution equation and
found that most of the photo-chemical observables, including the Mg/Fe-mass relation can
be reproduced in a scenario in which the more massive galaxies formed faster and with a
much more efficient star formation process with respect to the low mass objects.
PM04 suggested that a single galaxy should form outside-in, namely the outermost regions form earlier and
faster with respect to the central parts
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Dynamics of Elliptical Galaxies
Why are E’s not spherical?
Rotational flattening? (as with Earth at equator)
– If that were the explanation the rotational velocity
vrot, which is measurable in the relative Doppler
shift of absorption lines, would have to be of
about the same magnitude as the velocity
dispersion of the stars σv that is measurable
through the Doppler broadening of lines.
– for the rotational flattening of an axially
symmetric,oblate galaxy, we need:
– for luminous ellipticals one finds that, in general,
vrot <<σv, so luminous ellipticals are in general
not rotationally flattened.
– For less luminous ellipticals and for the bulges of
disk galaxies, however, rotational flattening can
play an important role
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The stars behave like a collisionless gas: elliptical
galaxies are stabilized by (dynamical) pressure, and
they are elliptical because the stellar distribution is
anisotropic in velocity space. This corresponds to an
anisotropic pressure
The dynamics of the orbits are determined
solely by the large-scale gravitational field of the
galaxy
pair collisions do not play any role in
the evolution of stellar orbits
>>age of Universe
Indicators of a Complex Evolution
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The isophotes (that is, the curves of constant surface brightness) of many of
the normal elliptical galaxies are well approximated by ellipses
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These elliptical isophotes with different surface brightnesses are concentric to
high accuracy, with the deviation of the isophote’s center from the center of
the galaxy being typically <1% of its extent
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However, in many cases the ellipticity varies with radius, so that the value for
is not a constant
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In addition, many ellipticals show a so-called isophote twist: the orientation of
the semimajor axis of the isophotes changes with the radius
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This indicates that elliptical galaxies are not spheroidal, but triaxial systems (or
that there is some intrinsic twist of their axes)
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the kinematics can be quite complicated.
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dust disks are not necessarily perpendicular to any of the principal axes
the dust disk may rotate in a direction opposite to the galactic rotation
ellipticals may also contain (weak) stellar disks.
Boxiness and Diskiness
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The boxiness parameter describes the deviation of the isophotes’ shape from that of an ellipse.
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Consider the shape of an isophote. If it is described by an ellipse, then after a suitable choice of
the coordinate system,
θ1 = a cos t,
θ2 = b sin t, where a and b are the two semi-axes of
the ellipse and t [0, 2π] parametrizes the curve.
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The distance r(t) of a point from the center is
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Deviations of the isophote shape from this ellipse are now expanded in a Taylor series,
where the term propt to cos(4t) describes the lowest-order correction that preserves the
symmetry
of the ellipse with respect to reflection in the two coordinate axes.
The modified curve is then described by
a4>0 disk like
a4<0 boxy
For most ellipticals |a4/a| <0.01
70% of the ellipticals are diskies
Schneider 2007
Shells and ripples
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In about 40% of the early-type galaxies that are not member galaxies of a cluster, sharp
discontinuities in the surface brightness are found, a kind of shell structure (“shells” or “ripples”).
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They are visible as elliptical arcs curving around the center of the galaxy
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Such sharp edges can only be formed if the corresponding distribution of stars is “cold”, i.e., they
must have a very small velocity dispersion, since otherwise such coherent structures would smear
out on a very short time-scale.
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These peculiarities in ellipticals are not uncommon indicators for shells can be found in about half
of the early-type galaxies, and about a third of them show boxy isophotes.
Correlations of a4 with Other Properties of Ellipticals
~1
therefore disky ellipticals are partially rotationally supported
• Boxy galaxies a4<0
<1
the flattening of “boxies” is mainly caused by the anisotropic
distribution of their stellar orbits in velocity space.
• Boxy galaxies have higher M/L ratio in their cores than the
average elliptical (the opposite is true for diskies)
• Boxies are strong radio and X-ray emitters while diskies are
not
• Boxies often have counter-rotating cores which is rarely
observed in diskies
Continuous transition from diskies to S0’s
• Disky galaxies a4>0
On Friday we will discuss the papers
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From dwarf spheroidals to cD galaxies: simulating the galaxy population in a
ΛCDM cosmology
Guo, Qi; White, Simon; Boylan-Kolchin, Michael; De Lucia, Gabriella;
Kauffmann, Guinevere; Lemson, Gerard; Li, Cheng; Springel, Volker;
Weinmann, Simone
2011MNRAS.413..101G
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The formation history of elliptical galaxies
De Lucia, Gabriella; Springel, Volker; White, Simon D. M.; Croton, Darren;
Kauffmann, Guinevere
2006MNRAS.366..499D
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Relaxation and stripping – The evolution of sizes, dispersions and dark
matter fractions in major and minor mergers of elliptical galaxies
Michael Hilz, Thorsten Naab, Jeremiah P. Ostriker,Jens Thomas,
Andreas Burkert and Roland Jesseit
2012MNRAS.425..3119