Download Elliptical galaxies

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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.
– Giant ellipticals are appreciably larger than spirals
– Dwarf ellipticals are appreciably smaller than spirals
– 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
The Faber–Jackson Relation
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the velocity dispersion in the center of ellipticals, σ0, scales with luminosity
The Faber–Jackson relation specifies a connection between the luminosity and a
kinematic property of elliptical galaxies.
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Various other relations also exist between the
parameters of elliptical galaxies
(e.g. log Re – MΒ etc)
Schneider 2007
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So, the inference is that there may well be a relation between the various parameters for
which the dispersion is smaller than that of the Faber–Jackson relation:
the fundamental plane.
(e.g. Primary component analysis)
The fundamental plane
The distribution of elliptical
galaxies in the three-dimensional
parameter space
Projections of the FP onto different
two parameter planes
Schneider 2007
FP relation – virial theorem
Virial theorem
which agrees with the FP relation
provided that
i.e. if the M/L ratio increases with M
Introductory remarks for elliptical formation
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Elliptical galaxies are the most massive stellar systems in
the local Universe and
They appear to define a homogeneous class of objects with
– uniformly old and red populations,
– Negligible amounts of gas,
– very little star formation.
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50 % or more of the stellar mass in the local Universe appears to be in
early–type systems and bulges
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Their deceptively simple appearance inspired a ‘classical’ formation
scenario in which they form in a single intense burst
of star formation at high redshifts (z ~ 5), followed by passive evolution of
their stellar populations to the present day (Partridge & Peebles 1967;
Larson 1975).
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This so-called monolithic scenario successfully explains
– the tightness of the fundamental scaling relations
(the colour–magnitude relation and the Fundamental Plane)
– the evolution of these relations as a function of redshift (Kodama et al. 1998; van
De Lucia et al. (2005)
Dokkum & Stanford 2003).
Other suggestions for elliptical galaxy formation
Ellipticals are more complex than what the simple monolithic scenario predicts
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Toomre & Toomre (1972) suggested that elliptical galaxies can form from major
mergers of massive disk galaxies.
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Detailed numerical simulations (Farouki & Shapiro 1982; Negroponte & White 1983)
showed that the merger of two spiral galaxies of comparable mass can indeed
produce a remnant with structural and photometric properties resembling those of
elliptical galaxies.
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In more recent years, a large body of observational evidence has been collected that
demonstrates that interactions and mergers indeed represent a common
phenomenon at high redshifts, and that these processes affect the population of
elliptical galaxies in the local Universe.
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Schweizer & Seitzer (1992) found evidence for bluer colours of elliptical galaxies
with increasing morphological disturbance
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Later studies using absorption–line indices have demonstrated that a significant
fraction of cluster early–type galaxies has undergone recent episodes of star
formation (Barger et al. 1996, Menanteau, et al. 2001; van de Ven, et al. 2003, Treu
et al. 2002 etc)
At least for a part of the elliptical galaxy population, a hierarchical formation scenario
in which larger spheroidals are assembled relatively late from the merger of late–type
galaxies of comparable mass.
Such a bottom-up formation scenario is naturally expected for the structure formation
process in cosmologies dominated by cold dark matter.