lecture3-2010galp.pptLongair, chapter 3
and variations of disk type & importance of bulges…
Hubble 1936, the Realm of Nebulae
Environment! Globular Clusters in Milky Way
~140 globular clusters, 65% <8kpc from centre
Mateo 2008, Garching workshop
metal rich
vrot = 43 +/- 29km/s
!los = 59 +/- 14km/s"
flattened from rotation dominated by random motions
Globular Clusters & galaxy formation and evolution
Metallicity dispersion is large; mean metallicity decreases with
increasing distance from galactic centre
metal rich
metal poor
Outer Halo: dSph
The Local Group
quiescently evolving dwarf irregulars
Near centres of mass:
Local Super-Cluster! What is a galaxy cluster?
Half the galaxies in the Universe are found in clusters or groups, systems of
galaxies that are a few Mpc across.
Within the central Mpc, clusters typically contain 50-100 luminous galaxies
(L> L* ~ 2 x 1010 L !
).
Most famous catalogues: Abell 1958 and it’s 1989 supplement, with 4073 rich
clusters, having at least 30 giant members within a radius of ~1.5h-1Mpc.
Galaxies in clusters are bound together by their mutual gravitational attraction: the
cluster is generally filled with hot interstellar gas, also retained by gravity.
Clusters differ from groups by having higher densities.
Cluster galaxies live in such proximity that they significantly affect each others
development.
What causes diversity of galaxy types?
There are a number of ways of reducing the “variables” in a study of galaxy
properties – and one is to look at a group or cluster of galaxies.
You remove uncertainties due to different distances of your sample of galaxies as
well as different environments.
Your sample completeness is easily defined.
HOWEVER – it is not clear that you obtain a complete sample of all types of
galaxies, and it may not even be a good “average” sample.
Virgo Cluster
closest rich cluster of galaxies, centred on giant elliptical galaxy M87
Velocity dispersion 715km/s
Virgo Cluster!
Global Properties!
Elliptical galaxies in Virgo (open symbols) & Coma (closed symbols)
Coma galaxies are shown 3.6 mag brighter as they would be at distance of Virgo
This trend could be explained if small elliptical galaxies were either younger or more
metal poor than large bright ones (or both).
What do colours mean?! Spectrum of an Elliptical galaxy!
U B V R I
What does it mean?!
Star Formation History
no particular concentration towards any centre
Global Properties!
Sd, Sm systems are fainter and bluer
Global Properties!
brightness
not efficient at turning HI into
stars
ngc4522 in Virgo
Virgo Cluster
Verheijen 2004
H I
HI content of galaxies in centre of Virgo
LESS than in the outskirts or in lower
density systems (Ursa Major)
Virgo than in the outskirts or in lower
density systems (Ursa Major)
Fraction of E & Sp
Morphology-Density Relation First large (55 clusters, 6000 galaxies) study of morphological segregation (Dressler
1980). The frequency of different galaxy types was found to vary as a function of the number density of galaxies in which they are found. Is this related to R? Or N?
Difficult to ascertain: N # R-1.
Dressler 1980 Effect of sub-structure?
Fraction Sp/E goes up moving
out from cluster centre fraction of Sp goes down
with size of cluster.
DENSITY of galaxies, although presumably
galaxies move through a range of densities, thus there must be coherent sub-structure.
study of poor groups (Postman & Geller
1984) - the centres of which have similar
densities to outer regions of clusters follow same relations as clusters.
galaxies with a nearby companion are more
likely to be Es (Whitmore, Gilmore & Jones
1993), so morphology-density a local
phenomenon.
-density relation over 6 orders of magnitude
in density.
What causes diversity?
Galaxies in clusters more likely to be Es or S0s than those in the field
Environment plays a role
Not all clusters are the same - large E fraction correlates to regular symmetric
clusters; low values to “ratty” ones Oemler (1974)
Also E/Sp varies with position in a cluster -> depends on density.
Fraction of spirals increases out from centre; essentially no spirals in cluster cores
MORPHOLOGY-RADIUS RELATION
Spirals closer to the centre have less gas than those further away
Why? Spatial segregation should give rise to kinematic differences - ie., spirals
follow more energetic orbits - ie., spirals at a given distance from centre of cluster
should have larger random velocities than E
Simulating Interacting Systems
ellipticals in dense environments has lead people to consider that
merging spirals result in an elliptical galaxy....
Josh Barnes 1998
Galaxy luminosity function Just as the distribution of stellar luminosities reflects the physics of star formation
and stellar structure, we might hope to learn about galactic evolutionary processes by studying the distribution of galaxy luminosities.
The galaxy luminosity fn. $(M), $(M)dM is proportional to the number of galaxies
that have absolute magnitudes in the range (M, M+dM):
Where % is the total number of galaxies per unit volume
The field galaxy luminosity function, in its simplest form, involves measuring the
apparent magnitudes of all the galaxies in some representative sample. The individual brightnesses are converted to absolute magnitudes by estimating the
galaxies distances usually by applying the Hubble law to their observed redshifts.
Short comings Malmquist bias - magnitude limited surveys - luminosity function distorted if function
has a finite spread in luminosity. Even if all galaxies have intrinsically identical luminosities , but a range of estimated absolute magnitudes due to errors in their
adopted distances.
Estimating distances using hubble law intrinsically approximate process. Particular
problem for nearby galaxies - local motions dominate over hubble flow. Particular problem for low luminosity galaxies - which can only be observed nearby. So faint
end of luminosity fns remains rather poorly defined.
Spatial structure - incomplete sampling of variations in galaxy distribution (filaments
vs. Voids).
In an attempt to find a general analytic fit to galactic luminosity functions,
Schechter (ApJ 203, p297, 1976) proposed the functional form:
Luminosity Functions of galaxies
Which can also be written (in terms of astronomical magnitudes):
In both forms & (the slope of the power-law at low luminosities) and L* (the
break luminosity) are free parameters that are used to obtain the best fit to the available data.
Local: &= -1.0 and M* B = -21
Virgo: &= -1.24 and M* B = -21 ± 0.7
i.e., this is NOT a universal luminosity function. It seems to depend upon
environment.
A power law with a high luminosity exponential cut-off
Press-Schecter Thus & sets the slope of the luminosity fn at the faint end L* or M* gives the
characteristic luminosity above which the number of galaxies falls sharply and $*
sets the overall normalisation of the galaxy density.
This formula was initially motivated by a simple model of galaxy formation (Press &
Schecter 1974 ApJ 187 425), but has proved to have a wider range of application
than originally envisaged.
Integration over previous eqn has limitation that it effectively predicts infinite number
of small faint galaxies (alpha lies close to ~-1)
However we know universe is finite (dark sky)
However only place we can easily detect low lum galaxies. They exist in large
numbers
Number of galaxies $(M) per 10Mpc cube between absolute magnitude
MR and MR + 1; vertical bars are errors
Schechter function
Galaxy luminosity function in the Virgo cluster (Sandage, Binggeli & Tammann 1985, AJ 90, 1759)
bright faint
Galaxy cluster LFs Different environment. Easier to obtain LF members lie in small region of sky. So
photometry can be obtained efficiently, and all members at same distance. Reducing distance errors. Only problem is rich clusters are rare. So typically at
large distances. Making it hard to detect fainter members.
Faint end slope in cluster significantly steeper than in the field: encounters don’t
end in mergers as often as in field – relative velocities are higher
$* larger in clusters
Jerjen & Tammann 1997
Relative numbers of different types This is usually represented by
the luminosity function '(M)dM. Defined to be the number of
galaxies in a particular sample that have absolute magnitudes
between M and M+dM.
B in
g g
environment is the sum of the individual
luminosity functions of each Hubble type.
Largest fraction in either
environment of all galaxies
are dwarfs (dE and Irr). Even though S and E the most
prominent in terms of mass
and luminosity.
Rotation curves allow mass determination. The constant rotational velocities in the
outer regions - suggests that mass increases linearly with distance from the centre. In stark contrast to the light distribution, which decreases exponentially over the
same distance. Meaning a rapidly increasing mass-to-light ration (M/L) and a hidden dark matter halo in spiral galaxies (Bosma 1981).
Spiral Galaxies: application of Gauss’s theorem to Newton’s law of gravity:
Elliptical Galaxies: application of virial theorem, assuming isotropic stellar
distribution
This kind of analysis has led to the prediction of large dark matter haloes around
elliptical galaxies (e.g., Côte et al. 2001, 2003), using globular clusters as tracers. The line-of-sight velocity dispersion remains remarkably constant out to the limits of
observation. This has the same explanation as flat rotation curves in HI. To bind globular clusters with large velocity dispersions at large radii means that the mass
within R must increase proportional to R.
Rotation Curves of Galaxies Surface Brightness profiles sample the distribution of luminous matter in a galaxy. This does not necessarily
tell us about the mass of the galaxy - about the presence and amount of DARK MATTER. The most direct
way to do this is via the rotation curve of the HI.
Bosma 1981
luminosity or Hubble type a number of correlations
are found: •! for increasing LB rotation curves tend to rise more rapidly with
distance from centre and peak at higher maximum velocity
(Vmax). •! for equal LB spirals of earlier type have larger Vmax. •! within a given Hubble type more luminous galaxies have
larger Vmax. •! for a given value of Vmax the rotation curves tend to rise
slightly more rapidly with radius for earlier type galaxies.
The fact that galaxies of different Hubble types, and
therefore different bulge-to-disk luminosity ratios, exhibit rotation curves that are very similar in form if
not in amplitude suggests that the shapes of the
gravitational potential do not necessarily follow the
distribution of luminous matter.
required for the development of a well ordered spiral pattern
Tully-Fisher
Internal dynamics of Ellipticals Source of galaxies shape? It might be thought that the internal dynamics of elliptical galaxies would be
relatively simple - the surface brightness distributions appear to be ellipsoidal, with a range of flattenings,
which it might be thought could be attributed to rotation.
This can be tested by measuring the mean velocities and velocity dispersions of the stars through out the
body of a galaxy. These measurements can be compared with the rotation and internal velocity dispersions
expected if the flattening can be attributed to rotation.
From Davies et al. 1983
Ellipticals rotate too slowly for centrifugal
forces to be the causes of their observed flattening.
Solid line: amount of rotation
necessary to account for
! of stars.
asymmetric spatial distribution and/or an isotropic velocity distribution of stars at all
points within galaxy must be wrong.
TRIAXIAL SYSTEMS
consequently anisotropic stellar velocity distributions
Kroupa, Tout & Gilmore 1993 MNRAS, 262, 545
Converting luminosity to mass
PDMF (present day mass function) number of stars observed today per unit mass per unit volume. This
needs to be corrected for the time evolution of the IMF up to the present day,
LF (luminosity function) currently observed number of stars observed per unit luminosity per unit volume
IMF (initial mass function) ((m, t), number of stars formed per unit volume at t=0
often approximated as a power law: ((m) dm = (0 m -&
IMF
PDMF