Docstoc

The Milky Way and other galaxies

Document Sample
The Milky Way and  other galaxies Powered By Docstoc
					The Milky Way and other
        galaxies
        Chapter 6
               Galaxies
Groupings of 107-1011
stars
Only locations where
stars exist
Galaxies do have
other components
other than stars
The Milky Way is a
spiral galaxy           http://spot.pcc.edu/~gvershum/
The Milky Way is a Spiral Galaxy




  http://getlitstaylit.files.wordpress.com/2009/06/spiral-galaxy.jpg
  Structure of the Milky Way
A central supermassive black hole
Flattened circular stellar disk at the center
Thinner gas-and-dust disk within the stellar disk
A stellar bulge or spheroid
A large, spheroidal halo of gas, stars and
globular clusters
Cosmic rays, energetic charged particles,
trapped by magnetic fields in the disk
A dark-halo, made of dark-matter, extending
beyond the visible components
    Structure of the Milky Way
A central supermassive black
hole
Flattened circular stellar disk at
the center
Thinner gas-and-dust disk
within the stellar disk
A stellar bulge or spheroid
A large, spheroidal halo of gas,
stars and globular clusters
Cosmic rays, energetic charged
particles, trapped by magnetic
fields in the disk
A dark-halo, made of
dark-matter, extending beyond
the visible components




                                     Figure 6.2
  Structure of the Milky Way
The sun is in the disk of the Galaxy
The term Milky Way refers to the diffuse white
color produced by the unresolved stars in the
disk, viewed across
R☼ = 26,000 ± 1,600 ly; v☼ = 220 km/s
Therefore, τ = 2π R☼/ v☼ = 2.25∙108 yr
From GM(R☼)/R☼2 = v☼/R☼, we can calculate
M(R☼), the mass inside the radius R☼. This is
approx. 1011 M☼. Since the about half of the
mass corresponds to stars and the average
mass of a star is 0.5 M☼, there are about 1011
stars in the Galaxy inside the sun’s orbit
The term Milky Way refers to the diffuse
white color produced by the unresolved
    stars in the disk, viewed across



                   from
                   http://www.nies.ch/sky/stars/Milky_Way.jpg
    Structure of the Milky Way
A central supermassive black
hole
Flattened circular stellar disk at
the center
Thinner gas-and-dust disk
within the stellar disk
A stellar bulge or spheroid
A large, spheroidal halo of gas,
stars and globular clusters
Cosmic rays, energetic charged
particles, trapped by magnetic
fields in the disk
A dark-halo, made of
dark-matter, extending beyond
the visible components




                                     Figure 6.2
                  The Disk
The mass distribution falls exponentially with distance
from the center (r) and with height above or below the
plane (z)
Ρ(r,z) = ρ0 exp(-r/rd) exp(-|z|/hd)
rd = 11,400 ± 1,600 ly (sun is in the outer regions)
hd = 1000 ly for stars, = 500 ly for gas/dust
For the sun, z = 100 ly above the disk
M(disk,<11,400 ly) = 1010 M☼
The mean density of stars (<n>=1010 M☼/V(rd,hd)) can be
used to obtain the mean distance between stars. <d> =
<n>-1/3 ≈ 1 pc ≈ 3 ly
                    The disk
From these numbers, we can use the familiar equations
to get the mean free path and the average time between
collisions: τ ≈ 7∙1018 years
This number is much greater than the age of the
universe, so most stars would never collide
In reality, the stars don’t move randomly. Gravitation
attracts stars to each other, so that there is a
gravitational focusing that increases the frequency of
collisions
τ = (<n>σeff vran)-1. vran (usually ~20 km/s) is the random,
non-orbital star velocity. <n> is the density of stars
(about 3 ∙10-62 m-3) and σeff is the effective cross section
due to gravitational focusing. σeff = σgeom (1+ve2/vran2).
σgeom = π(2r*)2 and ve is the escape velocity at the
surface of the star
                The Disk
Gravitational focusing increases the collision
cross section by a factor of 1000
Collisions are still rare in the disk
In the innermost regions of the Galaxy, the risk
of collision is much higher, especially if we
consider the geometric cross section to be not
the star size, but the size of the planetary
system
It is estimated that in these regions, 1% of all
planetary systems are disrupted by the close
approach of another star
                 The Disk
The presence of gas/dust is a key feature of
galaxies. In fact, we find it hard to observe large
portions of our own galaxy in the visible and IR
due to gas/dust
The spiral structure of galaxies is a permanent
feature even though stars closer to the center
move much faster than stars farther away
Although this should cause a breakage of the
spiral arrangement, it turns out that stars move
in and out of the spiral, but this remains
We talk about density waves, regions of higher
stellar and gas density.
    Structure of the Milky Way
A central supermassive black
hole
Flattened circular stellar disk at
the center
Thinner gas-and-dust disk
within the stellar disk
A stellar bulge or spheroid
A large, spheroidal halo of gas,
stars and globular clusters
Cosmic rays, energetic charged
particles, trapped by magnetic
fields in the disk
A dark-halo, made of
dark-matter, extending beyond
the visible components




                                     Figure 6.2
                  The Spheroid
Stellar bulge with size ~3000 ly and ρ ~ r-3
Stellar and gas halo out to ~160,000 ly
   About 200 globular clusters in the halo
Stars in the spheroid are old (10-14∙109 yr)
We know this from examination of the H-R diagram of
the population as a whole and from the relative fraction
of iron in the stars with respect to the sun: [Fe/H]/[Fe/H]☼
= 10-4.5 to 10-0.5
This implies that these stars were formed when the
abundance of heavy elements (produced in stars) was
lower, indicating that the stars formed early in the history
of the universe
    Structure of the Milky Way
A central supermassive black
hole
Flattened circular stellar disk at
the center
Thinner gas-and-dust disk
within the stellar disk
A stellar bulge or spheroid
A large, spheroidal halo of gas,
stars and globular clusters
Cosmic rays, energetic charged
particles, trapped by magnetic
fields in the disk
A dark-halo, made of
dark-matter, extending beyond
the visible components




                                     Figure 6.2
       The Galactic Center
Can be studied at
wavelengths outside
the visible and UV
range
Large density of stars
Includes young star
clusters, supernova
remnants and atomic
or molecular gases

                         http://arecibo.tc.cornell.edu/PALFA/images/galactic_center.jpg
            The Galactic Center
Sagittarius A, (d~1 AU, M~4∙106
M☼, see next slide), is located at
the center. The density of Sgr A
implies it is a supermassive black
hole
The luminosity from Sgr A stems
from the release of gravitational
energy by matter being accreted
Supermassive black holes appear
to be at the center of all galaxies
Their masses are proportional to
the stellar masses of the bulge
Mass range: 106-109 M☼
We don’t know if the black holes
resulted in the agglomeration of
material to form the galaxy, or if
the galaxy formed first and the
black hole was created in the
evolution of the galaxy
                                      http://arecibo.tc.cornell.edu/PALFA/images/galactic_center.jpg
               The Galactic Center
Figure 6.5




   The orbital periods of stars around Sgr A can be used to determine its mass
    Structure of the Milky Way
A central supermassive black
hole
Flattened circular stellar disk at
the center
Thinner gas-and-dust disk
within the stellar disk
A stellar bulge or spheroid
A large, spheroidal halo of gas,
stars and globular clusters
Cosmic rays, energetic charged
particles, trapped by magnetic
fields in the disk
A dark-halo, made of
dark-matter, extending beyond
the visible components




                                     Figure 6.2
               Cosmic Rays
Cosmic rays is an umbrella term for energetic charged
particles that arrive from space
The atmosphere blocks them, so they can’t be perceived
at the surface
The can be detected by using high altitude balloons or
by detecting short-lived particles, like muons, generated
when the cosmic rays impact with matter in the outer
atmosphere
They may have their origin in the solar system (sun), the
Galaxy (probably originated in supernova explosions), or
outside the Galaxy (from active galactic nuclei and
gamma ray bursts)
    Structure of the Milky Way
A central supermassive black
hole
Flattened circular stellar disk at
the center
Thinner gas-and-dust disk
within the stellar disk
A stellar bulge or spheroid
A large, spheroidal halo of gas,
stars and globular clusters
Cosmic rays, energetic charged
particles, trapped by magnetic
fields in the disk
A dark-halo, made of
dark-matter, extending beyond
the visible components




                                     Figure 6.2
            The Dark Halo
In astronomy/astrophysics, the term dark implies
something that cannot be detected via
electromagnetic radiation
The evidence for the existence of the dark halo
is indirect
Measurements of the rotation rate of stars
around the galactic center indicate that there is
more mass than what we actually observe
We can determine rotational velocities by
Doppler shift in lines of known frequency (and
do some corrections to account for angle with
line of sight or our own velocity)
             The Dark Halo
Figure 6.6 shows the
observed (solid) and
expected (dashed)
rotation curve (speed vs
distance to center) for
objects in the Galaxy
Unexpectedly, the curve
flattens out at about
50,000 ly and stays flat
even at the very far
reaches of galactic
material                     Figure 6.6
              The Dark Halo
In the solar system, most of the mass is located at the
center (the sun)
For a planet, and using Epot = Ekin, we can write GM☼m/r
= mv2/2, so that M~v2r/G
In the Galaxy, the mass is not located all at the center
(that’s why the speed initially increases with radius;
objects are moving faster because there is much more
“inner mass”, the farther from the center you go)
Using M(r)~v2r/G, v(r)=constant means that M(r)~r
In other words, in the regions where v(r) is flat, the mass
of the Galaxy grows linearly with r, out to the largest
distance we can measure, even though we can clearly
see that the amount of visible matter decreases
exponentially
 The Dark Halo: Dimensions
The flat region of the rotation curve extends to
~100,000 ly, or about 10 disc scale lengths.
This implies that the mass of the Galaxy is about
10 times that inside a disc scale length
In order words, 90% of the galaxy is dark (1012
M☼)
Since r~M(r)~ρ(r) r3, the mass density profile of
the dark matter is ρ(r)~1/r2
      What is Dark Matter?
The book goes over possible materials that can
constitute this “dark matter”. You can read some
of the discussion, but let’s briefly go over them
Most of the potential sources of dark matter
listed are baryonic matter
Baryonic matter is matter composed of three
quarks (i.e. proton, neutrons, and nuclei). Most
ordinary matter is baryonic in nature. Electrons
and neutrinos are not (they are leptons) and the
category of (unstable) particles called mesons
are not baryonic
        Baryonic Dark Matter?
Gas
   Atomic. Would have intense radiation in the microwave
   Molecular. Not completely ruled out yet, because it would not
    radiate if extremely cold. Yet, it would scatter light and this has
    not been observed
   Ionized gas. To be ionized, it must be hot, so it would emit
    X-rays
Dust
   Not good, because it would emit IR
   In other galaxies, we would observe dimming of background
    objects by the halos
   Finally, dust requires metals. Since the mass of the dark halo is
    10x that of visible matter, we would need that the mass of metals
    is 10x that of H and He. We don’t have any cosmological model
    that allows for this distribution of matter
          Baryonic Dark Matter?
Massive Compact Halo Objects (MACHOs). These are gravitationally
bound objects (stars, planets or black holes)
    Main-sequence stars: They would be visible
    Giant stars: Even more luminous
    Neutron stars: They are formed in supernova explosions. This means that most
     of the matter is ejected and would have to leave the galaxy, (otherwise the visible
     part would have higher mass), and we should observe a lot of metals in the halo,
     which we do not
    Black holes: Similar argument as in the neutron stars. They would also disturb
     the orbits of binary stars in the halo, and we haven’t seen that
    White dwarfs: We would expect an enrichment in the halo of intermediate mass
     metals (C, O, Ne) produced in the ejection of the outer layers of the stars, which
     we don’t see
    Brown dwarfs: Since they have low mass/luminosity, they were a leading
     candidate, but they have been ruled out by gravitational lensing experiments
Elementary particles
    Protons: They are ionized hydrogen (already ruled out)
    Neutrons: Have a half-life of about 900 s and decay (among others) to protons,
     which have already been ruled out
        Leptonic Dark Matter?
Electrons
   The Galaxy is electrically neutral. If electrons are dark matter,
    this would require equal numbers of protons (with a mass
    ~2000x larger), which we have seen is impossible
Massive neutrinos
   Neutrinos have a small mass. If it is large enough, due to their
    large numbers, they could account for the missing mass.
    However, they move at relativistic speeds, so it is hard to believe
    that they would remain in the dark halo
Sterile neutrinos
   Sterile neutrinos are hypothetical particles. Unlike ordinary
    neutrinos, which interact with other particles via gravity and the
    weak nuclear force, sterile neutrinos only interact via gravity
   Some models of matter predict that the sterile neutrinos would
    have much larger masses than regular neutrinos and could
    account for the bulk of the dark matter
           Dark Matter models
Dark matter theories predict the existence of particles,
not yet discovered, that interact only via gravity and the
weak force, and therefore would not be affected by
electromagnetism (they would be then, by definition,
dark)
Cold dark matter
   These particles move at low speeds (cold) and thus could be
    bound by the gravitational field of the galaxies
   Potential particle candidates are the axions, the neutralinos, and
    the WIMPs (Weakly Interacting Massive Particles). Neither have
    been observed
   Problems
       Cuspy Halo problem. The model predicts that the rotation curve
       peaks more strongly than observed
       Missing Satellites problem. The model predicts that the galaxies
       should have large number of satellite galaxies with mass about
       1/1000 the mass of the parent galaxy
           Dark Matter models
Hot Dark Matter
   Composed of particles that move at ultrarelativistic speeds
   Best candidate is the neutrino
   Big Problem: The smooth distribution of neutrinos in the early
    universe cannot explain the formation of galaxies
   Nowadays, HDM is only considered in combination with Cold or
    Warm Dark Matter theories
Warm Dark Matter
   A theory with intermediate properties between Cold and Hot
    Dark Matter
   Candidate particles: The sterile neutrino and the gravitino (a
    fermion with spin 3/2 which would be the supersymmetric partner
    of the graviton). The existence of the gravitino would bring
    problems when trying to reconcile predictions with observations
          Gravitational Lensing
General Relativity indicates that a
mass causes a disturbance in
space-time
The usual analogy used is that
empty space is like a flat sheet of
cloth or the surface of a mattress.
Light travels on straight lines on
these surfaces
But if you add a mass, this creates
a divot on the surface. A ray of
light would bend its trajectory to
accommodate motion through this
surface
As a matter of fact, the model
explains that planets orbit around
star by following a “linear” path
through this curved space-time


                                      http://www.astronomynotes.com/evolutn/grwarp.gif
      Gravitational Lensing
A distant object
located behind a
massive one can
have its light focused
by this effect
Thus, gravitational
lensing can be used
to carry out
                         http://en.wikipedia.org/wiki/File:Gravitational_lens-full.jpg
observations of
distant objects
          Gravitational Lensing
Let’s examine what happens when a single
beam of light travels near an object of mass
M, passing at a distance b
The beam gets deflected by an angle α,
which depends on M and b
If b>>rs (note the object doesn’t need to be
a black hole, but a Schwarzschild radius
can be calculated for any mass), and b is
much smaller than the distance from
source or observer to M, the equation for α
is:
           α = 4GM/(bc2) = 2rs/b
α is proportional to M (larger mass, larger
deflection) and inversely proportional to b
(closer to the mass, larger deflection).
The conditions outlined in the third bullet
point are referred to as the thin-lens
approximation
         Gravitational Lensing
The first test of general relativity consisted in measuring the
deflection caused by the sun on the light from a star as it passed
near the limb of the sun. This was done during an eclipse, of course
r☼ = b = 7.0∙108 m; M☼ = 2.0∙1030 kg; rs ~ 3 km; d☼ = 1.5∙1011 m.
Thus, b>>rs and b<< d☼
α = 2rs/b = 2 ∙ 3∙103 m/7.0∙108 m = 8.6∙10-6 rad
Thus, α = 1.8 arcsec
Life is a bit more complicated than this, because, in general, a
distance source can travel along different paths around the lens.
The lensed image is not a point, but a ring, called Einstein ring
But this is a good problem to have. You see, an alternative
consequence of this phenomenon is that all massive objects are
capable of producing gravitational lensing. We can identify the
lensing by observing the ring. So, when we see such a ring, we
know there is a lens, even if we can see the lens. This allows us to
use gravitational lensing to observe “dark” objects
  Simulation of Einstein Ring
The image to the right
shows the formation of an
Einstein ring around a
black hole
In the simulation, the black
hole (which normally would
be unseeable) travels in
front of an edge-on distant
galaxy
In the process, it
generates an Einstein ring
that can be used to infer
the presence of the black      http://en.wikipedia.org/wiki/File:Black_hole_lensing_web.gif
hole
Gravitational Lensing as a Test of
       Dark Matter Models
Paczyńsky (1986) proposed an
experiment to gauge whether
MACHOs could account for the
bulk of the mass of dark matter
He proposed studying the stars in
the Large Magellanic Cloud
(LMC), a satellite galaxy of the
Milky Way located approx.
160,000 ly away
At this distance, many of its stars
can be resolved individually
Given the mass of dark matter
and the expected mass of the
MACHO’s, we can predict the
expected number density of these
objects                               http://archive.stsci.edu/prepds/fuse_mc/LMC_3col_sm.jpg
Gravitational Lensing as a Test of
       Dark Matter Models
The light of individual stars in the LMC can be amplified by
gravitational lensing if a MACHO happens to pass in front of
the star
Knowing the number density of need MACHOs and the
number of stars visible in the LMC, the models predict that 1
in 106 stars should be significantly lensed by a MACHO in our
Galaxy
The length of a lensing event depends on the size of the
MACHO (e.g. if m=M☼, τ~6 months, while if m=MJupiter, then
τ~6 days)
Proposed experiment:
   Monitor 107 LMC stars for 5 years
   10 of the stars should be undergoing lensing at a given time
   The number and timescale of events would provide information
    on the size of the MACHOs (e.g. 10∙5 yr/0.5yr = 100 6-month
    long events if m=M☼; 2600 week-long events if m=MJupiter)
Gravitational Lensing as a Test of
       Dark Matter Models
The experiment started in 1991
After six years, only 15 lensing events (with τ=35-230
days (i.e. m~0.1-1 M☼)) were observed
The results indicate that no more than ~20% of the mass
of dark matter may consist of MACHOs (and this limit is
if each object is as massive as the sun)
In fact, it is entirely possible that the small number of
lensing events may have been caused directly by stellar
matter (both in the Galaxy and in the LMC)
So, it is expected that this number is not a realistic upper
limit (the upper limit is expected to be even lower)
Current thought is that non-baryonic cold dark matter is
the prime candidate for the missing mass
An aside: Extrasolar Planets
When looking in the direction of the
Galactic center, the number of stars
is so high that the observation of
lensing events is quite common
Figure 6.14 shows the observed
magnification due to such an event
by an intervening star
If this star (the one that acts as a
lens) has planets, they may cause a
perturbation in the magnification
curve (seen around day = +10 in the
figure)
Many extrasolar planets have been
detected via this microlensing
technique                              Figure 6.14
Modified Newtonian Dynamics
Starting in 1983, Mordehai Milgrom developed a theory that does not require
dark matter to explain the anomalous rotation rates of stars in galaxies
Subsequent contributions were made by Jacob Bekenstein. In the 2000s,
relativity was included in the formulations
Milgrom’s idea stems from the fact that Newton’s laws of motion have not been
tested in the very low acceleration regime that is characteristic in the flat portion
of the rotation curve
His original assumption was that F=m∙a is an approximation that works when a
is high. He postulated that the correct formula is:
                                 F = m∙a∙μ(a/a0)
where μ(a/a0) is a function of the acceleration a. a0 is a constant that defines
the onset of the “new dynamics”. When a/a0>>1, μ(a/a0) = 1 and the familiar
laws of physics apply.
When a/a0<<1, μ(a/a0) = a/a0, so the modified equation is
                                   F = m∙a2/a0
Milgrom determined a0 via several independent methods and found the results
cluster around a value of 2.0∙10-10 m/s2, which perhaps not coincidentally is
close to the value of the product of c and H0 (Hubble’s constant)
        This theory is called Modified Newtonian Dynamics (MOND)
Modified Newtonian Dynamics
The two expressions presented in the previous slide (F=m∙a and
F=m∙a2/a0) correspond to the limiting cases of normal and very
slow acceleration. Other values would follow an intermediate
expression
With this formulation, Milgrom was able to make predictions
(borne out by experiments) of rotation curves in galaxies and
clusters of galaxies




                                                    from
                                                    arXiv:0801.3133v2
                                                    [astro-ph]
Modified Newtonian Dynamics
“I review briefly different aspects of the MOND paradigm, with emphasis on
phenomenology, epitomized here by many MOND laws of galactic motion--
analogous to Kepler's laws of planetary motion. I then comment on the
possible roots of MOND in cosmology, possibly the deepest and most far
reaching aspect of MOND. This is followed by a succinct account of existing
underlying theories. I also reflect on the implications of MOND's successes
for the dark matter (DM) paradigm: MOND predictions imply that baryons
alone accurately determine the full field of each and every individual galactic
object. This conflicts with the expectations in the DM paradigm because of
the haphazard formation and evolution of galactic objects and the very
different influences that baryons and DM are subject to during the evolution,
as evidenced, e.g., by the very small baryon-to-DM fraction in galaxies
(compared with the cosmic value). All this should disabuse DM advocates of
the thought that DM will someday be able to reproduce MOND: It is
inconceivable that the modicum of baryons left over in galaxies can be
made to determine everything if a much heavier DM component is
present.”
This and image in previous slide from M. Milgrom, “The MOND paradigm”,
2008, arXiv:0801.3133v2 [astro-ph]. Accessible at:
              http://arxiv.org/PS_cache/arxiv/pdf/0801/0801.3133v2.pdf
from arXiv:0801.3133v2 [astro-ph]. Accessible at:
http://arxiv.org/PS_cache/arxiv/pdf/0801/0801.3133v2.pdf
                 Galaxy Types
Spiral galaxies. Some of them have a
central bar
Elliptical galaxies. Look like the
bulge, but there is no disk. Have old
stars and low amounts of gas/dust.       http://www.phys.ncku.edu.tw/~astrolab/mirrors/apod_e/image/0501/ngc1300_hst_c30g90.jpg


Stars move in orbits with large          http://www.astro.psu.edu/users/niel/astro1/slideshows/class21/009-m87-giantelliptical-typeE1.jpg


inclination and eccentricities
Irregular galaxies. Have ongoing star
formation and young stars.
We don’t know much about potential
dark halos in elliptical and irregular
galaxies. Not having a disk, we
cannot observe any anomalies in the
rotation curves at large distances
                                                                          http://www.astrographics.com/GalleryPrints/Display/GP0111.jpg
        Galactic Luminosities
The sizes, masses and luminosities of galaxies vary
significantly. However, it is fairly straightforward to measure
the latter
The Schechter function (luminosity function) describes the
number, φ(L), of galaxies per unit volume with a luminosity
between L and L+dL
             φ(L)dL ≈ φ(LЖ) (LЖ/L) exp(-L/LЖ) dL

Notice that the function decreases very fast with luminosity
because of the 1/L dependence and the exponential decrease
with L. In fact, galaxies with a luminosity of more than a few
times LЖ are rare. LЖ ≈ 2∙1010 M☼
The Milky Way and on of the other two large galaxies in the
Local Group, the Andromeda galaxy (or M31), have
luminosities roughly equal to LЖ
      Galactic Luminosities
The observed density of galaxies with
luminosities in the range LЖ/2 to 3LЖ/2 is of the
order of 10-2 Mpc-3
This means that there is, on average, one galaxy
with luminosity around LЖ for every 100 Mpc3
(which translates to an average distance of 5
Mpc between galaxies)
By using average galactic radii, we can predict
that collisions between galaxies happen with a
collision time of the order of 5∙1012 yr (500
galactic collisions over the age of the universe)
             Galactic Collisions
Once again, we have an
underestimation by not counting the
effect of gravity (which causes
clustering of galaxies and
gravitational focusing)
When they approach each other, the
gravitational pull of one galaxy is felt
differently by the components of the
                                           http://aaronvanetten.com/images/500_ngc4676_hst_c1.jpg
other due to the large difference in
distance. This creates tidal forces in
each galaxy
This tidal work can use up enough
kinetic energy that a collision may
result in a merger of both galaxies
It has been speculated that elliptical
galaxies have resulted from the
merger of two spiral galaxies

                                                 http://geology.com/news/images/antenna-galaxy.jpg
Active Galactic Nuclei and Quasars
 Most galaxies have supermassive black holes (106 – 109 M☼)
 that are surrounded by large amounts of gas and stars
 Therefore, matter can be (and is) accreted continuously
 Using the equation for Eddington luminosity (4.142):
            LE = 4πcGMmp/σT = 1.3∙1031 J/s M/M☼

 So, for the range of masses above, we can expect
 luminosities in range of 1037-1040 J/s
 The stellar luminosity of our galaxy is about 1037 J/s. This
 means that the central black hole can be at least as luminous
 (and often more luminous by several orders of magnitude)
 than the rest of the galaxy
 Approximately 1-10% of all galaxies have an Active Galactic
 Nuclei (recall that the Eddington luminosity is an upper value)
                AGN and Quasars
Different phenomena can be seen
    large luminosities from small areas
    wavelengths not associated to stellar
     phenomena
    jets of material ejected at relativistic
     speeds
    Doppler-broadening of emission bands
We can assign this behavior to black
holes, as opposed to clusters of stars,
based on the size (e.g. to double the
luminosity in a one hour period,
something that has been observed, the
source can’t be bigger than 1 light-hour
(7 AU)) or the mass of the source (e.g.
                                                http://www4.nau.edu/meteorite/Meteorite/Images/GalacticJet.jpg
when we see long, continuous jets, this
implies continuous emission of this
intensity. The mass needed to power
this using stars is much higher because
ηstar is much smaller than ηBH)
AGN and Quasars
            AGN and Quasars
AGNs can be classified into several groups of objects (Seyfert
galaxies, radio galaxies, BL-Lacertae objects, …). The most
important group is quasars
Quasars (quasi-stellar radio sources) were first identified as very
distant galaxies with AGN
They have luminosities near the Eddington limit
This requires a mass-to-energy conversion factor, η, of 0.057
and, for a black hole of about a billion solar masses, the
accretion of about 40 solar masses per year. The temperature of
the accreted matter at the inner radius of the disk would be of the
order of 250,000 K (this shows a bump in the UV and X-ray
ranges)
The fact that we don’t see quasars in nearby galaxies implies
that quasars were more common in the past (i.e. current AGN
have luminosities well below their Eddington limits). We don’t
know why, but quasar density reached a peak 1010 years ago
Groups and Clusters of Galaxies
Galaxies are not distributed
randomly throughout the universe.
They are located in groups (handful
of large galaxies) or clusters (larger
aggregations)
Our galaxy belongs to the Local
Group. The local group contains
three large galaxies: the Milky Way,
the Andromeda galaxy (M31,
2,500,000 ly away), and the
Triangulum galaxy (M33, 3,000,000
ly away). It also contains about 30
additional small galaxies
The nearest grouping is the Virgo
cluster, located approximately 50
million light years away

                    http://scienceblogs.com/startswithabang/upload/2009/07/the_last_100_years_1998_and_th/galaxy-cluster-abell1689-desk-1280.jpg
Groups and Clusters of Galaxies
The galaxies that form a cluster account for about 10% of the
(visible) total mass of the cluster
The rest of the visible mass is in the form of hot ionized gas at a
temperature of 2­10∙107 K, called the intracluster medium
Still, the total mass of visible (baryonic) matter is only about 15% of
the total mass estimated from galactic motions. The majority of the
mass is in the form of dark matter
At larger scales, galaxies are still not evenly distributed. We talk
about agglomerations of clusters into walls, sheets, filaments, or
bubbles and regions with lower than average galactic density called
voids. We consider now that the structure of the universe is like that
of a foam or a web, with aggregates of clusters (superclusters)
occupying the junctions in the web
Above ~100 Mpc (~300 Mly), the universe appears to be
homogeneous. This means that the mass contained in a sphere
with a 100 Mpc radius would be very similar to that contained in a
sphere of that size located somewhere else in the universe
      Groups and Clusters of Galaxies




http://arxiv.org/html/astro-ph/0306581v1   from Springel et al. Nature, 435 (2005), 629

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:13
posted:5/13/2012
language:English
pages:55