PH607 – 2 - Galaxies 2
Detailed structure of the Milky Way
The Inner Bar and Ring
Observed structure of the Milky Way's spiral arms
The Milky Way is thought to comprise a large barred spiral
galaxy of Hubble type SBbc (loosely wound barred spiral) with a
total mass of about 1012 solar masses, comprising 200-400
billion stars.
A BARRED SPIRAL: It was only in the 1980s that astronomers
began to suspect that the Milky Way is a barred spiral rather than
an ordinary spiral, which observations in 2005 with the Spitzer
Space Telescope have since confirmed, showing that the galaxy's
central bar is larger than previously suspected.
The galaxy's bar is thought to be about 9 kiloparsecs long,
running through the centre of the galaxy at a 44±10 degree angle
to the line between our sun and the centre of the galaxy.
It is composed primarily of red stars, believed to be ancient.
The bar is surrounded by a ring called the "5-kpc ring" that
contains a large fraction of the molecular hydrogen present in the
galaxy and most of the Milky Way's star formation activity.
Observed and extrapolated structure of the
spiral arms
Since the 1950s, astronomers have produced maps of the Milky
Way. The early models were based on radio observations of gas
in the galaxy, and suggested a spiral structure with four major starforming arms, called Norma, Scutum-Centaurus, Sagittarius
and Perseus.
Each spiral arm describes a logarithmic spiral (as do the arms of
all spiral galaxies) with a pitch of approximately 12 degrees.
The four major spiral arms all start at the Galaxy's centre:

2 and 8
– 3 kpc and Perseus Arm



3 and 7 - Norma and Cygnus Arm (along with a newly
discovered extension - 6)
4 and 10 - Crux and Scutum Arm
5 and 9 - Carina and Sagittarius Arm
There are at least two smaller arms or spurs, including:

11 - Orion Arm (which contains the solar system and the Sun
- 12)
Outside of the major spiral arms is the Outer Ring or Monoceros
Ring, a ring of stars around the Milky Way, which consists of gas
and stars torn from other galaxies billions of years ago.
Recent discoveries: 1. Just two arms!
For Release: June 3, 2008
Now, new images from NASA's Spitzer Space Telescope are
shedding light on the true structure of the Milky Way, revealing that
it has just two major arms of stars instead of the four it was
previously thought to possess.
The Spitzer findings make the case that the Milky Way has two
major spiral arms, a common structure for galaxies with bars.
These major arms, the Scutum-Centaurus and Perseus arms,
have the greatest densities of both young, bright stars, and older,
so-called red-giant stars.
The two minor arms, Sagittarius and Norma, are filled with gas and
pockets of young stars.
2. Extended structure.
1. Size. The Andromeda Galaxy (M31) extends much further than
previously thought. The disc of the Milky Way extends further is a
clear possibility and is supported by evidence of the newly
discovered Outer Arm extension of the Cygnus Arm.
2. Interaction. With the discovery of the Sagittarius Dwarf
Elliptical Galaxy in 1994 came the discovery of a ribbon of
galactic debris as the polar orbit of Sagittarius and its interaction
with the Milky Way tears it apart.
Similarly, with the discovery of the Canis Major Dwarf Galaxy in
2003, our closest neighbour, a ring of galactic debris from its
interaction with the Milky Way encircles the galactic disk.
3. Mergers. In 2005 the Sloan Digital Sky Survey of the northern
sky found a huge and diffuse structure within the Milky Way that
does not seem to fit within our current models. The collection of
stars rises close to perpendicular to the plane of the spiral arms of
the Milky Way. The proposed likely interpretation is that the
remains of a dwarf spheroidal galaxy is merging with the Milky
Way. This galaxy is tenatively named the Virgo Stellar Stream
and is found in the direction of Virgo about 10 kiloparsecs away.
Spiral galaxies in general:
Spirals are complex systems, more complex than elliptical.
 Wide range in morphological appearance
 Fine scale details – bulge/disk ratios, structure of
arms, resolution into knots, HII regions, etc.
 Wide range in stellar populations – old,
intermediate, young, and currently forming
 Wide range in stellar dynamics:
 “cold” rotationally supported disk stars
 “hot” mainly dispersion supported bulge &
halo stars
 Significant interstellar medium (ISM)
Kinematics: Spiral Galaxy Rotation Curve
As is typical for many galaxies, the distribution of mass in the Milky
Way is such that the orbital speed of most stars in the galaxy does
not depend strongly on its distance from the centre. Away from
the central bulge or outer rim, the typical stellar velocity is between
210 and 240 km/s.
Hence the orbital period of the typical star is directly proportional
only to the length of the path travelled.
This is unlike the solar system where different orbits are also
expected to have significantly different velocities associated
with them.
Rotational support: Like the Milky Way, external spiral galaxies are
supported against collapse by their rotation. (c.f. elliptical
galaxies, which are not).
Rotation curve: By using the Doppler shifts in spectral lines to
measure galaxies' line-of-sight velocity as a function of position,
we can measure their rotation curves (speed of material
following circular orbits around the centre of the galaxy as a
function of radius). We can derive this quantity from:
o
o
o
21cm emission from atomic hydrogen
Optical emission lines from hotter gas
Optical absorption lines from the stellar component
Most rotation curves look very similar: FLAT
M(r) = mass interior to radius R
V(r) = rotation speed
For our own galaxy, it is possible to obtain the "rotation curve" --the circular velocity as a function of radius --- of the inner part of
the Milky Way using the line-of-sight velocity of atomic hydrogen
as measured from the Doppler shift in its 21cm emission.
Combining the results from these methods, we obtain the following
estimate for the Milky Way rotation curve:
Visible Matter: Although the enclosed mass, M(r), continues to
grow apparently without limit, the enclosed luminosity, L(r),
tends to a finite limit as we reach the edge of the luminous
material in the galaxy.
There must therefore be significant amounts of dark matter which
continue to contribute to M(r) out to very large radii.
Out to the furthest point measured, typical galaxies have a
luminosity of L ~ 1010 L , and a typical enclosed mass of M ~ 1011
M .


The "mass-to-light ratio" M / L is hence ~ 10 solar
units.
Typical disk mass-to-light ratio: 0.5-2.0.
90% of the material in the galaxy is dark!
Example:
The Sun moves at about 220 km s-1 in a circular orbit around the
centre of the Galaxy, like almost all the stars near the Sun. We can
assume that all the matter is at the Galactic centre, a not too bad
approximation.
Let the speed be V0 , the mass of the Galaxy be MG and the
distance of the Sun from the Galactic centre be R0. Then the
centrifugal force due to rotational speed must balance the
gravitational force due to the mass of the Galaxy.
GMG/R02 = V02/R0
where G is the gravitational constant. Hence
MG = V02 R0/G
Substituting values of 8 kpc and 220 km s-1 for the Sun and G =
0.00430 M (km s-1)2 / pc, we get
MG = 1011 M
.
However, measurements of the rotation of the outer edge of the
Milky Way show that the stars out there also rotate at 220 km s-1,
out to about 20 kpc.
Thus, within a radius of 20 kpc we get a mass of
MG = 2x1011 M
If the light distribution of the Galaxy were proportional to the mass
distribution, then the two mass estimates above would imply that
the amount of light emitted by the 8 kpc region would be the same
as the region from 8 - 20 kpc...... whereas measurements show
that the 8 kpc region emits about 10 times more light than the 8 20 kpc region.
The major conclusion is that the distribution of emitted light is not
necessarily the same as the underlying distribution of matter.
The Galactic Centre
The centre of the Galaxy is a unique laboratory where we can study the
fundamental processes of strong gravity, stellar dynamics and star formation
that are of great relevance to all other galactic nuclei, with a level of detail that
will never be possible beyond our Galaxy.
Because of cool interstellar dust along the line of sight, the
Galactic Centre cannot be studied at visible, ultraviolet or soft Xray wavelengths. The available information about the Galactic
Center comes from observations at gamma ray, hard X-ray,
infrared, sub-millimetre and radio wavelengths.
The nucleus of the Milky Way contains a complex of gas,
dust, stars, supernova remnants, magnetic filaments, and,
almost certainly, a massive black hole at the very centre; it
lies in the direction of Sagittarius, around R.A. 17h 46m
and Dec. -28° 56'.
A few hundred parsecs (radio):
The galactic centre harbours a compact object of very large mass,
strongly suspected to be a supermassive black hole. Most galaxies are
believed to have a supermassive black hole at their centre.
Ever since black holes were suggested as the power sources for
Active Galactic Nuclei (AGNs) such as Seyfert Galaxies and
QSOs, we have speculated on whether the centre of our galaxy
might contain a black hole
"Galactic Centre" here will mean the central ~10 parsecs of the
Galaxy. It contains:
1. Young stars: the stellar population including evidence for star
formation there in the last 50 million years or even less
2. Interstellar material including both ionized gas (HII regions) and
molecular clouds which orbit the Centre in a ring with an inner
radius of about 2 pc. Hot dust is also observed.
3. Strong magnetic fields (milliGauss) as compared to elsewhere in
the Galaxy
4. A compact radio source called SgrA* which is quite unlike any
another radio source in the Galaxy.
5.. Radial velocities and proper motions of both stars and gas
which imply the existence of a large, unseen, compact object.
Large means a mass =~ 2.5 x 106 M
6.. Black hole. The discovery that the radio source SgrA*
corresponds to the dynamical centre of the Milky Way and
coincides with the large, dark mass has lead to the realization that
SgrA* is a black hole, albeit a puzzling one.
1)
Infrared stars: proper motion imply 3.7 x 106 Msun black
hole
http://astro.kent.ac.uk/mds/Modules/1011/PH607/Imagesmovies/gc-orbits3d_small.gif
blackhole.mov
Lying dead centre in the Galaxy is the Sagittarius A
Complex, which is believed to be associated with a black
hole, material in orbit around this object, and a nearby
supernova remnant.
Surrounding the galactic centre are narrow threads
known as nonthermal filaments (NTFs), the most
prominent of which are called the Arc, the Pelican, and the
Snake.
These seem to consist of magnetic flux tubes filled with
relativistic electrons, beaming synchrotron radiation, that
have been swept up from adjacent molecular clouds and
hurled along the field lines.
Another unusual structure in the nucleus is catalogued as
359.1-00.5 and appears to be a superbubble with a cluster
200 newborn stars at its heart.
Radio wavelengths have revealed a complex structure of ionized
gas called the 'mini-spiral'. The mini-spiral is broken down into
individual components including the Northern Arm, Western Arc
and Eastern Arm (see below).
The mini-spiral is surrounded by a thick ring of molecular material
called the Circumnuclear Disk (CND) which is from 2.5 to 4.8
parsecs (50 to 95 arcseconds) in size.
The Paradox of Youth
More than 100 OB and Wolf-Rayet stars have been identified there
so far. They seem to have all been formed in a single star
formation event a few million years ago.
The existence of these relatively young (though evolved) stars
there was of a surprise to experts, who would have expected the
tidal forces from the central black-hole to prevent their formation.
They are much too young to have migrated far, but it seems even
more improbable that they formed in their current orbits where the
tidal forces of the black hole act.
This paradox of youth is even more remarkable for stars that are
on very tight orbits around Sagittarius A*, such as S2.
One particular star, known as S2, orbits the Milky Way's centre so
fast that it completed one full revolution within the 16-year period.
Observing one complete orbit of S2 has been a crucial contribution
to the high accuracy reached and to understanding this region.
The scenarios invoked to explain this formation involve either star
formation in a massive star cluster offset from the Galactic Centre
that would have migrated to its current location once formed, or
star formation within a massive, compact gas accretion disk
around the central black-hole.
It is interesting to note that most of these 100 young, massive stars
seem to be concentrated within one or two disks, rather than
randomly distributed within the central parsec. This observation
however does not allow definite conclusions to be drawn at this
point.
Since the first near-infrared high-resolution observations of the
galactic centre in the beginning of the 1990s, the GC was regularly
monitored. However, in spite of all efforts, no unambiguous NIR
counterpart of SgrA* could be detected up to 2003.
On the 9th of May, during routine observations of the GC star
cluster at 1.7 microns with NAOS/CONICA at the VLT, we
witnessed a powerful flare at the location of the black hole.
Within a few minutes, the flux of a faint source increased by a
factor of 5-6 and fainted again after about 30 min. The flare was
found to have happened within a few milli-arcseconds of the
position of Sgr A*. The short rise-and-decay times told us that the
source of the flare was located within less than 10 Schwarzschild
radii of the black hole.
flare1movie.gif
http://astro.kent.ac.uk/mds/Modules/1011/PH607/Imagesmovies/gc-flare1movie.gif
http://astro.kent.ac.uk/mds/Modules/1011/PH607/Imagesmovies/milkyway-blackhole.mov