PC2491:
1.
Galaxies
Overview
The aim of this course is to understand the observed properties of galaxies in the hierarchical
structure formation model. This is a particular exciting time to study galaxies1 :
• Initial conditions of structure formation are now well determined (from cosmic microwave
background [CMB], supernovae, large-scale structure of galaxies, ...).
• there is an explosion of observational data (from radio to γ-ray), ground 8-10m class telescopes (KECK, VLT) and satellite missions (Hubble Space Telescope [HST], Chandra, XMMNewton).
• Theory lags behind and there are many unsolved problems, including the nature of dark
matter and dark energy.
• This field will remain active for many decades to come.
1.1.
Basic properties of galaxies
It is not easy to clearly define a galaxy as they have diverse properties. Fig. 1 shows some
Galaxy images, including the Milky Way (MW). But a galaxy usually contains:
• stars: main sequence stars (sun), giants and subgiants, and stellar remnants (such as white
dwarfs [WDs], neutron stars [NSs], black holes [BHs])
• Interstellar medium [ISM]: gas (hot ionised, warm, cold), cosmic rays, magnetic fields and
dust
• dark matter: stellar remnants and unknown dark matter particles
Types for galaxies
We will discuss more about the morphology and classification of galaxies in §1.2.
A number of extrinsic and intrinsic parameters are needed to describe a galaxy.
Extrinsic properties:
1
online material: http://www.jb.man.ac.uk/ ˜ smao/galaxy.html
–2–
M
ilkyW
ay
Fig. 1.— Examples of galaxies: COBE satellite image of the Milky Way (top left), NGC4731 (top
right), M51 (whirlpool galaxy; middle), M87 (bottom left, with MV ≈ −22, d = 18 Mpc) and M32
(bottom right, a dwarf galaxy companion to the Andromeda galaxy with MV = −16.4)
• position on the sky: Galactic longitude: l, Galactic latitude: b.
• radial velocity, or redshift (z): most galaxies have velocities receding from us as their spectral
lines are shifted toward redder wavelength. At low redshift, the receding velocities follow the
Hubble law: v = H0 d, where v is the recession velocity, d is the distance, and H0 is called
Hubble constant.
• distance (1pc=3.086 × 1016 m, and kpc, Mpc, Gpc)
00
• angular size, which is usually measured in arcseconds, with 1 =1/(180/π×60×60)=4.85×10−6
radian.
Intrinsic properties of galaxies:
• physical size (∼ 0.1kpc – 10kpc), which defines a dwarf → giant sequence of galaxies.
–3–
• luminosity (∼ 106 L − 1012 L ) is usually measured in solar luminosity, L , or absolute
magnitudes.
• Galaxies are extended objects. Their surface brightness measures the flux per solid angle
as a function of the position in the Galaxy. It is often measured in magnitudes per square
arcsecond, or L / pc2 . Note that the surface brightness is independent of distance (WHY??).
• colour, which defines a blue to red sequence of galaxies.
• metallicity (i.e., fraction of mass in elements heavier than helium) measures the chemical
composition of galaxies.
• dynamical properties: velocity dispersions for ellipticals and circular velocity for spiral galaxies
• central massive black holes
There are a number of correlations between the physical parameters. For example, the Tully-Fisher
relation for spirals and Fabor-Jackson law (fundamental plane) for elliptical galaxies. These provide
important clues about the formation and evolution of galaxies.
1.1.1.
Catalogues of galaxies and online resources
Appendix B in Binney and Merrifield (1998) provides a table of various catalogues.
The two most important historical ones are the Messier Catalogue (obtained around 1780s,,
which contains 100 objects; galaxies in this catalog starts with a letter ‘M’, e.g., Andromeda: M32)
and the New General Catalogue (which was originally obtained by Dreyer in 1880s, subsequently
revised in 1973 by Sulentic and Tifft. This galaxy catalogue contains 7800 galaxies, and their names
start with the letters ‘NGC’).
There are more recent catalogues with many more galaxies. In the optical, we have
• Las Companas redshift survey (26400)
• the Two Degree Field (2dF) survey which has ≈ 2.5 × 105 galaxies. Fig. 2 shows the spatial
distribution of galaxies in the 2dF survey. Notice that the galaxy distribution is not uniform,
but clustered, with filaments, great walls and voids.
• the SLOAN Digital Sky Survey (SDSS) contains 106 galaxies and 105 quasars in one quarter
of the sky.
There are many others surveys in the radio (e.g. NVSS and FIRST), X-ray (ROSAT) and
infrared (IRAS). It is clear that we are experiencing an explosion in the observational data, which
makes the study of galaxies an exciting data-driven field.
–4–
Fig. 2.— Positions of galaxies in the 2dF survey. The galaxies are shown as a function of their
angular position and distance (measured by redshift).
1.2.
Morphological classification of galaxies
Galaxies have diverse properties. A good classification scheme should not only provide common
terms to describe galaxies but also offer clues of how galaxies form. Hubble provided the first
meaningful scheme, with later modifications by de Vaucouleurs, Sidney van den Bergh and others.
We shall concentrate on the Hubble classification scheme (later refined by Sandage).
1.2.1.
Hubble classification scheme
Fig. 3 shows the famous Hubble tuning fork and Fig. 4 illustrates the scheme with real
galaxies.
Ellipticals
Elliptical galaxies have elliptical isophotes; they are almost featureless. They are classified
according to their ellipticities as Ex, where x = 10(1 − b/a), with b and a being the lengths of the
minor and major axes. Most ellipticals have types between E0 and E6. Fig. 5 shows two examples
of elliptical galaxies.
Spiral Galaxies: normal and “barred”
In disc galaxies, most of the luminosity comes from a thin layer. They often show striking
spiral patterns. Hubble classified them according to three criteria:
• relative bulge/disk size. It decreases from Sa → Sd
–5–
Fig. 3.— The famous tuning fork of the Hubble-Sandage classification of galaxies.
Fig. 4.— Illustration of Hubble-Sandage classification of galaxies using images of galaxies from the
Digitized Sky Survey (DSS).
–6–
Fig. 5.— M87 (E0) in the Virgo cluster and M110 (E6). Notice a large number of globular clusters
around M87.
• the openness of spiral arms. The spiral arms become more open from Sa → Sd galaxies.
• resolution of spiral arms into stars and nebulae. From Sa → Sd, the spiral arms are more
broken and resolved into stars and nebulae.
Fig. 6 illustrates the systematic changes of these physical properties with four galaxies.
Barred spirals:
Different from the “normal” spirals, barred spirals contain a significant “bar”-like structure.
Perhaps more than 30% of galaxies are barred. They also often contain dust lanes in the bar and
their spiral arms often start from where bar ends. Fig. 7 shows several examples of barred spirals.
S0 and Barred S0’s
A transition class from ellipticals to spirals. They have smooth central surface brightness
distribution, followed by a large region of gradually declining brightness (see Fig. 1.2.1 for several
examples). There is no obvious disk (spiral) structure in S0 galaxies. They contain relatively little
gas compared with other types of spiral galaxies.
Irregular Galaxies
Irregular galaxies do not easily fit in the description of either disc or elliptical galaxies. Fig. 9
shows two examples. Hubble further divided these into two categories.
–7–
Fig. 6.— Four spiral galaxies and their classifications: M104 (Sa, top left), M81 (Sb, top right),
M51 (Sc, top left) and NGC7793 (Sd, top right).
–8–
Fig. 7.— M83 (SBa, top left), NGC2523 (SBb, top right) and NGC613 (SBc, bottom).
–9–
Fig. 8.— NGC 3115 (S0, top left) and NGC 4371 (SB0, top right), NGC 1201 (bottom left) and
NGC 2859 (bottom right).
– 10 –
• Irr-I - asymmetric objects with no spiral arms, but with many O and B stars.
• Irr-II - asymmetric objects where the galaxy is more smooth with frequent dust lanes.
Fig. 9.— Two irregular type galaxies, LMC (Irr1, left) and M82 (Irr2, right).
Dwarf Galaxies
Dwarf galaxies have smaller physical sizes and lower luminosities. They can be further divided
into dwarf ellipticals (dEs), dwarf spheroidals (dSphs) with very low surface brightness and dwarf
irregulars (dIrrs). One example of each is shown in Fig. 10. It is still unclear how the different
classes, in particular, the dwarf spheroidals, form.
Ringed and peanut-shaped galaxies
One particular contribution de vaucouleurs is the recognition of rings. He classified the rings
as nuclear, inner and outer rings. Fig. 11 shows one example. We have also warped and peanutshaped galaxies (see Figs. 12 and 13). Warped galaxies are quite common; our own Milky Way is
warped.
Multi-wavelength view of galaxies
Galaxies may look very different in different wavelengths. Fig. 14 shows the picture of Milky
Way from the radio to gamma-ray. Notice how different the Galaxy looks in different bands.
Another striking example is shown for the M81 group (Fig. 15). While in the optical, this group of
galaxies looks normal, in atomic hydrogen, there are striking filaments/bridges which connect the
– 11 –
Fig. 10.— Three dwarf galaxies, UGC6456 (dIrr, top left), sculptor (dE, top right), and Leo1
(dSph, bottom).
– 12 –
Fig. 11.— A galaxy (NGC 6782) with inner and outer rings, from the Hubble Space Telescope.
member galaxies.
peculiar galaxies
5%-10% or so galaxies are peculiar, including starbursting galaxies (see Fig. 16 and 17) and
powerful active galactic nuclei (AGNs, see Fig. 18).
Starbursting galaxies often show highly irregular shapes, with jets, tails and rings. They often
involve tidal interaction or violent merging of galaxies.
Galaxies with active galactic nuclei contain bright point-like sources at their centres. They
contain a tremendous diversities of objects, ranging from radio galaxies to quasars.
1.3.
Statistical properties of galaxies
In this section we discuss some statistical properties of galaxies. These properties reveal
important clues about how galaxies form and evolve.
– 13 –
Fig. 12.— Two peanut shaped galaxies (NGC 128) and ESO597-36 in the Hickson compact group
87. Notice that both are in groups of galaxies, suggesting a tidal origin for their peculiar peanut
shapes.
Fig. 13.— Two warped galaxies. Notice the striking warped dust lane in the left panel. The right
panel shows the warped atomic hydrogen distribution in NGC 4013.
– 14 –
Fig. 14.— Multi-wavelength view of the Milky Way, ranging from the radio to the gamma-ray.
Fig. 15.— M81 group is shown in the optical (left) and atomic hydrogen (right). Notice the striking
irregular bridges that connect the galaxies in atomic hydrogen.
– 15 –
Fig. 16.— Antennae galaxies (NGC4038/NGC4039) in collision.
1.3.1.
luminosity function of galaxies
The luminosity function of galaxies is the number density of galaxies as a function of luminosity
or absolute magnitude. The observed luminosity functions follow the so-called Schechter shape:
α L
L
,
(1)
d
φ(L)dL = φ∗ exp(−L/L∗ )
L∗
L∗
Clearly the Schechter luminosity function depends on φ∗ , L∗ and α.
• At the faint end (L L∗ ), φ(L) ∝ (L/L∗ )α is a power-law. The observed α is usually
negative, which indicates a large number of faint galaxies.
• at the bright end (L L∗ ), φ(L) ∝ exp(−L/L∗ ), the number of very luminous galaxies drops
off exponentially.
• luminosity function varies as a function of types of galaxies, colour and environment of galaxies
(see §1.3.2 on the morphology-density relation)
• If we observe the galaxies in the B-band (a filter centred around 4000Å), φ∗ = (1.6 ± 0.3) ×
h3 10−2 Mpc−3 , α = −1.07 ± 0.07 and LB,∗ = (1.2 ± 0.1)h−2 × 1010 L .
• In K-band (a filter centered around λ ≈ 22, 000Å), observations find that φ∗ = (1.6 ± 0.2) ×
h3 10−2 Mpc−3 , α = −0.9 ± 0.2 and MK,∗ = −23.1 ± 0.2 + 5 log h
– 16 –
Fig. 17.— The upper nine images show nearby examples of merging galaxies in an approximate
time order, in data from telescopes at Lowell, Kitt Peak, and the Eurpoean Southern Observatory
(upper left to lower right: NGC 3690, NGC 520, NGC 6240, Arp 220, NGC 2623, IC 883, NGC
4194, NGC 7252, and NGC 7727). The lower panel shows faint galaxy pairs from HST images in
the Hercules 2 survey field. The comparison between nearby and distant samples indicates that the
galaxy merger rate has declined dramatically with cosmic time, pointing to an important role for
merging in galaxy evolution.
– 17 –
Fig. 18.— The giant radio galaxy Cygnus-A at redshift z = 0.0565. Cygnus A is an excellent
example of the so-called Fanaroff-Riley (FR) type II radio sources, characterized by faint, very
narrow jets, distinct lobes, and clear hot spots at the outer edges of the lobes.
We have defined a quantity h by
h=
H0
≈ 0.7.
100 km s−1 Mpc−1
(2)
Total number density
It can be shown that
Z
ntot =
∞
φ(L)dL = φ∗ Γ(1 + α),
(3)
0
where the special gamma-function is defined as
Z ∞
Γ(1 + α) =
xα exp(−x) dx.
(4)
0
Notice that if α < −1, it diverges as L → 0 – it means the Schechter form must break down at very
low luminosity. For α > −1, the integral converges.
Total luminosity density
The total luminosity density can be obtained by
Z ∞
ρL,tot =
Lφ(L)dL = φ∗ L∗ Γ(2 + α) ≈ 1.9 × 108 hL / Mpc3 .
(5)
0
Notice that the total luminosity density converges as the observed luminosity functions all satisfy
α > −2,
– 18 –
Fig. 19.— Luminosity functions from 2dF survey for different types of galaxies. The survey was
done in the bJ filter. The luminosity function for all the survey galaxies is well fitted by a Schechter
function: α = −1.2, φ∗ = 1.7×10−2 h3 Mpc−3 , and M∗ −5 log h = −19.7. Notice that the luminosity
functions change from Type 1 (passive, early-type ellipticals) to Type 4 (active, late-type spirals).
– 19 –
1.3.2.
Morphology-density distribution
The stronger clustering of ellipticals are also reflected in the so-called morphology density
relation in clusters of galaxies. See Fig. 20.
Fig. 20.— Morphology-density relation, see Dressler (1980). The left panel shows the fraction of
galaxies in different types as a function of projected surface density of galaxies, while the right
panel shows as a function of distance from the centre.
• The fraction of spirals decrease as the radius decreases, while the fraction of E/S0 remain
more or less constant
• The fraction of ellipticals increases while that of spirals drops as the surface density of galaxies
increase.
What causes the morphology-radius or morphology-density relation? There are several possibilities:
• disk galaxies hard to form at the centers of clusters
• evolution effects: spirals transformed into S0’s due to ram stripping or collisions; ram stripping
seems to be supported by the deficiency of HI in disk galaxies located in clusters of galaxies
• spirals can be turned into ellipticals due to mergers
Notice that the morphology-density relation can not be a purely positional effect as galaxies move
around and may also have something to do with substructures existing in clusters.
– 20 –
There is also another related effect: the Butcher-Oemler effect. It was found that clusters of
galaxies at redshift z=0.1-0.5 have more blue galaxies than nearby clusters. This clearly demonstrates that galaxies (in clusters) are not static objects, they are evolving as a function of time.
We will return to the formation scenario of disk and elliptical galaxies in later chapters.
Fig. 21.— M101 (Sc), NGC613 (SBc), NGC4261 (E3) and M81 (Sb). 1. Subjective, professionals
disagree with each other 2. effect of viewing angle. 3. neural networks.
– 21 –
1.4.
Schematic formation scenarios
The origin of the beautiful Hubble sequence is still not completely understood. Spiral galaxies
are rotationally supported, they contain a mix of old stars (in the bulge) and young stars in the
disk. Ellipticals are largely supported by random motions, and their stars are old.
Elliptical and disk galaxies form from very different scenarios. A schematic diagram is shown
in Fig. 22 for a spiral galaxy formation and Fig. 17 shows how elliptical galaxies might form.
1.4.1.
formation of disk galaxies
• Initial seed of fluctuations are seeded by quantum fluctuations in the inflationary field in the
early universe
• Gravitational instability amplifies the initial seed in density fluctuation
• Eventually dark matter haloes collapse to form filaments, walls and voids etc.
• Tidal forces of nearby haloes induce rotation of any given halo
• Baryons cool due to radiative processes at the centers of haloes.
• Eventually rotation arrests the collapse, and density high enough to form stars and spiral
arms → a disk galaxy
1.4.2.
formation of elliptical galaxies
Previously ellipticals are thought to form by monolithic collapse. But now it is thought that at
least some elliptical galaxies are formed by hierarchical mergers of two disk galaxies of comparable
masses. Fig. 17 illustrates the formation of elliptical galaxies using a sequence of observed merging
galaxies in the latter model.
– 22 –
Fig. 22.— The formation of disk galaxies in a cosmological framework.