Download Galaxy Classification Much of modern extragalactic astronomy deals

Galaxy Classification
Much of modern extragalactic astronomy deals with studying
how galaxies evolve with time. If galactic evolution can be understood, then parameters such as q0 , Ω0 , and Λ will follow, along
with data on the initial conditions for the formation of the universe. Before we can study evolution, however, we have to define
what a galaxy is, and how it is structured. Hence, the first step
to understanding galaxies is to classify them.
The most famous classification scheme, of course, is the Hubble
tuning-fork system, which starts out with spherical ellipticals, evolves to the transitional S0 (lenticular) galaxies, and then
branches out into Sa, Sb, and Sc spirals and barred spirals. Over
the years, this system has been built up, so that lenticulars are
now subdivided into S01 , S02 , and S03 (depending on the smoothness of the luminosity profile, and the amount of gas present in
the galaxy). In addition, there are now the additional symbols
“(r),” which indicates the presence of an inner ring, “R,” which
signifies the presence of an outer ring, and “(s),” which says that
the spiral arms begin at the end of a bar or are traced to the
galaxy’s center, rather than the galaxy’s inner ring. Needless to
say, this scheme isn’t very elegant.
A complementary classification system, which seeks to make a
parallel with stellar evolution, is the DDO (David-Dunlap Observatory) Luminosity Classification system of van den Bergh.
In this method, supergiant galaxies with well-developed bright
spiral arms and bars have the Roman numeral I (like supergiant
stars), and small, low-surface-brightness, irregular galaxies have
the roman numeral V. Of course, since one doesn’t usually know
a galaxy’s distance, it is somewhat difficult to estimate its true
luminosity. The system therefore assumes that the galaxies with
the most well-developed arms are also the most luminous. In the
Revised Shapley-Ames Galaxy Catalog (which lists the ∼ 1300)
brightest galaxies in the sky, Sandage adds the DDO luminosity classification onto his Hubble classification, so, for example,
the galaxy NGC 1097 is given the type RSBbc(rs)I-II. (In other
words, NGC 1097 is a very large barred Sbc spiral with an outer
ring, an inner ring, and arms that begin at the end of the galaxy’s
bar.)
A more computer-friendly system was devised for the 2nd Reference Catalog of Bright Galaxies by de Vaucouleurs. In his
system, galaxies are given a numerical T designation based on
compactness. The most compact elliptical galaxies are assigned
T = −6; normal ellipticals have T = −5, and lenticular galaxies have negative numbers near zero. Spiral galaxies start at
T = +1 for Sa, and proceed to T = +11 for those blue, irregular galaxies that are essentially extragalactic H II regions. In
de Vaucouleurs’ scheme, there is no difference between a normal
spiral and a barred-spiral galaxy.
In the 1970’s van den Bergh noticed that spiral galaxies in clusters seemed different from those in the field. In particular, many
cluster spirals seemed to have less gas and less star-formation
than their counterparts in low-density environments. (The S0
galaxies, which are spiral disks without arms or gas, are the extreme example of this phenomena.) Van den Bergh therefore
defined a system where Anemic Spirals occupied the transition
between regular spirals and lenticulars. In this system, sequences
would be Sa → Aa → S0a, Sb → Ab → S0b, etc. For these Anemic galaxies, it is as if something is quenching their active star
formation.
An interesting system that is still (partially) with us and has
(some) interesting uses is the classification system of Morgan.
The system has two components, a “concentration” component,
and a “form” component. The concentration part of the system
is the observed correlation between the types of stars present
in a galaxy, and how compact the galaxy is. Elliptical galaxies
have mostly old stars, and their integrated light is dominated by
K supergiants. These objects are also the most condensed systems; i.e., they are highly concentrated. Irregular galaxies with
no central mass condensation tend to have younger stars and a
corresponding earlier spectral type. Thus, Morgan defined an
“a-f-g-k” concentration index based on the spectral classification
of stars. For the form index, Morgan chose the (capital) letters
“S” for spiral, “B” for barred spiral, “E” for elliptical, “I” for
irregular, “Ep ” for elliptical peculiar (with dust), “D” for a rotationally symmetry without elliptical structure (i.e., a diffuse
system), “L” for low-surface brightness, and “N” for any system
with a small, brilliant nucleus (like a Seyfert galaxy). On top of
this, Morgan then added a number from 1 to 7 based on apparent inclination: face-on spirals were S1, while highly elongated
systems could be S7. Thus, a Morgan class might be kS4, fS1,
fgB1, or gkS7.
The Morgan system lives on today principally in the designation of “N” galaxies, which are sometimes used to refer to small
galaxies with an active galactic nucleus, and through the identification of some galaxies as “cD”. The cD classification (which was
actually defined about 5 years after the original Morgan paper)
refers to galaxies in the centers of clusters which have an elliptical galaxy-like core surrounded by a huge amorphous envelope of
stars. These systems are probably the largest collections of stars
in the universe; since some cD galaxies have multiple nuclei, they
have sometimes been described as “galaxies at lunch.”
Properties of Elliptical Galaxies
The first step in investigating the evolution of galaxies is to understand the properties of those galaxies today. We’ll start with
the elliptical galaxies. We can summarize their properties as follows:
1) In general, elliptical galaxies, as projected on the sky, have complete 2-dimensional symmetry. The question of whether these
objects are symmetric in practice, or are tri-axial is open (though
dynamical modeling suggests that triaxiality can only last for a
short time). Some ellipticals have “fine structure,” such as very
weak ripples, shells, and boxy (not elliptical) isophotes. These
signatures are weak, but real. In general, those ellipticals with
fine structure are slightly bluer than equivalent galaxies without
fine structure.
2) The apparent flattening of elliptical galaxies is given in the Hubble classification scheme as En, where n is defined in terms of
the apparent semi-major axis, a, and semi-minor axis, b, as
n = 10(a − b)/a
(23.01)
Ellipticals range in flattening from E0 (round) to E7. No elliptical is flatter than E7. The data are not consistent with the
hypothesis that all ellipticals are E7 and appear flattened by the
effects of random viewing angles. Most likely there is a spread of
flattenings centered around E3, or thereabouts.
3) Rotation is not important in most elliptical galaxies. That is, E7
galaxies are not flattened due to rapid rotation.
4) There is little or no star formation in elliptical galaxies. However,
the spectral energy distribution of some ellipticals turns up in the
ultraviolet. (In other words, since elliptical galaxies are made up
of old stars, the composite spectrum of an elliptical should look
like that of a ∼ 4, 000◦ K star. However, many ellipticals are
brighter at 1500 Å than they are at 2000 Å.)
5) There is very little cold interstellar medium in elliptical galaxies.
However, there is x-ray gas at a temperature of about T ∼ 106 K.
One can easily see where this gas comes from. The stars in an
elliptical galaxy must be losing mass. If the stars are moving
isotropically at σ ∼ 200 km s−1 , then the atoms of lost material,
if thermalized, will have a temperature of
1
3
mH σ 2 ∼ kT =⇒ T ∼ 106 degrees K
2
2
(23.02)
6) Ellipticals are almost always found in dense environments. The
few field ellipticals that exist may have swallowed their neighbors.
7) Most ellipticals have weak radial color gradients: they are redder
on the inside than they are on the outside. This may be due
to age (older stars have a redder turn-off mass), or metallicity
(metal-rich stars are intrinsically redder than their metal-poor
counterparts).
8) Elliptical galaxies populate a “fundamental plane” in luminositysurface brightness-velocity dispersion space. But there are many
other reflections of this plane. For example,
• Elliptical galaxy luminosity correlates with color. Large ellipticals are redder than small ellipticals.
• Elliptical galaxy color correlates with absorption line strength.
Redder galaxies have stronger absorption features.
• Elliptical galaxy absorption features correlate with the UV upturn. Galaxies with strong absorption features have larger UV
excesses.
• Elliptical galaxy UV excess correlates with the number of planetary nebulae in the galaxy. Galaxies with large UV excesses have,
relatively speaking, fewer planetary nebulae.
9) There are several laws that re-produce the observed luminosity
profile of an elliptical galaxy. The most famous is the de Vaucouleur r1/4 -law. Under the law, the surface brightness of an
elliptical galaxy, I (units of ergs cm−2 s−1 arcsec−2 ), as a function of the distance from the galaxy center, r, is given by
µ
log
I
Ie
(µ
¶
= −3.33071
r
re
¶1/4
)
−1
(23.03)
The key scaling variables in this equation are re and Ie . The
scaling length re is called the effective radius: it is the radius
that encloses half the total light from the galaxy. The variable
Ie is the surface brightness of the galaxy at radius re . Note that
with a little math, (23.03) can be transformed to the form
m = a + b r1/4
(23.04)
where m is the surface brightness of the galaxy (in magnitudes
per square arcsec), and a is the central surface brightness.