|
|
The Milky Way versus M31
By Tim Hunter
Introduction
M31, the Andromeda Galaxy, NGC224, is the nearest spiral galaxy
to our own galaxy, the Milky Way. It is often said to be the
most distant object one can see with the unaided eye, because it
is readily visible to the naked eye from a dark sky location.
The nature of the Milky Way and M31 were not realized until the
1920’s and 1930’s when work by Hubble and others showed that M31
is a vast stellar system similar to yet quite distinct from the
Milky Way. Both are large, luminous spiral galaxies which are
the dominate members of a small cluster of nearby galaxies known
collectively as The Local Group.
M31 besides being the closest
large galaxy to the Milky Way is a very important “laboratory”
for observation of galaxy dynamics and evolution. Baade’s work
on the concept of Population I and Population II stars was based
on his observations of M31 (Walterbos, 2000). Study of M31 has
yielded fundamental knowledge about star formation and
evolution, the distance scale, and evolution of the Universe
(Moore, 2002). Indeed, about 30 novae can be detected in M31 each
year, though the only supernova detected in M31 was in 1885.
For much of the last century M31 and the Milky Way have vied for
the title of the largest galaxy in the Local Group (Sky &
Telescope, 2000). This essay presents an overview of both
galaxies and contrasts their features, attempting to draw a
conclusion as to their relative sizes, luminosities, and masses.
In other words, which one is the first among equals - primus
inter pares? This is not an easy question to answer. The mass,
size, and luminosity of the Milky Way are particularly hard to
measure since we are imbedded in the Milky Way and can never
hope to see it with the same perspective with which we view M31.
Unfortunately, M31 is significantly inclined to our line of
sight, which complicates our ability to measure many of its
parameters.
The Milky Way is a large spiral galaxy which probably has a bar.
It often is classified as a type Sbc galaxy, but if it has a
bar, then it probably should be classified as a type SBbc
galaxy. The Solar System is 8 kpc from the Galactic center. The
structure of the Milky Way remains uncertain due to our
immersion within it. Radio studies of hydrogen atom radiation at
21 cm can delineate the gas clouds associated with the spiral
arms in the Milky Way. This work has also been supplemented by
CO observations which suggest there are four spiral arms in the
Milky Way (Moore, 2002).
M31 is classified as a type Sb spiral galaxy, and it lies 770
kpc from the Milky Way (Sparke, 2000). It is tilted 750 to the plane of the sky [figure
1A]. One of the first rectified pictures of the galaxy was
constructed at the Tuatenburg Observatory in Germany by Richter
and Weibrecht [figure 1B] (Sky &
Telescope, 1964). The spiral structure of M31 has been a matter
of some contention due to its inclination to our line of sight.
Arp in 1964 described M31 as a two armed spiral with the arms
trailing. Hodge regards this as the best fit for the galaxy’s
characteristics (Hodge, 1993; Arp, 1964). According to Arp (1964),
“…the pitch [of M31] steepens somewhat to the inside and becomes
slightly shallower to the outside.” Braun (1991) used the
neutral hydrogen content of M31 to trace “…two continuous,
trailing spiral arms…” This currently seems to be the best model
for M31’s structure.
Examination of wide field infrared images of M31 taken by the
1.3-meter Two Micron All Sky Survey (2Mass) showed M31's central
bulge is not simply a flattened sphere. It has a boxy
shape and theoretical modeling of this data showed M31 has a bar
25,000 light years long similar to the bar of the Milky Way [figure
1C] (Beaton, 2007).
The Local Group
M31 and the Milky Way are the dominant galaxies in a small
galaxy cluster known as the Local Group, a term first used by
Hubble in 1936 (Mateo, 2000). The Local Group contains
approximately 40 known members, although more, small faint dwarf
galaxies probably remain to be discovered in the group. Another
prominent member of the Local Group is M33, which is a beautiful
Sc spiral galaxy, though it is considerably smaller and less
luminous than the Milky Way and M31. Most of the galaxies in the
Local Group are irregular galaxies, dwarf irregular galaxies,
dwarf elliptical galaxies, and dwarf spheroidal galaxies (Sparke,
2000).
Most of the galaxies of the Local Group have low intrinsic
brightness and low surface brightness, making them difficult to
detect and study. A large number of low surface brightness
galaxies probably remain to be discovered throughout the
Universe and in the Local Group. The Milky Way hides some
members from discovery, and they will likely be discovered on
infrared and radio surveys (Mateo, 2000). Moreover, the boundary
of the Local Group is uncertain, and it is sometimes difficult
to tell for a particular galaxy whether it is bound to the Local
Group.
The best way to associate galaxies with the Local Group is to
see if they are physically bound to the M31-Milky Way system,
because the mutual gravitational attraction of these two
galaxies is strong enough to overcome the expansion of the
Universe (Sparke, 2000). In fact, M31 and the Milky Way are
approaching each other at >100 km/sec. They probably make up a
binary system and orbit around a common center of gravity (Mateo,
2000).
It is simple in theory to see if other galaxies are
physically bound to this binary system but difficult in practice
to determine this for small faint galaxies (Mateo, 2000). It is
likely that some galaxies now listed as being members of the
Local Group will later be found to be unassociated with it and
passing through on their way somewhere else, while other
galaxies not now considered to be part of the Local Group will
be found to be members of it.
The Milky Way has 11 known satellites [Table
IA], the most important of which are the Large and Small
Magellanic Clouds and the Sagittarius Dwarf Galaxy (Sparke,
2000). The satellite galaxies of the Milky Way lie nearly in the
same plane and may have formed out of a single gas cloud
captured by the Milky Way (Sparke, 2000).
The Large Magellanic
Cloud (LMC) has approximately 10% the luminosity of the Milky
Way, and it measures 14 kpc in longest dimension. It is the
prototype for the Sm class of Magellanic spirals. It is a
distorted spiral galaxy with a bar, while the Small Magellanic
Cloud (SMC) is ten times fainter and is an elongated cigar
shaped ellipsoid structure seen end on (Sparke, 2000). Both
Magellanic clouds are rich in gas and show active star
formation. A gaseous bridge connects the two galaxies, and a
large gas stream, The Magellanic Stream, trails from the SMC,
merges into the bridge between the Magellanic Clouds, and goes
into a “Leading Arm” running to the Milky Way. The Magellanic
Clouds orbit each other and orbit the Milky Way. They are on a
plunging eccentric orbit around the Milky Way and made a close
approach to the Milky Way 200-400 million years ago (Sparke,
2000).
The Milky Way is aggressively disrupting the Magellanic Clouds,
and at the same time is in the process of cannibalizing the
Sagittarius Dwarf Galaxy (Mateo, 2000). The SMC was probably
significantly disrupted by its close encounter with the LMC and
the Milky Way 200-400 million years ago (Spark, 2000; Mateo,
2000). The Magellanic Clouds are predicted to fall into the
Milky Way in a few billion years and will be totally disrupted
in the process (Mateo, 2000).
M31 also has at least 11 satellite galaxies [Table
IB]. These include two relatively prominent close galaxies,
M32 (NGC221) and M110 (NGC205) [figure
1A]. M32 is a low luminosity elliptical dwarf galaxy, and
M110 is a small elliptical galaxy. NGC147 and NGC185 are other
dwarf elliptical galaxies which are satellites of M31. Other
dwarf irregular or dwarf spheroidal galaxies that are satellites
of M31 include IC10, LGS3, AndI, AndII, AndIII, AndV, and AndVI
(Walterbos, 2000).
M110 is interacting with M31, which is
distorting M110 and pulling at its outer stars. M32 has a very
high central brightness, and it could be “a miniature version of
a normal or ‘giant’ elliptical galaxy” (Sparke, 2000). It may
have a large black hole at its center and be the remnant of a
much larger galaxy, perhaps a galaxy that underwent a past
disruptive interaction with M31. M32’s distance from M31 is
unknown, and its motion is not known well enough to determine if
it has undergone a recent interaction with M31 (Sparke, 2000).
There is a giant stream of metal rich stars within the halo of
M31. This stream could have M32 and M110 as its source. Both
galaxies have lost a large number of stars due to tidal
interactions with M31 (Ibata, 2001).
The halo of the Milky Way is deficient in metals compared to M31
(Reddy, 2003). Brown and colleagues (2003) found that 30%
of M31’s halo stars consist of a population of intermediate age
(6-8 billion years) of metal-rich stars in addition to a large
metal poor population of older (11-13 billion years) globular
cluster stars. This dual population composition of M31’s halo
“support the idea that galaxy mergers played an important role
in the formation of the M31 halo” (Brown, 2003).
On the other hand, most of the stars in the halo of the Milky
Way consist of mainly older metal poor stars. In the case of
M31, mergers of small galaxies with M31 may have blown newly
formed stars out into M31’s halo, or a larger galaxy merged with
M31, and some of its younger stars made their way into the halo
of M31, or the collision with a larger galaxy triggered new star
formation in M31’s halo (Reddy 2003). In any event, it seems
that M31 has undergone major galactic interactions more recently
than the Milky Way, though the Milky Way is currently ingesting
the small Sagittarius Dwarf Galaxy.
The metal poor halo of the
Milky Way places constraints on the number of galaxy
interactions the Milky Way has undergone in the past. It could
have interacted with up to 60 Carina like dwarf spheroidal
galaxies (dSphs) in the past, but it could not have interacted
with more than 6 Fornax like dwarf galaxies in the past 10
billion years (Unavane, 1996). According to Wyse (2001), “our
current understanding…implies a rather quiet evolution for the
disk [of the Milky Way], back to redshifts of order 2.”
Ways to Compare Galaxies
There several ways to compare one galaxy with another. For the
purposes of this essay, the following parameters will be used
to compare the Milky Way with M31: a) the number of globular
clusters associated with each galaxy, b) the mass and
distribution of gas within each galaxy, c) the number of stars
in each galaxy, d) the dynamics of their galactic centers, e)
the estimated size and luminosity of each galaxy, and f) the
estimated mass of each galaxy, including their luminous and dark
matter. These are common criteria for describing a galaxy, and
they are somewhat related to each other. It seems intuitive that
a more massive spiral galaxy ought to have a greater physical
size, be more luminous, contain more gas and stars, and have a
larger black hole in its center than a similar spiral galaxy
with less globular clusters, less gas and stars, and a smaller
black hole at its center. Whether these sweeping generalizations
are true or not is uncertain, but they are a good starting point
for comparing the Milky Way to M31.
|
