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Comparison of the Milky Way and M31
Globular Clusters
One measure of a galaxy’s relative size and prominence is the
number, size, and luminosity of its globular clusters. The
number of the globular clusters in the Milky Way is particularly
hard to estimate, since much of the Milky Way is hidden from our
view by dust and gas. In 1978 Sharov estimated the total number
of globular clusters in the Milky Way and in M31 as
approximately 500 for each galaxy. This number seems too large
for the Milky Way in view of later work. In 1991 Racine used the
Canada-France-Hawaii Telescope (CHFT) to obtain high resolution
CCD images of cluster candidates in the halo of M31. The total
population of globular clusters in M31 was estimated to exceed
that of Milky Way by a factor of ~ 1.8 (Racine, 1991). Barmby and
colleagues studied the globular clusters in M31 and estimated the
total number of globular clusters in M31 as 450 +/- 100, while
they and others estimated the total number of globular clusters
in the Milky Way as 180 +/- 20 (Barmby, 2000).
How many Milky Way globular clusters are hidden from our view is
debatable. Certainly, there are probably hidden globular
clusters in the disk of the Milky Way, particularly any small
globular clusters that are on the other side of the Galactic
disk. The disk plane and bulge of the Milky Way also must hide
some of the halo globular clusters in the Milky Way. However,
only a small portion of the halo is hidden in this manner, and
it is unlikely that there are very many Milky Way globular
clusters to be discovered. Not only does M31 have more globular
clusters than the Milky Way, but in some cases they may be more
luminous, and the more distant ones are farther out from the
galactic nucleus than those of the Milky Way (Hodge, 1993).
According to Bollinger, Hummels, and Rivera (2003), “…brighter
and more massive galaxies tend to have more globular clusters.”
This is not necessarily a well established relationship, but
probably is true in many cases. It is based on the assumption
that globular clusters formed with a similar distribution of
masses and luminosities with which their parent galaxies formed.
Because elliptical galaxies have more globular clusters per
luminosity than spiral galaxies, globular clusters provide clues
to the formation of galaxies (Bollinger, 2003).
Based on globular cluster numbers, M31 should be more
luminous and more massive than the Milky Way. Barmby and
colleagues also show that while there are some differences
between the Galactic globular clusters and those of M31, these
differences are relatively minor, and the globular clusters of
the two galaxies are similar except for the larger number and
wider distribution of them in M31. The relative percentages of
the globular clusters’ distributions in the halo and the disk of
each galaxy is nearly the same (Barmby, 2000). It therefore seems
unlikely the globular clusters of the Milky Way and M31 are
so different that they can not be used as a standard for
comparison of the galaxies with each other.
Gas and Dust
M31 has 4-6 x 109 MSun of neutral hydrogen. This is somewhat
more neutral hydrogen than is found in the Milky Way, but there
is less molecular gas in M31 than in the Milky Way, and the gas
mass/stellar luminosity of M31 is less than that of the Milky
Way (Sparke, 2000). The bulge of M31 contains some ionized gas
and scattered clouds of HI gas and dust, which show up as dark
nebulae. A star forming “ring of fire” circles the nucleus of
M31 at a distance of 9-10 kpc (Sparke, 2000; Hodge, 1993) [figures
2A and 2B]. This consists of molecular gas, neutral gas,
young disk stars, HII regions, and dust. Outside this ring dust
lanes and HII regions help trace out spiral arms.
The neutral
gas in M31 is concentrated in the ring of fire and extends
further out similar to the extension of gas in the Milky Way
beyond the visible disk. The outer parts of the gas disks in M31
and in the Milky Way are not flat but are warped into an S
shape. The stellar disks in M31 and probably in the Milky Way
are similarly distorted (Sparke, 2000).
CO mapping of the Milky Way seems to agree with far-infrared and
21 cm hydrogen surveys and can be used to estimate and map
molecular gas (Dame, 1999). The Milky Way molecular clouds in CO
have been almost completely mapped, and the entire disk of M31
has been partially surveyed in CO (Dame, 1999).
The total CO luminosity of M31 is only one-fourth that of the
Milky Way. According to work by Dame and colleagues (1993), “the
bulk of the CO emission in M31 is confined to a broad ring with
a radius of 10 kpc.” The mass of molecular gas in M31 is about
one-tenth that of its neutral hydrogen content (Dame, 1993).
Beyond 9 kpc, molecular gas content and the giant molecular
clouds (GMC) of M31 and the Milky Way are similar in size and
mass (Dame, 1997). “The molecular spiral arm S4 [of M31], which
lies near the peak of the Population I ring in M31, is
remarkably similar to the Carina Arm of the Milky Way, which
also lies at a galactocentric radius of 9 kpc. Both arms contain
GMC’s several hundred pc in size, spaced roughly every kpc along
the arm, and each containing several million solar masses of
molecular gas” (Dame, 1997). In the central 8 kpc, the
distribution of molecular gas is considerably greater in the
Milky Way than in M31. Beyond radii of 8 kpc, according to Heyer
and colleagues (2000) “…the properties of the molecular gas and
the processes which regulate its distribution are similar.”
The star formation rate of the Milky Way is greater than that of
M31. The mean star formation rate for the disk of M31 is
estimated to be approximately one solar mass per year (Williams,
2003). The star formation rate for the Milky Way is difficult to
judge from our perspective in the disk, but it is on the order
of 3 to 5 solar masses per year. The Milky Way produces two to
three times as many novae as M31, averaging 73 +/- 24 novae per
year (Liller, 1987).
Most of the dust in M31 is cold with a temperature of ~ 16K
(Hass, 1998). This is colder than the 19 K temperature of cold
dust in the Milky Way. The emission spectrum of M31 in the
mid-infrared is different than that of the Milky Way. There are
two dust populations, small and large grains, in both M31 and in
the Milky Way (Haas, 1998). There is much gas in the outer
regions of M31 as far out as 25 kpc from the center of M31 (Lequeux,
2000). The reddening of stars in M31 shows the dust/gas ratio in
M31 is about one-third of the Milky Way value (Lequeux, 2000).
The cold dust distribution in M31 is dominated by a ring at 10
kpc with a fainter outer one at 14 kpc with no clear spiral
pattern (Haas, 1998). The ring of dust is a large reservoir for
star formation, and M31 might be transitioning from an Sb type
spiral galaxy to that of a ringed galaxy (Haas, 1998).
Nuclei
At the very center of the Milky Way, there is a complex of hot
dense molecular clouds, Sagittarius A (Sgr A), that partially
surrounds an inner most star cluster (Sparke, 2000). Sgr A is the
strongest radio source in the Galactic plane near the Galactic
center. In 1974 Balick and Brown reported a strong radio
emission from a point source in Sgr A. This point source is now
known as Sagittarius A* (Sgr A*) (Balick and Brown, 1974).
Sgr A* has been intensively studied at radio, optical, infrared,
and X-ray wavelengths, and the location of Sgr A* has been very
precisely determined with infrared observations and Very Long
Baseline Array (VLBA) radio observations. There are several
stars in highly elliptical orbits around Sgr A*. Precise
measurements of one of the stars, S2, show Sgr A* has a mass of
2.6 to 3.0 million times that of the Sun (Genzel, 2003; Baganoff,
2003). There is an X-ray source coincident with Sgr A*. Because
this X-ray source has been observed to flare up in X-ray
brightness by a factor of up to ten over a few hours, it can not
be more than a few light hours in size (Baganoff, 2003). Sgr A*
is best explained by it being a black hole with an accretion
disk.
The nuclear region of M31 is somewhat different than that of the
Milky Way. In 1991 the Planetary Camera on the Hubble Space
Telescope showed the nucleus of M31 has a double structure [figure
3]. There are two bright spots in the nucleus of M31 with
the dimmer spot being at the exact center of the galaxy. The
brighter of the two spots corresponds to what had previously
been thought to be the center of M31 (Hodge, 1993). This brighter
spot corresponds to a bright dense grouping of several million
stars. It is offset by several light years from the exact center
of M31. It may be a remnant of a smaller galaxy that was
swallowed by M31 a billion years ago (Hodge, 1993).
In 1995 Tremaine proposed the nucleus of M31 is a thick
eccentric disk composed of gas, dust, and stars traveling on
orbits around a massive black hole. The black hole is centered
at the dimmer of the two nuclear bright spots (P2), and the
brighter bright spot (P1) is a cluster enveloped in the
associated accretion disk which travels on a Keplerian orbit
around the black hole at P2 (Tremaine, 1995). Tsvetanov and
colleagues showed in 1998 that the center of rotation lies
between the two brightness peaks P1 and P2. Their work “support
the hypothesis…that the nucleus of M31 is a thick stellar
eccentric disk orbiting around a massive black hole located at
P2” (Tsvetanov, 1998). The supermassive black hole (SMBH) in M31,
M31*, is approximately 10 times more massive than SgrA* and is
thousands of times more luminous in X-rays (Liu, 2001).
The considerable difference between the masses of the black
holes in the centers of the Milky Way and M31 raises the
question of the relationship, if any, between the estimated mass
of a galaxy and the mass of the black hole in the center of the
galaxy. In other words, do massive galaxies have massive central
black holes and do smaller galaxies have less massive central
black holes? The answer is that no one knows for sure.
There
does seem to be a loose relationship between the size of the
SMBH and the mass of the host galaxy. Work by Warner and
colleagues found there is a trend between SMBH mass and
metallicity in Active Galactic Nuclei (AGN), and there may be a
correlation between host galaxy mass and SMBH mass as well as
quasar metallicities (Warner, 2003). The black hole mass appears
to be related to galactic bulge mass and other properties
(Haring, 2003). The mass of the SMBH may also be related to the
age of the galactic stellar population (Merrifield, 2000). Older
galaxies may have black holes that are relatively larger for
their host galaxies, whereas younger galaxies may have smaller
black holes. Too few galaxies have been studied thus far to draw
firm conclusions.
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