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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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