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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.

 

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