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    Categories: Deep-Sky Objects

Deep-Sky Objects: Small Magellanic Cloud (SMC)

Image Credit: Large and Small Magellanic Cloud by Manfred Rudolf

The Small Magellanic Cloud (SMC) is a small dwarf irregular galaxy that has long been considered a true satellite galaxy of the Milky Way. However, while the SMC’s status as a dwarf galaxy is not disputed, as is the case with the Large Magellanic Cloud (LMC), its status as a satellite of the Milky Way is in serious doubt. Since the SCM is moving through space at a speed of 217 km/sec, relative to the galactocentric rest frame, most investigators are of the opinion that the SMC is moving too fast for the Milky Way to hold on to it, gravitationally speaking.

Note: The calculations that determined the SMC’s total space velocity took into account that the SMC and LMC are gravitationally bound and orbiting each other at a separation of 75,000 light years, and that their common center of mass is orbiting the Milky Way.

Quick Facts:

• Constellation: Tucana/Hydrus
• Coordinates: RA 00h 52m 44.8s|Dec. -72° 49′ 43″
• Distance: 200,000 light years
• Mass: 7 billion solar masses
• Object type: SB(s)m pec (Dwarf irregular galaxy)
• Apparent diameter: 5° 20′ × 3° 5′
• Effective diameter: 7,000 light years
• Apparent magnitude: +2.7
• Other designations: NGC 292, PGC 3085, Nubecula Minor

Visibility

Like the LMC, the SMC is best viewed from the Southern Celestial Hemisphere, but due to its low surface brightness, it is best to observe the SMC from a dark site that is free of light pollution. Observers that are located south of latitude 15°N will see the SMC as a hazy patch of light stretching across the constellations Tucana and Hydrus for a distance of just more than 4 degrees, which is equivalent to about 8 times that of the full Moon. Provided the sky is suitably dark, the LMC may also be observed about 20 degrees eastward of the SMC.

Notable Features

In terms of its appearance, many observers report that the SCM looks like a part of the Milky Way that had become detached from the galaxy. Furthermore, it has been hypothesized by a few prominent astrophysicists (astrophysicists D. S. Mathewson, V. L. Ford, and N. Visvanathan) that a close encounter with the LMC had actually broken the SMC into two discrete pieces and that the two parts are continuing to drift apart. According to this hypothesis, the smaller piece that had broken off the SMC is kind of “trapped” between the larger piece and the Milky Way, as seen from our perspective on Earth. Although the researchers have dubbed the smaller piece of the SMC “The Mini Magellanic Cloud”, there is as yet no definitive proof that the SMC had been split into two parts.

Physical Properties

Structure

In terms of its structure, the SMC contains a central bar that is thought by most investigators to have become as disorganized as it is due to repeated tidal interactions with both the Milky Way and the more massive LMC. It is perhaps worth noting that some doubts remain regarding the SMC’s irregular appearance:

a) Some investigators hold that the total space velocities of both the SMC and the LMC is suggestive of the fact that the small galaxies are being disrupted by the Milky Way as they pass it by, and that they are therefore not true satellites of the Milky Way.

b) Other investigators hold that the two small galaxies are only as close to the Milky Way as they are due to the relative positions they occupy during their current orbit of each other, while still other investigators believe that the two small galaxies are not gravitationally bound at all, and are therefore not orbiting each other. The latter case seems unlikely, however, given that both small galaxies are enveloped by a common cloud of hydrogen gas, and that an extensive bridge of dust and gas connects the pair of dwarf galaxies.

X-ray Emission

Like the LMC, no X-ray emission beyond ambient levels from the SMC was detected during initial X-ray surveys conducted during 1966. However, subsequent surveys conducted a few days later found X-ray emissions in other ranges. In fact, the SMC contains large numbers of bright X-ray binary stars, and recent bursts of star formation has created a large population of both “normal” massive stars, and high-mass X-ray binaries that are concentrated in the small galaxy’s central bar.

Star Formation

Like the LMC, the SMC is rich in dust and gas, although in the case of the SMC, the metallicity of the gas and dust is only about 10% that of the Milky Way. Nevertheless, recent studies on the star formation history of the SMC suggest that its first stars formed as long ago as 12 billion years, albeit at a relatively slow pace, as is indicated by lack of a definitive horizontal branch if the SMC history is plotted on an H-R diagram. Interestingly, recent studies have also identified a radial variation in the metal content of the SMC, which means that the central regions of the SMC are richer in metals than the outlying regions are.

Moreover, detailed analysis also suggests that although the rate of star formation was low during the SMC’s distant past, it nevertheless continued unabated until about 2-3 billion years ago, when it appeared to slow down. However, about 500 million years ago, the rate of star formation increased markedly, with a further increase that started about 100 million years ago.

Overall, these studies show that about 50% of all the stars that ever formed in the SMC formed longer than about 8.4 billion years ago. Between then and about 3 billion years ago, the star formation rate remained fairly constant, but several “spikes” in the star-forming rate at 2.5, 0.4, and 0.06 billion years ago respectively, appear to coincide with the times when the SMC was at perigee (nearest) with the Milky Way. This finding appears to provide evidence that tidal interactions with the Milky Way is the primary driving force behind bursts if star formation in the SMC, as opposed to any other possible cause.

 

Peter Christoforou :