Large Magellanic Cloud
|Large Magellanic Cloud|
The Large Magellanic Cloud
|Observation data (J2000 epoch)|
|Right ascension||05h 23m 34.5s|
|Declination||−69° 45′ 22″|
|Distance||162.98 kly (49.97 kpc)|
|Apparent dimensions (V)||10.75° × 9.17°|
|Apparent magnitude (V)||0.9|
|LMC, ESO 56- G 115, PGC 17223, Nubecula Major|
|See also: Galaxy, List of galaxies|
The Large Magellanic Cloud (LMC) is a nearby irregular galaxy (although it shows signs of a bar structure, and is often reclassified as a Magellanic type dwarf spiral galaxy), and a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs (≈163,000 light-years), the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal (~ 16 kiloparsecs) and the putative Canis Major Dwarf Galaxy (~ 12.9 kiloparsecs, though its status as a galaxy is under dispute) lying closer to the center of the Milky Way. It has a mass equivalent to approximately 10 billion times the mass of our Sun (1010 solar masses), making it roughly 1/100 as massive as the Milky Way, and a diameter of about 14,000 light-years (~ 4.3 kpc). The LMC is the fourth largest galaxy in the Local Group, after the Andromeda Galaxy (M31), our own Milky Way Galaxy, and the Triangulum Galaxy (M33).
While the LMC is often considered an irregular type galaxy (the NASA Extragalactic Database lists the Hubble sequence type as Irr/SB(s)m), the LMC contains a very prominent bar in its center, suggesting that it may have previously been a barred spiral galaxy. The LMC's irregular appearance is possibly the result of tidal interactions with both the Milky Way and the Small Magellanic Cloud (SMC).
It is visible as a faint "cloud" in the night sky of the southern hemisphere straddling the border between the constellations of Dorado and Mensa, and it appears from Earth more than 20 times the width of the full moon.
The very first recorded mention of the Large Magellanic Cloud was by the Persian astronomer `Abd al-Rahman al-Sufi (later known in Europe as "Azophi"), in his Book of Fixed Stars around 964 AD.
The next recorded observation was in 1503–4 by Amerigo Vespucci in a letter about his third voyage. In this letter he mentions "three Canopes, two bright and one obscure"; "bright" refers to the two Magellanic Clouds, and "obscure" refers to the Coalsack.
The LMC was long considered to be a planar galaxy that could be assumed to lie at a single distance from us. However, in 1986, Caldwell and Coulson found that field Cepheid variables in the northeast portion of the LMC lie closer to the Milky Way than Cepheids in the southwest portion. More recently, this inclined geometry for field stars in the LMC has been confirmed via observations of Cepheids, core helium-burning red clump stars and the tip of the red giant branch. All three of these papers find an inclination of ~35°, where a face-on galaxy has an inclination of 0°. Further work on the structure of the LMC using the kinematics of carbon stars showed that the LMC's disk is both thick and flared. Regarding the distribution of star clusters in the LMC, Schommer et al. measured velocities for ~80 clusters and found that the LMC's cluster system has kinematics consistent with the clusters moving in a disk-like distribution. These results were confirmed by Grocholski et al., who calculated distances to a number of clusters and showed that the LMC's cluster system is in fact distributed in the same plane as the field stars.
Determining a precise distance to the LMC, as with any other galaxy, was challenging due to the use of standard candles for calculating distances, with the primary problem being that many of the standard candles are not as 'standard' as one would like; in many cases, the age and/or metallicity of the standard candle plays a role in determining the intrinsic luminosity of the object. The distance to the LMC has been calculated using a variety of standard candles, with Cepheid variables being one of the most popular. Cepheids have been shown to have a relationship between their absolute luminosity and the period over which their brightness varies. However, Cepheids appear to suffer from a metallicity effect, where Cepheids of different metallicities have different period–luminosity relations. Unfortunately, the Cepheids in the Milky Way typically used to calibrate the period–luminosity relation are more metal rich than those found in the LMC.
In the era of 8-meter-class telescopes, eclipsing binaries have been found throughout the Local Group. Parameters of these systems can be measured without mass or compositional assumptions. The light echoes of supernova 1987A are also geometric measurements, without any stellar models or assumptions.
Recently, the Cepheid absolute luminosity has been re-calibrated using Cepheid variables in the galaxy NGC 4258 that cover a range of metallicities. Using this improved calibration, they find an absolute distance modulus of 18.41, or 48 kpc (~157,000 light years). This distance, which is slightly shorter than the typically assumed distance of 50 kpc, has been confirmed by other authors.
By cross-correlating different measurement methods, one can bound the distance; the residual errors are now less than the estimated size parameters of the LMC. Further work involves measuring the position of a target star or star system within the galaxy (i.e. toward or away from the observer).
The results of a study using late-type eclipsing binaries to determine the distance more accurately was published in Nature in March 2013. A distance of 49.97 kpc (162,983 light-years) with an accuracy of 2.2 per cent was obtained.
Like many irregular galaxies, the LMC is rich in gas and dust, and it is currently undergoing vigorous star formation activity. It is home to the Tarantula Nebula, the most active star-forming region in the Local Group.
The LMC is full of a wide range of galactic objects and phenomena that make it aptly known as an "astronomical treasure-house, a great celestial laboratory for the study of the growth and evolution of the stars," as described by Robert Burnham, Jr. Surveys of the galaxy have found roughly 60 globular clusters, 400 planetary nebulae, and 700 open clusters, along with hundreds of thousands of giant and supergiant stars. Supernova 1987a—the nearest supernova in recent years—was also located in the Large Magellanic Cloud. The Lionel-Murphy SNR is nitrogen-abundant supernova remnant (SNR) N86 in the Large Magellanic Cloud named by astronomers at the Australian National University's Mount Stromlo Observatory in acknowledgement of Australian High Court Justice Lionel Murphy's interest in science and because of SNR N86's perceived resemblance to his large nose.
There is a bridge of gas connecting the Small Magellanic Cloud (SMC) with the LMC, which is evidence of tidal interaction between the galaxies. The Magellanic Clouds have a common envelope of neutral hydrogen indicating they have been gravitationally bound for a long time. This bridge of gas is a star-forming site.
No X-rays above background were observed from the Magellanic Clouds during the September 20, 1966, Nike-Tomahawk flight. A second Nike-Tomahawk rocket was launched from Johnston Atoll on September 22, 1966, at 17:13 UTC and reached an apogee of 160 km (99 mi), with spin-stabilization at 5.6 rps. The LMC was not detected in the X-ray range 8-80 keV.
Another Nike-Tomahawk was launched from Johnston Atoll at 11:32 UTC on October 29, 1968, to scan the LMC for X-rays. The first discrete X-ray source in Dorado was at RA 05h 20m Dec −69°, and it was the Large Magellanic Cloud. This X-ray source extended over about 12° and is consistent with the Cloud. Its emission rate between 1.5-10.5 keV for a distance of 50 kpc is 4 x 1038 ergs/s. An X-ray astronomy instrument was carried aboard a Thor missile launched from Johnston Atoll on September 24, 1970, at 12:54 UTC and altitudes above 300 km (186 mi), to search for the Small Magellanic Cloud and to extend previous observations of the LMC. The source in the LMC appeared extended and contained the star ε Dor. The X-ray luminosity (Lx) over the range 1.5–12 keV was 6 × 1031 W (6 × 1038 erg/s).
The Large Magellanic Cloud (LMC) is in the constellations Mensa and Dorado. LMC X-1 (the first X-ray source in the LMC) is at RA 05h 40m 05s Dec −69° 45′ 51″, and is a high mass X-ray binary source (HMXB). Of the first five luminous LMC X-ray binaries: LMC X-1, X-2, X-3, X-4, and A 0538-66 (detected by Ariel 5 at A 0538-66); LMC X-2 is the only one that is a bright low-mass X-ray binary system (LMXB) in the LMC.
DEM L316 in the Large Magellanic Cloud consists of two supernove remnants. Chandra X-ray spectra show that the hot gas shell on the upper left contains a high abundance of iron. This implies that the upper left SNR is the product of a Type Ia supernova. The much lower iron abundance in the lower SNR indicates a Type II supernova.
View from the LMC
|This section needs additional citations for verification. (August 2012)|
From a viewpoint in the LMC, the Milky Way would be a spectacular sight. The galaxy's total apparent magnitude would be −2.0—over 14 times brighter than the LMC appears to us on Earth—and it would span about 36° across the sky, which is the width of over 70 full moons. Furthermore, because of the LMC's high galactic latitude, an observer there would get an oblique view of the entire galaxy, free from the interference of interstellar dust which makes studying in the Milky Way's plane difficult from Earth. The Small Magellanic Cloud would be about magnitude 0.6, substantially brighter than the LMC appears to us.
SNR B0544-6910 in the LMC. Credit: ESO
SNR 0543-689 in the LMC. Credit: ESO
N44 region in the Large Magellanic Cloud. Credit: ESO
DEM L 159 and two clusters KMHK 840 (top left) and KMHK 831 (bottom right). Credit: ESO
Image from the Wide Field Imager on the MPG/ESO 2.2-metre telescope at ESO's La Silla Observatory in Chile. Credit: ESO
The Large Magellanic Cloud galaxy in infrared light.
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