This is part of the HERMES Public Outreach pages, created by Juliette Voyez (Paris 7 University)
The M33 Galaxy
Contents
M33 Galaxy |
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Observation data |
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Constellation |
Triangulum |
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Right ascension |
01h 33min 50.8s (EquJ2000.0) (5) |
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Declination |
+30° 39' 37" (EquJ2000.0) (5) |
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Apparent dimension (V) |
70.8 × 41.7 arcmin (4) |
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Apparent magnitude (V) |
6.27 (4) |
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Characteristics |
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Type |
SA(s)cd (de Vaucouleurs system), Sc (Hubble sequence) |
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Inclination |
56° |
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Position Angle |
-22.5° |
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Astrometry |
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Helio Radial velocity |
-179 ± 3 km/s (4) |
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Redshift |
-0.000597 ± 0.000010 (4) |
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Galactocentric Velocity |
-44 ± 6 km/s (4) |
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Distance |
0.8470 Mpc (Cepheid variable method) (6) |
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Other designations |
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NGC 598, MCG +05-04-069, 1ES 0131+303, RX J0133.8+3039, PGC 005818 |
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Contents
Introduction
Messier 33 (NGC 598), also known as the Triangulum Galaxy, is a late spiral type galaxy (SA(s)cd according to the de Vaucouleurs system and Sc according to the Hubble sequence), third largest member of the Local Group. It is situated approximatively three million light-years away from us, in the Triangulum constellation. This galaxy is not so big compared to its apparent neighbors, the Andromeda Galaxy M31 and the Milky Way, but it corresponds more to the average size of spiral galaxies in the universe. Its disk mass (the stars) is roughly three billion solar masses and its dark mass is estimated to 50 billion solar masses (7).
LGS 3, the Pisces Dwarf, one of the small galaxies of the Local Group is suspected to be a satellite of M33, which itself may be a remote but gravitationally bound companion of M31 (their separation is about 750,000 light-years).
According to the NASA Extragalactic Database (NED, (7)), M33 is approaching us (our solar system) at a velocity of 179 km/s. When correction for our rotation around the Milky Way's Galactic Center is made, one finds that it is approaching our Galaxy at 24 km/s.
This galaxy can be seen with the naked eye under exceptionally good conditions and is often listed as the most distant object visible without any astronomy instrument.
General Information
The Triangulum Galaxy was probably discovered by Giovanni Battista Hodierna, an Italian astronomer at the court of the Duke of Montechiaro, before 1654, but it was independently rediscovered by Charles Messier in 1764 who catalogued it as M33. It was also catalogued independently by William Herschel in 1784. M33 was among the first "spiral nebulae" identified as such by William Parson, 3rd Earl of Rosse, and among the first "nebulae" identified as galaxies, in which Cepheid variable stars were found.
Also because of the cataloging of Herschel, the brightest and largest HII region (diffuse emission nebula containing ionized hydrogen) has obtained a NGC number of its own: NGC 604 (William Herschel's H III.150); it is situated in the northeastern part of the galaxy. This is one of the largest HII regions known at all, with a diameter of nearly 1500 light-years. Several other knots in the spiral arms of M33 have been assigned their own NGC catalog numbers: NGCs 588, 592 and 595 for example.
The pronounced spiral arms exhibit numerous reddish HII regions (including NGC 604), as well as bluish clouds of young stars. Baade has also discovered Population II stars, and globular clusters have been found. Although no supernovae have yet been detected in the Triangulum Galaxy, several supernova remnants have, and were cartographed by radio astronomers with high accuracy. At least 112 variables have been discovered in M33, including 4 novae and about 25 Cepheids.
In 2007, a black hole of about 15.7 sun masses was detected in the galaxy using data from the Chandra X-Ray Observatory. The black hole, named M33 X-7, orbits a companion star that eclipses the black hole every three and a half days. The companion star also has an unusually large mass, 70 times that of the Sun. This makes it the most massive companion star in a binary system containing a black hole (9). The same year another black hole was detected. With a mass between 24 and 33 solar masses, it is the most massive stellar black hole known (11).
It is interesting to underline that no supermassive blackhole (a mass higher than 2x10^4 solar masses) was detected in the center of the galaxy, but a X binary, M33 X-8, is stacked on the central core.
Observing M33 at different wavelengths
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FIG 1: The Electromagnetic Spectrum. |
Observing an astrophysics object at different wavelengths give us several information about it. A long wavelength radiation has to be connected to a cool region of the sky, therefore according to the electromagnetic spectrum (FIG 1), an object emitting in X-ray is much hotter than an object emitting in radio. In this section, we propose a detailed study of the M33 Galaxy at different wavelengths of the electromagnetic spectrum.
X-Rays
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FIG 2: M33 in X-ray, PSPC, ROSAT, 1990s. |
This X-ray image (FIG 2) shows a central point of emission at the galaxy center and some others points sources of X-ray radiation. X-ray sources are very energetic sources. In total, some 50 separate X-ray sources have been identified within M33. Most of these sources are probably associated with Population I stars (young stars). Roughly ten of the sources are supernova remnants.
Ultraviolet
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FIG 3: M33 in UV, UIT, ASTRO-1 Observatory, late 1990. |
With this ultraviolet image (FIG 3), one can easily trace out the spiral arms pattern of the galaxy. Indeed, star formation is concentrated in spiral arms and young stars are significant emitters of UV light.
Visible and Near Infrared
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FIG 4(a): M33 in visible light (left), 1991-2002, Malin/IAC/RGO. Photograph from Isaac Newton Telescope plates by David Malin. |
FIG 4(b): M33 in near-infrared, 2MASS. |
The visible light image (FIG 4(a)) shows that the stars in M33 are bluish, an indicator that the galaxy is populated primarily by young stars (older stars would appear redder). The supergiant HII region, NGC 604 appears as a pinkish nebula towards the upper left edge of the photo.
By looking at the optical (FIG 4(a)) and near-infrared (FIG 4(b)) images, one can trace out the relatively faint pattern of the spiral arms.
Mid and Far Infrared
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FIG 5: M33 in mid-infrared (left) and far-infrared (right), IRAS, 1983. |
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In these images (FIG 5) in mid (left-hand side) and far (right-hand side) infrared, red points out regions of brighter emission while blue corresponds to regions of lower emission. The two images clearly reveal a center core of bright emission, corresponding to the center of the galaxy. Other bright IR emission is seen along the spiral arms. Emission at both IR wavelengths is primarily due to new stars illuminating dust grains within the spiral arms. The bright region in the left hand side of the two images corresponds to the supergiant HII region, NGC 604.
Radio
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FIG 6: M33 in Radio. NRAO, 21 cm HI emission line, 2001. |
The radio image (FIG 6) maps the cool hydrogen gas within the galaxy. Because of its low temperature, this gas does not emit radiation in visible light and it cannot be seen with an optical telescope.
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FIG 7: Masked moment image showing the distribution, of CO J=1-0 emission from M33, FCRAO telescope, Heyer et al. 2004 (8). |
Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to trace the presence of H2 is CO. The ratio between CO luminosity and H2 mass is thought to be constant, so with this CO emission image (FIG 7), molecular clouds in M33 can be detected.