Messier 31 is more commonly referred to as the Andromeda Galaxy, and apart from our Milky Way, is arguably the most well-known galaxy. It is the nearest spiral galaxy to our own, and is one of the more than 20 galaxies comprising The Local Group of galaxies.
The first thing to notice is that the Andromeda Galaxy, because of its proximity, is huge. The field of view for each of the images above is about 2 degrees square. In other words, the width of each image spans the extent of four full moons. On a clear and dark winter night, Messier 31 can be seen as a fuzzy patch of light by the naked eye, and makes an inviting target for binoculars.
Astronomers have conducted intensive studies of the Andromeda Galaxy, in part to help increase understanding of our own Galaxy. Our viewpoint on the Milky Way originates from within the flattened spiral disk and is further complicated by the obscuring effects of dust and gas within that disk. One needs to view a galaxy from an external perspective to truly comprehend the large-scale structure within that galaxy. Hence the value of studying a sister spiral galaxy, like M31, that happens to be near.
Visible: DSS (left) and Visible: Jason Ware (right)
The visible-light images above reveal the general pinwheel characteristic of spiral galaxies: a bright central bulge, graceful spiral arms twisting about the bulge, and lanes of dust within the thin disk. Young and massive stars tend to be blue and the color image vividly shows that such stars are preferentially born within the arms of a spiral galaxy. The condensations of dust within M31 are typically mixed with molecular gas (which is best seen at radio wavelengths; see below). Such gas and dust are the raw materials from which future stars are born.
Two other Messier objects, both smaller dwarf elliptical galaxies, are also seen in the images above. Messier 32 is the companion immediately south of Andromeda, and Messier 110 is the galaxy located to the northwest (upper right). These small satellite galaxies are both gravitationally-bound to the much larger Andromeda Galaxy. They contain from 1-10 percent of the mass of M31 and are a factor of 10 or more smaller in spatial extent.
Mid-Infrared: Spitzer, Mid-Infrared: IRAS, and Far-Infrared: ISO
The infrared images shown above, obtained by different space-borne observatories, display similar aspects of Messier 31. While the brightest emission originates in the galaxy center, the most prominent features are the rings of infrared light circling the galaxy. It is within these rings that vast numbers of new stars are being born. One can also see traces of the inner spiral arms of M31.
The image to the left is a 24 micron view from the Spitzer Space Telecope. Spitzer detects dust heated by stars in the galaxy. Its multiband imaging photometer's 24-micron detector recorded approximately 11,000 separate infrared snapshots over 18 hours to create the new comprehensive mosaic. This instrument's resolution and sensitivity is a vast improvement over previous infrared technologies, enabling scientists to trace the spiral structures within Andromeda to an unprecedented level of detail. The galaxy's central bulge glows in the light emitted by warm dust from old, giant stars. Just outside the bulge, a system of inner spiral arms can be seen, and outside this, a well-known prominent ring of star formation.
The middle image represents a composite photograph taken by the Infrared Astronomical Satellite (IRAS) satellite at longer infrared wavelengths: 25 and 60 microns. Once again, the most luminous region is the center of the Andromeda Galaxy (shown in white). The ring is once again clearly visible. Moreover, the brightest condensations within the ring (depicted as white and red) correspond to the same bright enhancements seen in the Spitzer image.
The 175 micron IR image (above right) was taken by the European Space Agency's Infrared Space Observatory, which orbited from 1995 through 1998. The prominence of the central core is diminished, while the outer ring is still easily visible. Why is that? Mid- and far-infrared light is a tracer of dust. However, the temperature of the dust can vary, and depends on the amount of ambient visible and ultraviolet light absorbed and re-radiated by the dust particles. The peak emission from hotter dust occurs at shorter wavelengths than that from cooler dust. Because of the great density of illuminating stars near the center of a spiral galaxy, one can predict that the dust will be warmest in those regions. The coolest dust is likely to be in the outer regions of the galaxy disk, where the surrounding density of stars is less (see visible-light photos at top of page). This prediction is confirmed by comparing the IRAS and ISO photos. The bright central regions are seen best at the shorter wavelengths observed by IRAS. The ISO results measure cooler dust, and hence are more tuned to seeing the dust in the outer portions of the disk. Note how the IR emission even extends beyond the bright ring, to the outer edges of the galaxy disk.
To see what the far-infrared image would look like if we could see the Andromeda Galaxy in a face-on orientation face-on, click here for a mathematical re-projection of the data.
Mid-Infrared: IRAS and Radio: Effelsberg
The radio image of M31 (above right), obtained at a wavelength of 6 cm, is paired with the IRAS mid-IR photo (above left) for easy comparison of features. Red denotes the brightest regions within the radio data obtained by the largest radio telescope in Germany. The first thing one notices is that the central core of Andromeda and the ring are once again prominent. Astronomers have found that the spatial distribution of radio continuum (broadband) emission in spiral galaxies closely tracks that of far-infrared emission. The massive stars born in supergiant HII regions burn their fuel very fast, and have short lifetimes (by cosmological standards). Such stars often produce supernova explosions when they die, and these explosive events yield significant amounts of synchrotron radio emission. In the long continuum of astronomical time, the images in this Gallery essentially represent a near-simultaneous snapshot, revealing both star birth and star death occurring in the same era. Hence, we see both infrared and radio light. The physical emission mechanisms differ, but both arise from the same population of massive stars, albeit at different stages of their life.
The other notable features within the radio data are the bright blobs of emission scattered in an apparently random manner throughout the photograph. The fact that these point-like emission sources are not confined within Messier 31 immediately tells us that they are either in the nearby foreground (that is, within our Milky Way) or are in the distant background. Quasi-stellar objects (also known as quasars, or QSOs) are among the most luminous objects in the Universe and are found at great distances from us (billions of light years). Many quasars emit intense synchrotron radiation at radio wavelengths, usually powered by a central black hole. A search of the NASA/IPAC Extragalactic Database (NED), conducted within 2 degrees of the center of M31, reveals only four cataloged quasars. Since there are many more sources of radio emission in the field, we conclude that most of the observed quasars are still uncataloged by astronomers.
The GALEX ultraviolet image shows blue regions of young, hot, high mass stars tracing out the spiral arms where star formation is occurring, and the central orange-white "bulge" of old, cooler stars formed long ago. The red stars in this image are foreground stars in our own Milky Way galaxy.
At x-ray wavelengths (above right), the central regions of Messier 31 are
clearly seen. Again, this is primarily due to its proximity. Most
distant normal galaxies (like M31) are not strong x-ray sources.
on the other hand, are often strong emitters of x-rays. These types
of objects include quasars and various types of objects that are collectively
active galactic nuclei (AGN). The handful of the brightest point
sources seen scattered throughout the x-ray image is likely due to emission from distant QSOs
AGN in the background.