Multiwavelength images of the Andromeda Galaxy (M31)
This mosaic of images shows the Andromeda galaxy, also known as M31, the nearest major galaxy to the Milky Way, as observed by the two ESA space observatories Herschel (infrared) and XMM-Newton (X-rays). An optical view is shown for comparison.
Due to its proximity to us, Andromeda is ideally suited for investigations of star formation and evolution on the global scale of an entire galaxy similar to our own Milky Way with a degree of detail that cannot be achieved through observations of other, more distant galaxies.
Observing the Universe at far-infrared and submillimetre wavelengths, Herschel is sensitive to the cold material that represents a window on the early phases of the formation of stars.
The data collected by Herschel in the far-infrared domain (top right panel) probe the cold dust component of the interstellar medium (ISM), the mixture of (mostly) gas and dust from which new stars originate in galaxies. The dust, heated by young and massive stars as they form, shines brightly in the wavelength range explored by Herschel and traces the overall distribution of the ISM, revealing its intricate structure. The image highlights how the mixture of dust and gas in M31 exhibits a complex pattern organised in spiral arms and at least five concentric rings. In addition, a series of other smaller-scale features, such as bright arcs and dark "holes", disclose regions where the star formation activity appears to be more or less intense. The large rings of dust that encircle the centre of the galaxy may be the result of a smaller galaxy having collided with Andromeda some time in the past.
The image shown here, the most detailed ever obtained of M31 at a far-infrared wavelength of 250 micron (500 times longer than that of yellow light), was taken with the SPIRE instrument on board Herschel, and is based on a total observing time of 18 hours. The field of view of the image shown is 1.5×2 degrees, although a larger area was actually observed.
In contrast, XMM-Newton probes the X-ray portion of the spectrum and is thus sensitive to highly energetic phenomena typical of the latest evolutionary stages of stellar life. This short-wavelength radiation is either emitted by stars close to the end of their life cycle or by the remnants of stars that have already died.
Visible in the XMM-Newton image (bottom right panel) are hundreds of sources of X-rays, which mostly belong to two classes: supernova remnants (SNR), the remains of the powerful explosions through which massive stars end their life; and binary systems, pairs of objects consisting of a compact stellar remnant—a white dwarf, neutron star or black hole—exerting an intense gravitational pull onto a companion star, from which it strips material via an accretion disc.
Some of these binary systems—those comprising a white dwarf, referred to as novae—are also expected to give rise to supernova explosions (producing a different kind of object, the so-called type-Ia supernovae) and are thus extremely important in the global recycling of material within the galaxy, since the enormous amounts of mass and energy released by supernovae have a major impact on the surrounding, cold material from which stars are born.
The XMM-Newton image shown here has been generated from data obtained with the EPIC camera in the 0.3-7.0 keV energy range, corresponding to wavelengths of 1.8-41.3 Ångstroms. This deep map of M31 is the result of a few tens of observations performed between 2000 and 2010, with a total observing time of more than 2 million seconds, equivalent to over 20 days; different amounts of time were spent observing different regions of the galaxy.
Combined, the two images taken by Herschel and XMM-Newton (central panel) offer us a comprehensive view of the evolution of stars in the Andromeda galaxy, from the cold material where star formation takes place all the way to the remnants of stellar demises which, in turn, influence the ISM and contribute to shaping the birth of future generations of stars.
To complete the picture of the stellar populations of Andromeda, an optical image (top left) taken by astrophotographer Robert Gendler, shows the distribution of several hundred billion stars, currently in the main phase of their life cycle, that make up this massive spiral galaxy. All three images, at far-infrared, optical and X-ray wavelengths, are combined in a colour-composite (bottom left panel) showing the interplay of stars at all evolutionary stages and, thus, a unique view of the history of star formation within Andromeda: the current stars in the optical, in X-rays the stars that once were, and in the far-infrared the stars that will be.