ESA Science & Technology - Herschel and XMM-Newton composite image of the Andromeda Galaxy (M31)

This composite image 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 (shown here in grey) and XMM-Newton (shown here in red, green, blue and white, according to the energy of the different sources).

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 probe the cold dust component of the interstellar medium (ISM), the mixture of dust and, mostly, gas 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.

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, probing the X-ray portion of the spectrum, is 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.

The XMM-Newton data have been gathered by the EPIC camera in the 0.3-7.0 keV energy range, corresponding to wavelengths of 1.8-41.3 Ångstroms. In this image sources are shown with different colours, according to their energetic output: red corresponds to 0.3-0.7 keV, green to 0.7-1.2 keV and blue to 1.2-7.0 keV, whereas white corresponds to those sources that are detected with comparable intensity in all three energy intervals. 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.

Amongst the hundreds of sources revealed by XMM-Newton, those shown in red are low-mass objects, emitting only low-energy X-rays. Some of these sources, referred to as novae, are binary systems comprising a white dwarf star that is gradually accreting material from its companion. In these systems, the white dwarf may eventually grow massive enough to collapse catastrophically and explode as a so-called type-Ia supernova. In contrast, the objects shown in blue emit highly energetic X-rays and are likely binary systems in which a neutron star or a black hole, formed by the demise of a very massive star, rotates around a normal star. Other sources present in the image are supernova remnants (SNR), remains of the powerful explosions through which massive stars end their life.

By probing the latest stages of stellar evolution, which have also a major impact on the birth of future generations of stars via the mass and energy released by supernovae in their surroundings, XMM-Newton offer us an exceptional view of the evolution of stars in the Andromeda galaxy.

The combined view shows the interplay between the dust, seen at infrared wavelengths by Herschel, and the X-rays detected by XMM-Newton. The dust, in fact, blocks the lowest energy X-rays, whose sources are represented in red, and, if the amount of dust is sufficiently large, it can also block higher energy X-rays, namely those emitted by the sources represented in green. Therefore, it is possible to detect low- and intermediate-energy X-ray sources only where the dust leaves 'holes' along our line of sight; as the dust obscuration is stronger for low-energy X-rays, fewer low-energy X-ray sources (red dots) are detected with respect to intermediate-energy ones (green dots). The highest energy X-rays are less affected by the dust, thus their sources, represented here in blue, are detected almost everywhere across the galaxy.

Combined, these two images taken by Herschel and XMM-Newton offer us a comprehensive view of the evolution of stars in the Andromeda galaxy. By highlighting both the stars that once were, in the X-rays, and the stars that will be, in the far-infrared, the composite image tells the entire tale of star formation within Andromeda, 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 shape the birth of future generations of stars.