Face-On Spiral Galaxy Messier 74
Messier 74 appears in this image as the perfect archetype of a great design, face-on spiral galaxy, composed by a bright central core surrounded by faint and diffuse spiral arms. Spiral galaxies have flat shapes similar to dishes, causing a varying appearance depending on their inclination with respect to our line of sight. Therefore, when we look at an edge-on spiral galaxy, the central bulge outstands spheroidal in shape, while the spiral arms apparently display a “line” design. An archetype of edge-on spiral galaxy is NGC 891. An excellent example of spiral galaxy seen under an intermediate angle can be seen in the image of NGC 7331 offered by Calar Alto Observatory in a previous photo release.
The study of all known spiral galaxies has shown that stars on the outer parts of these systems orbit around the centre faster than predicted by Kepler laws, just as if there were more matter than deduced “weighting” the luminous objects (stars) seen in these island-universes. This unobserved matter, with so clear gravitational effects, is what astronomers called dark matter, an exotic and unknown kind of substance that does not emit light, and that constitutes one of the most intriguing challenges for modern astronomy.
Face-on spiral galaxies like M 74 are of high interest for researchers, as they let us look into them in high detail. Such overhead images are of interest to know how the population and behaviour of stars changes along the radial distance from the centre. As this picture clearly shows, the outer parts are bluer than the inner parts, revealing that, on average, the stars near the centre are older than the ones in the spiral arms, full of young stars produced by the still active processes of star formation. It is here, in the spiral arms, where one should expect a higher frequency of some specific types of supernovae, specifically those that mark the catastrophic end for massive, quick-living stars (such stars time ago disappeared in the aged centre of spiral galaxies). Indeed two such supernovae have been found in the spiral arms of this galaxy, in 2002 and 2003, respectively (supernova 2002ap was a peculiar Ib/c explosion, while supernova 2003gd was a type II event). Their light was useful to better calibrate the supernova-based measurement of distances, by comparing their implications with the results yielded by other methods based on classical variable stars.
Documentary Photo Gallery of Descubre Foundation (Descubre/DSA/OAUV), obtained at Calar Alto Observatory. Vicent Peris (OAUV), José Luis Lamadrid (CEFCA), Jack Harvey (SSRO), Steve Mazlin (SSRO), Ivette Rodríguez (PTeam), Oriol Lehmkuhl (PTeam), Juan Conejero (PixInsight). Image scale: 0.5” per pixel. The image comprises 15 arcminutes on the sky (half the apparent width of the full Moon). North up, East left. Entirely processed with PixInsight 1.6.
Color balancing of the image has been done in the same way as in the NGC 7331 image. By taking as white reference the whole light coming from the main galaxy, we can maximize the representation of color hues in the scene. Such color representation is not of universal validity, but relative to the specific chromatic content of this scene. This approach lets us to distinguish the different stellar populations inside the galaxy, from the younger and hot bluish stars in the outer regions, to the older and colder reddish ones at the central areas. The image accumulates a total exposure time of 19 hours through Johnson B, V and R filters, as well as in H-α emission light.
Photographing this galaxy in H-α light reveals a rich and complex spiral structure traced by clouds composed by ionised hydrogen, denoting the presence of intense stellar formation activity inside these areas. At the same time, the long integration time, linked to good seeing conditions, reveal aspects seldom displayed in such small portions of the sky. Not only the frame is overflowed by the spiral arms, but we can see in the background the redshifted light of the very distant —in time and space— galaxies that populated the universe several billion years ago.