Spiral Galaxy NGC 3628

NGC 3628 is a spiral galaxy and a member of a small, but conspicuous group of galaxies located about 35 million light-years away, toward the constellation of Leo (the Lion). The other distinguished members of this family, known collectively as the Leo Triplet, are two well-known prominent spiral galaxies, Messier 65 and Messier 66 (not seen on the image), which were both discovered in 1780 by famous French comet hunter Charles Messier. NGC 3628 is the faintest of the trio and escaped Messier’s observations with his rather small telescope. It was discovered and catalogued by William Herschel only four years later.

NGC 3628 hides its spiral structure because it is seen perfectly edge-on, exactly as we observe the Milky Way on a clear night. Its most distinctive feature is a dark band of dust that lies across the plane of the disc and which is visibly distorted outwards, as a consequence of the gravitational interaction between NGC 3628 and its bullying companions. This boxy or “peanut-shaped” bulge, seen as a faint X-shape, is formed mainly of young stars and gas and dust, which create the bulge away from the plane of the rest of the galaxy through their powerful motions. Because of its appearance, NGC 3628 was catalogued as Arp 317 in the Atlas of Peculiar Galaxies, published in 1966, which aimed to characterise a large sample of odd objects that fell outside the standard Hubble classification, to aid understanding of how galaxies evolve.

The depth of the image reveals a myriad of galaxies of different shapes and colours, some of which lie much further away than NGC 3628. Particularly noticeable is the fuzzy blob just in the centre of the image, which is a diffuse satellite galaxy. A number of globular clusters can be seen as fuzzy reddish spots in the halo of the galaxy. Also visible as bright spots near the lower edge of the image (the two blue star-like objects below the satellite galaxy) are two quasars, the central engines of distant and very energetic galaxies, billions of light-years away.

This image has been taken with the FORS2 instrument, attached to one of the ESO Very Large Telescope’s Unit Telescopes. It is a combination of exposures taken through different filters (B, V and R), for a total exposure time of just below one hour. The field of view is about 7 arcminutes across, which is why this large galaxy does not fit into the image.

Image Credit: ESO
Explanation from: https://www.eso.org/public/images/potw1026a/

Moon seen from the International Space Station

From his vantage point in low Earth orbit aboard the International Space Station, NASA astronaut Randy Bresnik pointed his camera toward the rising Moon and captured this beautiful image on August 3, 2017.

Image Credit: NASA

Tycho's Supernova Remnant

• An arc of emission just found in the Tycho supernova remnant provides evidence for what triggered the original explosion.

• Astronomers think that a shock wave created the arc when a white dwarf exploded and blew material off the surface of a nearby companion star.

• Tycho belongs to a category of supernovas that are used to measure the expansion of the Universe.

This new image of Tycho's supernova remnant, dubbed Tycho for short, contains striking new evidence for what triggered the original supernova explosion, as seen from Earth in 1572. Tycho was formed by a Type Ia supernova, a category of stellar explosion used in measuring astronomical distances because of their reliable brightness.

Low and medium energy X-rays in red and green show expanding debris from the supernova explosion. High energy X-rays in blue reveal the blast wave, a shell of extremely energetic electrons. Also shown in the lower left region of Tycho is a blue arc of X-ray emission. Several lines of evidence support the conclusion that this arc is due to a shock wave created when a white dwarf exploded and blew material off the surface of a nearby companion star (see accompanying illustration below). Previously, studies with optical telescopes have revealed a star within the remnant that is moving much more quickly than its neighbors, hinting that it could be the companion to the supernova that was given a kick by the explosion.

Other details of the arc support the idea that it was blasted away from the companion star. For example, the X-ray emission of the remnant shows an apparent "shadow" next to the arc, consistent with the blocking of debris from the explosion by the expanding cone of material stripped from the companion. This shadow is most obvious in very high energy X-rays showing iron debris.

These pieces of evidence support a popular scenario for triggering a Type Ia supernova, where a white dwarf pulls material from a "normal," or Sun-like, companion star until a thermonuclear explosion occurs. In the other main competing theory, a merger of two white dwarfs occurs, and in this case, no companion star or evidence for material blasted off a companion, should exist. Both scenarios may actually occur under different conditions, but the latest Chandra result from Tycho supports the former one.

The shape of the arc is different from any other feature seen in the remnant. Other features in the interior of the remnant include recently announced stripes, which have a different shape and are thought to be features in the outer blast wave caused by cosmic ray acceleration.

Image Credit: NASA/CXC/Chinese Academy of Sciences/F. Lu et al
Explanation from: http://chandra.harvard.edu/photo/2011/tycho2/