Le Galassie Di Seyfert


Le galassie di Seyfert prendono il nome Carl Seyfert, che fu il primo a descrivere questa classe di appartenenza nel 1943. Esse rappresentano circa il 10% di tutte le galassie e sono alcuni degli oggetti più studiati in astronomia, in quanto si pensa che possano essere tenute in vita dagli stessi fenomeni che si verificano nei quasar (gli oggetti con maggiore emissione di onde conosciute nell’universo) e sono più luminose delle altre. Anche se sono più vicine e meno luminose dei quasar, queste galassie hanno buchi neri supermassicci al loro centro, che sono a loro volta circondate da dischi di accrescimento di materiale in caduta. I dischi di accrescimento sono ritenuti essere la fonte della radiazione ultravioletta. Le righe di emissione e di assorbimento ultravioletto forniscono per le osservazioni una maggiore raccolta di dati della composizione del materiale di cui sono composte. Osservate in luce visibile, la maggior parte delle galassie di Seyfert sembrano normali galassie a spirale, ma quando vengono studiate sotto altre lunghezze d’onda, diventa chiaro che la luminosità dei loro nuclei è di intensità paragonabile alla luminosità di galassie intere, delle dimensioni della Via Lattea.

Taking centre stage in this new NASA/ESA Hubble Space Telescope image is a galaxy known as NGC 3081, set against an assortment of glittering galaxies in the distance. Located in the constellation of Hydra (The Sea Serpent), NGC 3081 is located over 86 million light-years from us. It is known as a type II Seyfert galaxy, characterised by its dazzling nucleus. NGC 3081 is seen here nearly face-on. Compared to other spiral galaxies, it looks a little different. The galaxy's barred spiral centre is surrounded by a bright loop known as a resonance ring. This ring is full of bright clusters and bursts of new star formation, and frames the supermassive black hole thought to be lurking within NGC 3081 — which glows brightly as it hungrily gobbles up infalling material. These rings form in particular locations known as resonances, where gravitational effects throughout a galaxy cause gas to pile up and accumulate in certain positions. These can be caused by the presence of a "bar" within the galaxy, as with NGC 3081, or by interactions with other nearby objects. It is not unusual for rings like this to be seen in barred galaxies, as the bars are very effective at gathering gas into these resonance regions, causing pile-ups which lead to active and very well-organised star formation. Hubble snapped this magnificent face-on image of the galaxy using the Wide Field Planetary Camera 2. This image is made up of a combination of ultraviolet, optical, and infrared observations, allowing distinctive features of the galaxy to be observed across a wide range of wavelengths. Notes A paper based on these observations was published in The Astronomical Journal in 2004, entitled "A Hubble Space Telescope Study of Star Formation in the Inner Resonance Ring of NGC 3081" by Ronald J. Buta, Gene G. Byrd, and Tarsh Freeman.

The NASA/ESA Hubble Space Telescope provides us this week with a spectacular image of the bright star-forming ring that surrounds the heart of the barred spiral galaxy NGC 1097. In this image, the larger-scale structure of the galaxy is barely visible: its comparatively dim spiral arms, which surround its heart in a loose embrace, reach out beyond the edges of this frame. This face-on galaxy, lying 45 million light-years away from Earth in the southern constellation of Fornax (The Furnace), is particularly attractive for astronomers. NGC 1097 is a Seyfert galaxy. Lurking at the very centre of the galaxy, a supermassive black hole 100 million times the mass of our Sun is gradually sucking in the matter around it. The area immediately around the black hole shines powerfully with radiation coming from the material falling in. The distinctive ring around the black hole is bursting with new star formation due to an inflow of material toward the central bar of the galaxy. These star-forming regions are glowing brightly thanks to emission from clouds of ionised hydrogen. The ring is around 5000 light-years across, although the spiral arms of the galaxy extend tens of thousands of light-years beyond it. NGC 1097 is also pretty exciting for supernova hunters. The galaxy experienced three supernovae (the violent deaths of high-mass stars) in the 11-year span between 1992 and 2003. This is definitely a galaxy worth checking on a regular basis. However, what it is really exciting about NGC 1097 is that it is not wandering alone through space. It has two small galaxy companions, which dance “the dance of stars and the dance of space” like the gracious dancer of the famous poem The Dancer by Khalil Gibran. The satellite galaxies are NGC 1097A, an elliptical galaxy orbiting 42 000 light-years from the centre of NGC 1097 and a small dwarf galaxy named NGC 1097B. Both galaxies are located out beyond the frames of this image and they cannot be seen. Astronomers have indications that NGC 1097 and NGC 1097A have interacted in the past. This picture was taken with Hubble’s Advanced Camera for Surveys using visual and infrared filters. A version of this image was submitted to the Hubble’s Hidden Treasures image processing competition by contestant Eedresha Sturdivant.