Čočková galaxie
Čočková galaxie (označení S0) je v Hubbleově klasifikaci galaxií přechodný typ galaxie mezi eliptickou a spirální galaxií. Není však vývojovým přechodem od jednoho typu k druhému. Jejich název je odvozen od vzhledu připomínajícího spojnou čočku při pohledu z boku.[1]
Čočkové galaxie se tak podobají spirálním galaxiím s výraznou středovou výdutí a s hladkým tenčím diskem bez spirální struktury. Středová výduť je vzhledem k disku větší než u spirálních galaxií, přesto mohou vykazovat centrální příčku. Někdy je však i pro zkušeného odborníka obtížné rozlišit čočkovou galaxii od eliptické galaxie.[1]
Čočkové galaxie jsou diskové galaxie (podobně jako spirální galaxie), které vypotřebovaly nebo ztratily většinu své mezihvězdné hmoty, a proto u nich probíhá jen velmi malá tvorba hvězd. Mohou si však ve svých discích zachovat značné množství prachu. V důsledku toho sestávají většinou ze starých hvězd (podobně jako eliptické galaxie). Navzdory morfologickým rozdílům mají čočkovité a eliptické galaxie společné vlastnosti, jako jsou spektrální vlastnosti a škálovací vztahy.
Stejně jako spirální galaxie mohou mít čočkové galaxie centrální příčkovou strukturu. Čočkové galaxie se dělí do podtříd buď podle množství přítomného prachu, nebo podle výraznosti centrální příčky. Třídy čočkových galaxií bez příčky se označují S01, S02 a S03, kde indexy označují množství absorpce prachu v diskové složce; odpovídající třídy pro čočkové galaxie s centrální příčkou jsou SB01, SB02 a SB03.[2]
Galaxie typu S01 nevykazují žádné známky absorpce prachu, zatímco galaxie S03 mají kolem svých eliptických složek v rámci svých disků zcela tmavý pás absorpce prachu. Absorpce prachu v galaxiích S02 je mezi těmito dvěma extrémy. U galaxií SB01 se příčka projevuje pouze jako dvě široké oblasti mírně zvýšené jasnosti po obou stranách centrální výdutě. U galaxií SB03 je příčka úzká a dobře ohraničená a táhne se zcela napříč čočkou. U galaxií SB02 je příčka středně výrazná.[2] Vlastnosti příček v čočkových galaxiích bohužel nebyly příliš podrobně prozkoumány. Pochopení těchto vlastností, stejně jako pochopení mechanismu tvorby příček, by pomohlo objasnit historii vzniku nebo vývoje čočkových galaxií.[3]
Čočkové galaxie se často vyskytují v supergalaxiích. Čím je větší hustota galaxií v supergalaxii, tím je vyšší jejich procentuální zastoupení. Tento statistický jev zůstává bez vysvětlení.[1]
Příklady čočkových galaxií
(c) ESA/Hubble, CC BY 4.0NGC 5308
(c) ESA/Hubble, CC BY 4.0NGC 4111
(c) ESA/Hubble, CC BY 4.0
(c) ESA/Hubble, CC BY 4.0NGC 1222
Odkazy
Reference
V tomto článku byl použit překlad textu z článku Lenticular galaxy na anglické Wikipedii.
- ↑ a b c JOSIP KLECZEK A KOL. Velká encyklopedie vesmíru. [s.l.]: Academia, 2002. Dostupné online. ISBN 80-200-0906-X.
- ↑ a b BINNEY, J.; MERRIFIELD, M. Galactic Astronomy. [s.l.]: Princeton University Press, 1989. ISBN 0-691-02565-7. S. 152-153. (anglicky)
- ↑ BLANTON, MICHAEL; JOHN MOUSTAKAS. Physical Properties and Environments of Nearby Galaxies. Annual Review of Astronomy and Astrophysics [online]. 2009. Čís. 47(1). arXiv 0908.3017. doi:10.1146/annurev-astro-082708-101734. Bibcode 2009ARA&A..47..159B.
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(c) ESA/Hubble, CC BY 4.0
The elegant simplicity of NGC 4111, seen here in this image from the NASA/ESA Hubble Space Telescope, hides a more violent history than you might think. NGC 4111 is a lenticular, or lens-shaped, galaxy, lying about 50 million light-years from us in the constellation of Canes Venatici (The Hunting Dogs).
Lenticular galaxies are an intermediate type of galaxy between an elliptical and a spiral. They host aged stars like ellipticals and have a disk like a spiral. However, that’s where the similarities end: they differ from ellipticals because they have a bulge and a thin disk, but are different from spirals because lenticular discs contain very little gas and dust, and do not feature the many-armed structure that is characteristic of spiral galaxies. In this image we see the disc of NGC 4111 edge-on, so it appears as a thin sliver of light on the sky.
At first sight, NGC 4111 looks like a fairly uneventful galaxy, but there are unusual features that suggest it is not such a peaceful place. Running through its centre, at right angles to the thin disc, is a series of filaments, silhouetted against the bright core of the galaxy. These are made of dust, and astronomers think they are associated with a ring of material encircling the galaxy’s core. Since it is not aligned with the galaxy’s main disc, it is possible that this polar ring of gas and dust is actually the remains of a smaller galaxy that was swallowed up by NGC 4111 long ago.
(c) ESA/Hubble, CC BY 4.0
This NASA/ESA Hubble Space Telescope image shows star clusters encircling a galaxy, like bees buzzing around a hive. The hive in question the an edge-on lenticular galaxy NGC 5308, located just under 100 million light-years away in the constellation of Ursa Major (The Great Bear).
Members of a galaxy type that lies somewhere between an elliptical and a spiral galaxy, lenticular galaxies such as NGC 5308 are disc galaxies that have used up, or lost, the majority of their gas and dust. As a result, they experience very little ongoing star formation and consist mainly of old and aging stars. On 9 October 1996, one of NGC 5308’s aging stars met a dramatic demise, exploding as a spectacular Type la supernova.
Lenticular galaxies are often orbited by gravitationally bound collections of hundreds of thousands of older stars. Called globular clusters, these dense collections of stars form a delicate halo as they orbit around the main body of NGC 5308, appearing as bright dots on the dark sky.
The dim, irregular galaxy to the right of NGC 5308 is known, rather prosaically, as SDSS J134646.18+605911.9
(c) ESA/Hubble, CC BY 4.0
This NASA/ESA Hubble Space Telescope image captures the galaxy Messier 84 — also known as NGC 4374 — an object from the Messier catalogue, published in its final version in 1781 by Charles Messier. This elliptical galaxy was discovered in March 1781 and lies about 60 million light-years away from Earth in the constellation of Virgo (The Virgin). The galaxy is part of the very heavily populated centre of the Virgo Cluster, a cluster which consists of more than 1000 galaxies. This image does not show the whole galaxy but only its very interesting centre, and is likely to be the best image of the region ever captured. Previous observations using Hubble’s Space Telescope Imaging Spectrograph (STIS) revealed a supermassive black hole in the centre of Messier 84. Astronomers found the supermassive black hole by mapping the motion of the gas and the stars which are caught in its grip. Next to its interesting centre Messier 84 is also known for its supernovae. Two supernovae have been observed within the galaxy. The first, SN1957 was discovered in 1957 and another, called SN1991bg, was discovered in 1991.
(c) ESA/Hubble, CC BY 4.0
NGC 1222, seen in this image taken with the Wide Field Camera 3 on board the NASA/ESA Hubble Space Telescope (HST), is a galaxy with a rather eventful story to tell. NGC 1222 has been described as a peculiar example of a type of galaxy known as a lenticular galaxy. Typically, this kind of galaxy would present a rather smooth appearance on the sky and would consist mostly of old, reddish stars. A bit dull, perhaps.
But NGC 1222 is certainly not a typical member of its class — and it’s anything but dull. Observations show the characteristic features of very recent star formation on a huge scale — an event known as a starburst. The reason for all this violent activity is caused by the fact that NGC 1222 is not alone. It actually contains three compact regions, each of which appears to be the central nucleus of a galaxy. Astronomers think that NGC 1222 is in the process of swallowing up two much smaller dwarf galaxies that strayed too close to it. It is likely that the encounter was the trigger for the starburst in NGC 1222, bringing in fresh supplies of gas that are now fuelling the burst of star formation.
Although its peculiarities were first seen in photographic images, these were not able to reveal the level of fine detail that can be recovered by Hubble. The image taken by Hubble allows us to see an astonishing amount of structure in this galaxy, emphasising its colourful history. Against the smooth background of old stars that was the original lenticular galaxy, we can clearly see dark filaments of dust and bright filaments of gas, both associated with the powerful star formation process.Autor: User:Joyce801, Licence: CC BY 4.0
Grid showing the location of early-type galaxies (elliptical E, ellicular ES, lenticular S0) relative to the late-type spiral galaxies. The horizontal axis shows the morphological type, primarily dictated by the nature of the spiral arms.
An image of the Cartwheel Galaxy taken with the NASA/ESA Hubble Space Telescope has been reprocessed using the latest techniques to mark the closure of the Space Telescope European Coordination Facility (ST-ECF), based near Munich in Germany, and to celebrate its achievements in supporting Hubble science in Europe over the past 26 years.
Astronomer Bob Fosbury, who is stepping down as Head of the ST-ECF, was responsible for much of the early research into the Cartwheel Galaxy along with the late Tim Hawarden — including giving the object its very apposite name — and so this image was selected as a fitting tribute. The object was first spotted on wide-field images from the UK Schmidt telescope and then studied in detail using the Anglo-Australian Telescope.
Lying about 500 million light-years away in the constellation of Sculptor, the cartwheel shape of this galaxy is the result of a violent galactic collision. A smaller galaxy has passed right through a large disc galaxy and produced shock waves that swept up gas and dust — much like the ripples produced when a stone is dropped into a lake — and sparked regions of intense star formation (appearing blue). The outermost ring of the galaxy, which is 1.5 times the size of our Milky Way, marks the shock wave’s leading edge. This object is one of the most dramatic examples of the small class of ring galaxies.
This image was produced after Hubble data was reprocessed using the free open source software FITS Liberator 3, which was developed at the ST-ECF. Careful use of this widely used state-of-the-art tool on the original Hubble observations of the Cartwheel Galaxy has brought out more detail in the image than ever before.
Although the ST-ECF is closing, ESA’s mission to bring amazing Hubble discoveries to the public will be unaffected, with Hubblecasts, press and photo releases, and Hubble Pictures of the Week continuing to be regularly posted on spacetelescope.org(c) ESA/Hubble, CC BY 4.0
The subject of this image is NGC 6861, a galaxy discovered in 1826 by the Scottish astronomer James Dunlop. Almost two centuries later we now know that NGC 6861 is the second brightest member of a group of at least a dozen galaxies called the Telescopium Group — otherwise known as the NGC 6868 Group — in the small constellation of Telescopium (The Telescope). This NASA/ESA Hubble Space Telescope view shows some important details of NGC 6861. One of the most prominent features is the disc of dark bands circling the centre of the galaxy. These dust lanes are a result of large clouds of dust particles obscuring the light emitted by the stars behind them. Dust lanes are very useful for working out whether we are seeing the galaxy disc edge-on, face-on or, as is the case for NGC 6861, somewhat in the middle. Dust lanes like these are typical of a spiral galaxy. The dust lanes are embedded in a white oval shape, which is made up of huge numbers of stars orbiting the centre of the galaxy. This oval is, rather puzzlingly, typical of an elliptical galaxy. So which is it — spiral or elliptical? The answer is neither! NGC 6861 does not belong to either the spiral or the elliptical family of galaxies. It is a lenticular galaxy, a family which has features of both spirals and ellipticals. The relationships between these three kinds of galaxies are not yet well understood. A lenticular galaxy could be a faded spiral that has run out of gas and lost its arms, or the result of two galaxies merging. Being part of a group increases the chances for galactic mergers, so this could be the case for NGC 6861. A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Josh Barrington.