Peculiar Lenticular Galaxy NGC 2685

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The very unusual galaxy NGC 2685, also known as the Helix Galaxy, is located about 40 million light-years away in the constellation Ursa Major. This image was captured by the Gemini North telescope, one half of the International Gemini Observatory, operated by NSF NOIRLab which is funded by the U.S. National Science Foundation.

NGC 2685 is a peculiar lenticular galaxy known as a polar ring galaxy. A ring of gas, stars, and dust orbits NGC 2685 perpendicular to the flat plane of the host galaxy. This odd crossing of planes is believed to be evidence of galaxy interactions, mergers, or tidal accretion events. Current research suggests that the present structure of NGC 2685 was formed when it captured material from another galaxy, which was strung out into an encircling ring.

This galaxy is one of our closest known polar ring galaxies and is therefore one of the easiest of its kind to study. Owing to its odd characteristics, it has been called the most unusual galaxy in the Shapley-Ames Catalog of Bright Galaxies and appears as number 336 in astronomer Halton Arp's Atlas of Peculiar Galaxies.

Image Credit: International Gemini Observatory/NOIRLab/NSF/AURA/L. Bassino

Image processing: J. Miller (Gemini Observatory/NSF's NOIRLab), M. Rodriguez (Gemini Observatory/NSF's NOIRLab), and M. Zamani (NSF's NOIRLab)

Image enhancement: Jean-Baptiste Faure

Spiral Galaxy IC 4633

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The subject of this picture from Hubble is the spiral galaxy IC 4633, located 100 million light-years away from us in the constellation Apus. IC 4633 is a galaxy rich in star-forming activity, as well as hosting an active galactic nucleus at its core. From our point of view, the galaxy is tilted mostly towards us, giving astronomers a fairly good view of its billions of stars.

However, we can't fully appreciate the features of this galaxy – at least in visible light – because it's partially concealed by a stretch of dark dust. This dark nebula is part of the Chamaeleon star-forming region, itself located only around 500 light-years from us, in a nearby part of the Milky Way galaxy. The dark clouds in the Chamaeleon region occupy a large area of the southern sky, covering their namesake constellation but also encroaching on nearby constellations, like Apus. The cloud is well-studied for its treasury of young stars, particularly the cloud Cha I, which has been imaged by Hubble and also by the James Webb Space Telescope.

The cloud overlapping IC 4633 lies east of the well-known Cha I, II and III, and has been called MW9 or the South Celestial Serpent. A vast, narrow trail of faint gas that snakes over the southern celestial pole, it’s much more subdued-looking than its neighbours. It's classified as an integrated flux nebula (IFN) – a cloud of gas and dust in the Milky Way galaxy that's not near to any single star, and is only faintly lit by the total light of all the galaxy's stars. Hubble has no problem making out the South Celestial Serpent, though this image captures only a tiny part of it. For a showy astronomical object like IC 4633, among the South Celestial Serpent's coils clearly isn’t a bad place to hide.

Image Credit: ESA/Hubble and NASA, J. Dalcanton, Dark Energy Survey/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA

Acknowledgement: L. Shatz

Image enhancement: Jean-Baptiste Faure

Globular Cluster NGC 1651

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This image shows a globular cluster known as NGC 1651. Like the object in another recent Picture of the Week, it is located about 162 000 light-years away in the largest and brightest of the Milky Way's satellite galaxies, the Large Magellanic Cloud (LMC). A notable feature of this image is that the globular cluster almost fills the entire image, even though globular clusters are only about 10 to 300 light-years in diameter (NGC 1651 has a diameter of roughly 120 light-years). In contrast, there are numerous Hubble Pictures of the Week that feature entire galaxies – which can be tens or hundreds of millions of light-years in diameter – that also more or less fill the whole image.

A common misconception is that Hubble and other large telescopes manage to observe wildly differently sized celestial objects by zooming in on them, as one would with a specialised camera here on Earth. However, whilst small telescopes might have the option to zoom in and out to a certain extent, large telescopes do not. Each telescope's instrument has a fixed "field of view" (the size of the region of sky that it can observe in a single observation). For example, the ultraviolet/visible light channel of Hubble's Wide Field Camera 3 (WFC3), the channel and instrument that were used to collect the data used in this image, has a field of view roughly one twelfth the diameter of the Moon as seen from Earth. Whenever WFC3 makes an observation, that is the size of the region of sky that it can observe.

The reason that Hubble can observe objects of such wildly different sizes is two-fold. Firstly, the distance to an object will determine how big it appears to be from Earth, so entire galaxies that are relatively far away might take up the same amount of space in the sky as a globular cluster like NGC 1651 that is relatively close by. In fact, there's a distant spiral galaxy lurking in this image, directly left of the cluster – though undoubtedly much larger than this star cluster, it appears small enough here to blend in with foreground stars! Secondly, multiple images spanning different parts of the sky can be mosaiced together to create single images of objects that are too big for Hubble's field of view.

Image Credit: ESA/Hubble and NASA, L. Girardi, F. Niederhofer

Image enhancement: Jean-Baptiste Faure

Interacting Galaxies Arp 72

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This image features Arp 72, a very selective galaxy group that only includes two interacting galaxies: NGC 5996 (the large spiral galaxy) and NGC 5994 (its smaller companion, in the lower left of the image). Both galaxies lie approximately 160 million light-years from Earth, and their cores are separated from each other by a distance of around 67 thousand light-years. Moreover, the distance between the galaxies at their closest points is even smaller, closer to 40 thousand light-years. Whilst this might still sound vast, in galactic separation terms it is really very cosy! For comparison, the distance between the Milky Way and its nearest independent galactic neighbour Andromeda is around 2.5 million light-years. Alternatively, the distance between the Milky Way and its largest and brightest satellite galaxy, the Large Magellanic Cloud (satellite galaxies are galaxies that are bound in orbit around another galaxy), is about 162 thousand light-years.

Given this, coupled with the fact that NGC 5996 is roughly comparable in size to the Milky Way, it is not surprising that NGC 5996 and NGC 5994 – apparently separated by only 40 thousand light-years or so – are interacting with one another. In fact, the interaction might be what has caused the spiral shape of NGC 5996 to distort and apparently be drawn in the direction of NGC 5994. It also prompted the formation of the very long and faint tail of stars and gas curving away from NGC 5996, up to the top right of the image. This "tidal tail" is a common phenomenon that appears when galaxies get in close together, as can be seen in several Hubble images.

Image Credit: ESA/Hubble and NASA, L. Galbany, J. Dalcanton, Dark Energy Survey/DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA

Image enhancement: Jean-Baptiste Faure

Irregular Galaxy UGC 5829

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This gauzy-looking celestial body is UGC 5829, an irregular galaxy that lies about 30 million light-years away. Despite there not being many observations of this relatively faint galaxy, it has the distinction of having a descriptive soubriquet: the Spider Galaxy. Perhaps the distorted galactic arms with their glowing, star-forming tips bring to mind the clawed legs of an arachnid. Somewhat confusingly, there is another, very similarly nicknamed but otherwise entirely distinct, galaxy known as the Spiderweb Galaxy. This galaxy has also been more extensively imaged (notably by Hubble), despite the fact that it lies about 300 times further from Earth than the Spider Galaxy does.

Fortunately, correct galaxy identification does not depend on casual given names. Rather, known galaxies are recorded in at least one catalogue – and often in several – such as the Uppsala General Catalogue of Galaxies, which gives the Spider Galaxy its more formal title of UGC 5829. This same galaxy also has several different designations in various other catalogues: it is, for example, LEDA 31923 in the Lyon-Meudon Extragalactic Database; MCG+06-24-006 in the Morphological Catalogue of Galaxies; and SDSS J104242.78+342657.3 in the Sloan Digital Sky Survey Catalogue. The Spiderweb Galaxy isn't recorded in all of the same catalogues – each is necessarily limited in scope – but it is included in the LEDA catalogue as LEDA 2826829. It is evidently simpler to not conflate the dull but distinct names LEDA 31923 and LEDA 2826829, than the fun but easily confused Spider and Spiderweb!

Image Credit: ESA/Hubble and NASA, R. Tully, M. Messa

Image enhancement: Jean-Baptiste Faure

Dwarf Irregular Galaxy I Zwicky 18

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The James Webb Space Telescope has captured a spectacular view of the galaxy I Zwicky 18 (I Zw 18) in this new image. The galaxy was first identified by Swiss astronomer Fritz Zwicky in the 1930's and resides roughly 59 million light-years from Earth.

This galaxy has gone through several sudden bursts of star formation. This galaxy is typical of the kinds of galaxies that inhabited the early Universe and it is classified as a dwarf irregular galaxy (much smaller than our Milky Way).

Two major starburst regions are embedded in the heart of the galaxy. The wispy brown filaments surrounding the central starburst region are bubbles of gas that have been heated by stellar winds and intense ultraviolet radiation unleashed by hot, young stars. A companion galaxy resides nearby to the dwarf galaxy, which can be seen at the bottom of the wider-field image. The companion may be interacting with the dwarf galaxy and may have triggered that galaxy's recent star formation. The orange blobs surrounding the dwarf galaxy are the dim glow from ancient fully formed galaxies at much larger distances.

This image was taken as part of a Webb programme to study the life cycle of dust in I Zw 18. Scientists are now building off of previous research with Hubble obtained at optical wavelengths, studying individual dusty stars in detail with Webb's equivalent spatial resolution and sensitivity at infrared wavelengths. This galaxy is of particular interest as its content of elements heavier than helium is one of the lowest of all known galaxies in the local Universe. Such conditions are thought to be similar to those in some of the first star-forming galaxies at high redshift, so the Webb study of I Zw 18 should shed light on the life-cycle of stars and dust in the early Universe.

Although previously believed to have only just recently begun forming its first generation of stars, the Hubble Space Telescope found fainter, older red stars contained within the galaxy, suggesting its star formation started at least one billion years ago and possibly as much as 10 billion years ago. The galaxy, therefore, may have formed at the same time as most other galaxies.

The new observations from Webb have revealed the detection of a set of candidate dusty evolved stars. It also provides details about Zw 18's two dominant star-forming regions. Webb's new data suggest that the dominant bursts of star formation in these regions occurred at different times. The strongest starburst activity is now believed to have happened more recently in the northwest lobe as compared to the galaxy's southeast lobe. This is based on the relative populations of younger versus older stars found in each of the lobes.

Image Credit: ESA/Webb, NASA, CSA, A. Hirschauer, M. Meixner et al.

Image enhancement: Jean-Baptiste Faure

Intermediate Spiral Galaxy NGC 5468 by Hubble and Webb

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This image of NGC 5468, a galaxy located about 130 million light-years from Earth, combines data from the Hubble and James Webb space telescopes. This is the most distant galaxy in which Hubble has identified Cepheid variable stars. These are important milepost markers for measuring the expansion rate of the Universe. The distance calculated from Cepheids has been cross-correlated with a Type Ia supernova in the galaxy. Type Ia supernovae are so bright they are used to measure cosmic distances far beyond the range of the Cepheids, extending measurements of the Universe’s expansion rate deeper into space.

Image Credit: NASA, ESA, CSA, STScI, A. Riess (JHU/STScI)

Image enhancement: Jean-Baptiste Faure