Interacting Galaxies Arp 142 as seen by Webb

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The distorted spiral galaxy at the center, the Penguin, and the compact elliptical galaxy at the left, the Egg, are locked in an active embrace. A new near- and mid-infrared image from the James Webb Space Telescope, taken to mark its second year of science, shows that their interaction is marked by a faint upside-down U-shaped blue glow.

The pair, known jointly as Arp 142, made their first pass between 25 and 75 million years ago – causing "fireworks", or new star formation, in the Penguin. In the most extreme cases, mergers can cause galaxies to form thousands of new stars per year for a few million years. For the Penguin, research has shown that about 100 to 200 stars have formed per year. By comparison, our Milky Way galaxy (which is not interacting with a galaxy of the same size) forms roughly six to seven new stars per year.

This gravitational shimmy also remade the Penguin's appearance. Its coiled spiral arms unwound, and gas and dust were pulled in an array of directions, like it was releasing confetti. It is rare for individual stars to collide when galaxies interact (space is vast), but the galaxies’ mingling disrupts their stars' orbits.

Today, the Penguin's galactic centre looks like an eye set within a head, and the galaxy has prominent star trails that take the shape of a beak, backbone, and fanned-out tail. A faint, but prominent dust lane extends from its beak down to its tail.

Despite the Penguin appearing far larger than the Egg, these galaxies have approximately the same mass. This is one reason why the smaller-looking Egg hasn't yet merged with the Penguin. (If one was less massive, it may have merged earlier.)

The oval Egg is filled with old stars, and little gas and dust, which is why it isn't sending out "streamers" or tidal tails of its own and instead has maintained a compact oval shape. If you look closely, the Egg has four prominent diffraction spikes – the galaxy's stars are so concentrated that it gleams.

Now, find the bright, edge-on galaxy at top right. It may look like a party crasher, but it's not nearby. Cataloged PGC 1237172, it lies 100 million light-years closer to Earth. It is relatively young and isn't overflowing with dust, which is why it practically disappears in Webb's mid-infrared view.

The background of this image is overflowing with far more distant galaxies. This is a testament to the sensitivity and resolution of Webb's infrared cameras.

Arp 142 lies 326 million light-years from Earth in the constellation Hydra.

Image Credit: NASA, ESA, CSA, STScI

Image enhancement: Jean-Baptiste Faure

Irregular Dwarf Galaxy NGC 4449

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Much of the visible matter in the Universe, the matter that makes up stars, planets – and us – is made inside stars as they complete their cycle of birth, life, and death. They are born from clouds of gas and dust, and when they die their remains are recycled back into the interstellar medium to be used as fuel for the next generation of stars. And in a not-so-distant corner of the Universe, 13 million light-years away in the constellation Canes Venatici, the beginning of this cycle is unfolding at an exceptional rate.

NGC 4449, also known as Caldwell 21, appears to be putting on a cosmic fireworks show in this image, captured with the Gemini North telescope, one half of the International Gemini Observatory, which is supported in part by the U.S. National Science Foundation and operated by NSF NOIRLab. The galaxy's billowing red clouds and sparkling blue veil are lighting up the sky with the colors of newly forming stars. It's classified as an irregular Magellanic-type galaxy, reflecting its loose spiral structure and close resemblance to the Large Magellanic Cloud – the prototype of Magellanic galaxies.

Stars have been actively forming within NGC 4449 for several billion years, but currently it is pumping out new stars at a much higher rate than in the past. This unusually explosive and intense star formation activity qualifies it as a starburst galaxy. While starbursts usually occur in the central regions of galaxies, NGC 4449's star formation is more widespread, evidenced by the fact that the youngest stars are both in the nucleus and in streams surrounding the galaxy.

This "global" starburst activity resembles the Universe's earliest star-forming galaxies, which grew by merging with and accreting smaller stellar systems. And like its galactic predecessors, NGC 4449's rapid star formation was likely ignited by interactions with neighboring galaxies. As a member of the M94 Group of galaxies – one of the closest galaxy groups to the Local Group, which hosts the Milky Way – NGC 4449 lies in close proximity to a handful of surrounding smaller galaxies. Astronomers have found evidence of interactions between NGC 4449 and at least two of these satellite galaxies.

One is a very dim dwarf galaxy that is actively being absorbed, as evidenced by a diffuse stream of stars extending to one side of NGC 4449. This "stealth" merger is nearly undetectable by visual inspection owing to its diffuse nature and low stellar mass. However, it possesses a large amount of dark matter, meaning its presence can be detected by the substantial gravitational influence it has on NGC 4449. The other object that provides hints of a past merger is a massive globular star cluster embedded within the outer halo of NGC 4449. This cluster is thought by astronomers to be the surviving nucleus of a former gas-rich satellite galaxy now in the process of being absorbed by NGC 4449.

As NGC 4449 interacts with and absorbs its smaller galactic companions, the tidal interactions between the galaxies compress and shock the gas. The glowing red regions scattered across this image showcase this process, indicating an abundance of ionized hydrogen – a telltale sign of ongoing star formation. A plethora of hot, young blue star clusters are emerging from the galactic ovens, fueled by the dark filaments of cosmic dust lacing throughout the galaxy. At the current rate, the gas supply that feeds NGC 4449's production of stars will only last for another billion years or so.

This image was released in celebration of the Gemini North telescope's 25th anniversary. On 25 June 1999 a dedication ceremony was held on Maunakea, Hawai‘i, to unveil the new world-class 8.1-meter telescope and reveal its first-light images, which at the time were some of the sharpest infrared images ever obtained by a ground-based telescope. Over the past two and a half decades Gemini North's large mirror, powerful suite of instruments and advanced adaptive optics have allowed astronomers to peer further and further into the cosmos. From capturing the first direct image of a multi-planet system to testing Einstein’s general theory of relativity – which helped astronomers earn the 2020 Nobel Prize – Gemini North has contributed greatly to humanity's understanding of the Universe.

Image Credit: International Gemini Observatory/NOIRLab/NSF/AURA

Image Processing: J. Miller (International Gemini Observatory/NSF NOIRLab), M. Rodriguez (International Gemini Observatory/NSF NOIRLab), T.A. Rector (University of Alaska Anchorage/NSF NOIRLab), M. Zamani (NSF NOIRLab)

Acknowledgements: PI: P. Frank Winkler (Middlebury College), Knox S. Long (STScI/Eureka Scientific Inc), and William P. Blair (Johns Hopkins University)

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

Irregular Galaxy ESO 245-5

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This image shows a densely packed field of stars, laid on top of a background of dust, gas, and light from more distant celestial objects. The stars take up so much of the field of view in this image that it is a little tricky to discern that you are in fact looking at most of a galaxy, known as ESO 245-5. This galaxy is a relatively close neighbour of the Milky Way, lying at the fairly modest distance of 15 million light-years from Earth in the constellation Phoenix.

Another reason that it is perhaps a little tricky to spot that ESO 245-5 is a galaxy is its apparent lack of structure. We frequently enjoy Hubble's spectacular images of spiral galaxies, which are so interesting to look at in part because of their seemingly extraordinarily ordered arms of stars, gas and dust. ESO 245-5, in contrast, is classified as an IB(s)m type galaxy under the system of galaxy classification known as the De Vaucouleurs system. The IB(s)m designation specifically means that the galaxy is irregular (I), barred (B), has a slight spiral structure ((s)), and is of the Magellanic type (m).

Irregular in this context is quite intuitive: the galaxy does not appear to have a regular, ordered structure. In fact, essentially the entire view here is covered by the stars of this galaxy. The second term means that the galaxy has a barred shape at its center: this is the dense stretch of stars that crosses through the center of this image. The third term says that there are hints of a spiral structure, but nothing clear or definitive (hence the "s" is bracketed). Finally, the last term indicates ESO 245-5's similarity to the Magellanic clouds, the two dwarf galaxies that are close neighbours of the Milky Way.

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

Image enhancement: Jean-Baptiste Faure

Irregular Galaxy NGC 6822 observed by Webb

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This image shows the irregular galaxy NGC 6822, which was observed by the Near-InfraRed Camera (NIRCam) and Mid-InfraRed Instrument (MIRI) mounted on the James Webb Space Telescope. As their names suggest, NIRCam and MIRI probe different parts of the electromagnetic spectrum. This allows the instruments to observe different components of the same galaxy, with MIRI especially sensitive to its gas-rich regions (the yellow swirls in this image) and NIRCam suitable for observing its densely packed field of stars.

NGC 6822 lies about 1.5 million light-years away, and is the Milky Way's nearest galactic neighbour that is not one of its satellites. It has a low metallicity, meaning that it contains low proportions of elements that are not hydrogen and helium. Metallicity is an absolutely key concept in astronomy, in part because elements other than hydrogen and helium are largely produced by stars over their lifetimes. Therefore, in the very early Universe (before the first generation of stars had been born, lived and died) everything had very low metallicity. This makes contemporary low-metallicity objects (like NGC 6822) objects of interest for understanding how processes such as the evolution of stars and the life cycle of interstellar dust likely occurred in the early Universe. This was the motivation for these observations of NGC 6822 with Webb: to better understand how stars form and how dust evolves in low-metallicity environments.

The study of NGC 6822 has an interesting history that long predates modern investigations with Webb. It was first discovered by E. E. Barnard, who presented his discovery in a very brief paper in 1884 in The Sidereal Messenger: a short-lived but important American monthly astronomical journal that was published between 1882 and 1891. As with many astronomical objects that appeared diffuse with telescopes of the time, NGC 6822 was miscategorised as an "exceedingly faint nebula".

Over the next few years, a series of confusions arose around NGC 6822 over its apparent size, brightness, and even what kind of object it was, because astronomers at the time did not properly account for how different the same object might look with different telescopes. Edwin Hubble, namesake of the Hubble Space telescope, went on to study NGC 6822 in depth and published a far more detailed paper of his own in 1925. This work was exceptionally important for humanity’s evolving understanding of the Universe, because, in Hubble's own words: "N.G.C. 6822, [was] the first object definitely assigned to a region outside the galactic system". This paper contributed to solving the debate that was raging amongst astronomers about the extent of the Universe at the time by demonstrating that there were astronomical objects that lay beyond the Milky Way. The study of this galaxy was notably continued by Susan Kayser, who was the first woman to receive a PhD in astronomy from Caltech. Her 1966 thesis remained the most thorough investigation of this galaxy until the 2000s. Now, the study of this key local galaxy is being continued by Webb.

This image shows a dense field of stars with clouds of gas and dust billowing across it. The clouds are patchy and wispy, dense and glowing parts obscuring the center of the image. Bright galaxies with various shapes and sizes shine through the gas and stars. Some of the star images are a bit larger than the rest, with visible diffraction spikes; two foreground stars are bright in the lower-right corner.

Image Credit: ESA/Webb, NASA and CSA, M. Meixner

Image enhancement: Jean-Baptiste Faure

Irregular Galaxy Arp 263

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The irregular galaxy Arp 263 lurks in the background of this image from the Hubble Space Telescope, but the view is dominated by a stellar photobomber; the bright star BD+17 2217. Arp 263 – also known as NGC 3239 – is a patchy, irregular galaxy studded with regions of recent star formation, and astronomers believe that its ragged appearance is due to its having formed from the merger of two galaxies. It lies around 25 million light-years away in the constellation Leo.

Two different Hubble investigations into Arp 263, using two of Hubble's third-generation instruments, contributed data to this image. The first investigation was part of an effort to observe the sites of recent supernovae, such as the supernova SN 2012A that was detected just over a decade ago in Arp 263. Astronomers used Hubble's powerful Wide Field Camera 3 to search for lingering remnants of the colossal stellar explosion. The second investigation is part of a campaign using Hubble's Advanced Camera for Surveys to image all the previously unobserved peculiar galaxies in the Arp catalogue, including Arp 263, in order to find promising subjects for further study using the James Webb Space Telescope.

The interloping foreground star, BD+17 2217, is adorned with two sets of criss-crossing diffraction spikes. The interaction of light with Hubble's internal structure means that concentrated bright objects such as stars are surrounded by four prominent spikes. Since this image of BD+17 2217 was created using two sets of Hubble data, the spikes from both images surround this stellar photobomber. The spikes are at different angles because Hubble was at different orientations when it collected the two datasets.

Image Credit: ESA/Hubble and NASA, J. Dalcanton, A. Filippenko

Image enhancement: Jean-Baptiste Faure

Dwarf Irregular Galaxy NGC 1705

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The dwarf galaxy NGC 1705 is featured in this image from the Hubble Space Telescope. This diminutive galaxy lies in the southern constellation Pictor, and is approximately 17 million light-years from Earth. NGC 1705 is a cosmic oddball – it is small, irregularly shaped, and has recently undergone a spate of star formation known as a starburst.

Despite these eccentricities, NGC 1705 and other dwarf irregular galaxies like it can provide valuable insights into the overall evolution of galaxies. Dwarf irregular galaxies tend to contain few elements other than hydrogen or helium, and are considered to be similar to the earliest galaxies that populated the Universe.

The data shown in this image come from a series of observations designed to unveil the interplay between stars, star clusters, and ionised gas in nearby star-forming galaxies. By observing a specific wavelength of light known as H-alpha with Hubble's Wide Field Camera 3, astronomers aimed to discover thousands of emission nebulae – regions created when hot, young stars bathe the clouds of gas surrounding them in ultraviolet light, causing them to glow.

This is not the first time that NGC 1705 has been imaged by Hubble – astronomers peered into the heart of the galaxy in 1999 using Hubble's workhorse camera at the time, the Wide Field Planetary Camera 2. This instrument was replaced with the Wide Field Camera 3 during the fifth and final Space Shuttle mission to Hubble in 2009, and the newer instrument has provided a richer and far more detailed portrait of NGC 1705 than the 1999 observation.

Image Credit: ESA/Hubble and NASA, R. Chandar

Image enhancement: Jean-Baptiste Faure

Dwarf Irregular Galaxy NGC 1427A

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What happens when a galaxy falls in with the wrong crowd? The dwarf irregular galaxy NGC 1427A is a spectacular example of the resulting stellar rumble. Under the gravitational grasp of a large gang of galaxies, called the Fornax cluster, the small bluish galaxy is plunging headlong into the group at 600 kilometers per second or nearly 400 miles per second.

NGC 1427A, which is located some 62 million light-years away from Earth in the direction of the constellation Fornax, shows numerous hot, blue stars in this newly released image obtained by the Hubble Space Telescope. These blue stars have been formed very recently, showing that star formation is occurring extensively throughout the galaxy.

Galaxy clusters, like the Fornax cluster, contain hundreds or even thousands of individual galaxies. Within the Fornax cluster, there is a considerable amount of gas lying between the galaxies. When the gas within NGC 1427A collides with the Fornax gas, it is compressed to the point that it starts to collapse under its own gravity. This leads to formation of the myriad of new stars seen across NGC 1427A, which give the galaxy an overall arrowhead shape that appears to point in the direction of the galaxy's high-velocity motion. The tidal forces of nearby galaxies in the cluster may also play a role in triggering star formation on such a massive scale.

NGC 1427A will not survive long as an identifiable galaxy passing through the cluster. Within the next billion years, it will be completely disrupted, spilling its stars and remaining gas into intergalactic space within the Fornax cluster.

To the upper left of NGC 1427A is a background galaxy that happens to lie near Hubble's line of sight but is some 25 times further away. In contrast to the irregularly shaped NGC 1427A, the background galaxy is a magnificent spiral, somewhat similar to our own Milky Way. Stars are forming in its symmetric pinwheel-shaped spiral arms, which can be traced into the galaxy's bright nucleus. This galaxy is, however, less dominated by very young stars than NGC 1427A, giving it an overall yellower color. At even greater distances background galaxies of various shapes and colors are scattered across the Hubble image.

The Hubble Space Telescope's Advanced Camera for Surveys was used to obtain images of NGC 1427A in visible (green), red, and infrared filters. These images were then combined by the Hubble Heritage team to create the color rendition shown here. Astronomers are using the data to investigate the star-formation patterns throughout the object, to verify a prediction that there should be a relation between the ages of stars and their positions within the galaxy. This will help them understand how the gravitational influence of the cluster has affected the internal workings of this galaxy, and how this galaxy has responded to passing through the cluster environment.

The disruption of objects like NGC 1427A, and even larger galaxies like our own Milky Way, is an integral part of the formation and evolution of galaxy clusters. Such events are believed to have been very common during the early evolution of the universe, but the rate of galaxy destruction is tapering off at the present time. Thus the impending destruction of NGC 1427A provides a glimpse of an early and much more chaotic time in our universe.

Image Credit: NASA, ESA, and The Hubble Heritage Team STScI/AURA

Acknowledgment: M. Gregg (Univ. Calif.-Davis and Inst. for Geophysics and Planetary Physics, Lawrence Livermore Natl. Lab.)

Image enhancement: Jean-Baptiste Faure

Dwarf Irregular Galaxy NGC 6822: Barnard's Galaxy

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Barnard's Galaxy, a dwarf irregular galaxy neighboring the Milky Way, is revealed in this stunning image from the Víctor M. Blanco 4-m Telescope at the Cerro Tololo Inter-American Observatory, operated by NSF's NOIRLab. The image reveals regions of intense star formation and a scattering of immense cosmic bubbles.

Despite its small size, the galaxy contains some spellbinding cosmic objects. Glowing red regions of star formation are scattered throughout Barnard's Galaxy and indicate that incandescent star birth is widespread. Several other regions are well known in their own right and have been well studied since they were first detected by Edwin Hubble in 1925.

Image Credit: CTIO/NOIRLab/NSF/AURA

Acknowledgments: P. Massey (Lowell Obs.), G. Jacoby, K. Olsen, C. Smith (NOAO/AURA/NSF) and T.A. Rector (NRAO/AUI/NSF)

Image processing: Travis Rector (University of Alaska Anchorage), Mahdi Zamani and Davide de Martin

Interacting Galaxies NGC 4194: the Medusa Merger

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The galaxy pictured in this Hubble picture has an especially evocative name: the Medusa merger. Often referred to by its somewhat drier New General Catalogue designation of NGC 4194, this was not always one entity, but two. An early galaxy consumed a smaller gas-rich system, throwing out streams of stars and dust out into space. These streams, seen rising from the top of the merger galaxy, resembles the writhing snakes that Medusa, a monster in ancient Greek mythology, famously had on her head in place of hair, lending the object its intriguing name.

The legend of Medusa also held that anyone who saw her face would transform into stone. In this case, you can feast your eyes without fear on the centre of the merging galaxies, a region known as Medusa's eye. All the cool gas pooling here has triggered a burst of star formation, causing it to stand out brightly against the dark cosmic backdrop. The Medusa merger is located about 130 million light-years away in the constellation of Ursa Major (The Great Bear).

Image Credit: ESA/Hubble and NASA, A. Adamo

Image enhancement: Jean-Baptiste Faure

Barred Spiral Galaxy NGC 2188

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The blue and orange stars of the faint galaxy named NGC 2188 sparkle in this image taken with the Hubble Space Telescope. Although NGC 2188 appears at first glance to consist solely of a narrow band of stars, it is classified by astronomers as a barred-spiral galaxy. It appears this way from our viewpoint on Earth as the centre and spiral arms of the galaxy are tilted away from us, with only the very narrow outer edge of the galaxy's disc visible to us. Astronomers liken this occurrence to turning a dinner plate in your hands so you see only its outer edge. The true shape of the galaxy was identified by studying the distribution of the stars in the inner central bulge and outer disc and by observing the stars' colors.

NGC 2188 is estimated to be just half the size of our Milky Way, at 50 000 light-years across, and it is situated in the constellation of Columba (The Dove). Named in the late 1500s after Noah's dove in biblical stories, the small constellation consists of many faint yet beautiful stars and astronomical objects.

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

Image enhancement: Jean-Baptiste Faure

Interacting Spiral Galaxy NGC 4490

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This image, taken with the Hubble Space Telescope, shows the galaxy NGC 4490. The scattered and warped appearance of the galaxy are the result of a past cosmic collision with another galaxy, NGC 4485 (not visible in this image). The extreme tidal forces of the interaction between the two galaxies have carved out the shapes and properties of NGC 4490. Once a barred spiral galaxy, the outlying regions of NGC 4490 have been stretched out, resulting in its nickname of the Cocoon Galaxy. This interacting pair with the smaller irregular galaxy NGC 4485 have also been catalogued as Arp 269. This system is an isolated analog of the Magellanic Clouds and is surrounded by en enormous hydrogen cloud. The extreme tidal forces of their interaction have determined the shapes and properties of the two galaxies. Once a barred spiral galaxy, similar to the Milky Way, NGC 4490's outlying regions have been stretched out. Virtually no trace of its past spiral structure can be seen from our perspective.

NGC 4490 has a high star formation rate (SFR) and is surrounded by an enormous HI feature stretching about 60 kpc north and south of the optically visible galaxies. Both the driver for the high SFR in NGC 4490 and the formation mechanism of the HI structure are puzzling aspects of this system. Mid-infrared Spitzer data showed that NGC 4490 has a double nucleus morphology. One nucleus is visible in the optical, while the other is only visible at infrared and radio wavelengths. The optical nucleus and the potential infrared visible nucleus have similar sizes, masses, and luminosities. Both are comparable in mass and luminosity to other nuclei found in interacting galaxy pairs and much more massive and luminous compared with typical non-nuclear star-forming complexes. The double nucleus structure could also explain why the galaxy system is surrounded by an enormous cloud of hydrogen. It is likely that NGC 4490 is itself a merger remnant, which is now interacting with NGC 4485. This earlier encounter provides both a possible driver for extended star formation in NGC 4490, and multiple pathways for the formation of the extended HI plume.

Image Credit: ESA/Hubble and NASA

Acknowledgements: D. Calzetti (UMass) and the LEGUS Team, J. Maund (University of Sheffield), and R. Chandar (University of Toledo)

Image enhancement: Jean-Baptiste Faure

Peculiar Galaxy NGC 3256

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This image shows the galaxy NGC 3256. This picture was taken with the Wide Field Camera 3 (WFC3) and the Advanced Camera for Surveys (ACS), both installed on the Hubble Space Telescope. NGC 3256 is an impressive example of a peculiar galaxy that is actually the relict of a galactic merger of two separate galaxies that took place in a distant past. The telltale signs of the collision are two extended luminous tails swirling out from the galaxy.

NGC 3256 belongs to the Hydra-Centaurus supercluster complex and provides a nearby template for studying the properties of young star clusters in tidal tails. The system hides a double nucleus and a tangle of dust lanes in the central region. The tails are studded with a particularly high density of star clusters. The galaxy is about 100 million light-years from Earth. As such, NGC 3256 provides an ideal target to investigate starbursts that have been triggered by galaxy mergers.

Image Credit: ESA/Hubble, NASA

Image enhancement: Jean-Baptiste Faure

Dwarf Irregular Galaxy NGC 1156

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The galaxy NGC 1156 resembles a delicate cherry blossom tree flowering in springtime in this Hubble Picture of the Week. The many bright "blooms" within the galaxy are in fact stellar nurseries – regions where new stars are springing to life. Energetic light emitted by newborn stars in these regions streams outwards and encounters nearby pockets of hydrogen gas, causing it to glow with a characteristic pink hue.

NGC 1156 is located in the constellation of Aries (The Ram). It is classified as a dwarf irregular galaxy, meaning that it lacks a clear spiral or rounded shape, as other galaxies have, and is on the smaller side, albeit with a relatively large central region that is more densely packed with stars.

Some pockets of gas within NGC 1156 rotate in the opposite direction to the rest of the galaxy, suggesting that there has been a close encounter with another galaxy in NGC 1156's past. The gravity of this other galaxy – and the turbulent chaos of such an interaction – could have scrambled the likely more orderly rotation of material within NGC 1156, producing the odd behaviour we see today.

Image Credit: ESA/Hubble, NASA, R. Jansen

Image enhancement: Jean-Baptiste Faure

Irregular Galaxy NGC 4485

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The Hubble Space Telescope has taken a new look at the spectacular irregular galaxy NGC 4485, which has been warped and wound by its larger galactic neighbour. The gravity of the second galaxy has disrupted the ordered collection of stars, gas and dust, giving rise to an erratic region of newborn, hot, blue stars and chaotic clumps and streams of dust and gas. The irregular galaxy NGC 4485 has been involved in a dramatic gravitational interplay with its larger galactic neighbour NGC 4490 – out of frame to the bottom right in this image. Found about 30 million light-years away in the constellation of Canes Venatici (the Hunting Dogs), the strange result of these interacting galaxies has resulted in an entry in the Atlas of Peculiar galaxies: Arp 269.

Having already made their closest approach, NGC 4485 and NGC 4490 are now moving away from each other, vastly altered from their original states. Still engaged in a destructive yet creative dance, the gravitational force between them continues to warp each of them out of all recognition, while at the same time creating the conditions for huge regions of intense star formation. This galactic tug-of-war has created a stream of material about 25 000 light-years long which connects the two galaxies. The stream is made up of bright knots and huge pockets of gassy regions, as well as enormous regions of star formation in which young, massive, blue stars are born. Short-lived, however, these stars quickly run out of fuel and end their lives in dramatic explosions. While such an event seems to be purely destructive, it also enriches the cosmic environment with heavier elements and delivers new material to form a new generation of stars.

Two very different regions are now apparent in NGC 4485; on the left are hints of the galaxy's previous spiral structure, which was at one time undergoing "normal" galactic evolution. The right of the image reveals a portion of the galaxy ripped towards its larger neighbour, bursting with hot, blue stars and streams of dust and gas.

This image, captured by the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope, adds light through two new filters compared with an image released in 2014. The new data provide further insights into the complex and mysterious field of galaxy evolution.

Image Credit: ESA, NASA

Image enhancement: Jean-Baptiste Faure

Dwarf Irregular Galaxy IC 4710

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Discovered in 1900 by astronomer DeLisle Stewart and here imaged by the Hubble Space Telescope, IC 4710 is an undeniably spectacular sight. The galaxy is a busy cloud of bright stars, with bright pockets – marking bursts of new star formation – scattered around its edges.

IC 4710 is a dwarf irregular galaxy. As the name suggests, such galaxies are irregular and chaotic in appearance, lacking central bulges and spiral arms – they are distinctly different from spirals or ellipticals. It is thought that irregular galaxies may once have been spirals or ellipticals, but became distorted over time through external gravitational forces during interactions or mergers with other galaxies. Dwarf irregulars in particular are important to our overall understanding of galactic evolution, as they are thought to be similar to the first galaxies that formed in the Universe.

IC 4710 lies roughly 25 million light-years away in the southern constellation of Pavo (The Peacock). This constellation is located in the southern skies and also contains the third-brightest globular cluster in the sky, NGC 6752, the spiral galaxy NGC 6744, and six known planetary systems (including HD 181433 which is host to a super-Earth). The data used to create this image were gathered by Hubble's Advanced Camera for Surveys (ACS).

Image Credit: ESA/Hubble and NASA

Acknowledgements: Judy Schmidt

Image enhancement: Jean-Baptiste Faure

Dwarf Irregular Galaxy NGC 4861

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The constellation of Canes Venatici (The Hunting Dogs), is home to a variety of deep-sky objects – including this beautiful galaxy, known as NGC 4861. Astronomers are still debating on how to classify it: While its physical properties – such as mass, size and rotational velocity – indicate it to be a spiral galaxy, its appearance looks more like a comet with its dense, luminous "head" and dimmer "tail" trailing behind. Features more fitting with a dwarf irregular galaxy.

Although small and messy, galaxies like NGC 4861 provide astronomers with interesting opportunities for study. Small galaxies have lower gravitational potentials, which simply means that it takes less energy to move stuff about inside them than it does in other galaxies. As a result, moving in, around, and through such a tiny galaxy is quite easy to do, making them far more likely to be suffused with streams and outflows of speedy charged particles known as galactic winds, which can flood such galaxies with little effort.

These galactic winds can be powered by the ongoing process of star formation, which involves huge amounts of energy. New stars are springing into life within the bright, colourful ‘head’ of NGC 4861 and ejecting streams of high-speed particles as they do so, which flood outwards to join the wider galactic wind. While NGC 4861 would be a perfect candidate to study such winds, recent studies did not find any galactic winds in it.

Image Credit: ESA/Hubble and NASA

Image enhancement: Jean-Baptiste Faure

Interacting Galaxies NGC 3447

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Some galaxies are harder to classify than others. Here, Hubble's trusty Wide Field Camera 3 (WFC3) has captured a striking view of two interacting galaxies located some 60 million light-years away in the constellation of Leo (The Lion). The more diffuse and patchy blue glow covering the right side of the frame is known as NGC 3447 – sometimes NGC 3447B for clarity, as the name NGC 3447 can apply to the overall duo. The smaller clump to the upper left is known as NGC 3447A. The trouble with space is that it is, to state the obvious, really, really big. Astronomers have for hundreds of years been discovering and naming galaxies, stars, cosmic clouds and more. Unifying and regulating the conventions and classifications for everything ever observed is very difficult, especially when you get an ambiguous object like NGC 3447, which stubbornly defies easy categorisation. Overall, we know NGC 3447 comprises a couple of interacting galaxies, but we're unsure what each looked like before they began to tear one another apart. The two sit so close that they are strongly influenced and distorted by the gravitational forces between them, causing the galaxies to twist themselves into the unusual and unique shapes seen here. NGC 3447A appears to display the remnants of a central bar structure and some disrupted spiral arms, both properties characteristic of certain spiral galaxies. Some identify NGC 3447B as a former spiral galaxy, while others categorise it as being an irregular galaxy.

Image Credit: ESA/Hubble and NASA

Image enhancement: Jean-Baptiste Faure

Irregular Galaxy NGC 2337

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This galaxy, known as NGC 2337, resides 25 million light-years away in the constellation of Lynx. NGC 2337 is an irregular galaxy, meaning that it – along with a quarter of all galaxies in the Universe – lacks a distinct, regular appearance. The galaxy was discovered in 1877 by the French astronomer Édouard Stephan who, in the same year, discovered the galactic group Stephan's Quintet. Although irregular galaxies may never win a beauty prize when competing with their more symmetrical spiral and elliptical peers, astronomers consider them to be very important. Some irregular galaxies may have once fallen into one of the regular classes of the Hubble sequence, but were warped and deformed by a passing cosmic companion. As such, irregular galaxies provide astronomers with a valuable opportunity to learn more about galactic evolution and interaction. Despite the disruption, gravitational interactions between galaxies can kickstart star formation activity within the affected galaxies, which may explain the pockets of blue light scattered throughout NGC 2337. These patches and knots of blue signal the presence of young, newly formed, hot stars.

Image Credit: ESA/Hubble and NASA

Image enhancement: Jean-Baptiste Faure