Image Archive: Galaxies

GOODS-S field (NIRCam image)

Wed, 29 May 2024 10:00:00 +0200

How did the first stars and galaxies form? The NASA/ESA/CSA James Webb Space Telescope is already providing new insights into this question. One of the largest programs in Webb’s first year of science is the JWST Advanced Deep Extragalactic Survey, or JADES, which will devote about 32 days of telescope time to uncover and characterize faint, distant galaxies. While the data are still coming in, JADES already has discovered hundreds of galaxies that existed when the Universe was less than 600 million years old. The team also has identified galaxies sparkling with a multitude of young, hot stars.

This infrared image shows a portion of an area of the sky known as GOODS-South, which has been well studied by the NASA/ESA Hubble Space Telescope and other observatories. More than 45,000 galaxies are visible here.

Using these and other data, the JADES team has discovered hundreds of galaxies that existed when the Universe was less than 600 million years old. The sheer number of these galaxies was far beyond predictions from observations made before Webb’s launch. The team also has identified galaxies that existed during a time known as the Epoch of Reionization, when the Universe underwent a transformation from opaque to transparent. Many of these galaxies shown unusually strong emission line signatures due to the creation of multitudes of hot, massive stars.

In this image, blue, green, and red were assigned to Webb’s NIRCam (Near-Infrared Camera) data at 0.9, 1.15, and 1.5 microns; 2.0, 2.77, and 3.55 microns; and 3.56, 4.1, and 4.44 microns (F090W, F115W, and F150W; F200W, F277W, and F335M; and F356W, F410M, and F444W), respectively.

The lights of a galactic bar

Mon, 27 May 2024 06:00:00 +0200

This week, an image of the broad and sweeping spiral galaxy NGC 4731 is the Hubble Picture of the Week. This galaxy lies among the galaxies of the Virgo cluster, in the constellation Virgo, and is located 43 million light-years from Earth. This highly detailed image was created using six different filters. The abundance of colour illustrates the galaxy's billowing clouds of gas, dark dust bands, bright pink star-forming regions and, most obviously, the long, glowing bar with trailing arms.

Barred spiral galaxies outnumber both regular spirals and elliptical galaxies put together, numbering around 60% of all galaxies. The visible bar structure is a result of orbits of stars and gas in the galaxy lining up, forming a dense region that individual stars move in and out of over time. This is the same process that maintains a galaxy's spiral arms, but it is somewhat more mysterious for bars: spiral galaxies seem to form bars in their centres as they mature, accounting for the large number of bars we see today, but can also lose them later on as the accumulated mass along the bar grows unstable. The orbital patterns and the gravitational interactions within a galaxy that sustain the bar also transport matter and energy into it, fuelling star formation. Indeed, the observing programme studying NGC 4731 seeks to investigate this flow of matter in galaxies.

Beyond the bar, the spiral arms of NGC 4731 stretch out far past the confines of this close-in Hubble view. The galaxy’s elongated arms are thought to result from gravitational interactions with other, nearby galaxies in the Virgo cluster.

[Image Description: A close-in view of a barred spiral galaxy. The bright, glowing bar crosses the centre of the galaxy, with spiral arms curving away from its ends and continuing out of view. It’s surrounded by bright patches of light where stars are forming, as well as dark lines of dust. The galaxy’s clouds of gas spread out from the arms and bar, giving way to a dark background with some foreground stars and small, distant galaxies.]

A jewel in the queen’s hair

Mon, 20 May 2024 06:00:00 +0200

This Picture of the Week shows the jewel-bright spiral galaxy NGC 4689, which lies 54 million light-years from Earth in the constellation Coma Berenices. This constellation has the distinction of being the only one of the 88 constellations officially recognised by the International Astronomical Union (IAU) to be named after an historical figure, Queen Berenice II of Egypt. The latin word ‘coma’ references her hair, meaning that NGC 4689 can be said to be found in the hair of a queen. Some people of Berenice’s time would have meant this quite literally, as the story goes that her court astronomer thought that a missing lock of Berenice’s hair had been catasterised (a word meaning ‘placed amongst the stars’) by the gods: hence the name of the constellation, Coma Berenices.

NGC 4689 holds an interesting — albeit less royal — place in modern astronomy too. The Universe is so incredibly vast that at a distance of a mere 54 million light-years NGC 4689 is relatively nearby for a galaxy. This image has been made using data from two sets of observations, one made in 2019 and 2024, both of which were made as a part of programmes that observed multiple ‘nearby’ galaxies. The 2024 observing programme is an interesting example of how Hubble — a relatively old but extraordinarily productive telescope — can support the work of the technologically cutting-edge Webb telescope. Observations collected by Webb stand to transform our understanding of how galaxies transform and evolve over time, by providing data of an unprecedented level of detail and clarity. However, thanks to their complementary capabilities, new observations from Hubble — such as those used to create this image — can assist the work done using Webb. In this case, the Hubble data were collected in order to get a more accurate grasp of the stellar populations of nearby galaxies, which is crucial to understanding the evolution of galaxies. Thus, NGC 4689 is playing an important role in developing our understanding of how all galaxies evolve. In fact, it is observed enough that it has been the subject of a Hubble Picture of the Week before, in 2020.

[Image Description: A spiral galaxy is viewed close up and fills most of the scene. It has a bright, glowing spot at the core, broad spiral arms that are covered by many dark threads of dust, and pink glowing spots across the disc that mark areas of star formation. The disc of the galaxy is surrounded by a faint halo that bleeds into the dark background.]

Lenticular dust in detail

Mon, 13 May 2024 06:00:00 +0200

Featured in this new image from the NASA/ESA Hubble Space Telescope is a nearly edge-on view of the lenticular galaxy NGC 4753. These galaxies have an elliptical shape and ill-defined spiral arms.

This image is the object's sharpest view to date, showcasing Hubble’s incredible resolving power and ability to reveal complex dust structures. NGC 4753 resides around 60 million light-years from Earth in the constellation Virgo and was first discovered by the astronomer William Herschel in 1784. It is a member of the NGC 4753 Group of galaxies within the Virgo II Cloud, which comprises roughly 100 galaxies and galaxy clusters.

This galaxy is believed to be the result of a galactic merger with a nearby dwarf galaxy roughly 1.3 billion years ago. NGC 4753’s distinct dust lanes around its nucleus are believed to have been accreted from this merger event.

It is now believed that most of the mass in the galaxy lies in a slightly flattened spherical halo of dark matter. Dark matter is a form of matter that cannot currently be observed directly, but is thought to comprise about 85% of all matter in the Universe. It is referred to as ‘dark’ because it does not appear to interact with the electromagnetic field, and therefore does not seem to emit, reflect or refract light.

This object is also of scientific interest to test different theories of formation of lenticular galaxies, given its low-density environment and complex structure. Furthermore, this galaxy has been host to two known Type Ia supernovae. These types of supernovae are extremely important as they are all caused by exploding white dwarfs which have companion stars, and always peak at the same brightness — 5 billion times brighter than the Sun. Knowing the true brightness of these events, and comparing this with their apparent brightness, gives astronomers a unique chance to measure distances in the Universe.

[Image Description: Lenticular galaxy NGC 4753 is featured with a bright white core and surrounding defined dust lanes around its nucleus, that predominantly appear dark brown in colour. A variety of faint stars fill the background of the image.]

A star forming factory

Mon, 06 May 2024 06:00:00 +0200

The celestial object showcased in this week's Hubble Picture of the Week is the spiral galaxy UGC 9684, which lies around 240 million light-years from Earth in the constellation Boötes. This image shows an impressive example of several classic galactic features, including a clear bar in the galaxy's centre, and a halo surrounding its disc.

The impetus for this Hubble image was a study into the host galaxies of Type-II supernovae. These cataclysmic stellar explosions take place throughout the Universe, and are of great interest to astronomers, so automated surveys scan the night sky and attempt to catch sight of them. The supernova which brought UGC 9684 to Hubble's attention occurred during 2020. It has faded from view in this image, which was taken in 2023.

Remarkably, the 2020 supernova in this galaxy isn't the only one that's been seen there — four supernova-like events have been spotted in UGC 9684 since 2006, putting it up there with the most active supernova-producing galaxies. It turns out that UGC 9684 is a quite active star-forming galaxy, calculated as producing one solar mass worth of stars every few years! This level of stellar formation makes UGC 9684 a veritable supernova factory, and a galaxy to watch for astronomers hoping to examine these exceptional events.

[Image Description: A spiral galaxy in the centre of a dark background, surrounded by a few distant galaxies and nearby stars. The galaxy is tilted diagonally and partially towards the viewer. Its disc is cloudy and threaded with dust, without clear arms. A bar of light extends across the disc from the glowing core. A faint halo of gas surrounds the disc.]

On the hunt for X-rays

Mon, 29 Apr 2024 06:00:00 +0200

Featured in this Hubble Picture of the Week this week is the dwarf galaxy IC 776. This swirling collection of stars new and old is located in the constellation Virgo — in fact, in the Virgo galaxy cluster — 100 million light-years from Earth. While a dwarf galaxy, it's also been classified as an SAB-type or ‘weakly barred’ spiral, one study naming it a “complex case” in morphology. This highly detailed view from Hubble demonstrates that complexity well. IC 776 has a ragged, disturbed disc that nevertheless looks to spiral around the core, and arcs of star-forming regions.

This image is from an observation programme dedicated to the study of dwarf galaxies in the Virgo cluster, searching for sources of X-rays in such galaxies. X-rays are often emitted by accretion discs, where material that is drawn into a compact object by gravity crashes together and forms a hot, glowing disc. The compact object can be a white dwarf or neutron star in a binary pair, stealing material from its companion star, or it can be the supermassive black hole at the heart of a galaxy, devouring all around it. Dwarf galaxies like IC 776, travelling through the Virgo cluster, experience a pressure from the intergalactic gas which can both stimulate star formation and feed the central black hole in a galaxy. That can create energetic accretion discs, hot enough to emit X-rays.

While Hubble is not able to see X-rays, it can coordinate with X-ray telescopes such as NASA’s Chandra, revealing the sources of this radiation in high resolution using visible light. Dwarf galaxies are thought to be very important for our understanding of cosmology and the evolution of galaxies. As with many areas of astronomy, the ability to examine these galaxies across the electromagnetic spectrum is critical to their study.

[Image Description: A spiral galaxy viewed tilted at a diagonal angle. The core and the disc of the galaxy are different colours, but are otherwise difficult to tell apart, with the disc having wispy, ragged edges and many arcs of glowing star-forming patches. A few distant galaxies can be seen in the background around the spiral galaxy, as well as several foreground stars.]

Links

Pan video of IC 776

Captured on glass

Mon, 22 Apr 2024 06:00:00 +0200

This Hubble Picture of the Week depicts the spiral galaxy ESO 422-41, which lies about 34 million light-years from Earth in the constellation Columba. The patchy, star-filled structure of the galaxy’s spiral arms and the glow from its dense core are laid out in intricate detail here by Hubble’s Advanced Camera for Surveys. Images of this galaxy have, however, a decades-long history.

The name ESO 422-41 comes from its identification in the European Southern Observatory (B) Atlas of the Southern Sky. In the times before automated sky surveys with space observatories such as ESA’s Gaia, many stars, galaxies and nebulae were discovered by means of large photographic surveys. Astronomers used the most advanced large telescopes of the time to produce hundreds of photographs, covering an area of the sky. They later studied the resulting photographs, attempting to catalogue all the new astronomical objects revealed.

In the 1970s a new telescope at ESO’s La Silla facility in Chile performed such a survey of the southern sky, which still had not been examined in as much depth as the sky in the north. At the time, the premier technology for recording images was glass plates treated with chemicals. The resulting collection of photographic plates became the ESO (B) Atlas of the Southern Sky. Astronomers at ESO and in Uppsala, Sweden collaborated to study the plates, recording hundreds of galaxies — ESO 422-41 being just one of those — star clusters, and nebulae. Many were new to astronomy.

Astronomical sky surveying has since transitioned through digital, computer-aided surveys such as the Sloan Digital Sky Survey and the Legacy Surveys, to surveys made by space telescopes including Gaia and the Wide-Field Infrared Survey Explorer. Even so, photographic sky surveys contributed immensely to astronomical knowledge for decades, and the archives of glass plates serve as an important historical reference for large swathes of the sky. Some are still actively used today, for instance to study variable stars through time. And the objects that these surveys revealed, including ESO 422-41, can now be studied in depth by telescopes such as Hubble.

[Image Description: A spiral galaxy, with a brightly shining core and two large arms. The arms are broad, faint overall and quite patchy, and feature several small bright spots where stars are forming. A few foreground stars with small diffraction spikes can be seen in front of the galaxy.]

Links

Pan video: ESO 422-41

Asteroid photobombs Hubble snapshot of Galaxy UGC 12158 (compass image)

Thu, 18 Apr 2024 16:00:00 +0200

This is an annotated NASA/ESA Hubble Space Telescope image of the barred spiral galaxy UGC 12158, with compass arrows, a scale bar, and colour key for reference. It looks like someone took a white marking pen to it. In reality it is a combination of time exposures of a foreground asteroid moving through Hubble’s field of view, photobombing the observation of the galaxy. Several exposures of the galaxy were taken, which is evidenced by the dashed pattern.

The asteroid appears as a curved trail as a result of parallax: Hubble is not stationary, but orbiting Earth, and this gives the illusion that the faint asteroid is swimming along a curved trajectory. The uncharted asteroid is inside the asteroid belt in our Solar System, and hence is 10 trillion times closer to Hubble than the background galaxy.

Rather than being a nuisance, this type of data is useful to astronomers for doing a census of the asteroid population in our Solar System.

[Image description: Annotated image of barred spiral galaxy UGC 12158 against the black background of space, with compass arrows, a scale bar, and colour key for reference. The galaxy has a pinwheel shape made up of bright blue stars wound around a yellow-white hub of central stars. The galaxy is tilted face-on to our view from Earth. A slightly S-shaped white line across the top is the Hubble image of an asteroid streaking across Hubble’s view. Indicated filters are expressed as: “F475W” in blue, “F606W” in green, and “F814W” in red. At the bottom left corner is a scale bar labelled “60,000 light-years” over “30 arcseconds.” At the bottom right corner, the “E” compass arrow points towards the 2 o’clock position. The “N” compass arrow points towards the 5 o’clock position.]

Asteroid photobombs Hubble snapshot of Galaxy UGC 12158

Thu, 18 Apr 2024 16:00:00 +0200

This NASA/ESA Hubble Space Telescope image of the barred spiral galaxy UGC 12158 looks like someone took a white marking pen to it. In reality it is a combination of time exposures of a foreground asteroid moving through Hubble’s field of view, photobombing the observation of the galaxy. Several exposures of the galaxy were taken, which is evidenced by the dashed pattern.

Rather than being a nuisance, this type of data is useful to astronomers for doing a census of the asteroid population in our Solar System.

[Image description: This is a Hubble Space Telescope image of the barred spiral galaxy UGC 12158. The majestic galaxy has a pinwheel shape made up of bright blue stars wound around a yellow-white hub of central stars. The hub has a slash of stars across it, called a bar. The galaxy is tilted face-on to our view from Earth. A slightly S-shaped white line across the top is the Hubble image of an asteroid streaking across Hubble’s view. It looks dashed because the image is a combination of several exposures of the asteroid flying by like a race car.]

The eponymous NGC 3783

Mon, 15 Apr 2024 06:00:00 +0200

This image features NGC 3783, a bright barred spiral galaxy about 130 million light-years from Earth, that also lends its name to the eponymous NGC 3783 galaxy group. Like galaxy clusters, galaxy groups are aggregates of gravitationally bound galaxies. Galaxy groups, however, are less massive and contain fewer members than galaxy clusters do: where galaxy clusters can contain hundreds or even thousands of constituent galaxies, galaxy groups do not typically include more than 50. The Milky Way is actually part of a galaxy group, known as the Local Group, which contains two other large galaxies (Andromeda and the Triangulum galaxy), as well as several dozen satellite and dwarf galaxies. The NGC 3783 galaxy group, meanwhile, contains 47 galaxies. It also seems to be at a fairly early stage of its evolution, making it an interesting object of study.

Whilst the focus of this image is the spiral galaxy NGC 3783, the eye is equally drawn to the very bright object in the lower right part of this image. This is the star HD 101274. The perspective in this image makes the star and the galaxy look like close companions, but this is an illusion. HD 101274 lies only about 1530 light-years from Earth, meaning it is about 85 thousand times closer than NGC 3783. This explains how a single star can appear to outshine an entire galaxy!

NGC 3783 is a type-1 Seyfert galaxy, which is a galaxy with a bright central region — so it’s particularly bright itself, as far as galaxies go. In this image it is recorded by Hubble in incredible detail, from its glowing central bar to its narrow, winding arms and the dust threaded through them, thanks to five separate images taken in different wavelengths of light. In fact, the galactic centre is bright enough to Hubble that it exhibits diffraction spikes, normally only seen on stars such as HD 101274.

[Image Description: A spiral galaxy, seen face-on to the viewer. The bright centre of the galaxy is crossed by a glowing bar, and it is surrounded by tightly-wound spiral arms, forming a circular shape with relatively clear edges. Faraway galaxies can be seen around it, along with a few bright stars, on a dark background. One star to the right of the galaxy is very large and extremely bright with long diffraction spikes around it.]

Hidden in a dark cloud

Mon, 08 Apr 2024 06:00:00 +0200

The subject of this week’s Picture of the Week 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 NASA/ESA/CSA 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 Description: A spiral galaxy seen nearly face-on. The disc is made up of many tightly wound spiral arms. They contain small strands of reddish dust, near the centre. On the left side, the disc features glowing patches of star formation. The whole right side, and part of the centre, is obscured by a large cloud of dark grey gas which crosses the image.]

Two’s company

Mon, 01 Apr 2024 06:00:00 +0200

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 Description: A large spiral galaxy with a smaller neighbouring galaxy. The spiral galaxy is wide and distorted, with colourful dust. Its companion lies close by it at the end of a spiral arm, to the lower left. A long, faint tail of stars reaches up from the right side of the spiral galaxy to the top of the image. Several small, distant galaxies can be seen in the background, as well as one bright star in the foreground.]

Links

Pan video: Arp 72

The Spider (and not its web)

Mon, 18 Mar 2024 06:00:00 +0100

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 Description: An irregular galaxy, consisting of a large central body of dull-coloured stars, with distorted arms around it. The arms are spotted with brightly glowing pink areas where stars are forming, and bluish gas that is brighter than the galactic core. Two large arms flank the left and right of the body, and smaller streams of stars emerge from the top. Other, distant, galaxies can be seen on the edges of the image.]

An unlikely spiral

Mon, 11 Mar 2024 06:00:00 +0100

This image shows LEDA 42160, a galaxy about 52 million light-years from Earth in the constellation Virgo. The dwarf galaxy is one of many forcing its way through the comparatively dense gas in the Virgo cluster, a massive cluster of galaxies. The pressure exerted by this intergalactic gas, known as ram pressure, has dramatic effects on star formation in LEDA 42160, which are presently being studied using the Hubble Space Telescope.

LEDA 42160 falls into the category of ‘Magellanic spiral galaxy’, or type Sm for short, under the de Vaucouleurs galaxy classification system. Magellanic spiral galaxies can be further sub-categorised as barred (SBm), unbarred (SAm) and weakly barred (SABm), where a ‘bar’ is an elongated bar-shape at a galaxy’s core. Generally speaking, Magellanic spiral galaxies are dwarf galaxies with only one single spiral arm. They are named after their prototype, the Large Magellanic Cloud, which is an SBm galaxy. Magellanic spiral galaxies are an interesting example of how galaxy categorisation is actually more nuanced than simply ‘spiral’, ‘elliptical’ or ‘irregular’.

[Image Description: A distorted dwarf galaxy, obscured by dust and by bright outbursts caused by star formation, floats roughly in the centre. A few distant galaxies are visible in the background around it, many as little spirals, and also including a prominent elliptical galaxy. A bright star hangs above the galaxy in the foreground, marked by cross-shaped diffraction spikes.]

A matter of perspective

Mon, 04 Mar 2024 06:00:00 +0100

Here we see NGC 4423, a galaxy that lies about 55 million light-years away in the constellation Virgo. In this image NGC 4423 appears to have quite an irregular, tubular form, so it might be surprising to find out that it is in fact a spiral galaxy. Knowing this, we can make out the denser central bulge of the galaxy, and the less crowded surrounding disc (the part that comprises the spiral arms).

If NGC 4423 were viewed face-on it would resemble the shape that we most associate with spiral galaxies: the spectacular curving arms sweeping out from a bright centre, interspersed with dimmer, darker, less populated regions. But when observing the skies we are constrained by the relative alignments between Earth and the objects that we are observing: we cannot simply reposition Earth so that we can get a better face-on view of NGC 4423!

Of course, celestial objects do not remain sedentary in space, but often move at extremely rapid velocities relative to one another. This might suggest that, should a galaxy be moving in a fortuitous direction relative to Earth, we might be able to view it from a substantially different perspective once it has moved far enough. This is theoretically possible, but the reality is that the distances in space are simply far too big, and human lifetimes far too short, for a noticeable difference in relative alignment to occur. In other words, this is more-or-less the view of NGC 4423 that we will always have!

[Image Description: A broad spiral galaxy is seen edge-on, so that its spiral arms can’t be seen. Visible dust and stars trace the disc of the galaxy, surrounded by a glowing halo above and below. The colour of the galaxy changes smoothly between the outer disc at the ends and the bulge in the centre. A few bright stars surround the galaxy on a dark background.]

A high amount of ram

Mon, 19 Feb 2024 06:00:00 +0100

This image features IC 3476, a dwarf galaxy that lies about 54 million light-years from Earth in the constellation Coma Berenices. Whilst this image does not look very dramatic — if we were to anthropomorphise the galaxy, we might say it looks almost serene — the actual physical events taking place in IC 3476 are highly energetic. In fact, the little galaxy is undergoing a process known as ram pressure stripping, which is driving unusually high levels of star formation within regions of the galaxy itself.

We tend to associate the letters ‘ram’ with the acronym RAM, which refers to Random Access Memory in computing. However, ram pressure has a totally distinct definition in physics: it is the pressure exerted on a body when it moves through some form of fluid, due to the overall resistance of the fluid. In the case of entire galaxies experiencing ram pressure, the galaxies are the ‘bodies’ and the intergalactic or intracluster medium (the dust and gas that permeates the space between galaxies, and for the latter the spaces between galaxies in clusters) is the ‘fluid’.

Ram pressure stripping occurs when the ram pressure results in gas being stripped from the galaxy. This stripping away of gas can lead to a reduction in the level of star formation, or even its complete cessation, as gas is absolutely key to the formation of stars. However, the ram pressure can also cause other parts of the galaxy to be compressed, which can actually boost star formation. This is what seems to be taking place in IC 3476: there seems to be absolutely no star formation going on at the edge of the galaxy bearing the brunt of the ram pressure stripping, but then star formation rates within deeper regions of the galaxy seem to be markedly above the average.

[Image Description: A dwarf spiral galaxy. The centre is not particularly bright and is covered by some dust, while the outer disc and halo wrap around as if swirling water. Across the face of the galaxy, an arc of brightly glowing spots marks areas where new stars are being formed. The galaxy is surrounded by tiny, distant galaxies on a dark background.]

Hubble traces 'string-of-pearls' star clusters in galaxy collisions

Thu, 08 Feb 2024 16:00:00 +0100

Contrary to what you might think, galaxy collisions do not destroy stars. In fact, the rough-and-tumble dynamics trigger new generations of stars, and presumably accompanying planets. Now the NASA/ESA Hubble Space Telescope has homed in on twelve interacting galaxies that have long, tadpole-like tidal tails of gas, dust, and a plethora of stars. Hubble's exquisite sharpness and sensitivity to ultraviolet light have uncovered 425 clusters of newborn stars along these tails — each cluster contains as many as one million blue, newborn stars.

Clusters in tidal tails have been known about for decades. When galaxies interact, gravitational tidal forces pull out long streamers of gas and dust. Two popular examples are the Antennae and Mice galaxies with their long, narrow, finger-like projections. This image depicts another example: galaxy Arp-Madore 1054-325.

A team of astronomers used a combination of new observations and archival data to get ages and masses of tidal tail star clusters. They found that these clusters are very young — only 10 million years old. And they seem to be forming at the same rate along tails stretching for thousands of light-years. "It's a surprise to see lots of the young objects in the tails. It tells us a lot about cluster formation efficiency," said lead author Michael Rodruck of Randolph-Macon College in Ashland, Virginia.

Before the mergers, the galaxies were rich in dusty clouds of molecular hydrogen that may have simply remained inert. But the clouds got jostled and bumped into each other during the encounters. This compressed the hydrogen to the point where it precipitated a firestorm of star birth.

The fate of these strung-out star clusters is uncertain. They may stay gravitationally intact and evolve into globular star clusters — like those that orbit outside the plane of our Milky Way galaxy. Or they may disperse to form a halo of stars around their host galaxy, or get cast off to become wandering intergalactic stars. This string-of-pearls star formation may have been more common in the early universe, when galaxies collided with each other more frequently.

[Image description: A Hubble Space Telescope image of galaxy AM 1054-325. It has been distorted into an S-shape from a normal pancake, spiral shape by the gravitational pull of a neighboring galaxy. Newborn star clusters have formed along a stretched-out tidal tail for thousands of light-years, resembling a string of pearls.]

Dim, but still distinct

Mon, 29 Jan 2024 06:00:00 +0100

This image of the spiral galaxy UGC 11105 is not as bright and vivid as some other Hubble Pictures of the Week. This softly luminous galaxy — lying in the constellation Hercules, about 110 million light-years from Earth — seems outshone by the sparkling foreground stars that surround it. The type II supernova which took place in this galaxy in 2019, while no longer visible in this image, definitely outshone the galaxy at the time! To be more precise, UGC 11105 has an apparent magnitude of around 13.6 in the optical light regime (this image was created using data that covers the heart of the optical regime, in addition to ultraviolet data). Astronomers have different ways of quantifying how bright celestial objects are, and apparent magnitude is one of them.

Firstly, the ‘apparent’ part of this quantity refers to the fact that apparent magnitude only describes how bright objects appear to be from Earth, which is not the same thing as measuring how bright they actually are. For example, in reality the variable star Betelgeuse is about 21 000 times brighter than our Sun, but because the Sun is much, much closer to Earth, Betelgeuse appears to be vastly less bright than it. The ‘magnitude’ part is a little harder to describe, because the magnitude scale does not have a unit associated with it, unlike, for example, mass, which we measure in kilograms, or length, which we measure in metres. Magnitude values only have meaning relative to other magnitude values. Furthermore, the scale is not linear, but is a type of mathematical scale known as ‘reverse logarithmic’, which also means that lower-magnitude objects are brighter than higher-magnitude objects.

As an example, UGC 11105 has an apparent magnitude of around 13.6 in the optical, whereas the Sun has an apparent magnitude of about -26.8. Accounting for the reverse logarithmic scale, this means that the Sun appears to be about 14 thousand trillion times brighter than UGC 11105 from our perspective here on Earth, even though UGC 11105 is an entire galaxy! The faintest stars that humans can see with the naked eye come in at about sixth magnitude, with most galaxies being much dimmer than this. Hubble, however, has been known to detect objects with apparent magnitudes up to the extraordinary value of 31, so UGC 11105 does not really present much of a challenge.

[Image Description: A spiral galaxy, with two prominent arms that are tightly wound around the brighter core. The arms disperse into a wide halo of stars and dust at their ends, giving the galaxy an oval shape. It is flanked by a number of bright stars in the foreground, each with a little cross over it due to light diffraction, and some distant background galaxies as well.]

Luminous in Lepus

Mon, 22 Jan 2024 06:00:00 +0100

This image shows the spiral galaxy IC 438, which lies about 130 million light-years from Earth in the constellation Lepus (the Hare). Lepus lies just south of the celestial equator (the ring around the middle of Earth that falls at right angles to its rotation axis). Appropriately, Lepus is flanked by the constellations Canis Major (the Greater Dog) and Orion (the Hunter), whilst Canis Minor (the Lesser Dog) lies very nearby, meaning that in artistic representations of the constellations, Lepus is often shown as being pursued by Orion and his two hunting dogs.

Lepus is one of the 88 constellations that are officially recognised by the International Astronomical Union (IAU). It is worth clarifying that, whilst the actual constellations themselves only comprise a handful of stars, the area of sky covered by those stars is often referred to using the name of the constellation. For example, when we say that IC 438 is in Lepus, we do not mean that the galaxy is part of the constellation — perhaps obviously, as it is not a single star, but an entire galaxy! Rather, we mean that it falls in the region of sky covered by the Lepus constellation stars.

The IAU’s 88 official constellations are by no means the only constellations ever described by humanity. Humans have been studying and naming the stars for a very long time, and different cultures of course have their own constellations. The IAU constellations are Eurocentric, with many taken from Ptolemy’s list of constellations. Collectively, the 88 constellations divide the night sky into 88 regions which completely cover it, so that the approximate location of any celestial object can be described using one of the 88.

The impetus behind Hubble examining this galaxy was a type Iax supernova that took place in 2017, a kind of supernova that arises from a binary system of two stars. While this data was obtained over three years after the supernova occurred, and so it’s not visible in this image, there’s still a lot to learn from studying the aftermath of supernovae like this one.

[Image Description: A large spiral galaxy seen close-up. The left side of the image shows the galaxy's core and its tightly-curled inner spiral arms. On the right side, one of the arms reaches down from above, curving across the dark background. There is a bright star inside the arc of the arm, and a couple more next to the galaxy.]

Gone but not forgotten

Mon, 15 Jan 2024 06:00:00 +0100

This image features a relatively small galaxy known as UGC 5189A, which is located about 150 million light-years away in the constellation Leo. This galaxy was observed by Hubble to study a supernova explosion in 2010 known as SN 2010jl. This particular supernova was notable for having been an exceptionally luminous supernova event. In fact, over a period of three years, SN 2010jl released at least 2.5 billion times more visible energy alone than our Sun emitted over the same timeframe across all wavelengths.

Even after supernovae have faded to non-observable levels, it can still be of interest to study the environments where they occurred. This can provide astronomers with valuable information: supernovae can take place for a variety of reasons, and understanding the environments in which they took place can help improve our understanding of the conditions necessary for them to be triggered. Furthermore, follow-up studies after supernovae can improve our understanding of the immediate aftermath of such events, from their potent effects on the gas and dust around them, to the stellar remnants they leave behind.

To this end, UGC 5189A has been observed many times by Hubble since 2010. This image is from data collected in three of the latest Hubble studies of UGC 5189A, which also examined several other relatively nearby galaxies that recently hosted supernovae — ‘relatively nearby’, in this context, meaning roughly 100 million light years away.

[Image Description: A galaxy that is flat and misshapen. Above and on its right it is covered by plumes of shining gas and dust, while its centre and left side are more dim and patchy. A trail of dark, dim dust stretches from below the galaxy up and off to the left, where there are three more bright patches. The background around the galaxy is quite dark, with only a few small background galaxies and one star visible.]

Hubble photographs home of farthest fast radio burst (annotated)

Tue, 09 Jan 2024 21:15:00 +0100

A Hubble Space Telescope image of the host galaxy of an exceptionally powerful Fast Radio Burst, FRB20220610A. Hubble’s sensitivity and sharpness reveals a compact group of multiple galaxies that may be in the process of merging. They existed when the Universe was only 5 billion years old. FRB 20220610A was first detected on June 10, 2022 by the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope in Western Australia, and confirmed to come from a distant origin by the European Southern Observatory’s Very Large Telescope in Chile.

[Image description: This image from the Hubble Space Telescope shows a field of blue, red, orange, yellow and white distant galaxies against the black backdrop of space. At image centre, a white inset box labelled “Host galaxy of FRB 20220610A” zooms in on a tight group of several galaxies of various elliptical shapes (to the far right). The white arrow inside the inset box points to the host galaxy of the exceptionally powerful fast radio burst 20220610A detected inside this galaxy group.]

Hubble photographs home of farthest fast radio burst (compass image)

Tue, 09 Jan 2024 21:15:00 +0100

[Image description: This image titled “Fast Radio Burst 20220610A HST WFC3” shows a field of blue, red, orange, yellow and white distant galaxies against the black backdrop of space. The white arrow inside the inset box points to the host galaxy of the exceptionally powerful fast radio burst 20220610A detected inside this galaxy group. The compass graphic at bottom right points to the object’s orientation on the celestial sphere. This image is a composite of separate exposures acquired by Hubble. The colour results from assigning different colours to each monochromatic (grayscale) image associated with an individual filter. The assigned colours listed in the upper right corner of the image are: red: F160W and blue: F606W.]

Hubble photographs home of farthest fast radio burst (clean image)

Tue, 09 Jan 2024 21:15:00 +0100

[Image description: This image from the Hubble Space Telescope shows a field of blue, red, orange, yellow and white distant galaxies against the black backdrop of space.]

When one plus one (eventually) equals one

Mon, 08 Jan 2024 06:00:00 +0100

This Hubble Picture of the Week features Arp 122, a peculiar galaxy that in fact comprises two galaxies — NGC 6040, the tilted, warped spiral galaxy and LEDA 59642, the round, face-on spiral — that are in the midst of a collision. This dramatic cosmic encounter is located at the very safe distance of roughly 570 million light-years from Earth. Peeking in at the corner is the elliptical galaxy NGC 6041, a central member of the galaxy cluster that Arp 122 resides in, but otherwise not participating in this monster merger.

Galactic collisions and mergers are monumentally energetic and dramatic events, but they take place on a very slow timescale. For example, the Milky Way is on track to collide with its nearest galactic neighbour, the Andromeda Galaxy (M31), but these two galaxies have a good four billion years to go before they actually meet. The process of colliding and merging will not be a quick one either: it might take hundreds of millions of years to unfold. These collisions take so long because of the truly massive distances involved.

Galaxies are composed of stars and their solar systems, dust and gas. In galactic collisions, therefore, these constituent components may experience enormous changes in the gravitational forces acting on them. In time, this completely changes the structure of the two (or more) colliding galaxies, and sometimes ultimately results in a single, merged galaxy. That may well be what results from the collision pictured in this image. Galaxies that result from mergers are thought to have a regular or elliptical structure, as the merging process disrupts more complex structures (such as those observed in spiral galaxies). It would be fascinating to know what Arp 122 will look like once this collision is complete . . . but that will not happen for a long, long time.

[Image Description: Two spiral galaxies are merging together at the right side of the image. One is seen face-on and is circular in shape. The other seems to lie in front of the first one. This galaxy is seen as a disc tilted away from the viewer and it is partially warped. In the lower-left corner, cut off by the frame, a large elliptical galaxy appears as light radiating from a point. Various small galaxies cover the background.]

So near, or so far?

Mon, 01 Jan 2024 06:00:00 +0100

What are we looking at when we study this image? A very distant galaxy that lies 19.5 billion light-years from Earth? Or a much closer luminous red galaxy that is a (relatively) small 2.7 billion light-years away? Or a third galaxy that appears to be fairly close to the second? The answer, perhaps confusingly, is that we are looking at all three. More precisely, we are looking at light emitted from all of those galaxies, even though the most distant galaxy lies directly behind the first as seen from Earth. In fact, it is that very alignment that makes the particular visuals in this image possible.

The central bright dot in this image is one of the closer galaxies, known by the lengthy — but informative — name of SDSS J020941.27+001558.4 (galaxy names in this format provide precise information about their location in the sky). The other bright dot above it — that appears to be intersecting a curving crescent of light — is SDSS J020941.23+001600.7, the second closer galaxy. And finally, that curving crescent of light itself is the ‘lensed’ light from the very distant galaxy. This is known as HerS J020941.1+001557, and it is also an interesting example of a phenomenon known as an Einstein ring.

Einstein rings occur when light from a very distant object is bent (or ‘lensed’) about a massive intermediate (or ‘lensing)’ object. This is possible because spacetime, the fabric of the Universe itself, is bent by mass, and therefore light travelling through spacetime is as well. This is much too subtle to be observed on a local level, but sometimes becomes clearly observable when dealing with curvatures of light on enormous, astronomical scales, for example, when the light emitted from a galaxy is bent around another galaxy or galaxy cluster. When the lensed object and the lensing object line up just so, the result is the distinctive Einstein ring shape, which appears as a full or partial circle of light around the lensing object, depending on how precise the alignment is. This partial Einstein ring is of particular interest as it was identified thanks to a citizen science project — SPACE WARPS — meaning that members of the public enabled the discovery of this object!

[Image Description: A field full of distant galaxies on a dark background. Most of the galaxies are very small, but there are a few larger galaxies and some stars where detail can be made out. In the very centre there is an elliptical galaxy with a brightly glowing core and a broad disc. A reddish, warped ring of light, thicker at one side, surrounds its core. A small galaxy intersects the ring as a bright dot.]