Starship Asterisk*

A close-in view of a spiral galaxy. It is seen face-on, showing its circular shape and tightly winding spiral arms. The galaxy glows brightly in the centre and dims to cool colours towards the edge. Dark, faint filaments of dust and brightly glowing, pink and orange bubbles of star formation mark the face of the galaxy. (Image Credit: ESA/Hubble & NASA, R. Chandar, J. Lee and the PHANGS-HST team

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This glittering image shows the spiral galaxy IC 5332, which lies about 30 million light-years away in the constellation Sculptor, and has an almost face-on orientation to Earth. To explain what is meant by ‘face-on’, it is helpful to visualise a spiral galaxy as an (extremely) large disc. If the galaxy is oriented so that it appears circular and disc-shaped from our perspective here on Earth, then we can say that it is ‘face-on’. In contrast, if it is oriented so that it appears squashed and oval-shaped, then we would say that it is ‘edge-on’. The key thing is that the same galaxy would look extremely different from our perspective depending on whether it was face-on or edge-on as seen from Earth. Check out these previous Hubble Pictures of the Week for examples of another face-on spiral galaxy and an almost edge-on spiral galaxy.

IC 5332 is designated as an SABc-type galaxy in the De Vaucouleurs system of galaxy classification. The ‘S’ is straightforward, identifying it as a spiral galaxy, which it clearly is, given the well-defined arms of bright stars and darker dust that curl outwards from the galaxy’s dense and bright core. The ‘AB’ is a little more complex. It means that the galaxy is weakly barred, which refers to the shape of the galaxy’s centre. The majority of spiral galaxies do not spiral out from a single point, but rather from an elongated bar-type structure. SAB galaxies — which are also known as intermediate spiral galaxies — do not have a clear bar-shape at their core, but also do not spiral out from a single point, instead falling somewhere in between. The lowercase ‘c’ describes how tightly wound the spiral arms are: ‘a’ would indicate very tightly wound, and ‘d’ very loosely wound. Thus, IC 5332 is quite an intermediate spiral galaxy on many fronts: weakly barred, with quite loosely wound arms, and almost completely face-on!

Image Credit: ESO/A. Ghizzi Panizza (www.albertoghizzipanizza.com)

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Standing majestically at the top of a hill at ESO’s La Silla Observatory, surveying the watercolour scenery of another sunset in Chile’s Atacama Desert, is the New Technology Telescope (NTT). The star of this Picture of the Week has been ticking along, making discovery after discovery, ever since it was inaugurated in 1989. Its home at La Silla sits at an altitude of 2400 metres and is far from sources of light pollution, giving the NTT uninterrupted views of the Universe.

This plucky telescope has a 3.58-metre primary mirror, which it uses to observe the cosmos in optical and near-infrared wavelengths of light. Currently, the NTT is equipped with two instruments: the Son of ISAAC (SOFI) and the ESO Faint Object Spectrograph and Camera 2 (EFOSC2). The former received its unusual name because of its similarity to an instrument on ESO’s Very Large Telescope (VLT, located further north) that went by the name ISAAC. Instruments like SOFI that observe infrared light can pierce through cosmic dust better than facilities observing at other wavelengths, peeling back a veil shrouding the Universe.

As for EFOSC2, it has had a rather nomadic life: it started operating at the NTT in 1989 and then it was relocated twice to other telescopes at La Silla before returning to the NTT in 2008. Having undergone several improvements over the years, it remains a highly versatile instrument at La Silla.

Image Credit: Gemini Observatory/NOIRLab/NSF/AURA/T. Matsopoulos

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From the vantage point of Gemini North, one half of the International Gemini Observatory, operated by NSF’s NOIRLab, a small rainbow can be seen sprouting from behind smaller peaks near the summit of Maunakea. One of the five volcanoes constituting the Big Island of Hawai‘i, Maunakea rises to an elevation of 4205 meters (13,786 feet). At that elevation, tropical clouds infrequently cover the summit, but they often roll past at lower altitude, as seen here, bringing some moisture to the dry area. These water droplets suspended in the air would typically be invisible, but their presence is betrayed when light is refracted through them. The droplets become like prisms, slowing and then separating the almost white sunlight into its component colors. The result is a spectacular spectrum of light — a rainbow — that is beautifully complimented by the red soil of the volcano and deep blue of the sky.

Two galaxies are prominent among many much smaller background galaxies in the darkness of space. The larger galaxy is an elliptical galaxy, radiating light in a perfectly even sphere from a bright centre. The smaller galaxy is a barred spiral, with arms that are wispy like fog connected to a bar crossing the galaxy’s shining core. The shape of the arms makes the smaller galaxy notably squarish (Credit: ESA/Hubble & NASA, M. West)

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This Picture of the Week prominently features two galaxies: NGC 3558 in the lower left, and LEDA 83465 in the upper right. Both galaxies lie roughly 450 million light years from Earth. The two galaxies are separated from one another by a distance of roughly 150 000 light years, which might sound vast, until we consider that our nearest galactic neighbour — the Andromeda galaxy — is a whopping 2.5 million light years distant from the Milky Way galaxy. In galactic terms, the two galaxies pictured here are practically on top of one another.

This is because they belong to a crowded and chaotic galaxy cluster known as Abell 1185, which is packed with galaxies that are interacting with one another via gravity. These galactic interactions have sometimes led to dramatic results, such as galaxies being torn apart completely. This fate has not befallen NGC 3558, which currently retains its integrity as both an elliptical galaxy and a low-ionisation nuclear emission-line region, or LINER. In fact, it probably attained its present form by devouring smaller galaxies in the cluster — galaxies much like LEDA 83465.

LINERs are a particular type of galactic nucleus or core, and are distinguished by the chemical fingerprints written into the light that they emit. As their name suggests, LINERs emit light which suggests that many of the atoms and molecules within these galactic cores have either been weakly ionised or not ionised at all. Ionisation is the process by which atoms or molecules lose or gain electrons. In galaxies, it is driven by a variety of processes — from shockwaves travelling through galaxies, to radiation from massive stars or from hot gas in accretion discs. In the case of LINERs, this means that many of the atoms and molecules within the galaxies have lost either a single electron, or have retained all their electrons. The mechanism that drives this weak ionisation in LINERs such as NGC 3558 is still debated amongst astronomers.

Image Credit: ESO/A. Ghizzi Panizza (www.albertoghizzipanizza.com)

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The white pearl on the left side of this Picture of the Week is one of the Exoplanets in Transits and their Atmospheres telescopes — ExTrA for short. Located at ESO’s La Silla Observatory at an altitude of 2375m, ExTrA is a French national project comprising three 60 centimetre telescopes that are working towards one goal: to detect Earth-sized planets in our home galaxy, the Milky Way.

ExTrA focuses on planets around very small stars, with a radius down to a tenth of the Sun. It relies on the so-called transit method, looking for slight dips in the light emitted from a star when a planet passes in front of it, like a tiny eclipse. The smaller the star is, the larger the dip an Earth-sized planet will cause. Each telescope observes a target star and four reference ones to calibrate the effects of Earth’s atmosphere. The telescopes can observe either the same patch of the sky or different ones; the light they collect is sent to a single infrared spectrograph that breaks it up into different colours or wavelengths, thus combining the classic transit method with spectral information.

Not only do ExTra telescopes make great exoplanet hunters, they also make pretty good picture frames. Just kidding, but can you see the telescope peaking through the frame of the door of the dome? It’s the ESO 3.6-metre telescope. Also, ExTrA are not the only triplets at La Silla: in this image we can also see the three BlackGEM telescopes.