Emission Versus Absorption: For this Picture of the Week, the NASA/ESA Hubble Space Telescope turned its powerful eye towards an emission line galaxy called NGC 3749. When astronomers explore the contents and constituent parts of a galaxy somewhere in the Universe, they use various techniques and tools. One of these is to spread out the incoming light from that galaxy into a spectrum and explore its properties. This is done in much the same way as a glass prism spreads white light into its constituent wavelengths to create a rainbow. By hunting for specific signs of emission from various elements within a galaxy’s spectrum of light — so-called emission lines — or, conversely, the signs of  absorption from other elements — so-called absorption lines — astronomers can start to deduce what might be happening within. If a galaxy’s spectrum shows many absorption lines and few emission lines, this suggests that its star-forming material has been depleted and that its stars are mainly old, while the opposite suggests it might be bursting with star formation and energetic stellar newborns. This technique known as spectroscopy, can tell us about a galaxy’s type and composition, the density and temperature of any emitting gas, the star formation rate, or how massive the galaxy’s central black hole might be. While not all galaxies display strong emission lines, NGC 3749 does! It lies over 135 million light-years away, and is moderately luminous. The galaxy has been used a “control” in studies of especially active and luminous galaxies — those with centres known as active galactic nuclei, which emit copious amounts of intense radiation. In comparison to these active cousins, NGC 3749 is classified as inactive, and has no known signs of nuclear activity.

Name: NGC 3749
Type: Early Universe : Galaxy
Category: Galaxies
Constellation: Centaurus
Distance: 140 million light years
Release date: 18 November 2019, 06:00
Credit: ESA/Hubble & NASA, D. Rosario et al.

A Close Relationship: Some galaxies are closer friends than others. While many live their own separate, solitary lives, others stray a little too close to a near neighbour and take their relationship to the next level. The two galaxies featured in this Picture of the Week, named NGC 6285 (left) and NGC 6286 (right), have done just that! Together, the duo is named Arp 293 and they are interacting, their mutual gravitational attraction pulling wisps of gas and streams of dust from them, distorting their shapes, and gently smudging and blurring their appearances on the sky — to Earth-based observers, at least. The NASA/ESA Hubble Space Telescope has viewed a number of interacting pairs. These can have distinctive, beautiful, and downright odd shapes, ranging from sheet music to a spaceship entering a sci-fi-esque wormhole, a bouquet of celestial blooms, and a penguin fiercely guarding its precious egg. Arp 293 is located in the constellation of Draco (The Dragon), and lies over 250 million light-years from Earth.

Name: Arp 293, NGC 6285, NGC 6286
Type: Local Universe : Galaxy : Type : InteractingEarly Universe : Galaxy : Type : Spiral
Category: Galaxies
Constellation: Draco
Distance: 250 million light years
Release date: 25 November 2019, 06:00
Credit: ESA/Hubble & NASA, K. Larson et al.

A Dramatic Demise: Some of the most dramatic events in the Universe occur when certain stars die — and explode catastrophically in the process. Such explosions, known as supernovae, mainly occur in a couple of ways: either a massive star depletes its fuel at the end of its life, become dynamically unstable and unable to support its bulk, collapses inwards, and then violently explodes; or a white dwarf in an orbiting stellar couple syphons more mass off its companion than it is able to support, igniting runaway nuclear fusion in its core and beginning the supernova process. Both types result in an intensely bright object in the sky that can rival the light of a whole galaxy. In the last 20 years the galaxy NGC 5468, visible in this image, has hosted a number of observed supernovae of both the aforementioned types: SN 1999cp, SN 2002cr, SN2002ed, SN2005P, and SN2018dfg. Despite being just over 130 million light-years away, the orientation of the galaxy with respect to us makes it easier to spot these new ‘stars’ as they appear; we see NGC 5468 face on, meaning we can see the galaxy’s loose, open spiral pattern in beautiful detail in images such as this one from the NASA/ESA Hubble Space Telescope.

Name: NGC 5468
Type: Local Universe : Star : Evolutionary Stage : SupernovaLocal Universe : Galaxy : Type : Spiral
Category: GalaxiesStars
Constellation: Virgo
Distance: 130 million light years
Release date: 2 December 2019, 06:00
Credit: ESA/Hubble & NASA, A. Riess et al. Acknowledgements: Judy Schmidt (Geckzilla)

Galactic Diversity: NGC 3175 is located around 50 million light-years away in the constellation of Antlia (The Air Pump). The galaxy can be seen slicing across the frame in this image from the NASA/ESA Hubble Space Telescope, with its mix of bright patches of glowing gas, dark lanes of dust, bright core, and whirling, pinwheeling arms coming together to paint a beautiful celestial scene. The galaxy is the eponymous member of the NGC 3175 group, which has been called a nearby analogue for the Local Group. The Local Group contains our very own home galaxy, the Milky Way, and around 50 others — a mix of spiral, irregular, and dwarf galaxies. The NGC 3175 group contains a couple of large spiral galaxies — the subject of this image, and NGC 3137 — and numerous lower-mass spiral and satellite galaxies. Galaxy groups are some of the most common galactic gatherings in the cosmos, and they comprise 50 or so galaxies all bound together by gravity. This image comprises observations from Hubble’s Wide Field Camera 3.

Name: NGC 3175
Type: Local Universe : Galaxy : Type : Spiral
Category: Galaxies
Constellation: Antlia
Distance: 50 million light years
Release date: 9 December 2019, 06:00
Credit: ESA/Hubble & NASA, D. Rosario et al.

Discs and Bulges: This image from the NASA/ESA Hubble Space Telescope shows IC 2051, a galaxy in the southern constellation of Mensa (The Table Mountain), lying about 85 million light-years away. It is a spiral galaxy, as evidenced by its characteristic whirling, pinwheeling arms, and it has a bar of stars slicing through its centre. This galaxy was observed for a Hubble study on galactic bulges, the bright round central region of spiral galaxies. Spiral galaxies like IC 2051 are shaped a bit like flying saucers when seen from the side; they comprise a thin, flat disc, with a bulky bulge of stars in the centre that extends above and below the disc. These bulges are thought to play a key role in how galaxies evolve, and to influence the growth of the supermassive black holes lurking at the centres of most spirals. While more observations are needed in this area, studies suggest that some, or even most, galactic bulges may be complex composite structures rather than simple ones, with a mix of spherical, disc-like, or boxy components, potentially leading to a wide array of bulge morphologies in the Universe. This image comprises data from Hubble’s Wide Field Camera 3 at visible and infrared wavelengths.

Name: IC 2051
Type: Local Universe : Galaxy : Type : Spiral
Category: Galaxies
Constellation: Mensa
Distance: 85 million light years
Release date: 16 December 2019, 06:00
Credit: ESA/Hubble & NASA, P. Erwin et al.