Anne’s Image of Today: Spiral Galaxy NGC 7090


July 14, 2014

NGC 7090, a spiral galaxy in Indus

NGC 7090, a spiral galaxy in Indus

Image Credit: ESA/Hubble & NASA; Acknowledgement: R. Tugral
Text: mostly by ESA/Hubble & NASA

NGC 7090 is a spiral galaxy, probably a barred spiral galaxy, located about 27 million light-years away from Earth in the southern constellation of Indus (the Indian), while it is receding from us at approximately 847 kilometers per second. The English astronomer John Herschel first observed this galaxy on 4 October 1834.

The galaxy is viewed edge-on from the Earth, meaning we cannot easily see the spiral arms, which are full of young, hot stars. However, our side-on view shows the galaxy’s disk and the bulging central core, where typically a large group of cool old stars are packed in a compact, spheroidal region. In addition, there are two interesting features present in the image that are worth mentioning.

First, we are able to distinguish an intricate pattern of pinkish red regions over the whole galaxy. This indicates the presence of clouds of hydrogen gas. These structures trace the location of ongoing star formation, visual confirmation of recent studies that classify NGC 7090 as an actively star-forming galaxy.

Second, we observe dust lanes, depicted as dark regions inside the disk of the galaxy. In NGC 7090, these regions are mostly located in lower half of the galaxy, showing an intricate filamentary structure. Looking from the outside in through the whole disk, the light emitted from the bright center of the galaxy is absorbed by the dust, silhouetting the dusty regions against the bright light in the background.

Dust in our galaxy, the Milky Way, has been one of the worst enemies of observational astronomers for decades. But this does not mean that these regions are quite blind spots in the sky. At near-infrared wavelengths — slightly longer wavelengths than visible light — this dust is largely transparent and astronomers are able to study what is really behind it. At still longer wavelengths, the realm of radio astronomy, the dust itself can actually be observed, letting astronomers study the structure and properties of dust clouds and their relationship with star formation.

The image was taken using the Wide Field Channel of the Advanced Camera for Surveys aboard the Hubble Space Telescope and combines orange light (coloured blue here), infrared (coloured red) and emissions from glowing hydrogen gas (also in red).

A version of this image of NGC 7090 was entered into the Hubble’s Hidden Treasures Image Processing Competition by contestant Rasid Tugral. Hidden Treasures is an initiative to invite astronomy enthusiasts to search the Hubble archive for stunning images that have never been seen by the general public. The competition is now closed.

Anne’s Image of the Day: Barred Spiral Galaxy UGC 12158


June 17, 2014

UGC 12158, a barred spiral galaxy in Pegasus

UGC 12158, a barred spiral galaxy in Pegasus

Image Credit: ESA/Hubble & NASA

UGC 12158 is a barred spiral galaxy of about 140,000 light-years across, located some 400 million light-years away from Earth in the northern constellation of Pegasus (the Winged Horse). It is speeding away from us at approximately 9,289 kilometers per second. This galaxy looks like our Milky Way’s identical twin in its structure, only it is almost 40% (!!) larger than our home galaxy, so it is really huge.

UGC 12158 is an excellent example of a barred spiral galaxy. Barred spirals, as the name suggest, feature spectacular swirling arms of stars that emanate from a bar-shaped center. Such bar structures are common, being found in about two thirds of spiral galaxies, and are thought to act as funnels, guiding gas to their galactic centers where it accumulates to form newborn stars. These aren’t permanent structures: astronomers think that they slowly disperse over time, so that the galaxies eventually evolve into regular spirals.

In this image we can also see a bright blue star just to the lower left of the center of the galaxy. This star is in fact a Type Ia supernova, called SN 2004ef, which was first spotted by the two British amateur astronomers Mark Armstrong and Tom Boles on September 4, 2004. The Hubble data shown here form part of the follow-up observations.

A Type Ia supernova is a result from the violent explosion of a white dwarf star (a compact star that has ceased fusion in its core). The white dwarf increases its mass beyond a critical limit (the Chandrasekhar limit) by gobbling up matter from a companion star. A runaway nuclear explosion then makes the star suddenly as bright as a whole galaxy, before gradually fading from view.

This picture was created from images taken with the Wide Field Channel of Hubble’s Advanced Camera for Surveys. Images through blue (F475W, coloured blue), yellow (F606W, coloured green) and red (F814W, coloured red) as well as a filter that isolates the light from glowing hydrogen (F658W, also coloured red) have been included. The exposure times were 1160 s, 700 s, 700 s and 1200 s respectively.

Anne’s Image of the Day: Spiral Galaxy NGC 4921


April 28, 2014

NGC 4921, a barred spiral galaxy in Coma Berenices

UGC 8134

Image Credit: NASA / ESA / Roberto Colombari / Hubble Legacy Archive

NGC 4921 (also known as UGC 8134) is a barred spiral galaxy, located about 320 million light-years away from Earth in the northern constellation of Coma Berenices (Berenice’s Hair), while it is speeding away from us at approximately 5,482 kilometers per second. It lies near the center of the Coma Cluster, a large cluster of galaxies that contains over 1,000 galaxies.

Visible in the galaxy are, from the center, a nucleus with a bright bar that is surrounded by a prominent ring of dark dust. Then a band that contains blue clusters of recently formed stars. The outer part consists of unusually smooth, poorly distinguished spiral arms. Surrounding the galaxy are several smaller companion galaxies, many unrelated galaxies in the far distant universe, and unrelated stars in our own Milky Way Galaxy.

This galaxy is categorized as an anemic galaxy because of its low rate of star formation, its remarkably diffuse spiral arms and low surface brightness. Nonetheless, it is the brightest spiral galaxy in the Coma Cluster.

NGC 4921 is low in non-ionized hydrogen gas. The distribution of the gas has also been deeply perturbed toward the south-east spiral arm and is less extended than the optical disk of the galaxy. This may have been caused by interaction with the intergalactic medium, which is stripping off the gas. This ram stripping, as it’s called, is common with spiral galaxies in dense clusters; as they move through the cluster environment.

On May 4, 1959, a supernova explosion, called SN 1959b, was observed in this galaxy. It reached an estimated peak magnitude of 18.5.

This image is taken with the Hubble Space Telescope.

Anne’s Image of the Day: The Whale Galaxy


April 23, 2014

NGC 4631, a barred spiral galaxy in Canes Venatici

The Whale Galaxy, Arp 281

Image Credit & Copyright: Dieter Beer & Patrick Hochleitner

NGC 4631 (also known as the Whale Galaxy) is an edge-on, barred spiral galaxy of about 140,000 light-years across that lies some 30 million light-years away from Earth in the small northern constellation of Canes Venatici (the Hunting Dogs), while it is receding from us at approximately 606 kilometers per second. This galaxy’s slightly distorted wedge shape gives it the appearance of a whale, hence its nickname.

This galaxy has a nearby companion, named NGC 4627, a dwarf elliptical galaxy which lies just above it. Together also called Arp 281, this interacting galaxy pair was listed in the Atlas of Peculiar Galaxies as an example of a “double galaxy”. Arp’s Atlas of Peculiar Galaxies is a catalog of 338 peculiar galaxies produced by Halton Arp in 1966.

NGC 4631 is also a gravitationally bound companion of NGC 4656 (a barred spiral galaxy of some 75 thousand light-years across and about 29 million light years away from Earth), which lies about half a degree to the southeast, and could be within half a million light-years of its neighbor. Their interaction is probably responsible for the curved end of the galaxy (which is listed as NGC 4657), and its nickname, the Hockey Stick.

NGC 4631,NGC 4627,NGC 4656 and NGC 4657 are all four part of the NGC 4631 Group of galaxies. Estimates of the number of galaxies in this group range from 5 to 27, and all studies identify very different member galaxies for this group.

The Whale Galaxy is a bright, large and extended galaxy with a very bright nucleus. It contains a central starburst, which is a region of intense star formation. The strong star formation is evident in the emission from ionized hydrogen and interstellar dust heated by the stars formed in the starburst.

The most massive stars that form in star formation regions only live for a short period of time, after which they explode as supernovae. So many supernovae have exploded in the center of NGC 4631 that they are blowing gas out of the plane of the galaxy. This superwind can be seen in X-rays and in spectral line emission. The gas from this superwind has produced a giant, diffuse corona of hot, X-ray emitting gas around the whole galaxy.

Because this nearby galaxy is seen edge-on from Earth, professional astronomers observe this galaxy to better understand the gas and stars located outside the plane of the galaxy. It can be seen with a small telescope. North is up in this image.

Anne’s Image of the Day: Spiral Galaxy IC 2560


March 27, 2014

IC 2560, a barred spiral galaxy in Antlia

IC 2560, a barred spiral galaxy in Antlia

Image Credit: Nick Rose, ESA/Hubble & NASA

IC 2560 is a barred spiral galaxy with a diameter of about 121,000 by 78,000 light-years, located over 110 million light-years away from Earth in the southern constellation of Antlia (the Air Pump). It is speeding away from us at approximately 2925 kilometers per second.

The constellation of Antlia was originally named antlia pneumatica by French astronomer Abbé Nicolas Louis de Lacaille, in honor of the invention of the air pump in the 17th century.

However, IC 2560 is a relatively nearby spiral galaxy, and it is easy to spot its spiral arms, its dark dust lanes and barred structure in this image. This galaxy is part of the Antlia Cluster, a group of over 200 galaxies held together by gravity. The Antlia cluster is unusual; unlike most other galaxy clusters, it appears to have no dominant galaxy within it.

IC 2560 is classified as a Seyfert II galaxy, a kind of spiral galaxy characterized by an extremely bright nucleus and very strong emission lines from certain elements, such as hydrogen, helium, nitrogen, and oxygen. The centers of Seyfert galaxies have active galactic nuclei (AGN), and usually contain supermassive black holes with masses between 10 and 100 million solar masses. Seyferts are classified as Type I or II, depending upon whether the spectra show both narrow and broad emission lines (Type I), or only narrow lines (Type II).

Indeed, the bright center of IC 2560 is thought to be caused by the ejection of huge amounts of super-hot gas from the region around a central black hole.

This image is taken with the Wide Field Planetary Camera 3 onboard the Hubble Space Telescope using an infrared, an optical and an ultraviolet filter. A version of this image was entered into the Hubble’s Hidden Treasures image processing competition by contestant Nick Rose.