Anne’s Picture of the Day: Galaxy Group HCG 44

March 26, 2013

Hickson Compact Group 44, a small group of galaxies in Leo

Hickson Compact Group 44, NGC 3190 Group

Image Credit: The MASIL Imaging Team

Hickson Compact Group 44 (also known as the NGC 3190 Group) is a small group of bright galaxies containing three spiral galaxies and an elliptical galaxy, located about 60-100 million light-years away in the constellation of Leo.

A Hickson Compact Group (HCG for short) is a collection of four or five gravitational bound galaxies in close physical proximity to one another, published in a list of 100 objects (462 galaxies) by the Canadian astronomer Paul Hickson in 1982. These groups usually contain large quantities of diffuse gas and are dominated by dark matter. Strong galaxy interactions result and merging is expected to lead to the formation of one, most likely elliptical, larger galaxy.

They are among the densest concentrations of galaxies known, comparable to the centers of rich galaxy clusters. Compact groups are worthy of intense study as they provide a rich opportunity to study galaxy interactions and mergers.

Compact groups are relatively short lived entities that form via mergers of galaxies within loose subsystems and groupings. Simulations predict that merging of the group members should proceed rapidly within one billion years. Hickson groups are therefore snapshots at various stages in this merging process. They may represent an intermediate stage between loose groups and individual galaxies. A better understanding of the nature of HCGs could help explain galaxy formation on a larger scale in the early Universe. Compact groups are surprisingly numerous, and may play a significant role in galaxy evolution.

The four members of Hickson Compact Group 44 are NGC 3190 (in the center), 3185 (the lower right), 3187 (the upper right), and NGC 3193 (the upper left). Two dwarf spiral galaxies named PGC 86788 (to the right of the center) and PGC 2806871 (the lower left) are also part of the group but are not included in the Hickson catalog. NGC 3190, together with NGC 3187 and NGC 3193 from Arp 316.

NGC 3190, the dominant member, is a striking spiral galaxy of some 75,000 light-years across with a characteristic dust lane that we see nearly edge-on. NGC 3187 is an S-shaped spiral with a prominent central bar. NGC 3185 is also a barred spiral, but its bar is subtle and much smaller relative to its overall size. NGC 3193 is a good example of an elliptical galaxy.

The galaxies in the Hickson Compact Group 44 are close enough that they interact with each other gravitationally. This causes the individual galaxies to become distorted and in some cases even share material.

If you look closely, you may notice a faint halo around NGC 3190, a considerable warping of its dust lane on the side nearer to NGC 3187, and also a very subtle smudge of light between NGC 3190 and NGC 3193, which is a bridge of stars being shared between the two giants as they embrace in their cosmic dance. Finally, NGC 3187 shows numerous tidal tails well above and below its disk plane, which are also signs of tidal encounters between members of HCG 44.

In this image you can also see a few bright foreground stars and hundreds of tiny yellowish and reddish objects, which are extremely distant galaxies, hundreds of millions of light-years away. North is up.

Anne’s Picture of the Day: Spiral Galaxy NGC 3190

December 23, 2012

NGC 3190, a spiral galaxy in Leo

NGC 3190, a spiral galaxy in Leo

Image Credit: ESO

NGC 3190 is a spiral galaxy of about 75,000 light-years across, located around 79 million light-years away in the constellation of Leo, while it is hurrying away from us at roughly 1271 kilometers per second. It is the largest member of the Hickson Compact Group 44, a small galaxy group that also includes the elliptical galaxy NGC 3193 and the two barred spiral galaxies NGC 3187 and NGC 3185.

The galaxies in the Hickson 44 group are either slowly merging to form a giant elliptical galaxy, or gravitationally pulling each other apart. Anyway, NGC 3190 shows signs of gravitational interaction. Its dust lane is warped on the side nearer to NGC 3187, and there is also a very subtle smudge of light between NGC 3190 and NGC 3193, what seems to be a bridge of stars being shared between the two giants as they embrace in their cosmic dance (not visible in this image).

NGC 3190, that we see nearly edge-on, has tightly wound arms that appear asymmetric around the center, likely caused by gravitational tidal interactions with other members of the group, while the galactic disk also appears warped. Finely textured dust lanes are surrounding the brightly glowing, compact active galactic nucleus, that is thought to host a supermassive black hole. The galaxy is surrounded by a faint halo.

In March 2002, a Type Ia supernova (SN 2002bo) was found in between the ‘V’ of the dust lanes in the southeastern part of NGC 3190. It was caught almost two weeks before reaching its maximum brightness, allowing astronomers to study its evolution. The image presented here was taken about a year after the maximum of the supernova which is 50 times fainter on the image than a year before.

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 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 category of supernovae produces consistent peak luminosity. The stability of this luminosity allows these supernovae to be used as standard candles to measure the distance to their host galaxies because the visual magnitude of the supernovae depends primarily on the distance.

In May 2002, another Type Ia supernova (SN 2002cv) was discovered, on the other side of NGC 3190. Two supernovae of this type appearing nearly simultaneously in the same galaxy is a rare event, as normally astronomers expect only one such event per century in a galaxy. SN 2002cv was best visible at infrared wavelengths as it was superimposed on the dust lane of NGC 3190, and therefore hidden by a large quantity of dust. In fact, this supernova holds the record for the most obscured Type Ia event.

This image of NGC 3190, in which many background galaxies are also visible, is a colour composite based on images taken with the Very Large Telescope, obtained on 26 March 2003. North is up and East is to the left.