Category Archives: galaxy interactions

astronomy, astronomy news, galactic history, galaxies, galaxy interactions

Disturbed Galaxies

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I Blame Gravity

The galaxies in this cosmic pairing, captured by the Wide Field Imager on the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile, display some curious features, demonstrating that each member of the duo is close enough to feel the distorting gravitational influence of the other. Courtesy ESO. Click to enlarge.

Take a look at the galaxies in this image. The one on the left, called NGC 3169, looks a little unsettled, not quite perfectly formed. The one on the right (NGC 3166) seems more blobby and its spiral arms aren’t quite as well-defined as, say, our Milky Way’s.

The reason they look this way?

Gravity. Both galaxies each have an extremely strong gravitational pull, and that plays a part in the cosmic dance they are undergoing.

As each galaxy feels the gravitational influence of the other, a push-pull tug-of-war is warping the spiral shape of one galaxy while fragmenting dust lanes in the other.

Spiral galaxies like NGC 3169 and NGC 3166 usually have arms of stars and dust that are arranged in a swirl around their central regions.  They stay in such configurations for quite a long time, until they have close encounters with other galaxies.

When galactic interactions happen, the combined gravity of the objects jumbles things up.  The classic spiral shape is stretched and pulled and sometimes torn apart, particularly when the galaxies merge. That’s what gravity does when massive systems of stars get close to each other during their mutual, lengthy cosmic dances.

The Advanced Camera for Surveys (ACS), the newest camera on NASA/ESA Hubble Space Telescope, has captured a spectacular pair of galaxies engaged in a celestial dance of cat and mouse or, in this case, mouse and mouse. Located 300 million light-years away in the constellation Coma Berenices, the colliding galaxies have been nicknamed "The Mice" because of the long tails of stars and gas emanating from each galaxy. Otherwise known as NGC 4676, the pair will eventually merge into a single giant galaxy. Courtesy Hubble Space Telescope.

Unlike the two galaxies shown in the Hubble image above, NGC 3169 and NGC 3166 aren’t yet in a full-out merger. Their close passage toward each other has only begun the transformation they may ultimately undergo. NGC 3169’s arms, shining bright with big, young, blue stars, have been teased apart, and lots of luminous gas has been drawn out from its disc. In NGC 3166’s case, the dust lanes that also usually outline spiral arms are in disarray. Unlike its bluer counterpart, NGC 3166 is not forming many new stars. In a few million years, these two galaxies could look very, very different — and, when their merger (if they have one) is complete, there’ll be an elliptical galaxy left where two majestic spirals once existed.  That’s what gravity will do to large-scale stellar systems!  For more information on this gorgeous image, visit the ESO web site writeup. There’s way more to these galaxies than meets the casual glance.

galaxies, galaxy interactions, galaxy shapes, Gemini Observatory

Not Your Typical Science Fair Project

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Australian Students and Gemini Observatory

Image of NGC 6872 (left) and companion galaxy IC 4970 (right) locked in a tango as the two galaxies gravitationally interact. The galaxies lie about 200 million light-years away in the direction of the constellation Pavo (the Peacock). Image credit: Sydney Girls High School Astronomy Club, Travis Rector (University of Alaska, Anchorage), Ángel López-Sánchez (Australian Astronomical Observatory/Macquarie University), and the Australian Gemini Office.

Remember back when you entered the science fair at school? I remember a winning entry that I did — something about yeast that my mother helped cook up.  I didn’t get to go get my ribbon for that one because I was at home with measles. Nowadays, kids get vaccinated for measles (the smart parents all do this), and so having spotty bumps on one’s skin is no excuse for missing out on a science fair award.

A group of students from Sydney Girls High School in Australia went way beyond science fair with their idea — to use Gemini Observatory to study a pair of interacting galaxies called NGC 6872 and IC 4970. They were participating in a country-wide contest to suggest scientifically interesting and aesthetically pleasing objects for the observatory’s telescope to snare with its 8-meter mirror.

The contest sponsors liked their proposal so much that it was selected as the winner, and the image at right was the result. The main instrument used to make this image was the Gemini Multi-Object Spectrograph (GMOS), in its imaging mode on the Gemini South telescope in Chile.

The primary galaxy in the image (NGC 6872) shows what happens when galaxies interact and their original structure and form is distorted. When galaxies like these get too close to each other, the mutual gravitational pull starts to distort their structures. Their spiral arms get stretched out to enormous distances, with streamers of starburst knots following along. In NGC 6872, the arms have been stretched out to span hundreds of thousands of light-years—many times further than the spiral arms of our own Milky Way galaxy.

Over hundreds of millions of years, NGC 6872’s arms will fall back toward the central part of the galaxy, and the companion galaxy (IC 4970) will eventually be merged into NGC 6872. The coalescence of galaxies often leads to a burst of new star formation. Already, the blue light of recently created star clusters dot the outer reaches of NGC 6872’s elongated arms. Dark fingers of dust and gas along the arms soak up the visible light. That dust and gas is the raw material out of which future generations of stars could be born, and possibly even countless numbers of planets.

The search for these dynamic changes in galaxy structure was what sold the selection committee on the students’ observation proposal. They wrote, “If enough color data is obtained in the image it may reveal easily accessible information about the different populations of stars, star formation, relative rate of star formation due to the interaction, and the extent of dust and gas present in these galaxies.”

The team also presented a more emotional perspective by looking at the impact this image might have on people trying to understand our place in the universe. When viewers consider this image “in contrast to their daily life,” the team explained, “there is a significant possibility of a new awareness or perception of the age and scale of the universe, and their part in it.”

When I look at this picture, I really envy those kids.  Imagine getting to have your science project executed on one of the world’s premier telescopes!