A picture-perfect pure-disc galaxy (w/ Video)

February 2, 2011
This picture of the spiral galaxy NGC 3621 was taken using the Wide Field Imager at ESO's La Silla Observatory in Chile. NGC 3621 is about 22 million light-years away in the constellation of Hydra (the Sea Snake). It is comparatively bright and can be well seen in moderate-sized telescopes. The data from the Wide Field Imager on the MPG/ESO 2.2-meter telescope at ESO's La Silla Observatory in Chile used to make this image were selected from the ESO archive by Joe DePasquale as part of the Hidden Treasures competition. Credit: ESO and Joe DePasquale

(PhysOrg.com) -- The bright galaxy NGC 3621, captured here using the Wide Field Imager on the 2.2-metre telescope at ESO’s La Silla Observatory in Chile, appears to be a fine example of a classical spiral. But it is in fact rather unusual: it does not have a central bulge and is therefore described as a pure-disc galaxy.

NGC 3621 is a spiral galaxy about 22 million light-years away in the constellation of Hydra (The Sea Snake). It is comparatively bright and can be seen well in moderate-sized telescopes. This picture was taken using the on the MPG/ESO 2.2-metre at ESO's in Chile. The data were selected from the ESO archive by Joe DePasquale as part of the Hidden Treasures competition. Joe's picture of NGC 3621 was ranked fourth in the competition.

This galaxy has a flat pancake shape, indicating that it hasn't yet come face to face with another galaxy as such a galactic collision would have disturbed the thin disc of stars, creating a small bulge in its centre. Most astronomers think that galaxies grow by merging with other galaxies, in a process called hierarchical galaxy formation. Over time, this should create large bulges in the centres of spirals. Recent research, however, has suggested that bulgeless, or pure-disc, spiral galaxies like NGC 3621 are actually fairly common.

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This zoom sequence starts with a view of the southern parts of the Milky Way. As we zoom in we can see the spiral galaxy NGC 3621, lying about 22 million light-years from us. The final detailed view shows a new image from the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory in Chile. The data used to make this image were selected from the ESO archive by Joe DePasquale as part of the Hidden Treasures competition. Credit: ESO/S. Brunier and Joe DePasquale. Music: John Dyson (from the album "Moonwind")

This galaxy is of further interest to astronomers because its relative proximity allows them to study a wide range of astronomical objects within it, including stellar nurseries, dust clouds, and pulsating stars called Cepheid variables, which astronomers use as distance markers in the Universe. In the late 1990s, NGC 3621 was one of 18 selected for a Key Project of the Hubble Space Telescope: to observe Cepheid variables and measure the rate of expansion of the Universe to a higher accuracy than had been possible before. In the successful project, 69 Cepheid variables were observed in this galaxy alone.

Multiple monochrome images taken through four different colour filters were combined to make this picture. Images taken through a blue filter have been coloured blue in the final picture, images through a yellow-green filter are shown as green and images through a red filter as dark orange. In addition images taken through a filter that isolates the glow of hydrogen gas have been coloured red. The total exposure times per filter were 30, 40, 40 and 40 minutes respectively.

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1 / 5 (2) Feb 02, 2011
Non-hierarchical galaxy formation more common than 'accepted' models permit?? Not surprising if galaxies grow from within: new matter and energy ejected periodically from the non-black 'black' hole in the core region. Likely many non-AGN galaxies have relatively small core stars, such as our Sagittarius 'A', of only a few million solar masses, and thereby are relatively quiescent in their outbursts, growing their galaxies at a slower rate. (Core ejection eruptions non-linearly proportional to local mass density.)

Such slower glowing galaxies are likely to produce more symmetric spirals, such as this pretty one. Hey, why not test this thesis?
1 / 5 (1) Feb 02, 2011
Given its relative brightness, one would expect that this galaxy formed in the near past, as opposed to some time closer to the BB.

If that is the case, perhaps the current speed of star-formation is what has prevented(so far) the formation of an AGN. With the solar wind of millions of stars pushing on the interstellar medium in pretty much every direction, it would be in a somewhat frothy state. Given time, though, one would expect a significant portion of the matter in the central region of this galaxy to eventually fall prey to increasing gravitational attraction, as its central stars age and die, shedding mass and velocity at the same time, and gradually spiralling centerwards, to eventually accrete into a growing black hole.

1 / 5 (1) Feb 23, 2011
And examining various spiral structures in "Spiral galaxies stripped bare" from Oct. 27, 2010.

If what is rotating (slowly) is the galactic core- rather than the entire galaxy - ejecting newly nucleated matter in preferred bipolar direction in the plane of the galaxy, we would see these nice spiral examples. Some cores rotate a little faster than others. Some stall out, or slow down the rotation, producing a bar structure. And some are fast, producing a more uniform spiral structure.

Stars in our galaxy have recently been show to be moving away from the core, rather than orbiting. Nucleation, periodic ejection, stellar condensation. Remember the recent Fermi bubbles.

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