November 24, 1999

Growth and Evolution of a Spiral Galaxy

Click on the image for a larger view.

Collage of images of the barred spiral galaxy NGC 1365.

The central image, taken from the ground in visible light, shows a thick bar that passes through the galaxy's central bulge. It also shows two arms that spiral in graceful arcs outward from the ends of the bar. Note the bright clumps of interstellar gas clouds and star clusters that characterize the bar and arms. Interwoven with the bright clouds and star clusters are regions of dark, light-absorbing dust.

The other two images show in more detail the central region of NGC 1365. The images were taken with the Wide Field and Planetary Camera 2 (WFPC2, green box) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS, red box) onboard the Hubble Space Telescope (HST).

The distance to NGC 1365 is estimated to be 60 million light-years.

Image Credit: Allan Sandage (The Observatories of the Carnegie Institution of Washington) and John Bedke (Computer Sciences Corporation and the Space Telescope Science Institute) (ground-based image); NASA and John Trauger (Jet Propulsion Laboratory) (WFPC2 image); and NASA, the European Space Agency (ESA), and C. Marcella Carollo (Johns Hopkins University and Columbia University) (NICMOS image).

The upper left image in the collage above (green box), taken with HST's WFPC2, registers mostly visible light; while the lower right image (red box), taken with NICMOS, is an infrared image. The information contained in these two images allows astronomers to determine several distinct components in the central region of NGC 1365.

Like many spiral galaxies, NGC 1365 hosts a compact, nearly spherical central nucleus, surrounded by a more extended, fuzzy-looking old component, the bulge, and by a disk of material. The bulge, which is best seen in the NICMOS image, bears a close resemblance to a small elliptical galaxy. Intermingled with the old stars of the galaxy's bulge, but also extending to the disk and far out along the spiral arms, are clusters of very bright, young stars, clouds of gas that are illuminated by the young stars, and enormous complexes of dark, light-absorbing dust, in many of which stars are currently forming.

The bar is the result of a dynamic instability in the disk of NGC 1365. Gas and dust are thought to be flowing inward along the bar toward the galaxy's inner region, enhancing the formation of dust clouds in that region and spawning the formation of new generations of stars, which, in turn, illuminate and brighten the clouds. As mass accumulates in the central region, it can, over the course of a few hundred million years, destroy the bar and thus prevent further accretion of material. Eventually, a new instability may develop, which causes the formation of another bar and further infall of matter toward the central region.

Many of the old stars in the central region of NGC 1365 are believed to date to the birth of the galaxy, roughly 10 billion years ago. This birth presumably started with the collapse of a huge intergalactic cloud (which consisted mostly of primordial hydrogen and some helium) and the formation of the galaxy's first generation of stars. Early on NGC 1365 probably resembled elliptical galaxies of that epoch. Subsequently, it accreted gas from the intergalactic medium, collided and merged with other gas-rich and probably small galaxies, and gradually evolved into the large spiral galaxy we see today.

Other large spiral galaxies, such as our own Milky Way, probably had a similar history as NGC 1365. In contrast, today's elliptical galaxies are believed to have grown much more rapidly by colliding and merging with bigger galaxies than did the spirals and forming most of their stars very early, often in dramatic starburst events (i.e., relatively brief phases of very intense star formation).

More Cool Stuff

For more information on the research reported here and additional information on the growth and evolution of galaxies, go to press release 99-34 of the Space Telescope Science Institute (STScI):

http://oposite.stsci.edu/pubinfo/pr/1999/34/index.html

We have featured galaxy collisions and star formation induced by such events before:

http://observe.ivv.nasa.gov/nasa/ootw/1998/ootw_980923/ob980923.html (Bright Ring of Young Stars, September 23, 1998)

http://observe.ivv.nasa.gov/nasa/ootw/1998/ootw_980617/ob980617.html (Galactic Cannibalism, June 17, 1998)

http://observe.ivv.nasa.gov/nasa/ootw/1998/ootw_980128/ob980128.html (Galaxies Under Construction, January 28, 1998)

http://observe.ivv.nasa.gov/nasa/ootw/1997/ootw_971112/ob971112.html (Fireworks in the Sky, November 12, 1997)

http://observe.ivv.nasa.gov/nasa/ootw/1997/ootw_970507/ob970507.html (Galaxy Encounter Spawns New Galaxies, May 7, 1997)

http://observe.ivv.nasa.gov/nasa/ootw/1996/ootw_960904/ob960904.html (Cosmic Collision, September 4, 1996)

As part of its Learning Technologies Project (LTP), NASA supports a number of educational Web sites that have excellent material on the space sciences:

http://observe.ivv.nasa.gov/nasa/education/edu/edudocs/topic_space.html