CFHT, Current Image of the Week


May 15th, 2000

Exploring the bar of NGC 2903

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Exploring the bar of NGC 2903

Credit: Image courtesy of P. Martin
Canada-France-Hawaii Telescope (Kamuela, Hawaii)

Roughly speaking there are as many galaxies in the Universe as there are stars in our Galaxies, something like 100 billions... Galaxies can have various shapes, from elliptical to spiral. Among the great family of spiral galaxies, about two third have a central stellar bar. These bars, first identified more than 100 years ago by astronomers visually observing "nebulae" with the largest telescopes available, play an important role in the evolution of galaxies. Bars are complex systems and thanks to new observations and sophisticated numerical models developed in the last 20 years, considerable progress has been made in understanding their morphology, dynamics, formation and evolution. Bars are, of course, principally formed from stars but some of them have also large amounts of interstellar dust, neutral hydrogen, ionized and cold molecular gas. They have also different morphological properties (e.g. amplitude, length, global shape). Our own Galaxy, the Milky Way, has a small bar in its center.

Bars affect galaxy evolution. Introducing a bar in a spiral disk, for instance from an interaction with another galaxy, will result in some dramatic changes for the host galaxy. When the bar is formed, motions of a large fraction of the stars in the galaxy become chaotic. For the interstellar gas clouds, these chaotic motions are also induced. Collisions between the clouds in the system disk become frequent and violent. Models show that the clouds then start moving all over the galaxy disk: there is a radial redistribution of the gas. In particular, a large quantity of gas will fall along the bar and might eventually reach the galaxy center. This is why it is believe that bars could be an important factor in feeding up massive black holes with interstellar material in the center of numerous galaxies. However, to better understand if this mechanism is plausible or not, it is necessary to study what happens to the gas falling along the bar toward the central region.

This week's image shows the central region of NGC 2903, a strong barred spiral galaxy observed by a team of astronomers using the CFHT and the integral field spectrograph OASIS. NGC 2903 is among a larger sample of barred spirals observed with OASIS to study star formation mechanisms in their bars. The red "spots" seen in the left image are mostly all regions of massive star formation, that is, clusters of stars formed from the gas clouds that were accumulated near the galaxy center after falling along the strong bar. OASIS allows the astronomers to derive the characteristics of these star forming regions from spectroscopy. Some example of these spectra are shown in the small red plots. From these spectral lines, physical proprieties like the density, the rate of star formation, the abundance, and the velocity (superimposed in white contours) of these regions can be studied. As shown here, OASIS has been used to study about 20 fields all along the bar of NGC 2903 allowing the astronomers to evaluate how these proprieties are changing according to the position within the bar. Each field of OASIS contains about 1200 spectra so in total, more than 20000 spectra were obtained on NGC 2903!!

Technical description:

These observations have been obtained in February 2000 at CFHT with OASIS, an integral field spectrograph built by Lyon Observatory. It is made of an array of 1200 micro-lenses, each of about 1.2 mm in diameter, located in front of a normal spectrographic stage. Each lens produces a small image of a portion of the field observed and a grism spreads the light for each image. Thus, 1200 spectra are observed at once on one CCD frame! When attached to the adaptive optics bonnette at CFHT, which corrects for the atmospheric turbulence, the instrument is used to obtain very precise spectroscopic information on small sources. OASIS is an unique instrument with capabilities still unsurpassed to study in great detail the central regions of galaxies...

next week: Arp 105: The Guitar, or the Art of Cosmic Recycling!

editors: François Ménard & Jean-Charles Cuillandre
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CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.