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January 10th, 2000

A Close Look at Arp 299

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A Close Look at Arp 299

Credit: Image courtesy of O. Lai¹ & D. Rouan²

¹Canada-France-Hawaii Telescope Corporation
²DESPA, Observatoire de Paris, France

(for more details see: Lai et al., 1999, Astronomy & Astrophysics, Vol. 351, p. 834-840)

Our Sun is located somewhere in one of the spiral arms of our Galaxy, the Milky Way. Our Milky Way itself is surrounded by other galaxies, like Andromeda (see the image of the week of Dec 6th, 1999 for example). These form what we call the Local Group. But somewhere out there, farther out, there are millions of millions of other galaxies and some of them are extraordinarily luminous, especially in the infrared, emitting more than one thousand billion times the luminosity of our Sun!!! For comparison, our Galaxy which is a normal but fairly large galaxy emits at least one hundred times less... These amazingly bright objects, called Ultraluminous Infrared Galaxies were discovered by the IRAS Satellite. Their geometry is in general complicated, resulting for interactions and/or collisions with other galaxies.

Understanding them still poses a challenge to the astronomers. For example, where do they take their energy from? A hidden new-born Quasar or an extreme flash of star formation? A group of French astronomers have recently used the CFHT to image Arp 299, one such Ultraluminous Infrared Galaxy. As expected, Arp 299 is a merging galaxy, i.e., two galaxies colliding together. An optical image showing this interaction (extracted from the NGS/Palomar Sky Survey) can be seen here. Arp 299 is also known as NGC 3690 or Mkn 171 A & B. One of the authors' images, is shown as this week's image. The most striking result is that Markarian 171 A, at left and the most active source of the merging system, appears elongated in the near infrared, at 2.2microns. This result likely rules out a new-born Quasar as the energy source because a Quasar would be too small and would remain unresolved. On the other hand, an intense zone of star formation is expected to be more extended and resolved, as detected on the image. Also, the high angular resolution provided by PUEO allowed to image and confirm the starburst nature of numerous other zones of intense star formation located in Mkn 171 B, on the right hand side on the image. They are seen as bright red knots.

All these results support the idea that a flash of star formation is the power source for the extremely bright Arp 299.

Technical description: This is a color-composite mosaic image that covers a square area of 40 arcsecond on a side. The individual frames were obtained on December 14-17th, 1997, with PUEO, the adaptative Optics Bonnette, attached to the Canada-France-Hawaii telescope. The images were obtained in the near-infrared part of the electromagnetic spectrum, with the KIR camera. The individual frames were obtained in two filters: the J and K bands at 1.25 and 2.2 microns, respectively. 4 images of 600sec make the final J image, and 8 images of 200sec were obtained in K-band. Here in the final color-composite image, J-band is coded in blue and K-band in red. The "blue" zones are brighter at J and trace the stars we see unobscured. The light from object burried deeper inside Arp 299 is absorbed. The K-band is more appropriate to probe deep inside and the embedded active star forming regions are brighter in K and color-coded in red here. The individual J and K images are shown in this panel for comparison.

next week: Cassiopeiae A: A Young Supernova Remnant

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CFHT is funded by the Governments of Canada and France, and by the University of Hawaii.