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Gemini Probes Red Stars in M82 Using Adaptive Optics

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A Canadian/Gemini team led by Tim Davidge (Herzberg Institute of Astrophysics) used the ALTAIR adaptive optics system with the Near Infrared Imager (NIRI) on the Frederick C. Gillett Gemini North Telescope to probe the red stellar content in the starburst galaxy M-82. The field studied is at a projected distance of 1 kiloparsec above the disk plane of the galaxy. This amorphous, nearly edge-on galaxy is the third brightest member of a nearby collection of galaxies called the M-81 group which is located about 12 million light-years from our Galaxy.

The interactions of galaxies in the M-81 group spur bursts of star formation in its members and M-82 appears to now be experiencing such a violent outburst. This activity in the disk of M-82 appears to drive a bipolar galactic wind that feeds gas to the outer regions of the galaxy. This work indicates that recent star formation in M-82 may not be restricted to the disk plane because of the possible detection of these bright asymptotic giant branch (AGB) stars away from the disk. Various mechanisms may explain star forming events away from the disk.

This image shows a wide-field view (from a sky survey) of the starburst galaxy M-82. A small inset image zooms in on a specific guide star used for the Gemini observations.
Figure 1. Image of a 10' x 10' field (from Digital Sky Survey) centered on the starburst galaxy M-82 showing the location of the star USNO 1575-03026118, which was used as the adaptive optics guide star for the Gemini observations. The 22" field image by ALTAIR/NIRI is comparable in size to the stellar disk of the star in this image.
This close-up of M-82 shows stars (likely AGB or RGB stars) with sharp details (FWHM = 0.08").
Figure 2. Portion of the final K' ALTAIR/NIRI image of the M-82 field. The inner circle has a radius of 3" and the outer circle has a radius of 8". The stars in this image have a FWHM = 0.08" and are AGB or RGB stars belonging to M-82.

The ALTAIR/NIRI observations were made over a 22" x 22" field using the f/32 mode of NIRI delivering an image scale of 0.022" per pixel. The data have an angular resolution of 0.08" FWHM. Individual AGB and red giant branch (RGB) stars are resolved.

 Based on the brightness of AGB stars and the brightness of the tip of the RGB, the authors conclude that the ALTAIR field they studied in M-82 is not dominated by an old population of stars. Instead, the brightness of the AGB stars suggest that they formed during intermediate epochs, possibly after the last major interaction with M-81 giving the stars in the ALTAIR field an age of about 1 billion years.

This plot compares the number of stars at different brightnesses (luminosity functions) in two regions: M-82 (solid line) and a nearby galaxy NGC 6822 (dotted line).
Figure 3. Completeness-corrected MK luminosity functions of the M-82 ALTAIR field (solid line) compared with the composite LF of outer disk fields in the nearby galaxy NGC 6822 (dotted line). Differences in LF indicate that the stars in the ALTAIR field of M-82 are younger than those in NGC 6822.

More details can be found in the paper "Deep Near-Infrared Imaging of a Field in the Outer Disk of M82 with the Altair Adaptive Optics System on Gemini-North", by T. J. Davidge, J. Stoesz, F. Rigaut, J.-P. Veran and G. Herriot, in PASP 116, 1-8, 2004 January.

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