A study of structure in M33 using adaptive optics

Abstract

FASTTRAC (Fast Adaptive Secondary for Tip-Tilt Removal by Automatic Centroiding) is a tip-tilt secondary system which increases the angular resolution of images taken at Steward Observatory's 90" Bok and 61" Bigelow telescopes. K band imagery is gathered with the facility infrared camera and wavefront sensing is done with a small format charge-coupled device (CCD). I examine desirable characteristics of wavefront-sensing CCDs and evaluate the performance of the device used in FASTTRAC according to those criteria. The main drawback of the device is its low quantum efficiency due to frontside illumination. The read noise of the system is adequate, particularly for FASTTRAC which is generally assigned to bright time. The increased angular resolution provided by FASTTRAC is desirable for imaging crowded fields, such as those found in Local Group galaxies. Stellar photometry is a more powerful tool for studying the structure of these galaxies than is surface photometry, in which the light from all types of stars is mixed together. In particular, the distribution of old stars is representative of the underlying mass distribution, so these stars may be used to measure the overdensity in the arms of spiral galaxies. FASTTRAC was used to observe fields in M33, the nearest spiral which is not seen edge-on. These fields were observed in a range of seeing conditions and with guide stars of varying magnitudes and positions relative to the fields of interest. I analyze the performance of FASTTRAC in these varying conditions and offer some advice to future FASTTRAC observers. I also analyze the crowding in the M33 fields and conclude that, to K∼16.5, it does not vary significantly with placement in or outside of a spiral arm. Therefore a coarsely-sampled, wide field survey of the populations of M33 will not be systematically biased by crowding. Therefore a survey covering 35' by 25' was conducted in I and K bands, covering all of M33 out to a deprojected radius of 16'. The resulting star catalogs reach a depth of about I=21.5 and Ks When plotted on a color-magnitude diagram, these stars separate into a young population of red supergiants and an older population of red giants. The giants are old enough to have experienced many orbits and are thus representative of the mass distribution of the galaxy. The fraction of K-band light which is "young" is only 5-10% indicating that surface photometry in general would not be terribly biased, but there are local variations in which the young component is much stronger. Fourier decomposition of the old catalog reveals significant amplitudes (up to 0.5), indicating that the galaxy does indeed contain local overdensities, which supports the basic idea of spiral density wave theory. The strongest component in M33 is one-armed or lopsidedness, with some power in the two-armed component and much less in the higher-order components. The two-armed component may in fact represent a bar in the inner few arcminutes.