Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994aas...185.2217g&link_type=abstract
American Astronomical Society Meeting, 185, #22.17
Astronomy and Astrophysics
Astronomy
Scientific paper
A new survey of UBV photometry and absolute proper motions to B ~ 22.5 was carried out in a small field at the SGP near NGC 288. The photometry was based on CTIO 4 m and 0.9 m CCD observations, and ultraviolet excesses and photometric parallaxes were obtained for several hundred stars. The proper motions were derived from the CTIO 4 m prime focus plates taken from 1981 to 1992. Using an astrographic plate as a flat field, the optical field angle distortion of the CTIO 4 m prime focus corrector was accurately modeled. The magnitude and color equations were carefully investigated and corrected using selected NGC 288 members as reference stars. The zero point of the absolute proper motion was defined by several hundred galaxies to an accuracy of +/-0.2 mas yr(-1) and the stellar tangential velocities and dispersions were derived. This has enabled us to probe the stellar spatial, kinematical and chemical distributions to very high vertical distances. It was shown that the thick disk as an intermediate component is necessary and distinct from the old thin disk and spheroidal halo. The thick disk has a scale height of 1245+/-95 pc, a local density ratio to the thin disk of 0.03+/-0.01, a mean asymmetric drift velocity of -60+/-5 km s(-1) , a mean metallicity of [Fe/H]=-0.58+/-0.06 and the thick disk to halo transition occurs at around z=5.5 kpc. The lack of a significant rotational velocity gradient and the small or zero metallicity gradient suggest that the thick disk is the product of some heating or merger event(s) that occured in the young Galaxy rather than a transitional phase between the thin disk and halo formation. For the halo, the local density ratio to the thin disk is 0.1% and its mean metallicity is -1.66+/-0.09. The mean rotational velocity is Vrot=32+/-7 km s(-1) , which indicates that the halo in mean is in prograde rather than retrograde motion. No metallicity and kinematical gradients and correlation were found for the halo, giving support to the chaotic formation picture of Searle & Zinn rather than the well-ordered smooth collapse scenario of Eggen, Lynden-Bell & Sandage. As a by product, the space motion of NGC 288 was determined to be (u,v,w)LSR=(31+/-15,-263+/-16,54+/-1) km s(-1) , indicating that it is moving on a retrograde orbit.
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