Mathematics – Logic
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........11p&link_type=abstract
Thesis (PH.D.)--THE JOHNS HOPKINS UNIVERSITY, 1995.Source: Dissertation Abstracts International, Volume: 56-06, Section: B, page
Mathematics
Logic
Cluster Luminosity
Scientific paper
The M81 globular cluster system (GCS) was sought and detected as an excess density within an 11 kpc galactocentric radius. Globular candidates were selected from a pool of 3,774 objects located in a 50^' diameter field centered on M81, for which photometric BVR, morphological and astrometric information have been measured. The photometric study has led to a revised distance of 3.2 +/- 0.4 Mpc for M81. A catalogue of all candidate globular clusters within a 50 ^' diameter around M81 is provided which contains 223 starlike and 31 globular-like images. Spectra for 82 of the brighter candidates have also been obtained. The total cluster population in M81 is estimated at NT = 210 +/- 30, implying a specific frequency S_{N } = 0.9, typical for early type spirals. The M81, M31 and Milky Way globular cluster luminosity functions reach their peak at the same location, M_sp {V}{*} = -7.5 +/- 0.3, and their spatial, (B-V) and (V-R) distributions are very similar. Using the (B-R)_0 index to trace (Fe/H), a weak dependence of mean metallicity with galactocentric distance is observed, as in the Galaxy and in M31. This effect is due to intrinsically redder globulars near the center of M81. Furthermore, there is suggestive evidence that 13 +/- 5 objects are disk-reddened clusters, such that A V(M81 disk) = 0.6 +/- 0.3 mag. The relative strengths of Hdelta , CaIlambda4227 and FeI lambda4045 absorption lines were used to distinguish stellar images of M81 globular clusters from stars in the Milky Way. 25 candidates have been confirmed globular clusters. The mean GCS metallicity is < Fe/H > = -1.5 +/- 0.3, which agrees well with the relation between the mean metallicity of a cluster system and the luminosity of the parent galaxy. This relation suggests that most globular clusters are created in unison with the formation and self-enrichment of the parent galaxy. Indeed, the combined data sets of the M81, M31 and Milky Way GCSs exhibit a significant chemical gradient Delta ({[ Fe/H] over logRproj/alpha}) = -0.45 +/- 0.07, as predicted in a dissipational collapse; no known bias was found to reproduce this effect. Assuming isotropic orbits, the projected mass of M81 within ~20 kpc is M ~ 1.6 +/- 0.5 times 1011M_odot implying M/L_{B_sp{T }{o}} ~ 10 +/- 3 M_odot/Lodot, in excellent agreement with values derived from the rotation curve of the gas (Rots 1975). A similar analysis for M31 and the Galaxy also confirms the isotropic hypothesis. The isotropic velocity ellipsoid reflects the mix of disk and halo globulars. Another effect may be the selection against globulars in radial orbits due to disk shocking.
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