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Ýlgili Makaleler
A CCD BVI Photometric Study of the Young, Highly Reddened Open Cluster NGC 6318
We present CCD BVI photometry for the southern open cluster NGC 6318.The sample consists of 9876 stars measured in an area of13.6m×13.6m, extending down to V~21.5 mag. Star countscarried out within and outside the cluster region allowed us to estimatethe cluster angular radius as ~8'. The comparison of the clustercolor-magnitude diagrams with isochrones of the Geneva group yieldsE(B-V)=1.20+/-0.05, E(V-I)=1.55+/-0.10, and V-MV=15.45+/-0.35for logt=8.20 (t=160 Myr) and Z=0.020. NGC 6318 is then located at2.1+/-0.5 kpc from the Sun and 30 pc below the Galactic plane. Using theWEBDA open cluster database, we examined the structure of the Galacticdisk along the line of sight of NGC 6318. Among the known clusters inthis direction, HM 1 and BH 222 are the farthest ones, while thoselocated between 1 and 2 kpc of the Sun appear to be more absorbed thanthose expected to follow a quasi-linear extinction law.
| Optical and Near-Infrared Color Profiles in Nearby Early-Type Galaxies and the Implied Age and Metallicity Gradients
We present results of an age and metallicity gradient analysis inferredfrom both optical and near-infrared surface photometry. The analysis isbased on a sample of 36 nearby early-type galaxies, obtained from theEarly Data Release of the Sloan Digital Sky Survey and the Two MicronAll Sky Survey. Surface brightness profiles were derived in each bandand used to study the color gradients of the galaxies. Using simplestellar population models with both optical and near-infrared colors, wemay interpret the color gradients in terms of age and metallicitygradients of galaxies. UsinggZ≡dlogZmet/dlogR andgA=dlog(age)/dlogR to represent the metallicity and agegradients, we found a median value of gZ=-0.25+/-0.03 for themetallicity gradient, with a dispersionσgZ=0.19+/-0.02. The corresponding valuesfor the age gradients were gA=0.02+/-0.04 andσgA=0.25+/-0.03. These results are in goodagreement with recent observational results, as well as with recentsimulations that suggest that both monolithic collapse and major mergershave played important roles in the formation of early-type galaxies. Ourresults demonstrate the potential of using multi-wave band colorsobtained from current and future optical and infrared surveys inconstraining the age and metallicity gradients of early-type galaxies.
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| The galactic halo - Globular clusters
The distribution of globular clusters is studied. Individual distancesare redetermined on the basis of available data and an adopted apparentmagnitude of +0.6 for RR Lyrae stars. Both metal-poor (H) andcomparatively metal-rich (D) clusters appear to have an essentiallyspherical distribution with the D clusters concentrated close to thegalactic center except for a few cases. Kinematic data are consistentwith the spatial density distributions if a circular-velocity value forthe local standard of rest of 200 to 225 km/s is adopted (for a solardistance from the galactic center of 9 kpc). The mass density in theglobular-cluster system declines sharply beyond 20 kpc from the center.Estimates of the mass of the halo are made. While it is quite possiblethat the mass of the halo exceeds that of the disk, there is no evidencethat the mass of the galaxy exceeds previous estimates by a substantialamount.
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