Astronomy and Astrophysics – Astronomy
Scientific paper
Nov 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998larm.confe.108o&link_type=abstract
"IX Latin American Regional IAU Meeting, "Focal Points in Latin American Astronomy", held in Tonantzintla, Mexico, Nov 9-13, 199
Astronomy and Astrophysics
Astronomy
Scientific paper
One of the most common problems in astronomy is that of distance determinations. Astronomers wish to find objects bright enough to be useful as standard candles. Undoubtedly, globular clusters are bright enough to be considered potentially interesting objects for distance determination. The obstacle arising from the broad range that they span in luminosity brightness can be avoided by observing samples of many of them, and then using the globular cluster luminosity function (GCLF) as a standard candle. A growing amount of observational evidence supporting the universality (at first order) of the GCLF has been acquired during the last tenure. In virtue of the progressive advance of astronomical technology, it has recently been feasible to reach and go beyond the turn-over in an increasing sample of galaxies. It is now possible to look for the second order differences between GCLFs, and its relations with their environments and metallicities. Today, some peculiarities of GCLFs are the target of investigation. Besides, the number of galaxies in which globular clusters are suitable for distance determination has been continuously increasing. From a practical point of view, the determination of GCLF parameters presents several troubles: First, one always observes the bright half of the luminosity distribution, and loses an usually significant fraction of the faint half. Second, the sample of globulars is unavoidably contaminated by interlopers, and this contamination must be removed by means of some statistical consideration. Hanes and Whittaker (1987) explored the use of maximum likelihood estimation techniques in the use of GCLFs as distance indicators. Their work was specifically focused to GCSs at the distance of the Virgo Cluster, whose galaxies began to be accessible to GCLF analysis by that time. Later, Secker and Harris (1993) performed further numerical experiments with their own maximum likelihood code. In view of the details of the GCLFs that now begin to be investigated, it would be interesting to know the feasibility of determining m0 and sigma with the sufficient precision. In this work, the credibility of GCLF parameter determinations, under a wider field of observational conditions, is investigated by means of Monte-Carlo experiments. The more relevant results are: Only when the sample is numerous, with very little contamination, and the peak is surpassed by, at least, two magnitudes, it is possible to obtain exact values for m0 and sigma. It is important to note that, in the cases of relatively high background contamination, although the turn-over has been surpassed by two magnitudes the results are uncertain enough to prevent the detection of differences of order 0.1 or 0.2 magnitudes in sigma. When m0 is equal to the limiting magnitude m_lim, then the resulting fitted parameters are affected by significant systematic errors. If m0-m_lim=-1, then the errors in m0 estimates tend just to -1, that is, is not possible to distinguish between the true turn-over and the lack of completeness. In these last cases, the fitting of a t_5 distribution instead a gaussian seems to give roughly better results, but this only occurs when the luminosity function is also a t_5. The fit of a t_5 to a gaussian generate GCLF also fails if the turn-over is not clearly surpassed.
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