Stellar population synthesis diagnostics

Abstract

Abstract. A quantitative method is presented to compare observed and synthetic colour-magnitude diagrams (CMDs). The method is based on a χ 2 merit function for a point (c i , m i ) in the observed CMD, which has a corresponding point in the simulated CMD within nσ(c i , m i ) of the error ellipse. The χ 2 merit function is then combined with the Poisson merit function of the points for which no corresponding point was found within the nσ(c i , m i ) error ellipse boundary.Monte-Carlo simulations are presented to demonstrate the diagnostics obtained from the combined (χ 2 , Poisson) merit function through variation of different parameters in the stellar population synthesis tool. The simulations indicate that the merit function can potentially be used to reveal information about the initial mass function. Information about the star formation history of single stellar aggregates, such as open or globular clusters and possibly dwarf galaxies with a dominating stellar population, might not be reliable if one is dealing with a relatively small age range.

Stellar Populations in the Phoenix Dwarf ([CLC]dIrr[/CLC]/[CLC]dSph[/CLC]) Galaxy as Observed by [ITAL]Hubble Space Telescope[/ITAL] WFPC2

“…There are many complications to extracting star formation histories from the distribution of stars in a CMD, and several groups have developed di †erent methods for doing so (e.g., Tolstoy & Saha 1996 ;Dolphin 1997 ;Hernandez, Valls-Gabaud, & Gilmore 1999 ;Ng 1998 ;Gallart et al 1999 ;Holtzman et al 1999). In the current paper, we analyze our CMD based on the methods of Holtzman et al (1999), in which, by using least-squares techniques, the number of stars in many regions of the CMD are Ðtted to a combination of basis functions of stellar populations of different ages and metallicities.…”

Automatic observation rendering (AMORE)

“…2.3), -a mockup version of the HRD Galactic Software Telescope (HRD-GST, see Sect. 2.4), -a statistical diagnostic tool to compare the observed and synthetic CMDs with each other (Ng 1998), and -the PIKAIA (extended version) and POWELL optimizers (see respectively Sects. 2.6 and 2.8), which search for the best fit between an observed and a synthetic CMD in a multiparameter space.…”

“…The method actually uses F χ as a loosely fixed "anchor", puts the outlier points in F P and then reduces the number of unmatched points by minimizing F P . We refer to Ng (1998) for additional details and a discussion of these functions. Suffice to say that for an acceptable solution both F χ and F P are about 1 or smaller 8 which on its turn can be relaxed to the condition F < ∼ 2 9 and thus yields 1 3…”

“…All tests, unless stated otherwise, use the synthetic population as described by Ng (1998): − a metallicity range, spanning Z = 0.005-0.030; − an age range from 8-9 Gyr; − an initial mass function with a Salpeter slope; − an exponential decreasing star formation rate with a characteristic time scale of 1 Gyr. 13 Red Hat R is a registered trademark of Red Hat Software, Inc. and Linux R is a registered trademark of Linus Torvalds.…”

“…In this paper we discuss the development and the tests of an automated analysis method, which fully employs the colour and magnitude information available about the stars populating the CMD. Our method is based on an implementation of the CMD diagnostics suggested by Ng (1998). The method uses, in contrast to other techniques (see Bertelli et al 1992;Gallart et al 1996Gallart et al & 1999Geha et al 1998Harris & Zaritsky 2001Hernandez et al 1999; Send offprint requests to: Y. K. Ng, e-mail: ykng@tpd.tno.nl Tables A1-A3 are only available in electronic form at http://www.edpsciences.org and at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/392/1129 Holtzman et al 1997Holtzman et al & 1999 the full, unbinned distribution of magnitudes and colours of the stars populating the CMD.…”