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The breaks and truncations in the luminosity profile of face-on spiral galaxies offer valuable insights in their formation history. The traditional method of deriving the surface photometry profile for face-on galaxies is to use elliptical averaging. In this paper, we explore the question whether elliptical averaging is the best way to do this. We apply two additional surface photometry methods, one new: principal axis summation, and one old that has become seldom used: equivalent profiles. These are compared to elliptically averaged profiles using a set of 29 face-on galaxies. We find that the equivalent profiles match extremely well with elliptically averaged profiles, confirming the validity of using elliptical averaging. The principal axis summation offers a better comparison to edge-on galaxies. [ABSTRACT FROM AUTHOR]

Context. The determination of stellar mctallicity and its gradient in external galaxies is a difficult task, but crucial for the understanding of galaxy formation and evolution. Alms. The color of the Red Giant Branch (RGB) can be used to determine metallicities of stellar populations that have only shallow photometry. We will quantify the relation between mctallicity and color in the widely used HST/ACS filters F606W and F8I4W. Methods. We use a sample of globular clusters from the ACS Globular Cluster Survey and measure their RGB color at given absolute magnitudes to derive the color-metal I icily relation. We especially investigate the scatter and the uncertainties in this relation and show Results. There is a clear relation between mctallicity and RGB color. A comparison with isochrones shows reasonably good agreement with BaSTl models, a small offset to Dartmouth models, and a larger offset to Padua models. Conclusions. Even for the best globular cluster data available, the mctallicity of a simple stellar population can be determined from the RGB alone only with an accuracy of 0.3dex for [M/H] ≳ -1, and 0.15dcx for [M/H] ≳ -1. For mixed populations, as they are observed in external galaxies, the uncertainties will be even larger due to uncertainties in extinction, age. etc. Therefore caution is necessary when interpreting photometric metallicities. [ABSTRACT FROM AUTHOR]

ABSTRACT The stellar haloes of galaxies can currently be studied either through observations of resolved halo stars or through surface photometry. Curiously, the two methods appear to give conflicting results, as a number of surface photometry measurements have revealed integrated colours that are too red to be reconciled with the halo properties inferred from the study of resolved stars. Several explanations for this anomaly have been proposed - including dust photoluminescence, extinction of extragalactic background light and a bottom-heavy stellar initial mass function. A decisive test is, however, still lacking. Here, we explain how observations of the halo of a nearby galaxy, involving a combination of both surface photometry and bright star counts, can be used to distinguish between the proposed explanations. We derive the observational requirements for this endeavour and find that star counts in filters VI and surface photometry in filters VIJ appear to be the optimal strategy. Since the required halo star counts are already available for many nearby galaxies, the most challenging part of this test is likely to be the optical surface photometry, which requires several nights of exposure time on a 4-8 m telescope, and the near-infrared surface photometry, which is most readily carried out using the upcoming James Webb Space Telescope. [ABSTRACT FROM AUTHOR]