Total magnitudes of Virgo galaxies - III. Scale errors in the Reference Catalogue of Bright Galaxies T system and light-profile distortion by resolution-degrading and differential-distance effects

Abstract

We investigate the BT magnitude scales of the Second and Third Reference Catalogues of Bright Galaxies, finding both scales to be reasonably reliable for 11.5 ≲Bt≲ 14.0. However, large‐scale errors of 0.26 and 0.24 mag per unit mag interval respectively are uncovered for early‐type galaxies at the bright ends, whilst even larger ones of 0.74 and 0.36 mag per unit mag interval are found for galaxies of all morphological types at the faint ends. We attribute this situation to several effects already discussed by Young et al. and Young (Paper I), including the application of relatively inflexible growth‐curve models that are only in a few specific cases appropriate to the galaxies concerned. Of particular interest to this study though, we find that the apparent profile shapes of giant galaxies in the Virgo direction of cz < 15 000 km s‐1 tend to be less centrally concentrated the greater their distance. This demonstrates that even for relatively nearby galaxies, the distortion of the overall shapes of light profiles by resolution‐degrading effects such as seeing and data smoothing, as originally predicted and modelled by Young & Currie and Young et al., is a significant effect. It is, therefore, not good practice simply to extrapolate the profiles of galaxies of identical intrinsic size and intrinsic profile shape (i.e. identical morphology) by means of the same growth‐curve model, unless the galaxies are known a priori to be at the same distance and unless their photometry is of the same angular resolution. We also investigate the total‐magnitude scale of the catalogue of photometric types of Prugniel & Héraudeau, finding it to be much more reliable than the BT one. However, we argue that photometric type is really a measure of apparent profile shape (i.e. intrinsic profile shape after scale reduction on account of distance followed by convolution with a seeing disc and often a smoothing function as well). Strictly, it should therefore only be applicable to comparisons between galaxies that are already known to be at similar distances provided that their photometry is also of similar angular resolution. Clearly, this must complicate attempts to construct quantitative morphological classification schemes for galaxies.

The Sheet of Giants: Unusual properties of the Milky Way’s immediate neighbourhood

“…Indeed, Prugniel & Heraudeau (1998) found that RC3 magnitudes were systematically dimmer by 0.06 mag than magnitudes measured with growth curves which were linear combinations of exponential and de Vaucouleurs profiles. Furthermore, Franco-Balderas et al (2004) found that RC3 magnitudes were 0.12 mag dimmer than the total magnitudes they measured, while Young (2004) found an apparent magnitude-dependent offset, with RC3 magnitudes being 0.03 mag brighter compared to their estimates at dim magnitudes and up to 0.5 mag dimmer at the brightest magnitudes. However, their bright galaxy sample is small, and no other study has found such a large offset.…”

“…This increases to 99.7 per cent if there is a 0.06 mag bias and 99.83 per cent if there is a 0.12 mag bias. When we implement the more extreme bias model argued for by Young (2004), the significance reaches 99.93 per cent.…”