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Astron. Astrophys. 333, 795-802 (1998)

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Distance indicators based on the luminosity-profile shapes of early-type galaxies-a reply

Christopher Ke-shih Young 1 and Malcolm J. Currie 2, 3

1 Beijing Astronomical Observatory, Chinese Academy of Sciences, Beijing 100080, P.R. China
2 Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, Scotland, UK
3 Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, England, UK

Received 5 November 1997 / Accepted 28 January 1998

Abstract

In a recent paper, Binggeli & Jerjen (1998) question the value of the extragalactic distance indicators presented by Young & Currie (1994 & 1995) and state that they have refuted `the claim that the Virgo dEs [dwarf-elliptical galaxies] ... are distributed in a prolate structure stretching from 8 to 20 Mpc distance (Young & Currie 1995).' even though no such claim was ever made.

In this paper, we examine Binggeli & Jerjen's claims that intrinsic scatter rather than spatial depth must be the main cause of the large scatters observed in the relevant scaling relationships for Virgo galaxies. We investigate the accuracy of Binggeli & Jerjen's photometric parameters and find that while their profile curvature and scalelength measurements are probably useful, their total magnitude and central surface-brightness measurements are not useful for the purpose of investigating scaling laws because they suffer from serious systematic and random errors. We also investigate Binggeli & Jerjen's criticisms of our (1995) analysis. We demonstrate that their test for strong mutual dependence between distance estimates based on the two different scaling laws is invalid because of its prior assumption of negligible cluster depth. We further demonstrate that the [relative] distance estimates on which their kinematical arguments are based cannot be meaningful, not only because of the seriousness of the photometric errors, but also because they are undermined by the prior assumption that depth effects can again be neglected.

Interestingly, we also find that Binggeli & Jerjen's own dataset does itself contain evidence for large depth. Using the observed correlation between scale-length and profile-curvature, (the only correlation that can be investigated meaningfully using their dataset), we find that the frequency distribution of residuals with respect to the best fitting curve deviates significantly from that expected from a uni-modal Gaussian distribution. Clearly, if as Binggeli & Jerjen claim, the very large scatter observed in this scaling relationship for Virgo galaxies (which is not observed for Fornax or Local Group ones) were intrinsic, one would expect a uni-modal Gaussian distribution.

Key words: cosmology: distance scale – galaxies: clusters: individual: Virgo – galaxies: elliptical and lenticular – galaxies: distances and redshifts – galaxies: fundamental parameters – galaxies: photometry

Send offprint requests to: C.K. Young, (c.young1@physics.oxford.ac.uk)

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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