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

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6. Virgo and Fornax dwarfs: a dichotomy?

Binggeli & Jerjen state that: `If the intrinsic dispersion of the n -M or the n -log [FORMULA] relation is much smaller for Fornax dwarfs than for Virgo dwarfs as it appears (which, however, might be caused by the incompleteness of YC's Fornax sample) we are in need of an explanation for this difference'.

In response to their criticisms that we excluded three suspected non-cluster members when investigating the scatter in our R -n relationship, we have re-measured the scatter in our R -n correlation without excluding any outlier. For a polynomial of the form [FORMULA], which has the advantage over Eq. 1 in YC95 of being monotonic, the scatter in R based on all 26 of the objects listed in Table 2 of YC95 is [in terms of magnitudes]: 0.55 mag. Allowing for a conventional Fornax-Cluster depth of 0.15 mag., but not making any allowance for possible foreground or background objects, places an upper limit on the intrinsic scatter of 0.53 mag. This is very much lower than the scatter found in the same relationship for Virgo galaxies (see Sect. 4). If, as Binggeli & Jerjen maintain, the much larger scatter observed for Virgo galaxies were intrinsic, we would therefore indeed be in need of an explanation as to why these Fornax dwarf galaxies differ so radically from their counterparts in Virgo.

As Binggeli & Jerjen concede, King (1966) profiles do not fit Virgo dwarf-elliptical galaxy profiles well. This suggests that tidal truncation is not a significant contributor to the luminosity profile shapes at the radii of interest. We therefore consider it unlikely that tidal effects could offer the explanation. Furthermore, on the basis of the colour information presented by Caldwell & Bothun (1987) and YC95, it is clear that most of the brighter Fornax and Virgo dwarf ellipticals have very similar colours, suggesting that they may well have very similar stellar populations and histories. We therefore remain to be convinced of Binggeli & Jerjen's suggestion that there is probably a dichotomy between Virgo and Fornax dwarfs.

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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