R. J. Dickens

A Critical Review of the Evidence for M32 being a Compact Dwarf Satellite of M31 rather than a More Distant Normal Galaxy

Since Baades photographic study of M32 in the mid 1940s, it has been accepted as an established fact that M32 is a compact dwarf satellite of M31. The purpose of this paper is to report on the findings of our investigation into the nature of the existing evidence. We find that the case for M32 being a satellite of M31 rests upon Hubble Space Telescope (HST) based stellar population studies which have resolved red-giant branch (RGB) and red clump stars in M32 as well as other nearby galaxies. Taken in isolation, this recent evidence could be considered to be conclusive in favour of the existing view. However, the conventional scenario does not explain M32s anomalously high central velocity dispersion for a dwarf galaxy (several times that of either NGC 147, NGC 185 or NGC 205) or existing planetary nebula observations (which suggest that M32 is more than twice as distant as M31) and also requires an elaborate physical explanation for M32s inferred compactness. Conversely, we find that the case for M32 being a normal galaxy, of the order of three times as distant as M31, is supported by: (1) a central velocity dispersion typical of intermediate galaxies, (2) the published planetary nebula observations, and (3) known scaling relationships for normal early-type galaxies. However, this novel scenario cannot account for the high apparent luminosities of the RGB stars resolved in the M32 direction by HST observations. We are therefore left with two apparently irreconcilable scenarios, only one of which can be correct, but both of which suffer from potentially fatal evidence to the contrary. This suggests that current understanding of some relevant fields is still very far from adequate.

Deep CCD Photometry of Omega Centauri

This paper presents a color magnitude diagram for the enigmatic cluster ω Centauri (NGC 5139 = C1328 - 472) tracing the main sequence down to V ∼ 21.5. The spread in color on the upper main sequence is confirmed as intrinsic to the cluster. The CCD observations were made using the SAAO 1 m telescope with the UCL CCD camera and the RGO CCD camera at the prime focus of the AAT.

Double-Mode RR Lyrae Stars in IC 4499

Thirteen double-mode RR Lyrae (RRd) stars, with mean magnitudes c =18.30 ± 0.10 and c =17.80 ± 0.15, have been identified in the variable-rich Oosterhoff type I globular cluster IC 4499. The stars have surprisingly uniform properties, and are considerably different from RRd stars found in Oo II systems. The mean first-overtone period (Fig.1) is =0.357 d ± 0.005 d (cf. =0.40 d for Oo II RRd stars), and the mean ratio of the first-overtone period to the fundamental period is = 0.7443 ± 0.0002. The mean double-mode pulsation mass for the 13 stars, using the King Ia (Y=0.279, Z=0.001) mass calibration, is 0.535 ± 0.003 M⊙. Such an average mass is 0.11 M⊙ smaller (i.e. ∼17% smaller) than that for RRd stars found in Oo II systems, and possibly ∼0.01 M⊙ smaller than the mean mass for the two RRd stars in M3 (it is important to note that the zero point of these mass determinations is uncertain by at least 15%). The metal abundances for the RRd stars, and for the system of RR Lyrae stars as a whole, are found to be consistent with = -1.38 ± 0.20, determined from ΔS spectroscopy. In the Peterson diagram (Fig.2), all known RRd stars now divide (apparently by mass) into two groups (split according to Oosterhoff type). With a reddening of EB-v=0.26 ± 0.03, the cluster distance modulus is (m-M)0=16.23 m ± 0.23 m .

On the mixed-mode RR lyrae variables in the globular cluster Ic 4499

Double-mode RR Lyrae variables have been found in several Elobular clusters, notably MI5 (Cox, Hodson and Clancy 1983 (CHC), and references therein) and M3 (GoransMiJ 1981), and in the Draco dwarf Ealaxy (Nemec 1985). Conversely. none at all have been found in some other clusters investiEated exhaustively, notably OmeEa Cen (Nemec et al 1986) and M5 (Nemec and Clement 1966). AlthouEh the incidence of double-mode behaviour in RR Lyraes is not well understood theoretically, the application of linear pulsation theory to sophisticated stellar models (J~rEenson and Paterson 1967, Peterson 1978, CHC) has shown that the ratio of the first-overtone period to that of the fundamental has been found to depend on the stellar mass.

The Two-Component Cluster in Centaurus

In a previous paper (Lucey et al 1980) we reported the existence of a double-peaked velocity distribution for the rich, nearby Centaurus cluster (α = 12h 46.9m, δ = −40° 57′). We have now obtained an enlarged data set for Centaurus consisting of 203 redshifts and GF photometry (g = IIIa−J + GG495, F = IIIa−F + RG630) for 329 galaxies which lie within 3° of the cluster’s centre. We have redshifts for all but 2 of the 99 galaxies brighter than G26.5 = 16.5 that lie within 1° of the cluster’s centre. The velocity histogram for the enlarged data set is shown (figure 1). The velocity distribution is bimodal and has best fit double gaussian parameters of: n1 = 123, v1 = 3041 km s−1, σ1 = 577 km s−1 and n2 = 57, v2 = 4570 km s−1, σ2 = 262 km s−1 where the velocity dispersions have been corrected for a measuring error of 100 km s−1. We have labelled the lower velocity and the higher velocity components Cen30 and Cen45 respectively. The sky distributions of the two components overlap completely. The galaxy distribution in the core region of the cluster is shown in figure 2.