Australia Telescope National Facility

Compact Stellar Systems in the Fornax Cluster: Super-massive Star Clusters or Extremely Compact Dwarf Galaxies?
M. J. Drinkwater, J. B. Jones, M. D. Gregg, S. Phillipps, PASA, 17 (3), 227.

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Discussion

We cannot say much more about the nature of these objects on the basis of our existing data. In ground-based imaging, they are intermediate between large GCs and small compact dwarf galaxies, so it becomes almost a matter of semantics to describe them as one or the other. The most promising way to distinguish between these possibilities is to measure their mass-to-light (M/L) ratios. If they are large, but otherwise normal, GCs, they will be composed entirely of stars giving very low M/L. If they are the stripped nuclei of dwarf galaxies we might expect them to be associated with some kind of dark halo, but we would not detect the dark halos at the small radii of these nuclei, so we would also measure small M/L values. Alternatively, these objects may represent a new, extreme class of compact dwarf elliptical (``M32-type'') galaxy. These would presumably have formed by gravitational collapse within dark-matter halos, so would have high mass-to-light-ratios, like dwarf galaxies in the Local Group (Mateo 1998). One argument against this interpretation is the apparent lack of M32-like galaxies at brighter luminosities (Drinkwater & Gregg 1998). If the compact objects are dwarf galaxies, they will represent the faintest M32-like galaxies ever found. They may also fill in the gap between globular clusters and the fainter compact galaxies in the surface brightness vs. magnitude distribution given by Ferguson & Binggeli (1994).

A further possibility is that these are small scale length ( $\sim 100$ pc) dwarf spheroidal galaxies of only moderately low surface brightness. While Local Group dSphs of equivalent luminosities generally have substantially larger scale sizes (and consequently lower surface brightnesses) (Mateo, 1998), Leo I for example has M_B = -11.0, and a scale length of only 110 pc (Caldwell et al., 1992), but as we discuss above this would be resolved in our existing imaging.

Our existing data will only allow us to estimate a conservative upper limit to the mass of these objects. If we say that the core radii of the objects are less than 75pc and the velocity dispersions are less than 400

${\rm\thinspace km}{\rm\thinspace s}^{-1}\,$ (the resolution of our 2dF spectra) we find that the virial mass must be less than

$10^{10}\hbox{M$_\odot$}$ . For a typical luminosity of M_B=-12 this implies that

M/L < 2 x 10³. This is not a very interesting limit, so we plan to reobserve these objects at higher spectral resolution from the ground and higher spatial resolution with the Hubble Space Telescope (HST) in order to be sensitive to

$M/L\approx 100$ . This will allow us to distinguish globular clusters from dwarf galaxies.

In order to demonstrate what we could measure with high-resolution images, we present two extreme possibilities in Figure 6: a very compact Galactic globular cluster and a dwarf galaxy with an r^1/4 profile (r_e=0.2 arcsec), both normalised to magnitudes of B=19 (V=18.4) and the Fornax cluster distance. We also plot the PSF of the Space Telescope Imaging Spectrograph (STIS) in the Figure for reference. The globular cluster profile is that of NGC 2808 (Illingworth & Illingworth 1976) with the radius scaled to the distance of the Fornax Cluster and the surface brightness then scaled to give the desired apparent magnitude. The globular cluster profile is very compact and will only just be resolved with HST, but it will clearly be differentiated from the dwarf galaxy profile.

**Figure 6:** Predicted radial surface brightness profiles of the compact objects in two extreme cases: (A) a Galactic globular cluster (Illingworth & Illingworth 1976) scaled 3 mag brighter in surface brightness, and (B) a compact dwarf galaxy with an r^1/4 profile. Both are scaled to have total magnitudes of B=19 mag; they are not corrected for instrumental PSF which is also shown (C).
$\begin{figure} \hfil \psfig{file=hst_glob_f2.ps,angle=0,width=9cm}\end{figure}$

In addition to measuring the size of these objects for the mass measurement, the radial surface brightness profiles may also give direct evidence for their origin and relationship to other kinds of stellar systems. For example, if they are the stripped nuclei of galaxies, the remnants of the outer envelope might show up in the HST images as an inflection in the surface brightness profile at large radius.

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Contents Page: Volume 17, Number 3

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